engineers_handbook.pdf
Short Description
Download engineers_handbook.pdf...
Description
the complete solution for concrete building systems
.cscworld.com/orion
HANDBOOK CSC Orion ™
Orion Documentation page 2
Disclaimer
Disclaimer CSC (UK) Ltd does not accept any liability whatsoever for loss or damage arising from any errors which might be contained in the documentation, text or operation of the programs supplied. It shall be the responsibility of the customer (and not CSC)
• to check the documentation, text and operation of the programs supplied, • to ensure that the person operating the programs or supervising their operation is suitably qualified and experienced,
• to ensure that program operation is carried out in accordance with the user manuals, at all times paying due regard to the specification and scope of the programs and to the CSC Software Licence Agreement.
Proprietary Rights CSC (UK) Ltd, hereinafter referred to as the OWNER, retains all proprietary rights with respect to this program package, consisting of all handbooks, drills, programs recorded on CD and all related materials. This program package has been provided pursuant to an agreement containing restrictions on its use. This publication is also protected by copyright law. No part of this publication may be copied or distributed, transmitted, transcribed, stored in a retrieval system, or translated into any human or computer language, in any form or by any means, electronic, mechanical, magnetic, manual or otherwise, or disclosed to third parties without the express written permission of the OWNER. This confidentiality of the proprietary information and trade secrets of the OWNER shall be construed in accordance with and enforced under the laws of the United Kingdom. Orion documentation: © CSC (UK) Ltd 2010 All rights reserved.
Orion software: © CSC (UK) Ltd 2010 All rights reserved.
Trademarks Orion™ is a trademark of CSC (UK) Ltd. Fastrak™ is a trademark of CSC (UK) Ltd. TEDDS® is a registered trademark of CSC (UK) Ltd. The CSC logo is a trademark of CSC (UK) Ltd. Autodesk and Revit are registered trademarks or trademarks of Autodesk Inc. in the USA and/or other countries. Microsoft and Windows are either trademarks or registered trademarks of Microsoft Corporation in the United States and/or other countries. Acrobat® Reader Copyright © 1987-2010 Adobe Systems Incorporated. All rights reserved. Adobe and Acrobat are trademarks of Adobe Systems Incorporated which may be registered in certain jurisdictions. All other trademarks acknowledged.
Friday 29 October 2010 – 14:42
Table of Contents
Orion Documentation page 3
Chapter 1
Overview .
Chapter 2
Modelling Techniques .
Chapter 3
. . . . . Introduction . . . . . Modelling Analysis and Design Flowchart Build the Model . . . . . Derive Beam Loads . . . . General Building Analysis . . . Design and Detailing . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
15 15 15 16 17 17 17
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18 18 19 19 20 23 24 25 26 28 29 29 31 31 32 35 39 39 41 41 42 42 43 43 44 44 44 45 46 46 47 47 49 49 49 49 50 50 50
Beam Loads and Load Decomposition Methods
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
51 51 51 52 53 54
. . . . Introduction . . . . . . . Modelling Inclined and Lowered Members . . Sloping and Lowered Slabs . . . . How To Drop Parts of a Slab Panel . . . . Sloping and Lowered Beams . . . . Sloping and Lowered Columns . . . . Sloping and Lowered Walls . . . . Working With Planes . . . . . Modelling Curved Axes and Beams . . . Curved Axes . . . . . . . The Curved Axis Generator . . . . . Curved Beams . . . . . . . The Curved Beam Generator . . . . How many segments to use? . . . . Linking Angled Beams. . . . . . Columns and Walls Spanning More Than One Storey Example Case Study . . . . . . User Defined Supports . . . . . What is a default support? . . . . . When might a default support be inappropriate? . Specifying a User Defined Support . . . Applying a User Defined Support . . . User Defined Supports - Trouble shooting . . Stepped Foundation Levels . . . . . Default Supports Method . . . . . Single Storey Example of the Default Supports Method . Two Storey Example of the Default Supports Method . User Defined Supports Method . . . . Example of the User Defined Supports Method . . Beams with Varying Depth . . . . . Example Case Study . . . . . . Pinned Member Ends . . . . . . To Pin a Single Column . . . . . To Pin Multiple Columns . . . . . To Remove the Hinges from the Columns . . To Pin a Single Beam . . . . . To Pin Multiple Beams . . . . . To Remove the Hinges from the Beams . .
General . . . Modifying Beam Loads Beam Loads Dialog .
. . .
. . .
. . . .
. . . .
. . . How to Define a New Point Load . How to Define a New Uniformly Distributed Load .
. . . . .
Orion Documentation page 4
Table of Contents
How to Define a New Partial Distributed Load .
. . . . Using the vertex table . . How to Define a New Load on an Inclined Beam . . Switching between Yield Line and FE load Decomposition. What is Load Decomposition? . . . . . Why switch to an FE method? . . . . . Example of FE Method for Slab Load Decomposition . Why retain the traditional (yield line) method? . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
54 54 56 56 58 58 58 60 65
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
66 66 66 66 66 67
. . . . . . . . Limitations - diaphragm modelling and inclined planes . Case Study 1 - single storey pitched frame . . . . Case Study 2 - storeys linked by inclined planes . . . Global Constraints . . . . . . . Pattern Loading . . . . . . . . Rigid Zones . . . . . . . . Rigid Zones – None . . . . . . . Rigid Zones – Reduced by 25% . . . . . Rigid Zones – Max . . . . . . . Discussion . . . . . . . . Rigid Links . . . . . . . . Shear Walls and Core Wall Systems . . . . 3D Effects . . . . . . . . . Continuous Beams . . . . . . . Effects of one Member on Another . . . . . Sway Effects . . . . . . . . Transfer Beams . . . . . . . . Stiffness adjustments . . . . . . . Flat Slab Construction . . . . . . . Transfer Levels . . . . . . . . Supports . . . . . . . . . Building Analysis Problems – Reviewing/Understanding . What are Errors? . . . . . . . . What are Warnings? . . . . . . . Building Model Validity Checking Errors . . . . Beam Load Analysis Errors . . . . . . Building Analysis Errors and Warnings . . . . Overview of Axial Load Comparison Report . . . Table 1 . . . . . . . . . Table 2 . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 68 . 68 . 68 . 69 . 70 . 71 . 72 . 72 . 73 . 74 . 77 . 80 . 80 . 81 . 83 . 84 . 85 . 86 . 87 . 89 . 89 . 90 . 92 . 94 . 95 . 95 . 96 . 96 . 96 . 98 . 98 . 98 . 99 . 99 .100 .101 .104 .105
. .
. .
. .
. .
. . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
General Building Analysis .
. . . .
. . . .
. . . . Slabs to define rigid diaphragms (Default Setting) . Single rigid diaphragm at each floor level . . No rigid diaphragm floor levels . . . Excluding Specific Slabs from Diaphragms . .
. . . . . . . .
Using the Load Generator
Chapter 4
Analysis Methods Introduction.
.
. .
General Building Analysis Eigenvalue Analysis . Staged Construction Analysis Finite Element Floor Analysis
Chapter 5
Introduction. . Structural Model .
. . Diaphragm Modelling .
. . .
Table of Contents
Orion Documentation page 5
. .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. 109 . 109 . 110
. . Introduction . . . . Eigenvalue Analysis Parameters . Controlling the Storey Mass . Model Stiffness . . .
. . . . .
. . . . .
. . . . .
. . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
114 114 114 114 116 116 117 117 118 119 119 119
. . . . . . . . Combining the entire structure into a single stage . Setting the Duration of Each Stage . . . Analysis Properties . . . . . . Modulus of Elasticity. . . . . . FE Merging . . . . . . . Simulaneously designing for FE merged forces and the results of a completely unstaged analysis. .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
120 120 120 122 122 122 124 125 125 126 126 126 127 128
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
129 129 129 130 131 131 131 131 131 132 132 134 134 134 135 135 135 136 137 138 139 141 144
Comparisons between tables 1 and 2 Table 3 Table 4
Chapter 6
. .
. .
. .
Eigenvalue Analysis .
Controlling the Number of Mode Shapes Required
Analysis
.
.
. . . Exporting to S-Frame .
. . . . Exporting to S-Frame Example . Graphical Results Numerical Results
. . . .
. . . . .
. . . . .
. . . . .
Correction of Self Weight in S-Frame (for Eigenvalue Analysis only)
Chapter 7
Staged Construction Analysis
. . . . . . . Simple Example . . . . . Staged Construction Modelling and Analysis. Model Creation . . . . . Staged Loading Creation . . . . Stage Control . . . . . . Combining multiple floors into a single stage .
Introduction
Chapter 8
.
Analysis and Design using FE
. . . Column / Shear Wall Model Type . Beam Stiffness Multiplier . . . Slab Stiffness Multiplier . . . Column and Wall Stiffness Multipliers . Cracking and Creep . . . . Include Column Sections in FE Model . Include Slab Plates in FE Model . . Consider Beam Torsional Stiffness . Include Upper Storey Column Loads . Upper Storey Column Loads Table . Floor Mesh and Analysis . . . Batch FE Chasedown . . . Meshing and Analysing your Model . If Slab Plates are NOT included . . If Slab Plates are included . . . Validity Checking . . . . Mesh Density . . . . . Mesh Uniformity . . . . Effect of Holes and Boundaries . . Mesh Sensitivity . . . . Introduction . . . Model Generation Options .
. .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
Orion Documentation page 6
Table of Contents
Reviewing Results – Contours and Strips . Worked Example – Beam and Slab Systems Introduction . . . . . FE Mesh Generation . . . . Review of Contouring Options . . Deflection Plots. . . . . Mx and My Plots . . . . M1 and M2 Plots . . . .
. . . . . . . . Plots Including Wood and Armer adjustments . Steel Reinforcing Requirement Contours . . Other Contouring Adjustment Options . . Slab Design . . . . . . Effects of Adjusting the Beam and Slab Stiffnesses .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . .
.144 .145 .145 .146 .148 .149 .149 .150 .150 .153 .154 .155 .159 .163 .164 .166 .166 .166 .167 .170 .171 .173 .175 .176 .176 .176
Building Sway and Differential Axial Deformation Effects .
. .
. .
. .
.177 .177
Effects of Wood and Armer Moment adjustments on a Regular Slab Reinforcement Design . Merging Beam Results .
. . What does this option do? .
. . . When might you use this option? . Example . . . . . Option 1 – A Plateless Model . Checking the Beam Designs . . Solution 2 – A Meshed Model . Checking the Beam Designs . . Merging Column Results . . What does this option do? . . When might you use this option? .
Chapter 9
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . .
.
.
.
.
.
.
. .
. Analytical Idealisations . . . . Deflection . . . . . . Cases where one wall option is Preferable Option to Check Both Ways . . . Wall Panel Design . . . . . Forcing Walls to resist all lateral loads . Adjust Model Stiffnesses . . . Pin the Columns . . . . Sway Effects Under Gravity Load . . Fully Framed Structures . . . Why does this sway happen? . . . Structures Incorporating Flat Slab Areas . Slab Loads – Yield Line Decomposition . Slab Loads – FE Decomposition . . Discussion . . . . . Load Eccentricity . . . . Construction and Creep Effects . . Discrete Cores . . . . . Results based on an FE Chase Down . Closing Summary . . . . .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
.178 .178 .178 .178 .179 .179 .180 .180 .181 .181 .181 .182 .184 .184 .186 .187 .187 .188 .189 .190 .191
.
.
.
.
.
.
.
.
.
.192
Introduction.
.
.
.
.
Chapter 10
Wall Modelling Considerations
Chapter 11
Sway Deflection Verification .
.
Table of Contents
Orion Documentation page 7
Introduction . . . . . . . . Comparison of Orion's alternative wall modelling options Compare analysis results using other software . .
Chapter 12
Overview of Bracing and Sway Sensitivity . Introduction
. Automatic Assessment of Sway Sensitivity . User Defined Bracing . . . . Classification Requirements of each code . BS8110 (similarly CP65 and HK-2004) . . . EC2
.
.
.
.
.
.
.
.
.
.
.
.
.
. . . . . . . Implementation of EC2 Classification in Orion Setting the Braced/Bracing Members . . Assessment of Sway Sensitivity . . .
. . . . .
. . . . . .
.
.
. . . .
. . . . . . . . .
Worked Example for a Sway Sensitive EC2 Structure Model Analysis Properties
.
.
ACI Classification (for comparison). EC2 Classification to Annex H
.
. . .
Application of Load Amplification Factors
Chapter 13
. . . .
. . . .
. . .
. . .
. . .
. . .
. . .
. . .
. 192 . 192 . 193
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
.
.
. . . . . . . . .
.
. . . . . . . . .
.
. . . . . . . . .
.
. . . . . . . . .
. . . . . . . . .
195 195 195 196 197 197 . ACI 318-02198 . . 199 . . 201 . . 201 . . 201 . . 202 . . 203 . . 203 . . 205 . . 207
Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings . 208 Introduction . . . . . . Opening Discussion . . . . . Example 20 Storey Building . . . . Traditional Analysis Results . . . . Emulating the Traditional Approach in Orion Sub-Floor Analysis (FE Chasedown) . .
. . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
208 209 211 212 214 215 Building Analysis, area factor adjustment methods 216 Why do the two models give such different answers? . 218 How do we eliminate this effect if we want to? . . 219 Increasing the column area factor . . . 220 What is a reasonable upper limit for the column area factor adjustment? 223 Detailed comparisons of the analysis results . . . . 223 Recommendation . . . . . . . . 225 Is it acceptable to simply emulate the traditional design result? 226 Ignoring differential axial displacements . . . . 226 Making allowances for differential axial displacements . . 226 What is a reasonable lower limit for the area factor adjustment? 227 What is the impact on the design when both upper and lower-bounds are taken into consideration? 227 Consider Case where AF = 2.7 is upper bound and AF = 1.5 is the lower bound: . . . . . 228 Consider Case where AF = 4.0 is upper bound and AF = 1.0 is the lower bound: . . . . . 228 Consider Case where FE Chasedown is upper bound and AF = 1.5 is the lower bound: . . . . 228 What is the impact on the design when Pattern Loading is Introduced? . . . . . 228 Conclusion on Design Impact . . . . . . . . . . . . 229 Overall Summary of Suggested Procedure . . . . . . . . . . 230 Closing Discussion . . . . . . . . . . . . . . 231 Typical Concerns . . . . . . . . . . . . . . 231 What answers are we trying to get? . . . . . . . . . . . 231 Fixed Values for Area Adjustment Factor . . . . . . . . . . 231 Side Effects on Lateral Load Analysis . . . . . . . . . . . 232 Will FE Chasedown also eliminate differential axial deformation? . . . . . . 232 Is this going to result in uncompetitive over design? . . . . . . . . 233 Could I avoid all this complication if Staged Construction Analysis were used? . . . . 233 DADE Analysis & Design Flowchart . . . . . . . . . . . 234
Orion Documentation page 8
Chapter 14
Table of Contents
Using Staged Construction Analysis to Emulate ‘Traditional’ Design Introduction. . Worked Example: .
. .
. .
. .
. .
. .
. .
. .
. . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
.236 .236 .236 .237 .237 .237 .237 .238 .239 .241
. .
Previous Results (from FE Analysis and Unstaged Building Analysis)
. . . . .
. . . . .
. . Building Analysis with Area Factor adjustment . Staged Construction Analysis Result . . Does Staged Analysis eliminate DADE? . . Use of Staged Construction as the upper bound solution . . .
. .
. .
. .
. .
. .
. .
. .
. .
.242 .242
. . . . . . Overview . . . . . . . Slab Strip Errors – Reviewing/Understanding .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
.243 .243 .243 .243 .243
. . . . 1. Check slenderness limits for lateral stability- Cl 3.4.1.6 . 2. Rectangular or flanged - Cl 3.4.1.5 . . . . 3. Analysis of Sections - Cl 3.4.4.1 . . . . . 4. Design for Bending- Cl 3.4.4.4 and Cl 3.4.4.5 . . . 5. Design for Shear- Cl 3.4.5 . . . . . . 6. Deflection Checks- Cl 3.4.6 . . . . . Worked Example . . . . . . . . The Design Model . . . . . . . Beam Design Settings . . . . . . . Analysis Results . . . . . . . . Performing the Design . . . . . . Design for Bending - Cl 3.4.4.4 . . . . . Design for Shear - Cl 3.4.5 . . . . . . Deflection Checks- Cl 3.4.6 . . . . . . Output Calculations . . . . . . .
. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
.244 .244 .245 .245 .245 .246 .246 .246 .249 .251 .251 .251 .252 .253 .254 .254 .256 .258 .259
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
.260 .260 .260 .260 .261 .262 .262 .263 .264 .264 .265 .265 .267
Sub-Floor Analysis (FE Chasedown)
.
.
Building Analysis without Area Factor adjustment
Chapter 15
Design and Detailing . Introduction.
Chapter 16
.
.
. .
. .
. .
Slab Design .
No. of Slabs and Beams along strip is not consistent! Creating Member… (but nothing seems to happen)
Chapter 17
Beam Design to BS8110 .
. . . . Beam Design Settings . . . The BS8110 Beam Design Process . Introduction.
Chapter 18
.
Beam Detailing .
. . Introduction. . . . The Design and Detailing Process Overview . . . . The Design Tab . . . The Parameters Tab . . The Bar Selection Tab . . The Curtailment Tab . . The Detailing Tab . . The Layers Tab . . . Overview of Patterns . .
. . . . . . . . . . . Pattern 1 – The Splice Bar Method . Pattern 2 – The Alternative Method
. . . .
. . . .
. . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
Table of Contents
Orion Documentation page 9
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
268 269 270 270 270 272 272 273 274 277 282 284 285 286 288 289 290 292 294 295 298 299
. . . 1. Braced or unbraced - Cl 3.8.1.5 . 2. Calculate effective height- Cl 3.8.1.6 .
. . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . 3. Check slenderness limits- Cl 3.8.1.7 & 3.8.1.8 . 4. Classify as short or slender- Cl 3.8.1.3 . . 5. If slender - calculate M_add- Cl 3.8.3.1 . . 6. Calculate minimum moments - Cl 3.8.2.4 . . 7a. If braced, calculate design moments about each axis - Cl 3.8.3.2 .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
301 301 302 302 303 303 303 303 304 304 304 304 305 306 306 307 309 310 310 311 312 312 312 314 314 315 316 317 319 321 322 322 322
Pattern 3 – The Hanger Bar Method The Bent-Up Pattern Method .
Detailed Example and Comparisons Overview . . . . Basic Setup . . . . Design Tab . . . . Parameters Tab . . . Bar Selection Tab . . . Curtailment Tab . . . Detailing Tab . . . . Initial Design and Drawing Creation Effects of Applying Preferences . Bar Spacing Maximisation Limiting the Bar Range Merging Bars .
.
Minimum Tension Lap
. . . .
. . . .
Stop Using 2nd Support and Span Bars
.
Extend and Merge End Bars
Extend Support Bars Symmetrically Standardise Link Size Uniform Links
Summary .
Chapter 19
.
. .
. . .
. . .
Column Design to BS8110
. Introduction . . . . The BS8110 Column Design Process
7b. If unbraced, calculate design moments about each axis - Cl 3.8.3.7 8. Calculate equivalent uni-axial design moments - Cl 3.8.4.5 . . 9. Member Design - Cl 3.8.4 . . . . . . .
Worked Examples
.
.
.
.
.
.
.
The Design Model . . . . . . . Column Design Settings . . . . . . Braced Rectangular Column Example . . . . Performing the Design . . . . . . 1. Braced or unbraced - Cl 3.8.1.5 . . . . 2. Calculate effective height- Cl 3.8.1.6 . . . . 3. Check slenderness limits - Cl 3.8.1.7 & 3.8.1.8 . . 4. Classify as short or slender - Cl 3.8.1.3 . . . 5. If slender - calculate Madd- Cl 3.8.3.1 . . . 6. Calculate minimum moments - Cl 3.8.2.4 . . . 7. Calculate design moments about each axis - Cl 3.8.3.2 . 8. Calculate equivalent uni-axial design moments - Cl 3.8.4.5 9. Member Design - Cl 3.8.4 . . . . . Cross check of the above solution . . . .
Bi-Axial Design Method Example . Braced Circular Column Example .
. . 1. Braced or unbraced - Cl 3.8.1.5 . 2. Calculate effective height- Cl 3.8.1.6 .
. . . .
3. Check slenderness limits - Cl 3.8.1.7 & 3.8.1.8
. . . . .
. . . . .
. . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . .
Orion Documentation page 10
Table of Contents
4. Classify as short or slender - Cl 3.8.1.3 . . . . 5. If slender - calculate Madd- Cl 3.8.3.1 . . . . 6. Calculate minimum moments - Cl 3.8.2.4 . . . 7. Calculate design moments about each axis - Cl 3.8.3.2 . 8. Calculate equivalent uni-axial design moments - Cl 3.8.4.5 9. Member Design - Cl 3.8.4 . . . . . .
. . . . . . .
. . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
.323 .323 .323 .324 .324 .325 .325 .325 .326 .326 .326 .326 .327 .328 .328 .328
. . . . . . . . . . . . . . Mesh and Minimum Reinforcement Requirements . Points to Consider . . . . . . Which Mesh Type is Better? . . . . . Overview of Possible Reinforcement Arrangements .
. . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . Design using Type A Mesh (not considering plain wall design option) . Design using Type A Mesh (considering plain wall design option) . Design using Type B Mesh (considering plain wall design option) . Limitation Copy/Paste Bars will not work . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
.333 .333 .333 .333 .333 .335 .335 .336 .338 .339 .340 .340 .340 .341 .341 .342 .343 .344 .345 .346 .347 .347
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
.348 .348 .348 .349 .350 .351 .351 .351 .351 .352 .352 .353 .353 .354 .354
Unbraced Circular Column Example 1. Braced or unbraced - Cl 3.8.1.5 .
. . 2. Calculate effective height- Cl 3.8.1.6 .
. . .
3. Check slenderness limits - Cl 3.8.1.7 & 3.8.1.8 4. Classify as short or slender - Cl 3.8.1.3 . . 5. If slender - calculate Madd - Cl 3.8.3.1 . . 6. Calculate minimum moments - Cl 3.8.2.4 . 7. Calculate unbraced design moments about each axis - Cl 3.8.3.2 8. Calculate equivalent uni-axial design moments - Cl 3.8.4.5 . 9. Member Design - Cl 3.8.4 . . . . . . .
Chapter 20
Wall Design and Detailing
Chapter 21
Foundation Design .
. Introduction. . . . . Conservatism in the design method Wall Design and Detailing Options . Design With End Zones . . Design Without End Zones . . Should I use End Zones? . . Plain Wall Design . . .
. . . . . . . . Option to use Single Layer of Reinforcement . Design with Mesh Reinforcement . . . The "Revert to Loose Bar" Option . . . How the "Revert to Loose Bar" Option Works . Limitation of "Revert to Loose Bar" Option . . Column Steel Details View . . . .
. . Foundation Design Settings . Foundation Depth . .
. . . . The Foundation Forces Table . Combining Columns and Walls for Shared Foundation Design . To combine multiple columns and walls . . . . . To ungroup columns and walls . . . . . . Calculation of the Combined Footing Design Forces . . . Creating a Typical Pad/Pile Footing for Multiple Foundations . To create a typical footing . . . . . . . Pad Footings . . . . . . . . . Defining a Pad Footing . . . . . . . Pad Footing Details . . . . . . . . Combined Pad Footings . . . . . . .
Introduction.
.
.
. . . . .
. . . . . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . . . . . . . . . . . .
Table of Contents
Orion Documentation page 11
Pile Caps .
.
.
. . . . . . . Strip Footings . . . Defining a Strip Footing . Strip Analysis Options . Defining a Pile Cap . Basic Design Procedure Limitations . . Larger Pile Groups . Pile Cap Details . Combined Pile Caps .
. . . . . . . . . .
Adjusting the Subgrade Coefficient Enveloping all Load Combinations .
Strip Footing Design. . Beam Design . . . Creating Wide Strip Footings Combined Strip Footings .
. . . .
Defining a Combined Strip Footing Analysis and Design . Raft (or Mat) foundations Piled Rafts . . . Defining a Piled Raft . Piled Raft Design .
Chapter 22
. . . . .
. . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
Solution Options for Inclined/Lowered Members Introduction
.
.
.
.
.
.
. . Overview of Solution Options and Limitations . . Inclined Beam and Slab Loads . . . . . Building Analysis Worked Example . . . . Diaphragm Modelling . . . . . . Simplified Load Decomposition . . . . . Analysis . . . . . . . . . Load Comparison Check . . . . . . Switching to FE Beam Load Decomposition . . . Design and Detailing . . . . . . Inclined Beam Design . . . . . . Inclined Beam Detail Drawings and Quantities . . . Inclined Column Design . . . . . . Inclined Column Detail Drawings and Quantities. . . Tapered Wall Design. . . . . . . Tapered Wall Detail Drawings and Quantities . . . Design and Detailing of the Inclined Slabs . . . Tapered Wall Modelling . . . . . . General Limitations - Inclined/Lowered Members . . Load decomposition for lowered slabs . . . . FE Analysis Worked Example . . . . . Introduction . . . . . . . . Example Model . . . . . . . FE Chasedown Analysis . . . . . . FE Model Generation . . . . . . Load Comparison Check . . . . . . Member Design based on FE Analysis . . . . Limitations - FE Analysis of Inclined/Lowered Members . Limitations - Finite Element Analysis and Building Sway . S-Frame Comparison . . . . . . Typical Test Model Results . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
355 355 357 357 358 359 359 360 360 364 364 369 370 372 374 375 375 375 375 376 376 377
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
379 379 380 380 381 381 383 383 384 385 387 387 387 388 388 388 389 389 389 392 392 397 397 397 397 397 399 400 401 401 402 403 404
Orion Documentation page 12
Table of Contents
.
.
. . .
. . .
. . .
. . .
. . .
. . .
. . .
.404 .405 .405
Overview of Solution Options for Transfer Levels
Discussion
Conclusions
Chapter 23
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . . . .
. . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
.406 .406 .406 .407 .407 .407
. Modelling and Analysis . . . . . Analysis Model Options . . . . Model Tab . . . . . . Stiffnesses Tab . . . . . . Settings Tab . . . . . . Analysis . . . . . . . Load Comparison Check . . . . Design and Detailing of the Transfer Beams . Discussion of Frame Analysis Results . . Gravity Loads (Mid-Pier Wall Modelling) . . Building Analysis Results . . . . Front Transfer Beam . . . . . Rear Transfer Beam . . . . . Frame Action . . . . . . Gravity Loads (FE Meshed Wall Modelling) . Building Analysis Results . . . . Front Transfer Beam . . . . . Rear Transfer Beam . . . . . Frame Action . . . . . . Limitations – Transfer Walls . . . . Supporting Beam to carry all Wall Load . . No supporting Beam – Wall to act as a Deep Beam . Beam and Wall to Work Together . . . . Limitations – Walls Supported by more than 1 Beam Analysis . . . . . . . . Results based on Mid-Pier Wall Idealisation . . Results based on Meshed Wall Idealisation . . Alternative Modelling Option – Split the wall . . Mid-Pier Model . . . . . . . Meshed Model . . . . . . . Summary/Recommendations . . . . Option 1 – Do not split the wall . . . . Option 2 – Split the wall . . . . .
Introduction. . . . . . . Understanding the Problem and the Limitations Where Beams support Columns and Walls . Where Slabs support Columns and Walls . Key Limitation . . . . . .
Chapter 24
Transfer Beams – General Method .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.409 .409 .409 .409 .410 .410 .411 .412 .413 .414 .414 .415 .416 .417 .418 .420 .421 .422 .423 .424 .425 .426 .427 .428 .432 .433 .434 .435 .436 .437 .438 .439 .439 .439
Chapter 25
Transfer Beams – FE Method, Option 1 (Simplest)
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
.440 .440 .441 .443 .444 .445 .447 .447
Modelling and Initial Analysis . The FE Analysis and Load Chase Down Axial Load Comparison . . . Merging Column Analysis Results . Column Design . . . . Merging Beam Analysis Results . Merging Options . . .
. . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
Table of Contents
Orion Documentation page 13
. . . Beam Design . . . . FE Chase Down with Duplicate Floors
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
448 448 450 452
Transfer Beams – FE Method, Option 2
Discussion of Merged Results Front Transfer Beam .
Chapter 26
Chapter 27
. . . .
Modelling and Initial Analysis . . The FE Analysis and Load Chase Down . Axial Load Comparison . . . Merging Column Analysis Results . . Merging Beam Analysis Results . . Discussion of Merged Results . . Front Transfer Beam . . . . Beam Design . . . . . Effect of adjusting Slab Stiffness Factor .
. . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
454 454 455 459 460 461 461 461 462 462
Solution Option for Transfer Slabs
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
464 464 465 466 466 469 470 470 476 477 477
.
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
478 478 478 478 478 479 479 480 480 482 482 486 492 498
. . . . . Check Shear on Series of Perimeters . Simple Examples . . . . Checking a Typical Internal Column . Performing the Check . . . Checking Maximum Shear Capacity .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
499 499 499 499 499 499 500 501 502 502
Modelling and Initial Analysis
. . . . . . . . . . . . The FE Analysis and Load Chase Down . Axial Load Comparison . . . Merging Column Analysis Results . . Merging Beam Analysis Results . . Column and Wall Positioning Slab Insertion . . . Building Analysis . . Alternative Modelling Option Concluding Note on Modelling
Chapter 28
Flat Slab Models Introduction
.
. .
. .
. .
Scope of Flat Slab Design in Orion
. . . . Example of a more Irregular Model Overview . . . . Braced Buildings
.
Un-Braced Buildings .
Slab Analysis, Design and Detailing Meshing Deflection
. .
. .
. .
. .
Bottom Steel Reinforcement Provision Top Steel Reinforcement Provision .
. . . . . . . . . . . .
Additional Notes on Bottom Steel Provision
Column Design
Chapter 29
.
.
Punching Shear Checks .
.
. Introduction . . . . BS8110 Design Code Requirements The Design Procedure . . Check Maximum Shear Capacity .
Orion Documentation page 14
Table of Contents
. . . Checking a Typical Corner Column . Column Drop Panels . . . . Dealing with Openings . . . Openings which have been modelled . Calculation of the Effective Shear Force . Allowing for Openings which have not been modelled . Final Batch Check and Output . . . . Concluding Notes . . . . . . . Limitations . . . . . . . Holes . . . . . . . . Dimension ‘x’ used at the face of the loaded area . . Slab Merging . . . . . . . Specification of Effective Slab Reinforcement . . Providing the Shear Reinforcement . . . . Punching Perimeters . . . . . . Walls . . . . . . . . Overlapping Perimeters . . . . . Discontinuous Columns . . . . . Advantages . . . . . . . EC2 Design Code Requirements . . . . The Design Procedure . . . . . . Check Maximum Shear Capacity . . . . Check Shear Capacity at the Basic Control Perimeter.
. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . Calculate Shear Reinforcement Required and Length of Outer Control Perimeter . Calculation of Magnification Factor . . . . . . . . Internal column . . . . . . . . . . . Edge column . . . . . . . . . . . . Corner column . . . . . . . . . . . . Calculation of W1 . . . . . . . . . . . . . . Requirements for Models to be Linked for Foundation Design . Procedure for Linking Existing Models into a Foundation Project . Linked Project Manager . . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
Check Shear on a Series of Perimeters
. Checking a Typical Edge Column . Providing Shear Reinforcement
Chapter 30
Linking and Merging Projects . Introduction. . . . Creating a Foundation Project
Chapter 31
. .
Reports and Drawing Output . Introduction. Orion Reports
. . Report Manager . Quantity Reports . Drawings . . Examples . .
. . . . . .
. . . . . .
. . . . . .
. . . . . . . .
. . . . . . . .
. . .
. . .
. . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.503 .505 .507 .510 .511 .514 .514 .516 .517 .519 .520 .520 .521 .522 .523 .527 .527 .527 .528 .528 .529 .529 .529 .529 .529 .529 .530 .530 .530 .531 .531 .532
. . . . . .
. . . . . .
. . . . . .
.534 .534 .536 .536 .536 .538
. . . . . . .
. . . . . . .
. . . . . . .
.539 .539 .539 .541 .541 .542 .543
Chapter 1 : Overview
Chapter 1
Orion Documentation page 15
Overview
Introduction The aim of this handbook is to provide added background information together with hints, tips, and examples all of which should help you to make the most of Orion. This manual is not written at a Getting Started level, and it is recommended that you have worked through basic training examples in order to become familiar with the system and terminologies used, before addressing the more complex detail provided here. Copies of the getting started and basic training manuals in pdf format are installed with Orion and can be accessed by clicking on the links below. • Orion Quick Start guide.pdf
• Orion Standard Training Manual.pdf (If the links do not work please browse to find the file name indicated above in the HELP sub-folder of the Orion Program Folder). It is particularly noted that the Standard Training Manual covers many topics in sufficient detail that no further mention is required in this document.
Modelling Analysis and Design Flowchart If you write down the main headings for what you expect a Concrete Building Modeller to do it will probably look pretty simple: 1. Provide a way to input/describe the model. 2. Analyse it. 3. Design it. 4. Produce Calculations. 5. Produce Drawings. If you have worked through the training course notes you will know that Orion lets you do all this. The following flow chart illustrates this basic sequence, but it also indicates options. The existence of such options can sometimes lead to confusion – which option should you choose? In this chapter we will try to deal with each heading in a little more detail, indicate when you might use the optional routes, and provide cross reference to other chapters with more detailed information and/or worked examples.
Orion Documentation page 16
Chapter 1 : Overview
Orion modelling, analysis and design flowchart 1. Build the model
2. Derive beam loads using the ‘Yield Line’ (tributary area) approach
3. Run the general building analysis to generate column, wall and beam design forces
Slab design based on tabulated code coefficients
2a - optional Derive beam loads based on a special FE model. Choose whether to use these loads selectively or on all beams
3a - optional Use sequential FE floor analyses to chase gravity loads down through the structure. Selectively merge/override column wall and beam design forces with those of the general building analysis
3b - optional Use same FE models to generate and merge alternative slab design forces
4.1 Beam design and detailing
4.2 Column/Wall design and detailing
4.3 Slab design and detailing
Build the Model As noted earlier, the primary source of help in this aspect of Orion usage is the Training Manual. Ordinary regular models can be constructed with speed and ease. Many chapters in this manual provide modelling information, hints and tips for more unusual circumstances.
Chapter 1 : Overview
Orion Documentation page 17
Derive Beam Loads This might incorrectly be regarded as part of the building analysis, it is not, slab loads are decomposed onto the supporting beams prior to building analysis. Examining the flowchart indicates that you have options here and it is important to understand what these are and when you might use them. The default load decomposition method is based on tributary areas, and is commonly known as the Yield Line Method. This method has limitations in circumstances such as: 1. When the slab boundaries are highly irregular 2. When there are significant holes defined in the slab 3. When there are eccentric concentrated point, patch, or line loads The alternative FE Method will deal with these more extreme conditions. A more detailed discussion and example is provided in the chapter Beam Loads and Load Decomposition Methods.
General Building Analysis A full 3D analysis model is derived from the physical information that you describe when constructing the model. This may sound simple but in fact there are numerous potential subtleties to consider here, items such as: 1. Does a full 3D analysis actually give the answers you expect? 2. How are diaphragms modelled? 3. How is pattern loading catered for? 4. How are walls modelled? 5. etc. These and other related topics are considered in the chapters General Building Analysis , Wall Modelling Considerations and Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings It is worth noting here that the 3D analysis model of a complete building is primarily a frame element model with an option to use FE meshing of shear/core walls. It does not include FE meshed floor elements. Since flat slab (or flat plate) structures do not include beams the basic 3D building analysis will not deal with these structures. The chapter Flat Slab Models describes the alternative solution provided for these circumstances.
Design and Detailing Much of the element design theory is covered in the following chapters: Beam Design to BS8110 , Column Design to BS8110 and Wall Design and Detailing . As beam and slab systems become more irregular, you may wish to turn to the optional FE analysis for the slab design, the chapter Analysis and Design using FE provides additional information in this regard.
Orion Documentation page 18
Chapter 2
Chapter 2 : Modelling Techniques
Modelling Techniques
Introduction The Standard Training Manual covers many of the modelling techniques required for a typical structure in sufficient detail that no further mention is required here. It can be accessed by clicking on the following link: Orion Standard Training Manual.pdf Note If the link does not work please browse to find the file name indicated above in the HELP sub-folder of the Orion Program Folder. More advanced modelling techniques may also on occasion be required. The following cases are discussed in this chapter: 1. Modelling Inclined and Lowered Members– By default columns and walls are vertical, beams and slabs are horizontal, however it is possible to specify otherwise to cater for inclined and lowered members. 2. Modelling Curved Axes and Beams - It is possible to generate axes on a curve to facilitate the definition of curved slab edges and beams. 3. Linking Angled Beams - If beams connect at an angle the program will attempt to automatically determine if they should be linked on the detail sheet. However if the default arrangement is unsatisfactory you can choose to revise it. 4. Columns and Walls Spanning More Than One Storey – Each column and wall only spans one storey in the building unless explicitly specified otherwise. If the column/wall spans more than one storey, the number of storeys should be defined using Len (Storey) field in the member properties dialog. This ensures the correct length is used in the slenderness calculations when the column/wall is designed. 5. User Defined Supports – These can be employed to model supports which occur above the common foundation level. They can also be used to model (linear elastic) ground springs. Note Currently user defined supports are only active in the Building Analysis model and not in the FE Analysis model. 6. Stepped Foundation Levels – Often buildings will be built on sloping sites, or they may have to accommodate split basement levels. These situations are catered for within Orion using either of two methods depending on the complexity of the modelling situation. 7. Beams with Varying Depth – Generally, beams will have constant section properties (width and depth) from the beginning of the member to the end, however there may be occasions when you need to change the beam depth part way along the member. 8. Pinned Member Ends – Columns and beams are by default fixed ended members, however to alter the way forces are distributed the user can introduce pins at specific locations within the model.
Chapter 2 : Modelling Techniques
Orion Documentation page 19
Modelling Inclined and Lowered Members Although beams and slabs are by default analysed with their centre-lines assumed to be at a common elevation, it is possible to raise or lower them out of the floor plane so that this is not the case. Similarly, although by default each column and wall is created vertically and each beam and slab is created horizontally, they can also be defined at other inclinations. This section describes how to define inclined and lowered members - it is important that you familiarise yourself with the associated limitations before you use them. For further details see the chapter Solution Options for Inclined/Lowered Members Note
The features described in this section are for the purpose of defining occasional sloping/lowered elements within a model which still contains distinct horizontal floor planes. These features are NOT intended to facilitate the modelling of structures with complex geometries in which the floor planes are not readily apparent.
Note
In some cases, using engineering judgement to make an allowance to the loading (to cater for the expected effects of the sloped/lowered elements) may actually be simpler than introducing the sloped/lowered elements themselves.
Sloping and Lowered Slabs To create a sloping slab panel you must first define a Plane to align it to. (see Working With Planes on page 26). Once the plane has been defined the slab can simply be moved into the new plane. A slab panel may be dropped using the Rel Level box in the Slab Properties dialog. Entering a negative value in the Rel Level box will drop the current slab beneath the general slab level by the amount specified.
Note that the slab design moments obtained (using either the moment coefficient strip method or Finite Elements strip method) would not be any different for a dropped slab panel in comparison to an identical panel which had not been dropped. This is because the level difference would not be recognized in the analysis.
Orion Documentation page 20
Chapter 2 : Modelling Techniques
However, because the slab strips would be cut differently in a dropped slab panel the reinforcement curtailment would be improved, as described later in this section. How To Drop Parts of a Slab Panel The example case below illustrates a partial drop. In order to drop a part of a slab panel, you need to insert dummy axes and dummy beams surrounding the drop panel. The original slab layout is shown in the figure below:.
In this case study we will define a partial drop with 2000 x 2000 dimensions at the upper right corner of slab panel 1S1.
Step 1: Inserting Dummy Axes to Define the Borders of the Drop Panel Select axis 2 and press the Axis Offset button to offset it by 2000 mm to left direction. Set the new axis label as 1a. Then, select axis B and offset it by 2000 mm to below direction. Set the new axis label as A1.
Chapter 2 : Modelling Techniques
Orion Documentation page 21
Set the new axes as Not To Plot, since we don't want to include these axes in the output drawings.
It is always good practice to shorten the dummy axes wherever they are not used to decrease the number of axis intersections.
Step 2: Inserting Dummy Beams Around the Drop Panel Now we will insert dummy beams that surrounds the drop panel. Since we will drop the panel by 200 mm and the thickness of the panel will be 150 mm, the dummy beams will have a depth of 350 mm. Before inserting the dummy beams, you have to delete the existing slab 1S1, otherwise Orion will not let you insert overlapping members.
Orion Documentation page 22
Chapter 2 : Modelling Techniques
Insert the beams 1B1A and 1B1B as shown in the figure below.
Step 3: Inserting Slab Panels Now you are ready to insert the slab panels. You have previously erased the slab 1S1 so you will need to re-insert it so that it excludes the area surrounded by the dummy beams. Then, you can insert the drop panel 1S1a, with the Rel Level defined as -200 mm as shown below. Additionally, if you are going to analyse the slabs using moment coefficients method, you have to set the Slab Type to be 9 – Four Edges Discontinuous
Chapter 2 : Modelling Techniques
Orion Documentation page 23
Step 4: Inserting Slab Strips Definition of the slab strips passing through the dropped panel needs additional care. The reinforcement of the dropped panel must be bent at the edges. Therefore, a strip must be inserted spanning only the dropped panel 1S1a, with end-conditions defined as Bob both at start and at end. Similarly, the slab strip that is inserted along slab 1S1 between the axes A1 and B must also have end-conditions defined as Bob both at start and at end.
Sloping and Lowered Beams Beams can be set above or below the storey level, or they can be inclined by using the del z boxes for each end of the beam in the Beam Properties dialog. Entering a negative value in the del z box will drop the beam end beneath the general slab level by the amount specified.
If multiple members are to be edited it can be more efficient to first define a Plane to align them to (see Working With Planes on page 26).
Orion Documentation page 24
Chapter 2 : Modelling Techniques
Sloping and Lowered Columns Column levels can be set above or below the storey level using the del z boxes in the Column Properties dialog (3D tab). Entering a negative value in the del z box will drop the column end beneath the general slab level by the amount specified.
Columns can be inclined by specifying, (from the Column Properties, (General tab), different axis intersections for each end of the column.
Columns can be associated with Planes (see Working With Planes on page 26) in order to raise or lower the column end relative to the storey level. Planes are not used to create inclined columns.
Chapter 2 : Modelling Techniques
Orion Documentation page 25
Sloping and Lowered Walls Wall levels can be set above or below the storey level for each end of the wall by using the del z box in the Shear Wall Properties (3D tab). Entering a negative value in the del z box will drop the wall end beneath the general slab level by the amount specified.
Walls can be inclined by specifying, (from the Properties - General tab) different axis intersections for the top and bottom of the Wall as shown below.
Walls can be associated with Planes in order to raise or lower the wall ends relative to the storey level. Planes are not used to create inclined walls.
Orion Documentation page 26
Chapter 2 : Modelling Techniques
Working With Planes Planes can be defined which may be offset from the storey level and which may also be inclined. They are inserted in a similar way to slabs. Initially, as shown below, they are placed horizontally at the storey level - the level and inclination being controlled by three node points at the corners of the plane.
If the default node points identified are not suitable, one, or all can be reselected using the appropriate Pick Point icon on the Plane Properties dialog. Once the required node points are displayed, the Z elevation of each can be updated in order to change the level, or inclination of the plane.
Having defined the plane and the members which are to be part of the plane, the next step is to Move Members to the Plane Definition. This command can be accessed from the right click
Chapter 2 : Modelling Techniques
Orion Documentation page 27
menu, (or from the Planes branch of the Structure Tree).
As shown below, all members contained within, or at the edge of the plane are adjusted to the plane.
Orion Documentation page 28
Chapter 2 : Modelling Techniques
Modelling Curved Axes and Beams It is possible to define beams and slabs that are curved in the floor plane. Curved beams by definition have to be placed on a curved axis. This axis can either be created via a 'Curved Axis Generator' or it can be generated automatically if the beam is formed using the 'Curved Beam Generator'.
If you look closely at a curved beam you will see that it is actually formed from a series of straight segments. You can specify how many segments to use when defining the beam.
Curved Beam formed from 6 segments
Curved Beam formed from 12 segments
Chapter 2 : Modelling Techniques
Orion Documentation page 29
Curved Axes Curved axes are required for the definition of curved beams and curved slab edges. They are formed from a number of linked straight axis segments which approximate to the curve required. Provided the curve has a constant radius it should be created using the 'Curved Axis Generator'. If you require a curve which doesn't have a constant radius you are restricted to placing and then linking each axis segment manually. The Curved Axis Generator Straight axes are created by simply clicking and dragging between two points. To create a curved axis, (or to generate multiple, or offset axes) you follow the same procedure, apart from you must press and hold down the Shift key while dragging between the points. When you let go of the mouse a dialog appears as shown below allowing you to define the degree of curvature and apply offsets, or repeat spacings.
Offset Options These options can be applied to both straight and curved axes. Instead of the axis passing through the points clicked, it is drawn offset by the amount specified.
Curved axis insertion methods Three methods exist for specifying the curve: Chord Offset; Centre Offset and Radius. The number of straight segments forming the curve can also be controlled as can the decision to draw the tangent segments external or internal to the curve. As you type in curve properties a preview is displayed on screen.
Orion Documentation page 30
Chapter 2 : Modelling Techniques
Preview with Axis Segments Drawn External
Preview with Axis Segments Drawn Internal
Insertion/Generation Options These options can be applied to both straight and curved axes. Multiple axes can be created at equal or varying spacings as required.
Chapter 2 : Modelling Techniques
Orion Documentation page 31
Curved Beams Curved beams are composed of a number of linked straight beam segments approximating to the curve required. They are defined using the 'Curved Beam Generator'. The Curved Beam Generator A straight beam is created by simply clicking and dragging between two points. A curved beam can be created in the same way, apart from you must press and hold down the Shift key while dragging between the points. When you let go of the mouse a dialog appears as shown below allowing you to define the degree of curvature.
Offset Options These options can be applied to both straight and curved beams. Instead of the beam passing through the points clicked, it is drawn offset by the amount specified.
Curved beam insertion methods Four methods exist for specifying the curve: Chord Offset; Centre Offset; Radius and 'Use Existing Curved Axis'. The first three methods are the same as those used for defining curved axes, the forth method only becomes active if both the start and end point clicked are linked by an existing curved axis Having specified the beam, a preview of how it will look is displayed on the plan view. Once you are happy that it is positioned correctly, click OK to generate it.
Orion Documentation page 32
Chapter 2 : Modelling Techniques
Insertion/Generation Options These options can be applied to both straight and curved beams. Multiple beams can be created at equal or varying spacings as required. How many segments to use? There is no definitive answer to this question for all cases - it will depend on the length of the beam and the amount of curvature introduced. The default of six segments will often prove sufficient, (we certainly wouldn't suggest using any less than six), but if you are in doubt you can check for yourself by examining the effect on the analysis result of introducing more segments. Note
By Increasing this number of segments a smoother curve is formed, however it also increases the size of the analysis model (potentially taking longer to solve). This more refined model may not significantly improve the accuracy of the result.
Example Case Study: The model shown below is analysed with the curved beam initially modelled with 6, then 12 and finally 24 segments. The resulting moments and deflections are then compared.
Chapter 2 : Modelling Techniques
Orion Documentation page 33
Analysis Results for different numbers of Segments
Six Segments
Twelve Segments
Twenty Four Segments
Model
Deflection (mm)
Hogging Moment (kN)
Sagging Moment (kN)
6 Segments
68
47.3
-14.6
12 Segments
45
47.0
-13.6
24 Segments
40
46.9
-13.4
Although from the above it can be seen that the deflections haven't converged on a stable answer, the hogging and sagging moments remain fairly constant. If the six segment model were adopted the beam would be designed for slightly higher moments than if a more refined model were adopted.
Beam End Conditions Hinges can be applied at either end of a curved beam by using the 'Update Beam End Conditions' command accessed from the right click menu.
Orion Documentation page 34
Chapter 2 : Modelling Techniques
Marking Cantilever Curved Beams Free ends can be specified at either end of a curved beam by using the 'Mark Free End of Cantilever Beam' command accessed from the right click menu.
Editing the Position of a Curved Beam Currently this is not possible. If the beam is not in the correct location you will have to first delete and then recreate it.
Editing Curved Beam Section Properties Currently this is not possible. If the sectional properties of the beam are not correct you will have to first delete and then recreate it.
Editing Curved Beam Member Loads Beam Member Loads can be applied by using the 'Edit Member Loads' command accessed from the right click menu. The dialog that is displayed only shows the loads applied on one beam segment at a time. The forward and backward arrows can be used to move from one segment to the next.
Note
If you change data for one segment then this change is saved when you move to another segment. If you press cancel you are only cancelling edits made to the current segment, edits applied to previous segments are not cancelled.
Chapter 2 : Modelling Techniques
Orion Documentation page 35
Linking Angled Beams When beam detail sheets are created, any beams which connect in a straight line are linked automatically. In the special case of beams meeting at angled intersections, it may not be immediately clear if they should be linked or not, particularly if multiple beams meet at the same point. The program will attempt to automatically determine the linking, however if the default arrangement is unsatisfactory you can choose to revise it. This is achieved by manually linking those axes on which you require the beams to appear as linked. Consider the example shown below, none of the highlighted beams are co-linear but it would make sense to link some of them on the detail sheet.
The the program chooses to automatically link the beams as follows:
Beams 1B3 and 1B4 are linked
Beams 1B1 and 1B5 are linked
Beam 1B2 is not linked
Orion Documentation page 36
Chapter 2 : Modelling Techniques
Note
The analysis is completely unaffected by the way the beams are linked together only the beam details are affected.
The resulting detail drawing is shown below:
Manually Linking the Intersecting Axes It would make more sense to link beams 1B1 and 1B2 on the detail sheet and have 1B5 detailed as a single span. This can be achieved by linking the intersecting axes A1 and A2 in the plan view. The axes are linked as follows: • Select the first axis to be linked (A1)
• • • •
From the Right Click menu choose Link Intersecting Axes Pick the axis to be linked to this axis (A2) The two axes are immediately linked together If you require, you can then continue to pick further intersecting axes to link to the end of this one
Chapter 2 : Modelling Techniques
Orion Documentation page 37
Once the axes have been linked only the first axis label is displayed. The linked axes can now be selected/unselected as a single entity.
If at any time you require to return the linked axes to their original unlinked state, this can be achieved by choosing Separate Linked Axes from the Right Click menu. After either linking or unlinking, although the analysis results are unaffected, a re-analysis is still required. This is because the data has to be stored in a different way in preparation for beam design and detailings. After re-analysis the affected beams should be re-designed. Having linked axis A1 to A2 the beams are now linked as follows:
Beams 1B3 and 1B4 are linked
Beams 1B1 and 1B2 are linked
Beam 1B5 is not linked
Orion Documentation page 38
Chapter 2 : Modelling Techniques
The revised detail drawing is shown below:
Note
It is not always possible to link intersecting axes - for example a Dir 1 axis can not be linked with a Dir 2 axis and vice-versa. In such cases it may be necessary to detail the beams individually.
Chapter 2 : Modelling Techniques
Orion Documentation page 39
Columns and Walls Spanning More Than One Storey Each column and wall only spans one storey in the building unless explicitly specified otherwise. If the column/wall spans more than one storey, the number of storeys should be defined using Len (Storey) field in the member properties dialog. This ensures the correct length is used in the slenderness calculations when the column/wall is designed.
Example Case Study In this example, some columns in the 5th storey span to the 3rd storey top level as shown below. The clear height of these columns is twice that of other columns at the 5th and 4th storeys and they will not be affected or restrained by any rigid diaphragm action that may exist at the 4th storey.
To define a column that spans two storeys 1. From the storey list go to the topmost storey that the column spans to, (in this example it is St05). 2. In the Graphic Editor, select the column spanning more than 1 storey.
Orion Documentation page 40
Chapter 2 : Modelling Techniques
3. Enter the number of storeys that the column spans in the Len (Storey) box. In this example enter 2 as shown below.
4. Press the Update button to apply the modification to the selected column. 5. Select the floor below, and delete the column that is already covered by the column in the upper storey. In this example select the 4 th storey and delete the corner column as shown below. Note that, if the column spans more than two storeys, this step must be repeated for all the lower floors covered by that column. P
P
Chapter 2 : Modelling Techniques
Orion Documentation page 41
User Defined Supports These can be applied to the lower end of any column or wall at any level to introduce an external support or spring. The translational (in global X, Y and Z) and rotational (about global X, Y and Z) degrees of freedom can be set to fixed, or free, or a spring stiffness can be assigned. Unless you specify and apply user defined supports, every column and wall in your model adopts a default support. Note Currently user defined supports are only active in the Building Analysis model and not in the FE Analysis model.
What is a default support? The support provided by default is dependant upon the storey level at which the column or wall stops. For columns and walls that stop at ST00 (i.e. the foundation) — Default = Fully Fixed Support For columns and walls that stop at ST01 and above — Default = No External Support In the simple model shown below, the grey shaded columns and walls are stopping at ST00, hence they each have fully fixed supports.
Whereas, the columns and walls that stop at ST01, (again shaded in grey below) have by default no external support. The loads within these columns will therefore be transferred directly into the members in the lower storey.
Orion Documentation page 42
Chapter 2 : Modelling Techniques
When might a default support be inappropriate? In typical models you will often find that default supports are all that is needed. However, certain situations might require you to specify and apply user defined supports. Cases where user defined supports could be necessary include: • Buildings with sprung, or pinned1 supports
• Buildings with stepped foundations Specifying a User Defined Support If a fixed support is inappropriate, a pinned or spring support can be defined as follows:
1. Choose Support Type Definitions from the Members menu. 2. Click the Add New button and enter a label to describe the new support type. 3. To define a translational release in a particular direction uncheck the x, y or z support box as appropriate. 4. To define a rotational release in about a particular axis uncheck the X-Rotation, Y-Rotation or Z-Rotation support box as appropriate. 5. To define a translational spring in a particular direction enter the spring stiffness in the appropriate x, y or z box, (having first unchecked the corresponding support box). 6. To define a rotational spring in a particular direction enter the spring stiffness in the appropriate X-Rotation, Y-Rotation or Z-Rotation box, (having first unchecked the corresponding support box). 7. Click OK to save the new support definition.
Footnotes 1. releasing the end of the member is a simpler technique for achieving a pinned connection to the support.
Chapter 2 : Modelling Techniques
Orion Documentation page 43
Applying a User Defined Support Having defined a new support type as above, it can then be assigned to a specific column or wall using the member properties dialog:
1. Select and load the properties for the column or wall. 2. Click on the 3D tab and choose the appropriate Support Type from the list. 3. Click on Update to save the change.
User Defined Supports - Trouble shooting In models where you have applied user defined supports we would recommend that you carefully review the analysis results to ensure they are working as you intended. Careless application of supports could have unexpected effects.
Mechanisms A mechanism may be introduced if, for example, you have applied a pinned support to a pin ended member.
Diaphragm restraint Typically, a rigid diaphragm exists within the floor slab. Hence, if a slab connects to the base of a column which has a user defined support applied, the support will be directly restraining the rigid diaphragm itself. This could inadvertently prevent lateral displacements from developing at that level even if this was not the original intention.
Load Paths User defined supports are assumed to transfer any reaction directly to the foundation. You should not apply a user defined support at an upper storey level unless there is means for this transfer to occur. A stepped foundation is an example of where a user defined support would be appropriate, whereas, a transfer column situation (i.e. where the column is supported by another member) is an example of where it is not.
Orion Documentation page 44
Chapter 2 : Modelling Techniques
Stepped Foundation Levels The model shown below illustrates a stepped foundation level. In such models care is required to ensure that the columns and walls are correctly supported. A fixed support is only automatically placed underneath each column and wall that is physically connected to the common foundation level (indicated by the grey plane ).
Depending on the model complexity, and the type of analysis carried out, one of two modelling methods may be appropriate for achieving this, we shall referto these as: • Default Supports Method
• User Defined Supports Method Default Supports Method All columns and walls that are supported on a foundation (irrespective of the foundation level) are initially created so that their lower end connects to the general foundation level (St00). The foundation level is then adjusted by raising or dropping the lower end of the columns and walls relative to the St00 level as required. Because the columns and walls connect to St00 they are all automatically provided with a default support. Note This approach can be used for models solved either by Building Analysis or FE Chasedown Analysis. Single Storey Example of the Default Supports Method In the model below the step occurs below the lowest floor level (St01). All the columns and walls are defined with Len (Storey) = 1. The only requirement is to vary the foundation level to form the step. This is achieved by raising, or dropping, the base level of the appropriate columns and walls .
Chapter 2 : Modelling Techniques
Orion Documentation page 45
Two Storey Example of the Default Supports Method In the model below the step occurs above the lowest floor level (St01).
The columns to the right of the model must initally be defined with Len (Storey) = 2 so as to enable them to connect to the foundation. (If this was not done they would be treated as unsupported.) Their base levels are then raised to the appropriate level by applying a dZ-Bot figure as described below.
To change the base level of selected columns and walls 1. In the plan view select the columns and walls to be changed. 2. Display the columns member table and change the dZ-Bot figure for all the selected columns. A negative figure will lower the base level below the common foundation level and a positive figure will raise it.
3. Repeat the above process for the walls changing both dZ-Bot, I and dZ-Bot,
Orion Documentation page 46
Chapter 2 : Modelling Techniques
User Defined Supports Method In this method it is not necessary for all the columns and walls to connect to the general foundation level (St00). User defined supports are applied at the floor level at which the foundation exists. The foundation level is then adjusted by raising or dropping the lower end of the columns and walls relative to the current storey. Note
This approach can only be used for models solved by Building Analysis. FE Chasedown Analysis does not currently recognise user defined supports.
Example of the User Defined Supports Method In the example below, the columns to the right, which are defined spanning from ST02 down to ST01 are to be supported on a raised foundation.
The exact level of the raised foundation is specified by applying a dZ-Bot figure to the columns which is measured from the current storey level (in the same way as described in the Default Supports Method). Because these columns do not connect to St00, they are initially unsupported. Therefore user defined supports will need to be manually defined and applied. For details of how to specify these supports refer to the section- User Defined Supports Note
If the raised foundation level coincides exactly with an existing storey level, it may be necessary to offset one from the other to avoid a user defined support being applied within a floor diaphragm. This can result in an illegal constraint at the joint in question.
Note
User defined supports can be either pinned, fixed, or spring bases as required.
Chapter 2 : Modelling Techniques
Orion Documentation page 47
Beams with Varying Depth Generally, beams will have the constant section properties (width and depth) from the beginning of the member to the end, however there may be occasions when you need to change the beam depth part way along the member. You can define a second beam along the axis in order to change the section properties. Naturally, you need to define a Non-frame dummy axis in order to create an axis intersection at the particular location that the beam change section properties, to insert the second beam.
“Non-frame” axis to create an intermediate intersection st
1 beam with section 250x500 2nd beam with section 250x750 1B25
1B25A 250x750
250x750
Changing the Depth of the Beam
Note that this procedure is known as Splitting Beams and while it is possible, it will have implications affecting the resulting beam detail. You will need to review and possibly edit the automatically produced detail to ensure it is acceptable.
Example Case Study In this example, there is a first floor beam shown below which has a varying depth. The beam changes depth from 500 mm to 750 mm after 1500 mm from its left insertion point. This is modelled using the following steps: 1. Before creating the beam, insert a grid line to cross the beam at the point where its depth will change. You can do this by offsetting the axis (say Axis 1) at the I end of the beam a distance 1500 mm. The name of the axis can be any dummy label (say 1x).
Orion Documentation page 48
Chapter 2 : Modelling Techniques
2. Stretch the end-points of the new axis 1x, so that it crosses only the grid line that the beam will be defined (say A).
“Non-frame” axis to create an intermediate intersection
1x
A 1500 mm 1
1x
2
3. Remove the check in the View Codes option of this dummy grid line, so that it is defined to be a ghost axis and check the Not to Plot option so that this axis will not be visible in any printed output. 4. Instead of creating a single beam, insert two beams, 1B25 from axis 1 to axis 1x and 1B25A from axis 1x to axis 2.
“Non-frame” axis to create an intermediate intersection
1x st
1 beam with section 250x500 2nd beam with section 250x750 1B25
1B25A 250x750
250x750 1x
Chapter 2 : Modelling Techniques
Orion Documentation page 49
Pinned Member Ends Columns and beams are by default fixed ended members, however to alter the way forces are distributed the user can introduce pins at specific locations within the model
To Pin a Single Column A column can be pinned at one or both ends, using the appropriate End-Condition button in the Column Properties dialog. The different conditions available are tabulated below: Fix-Fix (no hinges)
Pin-Pin (both ends pinned)
Fix top-Pin bottom (hinge at bottom end) Pin top-Fix bottom (hinge at top end)
To Pin Multiple Columns Multiple columns can be pinned using the Update Column End Conditions option in the shortcut menu accessed using the right mouse button after selecting one or more columns. After choosing the desired end condition a button menu appears for filtering options.
End Conditions Application Filter Options
Using this button menu, you can either apply the selected end condition to selected columns, or all columns in the current storey, or all columns in the building.
To Remove the Hinges from the Columns In order to remove the pin conditions of selected columns or all columns in the current storey or all columns in the building you can use the No Hinges (Fix-Fix) option of the Update Column End Conditions menu.
Orion Documentation page 50
Chapter 2 : Modelling Techniques
To Pin a Single Beam A beam can be pinned at one or both ends, using the appropriate End-Condition button in the Beam Properties dialog. The different conditions available are shown below: Fix-Fix (no hinges) Pin-Pin (both ends pinned) Pin-Fix Fix-Pin
To Pin Multiple Beams Multiple beams can be pinned using the Update Beam End Conditions option in the shortcut menu accessed using the right mouse button after selecting one or more beams. After choosing the desired end condition a button menu appears for filtering options.
End Conditions Application Filter Options
Using this button menu, you can either apply the selected end condition to selected beams, or all beams in the current storey, or all beams in the building.
To Remove the Hinges from the Beams In order to remove the pin conditions of selected beams or all beams in the current storey or all beams in the building you can use the No Hinges (Fix-Fix) option in the Update Beam End Conditions menu.
Chapter 3 : Beam Loads and Load Decomposition Methods
Chapter 3
Orion Documentation page 51
Beam Loads and Load Decomposition Methods
General Orion calculates the beam loads automatically using the slab area loads, beam wall load information and unit weight of the members. Calculated loads can be viewed and modified (if necessary) using the Edit Member Loads function on the right mouse click menu. The following distributed loads are calculated automatically: • Beam self weight as a uniformly distributed load (using the unit weight of concrete defined in the Project Parameters menu and the geometry of the beam),
• Dead and imposed (live) loads transferred from the adjacent slabs (as distributed function loads calculated based on the self weight and area loads of the slabs),
• User defined additional dead and imposed (live) loads (any additional loads that cannot be determined automatically can be defined by the user),
• User defined additional beam wall (dead) loads Dead and imposed point loads transferred by supported secondary beams do not show up when using Edit Member Loads and neither do point loads from supported columns and walls. This is because these members are themselves part of the 3D analysis model. The loads they transfer can only be seen in terms of steps in the shear force diagram. For more information on supported columns and walls refer to the chapter Overview of Solution Options for Transfer Levels.
Modifying Beam Loads When you select a beam and use a right mouse click to access the shortcut menu, you can select the Edit Member Loads option to view the beam vertical loads.
Orion Documentation page 52
Chapter 3 : Beam Loads and Load Decomposition Methods
Beam Loads Dialog The following buttons and fields are displayed on the Load Profile dialog.
Beam Load Editor
Load Table Each load in the Load Editor is represented as a row in the Load Table. A hand icon will appear in the right-most column to mark any manually defined user loads (none shown above). Typically, Uniformly Distributed Loads and Partial Distributed Loads may exist in this table. The selected load will be highlighted in the accompanying drawing to the right of the Load Table. You can only edit or delete manually defined user loads.
Edit Button You can edit an existing manually defined user load in the Load Editor by clicking on this button. Note that automatic loads will be loaded as Read-Only to this editor. You cannot make any modifications to automatic loads.
Delete Button You can delete an existing manually defined user load in the Load Editor by clicking on this button. Note that automatic loads cannot be deleted.
New Load Button You can define new manually defined user loads with the Load Editor by clicking on this button. For example, if you want to define a new point load, click this button to display the Load Editor, enter the Reference X, Dead Load and Imposed Load fields. Define the Load Transfer Side and close the dialog by pressing the OK button.
Slab Load Calculation Method This option allows you to swap between alternative methods for decomposing slab loads to establish beam loads. A detailed discussion of this is provided in the section Switching between Yield Line and FE load Decomposition later in this chapter.
Chapter 3 : Beam Loads and Load Decomposition Methods
Orion Documentation page 53
OK Button If you close the Beam Loads dialog by clicking the OK button after making any modifications, all the changes will be saved.
Cancel Button If you close the Beam Loads dialog by clicking the Cancel button after making any modifications, the changes made on the beam will be discarded.
How to Define a New Point Load You can do the following steps to define a new manual point load: 1. Press the New Load button to open the Load Editor. 2. Press the Point Load button at the top of the dialog to define a point load. 3. Press the From Above button in the Load Transfer Side group (default) at the bottom of the dialog. 4. Optionally, enter a label for the load. 5. Enter the Reference X. This is the distance of the point load from the reference point of the beam (which is the I-end). 6. Enter the load magnitudes into the Dead Load – G and Imposed Load – Q fields. 7. Press the OK button to insert the point load. Alternatively you can abort the process using the Cancel button.
Load Profile Editor Dialog (Point Load)
Orion Documentation page 54
Chapter 3 : Beam Loads and Load Decomposition Methods
How to Define a New Uniformly Distributed Load You can do the following steps to define a new manual uniformly distributed load (UDL): 1. Press the New Load button to open the Load Editor. 2. Press the Uniformly Distributed Load button at the top of the dialog to define a uniformly distributed load. 3. Press the From Above button in the Load Transfer Side group (default) at the bottom of the dialog. 4. Optionally, enter a label for the load. 5. Enter the Dead Load – G and Imposed Load – Q details. 6. Press the OK button to insert the uniformly distributed load. Alternatively you can abort the process using the Cancel button.
Load Profile Editor Dialog (UDL)
How to Define a New Partial Distributed Load You can define a partial load either by using the Load Generator or by directly defining the load points using the vertex table. Using the Load Generator 1. Press the New Load button to open the Load Editor. 2. Press the Partial Distributed Load button at the top of the dialog to define a partial distributed load. 3. Press the From Above button in the Load Transfer Side group (default) at the bottom of the dialog.
Chapter 3 : Beam Loads and Load Decomposition Methods
Orion Documentation page 55
4. Optionally, enter a label for the load. 5. Press the Load Generator button to open the Loading Options dialog. 6. Select a load shape by pressing the appropriate button in the top portion of the dialog. 7. Enter the values for the parameters defining the geometry and magnitude of the load. Here, x is the start distance to the load, and a, b, c are the widths of the load segments. 8. Enter the P values for load cases Dead Load – G and Imposed Load – Q.
Loading Options Dialog
9. Press the OK button to insert the partial distributed load. Alternatively you can abort the process using the Cancel button.
Load Profile Editor Dialog (Partial Distributed Load)
Note
The vertex table is filled with the details of the partial load you have defined.
Orion Documentation page 56
Chapter 3 : Beam Loads and Load Decomposition Methods
Using the vertex table Alternatively you can modify or directly define a new partial load by using the vertex table. For example, to add a partial 2.5 m width UDL with magnitude G=6 kN and Q=3.5 kN, starting from x=5 m you can use the following steps: 1. Select the 4 th vertex in the table. P
P
2. Press the Add Below button to append a new vertex to the end of the table. 3. Modify the x value to 5.0. 4. Press the Add Below button again to append another vertex to the end of the table. 5. Modify the x value to 7.5, the G value to 6.0 and the Q value to 3.5. 6. Press the Add Below button again to append another vertex to the end of the table. 7. Modify the x value to 5.0, the G value to 6.0 and the Q value to 3.5. 8. Press the Add Below button again to append a closing vertex to the end of the table. 9. Modify the x value to 7.5, the G value to 0 and the Q value to 0.
Load Profile Editor with a 2.5 m partial UDL entered to the end
10. Close the dialog by pressing the OK button to insert the partial distributed load.
How to Define a New Load on an Inclined Beam The following points should be noted:
• Loads applied via the Load Editor are always applied vertically, even if the beam is inclined. • For UDL and VDL loads the magnitude of the load, P (in kN/m) should be entered as a sloped load value.
Chapter 3 : Beam Loads and Load Decomposition Methods
Orion Documentation page 57
• The Load Editor requires all loads (including point loads) to be input using a horizontal projected distance rather than a distance measured on the slope. For example, a 5m long beam at a slope of 30 degrees would have a horizontal projected length of 4m. A point load at mid span would therefore be defined with a Reference X, of 2m as shown below.
Sloped load input using horizontal projected distance along the member
The load is analysed correctly at the mid point of the sloped length - this can be confirmed by displaying the loading diagram in the beam analysis results.
Sloped load output using inclined distance along the member)
Orion Documentation page 58
Chapter 3 : Beam Loads and Load Decomposition Methods
Switching between Yield Line and FE load Decomposition What is Load Decomposition? Slabs are not modelled as part of the general building analysis. To do this would require a fully meshed up FE model at every floor level in a multi-storey 3D model. For realistic structures, such models remain beyond the reasonable capacity of current computer hardware/ processors. Therefore, when you define slabs and loads on slabs, Orion has to work out what loading should be applied to the supporting beams. By default this is done in a traditional method based on tributary areas.
The boundaries of the tributary areas loading each beam can be viewed graphically as shown above. These look similar to potential Yield Lines, hence the name given to this default method. In this, as in many other areas, Orion effectively applies the sort of engineering methodology that has been used in hand calculations for many years.
Why switch to an FE method? This method has limitations in circumstances such as: 1. When the slab boundaries are highly irregular and particularly where some edges of a complex boundary are unsupported. 2. When there are significant holes defined in the slab. 3. When there are eccentric concentrated point, patch, or line loads.
Chapter 3 : Beam Loads and Load Decomposition Methods
Orion Documentation page 59
In such circumstances beam loads are more accurately calculated by using a Finite Elements Model.
Consider the simple 6 m by 4 m slab shown above. Eccentric concentrated point loads are applied and a large void also has been defined. The Traditional (Yield Line) method of load transfer would ignore the hole, and it would average out the point load by calculating an equivalent UDL to apply over the whole area of the slab. Although the load is not lost, its effect is independent of its proximity to any of the surrounding beams. This is all reasonably apparent when you examine the beam loads.
Orion Documentation page 60
Chapter 3 : Beam Loads and Load Decomposition Methods
The loading on beam 1B1 is a simple trapezoidal profile where the peak UDL is adjusted to account for the point loads. Based on this method, beam 1B2 sees an identical loading profile. When you run a building analysis, a completely symmetrical set of column loads is determined as shown below.
Example of FE Method for Slab Load Decomposition By using the optional Finite Elements Model, the point loads will be much more accurately distributed. The slab load will also be more accurately distributed, including the deductions and effects associated with the void. The procedure for generating optional beam loads based on an FE model is as follows: 1. Select the Load Decomposition by FE command from the Run menu as shown below.
Chapter 3 : Beam Loads and Load Decomposition Methods
Orion Documentation page 61
2. Click on the button Determine Loads Transferred from Slabs, this will open up an FE modelling window.
It is important to note the advice on mesh density that is given in the above dialog. 3. A plate size is suggested by default. Click the Generate Mesh icon and the slab is meshed automatically.
Orion Documentation page 62
Chapter 3 : Beam Loads and Load Decomposition Methods
In this case the suggested plate size results in 234 plates being generated, and although not well meshed around the opening this is probably sufficient in this simple example for the intended purpose. 4. Close this FE window and then also the Finite Element Analysis Form. This will result in the following dialog being displayed.
5. The FE results can be applied to selected beams, all beams in the current storey, or to the whole model. Choose Apply to All Beams in the Model. You are then returned to the Graphic Editor. Having completed the above process, FE information exists and has been applied to all beams in the model. You can review this on a beam-by-beam basis. Select any beam and open the beam loads dialog, it will have swapped to the Finite Elements Slab Load Decomposition Method. The new loading for beam 1B1 is shown below.
As expected the load intensity is peaking towards the ends where the point loads are applied.
Chapter 3 : Beam Loads and Load Decomposition Methods
Orion Documentation page 63
To utilise these new loadings it is necessary to reperform the building analysis. On completion we can see a very different distribution of column loads.
The total IL using the Yield Line method was approximately 267.6 kN. Using the FE method it has dropped to 265.5 kN, this is slightly lower because no load is applied to the hole. Note
If you change any slab loads or beam layouts, you must remember to reperform the Load Decomposition by FE command before reperforming the building analysis.
Orion Documentation page 64
Chapter 3 : Beam Loads and Load Decomposition Methods
Note
Axial load comparisons are performed at the end of each analysis. This is a comparison of applied loads vs. analysis reactions. Since you can choose decomposition methods on a beam-by-beam basis you can cause this comparison to show discrepancies. In the example above, if you choose finite element decomposition for beam 1B1 only, the comparison will be displayed as shown below.
The first applied loads table shows the totals for the loads applied to each slab, beam, etc. The second applied loads table shows the loading totals after decomposition. Since the decomposition methods have been mixed so that a worst case is used for every beam, the total loading after decomposition is higher. You can then see that this higher loading is maintained in the building analysis.
Chapter 3 : Beam Loads and Load Decomposition Methods
Orion Documentation page 65
Why retain the traditional (yield line) method? Having reviewed the example above this is a question you might ask. The main point to bear in mind is that the example is unusual/extreme. Concentrated loads and dominant openings are not the norm. The main disadvantage of the FE method is speed. When there are a large number of slabs, there are a correspondingly large number of plates required and the analytical models (even though it is one floor at a time) get big and hence slow. Comparisons of uniformly loaded slabs have shown excellent correlation between the traditional (yield line) and FE methods, so you may find it easier to continue using the traditional method for much of your work. If you think about the differences the FE method makes, you will be able to decide when it might be more appropriate - clearly the FE method is better if you have to deal with significant holes and/or isolated loading. Ultimately, neither method is going to suit everybody all of the time. For this reason you can pick and choose on a beam by beam basis if you wish, or you can design all the beams using one method and then check them using the other. When checking you can use the option to select new steel when previous bars are insufficient so that you end up with all beams designed for the worst case from either load decomposition method. Note
In the FE method beams are replaced by a line of fully fixed supports. As a consequence, in ordinary regular slabs both the yield line and FE method will share loads equally to internal and edge beams. In other words continuity is not being considered. This is a simplification that has long been accepted in hand calculations. A more extreme example of the above would be a cantilever slab. The beam adjacent to the cantilever takes the entire cantilever slab load, plus half of the internal slab load, for long cantilevers that can be a significant under estimate. The only way to account for continuity in such a case would be to run an FE chasedown analysis and then merge the resulting beam loads. This method deals with the continuity, a higher load is put on the beam adjacent to the cantilever and correspondingly less load is put on the next internal beam - some engineers may prefer this. For details of how to perform an FE chasedown analysis refer to the chapter "Overview of Analysis and Design Using FE".
Orion Documentation page 66
Chapter 4
Chapter 4 : Analysis Methods
Analysis Methods
Introduction Having constructed and loaded your model, an analysis will be required before member design1 can commence. The analysis method(s) used will depend on a number of factors; for certain structures (eg flat slabs) two or more analyses may be required. A brief summary of the analysis methods available within Orion is given below, these are then discussed in more detail in the next four chapters:
General Building Analysis A full 3D analysis is performed for all load cases and combinations, (vertical and lateral). The analysis model consists primarily of frame elements with an option to use FE meshing of shear/core walls. It does not include FE meshed floor elements. General Building Analysis can be used to determine: • Frame deflections/sway sensitivity
• • • •
Beam design forces (from both vertical and lateral combinations) Column and wall design forces (from both vertical and lateral combinations) Foundation forces (from both vertical and lateral combinations) In flat slab structures - the beam, column and wall design forces from lateral combinations.
A full description of this method is provided in the chapter General Building Analysis.
Eigenvalue Analysis An Eigenvalue Analysis can (optionally) be performed as part of General Building Analysis to determine natural frequencies and mode shapes. A full description of this method is provided in the chapter Eigenvalue Analysis.
Staged Construction Analysis This is a sophisticated method of solution which takes account of time dependant effects. It is used in place of general building analysis where it is felt necessary to model the time dependant properties of concrete. Staged Construction Analysis can be used to determine: • Frame deflections/sway sensitivity
• • • •
Beam design forces (from both vertical and lateral combinations) Column and Wall design forces (from both vertical and lateral combinations) Foundation Forces (from both vertical and lateral combinations) In flat slab structures - the beam, column and wall design forces from lateral combinations.
A full description of this method is provided in the chapter Staged Construction Analysis. Footnotes 1. the exception being slabs, which can be designed directly using tabulated code coefficients, where applicable.
Chapter 4 : Analysis Methods
Orion Documentation page 67
Finite Element Floor Analysis Analysis is performed for vertical load cases and combinations on a single floor at a time extracted from the 3D building. In the analysis model, slabs are represented by horizontal FE meshed floor elements and beams by horizontal frame elements. Supporting columns and walls are also modelled (optionally) as frame elements as are the columns and walls connecting with the slab from floors above. Sequential FE floor analyses can be performed to chase gravity loads down through the structure. The resulting column, wall and beam forces can be merged with those of the General Building Analysis. FE floor analysis is required for the solution of flat slab (or flat plate) structures. Finite Element Floor Analysis can be used to determine: • Floor deflections
• • • •
Slab design forces Beam design forces (for vertical combinations) Column and wall design forces (for vertical combinations) Foundation forces (for vertical combinations)
A full description of this method is provided in the chapter Analysis and Design using FE.
Orion Documentation page 68
Chapter 5
Chapter 5 : General Building Analysis
General Building Analysis
Introduction This chapter is split into two sections: • The Structural Model section below describes various modelling considerations and analysis options that have a significant impact on the building analysis result.
• The Building Analysis Problems – Reviewing/Understanding section discusses the error and warning messages you might encounter when running a building analysis. It also describes how you can perform a model validity check and how you can further cross check the analysis result using the Axial Load Comparison Report.
Structural Model A full 3D analysis model is derived from the physical information that you describe when constructing the model. This may sound like a relatively simple and comprehensive solution to the analytical modelling of any structure but in fact there are numerous difficulties, options, and even personal preferences to consider here. The following list introduces most of these items which are then discussed in more detail in the remainder of this section: 1. Diaphragm Modelling – The analysis model is a 3D frame (stick) model, slabs are only modelled in the sense that their diaphragm action is accounted for. What are the options relating to how/when diaphragms are included? How are inclined planes handled? 2. Global Constraints – What are they and when might you use them? 3. Pattern Loading – How is pattern loading catered for? 4. Rigid Zones – What are they and why would you use them? 5. Rigid Links – What are they? 6. Shear Walls and Core Wall Systems – How are walls modelled? 7. 3D Effects – A full 3D analysis will sometime give answers that you do not expect, is it wrong? 8. Stiffness adjustments – What options are available and when might you use them? 9. Flat Slab Construction – Dealing with structures that incorporate some degree of flat slab (flat plate) construction. 10. Transfer Levels – Dealing with beams and slabs that support discontinuous columns or walls. 11. Supports – Default supports and how to create new support types.
Chapter 5 : General Building Analysis
Orion Documentation page 69
Diaphragm Modelling In a typical building lateral resistance is provide at a few discrete points and it is assumed that applied lateral loads will be distributed to the lateral load resisting systems via floor diaphragm action. Within Orion diaphragm modelling is achieved using diaphragm constraints. A diaphragm constraint will maintain exact relative positioning of all nodes that it constrains, i.e. the distance between any two nodes constrained by a diaphragm will never change, therefore no axial load will develop in any member that lies in the plane of a diaphragm between any two constrained nodes. When running the building analysis the model options tab gives options as shown below.
For the diaphragm modelling there are 3 options: • Slabs to define rigid diaphragms (Default Setting),
• Single rigid diaphragm at each floor level, • No rigid diaphragm floor levels.
Orion Documentation page 70
Chapter 5 : General Building Analysis
The differences between these options can be demonstrated with the simple model below.
A single floor level has 2 separated slab areas that are linked by 2 beams. Within the main graphics window Orion will always indicate a single overall centre of mass for the entire floor level as shown below.
It is important to note that in the case of discrete diaphragms, or when there is no diaphragm at all, this overall central location is provided for information only. Slabs to define rigid diaphragms (Default Setting) If the building is analysed with this setting then Orion will find any discrete areas of interconnecting slabs and set up discrete diaphragms as appropriate. Separate notional loads are calculated and applied to each diaphragm area. These applied notional loads and the
Chapter 5 : General Building Analysis
Orion Documentation page 71
resulting sways can all be examined in the model and analysis results display. Perhaps the easiest way to see that 2 discrete diaphragms have been created is to look at an exaggerated view of deflections.
For each of the two diaphragm areas a separate centre of mass location is determined and the notional load is applied at that position. The view above shows the values for the Fy case in this example. Note that the mass of the walls increases the applied notional load in the left hand diaphragm area. Clearly the right hand area is moving independently to the left hand area which is better restrained by the walls as opposed to frame action. Single rigid diaphragm at each floor level If the building is analysed with this setting then Orion will find apply a single diaphragm constraint to every node at any given level. The existence of slabs is completely ignored/ irrelevant. A single notional load is calculated and applied at the overall centre of mass as shown below. Once again the easiest way to see the effect of this setting is to look at an exaggerated view of deflections.
This time we can see the entire level translating and rotating as a unit, since the centre of mass and hence the applied notional load is very eccentric to the core walls, the dominant effect in this example is one of rotation.
Orion Documentation page 72
Chapter 5 : General Building Analysis
No rigid diaphragm floor levels If you have defined slabs but for some reason you do not wish a diaphragm effect to be considered you can completely eliminate diaphragm constraints using this option. In this case notional loads are applied separately at every node in the floor level, these applied notional loads and the resulting sways can all be examined in the model and analysis results display. Once again the easiest way to see the effect of this setting is to look at an exaggerated view of deflections.
The frame that is restrained by the wall hardly moves at all, other frames move to differing degrees. Note that the frames which do not include walls have the same stiffness and so the differing deflections relate to differing notional loads. Excluding Specific Slabs from Diaphragms In the example above there was no slab defined in the area linking region between grids 4 and 5. What if there was a slab in this area as shown below?
If the linking slab was substantial you might consider that it maintains the diaphragm action between the two areas. However, as the link becomes more slender then at some point you will decide it cannot maintain diaphragm action between the two areas. In this case you can edit the properties of the slab and exclude it from diaphragm as shown above. In this case you
Chapter 5 : General Building Analysis
Orion Documentation page 73
can still use the default option Slabs to Define Rigid Diaphragms and the resulting deflections will be the same (ignoring small change due to additional notional load from the added slab) as were shown above for that option.
Limitations - diaphragm modelling and inclined planes Inclined diaphragms can be created and are catered for correctly. It is also possible to have multiple differently inclined diaphragms as long as they do not interconnect. However, it is not possible to cater for differently inclined and interconnected diaphragms. In such a situation a single diaphragm constraint is formed that constrains all nodes within it equally. This situation will trigger warnings during analysis as the results can be adversely affected. Note
If a warning appears during analysis, indicating that nodes constrained by a diaphragm do not lie in the same plane, it should not be ignored. You should review the diaphragm model and consider excluding any inclined slabs from it, this will often rectify the problem.
Orion Documentation page 74
Chapter 5 : General Building Analysis
Case Study 1 - single storey pitched frame
If either the Slabs to Define Rigid Diaphragm or Single Rigid Diaphragm at Each Floor Level option is used a warning message is displayed during the analysis indicating that the nodes constrained by the diaphragm do not all lie in the same plane. A single diaphragm is created within the pitched roof as shown below.
Chapter 5 : General Building Analysis
Orion Documentation page 75
On first viewing the above may not appear unreasonable, however, because the rigid arms that constrain all the nodes to move together are not co-planar there are incorrect side effects as demonstrated by the bending moment diagram below.
The diaphragm is holding up the mid-span node on the raking beams causing an unexpected hogging moment at that point. In addition, axial loads develop in the inclined beams (below) which would not happen if they existed in a properly inclined diaphragm.
Orion Documentation page 76
Chapter 5 : General Building Analysis
Considering a sway case (below), it seems that the diaphragm is constraining displacements in the horizontal plane only.
The degree to which the results are affected is related to the pitch of the inclined beams. As the pitch reduces the effect drops, however it should be noted that it is still significant even at relatively low pitches. A workaround in this example would be to re-run the analysis using the option No Rigid Diaphragm Floor Levels. The resulting bending moment diagram is then as shown below.
Chapter 5 : General Building Analysis
Orion Documentation page 77
Alternatively, exclude one of the slabs meeting at the apex (as described in the second case study below) so that there are two differently inclined but separate diaphragms. (You should find results are very similar to those obtained using the first workaround.) Case Study 2 - storeys linked by inclined planes In this example inclined planes have been used to define ramps between the storey levels.
If either the Slabs to Define Rigid Diaphragm or Single Rigid Diaphragm at Each Floor Level option is used a warning message is displayed during the analysis indicating that the nodes constrained by a diaphragm do not all lie in the same plane. Because the storeys are linked, a single diaphragm has been created constraining all the floor nodes as shown below.
Orion Documentation page 78
Chapter 5 : General Building Analysis
When a lateral load is applied, each floor moves by the same amount - obviously the model is not behaving as intended.
Diaphragm action is required to help resist the lateral loads, so specifying No Rigid Diaphragm Floor Levels is not an acceptable solution. The answer is to restrict diaphragm action to the horizontal floor planes only. This is achieved by excluding the inclined slabs from the diaphragm as shown below.
Chapter 5 : General Building Analysis
Orion Documentation page 79
Because the storeys are no longer linked, separate diaphragms are formed at each floor level as shown below.
By viewing the deflections under lateral load it can be seen that each floor is now moving independently of the other floors as intended.
Orion Documentation page 80
Chapter 5 : General Building Analysis
Global Constraints
You can use the options in the building analysis dialog shown above to apply general constraints to the floor translation/rotation: • X/Y and Torsion Permitted,
• X/Y Permitted, Torsion Prevented, • Only X Permitted, • Only Y Permitted, The default setting is X/Y (translation) and Torsion Permitted and it allows full/free movement of the floor. This means that the lateral load resisting systems you define will be required to stabilise the building. For the vast majority of buildings this is an appropriate choice. It is also possible to model a 2D frame in Orion, in such a case the permitted movement might be restricted to the in-plane direction, Only X, or Only Y as appropriate.
Pattern Loading Designs codes generally require that pattern loading is considered in the analysis/design of continuous beams. Orion has a method of automatically patterning both dead and imposed loads along continuous beam lines. This subject is discussed in some detail in the Standard Training Manual.
Chapter 5 : General Building Analysis
Orion Documentation page 81
Rigid Zones Rigid zones deal with two aspects of the analysis and design of concrete frames: 1. Design codes generally allow you to design for forces at the face of a support. 2. From an analytical modelling point of view it is commonly accepted that a simple centre-line model does not properly idealise the physical size of members. By modelling rigid zones we account for the extra stiffness that exist within the 3D block where the members physically interact with one another. The chapter dealing with Wall Modelling Considerations shows examples that take this second point to a more extreme level. Beams attach to the ends of walls (not the centre lines) and we are happy to models walls and beams in this way. Why would the same not be true where beams attach to columns? To demonstrate the effect of rigid zone modelling the following simple example will be used.
Orion Documentation page 82
Chapter 5 : General Building Analysis
A simple symmetrical structure is shown in the rendered view above. When running the building analysis you have a choice of 3 model options relating to rigid zones, none, reduced by 25%, and maximum as shown below.
The sections below describe what each of these options will do.
Chapter 5 : General Building Analysis
Orion Documentation page 83
Rigid Zones – None
The analysis results are as shown above. The beam bending moment diagrams extend right to the centre of the column, the beams will be designed for a support moment of 77.91 kNm and a span moment of 86.56 kNm. This is the most simplistic analysis model and is likely to be the most similar to any models you may have created in general analysis packages.
Orion Documentation page 84
Chapter 5 : General Building Analysis
Rigid Zones – Reduced by 25%
The analysis results are as shown above. Note that the beam bending moment diagrams do not extend right to the centre of the column, they stop at the face of the column. The column moment diagram also stop at the underside of the beams. No diagrams are shown on the rigid zone lengths. Importantly, for this model type, although the bending moment diagram starts at the face of the column the actual rigid length within the rigid zone only extends to 75% of this length (i.e. it is reduced by 25%). Comparing the moment diagrams to the case where no rigid zones are used we see that: 1. The span moment is reduced from 86.56 kNm to 76.44 kNm. Adding rigid zones at the ends of beams generally lifts the bending moment diagrams and hence will slightly reduce span moments. 2. Although the diagram has been lifted the support moment of 77.91 kNm has reduced slightly to 75.12 kNm because the moment is now being taken at the face rather than the centre-line of the column. 3. The column design moments are also reduced from 77.91 to 69.59 kNm.
Chapter 5 : General Building Analysis
Orion Documentation page 85
Rigid Zones – Max
The analysis results are as shown above. Once again the beam bending moment diagrams do not extend right to the centre of the column, they stop at the face of the column. The column moment diagram also stop at the underside of the beams. No diagrams are shown on the rigid zone lengths. Importantly, for this model type, the bending moment diagram starts at the face of the column AND the actual rigid length within the rigid zone extends to 100% of this length (i.e. it is max). Comparing the moment diagrams to the cases where no rigid zones and reduced rigid zones are used we see that: 1. The span moment is further reduced from 86.56 kNm to 76.44 to 73.2 kNm. Adding full rigid zones at the ends of beams increases the lifting of the bending moment diagrams. 2. In this case the diagram has been lifted so much that the support moment of 77.91 kNm which reduced slightly to 75.12 kNm using reduced rigid zones has now increased to 78.36 kNm. 3. The column design moments which reduced from 77.91 to 69.59 kNm have also increased again slightly to 72.28 kNm.
Orion Documentation page 86
Chapter 5 : General Building Analysis
Discussion It should be noted that the simplistic model used above to demonstrate rigid zones does not deal with the complexities of continuous beams and pattern loading and may to some degree exaggerate the effects of rigid zone modelling. A more typical four span continuous beam example is considered below.
Above – results when no rigid zones are used. Below – results when rigid zones are reduced by 25%, span moments reduced by 12% in end span, support moments reduced by around 10%.
Chapter 5 : General Building Analysis
Orion Documentation page 87
Below – results when maximum rigid zones are used, span moments reduced another 2–3% and support moments increase again by 2–3% but are still less than the moments when no rigid zones are used.
In general the use of rigid zones reduced by 25% will result in the maximum reduction in support moments combined with a less extreme reduction in the span moment. It is considered that this option will give maximum efficiency and will be the preferred option for most designers using Orion. NOTE: Although this option can often result in reduced support and span design moments, it should not be confused with moment redistribution. In theory there is nothing to these design forces being further reduced by the use of moment redistribution.
Rigid Links 3D analysis models are still centre-line models, members need to line up to connect to each other. When you construct a 3D model in a general analysis package you will naturally see where things do not line up and make adjustments so that connections are made. When working in Orion you tend to feel that you are working with the physical model rather than the analytical model that is derived from it. Sometimes members will not line up and in such cases Orion will attempt to make the required connections by adding rigid links.
Orion Documentation page 88
Chapter 5 : General Building Analysis
The simple example shown below should help clarify this.
The beams on grids 1 and 1a do not line up but are both supported by the same column. In this example this column is actually inserted at the intersection of 1a and A1. If we look at a 3D stick view we can see the frame that has been defined as shown below.
Chapter 5 : General Building Analysis
Orion Documentation page 89
The column appears to support one of the beams but not the other. However, when the analysis is carried out Orion will automatically connect the end of any beam that lies within the physical plan perimeter of any column or wall to that column or wall. So when this model is analysed a rigid link is added as shown below.
In this example the column was defined in an offset manner directly in line with one of the beams. To be more precise it could have been defined on a new grid on it’s own centre line and then rigid links would have been created from both supporting beams to the column. Note that rigid links should not be confused with rigid zones, rigid zones are not used in the above model but could have been. Note that rigid links will also be created where smaller offset columns sit on larger columns or walls in the floor below.
Shear Walls and Core Wall Systems There are optional methods for idealising walls within Orion. This subject is discussed at length in the chapter on Wall Modelling Considerations.
3D Effects In general our traditional engineering expectations are developed from considering simplified sub-models for analysis and design. When a full 3D model is created unanticipated effects sometimes creep in, is this wrong? Some simple examples will help demonstrate these effects.
Orion Documentation page 90
Chapter 5 : General Building Analysis
Continuous Beams Traditionally continuous beam lines are analysed and designed in isolation. The modelling of the support conditions in such cases is often unsophisticated. In the example below a series of 3 span secondary beams are supported at different points on primary beams and also directly by columns.
When analysed in 3D using unadjusted gross member properties the bending moments for a general UDL case are as shown below.
Chapter 5 : General Building Analysis
Orion Documentation page 91
Note how the end span moments for the internal beams (which all carry the same load) vary from 38.94 to 49.54 kNm. The smallest value occurs on the central column line and largest occurs where the beam is supported at the centre of the primary beam. The variation is not too significant but does start to indicate the importance of all the relative stiffnesses in a 3D model. However, note that hogging moments develop at the extreme ends of the secondary beams where they are supported by a primary beam. Investigation would show that these develop because torsional forces are developing in the primary beams. A traditional 2D continuous beam line analysis will generally model this support as a pin and no hogging moment will develop. In fact torsion is only usually considered in traditional hand calculations where it’s development is essential to the local stability of the structure. In generating the above results the default unadjusted properties of the rectangular beam sections were used in the analysis. These results are exactly what you would get from any general 3D analysis package. In Orion you have options to apply adjustment factors to the default properties of groups of elements as shown below.
These adjustments are discussed further in the next section, but it is worth noting here that by default Orion suggests adjusting the torsional stiffness factor of beams by a factor of 0.01 (i.e. reducing to 1%). This is suggested because in most cases this means that the results of a 3D analysis will be more compatible with the tried and tested analysis/design achieved using older 2D idealisations.
Orion Documentation page 92
Chapter 5 : General Building Analysis
When the model above is reanalysed using this setting the results are as shown below.
The hogging moments have disappeared and the end span moments have increased and become more consistent at around 67 kNm. Effects of one Member on Another The most likely 3D effect is one where the design forces generated are not what you expect. This sort of effect could happen in many different ways and becomes more likely as the structural arrangement becomes more complicated and irregular. However, it can be demonstrated with the very simple model shown below.
Chapter 5 : General Building Analysis
Orion Documentation page 93
A beam is selected along grid A at the front of the model. The loading, shear and moment diagrams for this beam are shown below.
The loads are slightly offset due to the triangular area of slab being supported, but the bending moment diagram shows an (unexpected?) sagging moment at the left end support and an (expected?) hogging moment at the right end support. Can this be correct?
Orion Documentation page 94
Chapter 5 : General Building Analysis
The explanation relates to the diagonal beam across the floor. It has been loaded with an extra UDL. When the structure is examined in the building analysis post processor we can see how it deflects and bends.
The heavily loaded diagonal beam is putting a big moment into the supporting columns and the joint is clearly rotating. This rotation cannot happen without some effect on the other connected beams. When viewed in this way the design moments in the beam do not seem unreasonable. Innumerable examples of this nature could be developed. The point is that if the design forces in a member seem wrong you cannot assume that the analysis is wrong. You need to review the results carefully and in particular look at the nature of deflections. Sway Effects Many structures will undergo a natural sway under purely vertical (gravity) loads. These sways can sometimes introduce significant changes to the expected moment diagrams in beams in much the same way as is shown in the example above. In such cases it is important to ensure that checks are made for combinations where notional load cases are applied in sympathy with the natural sway of the structure. In such situations where you might like to check for differences exposed by a 3D analysis of a floor in isolation, the chapter Analysis and Design using FE introduces this option. In more extreme cases buildings stabilised by shear and core walls will sway significantly under purely vertical load, this topic is discussed in more detail in the chapter Wall Modelling Considerations.
Chapter 5 : General Building Analysis
Orion Documentation page 95
Transfer Beams The way in which load accumulates in columns that are in turn supported on transfer beams in a full 3D analysis is often at odds with initial/traditional expectations. This subject is specifically dealt with in the series of chapters dealing with transfer levels and in particular the section Discussion of Frame Analysis Results in the chapter Transfer Beams – General Method.
Stiffness adjustments When you construct a model you set/control the analysis model properties by: • Using the Building Parameters options to globally set/change material properties.
• Applying releases (pins) to any member end. • Selecting any individual member and override the calculated section properties. When you run building analysis you then have options to apply global adjustment factors to the properties of members on a group by group basis as shown in the dialog below.
Some reasons why you might use these options could include:
Minimising Torsion in Beams In the 3D Effects section earlier in this chapter, the Continuous Beams example shows how torsions will often develop in 3D analysis. These torsions would not have been identified or designed for using more traditional subframe analysis. When the torsions are reduced or eliminated the major axis design moments usually go up. For this reason Orion defaults to the setting shown above which reduces torsional stiffness by a factor of 100.
Orion Documentation page 96
Chapter 5 : General Building Analysis
However, if you have a model that (unusually) actually relies on torsion for stability then using the above setting will often mean that extremely large deflections accrue.
Investigating Deflections You could use different values of Young’s Modulus to investigate different loading conditions – for example use the default short-term value when considering wind load conditions, then reanalyse with multipliers to reduce Young’s Modulus to allow for long-term effects in other cases and general design.
Flat Slab Construction It is worth noting here that the 3D analysis model of a complete building is primarily a frame element model with an option to use FE meshing of shear/core walls. It does not include FE meshed floor elements. Since flat slab (or flat plate) structures do not include beams the basic 3D building analysis will not deal with these structures on it’s own. Orion supports the analysis and design of Braced Flat Slab Structures by integrating the results of sequential finite element floor analyses (a top down procedure referred to as FE Chase down) with the results of the 3D building analysis. The chapter Flat Slab Models describes the solution provided for these circumstances.
Transfer Levels A transfer level is an upper level within the structure at which one or more columns or walls stop. The discontinuous column or wall is supported by beams or slabs such that it’s load is transferred to other columns/walls. This topic is discussed at length in a series of chapters and worked examples beginning with the chapter Overview of Solution Options for Transfer Levels.
Supports In a simple model such as the one shown below, all columns and walls finish at a common foundation level.
In such models, by default a fixed support is automatically placed underneath each column and each wall at the top of the common foundation level, (indicated by the grey plane).
Chapter 5 : General Building Analysis
Orion Documentation page 97
If a fixed support is inappropriate, a pinned or spring support can be defined using Support Type Definitions from the Members menu.
Having defined a new support type, it can then be assigned to a specific column or wall using the member properties as shown:
Note the special meaning of Support Type: Default For columns and walls that stop at ST00 - Default = Fixed Support For columns and walls that stop at ST01 and above- Default = No Support User defined supports are further discussed in the chapter Modelling Techniques. This chapter also discusses their application to stepped foundation levels.
Orion Documentation page 98
Chapter 5 : General Building Analysis
Building Analysis Problems – Reviewing/Understanding The majority of regular models constructed in Orion will run through building analysis without any messages. However for some models the building analysis process can display error or warning messages at various stages during execution. Tip
There is an option to run validation checks at any stage, if your building analysis is giving errors or warnings and you have not run the validation checks, it is recommended that you do so. This may expose the cause of problems.
Tip
Many building validation errors can be avoided by ensuring that the option to check the validity automatically each time a member is inserted is active. This is done by clicking on the Graphic Editor View Settings and on the Plan settings tab, and ensuring that you leave the option Don’t check Model During Member Insertion unchecked.
Note
On completion of analysis a series of comparisons are made between the loads that you applied and the results of load decomposition and subsequent analysis. If the differences exceed a small percentage variation warnings will be given. All such warnings should be investigated.
Note
Beyond the above, it is not anticipated that you need to read this section until such time as you are trying to understand or resolve errors or warnings.
What are Errors? Errors may indicate solution problems (errors) or modelling errors on physical/engineering grounds. Errors cannot be ignored, you should investigate until you understand the error message and if possible take action to remedy the situation or contact Technical Support.
What are Warnings? Once understood, warnings can often be ignored – the important point is that you take the time to review the messages and decide whether an action is warranted. Different Errors and Warnings will appear at the different stages of the Building Analysis Process.
Chapter 5 : General Building Analysis
Orion Documentation page 99
Building Model Validity Checking Errors Building Validation checks the eight conditions indicated in the dialog:
If any of these conditions occur an error message will be displayed which will clearly indicate the members that are invalid. You should return to the graphic editor to correct the error and then re-perform the building model validity check to confirm the problem has been resolved. On rare occasions a particular error message is displayed: Error: beam 1B157 is overlapping with beam 1B157
At first this does not appear to make sense since it suggests that a beam is overlapping with itself. On investigation you will find that there are two beams with identical labels at exactly the same location. If you encounter this problem you should simply delete one of the beams while checking that you have not lost any manually applied loads. Note that you will not immediately see the other beam after deleting the first one, but executing the redraw or regen function will make it reappear. Note: If there are no errors as a result of Building Model Validity Checking, it does not follow that your model is 100% error free. There are many other conditions that are not checked at this stage, which could result in your model being incorrect. These conditions are checked during later stages of the building analysis process.
Beam Load Analysis Errors There should be no errors encountered during Beam Load Analysis. In some complex arrangements where irregular slabs are supported by numerous beams and walls on a complex perimeter problems may be encountered. Please report any such problem to your support department. Note that these problems may be overcome by redefining the slab, or perhaps by passed by using the option for FE load decomposition.
Orion Documentation page 100
Chapter 5 : General Building Analysis
Building Analysis Errors and Warnings Just as in any general 3D analysis package, you can define releases as you create the structure and hence you have the flexibility to create mechanisms within the structure. Most unexpected analysis messages will relate to the existence of such mechanisms.
If you see messages about singularities as shown above than it usually means that too many pin ended members have been defined. It is recommended that you review the structural model in the Building Analysis postprocessor and eliminate the unnecessary releases. If there are so many releases that the structure is unstable then this will usually result in the message shown below.
This message should never be ignored unless you are dealing with a transfer slab structure – see below.
Chapter 5 : General Building Analysis
Orion Documentation page 101
The following warning messages are commonly displayed during Graphical Data Preparation:
The above message informs you that columns (and walls) exist at an upper storey but do not continue to foundation level and that the means of support at the transfer level are uncertain. (i.e. a column sitting on a beam does not generate such a warning.)
Action Required If it is intended that the columns are transfer columns and you are clear as to the means of support (e.g. a transfer slab) then you can ignore the warning provided that you have taken account of the related requirements outlined in the chapter Overview of Solution Options for Transfer Levels later in this handbook. If it is not intended to be a transfer column return to the graphic editor and remodel so that you have a support member in the desired location.
Overview of Axial Load Comparison Report
Orion Documentation page 102
Chapter 5 : General Building Analysis
A simple model can be created for a simple illustration of the Axial Load Comparison Report. The 3D view of the model is as above. The plan view of the model is as below.
General information regarding the model.
• Beam size:
250 mm (width) x 500 mm (depth)
• Column size:
500 mm x 250 mm
• Column Height:
2800 mm
• Slab Thickness:
175 mm – therefore self-weight = 4.2 kN/m2
• Loadings on slab:
self-weight (calculated automatically based on thickness of slab) SIDL = 1 kN/m2 Total Dead Load = 5.2 kN/m2 Imposed Load = 0 kN/m2
The following report is produced by Orion for this model. Note that there are 4 main sections (tables) of information the headings of which have been emboldened.
Chapter 5 : General Building Analysis
Orion Documentation page 103
AXIAL LOAD COMPARISON REPORT SUM OF APPLIED LOADS (Using Un-Decomposed Slab Loads): -----------------------------------------------------G - Dead Loads: Storey -------
Column
ShearWall
Beam
Slab
Rib
Total
(kN)
(kN)
(kN)
(kN)
(kN)
(kN)
---------- ---------- ---------- ---------- ---------- ----------
St01 -------
33.6
0.0
65.0
117.3
0.0
215.9
---------- ---------- ---------- ---------- ---------- ----------
Total
215.9
Q - Live Loads: Storey ------St01 -------
Column
ShearWall
Beam
Slab
Rib
Total
(kN)
(kN)
(kN)
(kN)
(kN)
(kN)
---------- ---------- ---------- ---------- ---------- ---------0.000
0.000
0.000
0.000
0.000
0.000
---------- ---------- ---------- ---------- ---------- ----------
Total
0.000
SUM OF APPLIED LOADS (After Decomposing Slab Loads): ---------------------------------------------------G - Dead Loads: Storey -------
Column
ShearWall
Beam
Slab
Rib
Total
(kN)
(kN)
(kN)
(kN)
(kN)
(kN)
---------- ---------- ---------- ---------- ---------- ----------
St01 -------
33.6
0.0
182.3
0.0
0.0
215.9
---------- ---------- ---------- ---------- ---------- ----------
Total
215.9
Q - Live Loads: Storey ------St01 ------Total
Column
ShearWall
Beam
Slab
Rib
Total
(kN)
(kN)
(kN)
(kN)
(kN)
(kN)
---------- ---------- ---------- ---------- ---------- ---------0.000
0.000
0.000
0.000
0.000
0.000
---------- ---------- ---------- ---------- ---------- ---------0.000
Orion Documentation page 104
Chapter 5 : General Building Analysis
BUILDING ANALYSIS COLUMN/SHEARWALL AXIAL LOADS: ----------------------------------------------Storey
G
Delta G
(kN) -------
(kN)
Delta Q
(kN)
(kN)
---------- ---------- ---------- ----------
St01 -------
Q
215.9
215.9
0.0
0.0
---------- ---------- ---------- ----------
Total
215.9
0.0
FINITE ELEMENTS ANALYSIS COLUMN/SHEARWALL AXIAL LOADS: -----------------------------------------------------Storey
G
Delta G
(kN) -------
Delta Q
(kN)
(kN)
---------- ---------- ---------- ----------
St01 -------
Q
(kN)
223.6
223.6
0.0
0.0
---------- ---------- ---------- ----------
Total
223.6
0.0
The purposes and validity of the comparisons that can be made between the sections of this table are discussed in the following sections. Table 1 SUM OF APPLIED LOADS (Using Un-Decomposed Slab Loads): -----------------------------------------------------Storey ------St01 -------
Column
ShearWall
Beam
Slab
Rib
Total
(kN)
(kN)
(kN)
(kN)
(kN)
(kN)
---------- ---------- ---------- ---------- ---------- ---------33.6
0.0
65.0
117.3
0.0
215.9
---------- ---------- ---------- ---------- ---------- ----------
Total
215.9
Q - Live Loads: Storey ------St01 -------
Column
ShearWall
Beam
Slab
Rib
Total
(kN)
(kN)
(kN)
(kN)
(kN)
(kN)
---------- ---------- ---------- ---------- ---------- ---------0.000
0.000
0.000
0.000
0.000
0.000
---------- ---------- ---------- ---------- ---------- ----------
Total
This table shows the sum of: • Member self-weights
• Applied loads on beams • Applied loads on slabs The slab loads are calculated based on the area as seen on plan – i.e they are not yet decomposed onto beams.
0.000
Chapter 5 : General Building Analysis
Orion Documentation page 105
The detailed calculations are therefore as follows: Columns Self Weight
Beams Loads
=
0.25 m x 0.5 m x 2.8 m(height) x 24 kN/m 3 x 4(nos)
=
33.6 kN
=
[(0.25 m(width of beam) x 5 m(length) x 4(nos)) x 0.5 m(depth of beam) x 24 kN/m 3 ] + [(0.25 m(width of beam) x 5 m(length)) x 1 kN/m 2 ]
P
P
Slab Loads
P
P
P
P
=
65 kN
=
[(5 m - 0.25 m(half width of beam x 2)) x (5 m - 0.25 m(half width of beam x 2))] x 5.2 kN/m 2 P
=
P
117.3 kN
Table 2 Differences between this table and table 1 are specifically intended to expose problems in slab load decomposition. Since there are two methods of slab load decomposition supported we will look at each of these separately.
Using Yield Line Decomposition SUM OF APPLIED LOADS (After Decomposing Slab Loads): ---------------------------------------------------G - Dead Loads: Storey -------
Column
ShearWall
Beam
Slab
Rib
Total
(kN)
(kN)
(kN)
(kN)
(kN)
(kN)
---------- ---------- ---------- ---------- ---------- ----------
St01 -------
33.6
0.0
182.3
0.0
0.0
215.9
---------- ---------- ---------- ---------- ---------- ----------
Total
215.9
Q - Live Loads: Storey ------St01 -------
Column
ShearWall
Beam
Slab
Rib
Total
(kN)
(kN)
(kN)
(kN)
(kN)
(kN)
---------- ---------- ---------- ---------- ---------- ---------0.000
0.000
0.000
0.000
0.000
0.000
---------- ---------- ---------- ---------- ---------- ----------
Total
Once again this table shows the sum of: • Member Self-weights
• Applied loads on beams • Applied loads on slabs
0.000
Orion Documentation page 106
Chapter 5 : General Building Analysis
The key difference in this table is that the slab loads are now decomposed and thus counted as UDL’s, VDL’s etc. on the supporting beams. Therefore, you will find that the slab loads become zero but the beam loads increase accordingly. Decomposed Slab Loads – Consider again the model and the decomposed slab loads shown below.
Chapter 5 : General Building Analysis
Orion Documentation page 107
The plan view shows the yield lines, strictly speaking these are really just load decomposition lines which are used to show the area of slab loading that will be attributed to each beam. This method of area load decomposition is commonly known as the Yield Line Method. Looking at the triangular load distribution generated on the above beam, the beam load calculation effectively becomes: Decomposed Slab Load
=
(A1 + A2 + A3 + A4) kN/m2
=
4 x 29.33 kN
=
117.3 kN
The beam loading profile and the above calculation clearly get much more complex when more irregular slab arrangements are used. The calculations reported in table 2 are therefore as follows: Columns Self Weight
Beams Loads
=
UNCHANGED
=
33.6 kN
=
65 kN + 117.3 kN
=
182.3 kN
Orion Documentation page 108
Chapter 5 : General Building Analysis
Using Finite Element Decomposition If slab loads are decomposed using the alternative FE method, the loadings from the slab are applied to the beam as a complex beam load as shown below.
SUM OF APPLIED LOADS (After Decomposing Slab Loads): ---------------------------------------------------G - Dead Loads: Storey ------St01 -------
Column
ShearWall
Beam
Slab
Rib
Total
(kN)
(kN)
(kN)
(kN)
(kN)
(kN)
---------- ---------- ---------- ---------- ---------- ---------33.6
0.0
182.0
0.0
0.0
215.6
---------- ---------- ---------- ---------- ---------- ----------
Total
215.6
Q - Live Loads: Storey ------St01 ------Total
Column
ShearWall
Beam
Slab
Rib
Total
(kN)
(kN)
(kN)
(kN)
(kN)
(kN)
---------- ---------- ---------- ---------- ---------- ---------0.0
0.0
0.0
0.0
0.0
0.0
---------- ---------- ---------- ---------- ---------- ---------0.0
Chapter 5 : General Building Analysis
Orion Documentation page 109
Comparisons between tables 1 and 2 Comparison between these 2 tables provides an indication of the success of the slab load decomposition. If the totals in table 2 are less than those in table 1 this would give an indication that loads have gone missing during the decomposition process in which case you should: 1. Check that slab loads are applied within slab boundaries 2. Visually Check Yield Lines – if they do not look right on the plan the decomposition is probably not right. 3. If Yield Lines are wrong, consider swapping to the FE Load Decomposition method. Note
Both FE and Yield Line decomposition decompose slab loads to beams. If there are areas without beams (where slabs are supported directly by columns – i.e. Flat Slabs) the decomposition process is guaranteed to lose load. In such cases full FE analysis must be used and the discrepancy between the totals in tables 1 and 2 should be seen as an indicator of this requirement.
Note
In all but the simplest of models, there will always be a small discrepancy between table 1 and 2. However, this can be ignored especially if the decomposed load in table 2 is slightly higher than the un-decomposed load in table 1.
Table 3 BUILDING ANALYSIS COLUMN/SHEARWALL AXIAL LOADS: ----------------------------------------------Storey
G (kN)
------St01 ------Total
Delta G (kN)
Q (kN)
Delta Q (kN)
---------- ---------- ---------- ---------215.9
215.9
0.0
0.0
---------- ---------- ---------- ---------215.9
0.0
The figures in this table reflect the results of the building frame analysis. The building analysis is a frame analysis where the beams are loaded with all the decomposed slab loads. Therefore the input is based on either the yield line or FE Load Decomposition method; whichever has been selected. When the analysis is complete the accumulated column loads on each storey are shown in the table. It is therefore appropriate to compare table 2 with table 3 which is in effect a comparison of analysis input with analysis results. If the totals are different the building analysis is incorrect in some way in which case you should check: • If there were warnings during the building analysis, have you ignored them and is it OK to ignore them?
• If transfer levels exist, check whether the discontinuous columns are properly supported.
Orion Documentation page 110
Chapter 5 : General Building Analysis
Note
Flat slab models are a good example of the case where there will be discrepancies and loads are lost, but this can all be ignored since Finite Element Analysis load Chase Down is required – see next section.
Note
This table is only available if an FE Chase Down (finite element analysis load chase-down) has been performed:
Table 4
When performing the FE Floor Analysis, there are 2 options available.
Option 1: Include Slab Plates in FE Model
When this option is active as shown above, a meshed up FE model is created as shown below.
Chapter 5 : General Building Analysis
Orion Documentation page 111
Slab loads are applied to all the individual shells and so the slab load decomposition is effectively inherent in the FE analysis of the floor. After analysis the total axial loads in columns and walls are derived at each level and reported in table 4 as shown below. FINITE ELEMENTS ANALYSIS COLUMN/SHEARWALL AXIAL LOADS: -----------------------------------------------------Storey
G
Delta G
(kN) -------
(kN)
Delta Q (kN)
---------- ---------- ---------- ----------
St01 -------
(kN)
Q
223.6
223.6
0.0
0.0
---------- ---------- ---------- ----------
Total
223.6
0.0
The total loads and self-weights reported will always be slightly different in FE analysis because this is a true centre-line model. In this simple example the loads effectively total up as follows: Column Self Weight
=
0.25 m x 0.5 m x 2.8 m(height) x 24 kN/m3 x 4(nos)
=
33.6 kN (this part is unchanged)
Beams Loads
=
[(0.25 m(width of beam) x 5 m(length) x 4(nos)) x 0.5 m(depth) x 24 kN/m3]
=
60 kN (i.e. self weight only – finishes loads are not applied to the beam)
Slab Self Weight
=
(5 m x 5 m) x 5.2 kN/m2
=
130.0 kN (see note below)
Total
=
60 kN + 33.6 kN + 130.0 kN
=
223.6 kN
Note
The area of the slab used in the FE model is different (greater) than the area used in tables 1 and 2 because the slab extends to the centre-line (not the edge) of the beam. This has two effects: • The self weight if the slab in the zone that overlaps with the beam is double counted – small extra load.
Orion Documentation page 112
Chapter 5 : General Building Analysis
• The finishes and imposed loads that apply on the outer edge of edge beams will not be counted – small loss of load. When this option is used it is appropriate to compare the totals in table 1 with table 4 (Load before decomposition and FE analysis results which embody both decomposition and analysis). Some degree of difference must be accepted for the following reasons: • In all models, there will be a slight difference for the reasons noted above.
• The FE method makes much better allowance for holes in slabs. If totals returned from FE Analysis (table 4) are significantly less than totals from table 1, the FE Analysis model may be incorrect. In such cases the following possibilities should be checked: • Are slab line loads applied outside the building perimeter or over voids.
• If the option to include column and wall sections in the model has been used, are slab line loads applied through columns or walls (the load inside the column or wall boundary is included in table 1 but is not taken into account in the FE analysis because the load could not physically exist.)
• As above, but checking for point loads applied inside column boundary. • Incorrectly modelled cantilever slabs. Note
The totalling of loads in table 4 will be incorrect if some columns are len(storey) 2 or more, but the results themselves are actually correct.
Option 2: Exclude Slab Plates in FE Model
Chapter 5 : General Building Analysis
Orion Documentation page 113
In this case the FE analysis is simply a 3D frame analysis where the decomposed slab loads are applied to the beams. In this case it is valid to compare table 2 with table 4 since this is checking that the decomposed loads which were intended to be applied to the FE models have been successfully applied.
Orion Documentation page 114
Chapter 6
Chapter 6 : Eigenvalue Analysis
Eigenvalue Analysis
Introduction An Eigenvalue Analysis can be performed as part of the Building Analysis in order to calculate natural frequencies and mode shapes; these will be dependent on storey mass and model stiffness. The Eigenvalue Analysis results can then be used for seismic design purposes and can also be of value if wind tunnel tests are required.
Eigenvalue Analysis Parameters Controlling the Storey Mass The storey mass for Eigenvalue analysis is always based on the dead load G plus a fraction of the live load Q.The degree to which the live load is assumed to participate is controlled by the participation factor (n), which can be specified on the Lateral Loading tab of the Building Parameters dialog.
Note
For Lateral Load calculations, the storey weight can be based on G or Q or G+nQ. The Live Load Participation Factor, (n) does not affect the Notional Load Calculation unless the G+nQ option is selected.
Chapter 6 : Eigenvalue Analysis
Orion Documentation page 115
The G and Q components of the Storey Mass/Weight can be derived from either the decomposed beam loads, or the undecomposed slab loads. The method used is controlled via the Settings tab of the Building Analysis Model Options dialog as shown below.
The mass/weight determined for the chosen option can be reviewed by hovering the cursor over the Center of Gravity of the form plan after running a Building Analysis.
C of G based on Un-decomposed Slab Loads, m = 553t
Orion Documentation page 116
Chapter 6 : Eigenvalue Analysis
C. of G. based on Decomposed Beam Loads, m = 106t
If all slabs transfer their loads to beams or walls, either option should produce a similar mass/ weight. This can clearly be seen not to be the case in the above (flat slab) example - the mass determined using the Decomposed Beam Loads option is significantly smaller than that from the Use Undecomposed Slab Loads option. In such models it is important that the latter option is always selected.
Model Stiffness The engineer should use section properties that are appropriate for the Eigenvalue Analysis. For columns and walls this could involve making global stiffness adjustments to model cracked section properties. The ACI code may be referred to for some guidance in this regard. These adjustments can be made via the Model Options/Stiffnesses tab of the Building Analysis dialog.
Controlling the Number of Mode Shapes Required These are set on the Lateral Loading tab of the Building Parameters dialog.
Chapter 6 : Eigenvalue Analysis
Orion Documentation page 117
Analysis The analysis produces results for both the Static and Eigenvalue Analysis that can be accessed simultaneously in the graphical post processor.
Graphical Results To view a mode shape, activate the Displacements button and select the mode shape required from the Loading menu as shown below. Animation can be activated if required from the Displacements menu.
Orion Documentation page 118
Chapter 6 : Eigenvalue Analysis
Numerical Results Numerical output from the analysis is accessed from the Report tab – Eigenvalue Results Report Button.
This button shows information relating to the frequencies and mass participations as shown below.
Chapter 6 : Eigenvalue Analysis
Orion Documentation page 119
Exporting to S-Frame If you export the model to S-Frame to cross check the Eigenvalue analysis, you will need to make a minor adjustment before running the analysis. This is necessary because the way S-Frame stores the mass information is slightly different to the way it is held in Orion.
Exporting to S-Frame Example In Orion – For a simple ‘table’ model the frequency of the first mode is given as 7.124Hz. The mass at the single floor level is 31.847 tonne (312.4kN). This mass includes for the weight of all columns and walls below the floor.
When the model is opened and analysed in S-Frame the frequency is 6.97Hz The difference is accounted for by the way the mass due to self weight is being modelled in S-Frame. Correction of Self Weight in S-Frame (for Eigenvalue Analysis only) Two materials are exported to S-Frame - The first material (MAT1) is assigned to the columns/walls and has a mass – Orion ignores this mass in the vibration calc because it has included it in the floor mass, S-Frame does not ignore this mass, therefore the mass is double counted in S-Frame. The second material (MAT2) used for beams has zero mass. In order to get compatible Eigenvalue analysis results between Orion & S-Frame you are required to switch off the mass of MAT1 in S-Frame. This is done by setting the force density to zero. If the density of the materials is set to zero in S-Frame the frequency for the first mode increases to 7.125Hz. Note
In S-Frame, for the G loadcase, selfweight of the structure is introduced by a gravitational factor - hence the above adjustment will invalidate the gravity case results if a static analysis is performed.
Orion Documentation page 120
Chapter 7
Chapter 7 : Staged Construction Analysis
Staged Construction Analysis
Introduction Staged construction analysis is a sophisticated method of solution which, if used with a reasonable degree of care can more closely reflect the real behaviour of the structure. A standard 3D analysis takes no account of time dependant effects - it assumes the structure has been constructed instantaneously. In the real world a concrete building has to be constructed in stages - typically one floor at a time. Depending on the number of storeys within the building a significant period of time can elapse from the commencement of first floor to completion of the top floor. Because concrete matures with time, this means that at the top floor the concrete has properties which ‘lag behind’ those of the first floor (and to a reducing degree each intermediate floor). Another aspect of the staged nature of concrete construction is that any settlement of existing lower floors which takes place prior to the pouring of the current floor is allowed for on site when establishing the current floor level. This effect can not be allowed for in a standard 3D analysis, however it is catered for automatically in a staged construction analysis.
Simple Example A model consisting of two blocks is constructed as shown below. The first floor plan and loading is identical in both blocks. A second floor framework is added to the right hand block but with no imposed load applied to that level.
A standard 3D analysis can be performed for the above, in which all members are analysed simultaneously with all the loads applied. A staged construction analysis can also be performed. If the option were selected to stage the imposed load only, the analysis would proceed as follows: • In the first stage, only the first floors are analysed for their imposed load.
• In the second stage, the second floor is added and the analysis repeated (for the undeformed model) but this time only for the imposed load at second floor level. Note
In this way, each stage can only affect the result in it’s own and previous stages.
Chapter 7 : Staged Construction Analysis
Orion Documentation page 121
Results for Q - Unstaged
In the unstaged result you can see that Q load applied at first floor is causing moments to develop in the structure above. If that frame existed when the load was applied then this is completely logical.
Results for Q - Staged
In the staged result no moments develop in the frame above the first floor members and in fact the forces in the first floor structure are identical in the single storey frame and the two storey frame. This is logical if all the load is applied before the frame above exists AND assuming that all deflection is instantaneous. Note
The effect of creep is not considered in the above - in reality the first floor beams would continue to deflect and the joints continue to rotate after the frame has been built and loaded. Therefore, although there are no moments in the second floor frame at the point at which it is constructed, the creep of the first floor structure should cause some moments to develop over time in the second floor.
Orion Documentation page 122
Chapter 7 : Staged Construction Analysis
Staged Construction Modelling and Analysis Model Creation No additional steps are required when creating a model for staged construction. Stages are created automatically, (each storey being treated as a separate stage). Stages can be adjusted once the model has been created as described in the section Stage Control.
Staged Loading Creation Staged loading is specified within the Load Combination Editor by clicking the Loading Generator button.
Using the controls you are able to stage any gravity load case and then create combinations of staged and unstaged cases. Stage Construction Cases — Check this box to activate staged construction. Stage Duration — Specify the default duration of each stage, this can subsequently be amended for individual stages - see Stage Control. Stage Construction Cases: G — Check this box to create an sG staged loadcase. Stage Construction Cases: Q — Check this box to create an sQ staged loadcase. Create New Combinations for Staged G and Q — When not checked one set of combinations is created which uses the staged cases, when checked two sets of combinations are created, one with the unstaged cases and the second with the staged cases.
Chapter 7 : Staged Construction Analysis
Orion Documentation page 123
The result of checking the Create New Combinations for Staged G and Q box is shown below:
Combinations 1 to 7 are not staged at all, combinations 8 to 14 use fully staged G + Q. Alternatively you may choose to have combinations of staged G and unstaged Q. As shown above, the Loading Generator populates the Load Combinations Editor with a column header for each staged and unstaged loadcase. The generated combinations are displayed in rows. Note You are not restricted to the automatically generated combinations and factors, you have complete control to add, delete, or modify the combinations as you see fit. In this way you can design for an envelope of staging assumptions. Note that you can even include staged pattern load cases! While in reality this may not seem entirely logical, it might actually be the best way of getting closest to the results generated by traditional sub-frame analysis. However, staged G plus unstaged Q might be preferred because they create the worst effects in different places and might actually produce a lower peak.
Orion Documentation page 124
Chapter 7 : Staged Construction Analysis
Stage Control Stages are defined within the Load Combination Editor by firstly clicking the Load Cases button. Next, highlight a staged loadcase, (for example SG as shown below) and click Edit.
Initially each floor level is considered to be a stage - so by default a 5 storey building is analysed in 5 stages as shown below:
You can if required specify different time durations for the individual stages. See Setting the Duration of Each Stage
Chapter 7 : Staged Construction Analysis
Orion Documentation page 125
Combining multiple floors into a single stage You are not restricted to having each floor analysed as a separate stage, several storeys can be combined into a single stage. For example you may choose to analyse the previous 5 storey building in just 2 stages:
• Stage 1 as displayed above comprises storeys 1 and 2, • Stage 2 comprises storeys 3, 4 and 5. To reduce the original five stages to two stages you would proceed as follows: 1. Stage 1 must always start at storey 1 so this is left as is, 2. Stage 2 starts at storey 3, when this is entered the screen is shown thus:
3. Stages 3 and 4 are not required and need to be deleted - this is achieved by clicking in the topmost stage (4) and putting "0" in the storey column.
4. Repeat once more so that you are left with stages 1 and 2.
Combining the entire structure into a single stage The same technique for deleting stages can be used to place the entire structure in a single stage. The reason for doing this is to allow the model to be simultaneously designed for patterned load combinations derived from the building analysis AND combinations derived from the FE merged load cases. See FE Merging for details.
Orion Documentation page 126
Chapter 7 : Staged Construction Analysis
Setting the Duration of Each Stage The duration of each stage controls the modulus of elasticity used at that stage - it is assumed that a normal cement is used so that Ec at age t is estimated by: Ec(t) = E(t)Ec(28)1 with E(t) = squareroot (cc(t) where (cc(t) = exp [0.25(1 - squareroot (28/t)] The significance of the stage duration on Ec is illustrated below: Duration 1 day
0.59 Ec(28)
Duration 15 days
0.96 Ec(28)
Duration 28 days
1.00 Ec(28)
Duration 99 days
1.06 Ec(28)
From the above it can be seen that changes due to duration do not have a large impact on the results after the first few days.
Analysis Properties
As per unstaged construction, factors are applied via the Model Options/Stiffnesses tab (shown above). These are applied across all construction stages. Modulus of Elasticity The Modulus of Elasticity Factor in particular requires careful consideration as it should be employed in order to make any allowances for creep and cracking. The value entered here is applied to the short term modulus of elasticity Ec(28) defined in Material dialog. As described in the previous section Setting the Duration of Each Stage further factors are then applied to the E value to account for the duration of each construction stage. Note
Any allowance for creep is only dealt with effectively in the context of an unstaged analysis..
Note
The E value adopted is very important in relation to sway sensitivity assessment for EC2 (and for other codes also if the ACI sway option is used).
Footnotes 1. Taken fron ‘Concrete Structures’ Amin Ghali, Renaud Favre, M. Elbadry
Chapter 7 : Staged Construction Analysis
Orion Documentation page 127
FE Merging When G and Q loads are merged from the FE analysis they replace all the G and Q loadcases so G-fe replaces G, GP11, GP12, and Q-fe replaces Q, QP11, QP12, etc. Staged loadcases are never replaced by FE merged results - the consequence of this on the combinations is as follows: • For combinations of entirely staged cases no merging takes place.
• For combinations of entirely unstaged cases - G, GP11, GP12, Q, QP11, QP12, etc. are replaced.
• For combinations that contain both staged and unstaged cases, only the unstaged cases are replaced. The latter situation is investigated in more depth below: Terminology: G and Q = results from 3D analysis G-s and Q-s = results from 3D analysis where staged construction has been considered G-fe and Q-fe = results merged from FE floor analysis where limitations noted above continue to apply. 1. Initially both G and Q are staged and the option to ‘Create New Combinations for Staged G and Staged Q’ is checked:
the result is that before merging there are two sets of combinations: G +Q G-s + Q-s In this case the merged FE forces would affect the unstaged combinations. Therefore: G-fe + Q-fe G-s + Q-s This may be considered to be an attractive mix of combinations by some engineers. 2. You may have chosen to edit the original combinations so that the first combination is based on staged rather than unstaged G, hence before merging the two sets of combinations are: G-s + Q G-s + Q-s FE merging only affects the unstaged results so in this case you would end up designing for an envelope that simultaneously considers G-s + Q-fe G-s + Q-s
Orion Documentation page 128
Chapter 7 : Staged Construction Analysis
3. You may have chosen not to stage Q at all in which case before merging there is a single combination: G-s + Q FE merging only affects the unstaged results so in this case you would end up designing for: G-s + Q-fe The above might or might not be considered adequate, that is for you to decide. Simulaneously designing for FE merged forces and the results of a completely unstaged analysis. Clearly the above introduces the possibility that FE merged forces can be designed for simultaneously with a combination where staged conditions are considered. What it does not allow is the possibility of simultaneously designing for FE merged forces and for the results of a completely unstaged analysis. However, a workaround can be employed to achieve this if you run a ‘single’ staged construction analysis for both G and Q as follows: • In the Load Generator, check the options to stage both G and Q and also to ‘Create New Combinations for Staged G and Staged Q’:
• Edit the two staged loadcases so that the entire model is put in the first stage and set the duration to 28 days.
• Before merging there are now two sets of combinations, (although the staged combination produces exactly the same result as the unstaged): G +Q G-s + Q-s FE merging only affects the unstaged results so you end up designing for an envelope that simultaneously considers G-fe + Q-fe G-s + Q-s (which in this case is identical to G + Q)
Chapter 8 : Analysis and Design using FE
Chapter 8
Orion Documentation page 129
Analysis and Design using FE
Introduction When you run FE Floor analysis you are presented with many options. In this chapter we aim to help make sense of these options by providing an overview of their meaning/effect and also indicate when each option would tend to be of most importance. Throughout this chapter we will also cross reference to more detailed examples that illustrate the use of each option in practice.
Model Generation Options When you run the FE Floor Analysis module the following dialog will immediately be displayed.
-
If you have previously run an analysis for the floor the settings will have been saved and will now be redisplayed. A brief introduction to each item/option is given below.
Orion Documentation page 130
Chapter 8 : Analysis and Design using FE
Column / Shear Wall Model Type You have a choice of 3 model types: 1. The short frame model. 2. Z Restrained (Pinned Support) Model. 3. Elastic Spring Element. As shown below, the short frame model option creates a 3D subframe for the floor including the columns/walls. The other two models do not include the columns/walls and so any fixity moments that develop between the floor system and the columns are excluded.
We strongly recommend that the default Short Frame Model is used unless/until you have some specific reason to try another option.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 131
Beam Stiffness Multiplier When the FE model is generated it uses the gross section properties of the un-flanged beam when calculating the inertia (I). Young’s Modulus (E) is taken as defined in the main building parameters. It is possible that you may want to increase I to account for flanged section properties. It is also possible that you may want to decrease I to account for cracking, creep and shrinkage if deflection is a concern. However, in framed systems deflection is generally not a concern and the advice given in Codes/Standards is that a consistent set of properties be used. Since the column properties are based on the gross section, it is felt that the default use of the gross section properties of the un-flanged beam is generally appropriate.
Slab Stiffness Multiplier As for beams, the FE model is generated using the gross section properties of the slab when calculating the inertia (I). Once again Young’s Modulus (E) is taken as defined in the main building parameters. When you are looking at a beam and slab system, it is considered highly likely that you will want to reduce the slab stiffness relative to the beam stiffness. Even with a flat slab system, you may also decide to adjust the slab stiffness. The reasons for this are best understood by reference to the following examples: • For Beam and Slab Systems, refer to the Worked Example – Beam and Slab Systems section later in this chapter.
• For Flat Slab Systems refer to the chapter Flat Slab Models and in particular the discussion of slab deflection estimation.
Column and Wall Stiffness Multipliers The FE model is generated using the gross section properties when calculating the inertia (I). and the Young’s Modulus (E) is taken as defined in the main building parameters. Typically you would only need to adjust the column stiffness for the purpose of making a cracking and creep allowance.
Cracking and Creep As previously stated the advice given in Codes/Standards is that a consistent set of properties be used. Hence if the Cracking and Creep function is used the calculated stiffness factor is applied by default to all member types.
Include Column Sections in FE Model This option allows you to create a more sophisticated model that idealises a rigid zone extending to the perimeter of the column sections. This modelling sophistication is really only of interest in flat plate models where it helps tackle the problem of extreme local hogging
Orion Documentation page 132
Chapter 8 : Analysis and Design using FE
moments that are generated over column heads in traditional analysis model idealisations. Refer to the chapter on Flat Slab Models for an example of the effect of this option. Note that activating this option may introduce more meshing difficulties during model generation. For beam and slab models this option has little positive effect and may only introduce difficulties, hence we recommend it is better not to activate this option for such models.
Include Slab Plates in FE Model This option allows you to create floor grillage models where the slabs are not modelled or meshed. In these models the beams are loaded using the same decomposed loads as are applied in the general building analysis. This modelling option has a various potential uses: 1. A fast solution option for Transfer Beams – Refer to the chapter Transfer Beams – FE Method, Option 1 (Simplest) for a worked example. 2. It allows you to investigate grillage effects that are not normally considered in any subframe analysis. For example the forces in a line of secondary beams can be influenced by the varying rigidity of primary support beams. As in the transfer beam example noted above, you have the option to merge results and check beams for the results of alternative analytical idealisations.
Consider Beam Torsional Stiffness The option to consider or exclude beam torsional stiffness for FE analysis has been included to allow you to model in line with traditionally accepted idealisations. For example, in hand calculations, or in any continuous beam design package, and in fact in Orion’s 2D sub-structuring approach to building analysis, a continuous secondary beam supported by other primary beams is assumed to receive vertical support, but rotational compatibility is ignored. i.e. a secondary beam will not introduce torsions in a supporting primary beam. In practice these torsions are known to exist, but are assumed not to be significant. When this option is not checked Orion’s 3D floor analysis will operate more in line with this commonly accepted idealisation.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 133
Note however that in some models excluding the beam torsional stiffness will allow a mechanism to develop.
In the example shown here a curved elevation is modelled with a series of straight beams. If the FE model is created without including beam torsional stiffness the deflections are extreme, indicating the presence of a mechanism.
Around the curve, moments in one element translate to a combination of moments and torsions in the adjacent elements due to the angular change.
Orion Documentation page 134
Chapter 8 : Analysis and Design using FE
When remodelled including beam torsional stiffness the deflections diagrams start to look much more reasonable as shown below.
Note
Orion does not consider torsion within beam design, so if significant torsions are developing these would need separate design checks. You can check torsion levels by reviewing the FE Analysis Output Report, or if you have S-Frame, you can export to that and review the results graphically.
Include Upper Storey Column Loads This option allows you to automatically include the column self weight and also reactions from the FE analysis of the floor above as loading input for the current floor. This enables an FE load chase down procedure that allows Orion to provide solutions for transfer beams/slabs. Refer to the chapters on these options starting with Overview of Solution Options for Transfer Levels for a full discussion and worked examples.
Upper Storey Column Loads Table This option displays a table allowing you to review the column loads being applied when the above Include Upper Storey Column Loads option is used.
Floor Mesh and Analysis This option opens the modelling window in which you can mesh and then refine the mesh for your floor model. This is discussed in a little more detail in the following section Meshing and Analysing your Model. If you have chosen to model without plates then meshing is unnecessary, you can simply enter and immediately exit this window in order to generate the beam element model. When you exit from this window the model will be automatically analysed.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 135
Batch FE Chasedown This option displays a table allowing you to specify all of the model generation options and mesh parameters required for every floor in the building. By clicking on OK you are then able to perform the chasedown procedure for all floors in a single batch. An option is provided for you to check and if necessary adjust the meshing at each floor. It is recommended that the meshing is reviewed and approved during the first run and also after any significant edits. If you need to abort the chasedown at any stage simply click on the cross at the top right of the analysis progress window shown below.
Meshing and Analysing your Model If Slab Plates are NOT included When you click the mesh generation option, you initially see an isometric view of the model. If you have chosen this model type you do not need to do anything in this window, the complete model is already created, you simply need to exit at which point the model will be automatically analysed.
Orion Documentation page 136
Chapter 8 : Analysis and Design using FE
If you wish, you can review the model by activating more of the view options from the menu on the left of the screen. The options you are most likely to switch on/off are also available as drop down iconised menus along the top of the screen - you should find these more convenient to use. However all of this (and more) can also be reviewed in the results window.
For detailed examples using this modelling option, refer to the chapter Transfer Beams – FE Method, Option 1 (Simplest) or to the following section on Merging Beam Results.
If Slab Plates are included When slab plates are included, you need to mesh up the model before exiting the mesh generation option. Prior to generating the mesh it is imperative that all columns have been positioned on slab boundaries - otherwise no connection can be formed between the slab and the column. It is also strongly recommended that each slab panel has been defined so that none of it’s internal angles are greater than 180 degrees. L shaped panels fall into this category - these can be avoided if you split the L shape into two or more rectangular panels. Overall it is important not to enter into FE meshing of floor slabs with an expectation that everything will always work first time. Meshing is often an inexact and iterative procedure that takes a little time to get right. The mesh generator will try to create a mesh for you within the parameters that you set, but it is not always possible. When a mesh is generated it is up to you to be confident that it is a reasonable mesh for the problem you are trying to assess. The sections below list the more common issues that seem to arise in relation to mesh generation. Meshing is also discussed in more detail in the following examples in this chapter, and also in the examples in the chapter Flat Slab Models.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 137
Validity Checking Note that the validity checks accessed from the main Building Analysis dialog will issue warnings about slabs that may cause problems during FE meshing and analysis.
These types of messages can be very helpful when trying to isolate meshing problems or mesh regularity issues as discussed in the following sections. Note that messages such as shown above are only warnings, they can be ignored provided you are happy with the meshing you generate. If you get errors referring to overlapping slabs and columns (such as shown below) these will significantly affect the FE models and cannot be ignored.
Orion Documentation page 138
Chapter 8 : Analysis and Design using FE
Mesh Density
Mesh density is strongly influenced by the target Plate Element Size that is provided as input before you press the Generate Model button. Orion suggests an initial default of 800mm, and for a simple regular layout such as shown above where the slabs are placed in squares with the supporting columns at the corners this default will be very reasonable. The mesh that gets generated will also be strongly influenced by the the Mesh Uniformity Factor which is accessed from the Model menu. In the above mesh (which was generated with a mesh unifomity of 100%) 6 plate segments are produced between the column heads. We recommend that you generally aim for around 6 to 8 plate segments between column heads.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 139
Mesh Uniformity If we alter the example above so that there are still 9 panels but they are not all the same size, then meshing with the default plate element size and keeping the mesh unifomity at 100% results in the mesh shown below.
This is not an ideal mesh. In several places there are only 4 segments between columns. At this point we can start to adjust the Plate Element Size and/or the Mesh Uniformity Factor to achieve a more satisfactory solution. By retaining a uniform mesh and reducing the plate size to achieve the objective of having a minimum of 6 plates between the closely spaced columns at top right, we must reduce the plate size to 500mm as shown below. This produces a total of about 1800 plates; the top right is now OK but we are generating more plates than we really need elsewhere.
Orion Documentation page 140
Chapter 8 : Analysis and Design using FE
If we swap to the other extreme, i.e. the least uniform mesh setting, and then adjust the plate element size; a reasonable mesh is achieved when the plate size is set at 2000mm as shown below. (Although in this example the uniformity factor is over-riding the plate element size setting, so that the maximum size is not being achieved anywhere within the slab.) The total number of plates required in this case is 1678.
The objective is to find a uniformity setting that allows you to reduce the number of plates because you do not have areas with significantly more plates than are really necessary.
The mesh above is achieved with a plate size of 800mm and a uniformity of 50%. It has 866 plates rather than 1678 and is equally as good for the important bottom left corner panel.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 141
Effect of Holes and Boundaries
Holes can not be placed on a slab boundary, but in the view above some holes have been added close to a boundary. In the panel containing one hole, the hole is positioned 25 mm from the edge of the panel. In the panel containing two holes, one is 25 mm from the edge of the panel, and the two holes are only 25 mm apart.
Orion Documentation page 142
Chapter 8 : Analysis and Design using FE
If we mesh this model with a plate element size of 800mm and the default mesh uniformity factor of 25% the result is as shown above. The meshing always generates nodes (edges) along panel boundaries and is therefore trying to create very small plates to work through the small gaps (as shown below).
The meshing shown here is actually useless for all practical purposes. In such circumstances Orion will sometimes generate coarse but unusable meshes, other times it may generate a message such as shown below.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 143
Either way you need to recognise that either the meshing density/refinement needs to be changed or the model needs to change. In the view below some small but important changes have been made.
• The slabs have been redefined so that there are no edges near to grid 10, this means there are no small gaps to deal with between the holes and fictitious edges along that grid line.
• The two closely spaced holes have been idealised as one larger hole thus avoiding the problem of the small gap between them. In practice, where there are a cluster of services openings this idealisation is not only helpful from an analytical point of view, it often proves to be more economic and flexible from a construction point of view.
This revised model will mesh easily and well as shown above.
Orion Documentation page 144
Chapter 8 : Analysis and Design using FE
Mesh Sensitivity In general the more elements you use the more precise the answers will be. Considering this in conjunction with the notes in the previous sections you may conclude that if/when in doubt the best thing to do is just ask for more plates and create a finer mesh. If you really are in doubt this is not a bad rule to follow, but this is sometimes also referred to as over meshing and it has a few drawbacks to consider: • As you add more and more elements you will find that the increased meshing is having less and less effect on the results – checking sensitivity to meshing changes is standard practice in FE modelling. In essence this is only a speed problem – meshing, analysis and presentation/interpretation of results all get slower as the number of plates in a model increases.
• Stress Point Intensification – with highly refined meshes you will begin to expose local effects (very high forces theoretically applying over very short distances) that are of no practical interest in RC design work. The most common example of this is the extreme hogging moments that traditional FE analysis tends to expose over column heads in flat slab models. The principle is however equally applicable to corners adjacent to holes etc.
Reviewing Results – Contours and Strips The various contouring and strip result options are best explained in the context of worked examples. The following example covers most of the options, additional notes are included in the examples in the chapter Flat Slab Models.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 145
Worked Example – Beam and Slab Systems Introduction
In this example we will work with the reasonably simple floor layout shown above. If you want to work through the example for yourself, you can load model DOC_Example_04. In order not to destroy the example for someone else you should then save the model with a new name before proceeding. One point to notice is that slab angles that have been defined in the angled wing of this model. The wing is angled at 40 degrees and by defining this angle in the slab information we are essentially designing the angle of the orthogonal reinforcing system that will be used in this wing. As we will see later this allows us to see much more relevant contouring details.
Orion Documentation page 146
Chapter 8 : Analysis and Design using FE
FE Mesh Generation
Start with the model settings as shown above. Later in this example we will look again at the effects of adjusting the slab stiffness multiplier, in this section we will focus on meshing – ensure the Plate Element Size is set to 1000mm and the Mesh Uniformity Factor is set to 100 then click on the Floor Mesh and Analysis button.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 147
Click on the mesh button and you should see a meshed model formed of 614 plates as shown above – this is NOT a good mesh. (See previous section). The mesh generator tries to refine the mesh adjacent to holes with the result that remote panels end up quite coarsely meshed as shown above. In beam and slab systems a reasonable rule of thumb is that there should be at least 6 and preferably 8 segments generated along the lengths of the beams. (In flat slab systems a similar principle can be applied to the stretches between columns.) There are two ways in which you can change the meshing. If you want to increase the refinement locally around the holes and corners you can reduce the mesh unifomity factor, or if you want to increase the refinement generally across the whole slab you can reduce the plate element size. The the mesh unifomity factor was set to 100, if we reduce this to 25 and re-mesh the result is as shown below.
This is a big improvement, however there are still a few beams with only 5 and 6 nodes along their length. Although the total number of plates used has increased to 2488, we might decide yet more plates are required, and ideally we need a bit more mesh refinement around the openings.
Orion Documentation page 148
Chapter 8 : Analysis and Design using FE
If we keep the mesh uniformity factor at 25 but reduce the plate element size to 600mm we get the mesh shown below.
This has had the desired effect. It has to be noted again that meshing is not an exact science, if in doubt you should reduce the plate element size and accept the slightly slower performance associated with the larger model. Having created an acceptable looking model, you can now exit from the FE preprocessor and the model will be automatically analysed.
Review of Contouring Options After analysis you can select the Analysis Post Processing option and begin to review the results. You can select from a number of contour displays and we will look at each of them now.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 149
Deflection Plots
In any analysis package it is always good practice to look at the deflections – if the deflections do not make sense the results will not either. In this case the contours look very reasonable. The model has an obvious angled line of symmetry, and the deflection contours are also symmetrical. The only point that might draw some attention is the deflections of the primary edge beams. They are not big deflections, but they are more significant than the slab deflections. Mx and My Plots
Orion Documentation page 150
Chapter 8 : Analysis and Design using FE
An Mx plot is shown above. Mx and My contours are displayed relative to a single global coordinate system. Since this model has an angled line of symmetry the contour pattern is not symmetrical. If you imagine X direction bars running from left to right (horizontally) in the above view then Mx moments are the moments that these bars will need to be designed to resist (but see also the later notes on design moments including Wood and Armer adjustments). Since it is unlikely that reinforcing bars will be positioned horizontally in the angled wing, these contours are of little value for that region. M1 and M2 Plots
In the introduction to this example it was noted that a slab angle (the reinforcing angle) has been defined for each slab panel. An M1 plot is essentially an Mx plot where the contours are displayed relative to a local coordinate system for each slab panel. The local system is defined by the angle of rotation applied to each slab panel. For this model this means that we can now see a symmetrical contour plot where the contours in each wing are relevant to the orthogonal reinforcing system that will be used in each wing. Notice that in any contour plot you can use the mouse pointer to highlight any node (as shown above) and the precise information about that node is displayed at the foot of the window. Plots Including Wood and Armer adjustments Wood and Armer adjustments take plate torsional moments into account to generate adjusted design moments. It is beyond the scope of this manual to present the derivation of the equations for these adjustments. If detailed background on this is required you should refer to slab/bridge design texts, or to the original papers: • Wood, R.H. “The reinforcement of slabs in accordance with a pre-determined field of moments” as published in Concrete, 2. February 1968, pp69-76,
Chapter 8 : Analysis and Design using FE
Orion Documentation page 151
• Armer, G.S.T. “Correspondence” as published in Concrete, 2. August 1968, pp319-320. It is noted that the implementation of the adjustments within Orion is limited to orthogonal reinforcing systems. The adjustments apply differently to hogging and sagging moments, so there are 4 possible contour diagrams that can be displayed.
An Md1-bot plot is shown above. This is essentially the M1 sagging (bottom) moments adjusted to account for plate torsion. The blue zones on the plot are hogging areas, the maximum sagging moment is 5.4, which is almost identical to the M1 sagging moment (5.3) shown previously.
Orion Documentation page 152
Chapter 8 : Analysis and Design using FE
An Md1-top plot is shown below. This is the adjusted M1 hogging (top) moments. The red zones on the plot are sagging areas, the maximum hogging moment is 8.2, which is a little higher than the M1 hogging moment shown above.
Since they are based on the M1 and M2 moments these design moment plots are also symmetrical for this floor system. The degree to which Wood and Armer adjustments affect the design moments is entirely dependent on the levels of Mxy (or M12) moments – these can be regarded as twisting moments. Regular beam and slab systems do not tend to have high twisting moments coincident with peak hogging/sagging moments and Wood and Armer effects are often of little significance. In this model you can find places where the effect I more significant.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 153
You can review the Mxy or M12 moment contours as shown below.
Corners are always areas where twisting is high. In this model there is also quite a high twist developing over the beams along the angled line of symmetry. We will see how this affects the strip design moments later in this example. Steel Reinforcing Requirement Contours
2 sets of steel area requirement contours are provided: • As contours – 4 options As1-bot, As2-bot, As1-top, As2-top These steel areas are based on the M1 and M2 moments (i.e. moments that do not include the Wood and Armer adjustments).
Orion Documentation page 154
Chapter 8 : Analysis and Design using FE
• As(d) contours – 4 options As(d)1-bot, As(d)2-bot, As(d)1-top, As(d)2-top These steel areas are based on the Md1 and Md2 moments (i.e. moments that include the Wood and Armer adjustments). It is recommended that unless you have some specific reason for ignoring the Wood and Armer adjustments you should always work with the As(d) contour results. Other Contouring Adjustment Options
Before accessing the postprocessor you will have noted the option to apply global adjustment factors to all Positive (Sagging) or Negative (Hogging) moments. This is not the same as moment redistribution, but it would potentially allow for redistribution style effects to be introduced. It should be noted that FE floor models do not include any pattern loading, it is not feasible/ logical to automate pattern loading to generate every possible worst case scenario for every conceivable irregular arrangement and any size of model. A more realistic use of these adjustments may be to amplify the sagging moments (perhaps 10 to 20%) if you are concerned about allowing for load patterning. This will be discussed further later in this example. Once you are in the post processor viewing moment contours you will also have noticed a drop down option relating to Col/Wall Node Interpretation. This has little effect on beam and slab systems and is a little more relevant to flat slab design. This option is discussed in more detail in the chapter on Flat Slab Models.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 155
Slab Design The design approach, which involves creating slab strips and then designing based on tabulated code coefficients, is covered in the training manual. Discussion there also extends to the option of using an FE strip. In this section we will look at some comparisons between the two approaches and also look a little closer at the FE modelling options and how these might affect design based on an FE analysis. For this sort of regular slab it would be considered quite reasonable to use the traditional strip method outlined in the codes. The importance of setting the slab type correctly is noted again, refer back to the training manual if you have any doubts on this. When you design the strip X1 on this basis the design moments are given in the output shown below.
Orion Documentation page 156
Chapter 8 : Analysis and Design using FE
Now we can change the strip to an FE Span Strip by updating the slab strip properties – first check the FE Strip option under the General tab then choose the appropriate type of FE strip on the FE tab as shown below.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 157
Now remesh the model and then go into the FE Postprocessor to review the analysis results.
In the view above the moment diagram is determined along the actual cut line, in the table below you can see the peak moments captured from the wider area of slab as indicated on the plan view in the background. We can compare the hogging and sagging moments in the table above with those reported using the coefficient method earlier. Where we now have a hogging moment of 7.5 kNm, it was 4.6 kNm. Where we now have a sagging moment of 4.8 kNm, it was 4.5. Before continuing it is worth emphasising that these peak moments indicated above are very close to or exceed the moments determined by the empirical code approach.
Orion Documentation page 158
Chapter 8 : Analysis and Design using FE
More interestingly, the peak hogging does not occur at the first internal support, at that support the FE analysis gives a moment of 4.4 kNm,whereas a traditional empirical approach gives 5.6 kNm. Traditional idealisations will assume that beams are rigid lines of support, but this support is deflecting. The deflection diagram for the strip cut line can also be viewed as shown below.
The relative beam deflections are obvious and in a stiffness analysis this will clearly have an impact on the design shears and moments. In effect we have built-in modelling of support flexibility in an FE analysis.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 159
Effects of Adjusting the Beam and Slab Stiffnesses The empirical code method takes no specific account of support flexibility, we can attempt to emulate this by adjusting the relative stiffnesses of the beams and the slabs.
Increase the beam stiffness multiplier to 1.5 (arguably to account for the flanges), and decrease the slab stiffness to 0.01. We would not suggest that such a large adjustment be used in practice.
Orion Documentation page 160
Chapter 8 : Analysis and Design using FE
In the view below we can see deflection contours and the moments determined on the strip.
These moments are actually more in line with the moments determined by the empirical approach. The hogging moments are more regular but the peak is still a good bit higher than determined by the empirical approach, 7.0 kNm as opposed to 5.6 kNm. The sagging moments are similar but a little lower than determined by the empirical approach, 3.2 kNm as opposed to 3.4 kNm being the most extreme variation. (Ignoring the angled slab). The empirical approach is deemed to include for load patterning and moment redistribution. BS8110 also advises that where the loads are not patterned, there should be a 20% redistribution of support moments with a resulting increase in sagging moments. In the current model that would mean reducing the 7.0 kNm hogging to 5.7 kNm and increasing the 4.4 kNm sagging (end span) to about 5.0 kNm, and the 3.2 kNm sagging (internal span) to about 4.4 kNm. This gives very good agreement on the hogging moments, but the sagging moments are higher than determined by the empirical approach. Note that for reasons made much clearer in the chapter on Flat Slab Models, the simple principles of redistribution as traditionally applied to 2D frames do not make sense within the more complex analytical geometry of a 3D FE model.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 161
Returning to the previous model where the beam and slab stiffnesses might be regarded as more reasonable, we did not have the extremely high hogging moments to start with. To some degree by actually accounting for the beam stiffness we have shifted the BMD reducing the hogging moments and increasing the sagging moments. Would it be safe to reduce the hogging moments further? Should the sagging moments be amplified? It is important that we do not regard the empirical approach as the correct answer, it is one possible method that comes with documented limitations. When we analyse a complete 3D subframe model we are dealing with a good analytical model, and it probably does not make sense to start making lots of adjustments to try and make it’s results fit with those derived from a table of coefficients. Before accessing the postprocessor you do have the option to apply moment adjustment factors as shown below.
We are unaware of any authoritative texts providing analysis and design guidance relating to FE modelling and analysis. In considering the requirements to adjust the moments we are probably considering serviceability rather than safety issues. This is all discussed in greater depth in the chapter on Flat Slab Models. In that chapter we suggest that you should be very cautious about applying negative adjustment factors of less that 1, it is suggested that you might apply an adjustment to the positive (sagging) moments of up to around 1.2. Once again, we emphasise that this is not the same thing as 20% moment redistribution. If we return to the original beam and slab stiffness settings you can then review the results after introducing a positive (sagging) moment multiplier as shown above.
Orion Documentation page 162
Chapter 8 : Analysis and Design using FE
In the view below the hogging moments are unchanged, but the sagging moments are factored up by 1.2.
Orion provides the tools to make such adjustments, but in the absence of any authoritative guidance on the subject we cannot make any definitive suggestions. In this particular example the unaltered moments seem to provide a reasonable set of design forces, when the option of amplifying the sagging moment is used an even safer set is generated. If in doubt we can only suggest that you consider making this sort of adjustment.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 163
Effects of Wood and Armer Moment adjustments on a Regular Slab
Refer back to the section Plots Including Wood and Armer adjustments in this chapter for an introduction to this. Consider the same strip (X1) swap to view the Design Moments, this diagram shows the hogging and sagging moments after applying Wood and Armer Adjustments. Note that in some places both a hogging and a sagging moment is generated. In this case the 1.2 factor is still being applied to positive (sagging) moments. Comparing with the previous diagram all the forces increase slightly, but the moment at the right hand end where the support beam is angled to the line of the strip (and to the line of the reinforcement) is more significantly increased. The adjusted design moments are never (by definition) less than the unadjusted moments. Designing based on adjusted moments is optional. Comparing the above with the moments calculated based on tabulate code coefficients, you might feel even more justified in not applying an amplification factor of as much as 1.2 to the sagging moments.
Orion Documentation page 164
Chapter 8 : Analysis and Design using FE
Reinforcement Design As you exit from the FE postprocessor a dialog is displayed.
The transfer option needs to be checked in order to pass FE results back to Orion for strip design. The interpolation option is of little significance in beam and slab work. It is more important in Flat Slab Models and it is therefore discussed in the chapter Flat Slab Models. You can choose whether or not to use the design moments including Wood and Armer adjustments. Back in the main Orion graphical editor, you can select one or several strips. You can then right click and select options to run a check design on any existing bars, or delete the bars and then update the strip to design new bars. You could for example design strips based on the empirical code method, and then update the slabs to FE Strips and check the steel provided on the basis of FE results.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 165
This may in fact be a very attractive option in many cases. In this example (since minimum steel requirements dominate) the original steel works for the revised moments from FE.
Orion Documentation page 166
Chapter 8 : Analysis and Design using FE
Merging Beam Results What does this option do? This option allows you to choose which results are to be used to design beams, on a beam by beam basis you can use either:
• The beam results obtained from the building analysis. • OR, the beam analysis results based on the FE analysis. In fact, you do not have to choose, you can design all the beams using the results from the building analysis and then check them all using results from the FE analysis.
When might you use this option? Examples of when merging might be appropriate include: 1. Floor layouts containing transfer beams. A full discussion of this plus worked examples is presented in a series of chapters beginning with the Overview of Solution Options for Transfer Levels. 2. Complicated floor layouts where the traditional Yield Line approach to load decomposition is inappropriate, and/or layouts that include significant cantilever beams or slabs. Note that for this condition you also have the option of using the FE Load decomposition and then continuing to work with the general building analysis. 3. In Flat Slab models where there are occasional beams, for example a perimeter edge beam.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 167
Example To illustrate grillage effects we can continue with DOC_Example_04. In order not to destroy the example for someone else you should then save the model with a new name before proceeding.
In this example we will focus on the selected beam, 1B17. At the lower end of this beam, its reaction is transferred to the long horizontal beam (1B2). At the top end the beam frames into a wall. After running the building analysis you can review the results. Beam 1B17 is supported at one end on beam 1B2. This beam can be seen to deflect vertically, and rotationally it will act like a pin, because the torsional stiffness factor used in the analysis was left at the default of 0.01.
Orion Documentation page 168
Chapter 8 : Analysis and Design using FE
At the other end Beam 1B17 does not deflect as it is supported by a wall.
The shears and moments in this beam resulting from this analysis are shown below.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 169
In addition to the above it is also worth noting the moments in the supporting beam 1B2 as shown below.
Alternative Beam Results can be generated in FE and merged with Building Analysis Results in either of two ways: 1. Using a Plateless model. 2. Using a fully meshed model. We will look at each of these in turn in the following two examples.
Orion Documentation page 170
Chapter 8 : Analysis and Design using FE
Option 1 – A Plateless Model Run the FE Floor analysis, use the settings shown below, then click on the mesh generation option to create the model, then exit and analyse.
1. After analysis click on the option to merge beam results and merge results for all beams. 2. Now review the merged results for beam 1B17.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 171
There is very good agreement between this result and that obtained by the building analysis. This is to be expected because in the plateless model the slab does not participate in the analysis. The slab loads are transferred directly to the supporting members based on the yield line distribution method. 3. You can make similar comparisons on other beams in this model to see how the design forces hardly change. On Beam 1B2 for instance, the moments are almost identical as shown below.
Checking the Beam Designs If you had designed all the beams based on the building analysis results you should be able to see a summary table along the lines shown below.
Orion Documentation page 172
Chapter 8 : Analysis and Design using FE
Having merged all the FE results, you can now carry out a batch check design using the option not to revise the reinforcing and the utilisation ratios hardly change. The moment in beam 1B18 increases marginally so that its utilisation ratio is just greater than 1.0, sufficient to register as a fail.
For this reasons illustrated in the Merging Beam Analysis Results section in the chapter Transfer Beams – General Method, we recommend that having analysed and designed all the beams using one method (e.g. Building Analysis) you should then run a check design for merged results to find any members that fail. At this point you should adjust (increase only) the steel in those members interactively until they pass. Then you should find that you can run a check design for either set of analysis results and everything should pass. Again, it is worth noting that one of the most compelling reasons for not simply using FE analysis, merging results and then designing for the merged results is that FE analysis does not deal with patterned load, everything is fully loaded. It is very worthwhile trying to use the general building analysis and then merge and check for FE results selectively.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 173
Solution 2 – A Meshed Model
Run the FE Floor analysis, use the settings shown previously, then click on the mesh generation option to access the FE Analysis preprocessor. Mesh up the model and you should see a mesh as shown below.
Exit from the preprocessor to analyse the model. After analysis click on the option to merge beam results and merge results for all beams.
Orion Documentation page 174
Chapter 8 : Analysis and Design using FE
Now review the merged results for beam 1B17.
In this model all the loads are applied to the shells, (no applied load will be displayed on the beam loading diagram). However, the effects of the loading from both the shells and the supported beam are clear to see on the shear force diagram.
Chapter 8 : Analysis and Design using FE
Orion Documentation page 175
In this case the design moments are in reasonably close agreement with those determined by both the Plateless model and the building analysis model. It is important to note that this result is related to the use of the slab stiffness multiplier that was set to 0.15. If we reset it to 1.0 and run the analysis again, the merged results for the same beam change as shown below.
The diagrams are similar, but the maximum shear and moments are all lower. This occurs because the slab is sufficiently stiff that it starts to carry a significant proportion of the load direct to the columns. Other aspects of this effect relating to slab design were discussed earlier this chapter. In general, if you are designing beams using FE analysis results with meshed up models, then you need to be very careful that the beam design forces are what you expect them to be. This can often be achieved by setting the slab stiffnesses to be low or extremely low and then perhaps ignore slab deflection results. Checking the Beam Designs Having merged the FE results for the meshed model the beam design can be checked in the same way as described for the plateless model.
Orion Documentation page 176
Chapter 8 : Analysis and Design using FE
Merging Column Results What does this option do? This option allows you to choose which results are to be used to design columns:
• The column results obtained from the building analysis. • OR, the column analysis results based on the FE analysis. When might you use this option? Examples of when merging might be appropriate include: 1. Floor layouts containing transfer beams. A full discussion of this plus worked examples is presented in a series of chapters beginning with the Overview of Solution Options for Transfer Levels. 2. In any model where you have determined a need to merge beam results the possibility of merging column results is also worth considering. 3. In Flat Slab models. Refer to the chapter Flat Slab Models for more information on this.
Chapter 9 : Building Sway and Differential Axial Deformation Effects
Chapter 9
Orion Documentation page 177
Building Sway and Differential Axial Deformation Effects
Introduction The following section of the Engineer’s Handbook contains five chapters primarily relating to the topics of Building Sway and Differential Axial Deformation Effects:
• The Wall Modelling Considerations chapter discusses: alternative analytical idealisations of walls, how model stiffness can be adjusted in order to force walls to resist all lateral loads, and also how differential axial deformations can produce sway effects under gravity load.
• In the Sway Deflection Verification chapter a sway deflection comparison is made between Orion and other analysis sofware.
• The Overview of Bracing and Sway Sensitivity chapter clarifies the bracing and slenderness classification procedures for columns and walls for each of the design codes. The subsequent design implications are also examined.
• The Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings chapter seeks to explain this 3D analysis effect, discusses whether it can be ignored for design purposes and also looks at how it can be catered for if necessary by designing for a wider envelope of forces.
• The Using Staged Construction Analysis to Emulate ‘Traditional’ Design chapter investigates if it is possible to use staged construction analysis to deal with differential axial deformation effects.
Orion Documentation page 178
Chapter 10
Chapter 10 : Wall Modelling Considerations
Wall Modelling Considerations
Analytical Idealisations Orion supports alternative analytical idealisations of walls. This topic is discussed in detail within a training presentation that is available in PDF format alongside this document and accessed via the link below. Shear Wall Modelling Presentation.pdf (If the link does not work please browse to find the file name indicated above in the HELP sub-folder of the Orion Program Folder) This presentation is given and discussed in detail during Orion Advanced Training Days.
Deflection For deflection estimation the above presentation demonstrates that the alternative analytical idealisations yield very similar results for any given input data. However, if deflection estimates are to be regarded as meaningful they have to be based on correct input and in this regard allowances must be considered in two areas: • Material Properties – Deflections are directly proportional to the Young’s modulus value you define as input, what is the correct value to use? A short term value may be appropriate for wind loads, but perhaps a long term value should apply to notional loads?
• Allowance for cracking – gross section properties are used by default, should some adjustment be made to allow for cracking? (It is understood that many engineers will adjust (reduce) stiffnesses by up to around 50% to allow for cracking, the ACI code may be referred to for some guidance in this regard.)
Cases where one wall option is Preferable As is shown in the training presentation, the alternative wall modelling options will generally produce very compatible results. In more unusual circumstances one option might be considered preferable to another, such circumstances would include:
Very Short Walls When walls are short the meshing may need to be very refined in order to generate more than one shell along the length of the wall. In such cases the mid-pier idealisation may be preferable.
Chapter 10 : Wall Modelling Considerations
Orion Documentation page 179
Very Long Walls For very long walls the rigid arms extending from the mid-pier element are very long and might not be sufficiently rigid to develop full out of plane effects. A good example of this would be the case where out of plane beams attach to the extreme ends of a long wall (on one side only). The torsional rigidity of the beam may not allow full beam end hogging moments to develop. In such cases the meshed wall idealisation may be preferable. More importantly however is the fact that a long wall is designed as a single entity. A meshed wall will allow varying deformations and concentrations of stresses along it's length, but the results are all re-integrated to create a single set of design forces on the entire wall length. In cases such as a basement wall that support a series of differently loaded columns and walls along it's length this averaging may not be desirable. Consideration should be given to breaking such a wall into a series of co-linear design panels that are then designed seperately (exactly as an engineer would have traditionally considered strips within such walls)
Extreme Layout Variation between Floors A good example of layout variation would be the case where a wall is broken into panels around door openings at each floor level. However, the door positions are moving from one floor to the next. The mid-pier element of wall panels in the upper floor are not therefore directly above the mid-pier elements in the floor below. • From an overall building analysis point of view this is not a problem.
• Provided that the mid-pier element of the wall above sits somewhere on the rigid arms of the wall below this is not a problem.
• In the special case where the mid-pier element of the wall above sits on a coupling beam over an opening in the wall below the mid-pier model will generate some very conservative design forces in the coupling beam. (As is discussed in relation to transfer beams in the chapter Overview of Solution Options for Transfer Levels and onwards). Hence in some specific circumstances the meshed wall idealisation may be considered preferable.
Option to Check Both Ways In considering the above it is worth re-emphasising that you always have the option to analyse and design using one modelling option for walls and then at some stage reanalyse using the other option and run a check design on all members. Any failures can then be isolated and examined.
Wall Panel Design The analysis phase described above determines Wall Panel Design Forces. Each wall panel is then independently designed for these forces. Therefore, when Orion designs a wall panel it is designing a discrete rectangular section. The design phase is discussed in detail in the chapters Column Design to BS8110 and Wall Design and Detailing
Orion Documentation page 180
Chapter 10 : Wall Modelling Considerations
Forcing Walls to resist all lateral loads Orion uses the flexural stiffness of columns and core walls to resist lateral loading. This means that sway effects introduce bending in both walls and columns/frames according to relative stiffnesses. Typically walls are much stiffer and will naturally attract the vast majority of lateral load. However, design codes tend to describe a more simplistic approach to lateral loading advising that columns may be designed as braced (and hence designed for gravity loads only) if walls or other bracing systems provide full restraint. In order to satisfy this some engineers feel it is important to ensure that the walls are designed to resist 100% of the lateral loading. If desired, this may be achieved in several ways in Orion. Adjust Model Stiffnesses
In the analysis model options you can adjust the relative stiffnesses of walls/columns/beams. You can make the walls stiffer and/or you can weaken the columns and beams. Several Points should be borne in mind if such adjustments are made: • If changing the wall stiffnesses it is better to apply a factor to the E value since this will apply to both the meshed and mid-pier modelling options.
• If you only change the column stiffnesses then you will change the way in which moments develop at beam/column interfaces. It makes more sense to make relatively equal adjustments to both the beams and the columns.
• If the above adjustment is made at not too extreme a degree (e.g. reduce beams and column stiffnesses by factor of 0.5), then the analysis results are probably valid for design of all members.
• If extreme adjustments are made than it is probably safest to use these analysis results only for a cross check on wall design. Therefore the approach to design might be:
• Make no adjustments, analyse and design all members. • Make required adjustments, analyse, and check wall designs only. • Alternatively the check design might be carried out on a copy of the model as described in the section below for very much the same reasons as stated below.
Chapter 10 : Wall Modelling Considerations
Orion Documentation page 181
Pin the Columns This approach seems to be used in practice but needs to be used with more caution than the stiffness methods described above. Fictitious pins are assumed and applied at the top and bottom of every column. It is recommended that a copy of the model is made and that the pins are created in the copy. After analysis design should be restricted to the wall members only, designing beams and columns for analysis results based on the existence of assumed pins could result in serious design flaws.
Sway Effects Under Gravity Load Most models will sway to a small degree when subjected to purely vertical loads. For most models/structures this effect is likely to be quite insignificant. For some models it could be more significant. In this section of this chapter we will show that: 1. This effect relates to differential axial compression within the lateral load resisting elements of the structure, primarily shear and core walls. 2. This effect is always exposed by any analysis of a fully framed structure. 3. This effect can be exposed in Flat Slab models which are stabilised by core walls. 4. There are many reasons why this effect will generally be insignificant in terms of design.
Fully Framed Structures
Orion Documentation page 182
Chapter 10 : Wall Modelling Considerations
The structure above is a very simple 10 storey model with a single C-Shape core which is completely offset to one side of the building. For the purposes of this example only, all the beams have been pinned at both ends thus eliminating any frame action which would provide sway stiffness. The structure is therefore completely reliant on the core walls for stability. The first point to note is that this is an extreme example. The view below shows how the building sways and twists under the action of notional loads.
The twisting effect is considerable and the maximum deflections at the top floor actually exceed 1 m. A real structure cannot be acceptably stabilised by such an eccentric C-Shape core. When we consider the dead load case (where only vertical loads are applied) lateral sway is also evident as shown below.
Why does this sway happen? In the view above the vertical stress contours are also shown in the core walls – the variation of stress along the length of the flanges of the C-shape core is evident.
Chapter 10 : Wall Modelling Considerations
Orion Documentation page 183
The design forces in the wall panels at first floor level are shown below. Note that the loads in the flanges 1W1 and 1W3 are higher than in the web 1W2. The average load per meter in these shorter flanges is therefore a lot higher.
Looking at this view it is possible to imagine how the effective centre of loading applied to the C-shape core is offset to the right and above it’s centroid (which will be quite close to the centre of rigidity shown in the view above). The eccentric load causes an unresisted curvature in the core and hence sway develops at the upper floors. The same effects are evident regardless of whether meshed or mid-pier idealisations are used. Before moving on to look at a very similar model but using flat slabs it is worth noting that the total Imposed Load in the three panels of the above core wall is 626 + 229 + 403 = 1258 kN.
Orion Documentation page 184
Chapter 10 : Wall Modelling Considerations
Structures Incorporating Flat Slab Areas
As is noted above the sway effect arises because of the eccentric application of load to a core wall. The magnitude of the sway is related to the magnitude and point of application of the axial load. Slab Loads – Yield Line Decomposition If this building is analysed using yield line decomposition of slab loads then a message is displayed to indicate that the building analysis has not captured all the applied loads.
Chapter 10 : Wall Modelling Considerations
Orion Documentation page 185
At this stage the analysis results displayed on plan will show no loads in columns and some loads in walls.
The amount of load in the walls will be incorrect, it could be too high or too low, in this case the total Imposed Load in the three panels of the above core wall is 275 +139 + 276 = 690 kN – a lot less than was derived in the model where pinned beams were included. Basically, as is well documented elsewhere, this building analysis is of no value as regards the dead and imposed load cases. For a flat slab model the design cases for the dead and imposed loads would usually come from the merged results of an FE chase down.
Orion Documentation page 186
Chapter 10 : Wall Modelling Considerations
Slab Loads – FE Decomposition However, the loading applied to the walls (not the columns) can be corrected by using the FE load decomposition option (refer to the section Switching between Yield Line and FE load Decomposition in the chapter Beam Loads and Load Decomposition Methods for more information on this option). After making this change the loads in the walls change as shown below.
The total Imposed Load in the three panels of the above core wall is now 768 – 11+ 695 = 1452 kN. As a total this is not too dissimilar to the 1258 kN total given for the model that included the pinned beams, slab continuity effects in the FE analysis mean that more load is attracted to the core in this case. This load is also more eccentric with the result that tension develops in the web (for this extreme example.
Chapter 10 : Wall Modelling Considerations
Orion Documentation page 187
Once again the sway of the structure for the gravity load case is evident in the analysis postprocessor view and the variation of stress along the length of the flanges of the C-shape core is also evident.
In this example the maximum sway that is developing is 65 mm, the discussion below should put this in some context.
Discussion Load Eccentricity As noted at the outset, the model used above is an extreme example. In order to generate the most extreme offset loading within the C-shape core it is positioned along on edge of the model. In this position this single core cannot stabilise the structure effectively. The sway under notional load cases exceeds 1000 mm. This structure is not something that would be built.
Orion Documentation page 188
Chapter 10 : Wall Modelling Considerations
Where structures are stabilised by a single core, the core is normally positioned more centrally and will tend to attract more uniform loading.
If the example is adjusted as shown above so that the core attracts load from all sides then the sway under gravity load only drops from 65 to 13 mm (everything else being equal). Even so this structure still undergoes very significant sway due to notional loads and would again probably not be something that would actually be constructed. (The open C-Shape of the core is very susceptible to twist.) Construction and Creep Effects In the event that sway induced by gravity loads seems significant and some estimation of the actual displacement is required then some thought should be given to the sequence of construction and to subsequent creep effects. The sway is occurring as a result of differential axial deformation within the walls. This axial deformation is a combination of immediate deformations each time a new floor is supported and ongoing creep deformations. Both of these effects are time dependent. When a floor load is added the concrete in the walls that support the floor will be younger in the floor immediately below and progressively older at lower floors. The younger concrete will see a greater immediate deformation and a greater long term creep deformation as a result of any applied load. A detailed discussion of this is beyond the current scope of this handbook, but it is worth noting that there would be a good deal of common background with the discussion of flat slab deflections accessed via the chapter on Flat Slab Models.
Chapter 10 : Wall Modelling Considerations
Orion Documentation page 189
A very sophisticated assessment of this effect would take account of construction sequencing and time dependent effects. In such circumstances the result needs to be assessed for sensitivity to variations in the assumptions on which they are based. To be fully accurate this analysis would need to take account of the fact that some of the deflection would be constructed out as the walls are constructed as vertical as is practical during construction. In fact the result achieved by any such complex analysis will lie somewhere between two extremes that might be predicted more readily be a simple linear static analysis using different E (Young’s Modulus) values for the concrete. A short term E value might be used to estimate a minimum value for the total deflection. In the case of walls cracking effects can probably be discounted so only additional creep effects need to be considered. It is likely that the net effect of creep will be something less than a doubling of instantaneous deflection. Hence the value determined above could be doubled, or another analysis could be made with E set to half of the short term value. Note
Where flat slab models are being considered it is common practice to use long term values of E set at around ¼ of the short term value. When this has been done deflection estimates of sway arising from the building analysis are likely to be too high, perhaps by a factor of 2 based on the suggestions above.
Discrete Cores Where structures have discrete cores or numerous discrete walls providing stability the effects will often counteract each other.
In the above view the model is mirrored so that there are two C-shape cores. This model does not sway at all in the X direction under gravity load and the vertical load contours indicate relatively uniform stresses within the cores.
Orion Documentation page 190
Chapter 10 : Wall Modelling Considerations
In this case the two cores lean against each other, the diaphragms at each floor stop any curvature from developing. The axial loads in the wall panels at first floor level are much more uniform as shown below.
The total Imposed Load in the three panels of the above core wall is now 435 + 663+ 324 = 1422 kN. On a panel by panel basis these are very different forces than were given by the un-mirrored model, but the total for the 3 panels is very similar to the 1452 kN given previously. Note that the major axis moments in the wall panels are also much reduced. Results based on an FE Chase Down For most models engineers will be happy to use the panel design forces merged from an FE analysis chase down. In this case the results for this core wall would then be as shown below:
Chapter 10 : Wall Modelling Considerations
Orion Documentation page 191
Once again the distribution of forces in the 3 wall panels is different, but the total Imposed Load is very similar at 561 + 391+ 473 = 1425 kN. The FE Chase down method does not deal with sway introduced by differential axial compression in the core walls over the full height of the building. It does however introduce more minor axis bending effects where the walls interact with the slabs. For many walls supporting flat slab floors the minor axis bending can be more significant in the panel design than variations in axial load and major axis moment that may be introduced by sway effects. For this simple 10 storey example building, despite the fact that the sway effects have been forced to be unrealistically extreme, the walls only require nominal reinforcement regardless of which set of design forces their design is based on. It is suggested that for flat slab models it is important to have always designed the walls for the loads generated based on an FE chase down. You then have the option of cross checking these designs for the results generated by a building analysis where wall loading has been determined using FE decomposition at each floor.
Closing Summary In this section we have been discussing Sway Effects Under Gravity Loads, it is important to emphasise that this relates entirely to sway that is generated in the presence of purely vertical applied loads. The possibility of sway developing due to differential axial shortening is something that would have been routinely ignored in hand calculations by most engineers looking at most structures. This has nothing to do with notional loads which are applied separately and will always be designed for provided you have included them in your design combinations. For framed models in Orion the effect is always included / exposed by the 3D analysis. For flat slab models the effect will not be exposed by an FE chase down but can be exposed if desired. It is emphasised however that we recommend walls are designed for the loads from an FE chase down and then a subsequent check design can be made for building sway effects if desired. For many reasons noted within the text of this section it is anticipated that this check will only produce more critical design in tall buildings and/or quite extreme examples.
Orion Documentation page 192
Chapter 11
Chapter 11 : Sway Deflection Verification
Sway Deflection Verification
Introduction In order to produce a sway deflection comparison against other analysis software, a 20 storey building is analysed in Orion using both mid-pier and meshed idealisations of walls. (The Example 20 Storey Building described in the later chapter on Differential Axial Deformation Effects is used for this purpose.) The results for lateral load cases are then compared against other analysis packages.
Example Model:
Comparison of Orion's alternative wall modelling options The model is initially analysed in Orion using a mid-pier idealisation of the wall panels. The analysis is then repeated using the alternative meshed FE shell model idealisation.
Chapter 11 : Sway Deflection Verification
Orion Documentation page 193
Shown below is a comparison of the two sets of results.
Mid-Pier Wall Model
Meshed Wall Model
Applied dead load
39187
39187
Decomposed dead load
39187
39187
Dead load reaction
39187
39187
Applied live load
11985
11985
Decomposed live load
11985
11985
Live load reaction
11985
11985
Wind X max (mm)
101.6
100.1
-1.5%
Wind X min (mm)
101.3
99.6
-1.5%
Wind X max resultant (mm)
101.8
100.3
-1.5%
Wind Y max (mm)
79.9
76.0
-4.9%
Wind Y min (mm)
35.9
35.8
0
Wind Y max resultant (mm)
81.7
77.7
-4.9%
Difference
It can be seen from the above that both midpier and meshed FE shell wall modelling options work equally well.
Compare analysis results using other software Orion models can be exported to other analysis software such as S-Frame or SAP. The models created for this example were exported and the results for both the mid-pier and meshed idealisations of walls are compared in the tables below.
Mid-Pier Wall Model Orion
S-Frame
SAP
Dead load reaction
39187
39187
Live load reaction
11985
11985
Wind X max (mm)
101.6
101.6
101.6
Wind X min (mm)
101.3
101.4
101.3
Wind X max resultant (mm)
101.8
101.7
Wind Y max (mm)
79.9
79.9
79.9
Wind Y min (mm)
35.9
35.9
35.9
Wind Y max resultant (mm)
81.7
82.5
Orion Documentation page 194
Chapter 11 : Sway Deflection Verification
Meshed Wall Model Orion
S-Frame
SAP
ETABS*
Dead load reaction
39187
39187
Live load reaction
11985
11985
Wind X max (mm)
100.1
100.2
100.1
104.3
Wind X min (mm)
99.6
100.0
99.9
Wind X max resultant (mm)
100.3
100.3
Wind Y max (mm)
76.0
76.2
75.9
79.9
Wind Y min (mm)
35.8
35.8
35.8
35.7
Wind Y max resultant (mm)
77.7
78.4
* — For the Etabs comparison the model was constructed by a 3rd party. In this model the default wall idealisation was not used, the walls were meshed more finely. CSC do not assume responsibility for the accuracy of this model.
Chapter 12 : Overview of Bracing and Sway Sensitivity
Chapter 12
Orion Documentation page 195
Overview of Bracing and Sway Sensitivity
Introduction The purpose of this chapter is to clarify the bracing and slenderness classification procedures for columns and walls for each of the design codes. The subsequent design implications are also examined.
Automatic Assessment of Sway Sensitivity Sway sensitivity is automatically determined in Orion when the design code is set to EC2. For other codes as discussed below, an assessment of sway sensitivity can also be made, however it should be noted that this is based on analytical results and the recommendations of the ACI code. Whenever sway sensitivity is assessed automatically you are advised to be aware of the limitations that apply, these can be viewed by clicking the ‘Limitations’ button on the Building Analysis/ Reports tab.
Orion Documentation page 196
Chapter 12 : Overview of Bracing and Sway Sensitivity
User Defined Bracing In both the BS8110 code and also CP65 there are no provisions for assessing sway sensitivity by analytical methods. Therefore, when using BS8110 or CP65 you need to check the option for User Defined Bracing for Columns and Walls and then apply the condition (braced or unbraced) that you deem appropriate for the building.
If you do not check this option then Orion will make an assessment of sway sensitivity based on analytical results and the recommendations of the ACI code.
Note however that this assessment is deflection dependent and that the ACI code gives guidance on appropriate adjustments to section/material properties to be used in the building analysis for the purposes of this assessment. Such adjustments will increase the deflection value used in the checks. Note that the check can result in different classifications for different storeys which is not a condition that is recognised by BS8110. We continue to advise that this assessment is used cautiously and that when the appropriate overall condition is determined that this should be applied as a user-defined classification (in accordance with BS8110).
Chapter 12 : Overview of Bracing and Sway Sensitivity
Orion Documentation page 197
Classification Requirements of each code The classification procedures in each of the codes are as follows:
BS8110 (similarly CP65 and HK-2004) Bracing Classification — In BS8110 columns (and walls in minor axis direction) are considered as braced if lateral stability is provided (predominantly) by walls or other stiffer elements. This classification remains a matter of engineering judgement. Global P-Delta Effects — it is an inherent assumption in the above that walls provide sufficient lateral stiffness that global sway of the building is small and hence "Big" P-delta effects can be ignored in braced structures. For un-braced structures there is no clear statement on whether or not global P-Delta is also considered ignorable or is simply considered to be adequately catered for in the amplification of design moments noted below. Slenderness Classification — this is based on the effective length. In braced structures effective lengths are < 1 and in un-braced structures effective lengths are > 1. It is considerably more likely that a member gets classified as slender when it has been classified as un-braced. Short (Non-Slender) Members will see no amplification of moment at all, even if they are un-braced. Slender Members (Members susceptible to P-Delta effects) • Braced-Slender Elements - additional moments are calculated based on effective length and are considered to be a maximum at around mid height. These moments are not added to the highest end moment so this may or may not end up being a critical design condition. This additional moment is clearly intended to cater for "little" P-delta effects (strut buckling)
• UnBraced-Slender Elements - additional moments are calculated based on effective length (which is longer and hence additional moments will be greater), and are considered to be a maximum at the member ends. The additional moment is added to the highest end moment so this will always end up being a critical design condition. It is assumed that this amplification of the critical design condition is intended to cater for both big and little P-delta effects. The advantage of the above procedure is that moment amplification in each column is related only to the classification and slenderness of that column. Where columns are unbraced this is not entirely logical and cl3.8.3.8 does provide an option where the average slenderness effect for an entire unbraced storey level can be used for all members at that level. Typically this would mean that members which are un-braced but not slender add to an average stiffening effect and so the design should be less conservative. This option is not applied in Orion because the design procedure would become highly iterative (the design of every column would affect the design of every other column at an un-braced level and moment amplification would need to be introduced to non-slender members).
Orion Documentation page 198
Chapter 12 : Overview of Bracing and Sway Sensitivity
ACI 318-02 When the design code is set to BS8110, CP65 or HK-2004; if you uncheck “User Defined Bracing for Columns and Walls”, a facility is made available for assessing the susceptibility of individual storeys to P-Delta effects. This uses the ACI method of classification during the building analysis. Bracing Classification — using the ACI approach each storey level within a building is classified as sway or non-sway. The code also provides a method allowing analytical assessment of this classification based on deflections arising from a linear analysis of the structure. Global P-Delta Effects — when a storey is classified as "non-sway" then it can be assumed that global P-Delta effects are small enough to be ignored at that level. When a storey is classified as "sway" then the frame analysis results need to be amplified in some way, options given are:
• A second order analysis (which would inevitably affect all members in the structure) • Approximate moment magnification methods (cl.10.13.2 appears to indicate that this moment amplification only needs to be applied to the slender members at each floor level (similar to BS8110) is this logical? - or should this amplify the sway moments in all columns and walls on a level by level basis?) Slenderness Classification — this is based on the effective length. At "Non-sway" levels effective lengths are < 1 and at "sway" levels effective lengths are > 1. It is considerably more likely that a member gets classified as slender when it exists at a "sway" level. Short (Non-Slender) Members will see no amplification of moment at all even if they are at "Sway" levels. Slender Members (Members susceptible to P-Delta effects) • Slender Elements at Non-Sway Levels - additional moments are calculated based on effective length and are considered to be a maximum at around mid height. These moments are not added to the highest end moment so this may or may not end up being a critical design condition. In essence the approach here is identical to that used for braced slender members in BS8110.
• Elements at Sway Levels - as noted above the end moments of all members may be amplified to account for Global P-Delta effects. If a member at such a level is classified as slender, the calculation of the magnified moment is not based on the effective length of each individual member, moment magnifiers are based either on the stability index for the floor (cl.10.13.4.2) or an assessment of the average buckling capacity of all members at the floor (cl.10.13.4.3 - similar to the optional method in BS8110). The additional moment is added to the highest end moment so this will always end up being a critical design condition. Additional check (cl.10.13.5) - having amplified the end moments there is a requirement to check that intermediate slenderness effects (using effective length = 1.0L) are not more critical.
Chapter 12 : Overview of Bracing and Sway Sensitivity
Orion Documentation page 199
While the method of moment amplification is different for slender members at sway levels, the general principles of moment amplification are the same in BS8110 and ACI and the terms used for classification are interchangeable: • BS8110 Braced = ACI Non-Sway
• BS8110 Un-Braced = ACI Sway The ACI has the advantage that the classification is not a matter of engineering judgement and also that it introduces the flexibility to mix both braced and un-braced classifications within one structure. The ACI amplifications are applied only to lateral load cases - this does not address the fact that sway will occur as a result of vertical loads applied to any unsymmetrical structure and hence ignores the possibility that significant P-delta effects could accrue due to this aspect of sway. However, for the majority of "building" type structures this simplification/assumption is likely to be acceptable. There does seem to be a question mark relating to the ACI approach for slender columns. If the sway moment amplification is made using the stability index then should the column be taken into design as a braced column using an effective length = 1.0 (because the unbraced (global P-Delta) aspect of slenderness has already been allowed for?). This seems much less conservative than the suggested implementation procedure for EC2 discussed below.
EC2 In EC2 similar terminologies are used but the meanings are different: • Cl 5.8.1 - Introduces concept of braced and bracing members.
• Cl 5.8.2 - Second Order Effects - this clause distinguishes between global effects (applying to the whole structure) and isolated member effects (slenderness). Bracing Classification — Bracing members are the members which are assumed to provide the lateral stability of the structure. Columns and walls that are not “bracing members” are classified as “braced”. Unfortunately there is an element of engineering discretion involved in this classification which will be discussed later. Global P-Delta Effects — there is some guidance on determining if these effects can be ignored (For the purposes of this discussion we will classify structures in which global P-Delta effects cannot be ignored as "sway sensitive"). Cl 5.8.3.3 (1) gives a simple equation that is only applicable in limited circumstances and is actually also difficult to apply. Initial calculations using this equation have suggested that it would be too conservative resulting in too many structures being classified as sway sensitive. Annex H provides slightly more general guidance. In order to automate the Annex H classification in Orion, the approach has been modified to become similar in principal to the ACI classification method. It is noted that a single classification gets applied to the entire sway resisting structure (the bracing members). If it is determined that global P-Delta effects
Orion Documentation page 200
Chapter 12 : Overview of Bracing and Sway Sensitivity
cannot be ignored (the structure is sway sensitive) then the approach becomes a user driven procedure, in which the sway loads are amplified in accordance with Annex H. This is a relatively simple procedure applied as follows: 1. View the sway sensitivity report to obtain the suggested load amplification factors. 2. Apply this amplification to the existing load combination factors. 3. Re-analyse using the option to over-ride further sway sensitivity assessment and design the structure as if it is not sway sensitive (because the global P-Delta effects are now catered for). Tests have indicated that the sway sensitivity assessment procedure described above results in a non-sway classification for the vast majority of structures . Note Although the classification applies to the bracing members, it is impossible to isolate these when analysing the structure, so P-delta forces (introduced by load amplification or P-delta analysis) will accrue in all members (braced or bracing, short or slender). Slenderness Classification — this is based on the effective length. For braced members effective lengths are < 1 and for bracing members effective lengths are > 1. It is considerably more likely that a member gets classified as slender when it has been classified as a bracing member. Short (Non-Slender) Members: • As noted above, if these members exist in a sway sensitive frame then there may have been some amplification of the design forces introduced during the general analysis procedure.
• no other amplification of moments is then applied. Slender Members (Members susceptible to P-Delta effects) • Slender Braced Members - additional moments are calculated based on effective length and are considered to be a maximum at around mid height. These moments are not added to the highest end moment so this may or may not end up being a critical design condition. In essence the approach here is identical to that used for braced slender members in BS8110 and ACI.
• Slender Bracing Members - as in BS8110 - additional moments are calculated based on effective length (which is longer and hence additional moments will be greater). Un-like BS8110 the additional moment does not have to be added to the highest end moment (because the end moment is already amplified if the structure is sway sensitive). In EC2 additional moments in slender members are introduced in the same way regardless of whether or not the member exists in a sway sensitive frame. In summary - it seems EC2 maintains a distinction between global P-delta effects and local slenderness effects which potentially results in a 2 stage amplification of moments. Once the sway sensitivity is assessed the global P-Delta effects are introduced in the analysis results as necessary. For the local slenderness effects the general principles of moment amplification in EC2 are very similar to those applied in BS8110: • EC2 Braced = BS8110 Braced
• EC2 Bracing = BS8110 Un-Braced (but we would expect that the EC2 amplification might be lower since the BS8110 amplification at this point mixes both global and local effects whilst in EC2 any global effects would already have been introduced).
Chapter 12 : Overview of Bracing and Sway Sensitivity
Orion Documentation page 201
Implementation of EC2 Classification in Orion Setting the Braced/Bracing Members EC2 requires the user to distinguish between the braced and the bracing members of a structure. This can be specified on the Lateral Drift tab of Building Parameters. Note this setting has nothing to do with assessing sway sensitivity which is dealt with separately.
The purpose is to identify Bracing Members in each Global Direction (The member types that contribute to lateral stability of the building). The default setting is as shown above, (columns considered to be braced; walls considered to be braced about their minor axis, but to provide bracing to the structure about their major axis).
Assessment of Sway Sensitivity Most of the guidance surrounding EC2 suggests/assumes that most buildings will be classified as non-sway. Essentially the expectation is that the assumption made in BS8110 design, that any building stabilised by shear/core walls is non-sway, will prove to be correct. Whether this proves to be true is somewhat irrelevant, the fact is that sway sensitivity classification has to be made and the Eurocode provides three options for doing this: 1. Use cl 5.8.3.3 (eq5.18) 2. Use guidance from Annex H 3. Do a P-Delta analysis and check that the change in results is less than 10% (cl 5.8.2), if true than you can revert to linear elastic analysis. Note
The Annex H guidance has been adopted for the first Eurocode release of Orion.
Orion Documentation page 202
Chapter 12 : Overview of Bracing and Sway Sensitivity
If the structure is classified as sway sensitive then there are two options for dealing with this: 1. Annex H - Application of increased horizontal forces. 2. Do a P-Delta Analysis In fact there is a third option which might be applied when an engineer discovers a building is sway-sensitive - they may find a way to add more shear walls and change the classification! Initially the P-delta option may seem attractive but it must be recognised that EC2 is very clear on the fact that realistic member properties accounting for creep and cracking must be used and the calculation of these properties becomes a unique procedure for every member. Note
For sway sensitive structures, the Annex H guidance has been adopted for the first Eurocode version of Orion.
Worked Example for a Sway Sensitive EC2 Structure
A model is constructed as shown above with two 3m wall panels providing stability in each direction. Floor to floor ht= 3.0 m Wall Length / Width= 3m / 0.2m Concrete Grade= C30/37 G= 7 kN/m2 (total including walls) Q= 2.5 kN/m2 Beams are provided for load collection only - they are pinned at both ends in order that lateral loads are focussed in the shear walls.
Chapter 12 : Overview of Bracing and Sway Sensitivity
Orion Documentation page 203
Model Analysis Properties The notes with eq H.8 indicate that cl 5.8.7.2 should be referred to - the stiffness of the members used in the analysis leading to the classification must be adjusted and Cl 5.8.7.2 is referred to for the adjustment. Cl 5.8.7.2 gives a procedure for calculation of Nominal Stiffness of compression members. Rigorous use of this clause would require iteration since the adjusted properties are member specific (load and reinforcement and even direction dependent). Simplified alternatives are given, the simplest of which still involves the use of theta-ef (the "Effective Creep Ratio") which remains a member specific calculation. Note
For the first Orion Eurocode version these adjustments are left entirely to the user.
Referring to eq 5.26, if we assume theta-ef is around 1.5 then the suggested approximate stiffness adjustment can be calculated: • Kc = 0.3 / (1 + 0.5*theta-ef) = approx 0.175 For the beams adjustments must be made to allow for creep and cracking - assume: • I-cracked = 0.5 I-conc
• (eqn5.25) Ecd-eff = Ecd / (1 + theta-ef) = Ecd / 2.5 • Therefore total adjustment to EI = 0.5/2.5 = 0.2. Overall it seems that initial adjustments might be as low as 0.15 to 0.2 EI for all members. To put this in perspective consider the slightly more concise advice given in the ACI. ACI suggests reducing stiffness (EI) by a factor of 0.35 (or 0.7 if the members can be shown to be uncracked). It is also noted that the 0.35 factor should be further reduced if sustained lateral loads are applied, it seems logical that notional loads should be regarded as sustained lateral loads. Therefore, a 0.2 adjustment factor may prove to be a little over conservative, but it is not wildly different to the ACI advice. Consider also that ACI classifies a building as sway sensitive when Q > 5% while EC2 allows this to increase to 10% - therefore, if the EC2 adjustment factor is around 0.175 compared to ACI factor of 0.35, then the classifications of the buildings would be almost identical. ACI Classification (for comparison)
Member Properties are adjusted before analysis as shown above. Note that the recommended column and beam adjustment is 0.7 and for cracked walls it is 0.35.
Orion Documentation page 204
Chapter 12 : Overview of Bracing and Sway Sensitivity
The report shows the structure is classified as sway-sensitive at all but the lowest floor level.
In the ACI only 5% second order effects are assumed to be ignorable. Q is the measure of this and at this point it is interesting to note that although Q is only marginally smaller than 0.05 at the lowest level, it becomes quite significantly greater at the top level. In fact, if we reduce this to a 4 storey building then the report below shows that the structure is still classified as sway-sensitive at the upper levels.
Note
As shown above, P-Delta effects can be proportionally higher at upper levels .
Chapter 12 : Overview of Bracing and Sway Sensitivity
Orion Documentation page 205
EC2 Classification to Annex H
Based on the discussion in Model Analysis Properties, member properties are adjusted before analysis as shown above. Note that although we are using 0.17, you may decide on a higher or lower value based on your engineering judgement. The report shows that the 5 storey structure is classified as sway-sensitive at all floor levels.
In EC2 10% second order effects are assumed to be ignorable. Q is the measure of this and so the actual check is that if Q > 0.1 then the classification is sway-sensitive. For the figures above we can see this is true at all levels. It is noted that although Q is only marginally greater than 0.1 at the lowest level, it becomes quite significantly greater at the top level.
Orion Documentation page 206
Chapter 12 : Overview of Bracing and Sway Sensitivity
In fact, if we reduce this to a 4 storey building then the report below shows that although Q becomes less that 0.1 at the lowest level, the structure is still classified as sway-sensitive at the upper levels.
Although the reduced section properties together with the increased ignorable P-Delta amplification limit means that the threshold for sway-sensitive/non-sway classification is very similar for the two codes, the amplification factors that apply to buildings that are classed sway sensitive are bigger (double) for EC2. EC2 does not seem to recognise the concept that a building can have different sway sensitivity at different levels, a single classification and amplification factor is applied to the whole building. This requirement is catered for in the report by including an extra line for ‘All‘ storeys. In the above 4 storey example the Q value calculated for “All” storeys is 0.1497 (therefore sway sensitive). Total deflection = 5.99mm Total Axial Load (F-V.Ed)= 30349 Total Shear Load (F-H.Ed)= 101.2 Total height = 12m Q = 1.5 (30349 * 0.00599) / (101.2 *12) = 0.1497 > 0.1 (therefore sway sensitive).
Chapter 12 : Overview of Bracing and Sway Sensitivity
Orion Documentation page 207
Application of Load Amplification Factors Provided the model is classified as non-sway no further adjustments are required - the member design is performed using the existing load combinations and factors. If (as in this case) the model is classified as sway-sensitive, the second-order effects must be accounted for in the design. As previously stated, the code provides two options for achieving this:
• Annex H - Application of increased horizontal forces. • Do a P-Delta Analysis In the current version of Orion the former approach is adopted - when the model is classified as sway-sensitive a load amplification factor is automatically applied to the existing design load combinations. The amplification factor, Delta-s, is calculated from the Q value for “All” storeys as follows: Delta-s = FH,Ed / FH,0Ed = 1 / (1-Q) In the original 5 storey example Q = 0.271. Hence the amplification factor displayed on the Horizontal Drift Classification Report is 1/(1-0.271) = 1.372. It is possible to over-ride this value if required and enter an amplification factor based on your own engineering judgement. To do this, re-display the Building Parameters, then from the Lateral Drift tab check the box to apply the ‘User-defined’ Sway Amplification Coefficient. You can then over-ride the automatically calculated value in one or both directions.
If you have applied user-defined sway amplification co-efficients, it is not necessary to re-analyse the building before the members are designed.
Orion Documentation page 208
Chapter 13
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Introduction Differential axial deformation will occur in the 3D analysis of any typical building frame. This deformation can significantly affect beam bending moments. For the purposes of discussion the effect is referred to as a "Differential Axial Deformation Effect" (DADE). During any analysis members subject to compression will shorten. In a concrete frame columns and walls under compression will both shorten. Highly stressed members will shorten by greater amounts than lower stressed members. Where two adjacent members shorten by a different amount there will be an effect (DADE) introduced in any members that attach to both the shortening members. Example:
Central column shortening by more than external columns introducing bending effects in connecting beams. CSC have previously sought to raise engineers awareness of this and other 3D analysis effects by illustrating them in seminars held around the world. This particular effect is known and studied by academia, one reference that provides a good overview is: Effects of Axial Shortening of Columns on Design and Construction of Tall Reinforced Concrete Buildings By M.T.R. Jayasinghe and W.M.V.P.K. Jayasena Available to download from the ASCE Research Library (http://www.ascelibrary.org)
Focusing on DADE, the key points of the discussion in these notes are: • Traditional methods of idealising sub-sections of structures for design purposes inherently completely ignore this effect. However, the existing building stock which is largely designed in this way appears serviceable, it seems that ignoring DADE has not caused any significant problems.
• An analysis of a complete 3D model of a structure will inherently introduce this effect. • Designers with experience in the traditional method look at the analysis results of a full 3D analysis and will quite rightly question the design forces that are displayed.
• There is a demand to use 3D analysis but at the same time find a way to achieve member design forces that are reasonably close to the forces that would always have been used. These notes demonstrate methods of achieving this.
• Checks are required to establish whether the method used has had an impact on sway deflections. If it does not then potentially designers can stop at this point on the basis that the traditional design approach has been reasonably emulated.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 209
• Potentially engineers will also want to consider that DADE should not be completely ignored. These notes show how a second analysis run could be made so that the initial design can be checked and strengthened as necessary. In essence this means that the structure ends up being designed for a wider envelope of design conditions (forces), clearly this must be a conservative approach.
• Concerns have been expressed that designing for this envelope could be uneconomic. For an example model it is shown that the additional reinforcement associated with this conservative approach is quite nominal. It is important to re-emphasise that although this discussion is in relation to Orion, the effect under discussion will occur in any analysis software. Equally, most of the suggested strategies for dealing with this effect could be applied in other software. Finally, the possibility that staged construction analysis will somehow easily deal with this issue is often raised. This point is covered in the closing discussion the key points being that: • Staged Construction is not a simple analysis technique.
• It does not generate results that emulate traditional design (i.e. DADE is not completely eliminated)
• If it is not used with a reasonable degree of care and understanding then a staged construction analysis can be a sophisticated and time consuming way of getting dubious answers.
Opening Discussion In these notes we will establish some traditional sub-frame results for beams at the 20th floor of a building and then look at how closely these results can be emulated using full 3D analysis. Several general discussion topics should be borne in mind as the results are compared. Traditional practice for the analysis and design of multi-storey concrete frames has involved a significant degree of idealisation of the structure: • Lateral load is often assumed to be exclusively resisted by a selected subset of members:
• Special models are constructed to determine the member forces which develop when lateral loads are applied.
• It is quite normal to ignore the sway that can be introduced when pure gravity load is applied to a non-symmetrical structure.
• For the remaining structure only gravity forces are considered: • Column and beam design is based on "sub-frame" analysis. • Typically sub-frame support points are assumed to provide rigid vertical support, the possibility of elastic deformation of the support is ignored.
• The result is that designs would be the same for the beams on the 20th floor as on the 1st floor.
Orion Documentation page 210
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
We present the results from the traditional analysis as a target "baseline" result. This should not be confused with being a "correct" result. A few thoughts on this: • A Quote: "Engineering (and some may think FE practice also) is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse, so as to withstand forces we cannot properly assess, this in such a way the public and (hopefully) the customer has no reason to suspect the extent of our ignorance." Structural Engineering Modelling and Analysis, by Arthur T. Murphy The Structural Engineer - 3rd Feb. 2004
• If a structure were loaded to full design load at all levels and the actual forces that developed in the members were measured what degree of variation with the forces assumed at design stage would be found to exist?
• If two engineers were independently given the same building frame and design brief would they both end up designing every element for exactly the same forces? Basically Engineers have traditionally idealised a very complex problem, applied safety factors, and established design forces which have been used successfully for many years. This is a good result, but should not be regarded as some sort of scientifically accurate result. Now there is a trend towards full 3 dimensional modelling and analysis of buildings. Creating a single analysis model for an entire building will inevitably have the following effects: • Lateral load is shared throughout the structure according to relative stiffness:
• Where robust core walls and shear walls exist these provide the vast majority of the resistance to these loads.
• The sway that occurs when pure gravity load is applied to any non-symmetrical structure will naturally be introduced.
• For the structure which predominantly resists gravity forces: • Elastic shortening of compression members (columns and walls) will occur. • As a result the analysis results for beams on the 20th floor will not be the same as those on the 1st floor. It is worth taking note of all of the above differences before we focus more specifically on sub-frame modelling and the inclusion/exclusion of DADE. The approach used is aimed at largely eliminating DADE, it does not eliminate natural sway effects. It must be acknowledged that sway effects will cause some unavoidable differences in the comparisons. There is no intention to suggest that the new 3D analysis model now creates scientifically accurate results, a great many simplifications, assumptions, etc. remain. The overall objective of the above discussion is to encourage consideration of the comparisons in these notes from an engineering standpoint.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 211
Example 20 Storey Building Example Model:
Storey Height Fdn to G/F
0.5m
G/F to 1/F
4.0m
1/F to Roof
3.05m per storey
Wall Thickness (mm) Fdn to 10/F
10/F to Roof
Shear Wall
300
225
Core Wall
350
250
Orion Documentation page 212
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Column Size (mm) Fdn to 10/F
10/F to Roof
C1
750 x 1250 (at Fdn to 1/F only)
C2
750 x 1250 (at Fdn to 1/F only)
C3
500 x 1500
400 x 1500
C4
500 x 1500
400 x 1500
C5
500 x 1500
400 x 1500
C6
500 x 1500
400 x 1500
Fdn to 10/F
10/F to Roof
B1 & B2
400 x750
300 x500
B3 & B4
400 x750
400 x500
B5 & B6
450 x700
300 x 450
B7, B8 & B9
350 x650
250 x 500
Beam Size (mm)
Slab Thickness 125mm
Height of Openings All openings to be 2300mm from floor level including the opening at G/F between columns C1 and C2. In a separate exercise it has been demonstrated that sway results for this model are in excellent agreement with results achieved using other software. Refer to the next chapter, Sway Deflection Verification for details.
Traditional Analysis Results As already discussed a 'traditional' design will historically have been based on the results of a series of discrete 2D subframe analyses: • Sub-frame analyses are carried out at each floor from top down (assuming fixed supports)
• Deflection compatibility between sub-models is ignored.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 213
• Column and wall forces are established by summing up the axial loads determined in each of the separate analyses. Or, in many cases the axial loads are determined by simply calculating an assumed supported floor area at each floor.
Both 2D and 3D subframe models can be created and analysed in Orion if required. To emulate the result that would be achieved by subframe analyses in both directions the top storey is extracted and analysed as a single floor model.
Orion Documentation page 214
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
The analysis results for this 3D subframe are shown below (mid-pier wall idealisation has been used).
To enable comparisons to be made with other approaches, the bending moment diagram can be plotted for the highlighted beams.
We shall consider this our "baseline" answer against which other approaches can be benchmarked. Once again it is important to note the comments made in the opening discussion about the assumptions inherent in the above result. It is a target baseline, it should not be regarded as some sort of perfectly correct result.
Emulating the Traditional Approach in Orion Orion has two basic analysis methods: • Sub-Floor Analysis Referred to in Orion as FE Chasedown analysis.
• Building Analysis Analysis of a full 3D model Since Orion's "FE Chasedown" analysis uses a sub-frame approach, we should expect to find it agrees closely with the "baseline" answer.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 215
Orion's 'building analysis' does not utilise sub-frames, instead it treats the entire structure as a single 3D frame in which the columns and walls are only fixed at the foundation level. We will now consider each of the above analysis methods in turn by comparing them against our "baseline" answer.
Sub-Floor Analysis (FE Chasedown) By choosing the option in Orion to perform an 'FE chasedown analysis', a batch process is initiated in which a separate analysis of all floors is performed one after the other, starting at the top of the building, and continuing to the ground level. For each analysis the calculated reactions from the floor above are treated as load input for the current floor. When this is run, the bending moment diagram for the comparison frame at the top storey is as follows:
This appears to be in very reasonable agreement with the baseline analysis result shown below.
Although the results are not exactly identical, the differences are generally small and are likely to be down to internal differences in the way FE beam and wall elements are assumed to connect. There is however a limitation to this approach - whilst in building analysis pattern load cases can be automatically derived and included, they are not considered in the FE chasedown analysis. Consideration of pattern loads would increase the peak design forces but two other factors could be taken into account: • Support Moments are taken at the centreline of supporting columns. Introducing rigid zones to the analysis and taking design moments at the face of supports would result in changes to design moments which are predominantly reductions where beams frame into columns.
• Moment redistribution is not considered.
Orion Documentation page 216
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Some engineers may wish to consider using the FE Chasedown results on the basis that the factors above have some counterbalancing effect. However, other engineers will prefer to know that pattern load has been considered and for this reason we tend to suggest that the area factor adjustment method which is illustrated in the next section may be considered preferable.
Building Analysis, area factor adjustment methods By choosing the option in Orion to perform a 'building analysis', a single frame model of the entire structure is created and analysed for all load cases and combinations. Prior to analysis, global adjustments can be made to member groups used in the analysis model, the default settings for these are as shown below:
If the analysis is run using the above default settings, the bending moment diagram for the comparison frame at the top storey is as follows:
There appears to be an anomaly, an unexpected sagging moment is occurring at the beam support.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 217
To gain a better understanding of why this is happening we need to think about the 3D nature of the analysis model. To begin with let's display the above diagram in 3D along with the other beams at the top floor:
Bending moments at the top floor of the building analysis model:
This result appears to disagree with our "baseline" answer obtained from the analysis of the top floor as a discrete model, which is shown below:
Bending moments in the 'control' model:
Orion Documentation page 218
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
This raises two immediate questions: • Why do the two models give such different answers?
• If we were to accept the results for the baseline model as being our target result, is there any way to adjust the building analysis model to bring it back in to line with this target? Why do the two models give such different answers? To resolve this we need to compare the two deflection diagrams.
Deflections under dead load for the baseline model:
Deflections under dead load for the top floor of the building analysis model:
In the baseline model the vertical deflections at the beam support points have been eliminated. This is not the case in the building analysis model - vertical deflections occur at the beam support points. Since stresses are greater in the columns, these deflections are greater at column support locations than at the wall support locations. The differential in these deflections is most obvious along the line of the frame on which the 'anomaly' in the bending moments was detected. (shown in dashed lines above.)
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 219
The differential tends to increase as you get higher in the building. Evidence for this can be seen in the change of shape of the bending moment diagram along the line of the highlighted frame when plotted for the entire building.
There is an obvious change in the bending moment between the ground and the 9th floor. There is another change at the 10th floor but this is due to a change in beam sizes. It is perhaps interesting to see that the effect is so noticeable over the first 9 floors. This is partly because the case of a beam spanning from a wall to a column and back to a wall is typically the most critical sort of location for exposing this effect. Differential axial deformations account for the different answers obtained from the two models. Traditional sub-frame analysis ignores this effect. Sub-frame analyses assume there is no vertical deflection at supports, whether it is appropriate to ignore this will be discussed later in these notes. For the time being we will assume that you want to ignore it, in which case we now need to determine if it is possible to eliminate it. How do we eliminate this effect if we want to? We have already seen that this effect is eliminated in Orion's FE chasedown analysis (because it is a sub-frame analysis).
Orion Documentation page 220
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
In order to eliminate it from Orion's building analysis, we need to reconsider the default stiffness settings applied to member groups in the analysis model.
There are probably several strategies of adjusting member properties that could be employed to try and reduce / eliminate DADE. 1. Make column vertical deflections consistent with the wall deflections by increasing the area factor applied to columns. 2. Eliminate column and wall vertical deflections completely by substantially increasing the area factor applied to columns and walls. 3. Adjust beam stiffnesses so that the differential vertical deflections induce much lower bending moments. 4. etc? After various trials it has been found that the first method (column area factor adjustment) seems effective without readily introducing unwanted side effects. Therefore, these notes now focus on this method.
Increasing the column area factor The objective is to increase the column area factor so that the differential axial deformation between columns and walls is reduced towards the point of being eliminated. By examining the deflected shape of the frame with the original area factor (AF) of 1.0, it is possible to estimate an average area factor which may achieve this.
Vertical Deflections under dead load at the top floor for AF = 1.0
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 221
If the model is re-analysed with the column AF 2.2 as calculated above, the revised deflected shape indicates a marked reduction in the differential axial shortening.
Vertical Deflections under dead load at the top floor for AF = 2.2
The above deflection diagram for AF 2.2 shows that some differential deformation still remains. This is explained by the fact the stiffer columns also attract more load (increasing it towards the value that the sub-frame analyses would determine). A second iteration of the AF calculation should reduce it. LH AF = 4.4/4.9 = 0.9, RH AF = 4.4/2.9 = 1.5 Average AF = (0.9+ 1.5)/2 = 1.2 Applying a further factor of 1.2 to the existing 2.2 the new AF becomes 1.2x2.2 = 2.65 - use 2.7. If the model is analysed with the column AF 2.7 the differential axial shortening is almost eliminated.
Vertical Deflections under dead load at the top floor for AF = 2.7
Results for these analyses can again be compared with the result from the baseline model.
Orion Documentation page 222
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
With the AF of 2.2 the BM's developed in beams B1 and B2 at the top floor are as follows:
When the AF is increased to 2.7 these become:
These can be compared with our baseline sub-frame result:
Applying a column AF of 2.2 reduces the effects of differential axial shortening and eliminates the sagging at the central column support. When the AF is increased to 2.7 the result is even closer to that of our baseline subframe. Using this slightly higher AF value of 2.7 ensures a conservative design at the column support. The same value would not be applicable. Arriving at this value is a matter of some judgement but the above demonstrates that a logical process can be applied. Whether the AF should be increased still higher would be a matter of engineering judgement. For example if the AF is increased to 4.0, the resulting bending moments are as follows:
With this higher area factor the hogging moment at the column increases further, however it is reduced at the wall supports.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 223
NOTE: An area factor of 4.0 is likely to be extreme for most buildings. In this example it has reversed the differential deformations - the walls are now shortening by more than the columns, as shown below:
Vertical Deflections under dead load at the top floor for AF = 4.0
What is a reasonable upper limit for the column area factor adjustment? The objective in setting an upper limit is to reduce or largely eliminate the differential axial deformation so that the design forces at each floor more closely resemble those obtained from a traditional approach. Although differential axial shortening is not taken into account in traditional subframe analysis methods, it will still exist to a greater or lesser extent in real buildings. Even taking into account the staged nature of construction there will always be some amount of differential shortening that actually does occur. The objective of setting the upper limit is therefore to obtain a realistic set of design forces, without necessarily being concerned about completely eliminating the effects of differential axial shortening.
Detailed comparisons of the analysis results The above approaches demonstrate that DADE can be at largely eliminated from the analysis results in attempts to emulate the results achieved by a traditional sub-frame approach. To establish a better understanding of the effects of adjusting the area factor we will now compare other aspects of the analysis results in more detail.
Gravity Loads When the area factor is used to counteract axial shortening, it affects the axial force distribution as well as the bending moments. This is illustrated in the table below, showing how the dead load results change at the base of the frame containing beams B1 and B2 as the area factor is increased: The axial and bending moment results which accrue at the bottom level of the walls and column along the frame under consideration can be tabulated as follows:
Orion Documentation page 224
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
AF 1.0
AF 2.2
AF 2.7
AF 4.0
FE chasedown (i.e. subframe)
Left Hand Wall Axial (kN)
-3219
-3091
-3065
-3025
-3042
Column Axial (kN)
-3233
-3954
-4103
-4342
-4112
Right Hand Wall Axial (kN)
-3044
-2862
-2828
-2776
-2889
Left Hand Wall Moment top (kNm)
-453
-439
-436
-432
-198
Column Moment btm (kNm)
-0.6
-1.6
-1.8
-2.2
7
Right Hand Wall Moment
-711
-555
-525
-479
7
btm (kNm) In this case the FE Chasedown results represent the result for the baseline model. As the area factor is increased there is a shift of axial load from the walls into the columns. This is to be expected. It is moving the results back towards those obtained for the baseline model. For the bending moments however it seems that this method results in much higher bending moments in the wall panels than the baseline model. This moment occurs for all AF levels, it is not introduced by the use of an increased AF. It is likely that this moment is developing as a result of sway effects that will occur in the full 3D analysis but are excluded from traditional sub-frame analysis.
Lateral Loads The deflections at the top of the building under lateral load can also be compared:
AF 1.0
AF 2.2
AF 2.7
AF 4.0
FE chasedown (i.e. subframe)
Wind X max (mm)
101.6
99.0
98.4
97.5
N/A
Wind X min (mm)
101.3
98.3
97.7
96.7
N/A
Wind X max resultant (mm)
101.8
99.2
98.4
97.7
N/A
Wind Y max (mm)
79.9
78.4
78.3
78.0
N/A
Wind Y min (mm)
35.9
35.0
34.8
34.5
N/A
Wind Y max resultant (mm)
81.7
80.4
80.3
80.1
N/A
It can be seen from the above that an increase in the area factor from 1.0 to the value of 2.7 or even the extreme value of 4.0 has very little impact on the deflections at the top of the building due to lateral load. This is because the columns are not significant in resisting sway deflection. If the deflections were changing then some increased caution in the use of the AF would be appropriate, this is noted again later.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 225
The axial and bending forces in the columns and walls at the bottom of the model can also be compared for the wind load cases. The effect that increasing the area factor has on these forces due to wind in X is illustrated below:
AF 1.0
AF 2.2
AF 2.7
AF 4.0
FE chasedown (i.e. subframe)
Left Hand Wall Axial (kN)
1113
965
937
895
N/A
Column Axial (kN)
1824
2160
2223
2321
N/A
Right Hand Wall Axial (kN)
-153
-225
-239
-261
N/A
Left Hand Wall Moment top (kNm)
681
652
651
648
N/A
Column Moment btm (kNm)
124
121
121
120
N/A
9572
9390
9356
9305
N/A
Right Hand Wall Moment btm (kNm)
Since the introduction of the AF had only a small effect on the deflection (comparing with AF=1.0), it has a similarly small impact on the distribution of forces.
Recommendation Based on the above it is recommended that where DADE is of concern and there is a desire to largely eliminate it, the strategy adopted for applying an AF adjustment should be: 1. After an initial analysis using AF = 1, examine the beam bending moments and the vertical deflections of columns and walls at the top floor of the structure. Also record sway deflections in both directions. 2. Using the approach demonstrated in this chapter, determine an appropriate AF adjustment to apply. 3. Re run the analysis and review the revised results at the top floor. If the DADE seems to have been largely eliminated continue with design of all column and wall members. 4. If the deflections are largely unchanged then you might choose to stop at this point, however, the discussion below should be considered. 5. If the deflections have changed significantly then it is strongly recommended that a second analysis run is made with a lower AF and that the columns and walls (at least) are checked for the results of this revised analysis. Within Orion this is a simple procedure and the implications of this are illustrated in the next section.
Orion Documentation page 226
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Is it acceptable to simply emulate the traditional design result? Ignoring differential axial displacements If you are happy to work on this basis then all you need to do is adopt one of the methods described previously for emulating the traditional design, taking note of the associated pros and cons.
FE Chasedown Analysis Within Orion there is an automatic batch procedure for achieving this. Despite the name "FE chasedown", the use of Finite Elements to model the slab is optional and unless there is a specific reason for doing so it is recommended that these are not used. This method will then very closely emulate the traditional analysis assumptions of sub-frame analysis for gravity loads. A building analysis is still required to determine results for the lateral load cases. Orion allows you to automatically merge the results of these two analyses to generate the design combinations for all members. Cons - No pattern loading and no rigid zones.
Building Analysis - Area Factor (AF) method Pros - Pattern loading is considered and rigid zones can also be introduced for economy. Cons - Applying a single area factor globally may not fully remove differential axial deformation at all locations.
Making allowances for differential axial displacements The inherent assumption within the traditional sub-frame approach is that differential axial displacement does not take place to a degree that would affect the design results. What a 3D analysis demonstrates is that this might not be the case. Therefore, having analysed the structure on the basis that differential axial displacements are ignored (using one of the methods described above), you have results that can be considered as an upper-bound solution. The next step is therefore to establish a lower-bound for which each member can then be checked. In effect this procedure will ensure that all the members are designed for a slightly wider envelope of forces between these two limits. Two questions tend to be raised when this approach is discussed. First there is the question of what values should be used for the upper and lower limits. Second there is a concern that this could end up being considered as uneconomic by comparison with traditional designs. Each of these questions is considered below.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 227
What is a reasonable lower limit for the area factor adjustment? Using a low AF allows the effects of differential axial deformation to be exposed. However, as is demonstrated earlier in these notes, an AF=1.0 will sometimes give results that seem completely incorrect when compared with expected values based on traditional sub-frame analysis. Clearly AF=1.0 can be used as a conservative lower limit. However, bearing in mind that traditional design methods have completely eliminated this effect, it is worth considering whether or not some slightly increased value could be easily justified. This topic is discussed in reasonable depth in CSC's 3D effects presentation which notes that: • More detailed consideration of the way in which creep and loading develop over time and through the construction phase dictates that some proportion of differential axial deformation being predicted is unrealistic. AF=1.0 will over estimate the effect of differential axial deformation.
• Consideration of ACI advice on analysis model properties which allow for the more likely case that beams are cracked while columns/walls are not would have the same effect as introducing an AF = 1.5. (ACI 318-08 cl 10.10.4.1 the precise clause number varies in older versions of the ACI, but the guidance appears consistent.) It was suggested in that presentation that a lower bound of somewhere between 1.5 and 2.0 is reasonable. 1.5 would be the more conservative value to use (since it widens the envelope between the lower and upper values).
What is the impact on the design when both upper and lower-bounds are taken into consideration? To establish this a comparative study can be undertaken using the test model. The total weight of beam, column and wall steel can be determined using one of the methods for emulating the traditional approach. (i.e. the upper-bound solution). The model can then be reanalysed, having made an allowance for differential axial displacements, (i.e. the lower-bound solution). The reinforcement provision already established is then checked for the new design forces and if necessary the provision is increased at any failing locations. The overall impact can be assessed in terms of the changes to the total steel reinforcement weight. The results of this exercise are as follows:
Upper Bound Emulation Method AF 2.2
AF 2.7
Design for Upper-bound Only
Add check for Lower-bound AF 1.5
Add check for Lower-bound AF 1.0
Total Beam Steel (kg)
44980
45334 (+0.8%)
Total Column & Wall Steel (kg)
80185
80211 (+0.03%)
80712 (+0.7%)
Total Beam Steel (kg)
44907
45273 (+0.8%)
46438 (+3.3%)
Total Column & Wall Steel (kg)
80192
80261 (+0.09%)
80762 (+0.7%)
Orion Documentation page 228
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Upper Bound Emulation Method AF 4.0
FE Chasedown
Design for Upper-bound Only
Add check for Lower-bound AF 1.5
Add check for Lower-bound AF 1.0
Total Beam Steel (kg)
45774
46528 (+1.6%)
47781 (+4.4%)
Total Column & Wall Steel (kg)
80187
80344 (+0.2%)
80845 (+0.8%)
Total Beam Steel (kg)
N/A
43847
45658 (+4.1%)
Total Column & Wall Steel (kg)
N/A
80545
82180 (+2.0%)
Note: the total beam reinforcement weight quoted doesn't include the weight of 2B10, 3B14 and 3B15 as these proved to be inadequate for the forces involved. Consider Case where AF = 2.7 is upper bound and AF = 1.5 is the lower bound: In this case the implications of considering the envelope are quite minimal. A 0.8% increase in beam reinforcement and a 0.01% increase in column and wall reinforcement is introduced. Consider Case where AF = 4.0 is upper bound and AF = 1.0 is the lower bound: In this case the envelope being considered includes extremes that have been suggested to be the most conservative at both ends. The result is that a 4.4% increase in beam reinforcement and a 0.8% increase in column and wall reinforcement is introduced. Consider Case where FE Chasedown is upper bound and AF = 1.5 is the lower bound: FE chasedown only provides results for the gravity loadcase, lateral load results must come from the full 3D building analysis. In this case the least overall reinforcement weight is determined. This is likely to be because sway effects introduced by gravity load get eliminated (as they are in a traditional sub-frame analysis).
What is the impact on the design when Pattern Loading is Introduced? In order to compare all the methods on a like for like basis, the reinforcement weight comparisons above did not include analysis or design for pattern load conditions. It was noted earlier that the FE Chasedown method in Orion does not currently cater for pattern loads. Pattern loads can only be considered by the AF method where the full 3D building analysis is used. The full 3D analysis also allows rigid zones to be introduced in which case design moments are taken at the face rather than the centreline of supports. Therefore, we can look at the overall impact of considering pattern loads but at the same time introducing the efficiency of rigid zones.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 229
The reinforcement weights when both pattern load and rigid zones are considered are as follows: ..
Upper Bound Emulation Method AF 2.7
AF 4.0
Design for Upper-bound Only
Add check for Lower-bound AF 1.5
Add check for Lower-bound AF 1.0
Total Beam Steel (kg)
40437
40964 (+0.8%)
42295 (+3.3%)
Total Column & Wall Steel (kg)
78778
78891 (+0.1%)
79517 (+0.9%)
Total Beam Steel (kg)
40951
41568 (+1.6%)
42839 (+4.6%)
Total Column & Wall Steel (kg)
78851
78980 (+0.2%)
79606 (+1.0%)
Compare the total weight requirement after designing for upper bound of 2.7 and lower bound of 1.5. For the beams the weight is now 40964kg, it was 45273kg before rigid zones were introduced. For the columns the weight is now 78891kg, it was 80261kg before rigid zones were introduced. The new reinforcement weights are also slightly lower than the weight previously shown when the FE chasedown method was used (43847 and 80545 respectively). For this example model, the AF method can be used with double benefit when compared to FE chasedown: 1. It eliminates any concerns about not having considered pattern loads 2. By introducing the efficiency of rigid zones it also results in the least overall reinforcement requirement.
Conclusion on Design Impact For the example model it seems that the impact of considering upper and lower bound solutions would have quite minimal effect on overall construction cost. For the columns and walls in particular it seems that the vast majority of members are unaffected. The FE chasedown method does not allow pattern loads to be considered at the upper bound. If this is a concern then the AF method must be used. In practice the AF method will be a little more time consuming to apply, but this example also demonstrates that allowing the use of rigid zones along with full pattern loading could have a net overall benefit when compared to the FE chasedown method.
Orion Documentation page 230
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Overall Summary of Suggested Procedure Assuming the Area Factor method is to be adopted the following steps summarise the entire procedure that should be undertaken.
• Determine if DADE is important for the structure under consideration. • After an initial analysis using AF = 1, examine the beam bending moments and the vertical deflections of columns and walls at the top floor of the structure.
• If effects are not a concern then no further action is required. • If steps need to be taken to eliminate DADE record sway deflections in both directions before proceeding.
• Initial Design - Emulate traditional hand calculation methods • Use the approach demonstrated in these notes to determine an appropriate AF adjustment to apply. (If it is your intention to perform the check design stage below then there is little harm in erring on the high side at this point)
• Re run the analysis and review the revised results at the top floor. If the DADE seems to have been largely eliminated continue with design of all column and wall members.
• Design all columns/walls/beams • If the sway deflections are largely unchanged then you might choose to stop at this point, however, if you prefer to cross check taking some account of DADE then proceed to next stage.
• If the deflections have changed significantly then it is strongly recommended that a second analysis run is made with a lower AF and that the columns and walls (at least) are checked for the results of this revised analysis.
• Check Design - Allow for differential axial deformations • Use a column area multiplier of between 1.5 and 2.0 (1.0 if you want to be more conservative)
• Run check design on all columns/walls/beams. (Check design on beams might be considered optional)
• Increase reinforcement on any failed members (needs to be done interactively on beam lines)
• Final Design and Submission Calculations • Return to the AF determined at the initial design stage above. • Run check design on all columns/walls/beams (all should pass if the above exercise was completed properly).
• Generate submission calculations: • Moment diagrams etc based on this analysis will be more familiar to the checking engineer.
• You will however have designed for larger moments in some places, so the steel may appear to be overprovided at those points. The above is illustrated in a DADE Analysis & Design Flowchart at the end of this chapter together with the possible option of using the FE chasedown procedure.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 231
Closing Discussion Typical Concerns The sorts of concerns expressed by engineers as they begin to consider this topic for the first time and then in greater detail are: 1. The suggested area factor is not a defined value, it is just a suggested range of values. I really want a fixed value or values that can always be used. 2. Making these adjustments to cater for gravity load may be logical but are there side effects? For example, will it cause completely different force distribution and building deflection under wind loading cases? 3. Does the FE chasedown option provide another (better?) way to eliminate differential axial deformation? 4. If I am designing for a wider envelope of forces am I not over-designing the structure? 5. Can this all be solved more accurately by using Staged Construction analysis?
What answers are we trying to get? Discussion of any of the above has to be prefaced by an acceptance and understanding of the fact that "correct" answers do not exist. This point is introduced in the opening discussion. The uncertainties associated with concrete as a material and with construction generally dictate this. These sorts of uncertainties (cracking, creep, etc) are noted in the seminar presentation. Traditionally answers have been achieved by methods involving very significant idealisation continuous beam/sub-frame analysis. Generally such designs would totally ignore the potential impact of differential axial deformation. Are these design forces to be considered as the "correct" answers, or should the potential impact of differential axial deformation be considered? Assuming it is agreed that differential axial deformation should not be completely ignored then we have a decision to make - do we always allow for axial deformation and abandon the previous methods where it was ignored? This is a really a matter for the general engineering community, but if we assume that there is concern about abandoning the results that have always been used with no particularly detrimental effect, then we inevitably end up having to consider designing for an envelope of forces derived from differing assumptions.
Fixed Values for Area Adjustment Factor A single fixed value could only be possible if a decision were made to either always ignore or always allow for differential axial deformation. For reasons noted above this does not seem to be a viable option and so we must really focus on whether a range with fixed upper and lower limits is possible.
Orion Documentation page 232
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
For the upper limit we are considering the case where differential axial deformation is being largely eliminated. Values of 3-5 are discussed in the presentation but this is actually something that can be quite structure dependent. However, it is also something where logic can be applied. If the deflections from an un-modified analysis are examined and a wall is deflecting down by 2mm while the columns adjacent to it are deflecting 5.5mm then the area factor required to eliminate this will be a little in excess of 5.5/2 = 2.75. (Bear in mind that the stiffer columns will attract increased axial load). In the example the relatively large size of the columns compared to the walls results in quite a low figure of around 2.7 being a reasonable upper limit, but use of values slightly higher than this would really only add a little to overall design conservatism. For the lower limit we are considering the case where differential axial deformation is allowed for. As is noted above a logical but very conservative lower limit is 1.0. For reasons detailed in the presentation we suggest that a 1.5 value is actually a very reasonable lower limit.
Side Effects on Lateral Load Analysis There will be some effect, but the following must be noted: • The area factor is only affecting the axial load deformation characteristics of the columns, the bending stiffness is not affected.
• Generally walls are providing the majority of lateral load resistance and so adjusting column properties has quite limited impact on deflections. Once again the impact of this is illustrated in the example. The percentage change in deflections is relatively small. A bigger question (not considered here) is the extent to which any analysis accurately predicts lateral deflection. The result is again entirely dependent on allowances made for creep and cracking. ACI guidance on this is more thorough and suggests varying member stiffness adjustments.
Will FE Chasedown also eliminate differential axial deformation? Yes - this is demonstrated in these notes. Instead of running the building analysis with the increased area factor, results from an FE chasedown could be used. There is however a limitation in this approach, pattern load cases are not considered during FE chasedown. For this reason we would suggest that the area factor adjustment method is not dismissed. In Orion pattern load cases are automatically derived and included in the full building analysis. We understand that in some other software this is not so easy to achieve and so perhaps it is not considered imperative that patterning is considered as well as eliminating differential axial deformation. In this case some engineers may prefer the option in Orion to use FE chasedown.
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 233
Is this going to result in uncompetitive over design? Traditional design tended to ignore the impact of differential axial deformation. If we now consider this effect, but continue to consider the traditional results as well, then the envelope of design forces will inevitably widen for some members which will result in more onerous design. However, for the example model considered the net effect of this proved to be quite minimal. Is this over design, or a more thorough and professional approach taking account of modern analytical capabilities? This is really another question for the engineering community.
Could I avoid all this complication if Staged Construction Analysis were used? This would not avoid complication - it would just mean that different complications need to be addressed. A single staged construction analysis can not deliver "correct" answers. The unknowns associated with the input for such an analysis dictate this. This is illustrated within our presentation, but the key point to consider is that this type of analysis means you have to define properties that would not normally be defined for a basic elastic-static analysis, and you would have to allow for uncertainties associated with these:
• Time varying properties of concrete - you have to be able to define what properties should be used at every time step (stage). This includes consideration of early age creep effects.
• You have to define the way in which loading becomes active through the stages. You must then layer on top of this the need to consider patterning of load. In this context there is load patterning along beam lines but also the need to consider loaded and unloaded floors through the height of the building. Can this be realistically achieved as part of a staged construction analysis of a large structure? As with any analysis, if it is not used with a reasonable degree of care and understanding then a staged construction analysis can be a sophisticated and time consuming way of getting bad answers. We would suggest that a staged construction analysis is another way of establishing an envelope of possibilities. The parameters applied during this process are not specified in any design code so this approach ought to be subject to a similar degree of judgment and scrutiny as is required for the area factor adjustment approach.
Orion Documentation page 234
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
DADE Analysis & Design Flowchart 09.
Which method to resolve DADE?
START
01.
FE1.
AF1.
yes
no
AF
Build/Edit the Model
FE2.
Determine upperbound AF 02
Are Rigid Zones set to ‘None’?
FE
Set Rigid Zones to ‘None’ and re-run Building Analysis
Derive Beam Loads
FE results can not be merged if rigid zones have been used
Yield Lines or FE decomposition for Beam Loads AF2.
Re-run Building Analysis using upperbound AF
03.
Decide on a lowerbound AF Conservatively use the default AF = 1.0 To reduce DADE, use 1.5 – 2.0
compare sway deflections with those obtained from lowerbound solution (step 05)
FE3.
Run FE Chasedown Analysis
DON'T use meshed floors DO include upper storey loads
FE4.
AF3.
04.
Decide if Rigid Zones are to be used When utilised in Building Analysis, rigid zones can produce a more economic solution
Design all members
Merge FE beam and column results
Because FE Chasedown considers gravity loads only, the lateral results from step 05. are still retained
This is a design for the upperbound analysis solution in which DADE has been minimised FE5.
Design all members
05.
Run Building Analysis This step is mandatory in order to produce analysis results for the lateral loads This step also produces a lowerbound analysis solution for gravity loads (in which DADE may exist)
This is a design for the upperbound analysis solution in which DADE are virtually eliminated
AF4.
Re-run Building Analysis using lowerbound AF
This re-instates the lowerbound analysis solution obtained in step 05
FE6.
Un-merge FE beam and column results
sizes not OK
06.
AF5.
Initial Design Check
Check all members Increase reinforcement in failed members
This re-instates the lowerbound analysis solution obtained in step 05
FE7.
Check all columns and walls
Increase reinforcement in failed members
sizes OK AF6. 07.
Is DADE a concern? no
yes
Re-run Building Analysis using upperbound AF and check all members
If step AF5 above was done properly all should pass at this point and design calculations should be saved
FE8.
Optionally - check all beams
Because the lowerbound solution caters for pattern load the user may want to consider it’s effect Increase reinforcement in failed beams
08.
Design All members FE9.
FINISH
Re-merge FE beam and column results and check all members If steps FE7 and FE8 above were done properly all should pass at this point & design calculations should be saved
Chapter 13 : Overview of Differential Axial Deformation Effects in 3D Analysis of Buildings
Orion Documentation page 235
Flowchart Notes.
Box No.
Comment
03.
Justification for increasing the AF may be derived from ACI advice on analysis model properties which allows for the more likely case that beams are cracked while columns/walls are not. Refer to What is a reasonable lower limit for the area factor adjustment? for an introduction to further discussion on this.
04.
See Rigid Zones for further details on the use of these.
06.
Inadequate initial member sizing can have significant impact on force distribution around the structure. It is important to review the member sizing at an early stage in order to prevent having to increase undersized members later on.
07.
Determine if DADE is important by examining
• how the beam bending moments change over the height of the structure, • the vertical deflections of vertical elements at the top floor see Why do the two models give such different answers? for details of how to do this. 08.
If DADE is not a concern, the design is now complete.
09.
It is down to user preference which method is adopted for minimising/ eliminating DADE, noting that:
• AF method - Applying a single area factor globally may not fully remove differential axial deformation at all locations.
• FE Chasedown method - No rigid zones, and for the upperbound solution no pattern loading. AF1.
See Increasing the column area factor for details of how to do this.
AF2.
The deflections at the top of the building under lateral load should be compared. Provided the differences are small the impact on the distribution of lateral forces will be small. See Overall Summary of Suggested Procedure for additional notes on this. If the high AF is affecting the deflections it is more important that the lower bound solution is checked for all members.
Orion Documentation page 236
Chapter 14
Chapter 14 : Using Staged Construction Analysis to Emulate ‘Traditional’ Design
Using Staged Construction Analysis to Emulate ‘Traditional’ Design
Introduction As described in the previous chapter a 'traditional' design will historically have been based on the results of a series of discrete 2D subframe analyses, in which: • Sub-frame analyses are carried out at each floor from top down (assuming fixed supports)
• Deflection compatibility between sub-models is ignored. • Column and wall forces are established by summing up the axial loads determined from each of the separate analyses. Alternative methods of emulating the above in Orion were investigated and the following methods were described: • Sub-Floor Analysis Referred to in Orion as FE Chasedown analysis.
• Building Analysis with Area Factor Adjustment Analysis of a full 3D model In this chapter a third alternative is explored via a worked example: • Staged Construction Analysis Analysis of a full 3D model in stages
Worked Example: In the previous chapter we established some traditional sub-frame results for beams at the 20th floor of a building and then looked at how closely these results were emulated using full 3D analysis., the purpose of the exercise being to quantify the effects of differential axial deformations. From the results it was possible to then formulate recommendations for emulating a ‘traditional design’. In this worked example the same model is used to examine 3D staged construction analysis and compare the results with those obtained previously. Once again the objective is to derive a means of emulating a ‘traditional design’.
Chapter 14 : Using Staged Construction Analysis to Emulate ‘Traditional’ Design
Orion Documentation page 237
Previous Results (from FE Analysis and Unstaged Building Analysis) A single beam line at the top floor was chosen for the purposes of comparison. Sub-Floor Analysis (FE Chasedown) The bending moment diagram below was obtained by performing an FE chasedown analysis. This type of analysis eliminates differential axial deformation effects (DADE) and is considered a reasonable base-line answer against which the other approaches can be compared
Building Analysis without Area Factor adjustment When a BuildingAnalysis was run using default settings, the bending moment diagram for the comparison frame was as follows, the effect of DADE is clearly apparent:
Building Analysis with Area Factor adjustment As discussed in Chapter 13 Area Factors can be applied to reduce DADE. With the AF set at 2.7 the following result was obtained
Orion Documentation page 238
Chapter 14 : Using Staged Construction Analysis to Emulate ‘Traditional’ Design
Staged Construction Analysis Result Currently this is only available for unfactored cases utilising 25% rigid arms:
The unstaged G+Q*F result (with 25% rigid arms and AF = 2.2) is:
• Super-posing these answers we get a staged combination result as follows: Solution Method
LH Support Moment
Span Moment
RH Support Moment
FE Chasedown
-100.4
67.4
-103.8
BA unstaged
-189
70
-16
BA AF 2.2
-126.2
62.9
-84.4
BA staged
-115.8
62.4
-95
The staged result initially appears to offer an as good or better means of eliminating DADE than is achieved through the Area Factor approach, (although comparison is difficult as the staged and AF results in the above table allow for rigid arms whereas the FE result does not). However, when the staged results are examined in more detail the conclusion is less clear cut.
Chapter 14 : Using Staged Construction Analysis to Emulate ‘Traditional’ Design
Orion Documentation page 239
Does Staged Analysis eliminate DADE? To answer this question, firstly we need to make a comparison between the unstaged and the staged bending moments that develop over the full height of the example building:
Unstaged Q - BMD Floors
Staged Q - BMD Floors
Each bending moment diagram shows a significant step change at the 10th floor - this can be accounted for by the step change in beam sizes that takes place at this level.
Orion Documentation page 240
Chapter 14 : Using Staged Construction Analysis to Emulate ‘Traditional’ Design
DADE is clearly affecting the unstaged analysis results from the second floor all the way through to the top floor of the building. The staged analysis seems to be largely unaffected by DADE at the upper floor levels, but this does not hold true from around the 10th floor level and below. (In this particular worked example it should be noted that the effect is being amplified due to the central column size being larger over the first 10 storeys.) The shape of the above bending moment diagrams can be understood more clearly by considering the extent by which the central column shortens in each analysis. In the unstaged analysis the column axial shortening must accumulate as you get higher in the building. The vertical deflection of the column at the top floor is directly affected by the axial shortening of the lower columns, which is occuring due to the load they each carry. Unstaged Analysis Deflections
\
1st Floor
10th Floor
Top Floor
In the staged analysis, the top floor forces and deflections are determined from the analysis of the final stage alone, in this stage only the top floor loads are applied and the loads carried by other floors have no influence - hence column shortening is not being accumulated from one floor to the next. Staged Analysis Deflections
1st Floor
10th Floor
Top Floor
So in the staged analysis, why are we seeing DADE occurring, particularly at the 10th floor and below? - To answer this you need to consider how the forces and deflections at the 10th floor are determined from the combined effect of the analyses of 10th stage to the final stage, (consisting of all the loads from the 10th floor to the top floor). This significant amount of load is being carried by a column which is 10 storeys high - there is undoubtably going to be considerable axial shortening taking place.
Chapter 14 : Using Staged Construction Analysis to Emulate ‘Traditional’ Design
Orion Documentation page 241
Lower in the building, at the 1st floor level the column is carrying a much greater load however, because it is so short the amount of axial dispacement would be minimal.
Conclusions Staged Construction does not eliminate DADE. (this particular example exacerbates the effect because of the change in sections - the effect may not be so evident in other models??). It moves the position where they are most significant (to building mid height ) and reduces them to some degree. This result is not technically wrong , but is it what engineers want? If you are happy to completely ignore differential axial displacements then all you need to do is adopt one of the methods described previously for emulating the traditional design, taking note of the associated pros and cons. If you are happy to take some account of DADE then staged construction may offer the best solution.
Use of Staged Construction as the upper bound solution The previous chapter discussed use of AF method to establish lower and upper bound solutions and designing for the resulting envelope. (You could also use ACI method of reducing inertias) In this chapter an alternative has been provided to the above using staged construction, in which the lower bound solution is an unstaged analysis and the upper bound solution is staged G+Q. (You might however consider closing the gap between these by taking the upperbound as staged G and unstaged Q).
Orion Documentation page 242
Chapter 15
Chapter 15 : Design and Detailing
Design and Detailing
Introduction The following section of the Engineer’s Handbook contains six chapters relating to the design and detailing of the various member types:
• Slab Design looks at slab strip errors and the actions required should you encounter them.
• Beam Design to BS8110 looks at Orion’s beam design process, relating it to the relevant code clauses. A worked example is included.
• Beam Detailing provides an overview of the beam detailing process, describing the available patterns and beam detailing settings. A worked example is used to illustrate how careful application of preferences can produce either: a minimum weight solution (where the details may be more complex), or a more standardised detail (with a slightly higher weight of reinforcement).
• Column Design to BS8110 looks at Orion’s column/wall design process, relating it to the relevant code clauses. Worked examples are included.
• Wall Design and Detailing looks at additional aspects of Orion’s design process that are unique to walls.
• Foundation Design describes the procedures required for defining and designing the various foundation elements available in Orion.
Chapter 16 : Slab Design
Chapter 16
Orion Documentation page 243
Slab Design
Overview Referring back to the Modelling Analysis and Design Flowchart in the initial Overview chapter, you can see that slab design may be based either on tabulated code coefficients, or on finite element analysis (FE) results. Design based on the tabulated code coefficients is covered in the training manual. Discussion there also extends to the option of using an FE strip, but design based on FE is covered in greater depth in this manual in the chapters Analysis and Design using FE and Flat Slab Models. In both cases, the design approach involves creating slab strips. Checks are applied as you create these strips to ensure they are valid. The following brief notes may be of some additional assistance (assuming that you have already completed the training examples successfully).
Slab Strip Errors – Reviewing/Understanding The Slab Strip creation process will fail and display an error message if you are attempting to define an invalid strip. The message is displayed on the status line at the bottom of the screen, No. of Slabs and Beams along strip is not consistent! This occurs if you have defined a strip that does not pass through alternating beams and slabs in the correct sequence. A typical occurrence of this is when attempting to create a strip that either begins or ends with a cantilever slab but you have set the end condition to either Bob or Slab. It could also occur if you have drawn a strip that spans across an open area such as a lift core where there is no slab. Finally, it could also occur if at some point along the strip it encounters two consecutive slabs without a beam separating them.
Action Required: For case 1 above, change the end condition to Cantilever and update the strip. For case 2, split the strip into two half strips. Draw one strip ending in the open area and then a second strip coming out of the open area. For case 3, redefine the slab panels so that the strip passes from beam to slab panel to beam etc. Creating Member… (but nothing seems to happen) This occurs if your strip doesn’t entirely traverse one entire slab.
Action Required: Interrupt the process by clicking on to another command on the Members toolbar, then redraw the strip but increase its length so that it entirely traverses at least one entire slab.
Orion Documentation page 244
Chapter 17
Chapter 17 : Beam Design to BS8110
Beam Design to BS8110
Introduction Orion designs beams bent about the major axis using the code clauses given in BS 8110-1:1997: Part 1 Section 3.4. The following table summarises the various stages of the beam design process:
Step
Calculation
Clause
1.
Slenderness limits for lateral stability
3.4.1.6
2.
If flanged - calculate effective width of flanged beam
3.4.1.5
3.
Analysis of Sections
3.4.4.1
4.
Design for Bending
3.4.4.4, 3.4.4.5
Calculate K/K’
3.4.4.4
Calculate z
3.4.4.4
Calculate x
3.4.4.4
Calculate As required
3.4.4.4
If K > K’ calculate As’ required
3.4.4.4
5.
6
Design for Shear
3.4.5
Calculate shear stress in beam, v
3.4.5.2
Calculate design concrete shear stress, vc
3.4.5.4
Determine shear reinforcement
3.4.5.3
Spacing of links
3.4.5.5
Deflection of Beams
3.4.6
Span/effective depth ratios
3.4.6.3
Modification of span/effective depth ratios for tension/compression reinforcement
3.4.6.5 3.4.6.6
Chapter 17 : Beam Design to BS8110
Orion Documentation page 245
Beam Design Settings You take control of the above design process by configuring the Beam Design Settings dialog to your preference.
For further details of the implications of adjusting the various settings see the chapter Beam Detailing
The BS8110 Beam Design Process 1. Check slenderness limits for lateral stability- Cl 3.4.1.6 A warning message is displayed during the member definition if the slenderness limit has been exceeded.
Orion Documentation page 246
Chapter 17 : Beam Design to BS8110
2. Rectangular or flanged - Cl 3.4.1.5 The choice of this setting is set via the Design tab of Beam Design Settings as shown below:
The option to use the rectangular section (rather than the flanged section) when the flange is in compression will generally result in slightly more conservative steel area requirements, however if minimum steel requirements apply then design using the flanged section will introduce greater steel requirements. If a flanged section is used the effective width is determined from Cl 3.4.1.5.
3. Analysis of Sections - Cl 3.4.4.1 All considerations stated in the Cl 3.4.4.1 are fulfilled. The simplified stress block for concrete as illustrated in Figure 3.3 is utilized.
4. Design for Bending- Cl 3.4.4.4 and Cl 3.4.4.5 Bending is checked for the top edge in three regions: 1. left (worst case forces in end region - 25% clear span) 2. middle (worst case forces in mid region - 50% clear span) 3. right (worst case forces in end region - 25% clear span)
Similarly, bending is checked for the bottom edge in three regions: 1. left (worst case forces in end region - 15% clear span) 2. middle (worst case forces in mid region - 70% clear span) 3. right (worst case forces in end region - 15% clear span))
Chapter 17 : Beam Design to BS8110
Orion Documentation page 247
The design moments for each of the six regions can be displayed by clicking the Design button shown below.:
The effective depth h’ to the centre of the bars is worked out separately at each location. This is automatically reduced if the steel doesn’t fit in a single layer.
Orion Documentation page 248
Chapter 17 : Beam Design to BS8110
You can control this calculation via the Design tab of Beam Design Settings as shown below:
Centre of Gravity of Steel Bars — the section effective depth will be determined based on the centre of gravity of the tension bars. In this method, the contribution of each tension bar in the section will be considered separately. Centre of Gravity of Layers — in this more conservative method the calculation of section effective depth is based on the average of the distances of the steel bar layers. Values for K, K’, x, As, and As’ are then all calculated for each region and are also shown in the same table. Where the ratio of K/K’ is less than 1.0 no compression steel is required.
Chapter 17 : Beam Design to BS8110
Orion Documentation page 249
5. Design for Shear- Cl 3.4.5 The design shear force at the column face, Vd is displayed in the same table as the design moments:
From the Design tab of Beam Design Settings you can specify if you want to design for this value, or (if the conditions of Cl. 3.4.5.10 are met) you may choose to design for the shear at ‘d’ from the column face.
Orion Documentation page 250
Chapter 17 : Beam Design to BS8110
If the latter option is adopted the design shear force at ‘d’ is then displayed and used in the design calculations as shown below:
In the above table, the maximum shear stress v is determined from Cl 3.4.5.2 and is always based on Vd (not Vd@d). This is cross checked against the maximum permissible shear stress v-max, which is defined in the beam design settings. vc is calculated using the equation in table 3.8. In BS8110 the concrete grade considered for shear is capped at 40N/mm2. Link requirements are established from Table 3.7 as follows - v is determined from either Vd or Vd@d depending on your choice in the beam design settings and Vnom – Nominal shear capacity (based on minimum link provision) is established. The maximum shear requirement is then checked at each end of the beam and if the nominal capacity is not sufficient additional links are provided over a distance (x) extending to the point where the nominal capacity is adequate (or beyond if user preferences dictate minimum lengths).
Chapter 17 : Beam Design to BS8110
Orion Documentation page 251
6. Deflection Checks- Cl 3.4.6 Basic span/effective depth ratios are calculated from Table 3.9 and modification factors applied according to Table 3.10 and 3.11. Note
The beam size is not automatically increased to satisfy the deflection criteria - it is highly unusual for beams to be sized so that deflection is a controlling factor. Also, it should be noted that increasing the steel is usually a highly uneconomic way of controlling deflection. It is more normal to conclude that the beam is not deep enough if beam deflection checks fail.
Worked Example The Design Model The example model Doc_Example_4 is opened and saved to a new name (so as not to destroy the original example). In the calculations that follow, the steel grade has been set to 500 with a steel material factor of 1.15. Bar diameters permitted for the longitudinal steel in the beams have been limited to 10, 12, 16, 20 and 25mm. The minimum link diameter is set to 10mm. The concrete grade is C40. Note that to obtain similar results you will need to apply the same settings. However, the actual design process would be identical irrespective of which material grades, material factors and bar diameters are used. The continuous beam on grid line 2 will be designed and the following calculations will focus on the design of beam 1B14.
Orion Documentation page 252
Chapter 17 : Beam Design to BS8110
Beam 1B14 span = 4m beam depth, h = 400mm beam width, b = 250mm cover = 20mm
Beam Design Settings The beam design settings initially adopted are as shown:
Settings on the Design tab
Note
Where the cover shown above has been set to 0 - as noted on the dialog, the cover is then based on code requirements - (i.e. a default cover of 20mm is applied). You may prefer to enter a value directly into each field to set the cover required.
Settings on the Parameters tab
Chapter 17 : Beam Design to BS8110
Orion Documentation page 253
Settings on the Bar Selection/Steel Pattern tab
Settings on the Bar Selection/Links tab
Analysis Results Having run a building analysis using default analysis settings, the design envelope for all combinations is as follows:
Orion Documentation page 254
Chapter 17 : Beam Design to BS8110
Performing the Design To perform the design calculations select beam 1B14, right click and choose Beam Reinforcement Design from the menu. This will display the data for the three beams on Axis 2, with beam 1B14 highlighted:
Each stage of the design process will now be examined in detail.
Design for Bending - Cl 3.4.4.4 Click the Design button to see the six design moments for 1B14 determined from the analysis moments shown previously:
The area of steel calculations are listed below the design moments. These can be confirmed with simple hand calculations using Cl 3.4.4.4. However, before this can be done we need to know the bar sizes used to accurately calculate the effective depth. Click the Steel Bars button to see the steel bars provided:
Chapter 17 : Beam Design to BS8110
Orion Documentation page 255
Effective depth to bar centre h’ (RHS top edge) = 400 - 20 - 10 - 12/2= 364mm h’ (other locations) = 400 - 20 - 10 - 10/2 = 365mm
Area of Steel Required: Top Edge - Left Hand Side K = M/bd2fcu = 0.011 K’ = 0.156 K/K’ = 0.07 < 1.0 compression steel not required z = d(0.5 + (0.25 – K/0.9)0.5) = 0.99d z = min (0.99d, 0.95d) = 347mm x = (d-z) / 0.45 = 40.6mm – OK As required = M / 0.87fyz = 98.0mm2 Minimum As from Table 3.25 0.13x250x400 =130mm2
Area of Steel Required: Top Edge - Right Hand Side K = M/bd2fcu = 0.022 K’ = 0.156 K/K’ = 0.14 < 1.0 compression steel not required z = d(0.5 + (0.25 – K/0.9)0.5) = 0.97d z = min (0.97d, 0.95d) = 346mm x = (d-z) / 0.45 = 40.4mm – OK As required = M / 0.87fyz = 196.7mm2 Note
Similar calculations to those shown above are performed for the other four regions.
Area of Steel Provided: The steel provision is dictated by the current Beam Design Settings. The bar selection and curtailment options chosen will greatly influence the weight of steel and the complexity of the resulting detail. For more information see the Beam Detailing chapter of this Handbook.
Orion Documentation page 256
Chapter 17 : Beam Design to BS8110
Design for Shear - Cl 3.4.5 On the Reinforcement Data dialog, click on any of the link fields for 1B14 to see the shear design calculations:
Left Hand Support Max shear stress from Cl 3.4.5.2 — v =Vd / bvd = 0.41 N/mm2 < v-max - OK vc is calculated using the equation in table 3.8 — d = 365mm, As = 157.1mm2 100As/b.d = 0.172 Note
As provided is used in the above calculation
vc = (40/25)0.33 0.79.(0.172)0.33 (400/365)0.25 /1.25 = 0.42
Right Hand Support Max shear stress from Cl 3.4.5.2 — v =Vd / bvd = 0.49 N/mm2 < v-max - OK vc is calculated using the equation in table 3.8 — d = 366mm, As = 226.2mm2 100As/b.d = 0.247 vc = (40/25)0.33 0.79.(0.247)0.33 (400/366)0.25 /1.25 = 0.48
Chapter 17 : Beam Design to BS8110
Orion Documentation page 257
Span vc is calculated using the equation in table 3.8 — d = 365mm, As = 235.6mm2 100As/b.d = 0.258 vc = (40/25)0.33 0.79.(0.258)0.33 (400/365)0.25 /1.25 = 0.48
Link requirements Determine if minimum bars at maximum spacing provide “minimum links” — Asv_req / sv = (0.4.bv.)/0.87fyv = 0.23 For minimum bars at maximum spacing, provide H10 links at 250mm centres Asv_prov / sv = 2 x 78.5 / 250 = 0.63 In this case OK - (if not OK decrease the spacing until requirement is met). Calculate vnom using the minimum links — capacity from vc vc b.d = 43.8kN capacity from links (0.87 fyv Asv_prov / (bv.sv)). (bv.d) = 99.8N vnom = 43.8 + 99.7 = 143.6 kN Determine if shear at the ends exceeds vnom — 40.7kN < 143.6kN In this case OK - hence minimum links apply for whole length of beam.
Orion Documentation page 258
Chapter 17 : Beam Design to BS8110
If the shear at either end had exceeded vnom - the distance, x to the point where vnom is adequate would be determined as shown below, and a calculation performed to determine the increased links to apply over that length.
Deflection Checks- Cl 3.4.6 Checks are based on the span values for sagging moment design. L/d = 4000/365 = 10.96 – OK Table 3.9. For a rectangular beam - basic permissible ratio = 26 Table 3.10– modification for tension reinforcement M/bd2 = 0.91 As req = 201.2 As prov = 235.7 fs = 2fy.As req / (3As prov) = 284.5N/mm2 Modification Factor = 1.44 Table 3.11– modification for compression reinforcement 100As’prov /bd = 0.172 Modification Factor = 1.05 Therefore adjusted Permissible L/d = 26*1.44*1.05 = 39.4 – OK
Chapter 17 : Beam Design to BS8110
Orion Documentation page 259
Output Calculations The following report shows the output for the beams along axis 2:
Orion Documentation page 260
Chapter 18
Chapter 18 : Beam Detailing
Beam Detailing
Introduction Orion has always included numerous beam design and detailing options that are all controlled from the main beam Settings and Parameters dialog that is shown below.
Given that there are tabs with sub-tabs and numerous options under each, the control of these settings can be very daunting. With each Orion release even more options tend to be added in order to respond to user demand to be able to apply more and more specific preferences. The objective of this chapter is to provide a brief overview of the options in general and then more specifically the reinforcement pattern options, the first of which is highlighted above.
The Design and Detailing Process Overview It is worth considering why so many options have developed and to do so requires some reflection on the design and detailing process. We can break this process into stages: 1. A model is created and analysed. 2. Analysis results are reviewed and design forces are established at critical cross sections. 3. Cross section design determines the areas of reinforcement required. 4. Bars are selected that provide at least sufficient reinforcement.
Chapter 18 : Beam Detailing
Orion Documentation page 261
5. The bars are arranged and curtailed so that the reinforcement provided is sufficient at all cross sections. 6. The information is conveyed on a detail drawing. This may initially seem simple enough but it soon becomes apparent that any two engineers or draughtsmen are quite likely to apply different preferences (personal preferences or company standards) and get different results at every stage of the process. The analysis stage is not the focus for this document but it is worth noting that Orion provides many analysis options, which will mean the analysis results differ, for example: • Option to use rigid zones (not only generates a more complex analysis model but also means that design moments at the face rather than the centre-line of the column may be used).
• Option to use flanged or rectangular section for the beams. • Options to adjust stiffnesses of beams, columns, etc. Having got the analysis results we head into design and that is where all the settings in the Beam Settings and Parameters dialog start to apply. The dialog is organised so that the main tabs introduce settings in the order in which you would apply them in the design process. The following subsections of this chapter provide an overview of the settings under each of the main tabs in turn.
The Design Tab
These settings are generally self evident, they will tend to have a slight influence on the values of As required that emerge from the design. For example the options to design for the shear at the column face and to use the rectangular section (rather than the flanged section) when the flange is in compression will result in slightly more conservative steel area requirements.
Orion Documentation page 262
Chapter 18 : Beam Detailing
The Parameters Tab
Again, these settings are generally self evident, they set limits on the ranges and spacing of bars which are considered when bars are being selected to provide reinforcement which at least meets the minimum requirements determined during design.
The Bar Selection Tab
In this tab we start to apply more specific preferences which will affect the way in which bars are selected to meet the As requirements determined in design.
Chapter 18 : Beam Detailing
Orion Documentation page 263
Standard Pattern 2 shown above is currently the most commonly used option. Many of the other options under this tab and also under the curtailments tab are more tuned to standard pattern 2. Later in this chapter we will look at each of the patterns in turn and at some of the other options (particularly relating to this tab and the following curtailment tab) in more detail.
The Curtailment Tab
In this tab we apply preferences as to how the reinforcement is curtailed. Although this is not under the Detailing tab, these sorts of preferences are more traditionally applied by the detailer than the designer.
Orion Documentation page 264
Chapter 18 : Beam Detailing
The Detailing Tab
In this tab all the preferences relate to detailing presentation options, i.e. changes here only relate to presentation and not to the reinforcement selection.
The Layers Tab
Settings in this tab control the layering, line types etc. to be used in the DXF file, which can be loaded into most general drafting packages.
Chapter 18 : Beam Detailing
Orion Documentation page 265
Overview of Patterns The concept of different basic patterns, which can then be optimised according to personal preference, is actually derived from the publication Standard Method of Detailing Concrete published by the Institution of Structural Engineers (UK) – August 1999. In the section on beam detailing (cl 5.4.20), two basic methods of detailing are introduced: • The Splice Bar Method – A pattern aimed at prefabrication of cages and minimisation of in situ steel fixing.
• The Alternative Method – A pattern more suited to in situ steel fixing. Orion’s Patterns 1 and 2 are based on these suggestions.
Pattern 1 – The Splice Bar Method
Orion Documentation page 266
Chapter 18 : Beam Detailing
The pattern is shown diagrammatically in the dialog above and in the typical detail below. The main attribute of this pattern is that top and bottom bars all stop at each side of columns/ supports and then splice bars are used to maintain the required continuity through the support.
All sorts of preferences can be applied to the pattern, generally this becomes a balance between complexity and material efficiency, this concept is explored in detail later in this chapter. Some general points worth noting at this stage are: • There are various options to merge bars through supports and between spans. If the objective of using this pattern is to create beam details such that discrete reinforcing cages for each beam can be constructed then lifted into position the merging options should all be switched off.
• Currently the pattern does not support top span bars that extend to the face of the support (link hanger bars), the top span bars will always stop short of the support. This may mean that the links towards each end of the cage are not fixed as securely as might be desired during lifting. Some extra (un-detailed) top bars may be required. In general it appears that this pattern has not yet been widely employed by Orion users, or if used it has generated very little feedback. However since the pattern does encourage off-site or low-level prefabrication it offers the possibility of reducing site risks by reducing work at height. There may even be financial benefits to be secured from factory production of cages. It is considered possible that the use of this pattern may become more dominant in the future.
Chapter 18 : Beam Detailing
Orion Documentation page 267
Pattern 2 – The Alternative Method
The pattern is shown diagrammatically in the dialog above and in the typical detail below. Currently the top bars are the same for this pattern and pattern 1. The bottom bars are different, the main difference being that they are allowed to run through the support and lap directly with the bars in the adjacent span.
All sorts of preferences can be applied to the pattern, generally this becomes a balance between complexity and material efficiency, this concept is explored in detail later in this chapter. Some general points worth noting at this stage are: • This pattern is the mostly commonly used and so many of the options/preferences introduced by user request tend to be more applicable to this case. The Detailed Example and Comparisons are therefore most applicable to this pattern.
• There are various options to merge bars through supports and between spans. Since this pattern is not intended to lend itself to prefabrication these options are more applicable.
Orion Documentation page 268
Chapter 18 : Beam Detailing
Pattern 3 – The Hanger Bar Method
The pattern is shown diagrammatically in the dialog above and in the typical detail below. Currently the bottom bars are the same for this pattern and pattern 2. The top bars are different, the main difference being that (link) hanger bars are used which can run through several spans. These bars provide any required span moment resistance. Additional top reinforcement is then added in as necessary through the supports.
Note that the detailing option to Re-Plot the Bars below the Beam has been used here to show how top hanger bars extend through span 1 and into the middle of span 2.
Chapter 18 : Beam Detailing
Orion Documentation page 269
All sorts of preferences can be applied to the pattern, generally this becomes a balance between complexity and material efficiency, this concept is explored in detail later in this chapter. Some general points worth noting at this stage are: • Since small bars extend right through the support where additional bars are also added, this pattern may tend to produce cluttered details at the support or even prove to be impractical from a spacing point of view. As with pattern 1, it appears that this pattern has not yet been widely employed by Orion users, or if used it has generated very little feedback.
The Bent-Up Pattern Method
This pattern is used in high seismic regions and is only shown here for completeness. The current version of Orion only supports the Turkish earthquake code (in the Turkish language version of Orion). In non-seismic regions use of this pattern is probably acceptable and is likely to produce a very robust structure, probably at the expense of both reinforcement weight and on-site construction complexity.
Orion Documentation page 270
Chapter 18 : Beam Detailing
For now it is recommended that Orion users in non-seismic regions do not use this pattern. In future releases when other international seismic design codes begin to be supported this pattern will be more important.
Detailed Example and Comparisons Overview As noted in the above sections, Orion offers lots of options to first choose a basic reinforcement pattern and to then apply preferences as to how bars are selected, curtailed and detailed. In general the application of any preference means that you are making a choice between: • A minimum reinforcement weight option where the details may be more complex and slightly more time consuming (more costly) to construct.
• A more standardised/simplified detail where the weight of reinforcement is slightly higher, but construction may be simplified. Ultimately a judgement on the balance between standardisation and minimum weight is applied in the hope of minimising the overall cost of construction. Traditionally engineers and detailers will apply their preferences in this regard with very little quantitative knowledge on either side of the equation. Orion offers much more scope to allow you to investigate the impact of different standardisation options so that you have the potential to make more considered judgements. In this section we will start by setting up a basic reinforcing pattern and find the weight of reinforcement that this generates for an entire floor. We will then start to apply preferences one at a time, and in each case look at how the details get simplified but also determine how much reinforcement weight gets added. Note
The procedure shown in this section can be applied to any Orion model (or more usually one floor of a larger model) in a matter of minutes or perhaps hours (depending on model size).
Basic Setup First we will get the design and detailing set up so that we have the base data on which further comparisons can be made. The floor we will look at is shown on the next page.
Chapter 18 : Beam Detailing
Orion Documentation page 271
It is a reasonably simple layout of nearly 50 beams with typical spans in the region of 5 to 7 m. The applied factored load is approximately 22 kN/m2.
The following subsections show the basic settings used for the initial design.
Orion Documentation page 272
Chapter 18 : Beam Detailing
Design Tab
The above settings are reasonably standard and will not be changed during any of the subsequent comparisons. Parameters Tab
In general the above settings are reasonably standard, but as will be shown in the subsequent comparisons, adjusting or restricting the range of bars sizes to be used can influence the efficiency of the design.
Chapter 18 : Beam Detailing
Orion Documentation page 273
Bar Selection Tab
On the Method sub-tab shown above the bar size minimisation option is selected. This is generally not the preferred option, but as will be shown it will actually lead to the minimum weight of steel being determined. Note that the steel bar area tolerance can be used to introduce an additional safety factor during preliminary design.
Pattern 2 is selected and the options to use extra bars locally to enhance the support and span moment capacities are both activated.
Orion Documentation page 274
Chapter 18 : Beam Detailing
The options that are not checked add constraints that will add reinforcement weight. The options in the Bent-up Bars tab are only relevant to the Bent-up Pattern.
The links options are set as shown above. Once again the options that are not checked add constraints that will add reinforcement weight. Curtailment Tab
Chapter 18 : Beam Detailing
Orion Documentation page 275
On the Steel Bars sub-tab shown above the option to lap the bottom bars outside the support is used. This seems to be the preferred option of most Orion users as it tends to reduce clutter within the column. Although merging will reduce reinforcement weight slightly these options are not activated yet, we will look at their effect later. The other options that are not checked add constraints that will add reinforcement weight.
On the Steel Bars 2 sub-tab shown above the options are not checked once again because adding these constraints will add reinforcement weight. Note that the minimum Tension Lap Factor. This factor works as follows:
• If you set the factor to 1.0, but for the lap in question the code minimum is 1.4, then we still use 1.4.
• If you set the factor to 1.4, but for the lap in question the code minimum is 1.0, then your user over-ride applies - 1.4 gets used.
Orion Documentation page 276
Chapter 18 : Beam Detailing
Options under the Bob Control sub tab shown above are self-explanatory. Note the setting for Min. Steel Bob Length shown above is 25 Dia. The actual minimum specified in BS 8666:2005 Table 2 varies depending on the bar diameter. For 10mm bars it is 12 Dia, for larger bar sizes it reduces further to around 10 Dia. (Potentially then you might consider reducing this for greater ecomony.) The Min Steel Bob Length is compared with the bob length that would be required to achieve the anchorage length, (which for design to BS8110 is taken from Table 3.27). Note that the anchorage length is measured from the inside face of the supporting member. The settings shown above are not changed within the comparisons in the next section.
Chapter 18 : Beam Detailing
Orion Documentation page 277
Detailing Tab As noted earlier, the options under this tab only affect the style of the display and not the content. These options will not be changed at all during the comparisons in the next section. The settings used for the comparisons are shown here with some brief discussion. In general these might be considered a good set of suggested settings, but it is best to experiment a little with all of these until you have optimised the detailing style to suit your own preferences.
On the General sub-tab shown above note the option to include bar marks. If you want to see the bar mark appear in the bar label in the usual style you must also ensure that the settings in the Graphical Editor Settings dialog are set as shown below.
Orion Documentation page 278
Chapter 18 : Beam Detailing
In the Beams sub-tab shown below we are using the option to refer to beams by the axes (grids) that they lie on and between.
Note
It is probably more usual to activate the option to put the beam label below the detail, but this option is automatically deactivated when one of the other options we will use is activated – see later.
Chapter 18 : Beam Detailing
Orion Documentation page 279
In the Sections sub-tab shown above we are choosing to see 3 sections on every beam, this can be cut down. We have also chosen to use the axis (grid) label as a prefix to every section and so we can restart the section numbers on every axis and still have a unique label on every section. Since the axis labels are also used to reference the beams everything can be easily cross-referenced. Note
If you choose to reference the beams using the beam labels (the numbers generated as each beam is added to the model), it would be better to add the beam label as a prefix to the section.
Orion Documentation page 280
Chapter 18 : Beam Detailing
In the Steel Bars and Links sub-tab shown above we have chosen to Re-plot the bars below the beam. This is an extra option that goes beyond normal detailing practice, it is not suggested in detailing guides perhaps because the extra drawing work it would generate would not be justified when the drawings are prepared by hand. In Orion you get this extra/alternative presentation of the reinforcement detail for no extra effort – see below.
At first glance the above does not look standard and perhaps this puts some people off. However the exploded reinforcement details provide extra (and clearer) information. We would suggest that many Orion users might seriously consider using this option as a new way of providing better information to the site. Alternatively, as we will see in the following section, Orion provides a schedule that gives the overall weight of reinforcement, but it is not a
Chapter 18 : Beam Detailing
Orion Documentation page 281
traditional bar bending schedule. If a traditional schedule is to be prepared the details need to be taken off the drawings that Orion prepares, the detailer who prepares the schedule might be able to do this more easily using the exploded view of the reinforcement.
The options under the Side Bars and Dimensioning (shown above) sub-tabs are all self-evident and will not be changed during the following comparisons.
Orion Documentation page 282
Chapter 18 : Beam Detailing
Initial Design and Drawing Creation
Using the filter options to filter to the floor of interest, a design is then run of all the beams in the floor. During this exercise care is taken to always reselect all bars during redesign.
Chapter 18 : Beam Detailing
Orion Documentation page 283
Next a beam detail sheet is created for all the beams (i.e. all the beams on this floor) and the option to include a quantity table is checked.
Clearly this sheet is not intended for printing, it is a quick way to get a lot of details into one DXF file which a detailer can then organise onto several drawings for final editing and then issue.
Orion Documentation page 284
Chapter 18 : Beam Detailing
Notice that at the top corner of the sheet there is a schedule that can be seen in partial close up below.
At the top there is a summary of reinforcement used, below that the data is broken down for each bar position (each bar mark) in two columns. For this initial design of all the beams on this floor the key points to be noted are: • Total Reinforcement Weight – 3246 kg
• Total bar marks used – 134 Effects of Applying Preferences We will now go on to examine how the details are simplified as we apply a range of preferences. As we do so we will also take note of how this adds reinforcement weight when compared with the above data for the initial design.
Chapter 18 : Beam Detailing
Orion Documentation page 285
Bar Spacing Maximisation
As noted earlier, the option to minimise bar sizes is not often used since this results in lots of small bars being used at close centres rather than a few larger bars at wider spacing. If we change to the more usual bar spacing maximisation method as shown above the weight of steel changes as shown below.
The weight of reinforcement increases to 3467 kg but the number of bar marks reduces to 121. Also note how the weight of H25 has gone up from 505 to 1130 kg while the H12 have dropped from 930 to 285 kg. In this case the weight goes up because the As provided (and hence weight used) changes in bigger steps. For example if As required is 510 mm 2 then the size minimisation method will pick 5 H12 (As = 566 mm 2 ), while the spacing maximisation method might pick 2 H20 (As = 628 mm 2 ). P
P
P
P
P
P
We would emphasise again that the size minimisation method is not generally used and so in the closing summary we have not used it as the base line figure for percentage comparisons.
Orion Documentation page 286
Chapter 18 : Beam Detailing
Limiting the Bar Range
This option is probably under utilised. In this example we will squeeze the maximum main bar diameter which can be used down to H20 from H25. Basically this means that in situations where 2 H25s were chosen we will now get 3 H20, etc. In addition there will be some saving due to shorter laps.
The weight of reinforcement decreases to 3324 kg and the number of bar marks decreases by 1 to 120.
Chapter 18 : Beam Detailing
Orion Documentation page 287
Note
If you try to restrict the maximum bar size too much lots of beams will start to appear as failed on the beam design summary – see below.
The beams have a utilisation ratio of less that 1 but are being shown as failing because the smaller bars (maximum size H16) at closer spacing are failing checks on minimum spacing. If the failing beams are filtered out and redesigned with the maximum bar size increased to H20 then in this model the weight of reinforcement can actually be optimised to 3234 kg but at the expense of going back up to 136 bar marks. Ideally you probably want to use a range where the vast majority of beams pass with one setting and then the remainder can be filtered out and will pass in a second run with a different setting. The degree to which this would be necessary would be quite structure dependent.
Orion Documentation page 288
Chapter 18 : Beam Detailing
Merging Bars
This option is widely used. Where bar groups of the same size meet they will be merged into one bar group in preference to lapping (provided the total bar length is less that the specified maximum bar length in the parameters tab). Therefore this option will reduce weight due to savings in laps but will result in the handling and fixing of longer bars on site.
In this example the weight of reinforcement decreases to 3272 kg and the number of bar marks decreases to 109. As we continue to simplify the detailing, later in the exercise this option will start to have a greater effect.
Chapter 18 : Beam Detailing
Orion Documentation page 289
Minimum Tension Lap It is common practise to standardise laps. This can be done by changing the the minimum Tension Lap Factor from 1.0 to 1.4. The result of this change is that where the code minimum of 1.0 could be used then your user over-ride applies - 1.4 gets used throughout.
The weight of reinforcement increases to 3348 kg - the number of bar marks remains the same at 109.
Orion Documentation page 290
Chapter 18 : Beam Detailing
Stop Using 2nd Support and Span Bars In the bar selection tab settings shown earlier, all the options relating to 2nd support and span bars were activated. The view below shows the sort of detail that this produces.
Note the shorter top support bars and the extra bottom span bars laid in to the central section of the span. If the options are now deactivated as shown below, the detailing becomes simpler.
Chapter 18 : Beam Detailing
Orion Documentation page 291
Notice how only one group of top support bars is used and that in the span all the bars extend full length of the span.
The weight of reinforcement increases to 3614 kg and the number of bar marks drops to 86.
Orion Documentation page 292
Chapter 18 : Beam Detailing
Extend and Merge End Bars
In the bar curtailment tab above all the options to extend the top and bottom span bars right in to the support have been activated. The options not to use any extra (shorter) bars locally at the end support are also activated. The view below shows the simplified detail that results in an end span.
Chapter 18 : Beam Detailing
Orion Documentation page 293
Comparing the view above with the view below (that was obtained when the end support curtailment options were all unchecked): • In the top view the H20 L bars in the top of the beam are extended and the short H16 span bars disappear.
• the bars in the bottom of the beam are unchanged - this option will not really have any effect on the bottom bars unless there are significant wind/sway moments that cause more steel to be required at the support than in the span.
The weight of reinforcement drops slightly to 3608 kg and the number of bar marks drops to 82.
Orion Documentation page 294
Chapter 18 : Beam Detailing
Extend Support Bars Symmetrically
In the bar curtailment tab above the option to extend the support bars symmetrically is activated. In the view below you can see how support bars are extended to a proportion of the span length (default 0.3 L may be adjusted to 0.25 L in line with BS8110 simplified detailing rules – the top span bars will by default be designed for the maximum hogging moment in the central section of the beam – see the design tab settings.)
When the bars are extended symmetrically the detail is arguably simpler (as shown below) with less potential for misplacement when fixing on site. However it is worth noting that this option will become impractical when the adjacent span lengths vary dramatically – it can mean that the top support bars start running right through the short span.
Chapter 18 : Beam Detailing
Orion Documentation page 295
The weight of reinforcement increases to 3640 kg and the number of bar marks remains unchanged at 82. Standardise Link Size
Orion Documentation page 296
Chapter 18 : Beam Detailing
General Note – When Orion is asked to Design Link Spacing at Supports it will check the span region and each support region. If it finds that the reinforcement required in either support region is the same as in the span region those two regions will be merged. The example below shows this case.
In the links tab above two additional options have been activated: • Select Symmetrical Links for Support Regions – This option means that when support regions are required at both ends the bar size (but not the same spacing) used at each end will be the same.
• Same Bar Size at Support and Span – This option means that the same bar size (but not the same spacing) will be used throughout the span of each beam.
Chapter 18 : Beam Detailing
Orion Documentation page 297
In the view above you can see how the second of these settings has forced the use of H10 links throughout this beam, but since the maximum link spacing is 300 mm this just means extra reinforcement weight. In the particular model being used here H8 bars were used almost everywhere, so the above two settings make almost no difference to the total steel quantity.
The weight of reinforcement increases to 3644 kg and the number of bar marks is unchanged at 82. On a different project it is quite likely that applying these settings will have a bigger effect on reinforcement weight.
Orion Documentation page 298
Chapter 18 : Beam Detailing
Uniform Links
The option to Design Link Spacing at Supports is not checked.
In the view above you can see how one link size and spacing is now used throughout this span. Applying this preference to this model causes a reasonably significant increase in reinforcement weight.
The weight of reinforcement increases to 3737 kg and the number of bar marks is still unchanged at 82.
Chapter 18 : Beam Detailing
Orion Documentation page 299
Summary In the detailed example in the previous section we have successively applied more and more bar selection and curtailment settings preferences and recorded how these changes affect the total weight of reinforcement in an entire floor. The following table summarises the findings.
Cumulative Adjustments
No of Bar Marks Used
Total Weight kg
Percentage Weight Added
Starting Model using Bar Size Minimisation
134
3246
-6%
Bar Spacing Maximisation
121
3467
Base Line
Limiting the Bar Range
120
3324
-4%
Merging Bars
109
3272
-6%
Standardise Laps
109
3348
-3%
Stop Using 2nd Support and Span Bars
86
3614
+4%
Extend and Merge End Bars
82
3608
+4%
Extend Support Bars Symmetrically
82
3640
+5%
Standardise Link Size
82
3644
+5%
Use Uniform Links
82
3737
+8%
We would suggest the main points to be drawn from this example are: 1. Most Orion users will use the Bar Spacing Maximisation Method hence the use of the 3467 kg weight as the base line in the above table. 2. Most users will probably not have examined the possibilities of limiting the preferred range of bars used in which case potential efficiencies may have been missed (in this case a saving of 4% in reinforcement weight). 3. If the weight after Limiting the Bar Range (3324 kg) were used as the base line figure then the net effect of introducing all the standardisation/simplification up to and including the option to Standardise Link Size is: • The number of bar marks reduces from 120 to 82 (more than 30% reduction)
• The weight increases to 3737 kg (a 12.5% increase) 4. We would suggest that the option to use Uniform Links throughout is used cautiously; it can add a lot of weight. 5. If the bar range is not so limited, but all the other preferences up to the option to Standardise Link Size are still applied, the total weight goes up from 3644 kg to: • If T25s are allowed – 3841 kg (an extra 5.5%)
• If T32s are allowed – 3927 kg (an extra 8%)
Orion Documentation page 300
Chapter 18 : Beam Detailing
Based on this example, the most important message is therefore the need to examine the effects of reducing the maximum size of bar to be used in the design. Finally, it is worth re-emphasising that all of these sorts of judgements have traditionally been made on something of a gut feel basis during the course of design and detailing. The automated procedures that Orion provides allow you to investigate options, quantify effects, and optimise the design for any project in a matter of minutes or perhaps an hour.
Chapter 19 : Column Design to BS8110
Chapter 19
Orion Documentation page 301
Column Design to BS8110
Introduction By default, Orion designs columns bent about a single axis, or bent about both axes using the code clauses given in BS 8110-1:1997: Part 1 Section 3.8. The following table summarises the various stages of the BS8110 column design process:
Step
Calculation
Clause
1.
Braced or unbraced?
3.8.1.5
2.
Calculate effective height using Part 2 of the code
3.8.1.6
3.
Check slenderness
3.8.1.3
4.
Classify as short or slender
3.8.1.3
5.
If slender - calculate Madd
3.8.3.1
6.
Calculate minimum moments
3.8.2.4
7a.
If braced - calculate design moments
3.8.3.2
7b.
If unbraced - calculate design moments
3.8.3.7
If using the BS8110 design method 8.
Calculate equivalent uni-axial design moments
3.8.4.5
(If using the Bi-Axial design method skip this stage) 9.
Member design
3.8.4
As indicated in the table, the program provides two design methods. The default method applies Cl 3.8.4.5 to convert bi-axial moments into an equivalent uni-axial design moment. Alternatively if the bi-axial design method is selected, the bi-axial moments calculated in step 7 are fed directly into the member design stage and a more rigorous solution technique developed from first principles is adopted. This can produce some economy, however because the neutral axis will lie on an incline the results of the design process will be more difficult to cross check.
Orion Documentation page 302
Chapter 19 : Column Design to BS8110
For poly-line columns, as shown below, the bi-axial design method will always be adopted. .
The choice of design method is set via the column design settings dialog shown below..
The BS8110 Column Design Process 1. Braced or unbraced - Cl 3.8.1.5 Globally, columns will be considered as braced if this option has been selected in the Building Parameters. Individual columns can have their braced/unbraced status modified within the
Chapter 19 : Column Design to BS8110
Orion Documentation page 303
Column Interactive Design, via the Slenderness tab as shown below:
For walls, the braced/unbraced status is applied in the same way as it is for columns. However, it should be noted that walls can always be considered as braced along their major axis (i.e dir 1).
2. Calculate effective height- Cl 3.8.1.6 The effective height is determined from the equation: le = lo A rigorous assessment of the effective length is undertaken using the formulae given in 2.5 of BS 8110-2:1985. Perhaps surprisingly, this can often result in a greater effective length than is determined from the Tables 3.19 and 3.20 of BS 8110-1:1997. Note: The beta value determined by part 2 can be edited and replaced by the value from the tables if required.
3. Check slenderness limits- Cl 3.8.1.7 & 3.8.1.8 An error message is displayed during the interactive design if the slenderness limit has been exceeded
4. Classify as short or slender- Cl 3.8.1.3 Columns and walls are considered as short when both the ratios lex/h and ley / b are less than 15 (braced) and 10 (unbraced), otherwise they are slender.
5. If slender - calculate M_add- Cl 3.8.3.1 In order to calculate the additional moment induced in the column it is required that factor K be determined. Although the code allows for K to be conservatively taken as 1.0, Orion calculates K using the equation 33 in the code: K = (Nuz - N) / (Nuz - Nbal) = 0.4 M2
7b. If unbraced, calculate design moments about each axis - Cl 3.8.3.7 The design moment is calculated in both directions as per figure 3.21
8. Calculate equivalent uni-axial design moments - Cl 3.8.4.5 Because there will always be at least a minimum moment acting in both directions, for rectangular columns the design moment will always be determined from equations 40 and 41 in the code. For Mx/h' >= My/b' Mx' = Mx + (beta * h'/ b' * My) For Mx/h' < My/b'
Chapter 19 : Column Design to BS8110
Orion Documentation page 305
My' = My + (beta * b'/ h' * Mx) For circular columns the moments in the two directions are resolved. M =
2
2
Mx + My
9. Member Design - Cl 3.8.4 With the design axial load and design moment established, the program determines the required steel area using the BS8110 stress block. The neutral axis of the cross section is determined and a bar size and spacing obtained to provide sufficient moment capacity. Each design combination is considered and the one that results in the largest steel area requirement is selected as being critical. Note
If the minimum area of steel is satisfactory for every combination, the program will record combination 1 as being critical, ( irrespective of the relative magnitude of loads in each combiation) .
Three methods of bar selection are available:
Fixed bar layout — The bar locations are defined by the user and the program determines the bar size required Bar Spacing Maximisation — The program determines the bar size and spacing with the aim to maximise the spacing. This is normally the preferred option. Bar Size Minimisation — The program determines the bar size and spacing with the aim to minimise the bar size.
Orion Documentation page 306
Chapter 19 : Column Design to BS8110
The maximum axial load is checked against Cl 3.8.4.3 or Cl 3.8.4.4. The program defaults to the more conservative capacity determined by Cl 3.8.4.4. The clause used can be changed via the BS8110 tab of the column design settings as shown:
Worked Examples The Design Model The example model Doc_Example_4 is opened and saved to a new name (so as not to destroy the original example). The copied model is then adjusted so that its storey height is increased to 5.5m and it is then re-analysed. In this model the steel grade is 460 and the steel material factor is 1.15. Bar diameters of 13mm are used and 12mm bar diameters are excluded. The minimum link diameter is set to 10mm. Note however, the actual design process would be identical irrespective of which steel grade and material factor and bar diameters are used.
Chapter 19 : Column Design to BS8110
Column Design Settings The column design settings initially adopted are as shown:
Design Parameters
Steel Bars - Layout/Selection
Orion Documentation page 307
Orion Documentation page 308
Steel Bars - Longitudinal Steel
Steel Bars - Links
Chapter 19 : Column Design to BS8110
Chapter 19 : Column Design to BS8110
Orion Documentation page 309
Braced Rectangular Column Example Column 1C8 will be used to demonstrate the design process for a rectangular column.
column dimension in direction 1b1 = 500 mm column dimension in direction 2b2 = 250 mm The clear height of column in the two directions takes account of the beams framing in to the top of the column. Lo1 = 5500mm - 500mm = 5000 mm Lo2 = 5500mm - 400mm = 5100 mm Note
As shown on the design screen above, if only 3 bars are placed in the x direction the default clear bar spacing limit of 200mm (as specified in the Column Design Settings) would be slightly exceeded. In the worked examples the Max. Column Steel Bar Spacing has been relaxed to 205mm in order that the above bar layout can be used.
Orion Documentation page 310
Chapter 19 : Column Design to BS8110
Performing the Design To perform the calculations click the Design button. This will design the column for all the highlighted design combinations. Design combination 1 is found to be critical and is highlighted in red in the table as shown below.
Each stage of this design process will now examined in detail.
1. Braced or unbraced - Cl 3.8.1.5 In this example the column has been defined as braced in both directions. This can be confirmed by clicking on the Slenderness tab.
Chapter 19 : Column Design to BS8110
Orion Documentation page 311
2. Calculate effective height- Cl 3.8.1.6 The effective length factors 1 and 2 that have been calculated are also displayed on the Slenderness tab as shown.
These effective length factors are calculated as follows:
In direction 1 Beam stiffness at top of column L = 5500mm b= 250mm d = 500mm kb1 =b*d3/(12* L) = 473484.8 mm3 Column stiffness kc1 = b2*b13/(12 * Lo1) = 520833.3 mm3 calculation using the formulae given in 2.5 of BS 8110-2:1985 c2 = kc1 / kb1 = 1.100 c1 = 1 (fixed base is defined in this example) cmin = min ( c1, c2 ) =1.000 eq3 = 0.7 +0.05 *( c1 + c2) = 0.805 eq4 = 0.85 +0.05 *( cmin) = 0.900 1 = min (eq3,eq4) = 0.805
In direction 2 Beam stiffness at top of column L =4250mm b= 250mm d = 400mm kb1 =b*d3/(12* L) = 313725.5 mm3 Column stiffness kc1 = b1*b23/(12* Lo1) = 130208.3 mm3
Orion Documentation page 312
Chapter 19 : Column Design to BS8110
Calculation using the formulae given in 2.5 of BS 8110-2:1985 c2 = kc1 / kb1 = 0.415 c1 = 1 (fixed base is defined in this example) cmin = min ( c1, c2 ) =0.415 eq3 = 0.7 +0.05 *( c1 + c2) = 0.771 eq4 = 0.85 +0.05 *( cmin) = 0.871 2 = min (eq3,eq4) = 0.771
Effective member lengths Le1 = 1 * Lo1= 4025 mm Le2 = 2 * Lo2 = 3931 mm
3. Check slenderness limits - Cl 3.8.1.7 & 3.8.1.8 60 * 250mm = 15000mn OK
4. Classify as short or slender - Cl 3.8.1.3 The classification is shown on the Column Reinforcement Design dialog.
5. If slender - calculate Madd- Cl 3.8.3.1 Depending on the classification the a and Madd values have been calculated accordingly and are displayed on the Slenderness tab.
In direction 1 a1 = 1/2000 * ( 1 * Lo1 /b1)2 = 0.032
Chapter 19 : Column Design to BS8110
Orion Documentation page 313
column is not slender, hence Madd1 = 0 kNm
In direction 2 a2 = 1/2000 * ( 2 * Lo2 /b2)2 = 0.123 Column is slender, hence: Calculate Madd K = (Nuz - N) / (Nuz - Nbal) = 0.4 M21 Mi1 = 0.4*M11 + 0.6*M21 = 23.17 kNm 0.4 * M21 = 22.96 kNm Hence Mi1 = 23.17 kNm Md1eff = Max (abs(M21),abs(Mi1+Madd1),abs(M11+Madd1/2), M1min) = 57.41 kNm
In direction 2 Smaller end momentM12 = 7.85 kNm Larger end momentM22 = -15.77 kNm Mi2 = 0.4 * M12 + 0.6* M22 >= 0.4 M21
Chapter 19 : Column Design to BS8110
Orion Documentation page 315
Mi2 = 0.4*M12 + 0.6*M22 = -6.32 kNm 0.4 * M22 = -6.31 kNm Hence Mi2 = -6.32 kNm Md2eff = Max (abs(M22),abs(Mi2 +Madd2),abs(M12+Madd2/2), M2min) = 15.77 kNm
8. Calculate equivalent uni-axial design moments - Cl 3.8.4.5 The effective design moment is calculated from equations 40 and 41
longitudinal bar dia = 13mm cover = 20mm links = 10mm Mx = Md2eff = 15.77 kNm My = Md1eff = 57.41 kNm h' = b2 - cover - links - dia/2 = 213.5 mm b' = b1 - cover - links - dia/2 = 463.5 mm Mx/h' = 73.9 kN My/b' =123.9 kN Mx/h' < My/b' hence My' = My + ( * b'/ h' * Mx) N/(b1*b2*fcu ) = 0.030 = 0.961 My' = My + ( * b'/ h' * Mx) = 90.3 kNm
Orion Documentation page 316
9. Member Design - Cl 3.8.4 Design forces N = 151.75kN My' = 90.39kNm Mx' = 0.00kNm
Solution determined by Orion:
Distance to neutral axisYbar = 64.84 mm Area of steel requiredAsreq = 650.8 mm2 Area of steel providedAsprov = 796.4 mm2
Reinforcement in Section
X1 = cover +links + dia/2 = 36.5 mm X2= 500mm /2 = 250.0 mm X3= 500mm - cover - links - dia/2 = 463.5 mm Ybar > X1, hence bars at X1 are in compression
Chapter 19 : Column Design to BS8110
Chapter 19 : Column Design to BS8110
Orion Documentation page 317
Calculate maximum axial load - Cl 3.8.4.3 or Cl 3.8.4.4.
In this example the design ultimate axial load is determined using Cl. 3.8.4.3. Nmax = 0.4*fcu * b1 * b2 + 0.75 * Asprov* fy = 2274.8 kN > N OK
Cross check of the above solution The solution can be cross checked using two basic equations given in standard texts. For example: W.H. Mosley and J.H. Bungey, Reinforced Concrete Design, (MacMillan)
1. Resolving forces vertically: N = FCC + FST + FSC Where: Fcc is Concrete compressive force FST is Steel tensile force FSC is Steel compressive force 2. Taking moments about mid-depth of section (should equate to zero): The applied moment My' must be balanced by the moment of resistance of the forces developed within the cross section.
Resolving forces vertically: N = FCC + FST + FSC Bars in tension are fully stressed, hence Total Tensile force in bars at X2 and X3
Orion Documentation page 318
Chapter 19 : Column Design to BS8110
FST = -4*Asreq/6 * 460N/mm2 /1.15 = -173.56kN Compressive force in concrete, using the BS8110 rectangular stress block FCC = 0.67/1.5 * fcu * (0.9*Ybar*h) = 260.67 kN Total compressive force in bars at X1 FSC = N - FST - FCC = 64.69 kN
Taking moments about mid-depth of section (should equate to zero): Distance to centre of concrete compression force XCC = 500mm/2 - 0.9*Ybar/2 = 220.8 mm Distance to centre of steel compression force XSC =500mm/2 - X1= 213.5 mm Distance to centre of steel tension force XST =(X3+X2)/2 -500mm/2 = 106.8 mm My' - XCC *FCC + XST*FST - XSC*FSC = 0.5 kNm The right hand side of the above equation should equate to zero to within an acceptable tolerance. To determine if this result is OK, recalculate the actual value of My' required for this to be the case and then compare the two. My'actual = XCC *FCC - XST*FST + XSC*FSC = 89.90 kNm My' / My'actual = 1.005 OK Note that the above cross check shows that if the Asrequired was actually the amount provided then the required capacity is just sufficient. In all cases Asprov will exceed Asreq to some degree. Orion reports the ratio: Asreq/Asprov as the utilisation ratio. Utilisation Ratio Asreq /Asprov = 0.82
Chapter 19 : Column Design to BS8110
Orion Documentation page 319
It is important to appreciate that 82% utilisation does not mean that 18% more load can be added. As is shown on the interaction diagram for this column, a great deal more axial load could be added.
Bi-Axial Design Method Example From the Column Design Settings dialog, change the design method to bi-axial.
Orion Documentation page 320
Chapter 19 : Column Design to BS8110
Then re-design column 1C8 once more.
It is important to note that design stages 1 to 7 are identical to the previous example, hence the effective design moments about each axis are unchanged: Md1eff = Max (abs(M21),abs(Mi1+Madd1),abs(M11+Madd1/2), M1min) = 57.41 kNm Md2eff = Max (abs(M22),abs(Mi2+Madd2),abs(M12+Madd2/2), M2min) = 15.77 kNm Instead of converting these to a uni-axial design moment (as per stage 8), an exact solution is determined using the bi-axial moments. The result is that the area of steel required drops from 650.8mm2 to 328.8mm2. Thus, this design method can obviously be seen to provide a more economical solution. The drawback is that because the neutral axis is no longer parallel to either face of the column verification is more difficult. The cross checks required do not lend themselves to hand calculation.
Chapter 19 : Column Design to BS8110
Orion Documentation page 321
Braced Circular Column Example From the Column Design Settings dialog, change the design method to BS8110-Cl.3.8.4.5.. Column 1C12 will be used to demonstrate the design process for a circular column.
Click on the Parameters button and change to fixed bar layout.
This will force the design to adopt the number of bars shown in the 'Qty' cell of the above table. In this example we will use 8 bars in the design, (Qty = 8). When the design is performed, the bar sizes will be adjusted to obtain an economic solution based on this layout. column diameterD = 500 mm The clear height of column in the two directions takes account of the beams framing in to the top of the column. Lo1 = 5500mm - 500mm = 5000mm Lo2 = 5500mm - 400mm = 5100mm
Orion Documentation page 322
Chapter 19 : Column Design to BS8110
1. Braced or unbraced - Cl 3.8.1.5
All the columns in this building should be considered as braced. If the column is designed as such, the effective length factors in the two directions are calculated as: 1 = 0.846 2 = 0.900
2. Calculate effective height- Cl 3.8.1.6 The values of 1 and 2 are calculated in the same way as in the previous rectangular column example. The full calculations will therefore not be repeated here. The column slenderness is calculated as: Le1 = 1 * Lo1= 4230 mm Le2 = 2 * Lo2 = 4590 mm
3. Check slenderness limits - Cl 3.8.1.7 & 3.8.1.8 60 * D = 30000 mm OK
Chapter 19 : Column Design to BS8110
Orion Documentation page 323
4. Classify as short or slender - Cl 3.8.1.3
5. If slender - calculate Madd- Cl 3.8.3.1
Short hence: Additional MomentMadd1 = 0 kNm Additional MomentMadd2 = 0 kNm
6. Calculate minimum moments - Cl 3.8.2.4
Applied Axial Load, N = 243.4 kN Minimum eccentricity, emin = min (0.05* D, 20mm) = 20 mm Mmin = N * emin = 4.87 kNm Note that when braced, combination 2 is identified as the critical combination.
Orion Documentation page 324
7. Calculate design moments about each axis - Cl 3.8.3.2
In direction 1 Smaller end momentM11 = -19.97 kNm Larger end momentM21 = 40.56 kNm As the column is short the effective moment is simply: Md1eff = Max (abs(M21), Mmin) = 40.56 kNm
In direction 2 Smaller end momentM12 = -39.85 kNm Larger end momentM22 = 80.60 kNm Md2eff = Max (abs(M22), Mmin) = 80.60 kNm
8. Calculate equivalent uni-axial design moments - Cl 3.8.4.5
Mx = Md2eff = 80.60 kNm My = Md1eff = 40.56 kNm Mx' = squareroot (Mx2 + My2) = 90.23 kNm
Chapter 19 : Column Design to BS8110
Chapter 19 : Column Design to BS8110
Orion Documentation page 325
9. Member Design - Cl 3.8.4 Solution determined by
Distance to neutral axisYbar = 88.30 mm Area of steel requiredAs = 520.3 mm2 Area of steel providedAs_prov = 1061.9 mm2 No. of bars providednbar = 8 Before cross checking this solution for equilibrium, we will first make the column unbraced.
Unbraced Circular Column Example The previous calculations are now repeated with the column specified as unbraced.
1. Braced or unbraced - Cl 3.8.1.5 To change the column to unbraced, uncheck the 'Dir 1 and Dir 2 braced' boxes on the slenderness tab as shown below and the click the Design button:
Orion Documentation page 326
Chapter 19 : Column Design to BS8110
When unbraced, the effective length factors in the two directions for this column change to: 1 = 1.439 2 = 1.684
2. Calculate effective height- Cl 3.8.1.6 When considered unbraced, the column slenderness is calculated as: Le1 = 1 * Lo1= 7195 mm Le2 = 2 * Lo2 = 8588 mm
3. Check slenderness limits - Cl 3.8.1.7 & 3.8.1.8 60 * D = 30000 mm OK
4. Classify as short or slender - Cl 3.8.1.3
5. If slender - calculate Madd - Cl 3.8.3.1
In direction 1 a1 = 1/2000 * ( 1 * Lo1 /D)2 = 0.104
In direction 2 a2 = 1/2000 * ( 2 * Lo2 /D)2 = 0.148
Chapter 19 : Column Design to BS8110
Orion Documentation page 327
Calculate Madd K = (Nuz - N) / (Nuz - Nbal) (250mm -X2), hence bars at X1 and X2 are in compression
Resolving forces vertically: N = FCC + FST + FSC Where: FCC is Concrete compressive force FST is Steel tensile force FSC is Steel compressive force Bars in tension are fully stressed, hence Total Tensile force in bars at X3, X4 and X5 FST = -5* Asreq/nbar * 460N/mm2 /1.15 = -211.3kN Tensile force per bar at X3, X4 and X5 FSTbar = FST /5 = -42.3kN
The area of concrete in compression (the shaded area above) is determined from the equation: A = R2 tan-1 (squareroot (R/r)2 -1) - r (squareroot (R2 - r2) R = D/2 = 250 mm r = R - Ybar = 146 mm A = 29568 mm2
Chapter 19 : Column Design to BS8110
Orion Documentation page 331
Compressive force in concrete, using the BS8110 rectangular stress block FCC = 0.67/1.5 * fcu * (0.9*A) = 475.5 kN Total compressive force in bars at X1and X2 FSC = N - FST - FCC = 20.9 kN Compressive force per bar at X1 and X2 FSCbar = FSC /3 = 7.0 kN
Taking moments about mid-depth of section (should equate to zero): For this hand calculation it has been assumed that the centre of concrete compression force is at 2Ybar/3 from the top of the section. The software would of course perform a rigourous calculation to determine the exact position of the centre of concrete compression force. Distance to centre of compression force XCC = R - 2*Ybar/3 = 180.7 mm My'actual = XCC *FCC + X1*FSCbar + 2*X2*FSCbar - 2*X4*FSTbar - X5*FSTbar = 121.0 kNm
Orion Documentation page 332
My' / My'actual = 0.98 OK Utilisation Ratio Asreq /Asprov = 0.80 OK
Chapter 19 : Column Design to BS8110
Chapter 20 : Wall Design and Detailing
Chapter 20
Orion Documentation page 333
Wall Design and Detailing
Introduction Orion designs walls in the same way as columns, therefore for general details of the design process refer to the previous chapter - Column Design to BS8110. In this chapter additional aspects of the process that are unique to walls are examined.
Conservatism in the design method The analysis phase described inthe chapter Wall Modelling Considerations determines Wall Panel Design Forces. Each wall panel is then independently designed for these forces. Therefore, when Orion designs a wall panel it is designing a discrete rectangular section. In the cases of Core Walls (where there are several interconnected panels), this design approach will be conservative for two reasons: 1. Core walls introduce flanges, and just as for beams these flanges will enhance the bending capacity when they are considered (and when they act in compression). The concrete stress block develops over a reduced depth and so a bigger lever arm develops between the concrete and steel forces, hence greater moment capacity. 2. As is shown below, the detailing where panels overlap must be manually edited so that the sum of the requirements for each panel are provided in the overlapping zone. Where the critical design case determining the requirements in each panel is different it will be conservative to provide the sum of the two.
Wall Design and Detailing Options As with columns, preferences can be applied to the design and there are a number of aspects of this that are worth highlighting.
Design With End Zones Choosing to use End Zones makes a significant difference to the way reinforcement is provided within the section - heavier bars will be concentrated in a zone at each end of the wall and smaller bars will be used in the mid section.
Orion Documentation page 334
Chapter 20 : Wall Design and Detailing
In the column/wall design settings you can choose between shear wall reinforcing styles – shear walls with or without End Zones.
The column application plan view below shows the reinforcing for wall panels when end zones are requested. Note that there is significant overlapping of the bars where the panels overlap.
Chapter 20 : Wall Design and Detailing
Orion Documentation page 335
As noted earlier, this is one of the areas where manual adjustment of the detailing is required. The detailer needs to look at the total reinforcement required in the overlapping zone and provide the same total in a revised corner detail. For example, at the corner of walls 1 and 2, wall 1 has 7 T20 bars and wall 2 has 13 T16, the detailer might change this to 10 T25. As noted previously this summing of the individual panel requirements is a safe engineering approach.
Design Without End Zones
The view above shows the column application plan view after redesigning all the walls having changed to the design setting where End Zones are not considered. When end zones are not used the same bar size will be used throughout the length of the wall with the exception of the end corner bars which may be larger. Once again there is a certain amount of overlapping reinforcing between wall panels that needs to be rationalised, but this tends to be a simpler process where end zones are not used.
Should I use End Zones? The subject of zoned reinforcement (or boundary elements) tends to be dealt with under the heading of Earthquake Resistance in building codes and texts. However it is clearly logical that zoning of reinforcement in the ends of wall panels is a valid and potentially more efficient approach to wall design wherever lateral loading is generating significant in-plane moments. The main reason why the option not to consider end zones becomes more efficient at lower loading levels is that minimum steel requirements start to dominate. Within Orion, when zoned reinforcement is used then minimum reinforcing requirements within the zones are applied. These minimum levels mean that the overall minimum reinforcing requirement for the wall is around 0.52%. When end zones are not used the standard BS8110 code limit of 0.4% is applied.
Orion Documentation page 336
Chapter 20 : Wall Design and Detailing
Note
Even when end zones are not used Orion will still tend to suggest a larger bar size at the extreme ends of the wall. Sometimes this might be needed because it increases the supplied steel just enough to exceed the minimum steel requirement. Sometimes it may also help to satisfy detailing (maximum spacing) requirements across the end of the wall. However Orion will often provide larger bars here for no other reason than it is considered Good Practice. In the case of intersecting walls it is particularly felt that this is good practice. However, it is acknowledged that some engineers would prefer to use a single main bar size throughout and the option to set this preference will be added in a future release of Orion. For now it is possible to interactively adjust the sizes of the end bars to achieve this objective.
The British Standard does not really acknowledge or provide guidance on End Zones. It is written from the point of view that walls will be quite uniformly reinforced, hence many Orion users will prefer to use the option NOT to consider end zones. As is demonstrated in the above example, for many structures this will actually give a lesser reinforcing requirement. The only argument that can be set against this approach is in the case where the walls are actually resisting significant in-plane moments. In such cases not only does the end zone approach become more efficient, it also puts the steel where the strain is and so potentially limits cracking more efficiently.
Plain Wall Design In the column/wall design settings you can choose to Use Plain Wall Design where it is applicable.
BS8110 cl 1.3.4.7 defines a plain wall as one that contains either no reinforcement, or insufficient to satisfy the criteria in cl 3.12.5. Any reinforcement supplied is ignored when considering the strength of the wall. Where no reinforcement is required in the design of the wall cl 3.9.4.19 indicates that if reinforcement is provided at all, it should be at least 0.25% (for Gr460/500 steel) and 0.3% (for Gr250 steel) Therefore, if design can be based on no reinforcement and this indicates a capacity in excess of that required for the design combinations, a lower minimum reinforcement level can be used. Note
Where reinforcement is less than 2% design will automatically exclude ties.
Chapter 20 : Wall Design and Detailing
Orion Documentation page 337
Orion will determine if a wall section is in compression throughout for each applied load combination. When compression throughout is proven then design will utilise this lower minimum steel provision. The "detailed design report" option includes a statement that wall is in compression throughout and that plain wall design applies as shown below:
Although plain wall design applies, the capacity of the section (and hence the utilisation ratios) are still based on the provided area of steel. The final calculations, interaction curves, etc relate to design of the section using the actual supplied reinforcement. These calculations also show the NA position calculated for the critical combination.
This NA position effectively reflects the theoretical capacity moment for the given axial load and the required reinforcement. Therefore this may often indicate that the NA lies in the section hence implying that the section is not in compression throughout - when designed as a plain wall this is a false implication and can be ignored.
Orion Documentation page 338
Chapter 20 : Wall Design and Detailing
Option to use Single Layer of Reinforcement This option may be appropriate in thinner walls. In the column/wall design settings you can use the Max. Width for Single Layer Walls box to specify the maximum thickness of wall in which single layer of reinforcement is to be used.
The single layer may be either mesh or loose bars and it may be used in conjunction with end zone reinforcement if required.
Chapter 20 : Wall Design and Detailing
Orion Documentation page 339
Design with Mesh Reinforcement The option to consider use of mesh reinforcement in walls is available and initially controlled via Material Property Settings.
In the column/wall design settings you can then set preferences relating to mesh reinforcement. The settings shown below would be quite reasonable in this regard.
Orion Documentation page 340
Chapter 20 : Wall Design and Detailing
Note the Min. Wall Length for Mesh Steel - a wall of this length or greater will attempt to adopt mesh reinforcement - provided you have chosen to use a fabric for wall web steel in the material properties dialog. You should set the mesh width to that of the standard panel width you intend to use. The "Revert to Loose Bar" Option The option to use loose bars (shown checked above) applies during the main steel design routines when mesh has been specified. If no mesh can be found with sufficient main steel reinforcement to meet the design requirements then the wall is set to use loose bar and re-designed on that basis. How the "Revert to Loose Bar" Option Works In the properties for any wall, the Web Steel (Longitudinal) property will initially be set as "default". The default setting results in loose bars being used if the wall in question is shorter than the minimum length for mesh steel with mesh steel being used for longer walls. However, in the latter case, if it is determined that no available mesh is adequate and the "Revert to Loose Bar" option has been checked then the “Web Steel” property for the wall be re-set to a relevant non-default value as shown below.
Once this is done then loose bar will always be selected for that wall until the property is re-set back to default. Re-setting these properties would be tedious if done interactively so when running batch design we have added the option to "Try Using Mesh in all Walls". When this is checked the properties are re-set to default before the batch design and so if the design forces have changed mesh could be selected in a wall where loose bar has previously been selected. Limitation of "Revert to Loose Bar" Option The design will not revert to loose bars in the following cases: 1. If the main bars of a mesh provide sufficient main reinforcement but subsequently it is found that lateral bars do not provide sufficient transverse reinforcement, then the design will show the wall as failed and will not automatically revert to using loose bar. Generally this possibility only applies to B series meshes in thicker walls (as indicated by the * in the Minimum Reinforcement Requirement tables below).
Chapter 20 : Wall Design and Detailing
Orion Documentation page 341
2. If the main bars of a mesh provide sufficient main reinforcement to meet combined Axial load and bending, but subsequently it is found that the limiting axial load is not sufficient, then the design will show the wall as failed and will not automatically revert to using loose bar. This requires walls to be subject to unusually high axial loads and should occur very rarely. Column Steel Details View The view below shows the column steel details view for a wall with mesh reinforcement. This view is obtained from right mouse click menu within the column application plan view.
Mesh and Minimum Reinforcement Requirements It is important to appreciate that standard meshes will not be suitable in all walls, in many cases meshes will not even provide the required minimum levels of reinforcement required by the code. The tables below should help clarify this.
• Plain walls - min horizontal and vertical reinforcement is 0.25% (basically an anti-crack provision)
• Reinforced Walls - min vertical reinforcement increases to 0.4%. These minimums dictate the extent to which meshes can ever be used in wall design.
Orion Documentation page 342
Chapter 20 : Wall Design and Detailing
Minimum Meshes in Walls with a Single Layer of Reinforcement Wall Thickness
Plain Walls
Reinforced Walls
A series mesh
B series mesh
A series mesh
B series mesh
125mm
A393
*
-
*
150mm
A393
*
-
*
175mm
-
*
-
*
200mm
-
*
-
*
Minimum Meshes in Walls with a Double Layer of Reinforcement Wall Thickness
Plain Walls
Reinforced Walls
A series mesh
B series mesh
A series mesh
B series mesh
125mm
A193
B196
150mm
A193
B196
A393
B385
175mm
A252
B503
A393
B503
200mm
A252
B503
-
B503
250mm
A393
*
-
*
300mm
A393
*
-
*
B283
* In these cases no mesh is adequate because no mesh provides sufficient horizontal reinforcement to meet the required minimum percentage. Points to Consider Based on the above it is clear that: 1. If using the option to consider a single layer of reinforcement in thinner walls then only an A393 mesh is ever suitable in this situation and this only applies when the plain wall design option is selected and applicable. Therefore:
• If B-Series mesh is specified mesh will never be selected. • If the plain wall design option is not activated then mesh will never be selected. • If the setting for maximum thickness of single layer walls is set to anything much greater than 150mm, then meshes will never be selected in the thicker walls. (It is considered that this setting should generally be set to 150mm or less)
Chapter 20 : Wall Design and Detailing
Orion Documentation page 343
2. For walls thicker than 150mm where double layer reinforcement is being used:
• If the plain wall design option is activated then potentially A-series meshes can be selected in quite thick walls.
• However, where plain wall design is not selected or not applicable, then A series meshes will not be picked in any of the thicker walls.
• If B-series meshes are selected then mesh will never be picked in walls thicker than 200mm. (This is because no B mesh satisfies the min horizontal reinforcement requirement for thicker walls). 3. Where no mesh is suitable the design will either fail (which could be many/most walls in a building) or design may revert to using loose bar if that option has been activated.
Which Mesh Type is Better? Currently you are required to choose either A-Series or B-Series meshes, they cannot be mixed within one project. In the specific case where wall loads dictate that significantly more that min reinforcement is required then the B-series is better. However, this is not necessarily the most common condition for walls. If B-series is selected then there is the possibility that many wall designs will fail (because of limitation noted above). Additionally there is no point in attempting to consider single layer reinforcement while also selecting B-Series mesh as the default reinforcement because all walls thin enough for single layer reinforcement to apply will simply fail (again because of limitation noted above). Since A-Series meshes are not greatly affected by the limitation (and hence a design can be achieved for most walls regardless of thickness) it is considered that in general it will be preferable to use the A-Series meshes. Where plain wall design applies these meshes are also more cost effective since they do not over-provide reinforcement in the vertical direction simply in order to achieve min reinforcement in the horizontal direction.
Orion Documentation page 344
Chapter 20 : Wall Design and Detailing
Overview of Possible Reinforcement Arrangements
The options shown above allow for a large range of reinforcement configurations to be considered automatically. If a single layer option is selected (as highlighted above) it will only be used in walls thinner than a specified max thickness (shown as 150mm above), for thicker walls a double layer of mesh will be used. The example below demonstrates the range of design outcomes that are possible..
Chapter 20 : Wall Design and Detailing
Orion Documentation page 345
A simple model is created with 8 walls (as above) set with lengths and widths such that different design results are expected:
• Walls 1 - 4 are 150 mm thick - with max thickness of single layer walls set to 150 mm we expect to see single layer reinforcement in all these walls. Walls 4 - 8 are 175 thick so double layer reinforcement is used.
• Min length of wall in which mesh can be used is set to 1300mm - so we only ever expect to see mesh in walls 3,4,7 and 8.
• W1 (900x150) and W5 (1050x175) - are both short walls - setting allows use of end zones in short walls.
• W4 is identical size to W3 - the loads in this wall have been increased in order to demonstrate that when mesh is specified as the default preference, the design will revert to using loose bars if the largest available mesh is not adequate. (Same applies to W8 and W7) View above shows design result when plain wall design is not active and when loose bar is set as the general preference - a successful design is achieved for all walls. (Note that the reinforcement requirement in wall 8 exceeds 2% resulting in the automatic introduction of tie bars in the wall.) View below shows how design changes if the plain walls design option is activated.
NOTE: Review of the designs at this point confirms that plain wall design is applicable to walls 1,2,3,5,6,7 and hence the steel supplied in these walls drops as shown above.
Design using Type A Mesh (not considering plain wall design option) Refer to the Mesh and Minimum Reinforcement Requirements table earlier in this chapterthis indicates that the heaviest mesh (A393) will not be sufficient in single layer walls, but that A393 may be OK as double layer mesh in the 175 thick walls.
Orion Documentation page 346
Chapter 20 : Wall Design and Detailing
Change default material to A-mesh and run design.
Unexpectedly in 1W3 A393 is selected, this is achieved because heavy end bars are selected to increase the area of steel provided to beyond 0.4%, this would not be possible on longer walls. In walls W4 and W8 the heaviest available meshes are not adequate and so the design has successfully reverted to the use of loose bar.
Design using Type A Mesh (considering plain wall design option) Refer to the Mesh and Minimum Reinforcement Requirements table earlier in this chapterthis indicates that (A393) would be OK as single layer mesh in the 150 thick walls if plain wall design applies. Check the option to consider plain wall design and re-design.
In W3, A393 mesh is still used, but the end bar sizes reduce. In W7, A252 is now used instead of A393.
Chapter 20 : Wall Design and Detailing
Orion Documentation page 347
Design using Type B Mesh (considering plain wall design option) Refer to the Mesh and Minimum Reinforcement Requirements table earlier in this chapter this indicates that B-mesh will never be suitable in any single layer wall. B503 may be OK as double layer mesh in the 175 thick walls. Check the option to consider plain wall design and re-design..
As expected design fails in walls W3 and W4. For reasons stated earlier (Limitations of Revert to Loose Bar Option), since these failures relate to the failure of any mesh to provide minimum horizontal steel, we do not expect the design to revert to use of loose bar in this case.
Limitation Copy/Paste Bars will not work Note that if you copy bars from a wall with mesh reinforcement to a wall that has loose bar reinforcement (or vice versa) the procedure will result in a confused status where check designs fail and the walls need to be re-designed.
Orion Documentation page 348
Chapter 21
Chapter 21 : Foundation Design
Foundation Design
Introduction In addition to building superstructure analysis and design, Orion includes foundation design calculations for the following: • Pad Footings /Combined Pad Footings,
• Pile Caps / Combined Pile Caps, • Strip Footings / Combined Strip Footings, • Raft (or Mat) foundations / Piled Rafts Foundation Design Settings The allowable soil stress and the coefficient of subgrade reaction are set in the Building Parameters – General tab.
Some further detailed preferences are set in the Graphical Editor Settings – Foundations tab.
Note in particular the Allowable Soil Stress Ultimate Strength Factor - in Orion the foundation design procedures assume that the permissible bearing pressure is used in conjunction with ultimate loads. Hence if the allowable soil stress is given as 200 kN/m2 the program then checks factored combinations against a factored permissible bearing pressure of 1.4 x*200 = 280 kN/m2. This is usually conservative.
Chapter 21 : Foundation Design
Orion Documentation page 349
Foundation Depth The foundation depth is set up globally for the whole structure a via the Edit Storey dialog.
In order to create detail drawings including starter bars, a reasonable (non zero) foundation depth is required. Although the foundation depth is added onto the column length displayed in the 3D window, it does not effect the column design length.
A column kicker depth cn be set in in Column Design Settings Settings – Detail Drawings General tab. If different footing heights have been specified in the same model, the undersides are all set out at the foundation depth - the topsides will thus vary in elevation.
Orion Documentation page 350
Chapter 21 : Foundation Design
The Foundation Forces Table This table displays the foundation design forces for each column and wall. To display the table, right click on either the Pad & Pile Bases, or Strip Footings branch of the Structure Tree - these branches are only available when the current storey is St00.
The forces and moments displayed are initially those determined from the analysis. If required they can be revised in one of two ways: Applying a Force Override — To apply a Force Override simply overtype the displayed value with the override value. Columns which have had a force override applied are indicated by a tick in the Edited checkbox. If you remove the tick the value determined from the analysis is restored. Added Axial Load — If you require, an extra axial load can be added to the value displayed. The extra load is entered in the Added Axial Load column. If you want to add the same amount of axial load to all columns this can be achieved by clicking the 'Apply added axial load to all columns' button. This feature can be particularly useful for piled foundations. Note
If the loads are edited via either of the above methods, the revised values are the ones displayed on the plan view at foundation level.
Chapter 21 : Foundation Design
Orion Documentation page 351
Combining Columns and Walls for Shared Foundation Design It is possible to group columns/walls together so that their combined action is supported by a single foundation. Grouping of columns/walls in this way enables the design of:
• Combined Pad Footings • Combined Strip Footings • Pilecaps supporting multiple columns/walls To combine multiple columns and walls 1. Display the foundation level, (ST00) in the Plan View. 2. Select the columns and/or walls that are to be part of the group. 3. From the right click menu select Combine Selected Columns and Walls for Shared Foundation Design.
4. The members are now grouped together for the purposes of foundation design. To ungroup columns and walls 1. Select any member in the group (this selects all members of the group). 2. From the right click menu select Remove Foundation Column Grouping. 3. The members are now ungrouped. Calculation of the Combined Footing Design Forces The axial forces in the individual group members are summed together to provide the design axial force for the footing. The design moment is calculated as the sum of the moments in the individual columns, plus an additional moment due to eccentricity of the axial force in each member from the geometric centroid of the group.
Orion Documentation page 352
Chapter 21 : Foundation Design
In the example above two pinned columns are supported on a single pad base. The combined design forces for load combination 1 are thus: N = 19.4kN + 345.1kN = 364.5kN Mx = 345.1kN x 0.9m - 19.4kN x 0.9m = 293.1 kNm My = 0 kNm
Creating a Typical Pad/Pile Footing for Multiple Foundations If you insert multiple pad footings or pile caps simultaneously, you are given the option to create a standard footing at all locations which will then be designed for the worst case loading. To create a typical footing 1. Display the foundation level, (ST00) in the Plan View. 2. Select the columns and/or walls required 3. Display the right click menu and select Insert Pad Footing or Insert Pile Cap. 4. Check the option Create Typical Footing for the Selected Columns then click OK
Chapter 21 : Foundation Design
Orion Documentation page 353
Pad Footings The footing is sized to ensure the soil is not overstressed. If the applied moment is greater in one direction, it will usually be rectangular, although you can restrict the design to a square footing if you prefer. Reinforcement to resist bending is determined in each direction and punching and shear checks are performed. If required, more than one column can be supported by a single footing. For details refer to the section on Combined Pad Footings
Defining a Pad Footing
The requirements/procedure for defining a pad base are in the help system. The steps involved are summarised below: 1. A valid building analysis (or FE chase down procedure) must have been completed so that the relevant foundation design loads exist. Column reactions are simply fed into the pad base design module, there is no more complex interaction of superstructure and the foundation analysis models. 2. Display the foundation level, (ST00) in the Plan View. 3. Select the column under which the pad base will be inserted. More than one column may be selected. In that case, the calculated pad base (based on the effects transferred from all the selected columns) can optionally be inserted to all selected columns. 4. Display the right click menu and select the Insert Pad Footing option. 5. The Pad Base Properties menu will be displayed. Check the parameters in this menu and then press the Analysis button to design the footing. 6. Use the up arrow buttons located to the right of the "Lx" and "Ly” fields to decide on the desired footing size. 7. Press OK to close the Pad Base Design dialog, then press OK once more to complete the insertion of the footing. The base is then displayed on the Plan View as shown above.
Orion Documentation page 354
Chapter 21 : Foundation Design
Pad Footing Details Footing elevations and sections can be created as Foundation Detail Sheets. as shown below.
Combined Pad Footings A combined pad footingcan be used when two (or more) columns are close enough to each other to be supported on a single foundation. Before a combined pad footing can be defined you must first group the columns together so that their combined action is applied to the footing. For details of how to group the members together see the earlier section Combining Columns and Walls for Shared Foundation Design. Once the columns have been grouped together you simply proceed to define the footing in an usual way. see the earlier section Defining a Pad Footing.
Chapter 21 : Foundation Design
Orion Documentation page 355
Pile Caps Defining a Pile Cap
The requirements/procedure for defining a pile cap are summarised below: 1. A valid building analysis (or FE chase down procedure) must have been completed so that the relevant foundation design loads exist. Column reactions are simply fed into the pad base design module, there is no more complex interaction of superstructure and the foundation analysis models. 2. Display the foundation level, (ST00) in the Plan View. 3. Select the column under which the pile cap will be inserted. More than one column may be selected. In that case, the calculated pile cap (based on the effects transferred from all the selected columns) will optionally be inserted to all selected columns. 4. Display the right click menu and select the Insert Pile Cap option. 5. The Pile Cap Design dialog will be displayed as shown below.
Orion Documentation page 356
Chapter 21 : Foundation Design
Define the pile and pile cap data as required. The pile cap dimensions can be calculated automatically - if entered manually the initial values may be rounded up or down during the analysis process, (if this happens try modifying the inital value and analysing again). Note The reinforcement defined here is neither designed or checked in any way 6. On the Loads tab you can use the existing combination data or input manually defined loads. Surcharge is also applied on this tab.
7. On the Parameters tab you can specify the minimum number of piles, spacing requirements and other design parameters.
Note that the max pile spacing multiplier only applies to walls , so as to achieve efficient transfer of load through the pile cap.
Chapter 21 : Foundation Design
Orion Documentation page 357
If you check the box to allow a tapered footing, then the corners will be chamfered (as illustrated by the footing shown on the right at the start of this section). 8. Press the Analysis button to design the pile cap. 9. A report is displayed as shown below. If required, a column forces table and the pile axial forces can be added to this report - options for this exist on the Parameters tab.
10. The number of piles used can be adjusted ‘on the fly’ by clicking the (+)Increase or (-)Decrease buttons. It is not possible to decrease to less than the original number of piles calculated. In other words, (+)Decrease can only be used to reduce an (+)Increase. 11. Click OK to insert of the Pile Cap. The footing is then displayed on the Plan View.
Basic Design Procedure The basic design process involves a simple procedure: 1. Set number of piles (Start with 1 pile) 2. Find a valid pile arrangement (setting out) for current number of piles 3. Check the pile arrangement - if piles fail then go back to step 1 and increase number of piles by 1, keep iterating until a number of piles is determined that passes.
Limitations Within the above apparently simple process there are a number of limitations to be considered: 1. Piles are positioned on a fixed spacing basis, spacing is not adjusted (increased) to resist bending, the number of piles is always increased. 2. Pile groups are centred on the supported member - No offset grouping to resist applied moments.
Orion Documentation page 358
Chapter 21 : Foundation Design
3. No ability to model ground beam systems in such a way that analysis is performed and additional loads are introduced to the pile caps. 4. Design relates to pile selection only, no design is carried out for the pile cap: • No shear checks or cap depth checks
• No reinforcement design of any type. 5. Design is not based on Working Load Combinations. The Pile Capacities are factored during the design so they can be used in conjunction with ultimate loads as follows: • The pile SWL is defined during input.
• Design is carried out using factored combinations with a factor applied to SWL (default 1.4). The Factor applied to SWL is specified on the Foundations tab in Graphic Editor Settings . Note There is potential for a significant error if you incorrectly provide the ultimate design capacity of the pile (which may be 3 times SWL) 6. No general summaries (pile schedules).
Larger Pile Groups Larger pile groups are achieved automatically as shown below.
Currently there is no way to apply preferences to prevent non-preferred layouts from being proposed and no way to interactively define an alternative preferred layout.
Chapter 21 : Foundation Design
Orion Documentation page 359
Pile Cap Details Pile cap elevations and sections can be created as Foundation Detail Sheets. as shown below.
Combined Pile Caps A combined pile cap can be used when two (or more) columns are close enough to each other to be supported on a single foundation. Before a combined pile cap can be defined you must first group the columns together so that their combined action is applied to the footing. For details of how to group the members together see the earlier section Combining Columns and Walls for Shared Foundation Design. Once the columns have been grouped together you simply proceed to define the pile cap in the usual way. see the earlier section Defining a Pile Cap.
Orion Documentation page 360
Chapter 21 : Foundation Design
Strip Footings For the most part strip foundation design is relatively straightforward. However, Orion offers some options in relation to strip analysis and the existence of such options can introduce uncertainty in some cases. This section therefore focuses primarily on the strip analysis options that are offered during the course of strip analysis and design.
Defining a Strip Footing
The requirements/procedure for defining a strip are in the help system. In this example we will consider a strip running between columns C2, C5, C7 and C9 as shown above. The steps involved are as follows: 1. A valid building analysis (or FE chase down procedure) must have been completed so that the relevant foundation design loads exist. Column reactions are simply fed into the foundation design modules, there is no more complex interaction of superstructure and foundation analysis models. 2. Beams are defined between the columns along this line (as shown in the previous screen shot) Note that these beams will be required to resist the longitudinal moments developed along the strip. Hence the sizing of these beams needs to be realistic and getting the size right may prove to be an iterative and interactive procedure – we are starting here with a beam strip 800 wide and 800 deep.
Chapter 21 : Foundation Design
Orion Documentation page 361
3. The 3 beams need to be selected and then a right click exposes the option to Insert Strip Footing and the strip footing design dialog appears as shown below.
Orion Documentation page 362
Chapter 21 : Foundation Design
4. The strip design is performed, as discussed in more detail in the following sections. When the design is saved the strip appears on the Plan view at foundation level as shown below
5. The foundation beam design is then carried out in a similar manner to the design of superstructure beams, but it is a 2 stage process: a. First use the Create/Update Footing Beam Records option as shown above. (This transfers the strip analysis results data into the beam design module.) b. Then run the Foundation Beams design option. In this case only one strip has been defined and so the beam design dialog appears as shown below.
Chapter 21 : Foundation Design
Orion Documentation page 363
6. You can design the foundation beams interactively or in a batch mode in exactly the same way as you would for beams in the supported building and drawings can be created/ exported in the usual way.
7. Elevations and sections are created as shown above, it is probably best to activate the option to label bars within the section since this means that the cantilever footing steel is also noted up as shown below. It is also suggested that it is better to use the detailing option to Re-plot the bars below the beam resulting in a display as shown above. The foundation beam detailing is not yet as automated as the mains structure beam detailing and this ensures that the maximum amount of information is readily available when final edits are made.
We will now examine some of the options and the design philosophy in a little more detail.
Orion Documentation page 364
Chapter 21 : Foundation Design
Strip Analysis Options
There are various options at this point –the left and right extension length and the footing depth are not determined or optimised automatically during design these could be set now or reset after an initial design is examined. The assumed design philosophy is that a footing cantilevers out to each side of the 800 by 800 longitudinal beam, in the example above the assumed initial depth for this footing is 400 mm. If the footing width is set to zero a minimum width will then be calculated automatically. Note that the axial loads listed in the table of design information relate to one currently selected load combination. A design carried out at this point is for the one selected combination only. To design for all combinations the design envelope option must be checked. We will look at this later, for now we will make analysis comparisons based on the G+Q*F combination only. Adjusting the Subgrade Coefficient The analysis method accounts for the assumed elastic deformation of the soil and the model used is that of a beam supported by springs. The springs model the stiffness of the soil which you define by setting the subgrade coefficient. The subgrade coefficient defined in the building parameters is displayed here and can be edited.
Chapter 21 : Foundation Design
Orion Documentation page 365
When you click on design a tabbed dialog is displayed, on the first tab calculations are shown for review/printing, on the second tab diagrams are displayed for review/printing. The diagrams for this strip are initially as shown below.
Note that the bearing pressures are varying, with peak pressures occurring under the load positions. The permissible bearing pressure was given as 200 kN/m2. Since the applied column loads are factored the design is aiming to keep the peak bearing pressure below a factored allowable pressure which is conservatively assumed to be 1.4*200 = 280 kN/m2. (The 1.4 factor can be adjusted in the foundation settings.)
Orion Documentation page 366
Chapter 21 : Foundation Design
In this initial run this has been slightly exceeded, but as is shown below, some degree of manual optimisation is always required.
When the initial design is reviewed you can see that the footing width is not a practical sort of number – you might change this to something like 3000 mm. You might also think that the 1 m extensions are a bit long, let’s reduce those to 800 mm. The revised diagrams are then as shown below.
Chapter 21 : Foundation Design
Orion Documentation page 367
You may not be confident of what value to define for the sub-grade coefficient, the table below indicates the potential range of values that might be considered.
We can easily examine the effect of changing this coefficient, the diagram below shows the result when this strip is rechecked with the coefficient reduced from 80000 to 8000 .
This results in a more uniform bearing pressure with lower peak values. By assuming weaker soil springs we are in effect considering the footing as being relatively more rigid and so the pressures get spread more evenly. If the design were carried out on this basis a narrower strip footing could be used. But notice that the span bending moments in the strip have increased significantly while the moments under the columns have reduced. In fact this result is probably more akin to the results achieved in traditional hand calculations.
Orion Documentation page 368
Chapter 21 : Foundation Design
In practice you may want to design for a range of possibilities, this can be achieved automatically in Orion by using the step option as shown below.
We have asked the design to consider a range of subgrade coefficients from 8000 to 80000 and the resulting diagrams for this one load combination are then as shown below.
The bearing pressure diagram shows a single case and indicates the maximum bearing pressure from all the cases considered. The diagrams indicate show the envelope of design conditions that have been developed.
Chapter 21 : Foundation Design
Orion Documentation page 369
Enveloping all Load Combinations As noted earlier it would be normal to do this from the outset, the Design Envelope option is now checked as shown below.
For this example this results in the following revised diagrams:
Orion Documentation page 370
Chapter 21 : Foundation Design
As noted previously the pressure diagram displayed at this point is not necessarily the most critical one, but the maximum pressure is shown. The envelope diagrams for design forces have now widened to include the worst cases for all load combinations and the required range of sub-grade coefficients. In practice you may not want to consider such an extreme range of sub-grade coefficients as have been used here, but the points to note are: • Higher sub-grade coefficients result in higher local bearing pressures and hence potentially wider footings.
• Lower sub-grade coefficients result in some higher strip design forces which may be closer to those that you would expect on the basis of traditional hand calculations. Note
General Limitation – The analysis never attempts to take any account of column base moments transverse to the strip direction. It is anticipated that such forces will not be allowed to develop where the intention is to use a strip footing – i.e. it is assumed that the column bases will have been defined as pinned or that the moments are nominal. If you wish to consider wide strips where biaxial moments get applied from the columns the FE foundation design option should be used.
Strip Footing Design We will continue with the same example used to demonstrate the analysis options in the previous section.
The above cross section illustrates the two aspects of the strip design: 1. The longitudinal beam that spreads the column point loads along the length of the strip – in this case an 800 x 800 beam. 2. The cantilever edges that in effect spread the line load developed by the beam out to the edges of the strip. Within Orion this part is referred to as the footing design.
Chapter 21 : Foundation Design
Orion Documentation page 371
When the design is run from Strip Footing dialog shown above, the results dialog appears as shown below.
In the previous section we looked at the results shown on the diagrams tab, here we are looking at the numerical results. In the previous section we saw that the maximum bearing pressure was established as 278 kN/m2. This peak figure is repeated in the numerical results above.
Orion Documentation page 372
Chapter 21 : Foundation Design
The design of the steel required in the cantilever footing is carried out at this point. Note that the cantilever is considered as the distance from the face of the beam and not the centre-line of the strip and that the peak bearing pressure is used. For the bar size defined in the input dialog, the minimum spacing requirement is determined. You can change the bar size and the footing depth until you have an arrangement that you are comfortable with. These sorts of changes do not affect the bearing pressures.
Beam Design
Scrolling further down the strip design results you will see the design forces for the beam being reported at the end, but the beam design is not carried out at this point. These are the forces that are transferred to the beam design module when you Create/Update Footing Beam Records. You may notice that the design moments noted above for transfer are not identical to the moments indicated on the envelope diagram in the previous section. This is because alternative Minimum Moments are also calculated and shown in the results at the top of the data shown above.
Chapter 21 : Foundation Design
Orion Documentation page 373
If the minimum moments exceed the analytical results then they are used in design. The minimum moments are based on average bearing pressures (W) and are calculated as follows: • Minimum Span Moment = WL2/16 (where L is the span length)
• Minimum Support Moment = WL2/12 (where L is the longer of the 2 spans to each side of the support in question) In cases with multiple equal spans these minimums are unlikely to dominate significantly if at all. In cases with unequal spans they may be more significant. In this example the minimum moments are greater in several cases. Application of such minimums may be regarded as good practice since they ensure that the foundation strip is more capable of dealing with loading variations (pattern loads) and variable ground conditions (soft spots). If you feel that these minimums are introducing too much conservatism you can adjust them in the Foundation Design Settings dialog shown at the start of this chapter. Having transferred the design forces the beam is designed in the same way as superstructure beams. Design reports are available (as shown below) as are options to print/export reinforcement details.
Orion Documentation page 374
Chapter 21 : Foundation Design
Creating Wide Strip Footings
Strip footings with deep stiffening beams are more structurally efficient but may be considered more costly to construct in some circumstances. Many engineers prefer wide rectangular strip footings as indicated above, particularly for lower rise construction. Wide strip footings can be generated in Orion, but the following should be noted: 1. You need to start by defining wide beams between the supported columns, and then select and insert the strip footing in the usual way. 2. These strips tend to be shallow and therefore flexible – so you may find a wide variation of bearing pressures on stiffer ground (when the sub-grade coefficient is higher). 3. When you do the design you do not want a cantilever footing to be introduced, you will probably need to keep changing the beam width until an acceptable solution is achieved. 4. Links are introduced to satisfy longitudinal shear only. As with superstructure beams there is a maximum of 3 links (6 legs) across the section. For wide footings this may not be sufficient. 5. No design across the width of the strip is carried out – i.e. the links are not designed to resist any transverse bending. 6. Punching Shear under the columns may be a more critical design condition than shear across the full section of the beam. This it is NOT checked at all.
Chapter 21 : Foundation Design
Orion Documentation page 375
Combined Strip Footings A combined strip footing is commonly used when two lines of closely spaced columns are themselves close enough to each other to be supported on a single foundation. An example might be underneath movement joints exist in the superstructure. Defining a Combined Strip Footing Before a combined strip footing can be defined you are required to first of all group the adjacent columns together so that their combined action is applied to the footing. For details of how to group the members together see the section Combining Columns and Walls for Shared Foundation Design earlier in this chapter. Once each group of columns along the strip has been defined you simply proceed to define the footing in an identical way to an ordinary strip footing. see the previous section Defining a Strip Footing. In summary the steps involved are as follows: 1. A valid building analysis (or FE chase down procedure) must have been completed so that the relevant foundation design loads exist. Grouped column reactions are simply passed into the foundation design module, there is no more complex interaction of superstructure and foundation analysis models. 2. Beams are defined between the columns along the line joining the grouped columns. Note that these beams will be required to resist the longitudinal moments developed along the strip. Hence the sizing of these beams needs to be realistic and getting the size right may prove to be an iterative and interactive procedure. 3. The beams need to all be selected and then a right click exposes the option to Insert Strip Footing and the strip footing design dialog appears. Analysis and Design For details of the analysis and design procedure, refer to the notes in the earlier section Strip Footings.
Raft (or Mat) foundations This topic is discussed within the standard training course notes, available in PDF format alongside this document and accessed via the link below. Orion Standard Training Manual.pdf
Orion Documentation page 376
Chapter 21 : Foundation Design
Piled Rafts Defining a Piled Raft
The requirements/procedure for defining a piled raft are summarised below: 1. A valid building analysis (or FE chase down procedure) must have been completed so that the relevant foundation design loads exist. Column reactions are simply fed into the FE raft foundation analysis module, there is no more complex interaction of superstructure and the foundation analysis models. 2. Display the foundation level, (ST00) in the Plan View. 3. Select the slabs under which the piles will be inserted. More than one slab may be selected. 4. Display the right click menu and select the Insert Pile option. 5. Click on the desired setting out point in the Plan View.
Chapter 21 : Foundation Design
Orion Documentation page 377
6. The Pile Properties dialog will be displayed as shown below.
Define the pile and pile spacing data as required. A preview of the resulting pile layout is displayed in the Plan View. 7. Click OK to create the raft.
Piled Raft Design After completing the placement of piles in the raft, in order to determine the individual pile loads the next stage is to perform an FE analysis of the foundation. Note that when preparing the FE model of the foundation you are given the option to ignore the bearing capacity of soil. On completion of the FE analysis the calculated pile loads are checked against the pile safe working loads (factored in the vertical case) and each pile is given a pass or fail status. The calculated pile loads and pass/fail statuses are displayed in the FE post-processor by clicking the 'Display Pile Forces' button on the Nodes pull down menu.
Orion Documentation page 378
Chapter 21 : Foundation Design
Where load exceeds capacity the pile node is shown red, where it is greater than 90% the pile node is orange, for < 90 % the pile node is green.
Chapter 22 : Solution Options for Inclined/Lowered Members
Chapter 22
Orion Documentation page 379
Solution Options for Inclined/Lowered Members
Introduction The objective of this chapter is to demonstrate the solution options available for structures containing inclined/lowered members, of which any of the following are possible: • sloping and/or lowered slabs
• sloping and/or lowered beams • sloping and/or lowered columns • sloping and/or lowered and/or tapered walls Specific attention is drawn to the additional limitations that apply when any of the above have been employed and to those areas where extra care during modelling is necessary.
Consider the model shown above, which features a number of inclined members: • inclined beams, slabs and columns have been defined at storey 2,
• these columns and also the end wall have been lowered beneath the general floor level, • a tapered wall has been defined at storey 1. Note
The features described in this chapter are for the purpose of defining occasional sloping/lowered members within a model containing distinct horizontal floor planes. These features are NOT intended to facilitate the modelling of structures with complex geometries in which the floor planes are not readily apparent.
The method used for creating inclined/lowered members is described elsewhere - see Modelling Inclined and Lowered Members
Orion Documentation page 380
Chapter 22 : Solution Options for Inclined/Lowered Members
Overview of Solution Options and Limitations The presence of inclined/lowered members in a structure does not in itself preclude the use of either the Building Analysis or FE Chasedown Analysis method of solution. However you must be aware of the limitations that apply to each - particularly if the latter method is adopted. This chapter uses a Building Analysis Worked Example to illustrate the first solution method. This is then followed by a list of the general limitations - see General Limitations - Inclined/ Lowered Members. For those structures for which an FE Chasedown Analysis solution is required, a second worked example is included - see FE Analysis Worked Example following this is a list of those limitations applicable specifically to FE- see Limitations - FE Analysis of Inclined/Lowered Members. If an FE chasedown analysis is performed a further item to be considered is the suitability of the analysis model for accurately determining building sway - this is discussed in the section: Limitations - Finite Element Analysis and Building Sway
Inclined Beam and Slab Loads The following points should be taken into consideration when applying loads to inclined beams and slabs: • Gravity loads applied to inclined beams and slabs are always applied vertically, (not perpendicular to the slab).
• Area and line loads applied via the Slab Load tool are always applied to the projected slab area and therefore take no account of the incline.
• For slab loads defined via the slab properties dialog (including slab self weight) - whether or not the incline is taken into account when determining the loaded area will depend on the method of solution:
• building analysis takes the incline into account (irrespective of whether yield line, or FE beam load decomposition is used).
• FE analysis ignores the incline.
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 381
Building Analysis Worked Example
The model described at the start of this chapter is used in this example.
Diaphragm Modelling One of the key issues to consider for any model which incorporates inclined planes is the diaphragm modelling. There is a potential for errors to result if those nodes being constrained by a diaphragm do not all lie in the same plane. A warning is displayed during the building analysis if such a situation exists:
In this example there is a potential problem at storey 2. The deviation from a common plane is small and the error introduced will not be too great, however, for the purpose of the example it is convenient to remove the problem. Corrective action requires the physical exclusion of the two inclined slabs from the floor diaphragm as shown below.
Orion Documentation page 382
Chapter 22 : Solution Options for Inclined/Lowered Members
Note that the removal of slabs from a diaphragm in this manner is only effective if the Storey Diaphragm Model is set to Slabs to Define Rigid Diaphragm in the Building Analysis Model Options. The diaphragm model that results as a consequence of the above settings is shown below, note that all nodes constrained by each diaphragm are constrained within a single plane:
For further guidance on the topic of working with diaphragms and inclined planes see Diaphragm Modelling
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 383
Simplified Load Decomposition For any storey containing inclined beams and slabs irrespective of the load decomposition method the following simplification is always applied: All inclined (or lowered) beams and slabs are considered to be in the same plane for the purposes of load decomposition. In this example, although the slabs at storey 2 contain two slopes, the entire floor remains continuous, hence irrespective of the load decomposition method adopted the simplification described above has no adverse effects. Where models contain inclined (or lowered) beams or slabs, it remains the user's responsibility to ensure that whichever decomposition method is applied, the above simplification does not adversely affect the results of the building analysis. Refer to Load decomposition for lowered slabs in the General Limitations section for an example of when corrective action is necessary.
Analysis A building analysis is to be performed using yield line load decomposition.The diaphragm model is set as Slabs to Define Rigid Diaphragm and the other Analysis Model settings are as shown below.
When the building analysis is run it completes with no errors or warnings.
Orion Documentation page 384
Chapter 22 : Solution Options for Inclined/Lowered Members
Load Comparison Check For all structures it is worth checking the axial load comparison report available after analysis.
This report shows up to 4 tables, the first indicates the sum of loads as they are applied to the structure (the "un-decomposed" loads). The second indicates the sum of loads as they are applied to beams at each level (after "decomposing" slab loads). The third indicates the total column/wall loads derived at each level after the building analysis. The fourth (not shown above) indicates the total column/wall loads derived at each level if an "FE Chase Down" has been performed.
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 385
In the example above you can see that no significant loading discrepancies have occurred in either the load decomposition phase or the building analysis phase, the analysis is complete. The following points should be noted: • The reduction in self weight due to the wall and two columns being lowered below the general floor level at storey 2 is accounted for.
• The slight increase in self weight where beams, slabs and columns have been inclined is accounted for.
• For the two inclined slabs, the sloped area rather than the projected area has been used when calculating the full area dead and live loads.
• The total column/wall loads reported in the third table are adjusted to take into consideration any non-vertical columns or walls. Only the axial force component is adjusted, no allowance is made for the inclusion of a vertical shear component. This may result in a slight discrepancy between the second and third tables. Switching to FE Beam Load Decomposition For the purposes of load decomposition irrespective of the method used, inclined/lowered beams and slabs are considered to be in the same plane, this is clearly apparent when viewing the FE load decomposition model for storey 2 as shown below:
If the analysis is re-run having been loaded on this basis, the same adjustments for inclined members are made as were described previously and hence the building analysis result should be similar to that obtained on the basis of yield line decomposition.
Orion Documentation page 386
Chapter 22 : Solution Options for Inclined/Lowered Members
The axial load comparison report based on FE beam load decomposition is as follows:
Although there is a slight increase in the load applied when the FE decomposition method is used, this is due to a slight conservatism in the FE decomposition method generally (and is unrelated to the presence, or otherwise, of inclined members).
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 387
Design and Detailing All beams, columns and walls can be designed automatically as part of a single batch building analysis and design if requested, as shown below.
Inclined Beam Design The sloped beam length is used for design. The design procedure is identical to that for horizontal beams as covered in the chapter Beam Design to BS8110. Inclined Beam Detail Drawings and Quantities The sloped beam lengths are also taken into account in the detail drawings and steel quantities. Details along the lines of those shown below are produced automatically.
Note
The concrete and formwork quantities do not take account of the sloped length of the beams.
Orion Documentation page 388
Chapter 22 : Solution Options for Inclined/Lowered Members
Inclined Column Design Inclined columns are designed and detailed in a similar manner to vertical columns. The inclined column length is used in the design calculations. In this example, the two inclined columns have been dropped below the general slab level, which shortens their designed length (by 500mm), however, because they are inclined their length increases slightly, hence the actual design length is only shorter by 423mm. The design procedure is described in the chapter Column Design to BS8110 Inclined Column Detail Drawings and Quantities The Column Application Plan for storey 2 is shown below:
Each section is drawn perpendicular to the face of the member - for the inclined columns the sections are therefore inclined as opposed to horizontal. By examining the longitudinal steel details, it is clear that the drop has been accounted for, (the bar lengths differ by exactly 500mm). However, it is also clear that these lengths do not take into account column inclines (if they did the H16-2 bars would be slightly longer). This limitation also applies in the column steel quantity reports. Note
Similarly, the concrete and formwork quantities take into account the column drops but do not take account of the column inclines.
Tapered Wall Design The tapered wall is designed as a rectangular section of constant cross-section. Tapered wall properties are always derived from the cross-section at the top of the wall. Note If a tapered wall is specified where the cross section at the top is larger than at the base, the resulting design would be unconservative.
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 389
Tapered Wall Detail Drawings and Quantities The detailing and quantities produced for the tapered wall require attention as the reinforcement is provided to suit the top cross-section only as shown below.
Note
The tapered parts of the wall are not included in the concrete quantities. The formwork also follows the same sizing ignoring the taper.
Design and Detailing of the Inclined Slabs Inclined slabs are designed and detailed as if they were horizontal. The effective spans used for calculating the moment co-efficients, the deflection checks and the bar lengths are the horizontal projections of the inclined span lengths. The concrete and formwork quantities also assume a horizontal slab and consequently give lower quantities for each.
Tapered Wall Modelling A mid-pier idealisation has been used for the all the walls in the analysis model. In this section we shall investigate the consequences of switching to an FE idealisation for the tapered wall.
Orion Documentation page 390
Chapter 22 : Solution Options for Inclined/Lowered Members
Gravity Loads (Mid-Pier Wall Modelling) The axial loads developed in the tapered wall at foundation level are shown below (by activating the appropriate display setting).
Gravity Loads (FE Meshed Wall Modelling) If the building analysis is repeated using FE meshed walls the axial loads developed in the columns and walls at foundation level are shown below.
Although the overall results are only affected very slightly, there is a local increase in the axial force due to dead load in the tapered wall.
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 391
This increase is attributable to the chosen method of modelling. The mid-pier modelling of the tapered wall uses an average width defined by top width which means a reduction in the wall self weight used in the analysis.
The section of the wall used in the analysis based on mid-pier modelling is shown hatched, above.
The FE shell model gives a closer approximation to the actual self weight of the tapered wall. We would not suggest that this is sufficient reason to use FE Meshed Wall modelling generally. Tapered walls are unlikely to feature in a typical model and even if they do, the amount of error introduced by the mid-pier idealisation is quite small - the benefits of simplicity and speed of solution that are offered by the mid-pier wall solution should not be ignored. Note
It is possible to specify mid-pier modelling in the Building Analysis options menu, but then to locally over-ride this setting for an individual wall, via the 3D tab of the Shear Wall Properties dialog.
Orion Documentation page 392
Chapter 22 : Solution Options for Inclined/Lowered Members
General Limitations - Inclined/Lowered Members Note
If an FE chasedown analysis is performed, a further set of limitations are also applicable, these are described in the section: Limitations - FE Analysis of Inclined/Lowered Members
The following limitations always apply when using inclined members: 1. If the end of a beam, or the edge of a slab has been lowered so that a vertical gap exists between it and other members at the same storey, it is still assumed to be physically connected to the other members for the purpose of beam load decomposition. 2. Slab partial area loads are always applied to the projected slab area and therefore take no account of the slab incline. 3. Slab line loads are always applied to the projected slab length and therefore take no account of the slab incline. 4. Diaphragm modelling - There is a potential for errors to result if those nodes being constrained by a diaphragm do not all lie in the same plane. (A warning is displayed during the building analysis if such a situation exists.) 5. Axial load comparison report - Although the column/shear wall axial loads totals shown in the report are adjusted to take into consideration any non-vertical columns or walls, only the axial force component is adjusted, no allowance is made for the inclusion of a vertical shear component. This may result in a slight discrepancy between the total of the axial loads when compared to the sum of applied loads. The discrepancy only exists in the axial load comparison report and doesn't affect the column design. 6. Column Detailing - bar lengths are not increased to take into account sloped member lengths. 7. Concrete and formwork quantities are not increased to take into account sloped member lengths. 8. Tapered Wall Design - The tapered wall is designed as a rectangular section of constant cross-section. Tapered wall properties are always derived from the cross-section at the top of the wall. 9. Tapered Wall Detailing - Reinforcement is provided to suit the top cross-section only.
Load decomposition for lowered slabs The consequences that can result from the first of the limitations mentioned above can best be understood by way of a further example. Consider the model below, which although it only consists of a single storey has a number of lowered beams and a lowered slab, (created by defining a lowered plane):
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 393
Note that there are two transverse beams in the middle bay, one directly underneath the other. The structure may appear valid but a problem can be seen in the load decomposition views.
Yield line decomposition
Both transverse beams in the middle bay are considered to be in the same plane for the purposes of load decomposition - hence they are both conservatively loaded by the slabs from both sides.
Orion Documentation page 394
Chapter 22 : Solution Options for Inclined/Lowered Members
FE load decomposition
When FE decomposition is used, once again both transverse beams in the middle bay are considered to be in the same plane - however using this method one of the two beams attracts all of the slab load while the other is unloaded (apart from it's own self weight) - using this method there is potential for some beams to be under designed.
Suggested Workaround The recommended method of modelling in this scenario is to use floors defined by separate storey levels and limit the use of planes to sloping and minor offsetting of slabs.
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 395
Alternative Workaround - Returning to the single storey method with a lowered plane, but with 2 grid lines defined at close proximity so each beam is on a different grid line (and hence receives load from one slab only). The program automatically detects that both beams are connected to the same columns. Plan layout showing the additional grid lines.
The 3D view showing the beams correctly modelled at different levels
Orion Documentation page 396
Chapter 22 : Solution Options for Inclined/Lowered Members
The yield line decomposition
The FE load decomposition
From both the above load decomposition diagrams, it can be seen that the correct loads are applied to each beam in the centre of the model.
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 397
FE Analysis Worked Example Introduction FE chasedown analysis is permitted for structures containing inclined members, however it is strongly recommended that you familiarise yourself with the associated limitations as described at the end of this section before doing so. If an FE chase down analysis is performed for a structure containing inclined members it remains the user's responsibility to ensure that the analysis method is appropriate.
Example Model The model from the building analysis example is re-used:
FE Chasedown Analysis The analysis uses an idealised model in which sloped floor planes are adjusted to the horizontal. The following simplifications are applied: • Inclined beams and slabs are modelled horizontally at the general floor level.
• Lowered (or raised) beams and slabs are modelled horizontally at the general floor level. • Any columns or walls which have had their top ends lowered (or raised) are modelled as if they still connect at the general floor level.
• tapered walls are modelled as rectangular walls, based on the top cross section of the wall. FE Model Generation The analysis models are created with a reduced slab stiffness multiplier (0.15) the aim of the reduction being to transfer most of the slab load directly into the beams - (see Worked Example – Beam and Slab Systems in the Analysis and Design using FE chapter). When creating the analysis models the following warning message is displayed:
Orion Documentation page 398
Chapter 22 : Solution Options for Inclined/Lowered Members
The message is self explanatory relating to the horizontal idealisations used. The FE Analysis model for storey 2 is shown below, note that the floor plane is horizontal.
The FE Analysis model for storey 1 is shown below. Note that the inclined columns are positioned vertically at the point where they connect to the floor. The loads being applied from the storey above via these columns are displayed.
p
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 399
Load Comparison Check The axial load comparison report based on FE chasedown analysis is as follows:
Partly because of the previously mentioned simplifications, the sum of axial loads for the FE Analysis do not exactly match the previous Building Analysis results.
Orion Documentation page 400
Chapter 22 : Solution Options for Inclined/Lowered Members
Member Design based on FE Analysis Beam design Extreme care is required when merging the FE results for inclined beams with their respective building analysis results. The building analysis model uses the sloped member length, whereas the FE model uses the shorter horizontal length. This can significantly affect the reliability of the results - the steeper a beam is sloped the more unreliable the results become. During the merging process you are warned which beams are potentially affected.
When any beam is designed, the diagram moments are automatically cross checked against the analysis moments. If merged FE results are used for steeply inclined beams, there is a high likelihood that a mis-match will occur. In such circumstances, because the design forces are invalid the beam fails. If you attempt to examine the results an error will be displayed similar to the one shown below.
When the beams in the example model are re-checked after merging the FE results no analysis/diagram moment mis-matches occur (because the beam slopes are small). Although the beam utilisation ratios change, the reinforcement previously determined adequate for the building analysis results remains adequate for all but one of the beams.
Column Design For columns (and walls) a similar disparity exists - the building analysis model uses the sloped member length, whereas the FE model uses the shorter vertical length. However, in this situation an analysis/diagram moment mis-match will not arise - this is because the design only requires the forces at the ends of the column - the forces at other points along the member length are not considered. In the example model, when the columns and walls are re-checked after merging, again the utilisation ratios change. This result is not unexpected as different axial loads and moments are bound to be generated by the different analytical approaches.
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 401
Limitations - FE Analysis of Inclined/Lowered Members Note
See also the general limitations for inclined members General Limitations Inclined/Lowered Members
The following limitations apply when performing an FE Analysis of a model containing inclined/lowered members: 1. Sway effects are not catered for in the FE Analysis model - these may be significant in models containing inclined elements. This limitation is examined in detail in the next section: see Limitations - Finite Element Analysis and Building Sway 2. Inclined beams and slabs are modelled horizontally at the general floor level. 3. As a result of the above, loads applied to inclined beams and slabs are calculated on the projected rather than sloped lengths. 4. Lowered (or raised) beams and slabs are modelled horizontally at the general floor level. 5. Any columns or walls which have had their top ends lowered (or raised) are modelled as if they still connect at the general floor level. 6. The self weight of a sloped beam is calculated based on the sloped length and used in the idealised model, where it will be shown shorter. 7. Inclined raft foundations are modelled horizontally.
Limitations - Finite Element Analysis and Building Sway All FE Floor models in Orion (irrespective of whether they contain inclined members or not) are translationally restrained in the floor plane. This idealisation is common to any traditional subframe model. Building sway is assumed only to affect the sway resisting part of the structure, typically the shear walls and core walls. In addition, if any inclined columns exist they are modelled as vertical. As an example, consider the structure shown below, consisting of four 8m bays in both directions, inclined columns are defined in the last bay:
Orion Documentation page 402
Chapter 22 : Solution Options for Inclined/Lowered Members
The FE analysis model for this is as shown below, note the inclined columns in the last bay.
Given the above, a potential concern is worthy of discussion: • Gravity loads in the inclined columns will induce a lateral force that has to be resisted by the stabilising structure (typically shear walls). Could this sway effect appreciably change the moments in the inclined columns?
S-Frame Comparison To investigate how significant this concern may be it is necessary to perform a comparative study. The results from the Orion model for varying degrees of column slope are compared to those obtained after exporting the model to S-Frame (a general analysis program). In the S-Frame model multiple analysis runs are performed having removed the translational restraint at the floor level.
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 403
The affect of varying the lateral stiffness is also investigated by changing the length of the walls and also by remodelling them using shells as shown below:
The moment at the top of one of the inclined corner columns is recorded for 3 different column inclines (45, 30 and 15 deg) and for a wall length of 1.5m and then 3m. These results can be compared to the Orion model results (in which the floor is translationally restrained). Typical Test Model Results
Orion model: Column Top Moments Floor translationally restrained
Column slope
Wall model
sway direction (kNm)
perpendicular to sway (kNm
0 degrees
Short wall (1.5m)
126.6
-126.5
Long wall (3.0m)
126.3
-126.7
Short wall (1.5m)
126.0
-128.2
Long wall (3.0m)
125.7
-128.1
Short wall (1.5m)
117.4
-121.4
Long wall (3.0m)
117.0
-121.3
Short wall (1.5m)
101.6
-102.9
Long wall (3.0m)
101.3
-102.8
15 degrees
35 degrees
45 degrees
Orion Documentation page 404
Chapter 22 : Solution Options for Inclined/Lowered Members
S-Frame model: Column Top Moments Column slope 15 degrees
translationally restrained
translationally unrestrained
sway direction (kNm)
perpendicular to sway (kNm
sway direction (kNm)
perpendicular to sway (kNm
Short wall
126.7
-128.9
113.3
-127.3
Short wall (meshed)
-
-
115.2
-127.4
Long wall
126.4
-128.8
117.1
-127.5
Long wall (meshed)
-
-
118.8
-127.6
Column slope 30 degrees
translationally restrained
translationally unrestrained
sway direction (kNm)
perpendicular to sway (kNm
sway direction (kNm)
perpendicular to sway (kNm
Short wall
118.1
-122.3
89.7
-119.3
Short wall (meshed)
-
-
95.6
-119.4
Long wall
117.8
-122.2
98.4
-120.0
Long wall (meshed)
-
-
103.2
-120.2
Column slope 45 degrees
translationally restrained
translationally unrestrained
sway direction (kNm)
perpendicular to sway (kNm
sway direction (kNm)
perpendicular to sway (kNm
Short wall
102.3
-103.8
55.0
-100.3
Short wall (meshed)
-
-
65.6
-9100.9
Long wall
102.1
-103.8
69.7
-101.4
Long wall (meshed)
-
-
78.0
-101.8
Chapter 22 : Solution Options for Inclined/Lowered Members
Orion Documentation page 405
Discussion By modelling the floor as translationally restrained, the Orion FE model does not cater for building sway. The significance of this can initially be examined by comparing for the "short wall" model the Orion results with the translationally unrestrained S-Frame results. These show that the elimination of building sway in the Orion model in this example produces a conservative answer for the column under investigation. In this example the degree of conservatism increases as the amount of sway increases (i.e. as the column slope increases 12% for a 15 degree slope rising to nearly double the moment for a 45 degree slope.) By making the comparison using the stiffer "long wall (meshed)" model instead, the conservatism is 7% for a 15 degree slope rising to 25% for a 45 degree slope. Several Points should be noted: 1. Even 2 discrete 3m long panels is probably providing an unrealistically low resistance to sway and hence this example is probably exagerating this effect - in a practical structure it is likely to be much smaller. 2. However, the sway effect would not necessarily always reduce column moments. In cases where ignored sway might increase the moments this could be offset against the conservatism. 3. In an un-braced structure (where sway stability relies on the interaction of columns and slabs) this effect could not be ignored. As is noted elsewhere, unbraced flat slab construction is beyond the intended scope of Orion. 4. Finally, inclined columns will clearly cause tensions and compressions to develop in the slab. This effect will not develop in the idealised Orion model. Some consideration should be given to this to ensure that tying details are adequate. A final more general point is that the comparisons made also highlighted the increased modelling sensitivities that arise when inclined members meet FE meshes. This issue is very similar to the issues that arise when beams intersect with walls. There is extensive discussion of this in the wall modelling presentation accessed from the wall modelling chapter.
Conclusions Sway effects are catered for in Orion's Building Analysis, however they are not catered for in its Finite Element Analysis of a sub-floor structure. (In exactly the same way as a traditional sub-frame analysis would ignore sway effects arising from vertical loading). Although in this example the effect was conservative, this may not always be the case for other structural geometries.
Orion Documentation page 406
Chapter 23
Chapter 23 : Overview of Solution Options for Transfer Levels
Overview of Solution Options for Transfer Levels
Introduction A transfer level occurs wherever a column or wall is supported by a beam or slab. Consider the simplified model shown here.
In this model there are walls along the rear and side elevations that stop at first floor level. There are also two columns on the front elevation that sit on a first floor level transfer beam. During a general building analysis of this structure, a completely coherent 3D analysis model can be constructed and solved. Orion will not any issue warning or error messages for a structure as simple as this one. However, in any model containing transfer levels there are potential problems and limitations to bear in mind. Orion provides more than one way of dealing with such models. This chapter is intended to introduce the options available to you and the potential limitations.
Understanding the Problem and the Limitations Broadly speaking there are two cases to consider here, we will now briefly introduce each in turn and cross refer to examples in subsequent chapters that illustrate the issues and suggested solutions in greater detail.
Chapter 23 : Overview of Solution Options for Transfer Levels
Orion Documentation page 407
Where Beams support Columns and Walls As noted above, many models containing transfer beams can be analysed and designed without any apparent difficulty. The first part of the example in the chapter, Transfer Beams – General Method illustrates this point. The chapter goes on to illustrate the issues of which you need to beware: • 3D Analysis Effects – Any traditional 3D linear elastic analysis of a structure may not give results that are in line with your engineering expectations.
• Walls supported by more than one beam (An example to show an arrangement that needs to be handled with caution). Subsequent chapters go on to illustrate the optional FE Chase Down methods for assessing transfer beams: • A simpler/faster way that you would use for more regular buildings and in particular where you do not intend to make use of FE analysis results for slab design. A worked example for this is provided in the chapter Transfer Beams – FE Method, Option 1 (Simplest).
• A more complex variation of the method above which uses the meshed up FE Floor models. If it is your intention to use FE results for slab design, then you will find it easier to use this method since it reutilises the same floor models. A worked example for this is provided in the chapter Transfer Beams – FE Method, Option 2. In these examples we show how beams can initially be designed using one method and then cross checked and amended using another.
Where Slabs support Columns and Walls In such circumstances the only method for designing the slabs to resist the axial loads from the discontinuous columns is an FE Chase Down. A worked example for this is provided in the chapter Solution Option for Transfer Slabs.
Key Limitation Before delving into the detail of the specific examples in the following chapters, it is worth noting the key limitation that continues to apply to structures containing transfer levels. The FE Chase Down method does not deal with sway (lateral) loads in any way. Sway resistance is always assessed using the building analysis and is therefore subject to the limitations of that analysis. If the columns/walls that are discontinuous are an important element of the sway resistance system for your building then you will need to consider your design with some care. For example: 1. If continuous walls brace your building then columns would be regarded as braced for the purposes of design. Such columns are almost unaffected by sway loads and designed exclusively for gravity loads. Where such columns stop at transfer levels, Orion’s iterative building analysis and/or FE Chase Down methods will collect all the gravity loads and apply them to the beams or slabs as appropriate. This sort of condition is not a concern.
Orion Documentation page 408
Chapter 23 : Overview of Solution Options for Transfer Levels
2. It is much less common to see un-braced columns – i.e. buildings where sway resistance is provided by frame action between columns and beams/slabs. In such cases concerns relating to discontinuous columns would be greater – the same issues as described for walls below would apply. 3. Discontinuous walls are more of a concern. Walls generally attract more significant sway loads thus generating moments (or couples) at the transfer level. In the chapter Transfer Beams – General Method examples show how this condition is catered for analytically while the design may prove to be conservative. However, in such circumstances it is recommended that an increased level of attention is paid to the parallel checks that are required when using any analysis/design software.
Chapter 24 : Transfer Beams – General Method
Chapter 24
Orion Documentation page 409
Transfer Beams – General Method
Modelling and Analysis
If you want to work through this example for yourself, you can load model DOC_Example_01. In order not to destroy the example for someone else you should then save the model with a new name before proceeding.
Analysis Model Options Having loaded this model you should confirm that analysis model option settings are made as shown below. Model Tab
Orion Documentation page 410
Chapter 24 : Transfer Beams – General Method
The default settings are used as shown above. Stiffnesses Tab
The default settings are used as shown above. Settings Tab
The default settings are used as shown above. Note that the option to Issue Warnings for Unsupported Columns …. is also checked at the moment.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 411
Analysis
Check the options as shown above and start the building analysis which should then run with no error or warning messages and be completed as shown below.
Orion Documentation page 412
Chapter 24 : Transfer Beams – General Method
Load Comparison Check For all structures it is worth looking at the axial load comparison report available after analysis.
This report shows up to 4 tables, the first indicates the sum of loads as they are applied to the structure (the un-decomposed loads). The second indicates the sum of loads as they are applied to beams at each level (after decomposing slab loads). The third indicates the total column/wall loads derived at each level after the building analysis. The fourth (not shown above) indicates the total column/wall loads derived at each level if an FE Chase Down has been performed. In the example above you can clearly see that no loading discrepancies have occurred in either the load decomposition phase or the building analysis phase, the analysis is complete.
Chapter 24 : Transfer Beams – General Method
Note
Orion Documentation page 413
In the analysis settings you have the option to define an Axial Load Comparison Tolerance, in this example it was set to the default 5%. If the overall totals in the above tables vary by more than this amount Orion will issue warnings at the end of the analysis.
Design and Detailing of the Transfer Beams All beams and columns, including the transfer beams and columns can be designed automatically as part of a single batch building analysis and design if requested as shown below.
Transfer beams are designed and detailed like any other beam; this procedure is controlled by the beam design settings and preferences as covered in the Orion Training Manual. Details along the lines of those shown below are produced automatically.
The columns above the beam are shown automatically and slab and beam lines behind the beam are optionally dotted in.
Orion Documentation page 414
Chapter 24 : Transfer Beams – General Method
Discussion of Frame Analysis Results
Gravity Loads (Mid-Pier Wall Modelling) The axial loads developed in the columns and walls at second floor level are shown below (by activating the appropriate display setting).
Does it seem strange that the internal columns along grid A do not develop higher axial loads than the corner column?
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 415
Building Analysis Results
The results can be reviewed graphically to see shear force diagrams as shown above and bending moment diagrams as shown below. Note how the side and rear walls are idealised with a single mid-pier element.
Orion Documentation page 416
Chapter 24 : Transfer Beams – General Method
Front Transfer Beam
The member force diagrams for the front transfer beam are shown above. It all looks very reasonable – a peak sagging moment of 579 kNm, and end shears of up to 336 kN. Notice that the loading diagram does not show applied loads at the transfer column positions. The transfer columns are a part of the analysis model and so the loads they transfer can only be seen in terms of the steps in the shear force diagram, which are clearly visible. Notice the small steps in the bending moment diagram under the supported column positions, these are a small indication of the frame action being developed with the columns themselves, this step could be eliminated by pinning the bottom of the transfer columns. It is noted that there is currently no (easy) way to account for IL reductions within the columns that load the transfer beam, in beams supporting columns with many levels above this means that the transfer beam design will be conservative by default.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 417
Rear Transfer Beam
The member force diagrams for the rear transfer beam are shown above. Again, it all looks very reasonable – a peak sagging moment of 1155 kNm, and end shears of up to 416 kN. Note that the wall is idealised as single mid-pier element therefore it’s load is concentrated at the centre of the beam as can be seen from the shear force diagram. Clearly this idealisation will tend to result in conservative design of the transfer beams.
Orion Documentation page 418
Chapter 24 : Transfer Beams – General Method
Frame Action Now consider the moment diagrams for the continuous beam line at second floor level above the front transfer beam.
Perhaps not what some might initially expect – there is no hogging across the transfer column positions. This is a logical result of any full 2D or 3D analysis, the transverse beam is deflecting, hence the supported columns are deflecting and this has an effect on the beams above. In essence the loads are being shared according to the stiffnesses of the beams at all levels. This effect makes more sense when the deflections for the frame are viewed as shown below.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 419
This also explains the apparently low axial loads noted in the transfer columns at the start of this section. It is an entirely logical result the same as would be given by any general 3D frame analysis package. It is probably not the same result as would be derived by traditional hand calculation methods. An engineer carrying out hand calculations would probably consider each floor independently designing the continuous beam lines on the assumption that the columns provide a rigid support producing a moment diagram along the lines shown below.
This may be more in line with traditional expectation but it has not considered the vertical deflection of columns supported by beams or slabs at a lower level. A 3D analysis inherently considers this effect. Is there a correct answer – what is it? Since the answer is related to deflection it relates to stiffness assumptions. This can become an extremely complex subject which is discussed in some detail in the context of Flat Slab Analysis and Design in the chapter Analysis and Design using FE of this handbook. A very sophisticated assessment would take account of construction sequencing and time dependent effects. In such circumstances the result needs to be assessed for sensitivity to variations in the assumptions on which it is based. In fact the result achieved by any such complex analysis will lie somewhere between the two extremes shown above. A simpler approach to satisfying these extremes is to ensure that your design covers for both possibilities. This can be achieved in Orion in one of two ways. 1. Carry out a building analysis on the basis described above and design all members. 2. EITHER: Edit the properties of the transfer beam and artificially increase it’s stiffness (by increasing both the inertia and the shear area) and hence largely eliminate transfer beam deflection. Reanalyse and examine results to see that this has had the desired effect then run a design check on all members in the structure – if any member fails investigate and increase reinforcement accordingly.
Orion Documentation page 420
Chapter 24 : Transfer Beams – General Method
3. OR, as an alternative to 2. above: In Orion it is possible to force the entire vertical load to be carried by the transfer beam by using either of the FE methods, further discussion of this analysis result is picked up in the Discussion of Merged Results section in the chapter Transfer Beams – FE Method, Option 1 (Simplest). In essence this emulates the traditional hand calculation approach.
Gravity Loads (FE Meshed Wall Modelling) If the building analysis is repeated using FE meshed walls the overall results will be affected slightly and there will be some significant local differences which are discussed in this section. The axial loads developed in the columns and walls at second floor level are shown below (by activating the appropriate display setting).
There are very small changes in the distribution of axial loads in the columns and walls. The effect where the internal columns along grid A do not develop higher axial loads than the corner column still applies.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 421
Building Analysis Results
Once again, the results can be reviewed graphically to see shear force diagrams as shown above and bending moment diagrams as shown below. Note how the side and rear walls are now idealised with a mesh of shell elements.
Orion Documentation page 422
Chapter 24 : Transfer Beams – General Method
Front Transfer Beam
The member force diagrams for the front transfer beam are very similar to those given by the mid-pier idealisation. The peak sagging moment has increased from 579 kNm to 584 kNm, and the maximum end shears has decreased from 336 kN to 326 kN. Overall the change from mid-pier to meshed wall modelling has had no significant impact on the design forces developing in this beam despite the fact that one end of the beam interacts with a wall.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 423
Rear Transfer Beam
In this case the shear forces at the ends of the beam are almost unaffected by the change for mid-pier to meshed wall idealisation. The maximum end shear decreases from 416 to 410 kN. However, over the length of the wall we can see a series of step changes in the shear force diagram as the wall interacts with the beam. This interaction also appears to have the effect of smoothing the BMD and the peak sagging moment drops from 1155 kNm to 947 kNm. Within the length of the wall the wall itself will provide a much deeper compression zone thus reducing steel tension reinforcement requirements. Within Orion the beam is designed in isolation ignoring this interaction of beam and wall elements. Orion’s design for a moment of 947 kNm could therefore still be argued to be conservative. This situation needs a little more consideration as the wall becomes proportionately longer in relation to the transfer beam. This is discussed further in the Limitations – Transfer Walls section later in this chapter.
Orion Documentation page 424
Chapter 24 : Transfer Beams – General Method
Frame Action Consider again the moment diagrams for the continuous beam line at second floor level above the front transfer beam.
The numbers change slightly but the same effect occurs, there is no hogging across the transfer column positions. As before this effect makes more sense when the deflections for the frame are viewed as shown below.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 425
Limitations – Transfer Walls
Consider the wall at the left hand side of the above model. It stops at first floor level and a beam is placed under this wall despite the fact that there is a column under each end of the wall. On the whole there seems to be 3 engineering approaches adopted in such situations: 1. A beam is defined, so the engineer expects the beam to carry all of the wall load. 2. No beam is defined – the wall must be designed to act as a deep beam spanning between the supporting columns. 3. A beam is defined but only for detailing reasons, or perhaps to provide an inbuilt construction stage support for the wall above. In essence the engineer expects the beam to be ignored or to carry a very small proportion of the load and that the wall should then be designed as a deep beam for the greater part of the load. Orion provides different levels of support for each of these approaches, we will consider each in turn in more detail.
Orion Documentation page 426
Chapter 24 : Transfer Beams – General Method
Supporting Beam to carry all Wall Load
As was shown earlier in this chapter, if the model is analysed using the mid-pier idealisation of walls then the load is concentrated at the centre of the wall and the beam is designed for a moment that would normally be regarded as conservative.
In this case the moment is 905 kNm and the sum of the end shears is 715 kN. If you want to ensure that a supporting beam is conservatively designed to support all the loads from a wall above we recommend that you use the mid-pier idealisation. If this is going to be too conservative an approach then the meshed wall modelling option can be used provided the points made in the Beam and Wall to Work Together section below are taken into account.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 427
No supporting Beam – Wall to act as a Deep Beam Analytically Orion supports this option with no difficulty. Make a copy of DOC_Example_01 and delete the beam under the wall. Analyse using the mid-pier idealisation and the analysis will proceed with no errors or warnings. When the analysis model is viewed as shown below you can see this is because Orion creates rigid arms at every floor level of a wall and the arms at first floor level connect to the supporting columns at each end of the wall. The axial loads that develop in the supporting columns can be seen in the view below.
Orion Documentation page 428
Chapter 24 : Transfer Beams – General Method
Swapping to a meshed wall and reanalysing similar results can be seen.
Once again it is worth noting that even in meshed walls Orion inserts beam elements at each floor level for numerous reasons discussed elsewhere. In the absence of such elements (and in particular in more complex models which include transfer walls) the comparison of results between the meshed and mid-pier idealisation would not be so good. LIMITATION – The model has been solved analytically, but Orion does not automatically recognise walls which require deep beam design. In the current version it remains essential that a manual design is carried out for all Transfer Wall Panels.
Beam and Wall to Work Together If the original model is reanalysed using meshed walls the results for the same beam change as shown below.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 429
The moment has dropped from 905 kNm to 263 kNm and the sum of the end shears has dropped from 715 kN to 562 kN. Given that the mid-pier model generates a single central point load where a UDL might be a more realistic we should expect the reduction in moment to be at least in the ratio of WL/4 reduced to WL/8 – i.e. something less than 50% for this single span example. In fact the moment has dropped by nearly 70%. The reduction may not be as significant for more complex situations with continuous transfer beams. The stepped shape of the shear force diagram is indicative of the interaction between the shells and the beam. The drop in moments and shears is an indication that the meshed wall has a stiffness in it’s own right and is carrying loads direct to the supporting columns. In fact, these are probably still conservative design forces for the supporting beam. When Orion constructs the analytical model of meshed walls it creates relatively rigid beams at each floor level and where these beams overlap with real beams it replaces the properties of the real beam with those of a more rigid beam. Therefore the more rigid beam is going to attract a greater share of the design forces and the design will tend to be conservative.
Orion Documentation page 430
Chapter 24 : Transfer Beams – General Method
You have the option to apply adjustments to the rigid beams used in these walls using the analysis option to adjust the moment of inertia as shown below.
Note that a Modulus of Elasticity adjustment will not affect the merged beam because it retains the material properties of the transfer beam. If a 0.2 factor is applied as shown above then the results in the transfer beam change as shown below.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 431
The moment has dropped from 263 kNm to 152 kNm and the sum of the end shears has dropped from 562 kN to 478 kN. Caution
A great deal of effort has gone into establishing the rigid member properties that are generated by Orion. As is shown elsewhere Orion will generate very compatible results using either the mid-pier or meshed models when the default properties are accepted. Applying significant adjustments to these multipliers could seriously affect the overall validity of the analysis.
In conclusion, by using the meshed wall option the transfer beam and wall will both carry load. As is shown in this example the design forces developed in the transfer beam are still likely to be conservative. However, the wall is required to carry some of the load and in this respect the same limitation as is noted for the case where there is no supporting beam is applied. LIMITATION – Where meshed walls are supported by transfer beams, the model is solved analytically, but Orion does not automatically recognise that such walls require some degree of deep beam design checking. In the current version it remains essential that a manual design is carried out for all such Transfer Wall Panels. However, assuming the transfer beam reinforcement can be considered as tension reinforcement in the deep beam design, it is considered likely that manual design checks in such situations will tend to show that little or no additional reinforcement is required.
Orion Documentation page 432
Chapter 24 : Transfer Beams – General Method
Limitations – Walls Supported by more than 1 Beam
In the above view you can see a wall supported at first floor level but it sits on two different transfer beams. If you want to work through this example for yourself, you can load model DOC_Example_07. In order not to destroy the example for someone else you should then save the model with a new name before proceeding. The first floor plan view is shown below.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 433
Analysis You can immediately launch an analysis of this building with the settings as shown below and using the mid-pier idealisation for walls.
At the start of the analysis there is a message as shown below.
The message is not completely clear as to the problem. Orion is checking to ensure that where a wall is not supported by at least 2 columns that it is supported by the same beam at both ends. However, there are many permutations of this sort of support condition and not all will generate warnings. In fact the analysis will continue to run quite happily. It is better to understand the issue which can be explained by examining the results.
Orion Documentation page 434
Chapter 24 : Transfer Beams – General Method
Results based on Mid-Pier Wall Idealisation
In this case the wall is symmetrically positioned over a supporting column below. Hence the mid-pier element of the wall sits directly on this column. This is likely to mean that lower moments and shear forces develop in the transfer beams to each side of the column. These forces are shown below for the Factored G + Q combination.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 435
Results based on Meshed Wall Idealisation
In this case the meshed wall elements will obviously be interacting with the supporting beams. The forces that develop in the transfer beams are shown below for the Factored G + Q combination.
The span moment only increases slightly from 145 to 158 kNm. However the hogging moment increases greatly from 649 to 1374 kNm and the end shears increase from 237 to 789kN.
Orion Documentation page 436
Chapter 24 : Transfer Beams – General Method
As was noted earlier in the chapter, these are likely to be conservative transfer beam design forces due to the rigid link modelling that is used at the bottom of the meshed wall. However, the wall itself needs to be considered as an inverted deep beam hogging over the support. Once again this requires some manual check design calculations.
Alternative Modelling Option – Split the wall In this workaround the model is adjusted, the wall is split into two panels directly above the supporting column.
The split walls are defined face to face – no wall extension is defined at the interface. With the edits made, the building analysis can be rerun for each of the wall modelling options. Using this alternative modelling the analysis will run with no error/warning messages. We can now examine the effects that this has on the results.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 437
Mid-Pier Model
Now the mid-pier elements of the two separate walls are sitting part way along each of the transfer beams. The forces that develop in the transfer beams are shown below for the Factored G + Q combination.
The span moment only increases slightly from 145 to 168 kNm. However the hogging moment increases greatly from 649 to 1584 which is now higher than the 1374 kNm given by the meshed wall. The end shears increase from 237 to 865 which is again greater than the 789kN given by the meshed wall.
Orion Documentation page 438
Chapter 24 : Transfer Beams – General Method
Meshed Model
The model looks identical to that generated for the un-split wall. In other examples it may differ slightly because the meshing would not break down to the same sizes. In fact the properties of the rigid elements created within the walls are related to the dimensions of the wall so we may see slight differences in the results shown below.
However, in this example the results for the split wall when meshed are identical to those given when the wall was not split.
Chapter 24 : Transfer Beams – General Method
Orion Documentation page 439
Summary/Recommendations Where a transfer wall has numerous points/lines of support there are two basic options: Option 1 – Do not split the wall The analysis may give warnings. In this case the Mid-Pier idealisation may not be appropriate (regardless of whether or not warnings are given). The meshed idealisation may be preferable but might not be an upper bound. An advantage of this method is that the wall is then designed as a single long element. Consideration should be given to manual design checks regarding the design of the transfer wall panel as a deep beam. Option 2 – Split the wall In this case the analysis will start to run without error messages. The mid-pier idealisation will tend to provide a most conservative upper bound for design of the transfer beam. The meshed idealisation will generate the same or very similar results as it did when the wall was not split. A potential disadvantage of this method is that the wall is then designed as a series of discrete elements and some manual adjustment of the design details will be required to reintegrate. However, for very long walls (particularly those with varying load intensity) this may be deemed appropriate and in fact something of an advantage. Consideration should again be given to manual design checks regarding the design of the transfer wall panel as a deep beam.
Orion Documentation page 440
Chapter 25
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Transfer Beams – FE Method, Option 1 (Simplest) This chapter and the chapter Transfer Beams – FE Method, Option 2 describe the alternative methods that have been introduced in Orion to help deal with transfer levels. Both these methods utilise 3D analysis on a floor-by-floor basis. This method is the simplest and has least potential for errors arising from modelling anomalies. It is fast because it avoids the need for large meshed up models incorporating 1000’s of plate elements. However, if it is your intention to use FE meshing and analysis for slab design work you should probably use option 2, which is described in the chapter Transfer Beams – FE Method, Option 2.
Modelling and Initial Analysis
If you want to work through this example for yourself, you can load model DOC_Example_01. In order not to destroy the example for someone else you should save the model with a new name before proceeding. This is the same model as was used for illustration of the General Method in the chapter Transfer Beams – General Method.
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Orion Documentation page 441
The FE Analysis and Load Chase Down Choose the option to run Finite Element Floor Analysis. The FE analysis dialog includes various features/options which influence the analysis model that is generated for the selected floor. These options are discussed in some detail in the chapters Analysis and Design using FE and Flat Slab Models later in this handbook.
In order to chase the loads down through the building using the simpler modelling option you must start at the top floor level and work downwards. At each level you must set/consider the options as shown above, noting that: • This model has no duplicate floors, duplicates can be accommodated but there are rules to note, see the section FE Chase Down with Duplicate Floors section later in this chapter for more information.
• Removing the check against the option to Include Slab Plates in FE model means that you work with a simple stick model as shown later.
• Including or excluding beam torsional stiffness will make little difference to the overall validity of the load chase down. (However, engineers have traditionally calculated forces in floor grillage systems without allowing for torsion and Orion does not consider torsion within beam design, so let’s exclude it for the purposes of this example.)
• Checking the option to Include Upper Storey Column Loads means that the reactions established by FE analysis of the level above will be reapplied as loads within the model you create for the current level. It also adds in the self-weight of the columns for the current level. You can review these loads by clicking on the Column Loads Table button.
• Since there will be no slab elements, adjusting the slab stiffness multiplier will have no effect, you could however adjust the relative stiffness of the beams. Orion uses the gross sectional area of the beams (ignoring flanges) and columns by default, so you might make this adjustment to allow for the flanges.
Orion Documentation page 442
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
You can run the FE Chase Down manually starting at the top level by making the appropriate model generation settings and clicking the mesh generation button. This creates the model for the top level and opens a window so that you can review it. Simply exit from this window to analyse. Repeat this process for each non duplicate level in the building. Alternatively you can run the process automatically by clicking on the Batch FE Chasedown button. The batch method also allows you to review and adjust the mesh at each floor level if required. After analysis, options have been added to allow graphical review of the applied loads. The picture below shows the accumulated dead loads applied at the first floor level in this example model.
Note that the dead loads shown in the discontinuous columns above are similar, but slightly higher than the 63 to 68 kN loads determined in the chapter Transfer Beams – General Method.
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Orion Documentation page 443
Axial Load Comparison Having completed an FE Chase Down (sequential FE analysis of all floors starting at the top and working down) you can review the resulting column loads in the Axial Load Comparison Report accessed from the main building analysis dialog.
For this model there are no discrepancies, as you will see in the chapters Overview of Solution Options for Transfer Levels and Transfer Beams – General Method this is not always the case. The most important comparison when reviewing the results of the FE Chase Down (shown in the last table in the report above) is with the first table (the un-decomposed slab loads).
Orion Documentation page 444
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Merging Column Analysis Results
Having completed the sequential floor analysis column design forces can be updated by merging the results from FE with those from the main building analysis. Column results are merged using the appropriate button on the postprocessor tab of the FE analysis dialog. Lets look at how the results change before and after merging.
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Orion Documentation page 445
The view above shows column loads before merging. These are the loads from the general building analysis carried out using the mid-pier idealisation for walls and with rigid zones set to none.
The view above shows column loads after merging. Obviously the loads on the transfer beams have increased, but as the axial load comparison shows, the total load is the same at all levels.
Column Design Having merged the column results, the columns can be designed in the usual way. Note that you have the option to design them based on building analysis results and then check them based on the merged FE results. The view below shows the column design summary based on the building analysis results. Note that the reinforcement and utilisation ratios may be different on your machine because of different design settings/preferences that you may have set.
Orion Documentation page 446
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
After merging the analysis results from the FE Chase Down and using the batch mode check design option, the reinforcement is unchanged, but the utilisation ratios change as shown below.
Some utilisation ratios are higher, some are lower, it is simply a function of the different axial loads and moments that are generated by the different analytical approaches. Clearly one wall is now failing. This is occurring since the FE chase down is generating higher moments in the wall because of the increased bending in the transfer beam that is connected to it. Any failed members can be filtered out and selectively redesigned. Note that you could also have used the options to create and save reports so that you have a complete set of column design calculations, one set based on the building analysis, a second set based on the FE Chase Down.
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Orion Documentation page 447
Merging Beam Analysis Results Merging Options As with the column results above, the beam results are merged using the appropriate button on the postprocessor tab of the FE analysis dialog.
When you merge beam results a message box will appear confirming whether or not the merge has been successful at each level.
It should be noted that when results are merged there will no longer be any pattern load cases results for the beams.
Orion Documentation page 448
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Discussion of Merged Results Front Transfer Beam The chapter Transfer Beams – General Method included a section Discussion of Frame Analysis Results in which frame analysis effects were introduced. In this section we can make some comparisons with those results. The member force diagrams for the front transfer beam are shown below.
Note that the beam label is annotated (FE) indicating that the diagrams are based on the results of FE analysis. Once again, the shear force diagram clearly indicates the presence of high point loads at the expected positions. The diagrams are quite similar to those shown in the chapter Transfer Beams – General Method, in this case the shear forces and moments are a little higher (maximum sagging moment increasing from around 579 kNm to 712 kNm). The left hand end hogging moment has increased from around 258 to 360 kNm. The transfer of this force into the wall is the explanation for the wall failure noted above. The maximum shear force has increased from around 336 kN to 377 kN. All this is to be expected since the FE chase down accumulates all the loading in the columns and applies it to the transfer beam. There is no frame action that generates sharing of load back up to the floor above. In fact, this approach probably more closely emulates traditional hand calculations. It is noted that there is currently no (easy) way to account for IL reductions within the columns that load the transfer beam, in beams supporting columns with many levels above this means that the transfer beam design will be conservative by default. If it is essential to try
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Orion Documentation page 449
assess the effect of IL reduction this can be achieved by creating a copy model with a new set of load combinations where the IL factor is reduced from 1.6 to say 0.96 if a 40% reduction is required. This model can then be only be used for the design of the transfer beams. Now we can compare the diagrams for the continuous beam line at second floor level above the transfer beam.
These results may look more in line with initial expectations than those shown in the chapter Transfer Beams – General Method, but what is the correct answer? Some might say that the sequential nature of construction means that the above style of diagrams is more appropriate for the self-weight, this would mean that the correct answer lies somewhere in the middle of the two extremes. This particular model may be an extreme example of this effect, but we would recommend that the possible effects of load redistribution be quite carefully considered in any transfer situation. Once again, the check design mode for beams can be used, so if you wanted to design for one extreme and then check for the other you can do it as shown below.
Orion Documentation page 450
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Beam Design The design summary for the beams in this model based on the building analysis results in the chapter Transfer Beams – General Method is shown below.
Having merged the beam results the design and detailing can again proceed as for any other beam, the summary below shows the changes after running a batch mode check design without allowing any of the reinforcing to change.
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Orion Documentation page 451
Now lots of the beams fail, so we can rerun the batch design and use the option to increase steel only when required, if you do that the summary may change as shown below to show that everything passes.
However, in the batch mode when a beam fails all the steel in the beam is reselected, so if you un-check the merge option and then check the beams again you might find that beams which previously passed now fail, as shown below.
For this reason we recommend that having analysed and designed all the beams using one method (e.g. Building Analysis) you should then run a check design for merged results to find any members that fail. At this point you should adjust (increase only) the steel in those members interactively until they pass. Then you should find that you can run a check design for either set of analysis results and everything should pass. Finally, it is worth noting that one of the most compelling reasons for not simply using FE analysis, merging results and then designing for the merged results is that FE analysis does not deal with patterned load, everything is fully loaded. It is very worthwhile trying to use the general building analysis and then merge and check for FE results selectively.
Orion Documentation page 452
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
FE Chase Down with Duplicate Floors In the simple example in this chapter information has been defined at all three levels of the building. It is possible to have duplicate floor and have these dealt with automatically as part of the FE Chase Down procedure without the need to mesh and analyse them. However, there are more restrictions on what can be regarded as valid duplicate when the FE subframe models are involved. The FE model for each floor includes the columns and walls above and below that floor, so for the models to be identical the wall and column arrangements above and below identical floors must also be identical.
Consider the 10-storey model above. The folders indicate that information is defined at upper levels 10, 9, 6, and 1. From a beam layout point of view level 9 might be identical to level 10. However there are no columns above level 10 so the FE models at these two levels will never be identical. Hence it is always necessary to create information and generate the FE models at the top 2 levels in any model if an FE Chase Down is to be used. In the model above 7 and 8 are valid duplicates of level 9.
Chapter 25 : Transfer Beams – FE Method, Option 1 (Simplest)
Orion Documentation page 453
Level 6 has information, the model takes up an increased plan area and new columns start from this level. For the same reasons as at the top two floors, level 5 is not a valid duplicate of level 6. If you try to work through an FE Chase Down for this model then you will get a message as shown below when you try to create the model at level 1.
By copying storey information from level 6 to level 5 this model will have information at enough floors for an FE Chase Down to be completed. Note that information is always required at level 1 and therefore as an absolute minimum in any model information is required at 3 levels, Top, 2nd Top, and First.
Orion Documentation page 454
Chapter 26
Chapter 26 : Transfer Beams – FE Method, Option 2
Transfer Beams – FE Method, Option 2 This chapter and the chapter Transfer Beams – FE Method, Option 1 (Simplest) describe alternative methods that have been introduced in Orion to help deal with transfer beams. Both these methods utilise 3D FE analysis on a floor-by-floor basis. This 2nd option is simply a variation of the method described in the chapter Transfer Beams – FE Method, Option 1 (Simplest), but this time using the meshed up floor models. This option is therefore a little more complex, slower, and has a bit more potential for errors arising from modelling anomalies. However, if it is your intention to use FE meshing and analysis for slab design work you will probably find it easier to stick to this option rather than trying to swap back and forth between the two.
Modelling and Initial Analysis
If you want to work through this example for yourself, you can load model DOC_Example_01. In order not to destroy the example for someone else you should save the model with a new name before proceeding. This is the same model as was used for illustration of the other methods in the previous chapters. There is no need to make special changes in support of this new method. Refer to the opening notes in the chapter Transfer Beams – FE Method, Option 1 (Simplest) for an overview of the problem.
Chapter 26 : Transfer Beams – FE Method, Option 2
Orion Documentation page 455
The FE Analysis and Load Chase Down Choose the option to run Finite Element Floor Analysis. The FE analysis dialog includes various features/options which influence the analysis model that is generated for the selected floor. These options are discussed in some detail in the chapters Analysis and Design using FE and Flat Slab Models later in this handbook.
In order to chase the loads down through the building you must once again start at the top floor level and work downwards. At each level you must set/consider the options as shown above, noting that: • This model has no duplicate floors, duplicates can be accommodated but there are rules to note, see the section FE Chase Down with Duplicate Floors at the end of the chapter Transfer Beams – FE Method, Option 1 (Simplest).
• Checking the option to Include Column and Wall Sections in the Model means that a more sophisticated model which idealises a rigid zone extending to perimeter of the column or wall is used. This modelling sophistication is really only of interest in flat plate models and may introduce more meshing difficulties. For beam and slab models it is better not to activate this option.
• Checking the option to Include Slab Plates in FE model means that you will need to mesh up the floor slab as shown later.
• Including or excluding beam torsional stiffness will make little difference to the overall validity of the load chase down. (However, engineers have traditionally calculated forces in floor grillage systems without allowing for torsion and Orion does not consider torsion within beam design, so let’s exclude it for the purposes of this example.)
• Checking the option to Include Upper Storey Column Loads means that the reactions established by FE analysis of the level above will be reapplied as loads within the model you create for the current level. It also adds in the self-weight of the columns for the current level. You can review these loads by clicking on the Column Loads Table button.
Orion Documentation page 456
Chapter 26 : Transfer Beams – FE Method, Option 2
• The theoretical stiffness of the slab relative to the beams can make a big difference to the load paths and influence the design moments determined in the beams. Refer to the chapter Analysis and Design using FE for more detailed discussion of this effect. For this example set the slab stiffness multiplier to 0.2. At the end of this chapter we will also look at the effect of changing this setting for this model. You could also adjust the relative stiffness of the beams. Orion uses the gross sectional area of the beams (ignoring flanges) and columns by default, so you might make this adjustment to allow for the flanges. In this example leave the beam stiffness multiplier set to 1.0. At each level in turn you click on the mesh generation button. This takes you into the FE Preprocessor where you can mesh up your model. You can use the mouse controls to zoom/ pan/rotate the model until you see a view as shown below for the third floor level.
When you first access any model Orion will suggest a default number of plates. Meshing is not an exact science, to a large degree the more plates the better, but as a rule of thumb you should aim to see at least 6 to 8 nodes generated along the length of each beam. In this model changing from the initial default to 600 plates, the resulting mesh
Chapter 26 : Transfer Beams – FE Method, Option 2
Orion Documentation page 457
is as shown below.
Note
Click buttons in the menu bar to toggle on and off the display content.
You can keep changing the suggested number of shells and re-mesh to review the result until you are happy. Since we have 6 nodes along the length of the middle beam at the front we will use this mesh density. Refer to the chapter Analysis and Design using FE for more discussion on mesh density in FE. When you exit from the preprocessor the model of this floor will be automatically analysed. Repeat the modelling options and meshing at the second and then the first floor levels. Alternatively you can run the above process for all floors automatically by clicking on the Batch FE Chasedown button. The batch method also allows you to review and adjust the mesh at each floor level if required.
Orion Documentation page 458
Chapter 26 : Transfer Beams – FE Method, Option 2
After analysis, the postprocessing option allows graphical review of the applied loads and other results. The picture below shows deflection contours together with the applied nodal loads for the Dead Load case.
Note that the dead loads shown in the discontinuous columns above are quite similar to those determined in the chapter Transfer Beams – FE Method, Option 1 (Simplest). Within the FE Post Processor you can of course also review contour diagrams for all sorts of shell results. Once again you should refer to the chapters Analysis and Design using FE and Flat Slab Models later in this handbook for more detailed discussion.
Chapter 26 : Transfer Beams – FE Method, Option 2
Orion Documentation page 459
Axial Load Comparison Once again, having completed an FE Chase Down (sequential FE analysis of all floors starting at the top and working down) you can review the resulting column loads in the Axial Load Comparison Report accessed from the main building analysis dialog.
For this model there are small discrepancies between the loads developed in the FE Chase Down (shown in the last table in the report above) when compared with the sum of applied loads in the first table (the undecomposed slab loads). These differences are explicable and unavoidable. The FE model is a traditional centre-line analysis model. For the meshed up model the loads are applied to the plates, the plates are then supported by the beams and columns. The model is constructed on outer grids spaced at 5 m by 8 m, so the total plan area for the centre-line model is 40 m 2. P
P
Orion Documentation page 460
Chapter 26 : Transfer Beams – FE Method, Option 2
Consider the imposed loads: the imposed load at each level is 2.5 kN/m 2, so the load at each floor is 200 kN. However, when Orion calculates beam loads using the yield line decomposition method in preparation for the general building analysis it takes account of the size of the beams and adds in imposed loads on the outer half of the perimeter beams. So in this example the FE Chase Down finds a little less imposed load than the building analysis. P
P
Consider the dead loads: in the FE analysis the slabs exist right up to the centre line of the beams. However, when Orion calculates beam loads using the yield line decomposition method in preparation for the general building analysis it takes account of the size of the beams and only considers the self weight of the slab up to the face of the beam. So in this example the FE Chase Down effectively double counts a small amount of slab weight where the slab and beam overlap and hence finds a little more dead load than the building analysis. Using either of these methods it is quite likely that Orion has been more rigorous in decomposing and maintaining loads than most Engineers would have deemed necessary in hand calculations or more traditional approaches. A more detailed discussion of the Axial Load Comparison can be found in the chapter General Building Analysis.
Merging Column Analysis Results
Having completed the sequential floor analysis column design forces can be updated by merging the results from FE with those from the main building analysis. You can then design columns or check previously designed columns for these new slightly different loads. This procedure is identical regardless of whether the floors have been meshed and can therefore be followed for this meshed up version of the model by referring back to the section Merging Column Analysis Results in the chapter Transfer Beams – FE Method, Option 1 (Simplest).
Chapter 26 : Transfer Beams – FE Method, Option 2
Orion Documentation page 461
Merging Beam Analysis Results As with the column results above, the beam results are merged by checking the appropriate button on the postprocessor tab of the FE analysis dialog shown above.
Discussion of Merged Results Front Transfer Beam The chapter Transfer Beams – FE Method, Option 1 (Simplest) included a section Discussion of Merged Results in which comparisons were made with the results obtained using the simpler FE based option and the general method. In this section we can make some further comparisons with those results. The member force diagrams for the front transfer beam are shown below.
Note that the beam label is annotated (FE) indicating that the diagrams are based on the results of FE analysis. Once again, the shear force diagram clearly indicates the presence of high point loads at the expected positions. The diagrams are quite similar to those shown in the previous chapter, in this case the shear forces and moments are a little higher (maximum sagging moment increasing from around 712 kNm to 741 kNm). The left hand end hogging moment has decreased from around 360 kNm to 306 kNm. The maximum shear forces have also decreased slightly. The notes in the chapter Transfer Beams – FE Method, Option 1 (Simplest) regarding the way these FE chase down methods concentrate the loading in the lowest beam are equally applicable here.
Orion Documentation page 462
Chapter 26 : Transfer Beams – FE Method, Option 2
We can again compare the diagrams for the continuous beam line at second floor level above the transfer beam.
These results may look to be in line with initial expectations but you should refer again to the discussion in the chapter Transfer Beams – FE Method, Option 1 (Simplest) for an alternative view on this.
Beam Design Having merged the beam results you can then design the beams or check previously designed beams for these new slightly different loads. This procedure is identical regardless of whether the floors have been meshed and can therefore be followed for this meshed up version of the model by referring back to the Beam Design section in the chapter Transfer Beams – FE Method, Option 1 (Simplest).
Effect of adjusting Slab Stiffness Factor To illustrate this effect, repeat the sequential FE modelling and analysis this time using a slab stiffness factor of 1.0. On completion you can merge and compare the column forces, and as you might expect, you should find that the column force distribution is not greatly affected.
Chapter 26 : Transfer Beams – FE Method, Option 2
Orion Documentation page 463
However, lets now look at the beam design forces that will get merged. The member force diagrams for the front transfer beam are shown below.
The hogging and sagging moments on the transfer beam have both reduced slightly. The shears at each end of the beam have also dropped. In this example the differences are not very dramatic, but in other examples you may well find that they can be. The effect that is occurring is that the slabs are carrying a proportion of the load straight to the supporting columns and by stiffening the slab the effect gets bigger. Once again you should refer to the chapter Analysis and Design using FE for a more detailed discussion of this and other aspects of FE analysis.
Orion Documentation page 464
Chapter 27
Chapter 27 : Solution Option for Transfer Slabs
Solution Option for Transfer Slabs The chapter Transfer Beams – FE Method, Option 2 showed the option to load a meshed up floor model with column loads accrued from upper levels. Since there is no necessity for a column node to sit on a beam, this solution simply reutilises the same sort of meshed floor model option at the transfer slab level.
Modelling and Initial Analysis For Transfer slabs the solution method is essentially the same as for transfer beams. Consider a revised version of the simplified model used in the previous transfer beam examples.
If you want to work through this example for yourself, you can load model DOC_Example_02. In order not to destroy the example for someone else you should save the model with a new name before proceeding. All the beams have been removed at first floor level and a deep flat (transfer) slab has been created.
Chapter 27 : Solution Option for Transfer Slabs
Orion Documentation page 465
There are a few points to note about the modelling of this simple layout that can often have greater implications in larger and more complex models:
Column and Wall Positioning Note that every column and wall sits at the edge but not necessarily at a corner of the slab panel. It is important that you do not define slabs that simply span over or under internal columns and walls. You can check whether you have defined such slabs accidentally by running a building validity check, which will generate warnings for any such clashes.
Orion Documentation page 466
Chapter 27 : Solution Option for Transfer Slabs
Slab Insertion The default slab insertion method is Beam Region in which case Orion will look for a closed perimeter of beams around the area in which you click and insert a slab extending to that perimeter. This option does not work where there are no beams.
For Flat Slab models you need to change the insertion method to either Axis Region or Pick Axes. Using Axis Region Orion will look for a closed perimeter of axes (grids) around the area in which you click and insert a slab extending to that perimeter. If you did that for the model shown above you would end up with 3 slabs. That would not necessarily be a problem, but one slab will do. Using the axis region method you can merge these slabs as you insert them if you press and hold the control key while you click in each of the 3 areas. For more complex boundaries you might use the Pick Axes option in which case you pick the axes that define the boundary of a slab by clicking on them one at a time. Overall it is better to have larger and hence fewer slabs and to avoid small or thin infill strips. This will generally result in better meshing in the FE module, refer to the next chapter Analysis and Design using FE for more guidance on this.
Building Analysis Run the building analysis as described in the Modelling and Analysis section in the chapter Transfer Beams – General Method. During the analysis errors will be displayed as shown below, but you will have the option to continue.
Chapter 27 : Solution Option for Transfer Slabs
Orion Documentation page 467
The first message shown below lists columns and walls that the system regards as unsupported. This basically means columns and walls that stop at an upper floor level and which do not sit on a beam.
Press OK and then the message below appears.
Note that the error suggests that the analysis results will not be reliable – this cannot be ignored if you expect to use the building analysis results for design. In the case of any flat slab structure and in particular transfer slabs you will use FE Chase Down analysis so you will not going to be relying on these results for gravity load design, press Yes to continue. At the end of the analysis you should see an axial load comparison warning.
The first warning is telling us that only 35% of the applied load is being applied successfully decomposed onto beams and hence included in the building analysis. Essentially this is also telling us that the building analysis results are of little value.
Orion Documentation page 468
Chapter 27 : Solution Option for Transfer Slabs
At this point it is worth examining the structural model that has been created and analysed.
The problem is very obvious, the two front columns and the wall at the rear do not sit on anything, so the loads in these frames actually reach the end wall and columns by virtue of beam and vierendeel action at the upper level. This is not a mechanism, it is potentially a valid structure, but it is ignoring the transfer slab and so it is not the structure we want to consider here.
Chapter 27 : Solution Option for Transfer Slabs
Orion Documentation page 469
Alternative Modelling Option Clearly this is quite a regular model and in such cases you can make use of the option to define Support Band beams as shown below.
Band beams are defined along grids A and B. Support band beams are in a sense fictitious beams that allow you to generate a coherent building analysis model but these beams are not passed to the FE model, so they do not affect the FE results. For this particular model the edits shown above mean that the Building Analysis will run with no errors or warnings and the axial load comparison will be perfect – no missing loads. This may initially seem to be the best option for dealing with transfer slabs, and for simple/ regular models it probably is. However, most of the real models we see, and especially those involving flat slabs and transfer slabs, are not regular. It can often be very difficult to insert a logical system of band beams.
Orion Documentation page 470
Chapter 27 : Solution Option for Transfer Slabs
Concluding Note on Modelling Regardless, of how a structure like this is modelled, the building analysis results should all be treated with a good degree of caution. In this simple example the discontinuous columns and walls clearly participate in the lateral load resistance of the structure and none of the modelling options above really deal fully with that issue.
The FE Analysis and Load Chase Down In order to chase the loads down through the building using this model, you could stick to exactly the same sequence and settings as were used in the chapter Transfer Beams – FE Method, Option 2. However, let’s take this opportunity to illustrate that you can swap between the simpler and more complex modelling options at different levels: Start at the top level and work downwards. At the 3rd and then the 2nd floor levels use the grillage (unmeshed) model option as described in the chapter Transfer Beams – FE Method, Option 1 (Simplest). When you get to the first floor check the option to Include Slab Plates in FE model and use the settings and mesh density as described in the chapter Transfer Beams – FE Method, Option 2.
Chapter 27 : Solution Option for Transfer Slabs
Orion Documentation page 471
After analysis use the options for graphical review of the applied loads. The picture below shows the accumulated dead loads applied at the first floor level in this example model together with the deflected contours for that case.
Although a little difficult to see in the above view, the column loads are the same as determined in the chapter Transfer Beams – FE Method, Option 1 (Simplest). This is as expected, since the 3rd and 2nd floor FE models are identical in this chase down, and the analysis at those levels is unaffected by what goes on at the first floor. U
More importantly, after analysis, you can see moment and deflection contours for the transfer slab that take account of the supported column and wall loads. At this point we enter into the general topics associated with FE analysis and Flat slab design which are described in much more detail in the following two chapters, Analysis and Design using FE, and Flat Slab Models.
Orion Documentation page 472
Chapter 27 : Solution Option for Transfer Slabs
For this example we will look quickly at some of the results, which can quickly be exposed.
Chapter 27 : Solution Option for Transfer Slabs
Orion Documentation page 473
In the view above we can see a contour diagram in the background showing design sagging (tension in bottom) moments generated in the longer span direction of the slab. On top of that is a section view showing design moments on a line (red dots along the centre of the strip) cut in the same direction. Note the high local moment at the column head at the right had end of the line.
Orion Documentation page 474
Chapter 27 : Solution Option for Transfer Slabs
By changing to an Integral strip, we can see the results captured and averaged over a specified the width of strip, in this case a 1 m wide strip at the edge of the slab. This shows how the high moment at the RH end is a local effect and in fact higher average hogging moments are generated at the slab to wall boundary at the other end.
Chapter 27 : Solution Option for Transfer Slabs
Orion Documentation page 475
This next view shows the top and bottom reinforcing requirements averaged over this 1 m wide strip. The view below shows As required contours for bottom steel in direction 1.
Note
While in some respects a deep slab such as illustrated in this example can be treated as any other flat slab it is noted that such slabs may well also require some different/extra design and detailing considerations in areas such as deflection, punching, and perhaps reinforcement bundling. It is recommended that the details of such important structural elements be thoroughly cross checked.
Orion Documentation page 476
Chapter 27 : Solution Option for Transfer Slabs
Axial Load Comparison Once again, having completed an FE Chase Down (sequential FE analysis of all floors starting at the top and working down) you can review the resulting column loads in the Axial Load Comparison Report accessed from the main building analysis dialog.
For this model (before introduction of any band beams at first floor level) we still have the discrepancies occurring during load decomposition and building analysis that are discussed earlier in this chapter. However, the results for the FE Chase Down (shown in the last table in the report above) compare perfectly with the sum of applied loads in the first table (the undecomposed slab loads). This means that the FE Chase Down procedure appear to have been successful and we can now continue to merge column and beam design forces.
Chapter 27 : Solution Option for Transfer Slabs
Orion Documentation page 477
Merging Column Analysis Results
Having completed the sequential floor analysis column design forces can be updated by merging the results from FE with those from the main building analysis. You can then design columns or check previously designed columns for these new slightly different loads. This procedure is identical regardless of whether the floors have been meshed and can therefore be followed for this meshed up version of the model by referring back to the section Merging Column Analysis Results in the chapter Transfer Beams – FE Method, Option 2.
Merging Beam Analysis Results As with the column results above, the beam results are merged by checking the appropriate button on the postprocessor tab of the FE analysis dialog shown above. The notes in the previous chapters regarding the way these FE Chase Down methods concentrate the loading in the lowest beam are equally applicable here, but in this case it applies to the slab. For the beams at the upper floors the design moments from the building analysis may or may not be meaningful, it will depend on which of the modelling options discussed earlier in this chapter you have used. If they are not useful you will be restricted to designing the beams for the merged FE results. It is likely that the use of the alternative modelling option (earlier in this chapter) is most likely to generate meaningful design information in the building analysis phase.
Orion Documentation page 478
Chapter 28
Chapter 28 : Flat Slab Models
Flat Slab Models
Introduction When tackling the subject of flat slabs some fundamental issues routinely arise: The building codes provide simplified methods of dealing with regular flat slab models. In practice a large proportion of models do not seem to fit within these limitations and so engineers turn to more advanced FE methods. Having turned to more advanced FE methods in order to look at models with more irregular geometry, engineers are often faced with FE results that seem unreasonable to them. Then there is the need to consider compliance with deflection limits. These topics and more are all discussed in detail within a training presentation that is available in PDF format alongside this document and accessed via the link below. Orion Flat Slab Training.pdf (If the link does not work please browse to find the file name indicated above in the HELP sub-folder of the Orion Program Folder). The above presentation extract is given and discussed in detail during Orion Advanced Training Days. The extract assumes that the content of the chapter Analysis and Design using FE of this handbook has already been covered. The remainder of this chapter focuses on a practical example in which the techniques described in the presentation are put to use to design an irregular model. Orion is also able to undertake the Punching Shear Checks associated with flat slab design. The implementation of these checks and associated limations are discussed in the succeeding chapter.
Scope of Flat Slab Design in Orion Braced Buildings Orion’s FE module for flat slab analysis and design is aimed primarily at Braced Buildings as defined in the relevant design code. Typically this means that the lateral stability of the building should be provided by shear walls and that the slabs are then only designed for Gravity Load effects. Un-Braced Buildings In this more unusual case lateral stability is provided by interaction between the slabs and the columns. This sort of construction appears to be less common and is usually restricted to relatively low rise buildings. In such cases the sway design is beyond the current scope of Orion. However, some engineer’s have still used Orion for a gravity load design and then made separate checks/assessments of the sway effects. Where the structural grid is sufficiently regular these checks have sometimes
Chapter 28 : Flat Slab Models
Orion Documentation page 479
made use of Orion’s Band Beam option which models fictitious beams within the slab. However, it must be accepted at the outset that this form of design will require more thoughtful, interactive, and time consuming design involvement by the engineer.
Example of a more Irregular Model Overview The training presentation concentrates on regular models so that comparisons can be made between traditional code methods and results achieved using FE. In this example a more irregular problem will be considered.
Consider the model shown above. A core wall system is assumed to brace the structure and therefore the flat slabs and peripheral columns are to be designed for gravity loads only.
Orion Documentation page 480
Chapter 28 : Flat Slab Models
Although it looks like a simple layout, it would actually be very difficult to apply the idealisation of column and middle strips and hence it becomes almost impossible to apply the code’s simplified design methods.
Slab Analysis, Design and Detailing The methods illustrated in the training presentation for a simple/regular model are in principle equally applicable to this more irregular model. However, we will work through the steps again to emphasise some points of note. If you want to work through the example for yourself, you can load model DOC_Example_06. In order not to destroy the example for someone else you should then save the model with a new name before proceeding. Meshing
Ensure that the concrete material properties are adjusted so that a long term E value is used, in this case C40 concrete is used with E set to 7000 N/mm2. At the 4th floor level use settings as shown above.
Chapter 28 : Flat Slab Models
Orion Documentation page 481
You could adjust the mesh density from the default down to a lower setting that you think might be acceptable. However, using the default 1500 shells and setting the regularity factor to 1.0 produces a mesh as shown below which is clearly more than adequate.
Analyse the model and then access the postprocessor setting the Positive (sagging) Moment Factor to 1.2.
Orion Documentation page 482
Chapter 28 : Flat Slab Models
Deflection
Estimated total deflection is peaking at around 70 mm along the perimeter lengths between columns. The distance between these columns is about 11.3 m therefore we should be looking to see total deflection restricted to around 11300/250 = 45 mm. A 300 thick slab is probably not adequate for these long edges. However we will continue with the example unaltered. Bottom Steel Reinforcement Provision Before starting to review reinforcement requirements right click in the graphics area and set the effective depth information to some reasonable values.
Chapter 28 : Flat Slab Models
Orion Documentation page 483
We will start by assuming that the whole slab is to be reinforced orthogonally in the Global X and Y directions. At this point the slabs reinforcement angles have not been adjusted, so direction 1 steel is aligned with global X, which runs from left to right in the view below.
Orion Documentation page 484
Chapter 28 : Flat Slab Models
This view shows bottom steel requirements in direction 1 and as might be anticipated the peak requirements are occurring along the longest free edges. In direction 2 (below) we see a very similar situation and a very similar peak requirement of just over 1300 mm2/m.
In practice you may want to consider the reinforcing requirements if the main steel along the angled edges is aligned to the edge. This can be done and will be shown in the Additional Notes on Bottom Steel Provision section at the end of this example.
Chapter 28 : Flat Slab Models
Orion Documentation page 485
We can now use the custom contours option to show where different steel reinforcement provisions would be adequate. In general the strategy would be to decide on some general lower level of reinforcement to be provided continuously throughout the slab and identify the regions where an increased provision is required.
In the view above we have set the lower general provision to T12 at 200 and we can see that this is sufficient over a large proportion of the slab. We have then considered the possibility of laying in extra T16 at 200 (providing T12 and T16 alternate bars) and the view shows that this is adequate everywhere else. These requirements can be simply communicated to the detailer by exporting the above contours to the main graphical editor and then to DXF. The same procedure would be repeated for the bottom steel in both directions.
Orion Documentation page 486
Top Steel Reinforcement Provision
Chapter 28 : Flat Slab Models
Chapter 28 : Flat Slab Models
Orion Documentation page 487
As expected, a review of the top reinforcing requirements shows that in theory no steel is required over large areas of the slab and that the hogging moments intensify rapidly over the column heads and the core wall. However, many (most?) engineers would tend to provide minimum reinforcement throughout the top of an irregular flat slab and so the user-defined contours shown below show this as the minimum level.
In this case we can see T10 at 200 will be provided everywhere. Then we see large areas where T16 at 200 would need to be added leaving relatively small areas where a higher provision is necessary. As is covered in the training presentation, the only practical way to deal with these peak requirements is to integrate results in strips cut across the column and wall heads.
Orion Documentation page 488
Chapter 28 : Flat Slab Models
In the view below a 2 m wide strip is cut across the head of the most critical internal column.
Based on the proportions and span of the slabs involved this might be considered as a reasonable strip width and the average steel requirement in this width is 1772 mm2/m. However, if this steel were provided, it must be provided over at least the 2 m width of the strip. Clearly this strip strays beyond the contour boundaries where a much lower steel requirement has been shown to be adequate.
Chapter 28 : Flat Slab Models
Orion Documentation page 489
As an alternative it is more conservative to cut a narrower strip as shown below.
This strip is cut just within the boundaries where the lower provision of T10@200 + T16@200 is shown to be adequate. The average requirement in this narrower strip has increased to 2187 mm2/m. (This is also more in line with the rule of thumb expectation that the peak hogging steel provision will be in the order of double the peak sagging steel provision).
Orion Documentation page 490
Chapter 28 : Flat Slab Models
The same principle is applied at the walls.
Peak requirements tend to occur around the ends/corners of walls, and a little unusually in this example the peak requirement at the wall corner is actually higher than at the column. Once again there would be arguments for cutting a wider strip and determining a lower
Chapter 28 : Flat Slab Models
Orion Documentation page 491
average steel requirement in the vicinity of the wall, but given that we are working with a standard 200 spacing elsewhere, the choice here lies between T20@200 and T25@200 – clearly T25@ 200 are required.
In order to achieve standardisation throughout you may then choose to provide T25 at 200 as a standard patch of reinforcement across all column heads. Once again this information can be communicated to the detailed by adjusting the user defined contours as shown above. The yellow patches indicate the minimum zones in which the peak reinforcement is required. However, it would probably be good practice to apply a standard minimum patch throughout, in this case something like a minimum of 8 T25@200 across all column heads. To be effective these bars must also extend a full anchorage length beyond the point where they are required.
Orion Documentation page 492
Chapter 28 : Flat Slab Models
Additional Notes on Bottom Steel Provision
It was noted that for this slab some of the highest sagging moments and hence the greatest bottom steel requirements occur along the angled slab edges. This requirement was being established on the assumption that direction 1 and 2 steel will be provided in the global X and Y directions throughout. The steel requirement at this point is high in both directions.
Chapter 28 : Flat Slab Models
Orion Documentation page 493
If you check you will find high design moments in both directions at these points, and if you look at the unadjusted moments you will find that Mxy is very high and that the Wood and Armer adjustment is having a big impact on the design moments for reinforcement if it is not going to be provided parallel to the free edge.
Logically you would expect that steel provided parallel to the free edge would be the most efficient solution in an area like this.
Orion Documentation page 494
Chapter 28 : Flat Slab Models
There are 2 ways in which this requirement can be investigated in Orion.
When you cut a strip at an angle as shown above you will automatically be looking at moments and hence reinforcing requirements along the cut line.
Chapter 28 : Flat Slab Models
Orion Documentation page 495
Alternatively you can define the intended reinforcing angles in any/every slab independently and view contours on that basis. In the view below, you can see how the reinforcing angle has been reset in the four panels adjacent to these edges.
Orion Documentation page 496
Chapter 28 : Flat Slab Models
After re-meshing and reanalysis, you will see contours as shown below.
Chapter 28 : Flat Slab Models
Orion Documentation page 497
The direction 1 steel area requirements are shown above appear very similar to the requirements shown earlier before this adjustment was made. The direction 2 requirements shown below are quite different. Direction 2 is perpendicular to the free edges, and as expected the reinforcement requirement becomes nominal at the edges where a peak was previously exposed.
In other examples the feature that allows contours to be simultaneously displayed relative to different axis systems can seem a little strange until you get used to them, refer back to the Review of Contouring Options in the chapter Analysis and Design using FE for more detail if necessary. X
UX
In terms of the weight of reinforcing provided, it would be more efficient to design a perimeter ring and then place orthogonal infill steel, but this may not be the fastest/simplest or even the cheapest construction solution. Using Orion as shown above you can quite easily investigate such options and you could even create extra slabs to define a ring beam zone. As was noted in this example and in the training presentation, it is relatively easy to generate the design information (the information that can be passed to a detailer) in Orion. This example should reinforce the view that for flat slabs it may be better to restrict your use of Orion to this level of detail and then revert to traditional detailing.
Orion Documentation page 498
Chapter 28 : Flat Slab Models
Column Design By following the procedure for sequential FE floor analysis and then export of column design forces described in the chapter Transfer Beams – FE Method, Option 2 column loads can be made available for design.
The view above shows that axial loads and moments are transferred from the FE analysis (short frame model) and that the braced columns can now be designed in Orion including for IL reduction factors if desired. Since there are no beams attached to these columns Orion assumes a small strip of slab is effective as the beam and calculates effective length factors on this basis.
Chapter 29 : Punching Shear Checks
Chapter 29
Orion Documentation page 499
Punching Shear Checks
Introduction In Orion R13.0 a basic interactive punching shear check was introduced. Since then some enhancements have been made which we believe have made it a more attractive design aid. However, the nature of the design code requirements tend to dictate that punching shear checks can not be safely automated in batch runs. For this reason Orion’s punching shear check is an interactive design tool, you need to use it carefully on one column at a time to review/establish the correct design settings for each column. Having done so rechecking can be carried out in a more automatic fashion. In this chapter we will show what this check can do for you and illustrate the limitations of which you need to be aware.
BS8110 Design Code Requirements The Design Procedure In BS8110 section 3.7.6 covers the requirements for checking punching shear. Orion’s punching shear check cannot be used without a reasonable understanding of the code’s requirements. The following notes provide a brief overview of these then in the next section of this chapter we will use specific examples to go into more detail and illustrate limitations. BS8110 cl 3.7.7.6 describes a design procedure; the notes below expand on this.
Check Maximum Shear Capacity This is a check at the face of the loaded area, refer to BS8110 cl 3.7.6.4. By default Orion will perform this check, but limitations do apply.
Check Shear on Series of Perimeters The first perimeter is at 1.5 d from the face of the column (the loaded area) subsequent perimeters are at 0.75 d increments – i.e. 2.25 d, 3.00 d, 3.75 d, etc. To make an accurate check on each perimeter the following needs to be known/controlled: • Is it an internal/edge/corner condition?
• How much main reinforcement is effective? • Are there holes, for which you need to account? For each perimeter three results are possible: • The check fails with v > 2 vc in which case a required area of shear reinforcement cannot be calculated so the whole checking procedure terminates.
Orion Documentation page 500
Chapter 29 : Punching Shear Checks
• The check fails but the required area of shear reinforcement can be calculated and the checking procedure moves on to the next check perimeter.
• The check passes – in which case no shear reinforcement is required and no further checks on subsequent perimeters are required. Note
Where shear reinforcement is found to be required on a Check Perimeter it is important to note that it is then provided on (and shared between) two reinforcing perimeters as discussed later in this chapter.
Simple Examples In this section we will look at a series of relatively simple examples using the model shown below.
However, although it looks simple, this model is sufficient to illustrate most aspects of the checking procedure including: • A typical internal column.
Chapter 29 : Punching Shear Checks
Orion Documentation page 501
• Typical edge columns with varying slab cantilevers. • Typical corner columns with varying slab cantilevers. • The effects of openings. The model is set out on a regular 7.5 m grid, the slab is 300 thick with 0.5 kN/m2 finishes and 5.0 kN/m2 imposed load. The columns are 400 mm square.
Checking a Typical Internal Column
Orion Documentation page 502
Chapter 29 : Punching Shear Checks
Performing the Check In the view above column 2C9 is selected, right click and select the Column Punching Check option to expose the punching check dialog as shown. The options this exposes will generally be discussed in more detail as we go through the examples, but in summary they are:
Floor Slab Merging Options • These options would be used in cases where the punching perimeters are not found (generally in cases of complex slab geometry), refer to the section Slab Merging for more details. In the first instance these options can be ignored.
Column Punching Perimeter Options • Rectangular/Polyline Column – allows you to choose the shapes of punching perimeters that will be considered. BS8110 only considers idealised rectangular perimeters as highlighted, so all checks will be made using this setting. Note
For circular columns, the same principle applies - BS8110 only considers idealised rectangular perimeters so checks should be made using the rectangular as opposed to the circular setting.
• Check Column Perimeter and Check Column Drop Panel Perimeter (only active if a drop has been defined) – These options allow the check on maximum shear capacity at the face of the loaded area to be toggled on and off.
• Periphery Reduction Amount – An allowance for holes that have not been modelled or may be cut in the future – see the section on Dealing with Openings.
• Include Load Within Punching Perimeter – When this option is checked the shear load considered on each successive punching perimeters will reduce (due to the increasing area of loading that falls within the punching perimeter. When un-checked the shear load will be the same on all perimeters – i.e. the check becomes more conservative.
Column Location • As will be illustrated in the following examples, it is the user’s responsibility to indicate the location (internal/edge/corner) and hence the more applicable shear checks as detailed in BS8110 cl 3.7.6.2 and 3.7.6.3.
Slab Reinforcement X (and Y) • The effective area of tension reinforcement in each direction must be input manually. Checking Maximum Shear Capacity This is a check at the face of the loaded area, refer to BS8110 cl 3.7.6.4.
Chapter 29 : Punching Shear Checks
Orion Documentation page 503
If no changes are made and the column is checked with the settings as shown the result is as shown below.
The output indicates the design forces and then makes the first punching check (at the face of the loaded area). For this and most reasonable examples this check passes. The red perimeter is indicating that the checks then fail at the next perimeter – see below. Check Shear on a Series of Perimeters Continuing with the same example, the code requires the first perimeter check to be made at 1.5 d from the face of the column (the loaded area). Note that Vt is slightly reduced for this perimeter, from 1139.5 kN at the face of the column to 1112.8 kN (because the option to include the load within the punching perimeter is active – i.e. we have included it’s exclusion.)
Orion Documentation page 504
Chapter 29 : Punching Shear Checks
The output above indicates that v > 2vc – reference should be made to BS8110 cl 3.7.7.5, once v exceeds this level justification of shear resistance moves beyond the limits of the code. Hence in this situation Orion simply indicates the perimeter in red on the plan view and the output suggests that something must be done to increase vc (or reduce v). The shear capacity vc is influenced by concrete grade and provided steel. For this check we accepted the default bars T8@250. Clearly a much higher steel provision would be normal across a column head so we can now rerun the check with more realistic steel – say T20@200 (probably still a little low).
The result for this revised check is shown above. The 1st perimeter at 1.5 d is checked and it fails, and Asv required is determined as 1088 mm2. The 2nd perimeter at 2.25 d is checked and it fails, and Asv required is determined as 1445 mm2.
Chapter 29 : Punching Shear Checks
Orion Documentation page 505
The 3rd perimeter at 3.0 d is checked and it passes – so no further perimeters need to be checked. The plan view reflects these key results – the first two perimeters are shown red and the Asv requirement is written on each perimeter. The 3rd perimeter is shown green. This sort of red, red, green series of perimeters is ultimately what you will see against all of the columns. Note
Since the punching shear perimeter length increases and the shear load decreases on each successive perimeter you may expect to see a reducing Asv requirement on the successive perimeters. For higher shear loads this will be true, however, where nominal shear reinforcement is required (Asv min = 0.4ud/0.95fyv) the requirement will increase as a function of perimeter length (u).
Note
The input of main reinforcement has 2 effects. The bar size influences the value of effective depth (d) used in the checks, and the spacing sets the total effective tension steel (Ast). In practice you might actually have alternate bars of different sizes providing a total Ast (say 1850 mm2/m for the purposes of an example). In this case you would simply use the larger bar size and adjust the spacing so as to achieve the correct Ast value as shown below. It is the Ast value that is important, the spacing is irrelevant in the context of this check.
Providing Shear Reinforcement Orion will create a plan view showing all the failed perimeters and the associated Asv requirements.
Note that on 2C10 the very slightly higher punching load is sufficient to mean that the Asv requirement on the first perimeter is greater than the minimum requirement which applies on 2C9. On the second perimeter however the minimum requirement applies to both columns.
Orion Documentation page 506
Chapter 29 : Punching Shear Checks
In practice punching shear resistance may be provided in different ways using various proprietary systems. If it is to be provided using links, the detailing requirements of BS8110 cl 3.7.7.6 and fig 3.17 must be adhered to. This remains a detailing exercise but the following notes indicate the points to watch – continue to consider column 2C9: Check Perimeter at 1.5 d — The 1088 mm2 required at the first check perimeter should be provided on at least 2 reinforcement perimeters: At 0.5 d – at least 40% i.e. approx. 435 mm2 At 1.25 d – the balance of approx. 653 mm2 Check Perimeter at 2.25 d — The 1445 mm2 at this second check perimeter must again be provided on 2 reinforcement perimeters the first of which is at 1.25 d and is therefore common to the previous check perimeter. At 1.25 d – at least 40% i.e. approx. 578 mm2 – but 653 mm2 already provided above. At 2.0 d – the balance of approx. 1445 – 653 = 792 mm2 is required. Now consider the detailing requirements on each perimeter. The main restriction to note is that the maximum spacing of link legs along any perimeter is 1.5 d which in this example is a spacing of 380 mm. The spacing of main bars is likely to further dictate the maximum spacing that can be used – say main bars are at 100 crs then maximum reasonable spacing is 300 mm. The table below indicates the minimum area of shear reinforcement that will be provided on each perimeter if a 300 mm link leg spacing is used.
Perimeter at (distance from face of column)
Minimum Asv req’d mm2
Perimeter Length (mm)
Min No of Link Legs (at 300 mm crs)
0.5 d
435
2620
1.25 d
653
2d
792
Min Area of shear reinforcement provided assuming min bar size of: T6
T8
T10
9
254 mm2
452 mm2
707 mm2
4150
14
396 mm2
704 mm2
1100 mm2
5680
19
537 mm2
955 mm2
1491 mm2
It seems that T8 legs at 300 will be required on each of 3 reinforcing perimeters. Note
There are some further points to consider in relation to the provision of shear reinforcement around holes. See further comments in the section Dealing with Openings.
Chapter 29 : Punching Shear Checks
Orion Documentation page 507
Checking a Typical Edge Column
In the above view a column close to an edge has been selected and a check has been made using the same settings as were used for the internal columns above. The view shows how Orion has correctly detected the shortest punching perimeter in accordance with BS8110 figure 3.19, however, clauses 3.7.6.2 and 3.7.6.3 require that a distinction is made between internal and edge columns. This affects the calculation of Veff in both directions. Orion cannot make this distinction automatically for all potential geometries, and so it never attempts to make it at all. THE USER MUST ALWAYS CONTROL THIS SELECTION.
Orion Documentation page 508
Chapter 29 : Punching Shear Checks
In this simple case the red and green perimeters clearly indicate that the free edge is close enough that this check should be made as an edge condition on all perimeters. If we rerun the check having changed to the appropriate column location setting the result changes as shown below.
The result looks similar, but the Asv requirements have actually changed. In actual fact in this example the requirement on the first perimeter has reduced, and on the second perimeter min requirements continue to apply. In fact, because of the way Orion calculates the dimension x used in the calculation of Veff (see later discussion) this is likely to be the case, but this result cannot be guaranteed for all edge conditions.
Chapter 29 : Punching Shear Checks
Orion Documentation page 509
As an example of where some care needs to be taken with the selection of edge conditions, consider the column shown below.
The first perimeter is that of an internal column, the second is an edge condition. When checked as an internal column the Asv requirement shown above is 1088 mm2 (min reinforcement). When checked as an edge column the calculation of v changes, but the minimum Asv requirement continues to apply – i.e. the result is unchanged in this example. Beyond the requirement to consider and apply the appropriate check there is no difference between the checks for internal and edge columns.
Orion Documentation page 510
Chapter 29 : Punching Shear Checks
Checking a Typical Corner Column
The example shown above illustrates a simple corner column case. The points made for the edge column in the previous section are equally applicable here, the obvious difference being that the corner column location must be selected. In this example, the corner column with significant overhangs passes the punching shear check with no reinforcement being required.
Chapter 29 : Punching Shear Checks
Orion Documentation page 511
Column Drop Panels
In the view above column 2C10 has been selected and a 2m square by 500mm deep drop panel has been defined at the top of the column. The purpose of a drop panel is typically to remove the need for additional punching shear reinforcement. This can be achieved by adjusting the geometry of the column drop panel.
When the checks are performed the objective is therefore to obtain two green perimeters as shown above. The inner perimeter (at 1.5d from the column) confirms that the drop is sufficiently deep and the outer (at 1.5d from the drop) confirms that it is sufficiently wide.
Orion Documentation page 512
Chapter 29 : Punching Shear Checks
Check Punching around the Column Perimeter For the inner perimeter shown above to be determined (and an output report created for it), the option Check Column Perimeter has to be ticked on punching check dialog. When the calculation is performed punching checks are carried out at the face of the loaded area and at the first perimeter, (both checks taking account of the increased slab depth). A red perimeter indicates that the depth of the drop panel should be increased.
Note
Depending on the size of panel, it is possible for the first perimeter at 1.5d to occur outside the drop panel. The calculations are then performed at this perimeter based on the increased slab depth. however, in such a case the calculation becomes irrelevant and can be ignored.
Chapter 29 : Punching Shear Checks
Orion Documentation page 513
Check Punching around the Drop Panel Perimeter The Check Column Drop Panel Perimeter option must be ticked in order to check the outer perimeter beyond the drop panel. This time when the calculation is performed, a check is carried out at the face of the drop panel (taking account of the increased slab depth) and subsequent perimeters are then checked beyond this for the unthickened slab. A red perimeter indicates that the width of the drop panel should be increased.
Note
For an output report to be created for the results around the drop panel perimeter (as shown above), the option Check Column Perimeter should be left unticked. If both options Check Column Perimeter and Check Column Drop Panel Perimeter are ticked, all the required checks are performed and the graphical display of the perimeters is created accordingly, however, the accompanying report will only display the results around the column perimeter.
Orion Documentation page 514
Chapter 29 : Punching Shear Checks
Dealing with Openings Openings which have been modelled
In the view above a small hole has been positioned close to a column. On the punching perimeters that are drawn you can then see sections in black (rather than red or green) indicating the lengths on the check perimeters that are excluded. The excluded length is established as the length between lines radiating from the centre of the column past each of the extreme edges of the hole.
Chapter 29 : Punching Shear Checks
Orion Documentation page 515
In actual fact this is quite a small hole (150 mm square) and is actually something we would not recommend complicating the FE modelling with. We would tend to recommend that you allow for something like this by using the Periphery Reduction Amount option. Before moving on to that, it is worth noting just how significant the reductions due to this (any) small hole can be.
Examining the calculations above, note that: 1. At the face of the column –the perimeter length for the maximum shear capacity check is never reduced, although this is rarely critical by the time other checks are passing, you may want to consider the possible effect of some reduction here. 2. At 1.5 d from face – The perimeter length is reduced by 393 mm (from 4660 mm to 4267 mm). 3. At 2.25 d from face – The perimeter length is reduced by 522 mm (from 6190 mm to 5668 mm).
Orion Documentation page 516
Chapter 29 : Punching Shear Checks
4. At 3.00 d from face – The perimeter length is reduced by 651 mm (from 7720 mm to 7069 mm). As expected the reduction length is increasing. The main point to note is that a small hole of 150 mm side quite close to the face of the column reduces the punching perimeter by much longer lengths. Note
Providing Shear Reinforcement – The reported Asv requirements relate to the effective perimeter length even though there is an unbroken full perimeter. The detailing guidance in BS8110 does not indicate whether the required reinforcement should be provided on a similarly shortened perimeter. An option to allow for this would be to factor up the Asv requirement by the ratio of the full perimeter to the reduced perimeter and then provide reinforcement on the full perimeter.
Note
Due to the almost infinite variations of intersecting slab shapes and hole shapes and positioning, it is quite possible that Orion will have difficulties accurately predicting punching perimeter reductions, this point is illustrated in the concluding notes.
Calculation of the Effective Shear Force During punching shear checks the identified transfer force (Vt) is adjusted to allow for variations induced by transfer moments (Mt). Referring to BS8110 clauses 3.7.6.3 and 3.7.6.4, the applicable cases and equations are:
Internal Columns: •For bending in both directions: Veff = Vt (1 + 1.5 Mt/(Vt*x))
(eq25)
Edge Columns •Where bending is about an axis parallel to the free edge: Veff = 1.25 Vt
•Where bending is about an axis perpendicular to the free edge: Veff = Vt (1.25 + 1.5 Mt/(Vt*x))
(eq26)
Corner Columns •For bending in both directions: Veff = 1.25 Vt BS8110 defines x as the length of the side of the perimeter considered parallel to the axis of bending. There is no guidance as to whether or not this dimension should be shortened if openings affect the perimeter. There is no guidance on what to do when the length is shortened on one face but not on the other. Where opinions have been sought on this there is a view that x should not be shortened to account for openings, however no authoritative guidance has been found to this effect. Looking to the above example, for direction 1 bending on the first perimeter one side is shortened by 393 mm. The most conservative view would be that x should be taken as 1165 393 = 772 mm. In Orion x is taken as the average of the two relevant sides, in this case (1165 + 772) / 2 = 968 mm. This is believed to be a reasonably conservative interpretation of the code requirements.
Chapter 29 : Punching Shear Checks
Orion Documentation page 517
Allowing for Openings which have not been modelled As noted above the modelling of small holes creates unnecessary complication and is often best avoided. In many projects there may also be a need to allow for holes without actually knowing for sure where they are. This can be achieved using the option to specify a periphery reduction. Using the same example as above and initially applying a reduction length of 393 mm (the same as Orion calculated for the first perimeter) the result is as shown below.
Points to note are: 1. Looking at the detail of the calculations you may note that x is not reduced to account for the existence of un-modelled holes. This avoids what some may regard as the over conservative adjustments to x which Orion applies when the holes are modelled.
Orion Documentation page 518
Chapter 29 : Punching Shear Checks
2. Despite the above, for the first perimeter the calculated Asv requirement is identical. This is because the design in direction 2 is dominating and in that direction x was not reduced when the hole was modelled. 3. For the second perimeter the Asv requirement is a little higher because using this option the length reduction is constant on all perimeters (hence it is 393 mm rather than 522 mm). On this perimeter min Asv requirements are being used and so the requirement increases as the perimeter length increases. Given that we are only allowing for the possibility of a hole, we should check the requirements when no hole is allowed for as shown below.
In essence the above is exemplifying the note made earlier regarding the practicalities of providing the shear reinforcement. If the Asv requirements obtained by excluding a perimeter length are factored up by the ratio of the full perimeter to the reduced perimeter so that reinforcement can then be provided on the full perimeter a very reasonable result will be achieved, perhaps erring on the side of conservatism. In conclusion, for this model if an allowance had to be made for the possibility of holes (up to say 150 mm) adjacent to any column, it would be quite reasonable to specify a periphery reduction of around 400 mm to all columns rather than take the time to attempt to model every possible hole in detail. The reported Asv requirement might then be factored up slightly to allow for the point made above.
Chapter 29 : Punching Shear Checks
Orion Documentation page 519
Final Batch Check and Output Having set up the punching design information at each column location it is possible to rerun a batch check on punching shear for all columns as shown below.
Orion Documentation page 520
Chapter 29 : Punching Shear Checks
When accessed this option will show a table where the key design information can be reviewed and adjusted if necessary.
So for the final run on this model a perimeter reduction of 400 mm is applied to all columns.
Concluding Notes Limitations Three points emphasised throughout this chapter are: 1. The punching shear checks required by design codes are quite empirical by nature and engineering judgement is often required as to which part of the checks are most applicable. 2. Potentially there are an almost infinite number of variations of intersecting slab shapes, hole shapes, discontinuous edges, etc. It is quite possible that Orion will have difficulties accurately predicting punching perimeters in many circumstances. 3. As a result, the checks must initially be carried out on one column at a time so that the engineer can take proper control over the input and review the results. The areas in which we would particularly highlight the limitations of the current checking are:
Chapter 29 : Punching Shear Checks
Orion Documentation page 521
Holes It is possible that Orion will have difficulty identifying correct punching perimeters as the geometry of slabs and openings become more complex. The example below shows the sort of issue that could occur. Two holes are positioned near to a column. Orion has identified the circular hole outside the check perimeters and has reduced the perimeters accordingly, the black sections of the perimeter lines can be seen on close examination.
For the rectangular opening however, reduced lengths are also shown that extend above and below the edges of the opening. The above view was created using an older release of Orion. However, there is some debate as to whether these extended exclusions are actually required, and in fact introducing these extensions was found to cause other difficulties.
Orion Documentation page 522
Chapter 29 : Punching Shear Checks
The view below shows the punching perimeters that are now generated in the current release.
There are no extensions to the excluded lengths where a check perimeter is actually broken by a hole, the splayed reduction of length is only applied to unbroken perimeters. Note that an additional periphery reduction is still applied – this value could be further increased if you wished to make more allowance for extended exclusions at the sides of the hole. If you identify or are concerned about problems of this nature we would appreciate being sent the example so that we can look into the matter. In the meantime we would suggest using the options to either simplify the holes or to remove them and specify a periphery reduction length. Dimension ‘x’ used at the face of the loaded area At the face of the loaded area you will find that value of “x” used is the same as at the first check perimeter. It is not taken as just the column dimension. The justification for this assumption is discussed below: We found that when you do the check using the actual column dimensions then columns in fairly ordinary situations start to fail. When we talked to people in the industry about this we found that in practice they tended to ignore this check completely (assuming it to be ok by inspection) or if they did it they just used the actual shear force and did not bother to amplify it at all, or sometimes that they just assumed the 1.15 amplification noted in Clause 3.6.7.2 would cover it. Everyone felt that the check seemed incorrect (over conservative) when applied more rigorously using the column dimensions as "x" at the face of a column.
Chapter 29 : Punching Shear Checks
Orion Documentation page 523
Basically as you approach the column "x" decreases rapidly and so the amplification calculated in Veff becomes extreme. Internal columns with ordinary transfer moments are treated as worse cases than edge columns with very high transfer moments where the amplification of Veff is a fixed value of 1.25Vt. It also seems to work back to front in terms of the way it amplifies V - for example using a 600x300 column, if the same transfer moment is applied in both directions the amplification of Veff will be greater when bending is in the strong direction (because x=300) than when bending in the weak direction - seems counter intuitive? Furthermore there is the complication that “x” is unquantifiable for irregular and even circular column perimeters. After investigations at CSC and external consultations, the conclusion arrived at was that making the same amplification as at the first check perimeter seemed reasonable and probably more conservative than what had previously been done/assumed in practice for many years. Slab Merging For Orion to be able to resolve the punching perimeter around any column all of the slabs inserted around that area need to be merged into one. For simple geometries this is not a challenge. For more complex cases it is a significant challenge where improvements have been made on an ongoing basis. Where difficulties are encountered the floor slab merge options may help to either resolve the issue completely, or isolate the problem so that the modelling can be adjusted. The examples below are exaggerated in some respects but they do show how the features work.
Orion Documentation page 524
Chapter 29 : Punching Shear Checks
Notice that some of the punching perimeters in the slab area shown above are clearly incorrect. The option to Hatch Floor Area can be used as shown below and this shades all the slab areas that Orion has managed to merge.
Clearly one of the slabs has not been included, the reasons why this might happen vary, but usually become apparent if the boundary of the slab is examined in close up. In this example the slab and some of the adjacent small strips do not meet properly, the slabs can be redefined and the merged floor area and punching perimeters are all then resolved as shown below.
Chapter 29 : Punching Shear Checks
Orion Documentation page 525
Note
In general it is good practice to avoid thin strips of slab and have fewer slabs wherever possible. Not only does it avoid the sort of problem shown above, it is also less likely to cause meshing difficulties during the FE analysis.
Tip
The Slab Merging option can be used at any stage during model creation, therefore you can check your model for potential problems as you create it rather than finding problems later on when much more editing (and reanalysis) may be required to eliminate them.
A second typical problem occurs where lots of grids meet at or near a single point. The view below shows a column where 6 grids are meeting in close proximity and the punching shear check is showing a long section of black (missed) perimeter.
Orion Documentation page 526
Chapter 29 : Punching Shear Checks
When the hatching option is used you can begin to see more clearly how the slabs have not been defined so that a triangle has been missed within the column. (This sort of problem usually occurs in smaller triangles than shown here.)
If we now check the option to include the column wall and beam edges and then re-hatch, the area inside the column is considered as another slab area and so the merged boundary works round the perimeter of the column. This will potentially avoid all problems relating to local inaccuracies where grids meet.
Chapter 29 : Punching Shear Checks
Orion Documentation page 527
If the punching shear check is them made with this option active the perimeters seem much more reasonable. Note that in this example the missed triangle actually extends outside the column and so is considered as a hole with the resultant shortening of the punching perimeter. Specification of Effective Slab Reinforcement When defining the effective slab reinforcement you can only provide one value which will apply at all the check perimeters. In practice it is possible that lesser values of effective tension reinforcement may be applicable at perimeters further from the column perimeter. In such cases results from multiple checks may need to be used, or the more conservative (lower) value could be used for a single check across all perimeters. Providing the Shear Reinforcement Orion provides the design checks and information to allow the detailing to commence. As noted within the examples, the detailing is not necessarily a trivial matter, especially when allowances for holes need to be made. Punching Perimeters The displayed punching perimeters should look correct and complete. No examples of incorrect perimeters are shown in this document (other than the debatable perimeters shown above). The presence of holes and/or complex irregular slab geometry may trigger problems in the perimeter identification. Where holes appear to be the source of the problem it is again worth considering removing the holes and using the option to specify a periphery reduction. Where slab geometry appears to be the source of the problem it is possible that the slabs can be redefined locally to avoid the problem.
Orion Documentation page 528
Chapter 29 : Punching Shear Checks
Walls
Checks on walls are made but should be viewed with particular caution. The example above shows 3 discrete walls being checked. In this example the perimeter of the wall on grid D looks suspicious (line being drawn outside the slab). On closer checking you would find that the effective length of the punching perimeter is actually correct. It is particularly advisable to check the lengths used for the punching perimeters in the case of walls, and it is also noted that overlapping perimeters (see Overlapping Perimeters below) will almost always apply where core walls are used. In addition there could well be some debate as to the applicability of a concentrated punching load check in the case of long walls, BS8110 provides no additional guidance on this. Could there be a stress concentration at the ends that is not being checked? Overlapping Perimeters Where supports are closely spaced and perimeters overlap the code requires that a single combined perimeter be checked. Orion does not identify such conditions and will simply draw overlapping perimeters on the screen. If this condition occurs additional hand calculations are required to substantiate the design.
Chapter 29 : Punching Shear Checks
Orion Documentation page 529
Discontinuous Columns In the special case where a slab supports a column or wall Orion will not make any checks at all. The case remains beyond the current scope and additional hand calculations are required to substantiate the design.
Advantages Checking punching perimeters by hand can be a very tedious and time-consuming task. It is not yet something that we can completely automate in Orion, and so it must still be treated as an interactive activity. This interactive checking procedure will also be subject to limitations in many projects. However, we hope that this chapter still illustrates the potential for significant time saving if punching shear checks are made using Orion. It is especially noted that once the initial checks have been made (thus setting up the correct check data), subsequent rechecking can be carried out very quickly indeed. It is also noted that by following the training guidance notes and making checks as models are constructed limitations can be determined earlier and hence time can be saved overall. We look forward to receiving comments on this feature so we can continue to refine it to meet the demands of designers.
EC2 Design Code Requirements The Design Procedure In EC2 section 6.4 covers the requirements for checking punching shear. Orion’s punching shear check cannot be used without a reasonable understanding of the code’s requirements. The following notes provide a brief overview of these. Cl 6.4.3 describes the basic design procedure; shear capacity is checked at the face of the column and then at the basic control perimeter. If shear reinforcement is required a further perimeter is established at which shear reinforcement is no longer required.
Check Maximum Shear Capacity This is a check at the face of the loaded area, refer to clause 6.4.5(3) of EC2. The length of the shear perimeter at the column face may be calculated in accordance with clause 6.4.5(3). Note that this only provides specific design guidance for rectangular columns with this guidance being further limited, for the case of edge and corner columns, to those cases where the edge(s) of the slab coincide with the edge(s) of the column. Therefore for other column shapes and scenarios Orion has to modify the equations presented in EC2. It is considered that the equations used by Orion will result in either the correct perimeter length being obtained or a conservative value (i.e. an underestimate of the perimeter length).
Check Shear Capacity at the Basic Control Perimeter The basic control perimeter is at 2d from the face of the column (the loaded area), where d is the average effective depth of the tension reinforcement.
Orion Documentation page 530
Chapter 29 : Punching Shear Checks
To make an accurate check on this perimeter the following needs to be known/controlled: • Is it an internal/edge/corner condition?
• How much main reinforcement is effective? • Are there holes, for which you need to account? The shear stress at the modified basic control perimeter, vEd,1 is calculated from Eqn 6.38 as follows: vEd,1 = Vt / (u1d) The punching shear resistance of slab without punching shear reinforcement, vRd,c is then calculated at the basic control perimeter. Three results are possible:
• If vEd,1 > 2vRd,c the slab thickness is inadequate, the check has failed and no further calculations possible for the particular column. else
• If vEd,1
View more...
Comments
