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Disclaimer
Table of Contents
Building Designer Documentation page 5
Engineer’s Handbooks Building Designer Engineer’s Handbook Chapter 1
Modelling and Analysis .
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Chapter 2
Building Effective Models in Fastrak Building Designer .
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Grid Lines . . . . . . Attributes . . . . . . Loading Self Weight of Concrete Slabs . Loading applied to slabs . . . Staged modelling and design . . Simple Construction is still the best method
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Simple Wind Loading
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Overview of Construction Types .
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Member Beams and Member Columns . Section/Material Properties . . Analytical Properties (End Releases) . General Beams . . . . . Creating General Beams . . . Analytical Properties (End Releases) . Design Properties . . . . General Columns . . . . Creating General Columns . . . Analytical Properties (End Releases) . Design Properties . . . . Shear Walls . . . . . Creating Shear Walls. . . . Shear Wall Limitations . . . Analytical Properties . . . Transfer Shear Walls . . . . Getting the Analysis Model Right . . End Releases . . . . . Axial Releases . . . . . Member Orientations . . .
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Table of Contents
Supports and Base Fixity . Reviewing the Analysis Results
Practical considerations
Chapter 5
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Overview of Analysis And Design Procedures
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Construction Levels, Floors and Diaphragms
Controlling the design procedure . . Why is it an iterative procedure? . . Controlling the iterative procedure . . Speeding up iterative analysis and design Limiting the iterations . . . .
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Leaving the Start from beginning of order file on each pass option un-checked
Initial Review of Analysis Results Maximum Nodal Deflections Sway Sensitivity . . . Loading Summary . . Review of Selected Sections . Review Analysis Results .
3D Analysis Effects
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Continuous Beam Example . Braces Carry Gravity Loads Example
Interactive Design.
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Backspan Beams . . General Points to Note . Pattern Loading . . Transfer Beams / Levels
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Chapter 8
Sway Resistance .
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Connections Integration . . . General Limitations . Moment Connections .
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Issues and Limitations
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Foundation loads .
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. . . Introduction. . . . . Using Bracing . . . . Using Steel Moment Resisting Frames Using Other Moment Resisting Frames Using Shear Walls . . . .
Introduction. . Simple Connections
Chapter 10
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Is it a Floor? . . . Diaphragm Modelling . Single diaphragm. . Slab items defined. . No diaphragm. . .
Chapter 7
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Table of Contents
Building Designer Documentation page 7
Vertical cross bracing .
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Chapter 11
Sign Conventions .
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Building Designer Advisory Note – Designing for Second-order Effects Chapter 12
Summary .
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Chapter 13
Introduction
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Basic concepts .
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Chapter 15
Second-order options
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Analysis options . . Design combinations .
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Analysis
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Design
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Other issues
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References .
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. Determination of λ cr Connection stiffness Base stiffness . .
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Building Designer Advisory Note – Integrated connection design
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Table of Contents
Chapter 20
Introduction
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Chapter 21
Practical Applications
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Simple Connections . Moment Connections . Column Base Connections
Chapter 22
Scope .
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Chapter 23
Limitations and Assumptions .
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Chapter 24
Analysis
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Chapter 25
Ultimate Limit State .
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Serviceability Limit State .
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Simple Connections . . . . . . Column Base Connections
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Accidental Limit State Structural Integrity Fire Limit State .
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Building Designer Advisory Note – Definition and Design of Trusses and Truss Members Chapter 28
Introduction
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Practical Applications
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Scope .
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Table of Contents
Chapter 31
Building Designer Documentation page 9
Limitations and Assumptions Limitations Assumptions Supports – Restraints –
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Chapter 32
Analysis
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Chapter 33
Ultimate Limit State – Strength .
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. . . Lateral Torsional Buckling Resistance, Clause 4.3 – . Lateral Torsional Buckling Resistance, Annex G – . Compression Resistance – . . . . . Member Buckling Resistance, Clause 4.8.3.3.1 – . Member Buckling Resistance, Clause 4.8.3.3.2 – . Member Buckling Resistance, Clause 4.8.3.3.3 – .
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Classification – . . Shear Capacity – . Moment Capacity – . Axial Capacity – . . Cross-section Capacity –
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Chapter 34
Ultimate Limit State – Buckling
Chapter 35
Serviceability Limit State
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Chapter 36
Member End Fixity and Supports .
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Chapter 37
Miscellaneous .
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Wind Modeller Engineer’s Handbook Chapter 38
Introduction
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Chapter 39
Scope .
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Chapter 40
Limitations
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Chapter 41
Applying Walls and Roofs
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41.1 Applying Walls . 41.2 Applying Roofs .
Chapter 42
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Running the Wind Wizard
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Chapter 43
Table of Contents
Creating Wind Zones on the Building . 43.1 Basic Geometry 43.2 Wall Zones . 43.2.1 43.2.2 43.2.3 43.2.4
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Wall Type Windward Walls Leeward Walls . Side Walls .
43.3 Roof Zones .
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43.3.1 Direction . 43.3.2 Automatic Zoning 43.3.3 Non-Automatic Zoning
43.4 User Modification of Zones
Chapter 44
Chapter 45
Load Decomposition .
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Simple Beam Engineer’s Handbook Chapter 1
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Practical applications . Designing a beam . Checking a beam . Worked Example . . Design Pass 1 . . Design Pass 2 . . Design Pass 3 . .
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Chapter 2
Scope .
Chapter 3
Theory and Assumptions .
. . Scope of simple beam Beam . . Steel sections . Web openings .
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Analysis method . Design method .
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Chapter 4
References .
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Building Designer Documentation page 11
Further information – Westok Beams .
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Composite Beam Engineer’s Handbook Chapter 1
Introduction
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Chapter 2
Scope .
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. . To invoke non-composite design in Building Designer . To invoke non-composite design in Composite Beam . Automatic transverse shear reinforcement design . . Bar spacing as a multiple of stud spacing. . . . Controlling the bar spacing directly. . . . .
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. . Practical applications . Designing a beam . Checking a beam .
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Westok Technical Support and Design Service Steel sections . . . . . . Web openings . . . . . . Profiled metal decking . . . . Precast concrete slabs . . . . Concrete slab . . . . . . Shear connectors . . . . . Reinforcement . . . . . Fibre Reinforced Concrete . . . . Construction stage restraint conditions . Construction stage loading . . . Composite stage loading . . . . Construction stage design checks . . Composite stage design checks . . .
Error messages and limitations
Chapter 3
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Design Aspects .
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Automatic transverse shear reinforcement design with Fibre Reinforced Concrete
Specify the stud spacing at the start of automatic design Worked Example . . . . . . . Without transverse shear reinforcement . . . Design Pass 1 . . . . . . . . Design Pass 2 . . . . . . . . Design Pass 3 . . . . . . . .
Chapter 4
Theory and Assumptions Analysis method. Design method . Construction stage
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Lateral torsional buckling checks Deflection checks . . . Torsion for ASB and SFB beams .
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Table of Contents
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Theory and Assumptions – Westok beams . . . . . . . . . . . . . . . . . . . . .
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Equivalent steel section - Ultimate limit state (ULS) Section classification (ULS) . . . . Member strength checks (ULS) . . . Shear connectors (ULS) . . . . Section properties - serviceability limit state (SLS) Stress checks (SLS) . . . . . Deflection checks (SLS) . . . . Natural frequency checks (SLS) . . .
Chapter 5
Construction stage Classification . Vertical shear .
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Composite Stage . Classification . Vertical shear .
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Horizontal Shear Longitudinal shear Moment Capacity Web Post Flexure and Buckling Vierendeel Bending . . Deflections . . . Service Stresses . . . Natural frequency . .
Chapter 6
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References & Further Information .
. References . . . . . . . Further information – Bison precast concrete slabs Further information – Westok Beams . .
General Beam Engineer’s Handbook Chapter 1
Introduction and application . Practical applications . Designing a beam . Checking a beam . Worked Example . . Design pass 1 . . Design pass 2 . . Design Pass 3 . .
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Chapter 2
Scope .
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Limitations .
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Assumptions
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Chapter 4
Analysis
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Chapter 5
Ultimate Limit State – Strength .
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. . . Building Modeller object General Beam . .
Classification
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Ultimate Limit State – Buckling
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Serviceability Limit State
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Chapter 8
Member End Fixity and Supports .
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. . Lateral Torsional Buckling Resistance, Clause 4.3 Lateral Torsional Buckling Resistance, Annex G Compression Resistance . . . . Member Buckling Resistance, Clause 4.8.3.3.1 Member Buckling Resistance, Clause 4.8.3.3.2 Important Note . . . . . Member Buckling Resistance, Clause 4.8.3.3.3
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General Beam Stand-alone . Building Designer . .
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Design Procedure .
Simple Column Engineer’s Handbook Chapter 1
Introduction and application.
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Design of concrete filled columns
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Practical applications . Designing a column . Checking a column . Worked Example . Design pass 1 . . Design pass 2 . . Design Pass 3 . .
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Proposed method Points to Note .
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Chapter 3
Table of Contents
References and further information References .
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General Column Engineer’s Handbook Chapter 1
Introduction and application . Practical applications . Designing a column . Checking a column . Worked Example . . Design pass 1 . . Design pass 2 . . Design Pass 3 . .
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Chapter 5
Ultimate Limit State – Strength
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Classification
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Ultimate Limit State – Buckling
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Serviceability limit state .
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Design Procedure
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. . Lateral Torsional Buckling Resistance, Clause 4.3 Lateral Torsional Buckling Resistance, Annex G Compression Resistance . . . . Member Buckling Resistance, Clause 4.8.3.3.2 . Important Notes . . . . . Member Buckling Resistance, Clause 4.8.3.3.3 . .
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Chapter :
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Engineer’s Handbooks
Introduction This part of the documentation system contains essential engineering information relating to the individual design engines for each member type which is available in Building Designer. The following chapters deal with each object type in turn, and include essential tips on using the applications, as well as advanced engineering information pertaining to the applications, and references to the documentation on which the design requirements are based. We recommend that all engineers using this software adopt the advice of SCOSS* and CROSS in their comments on the use of computer software - their advice is "to know the scope of the software, check results, ...., and check that the computer aided design is to the relevant codes of practice. Designers must remember that it is they, not software suppliers, who are responsible for design." (CROSS Newsletter No. 1 - Nov 2005) Based on this advice, we strongly recommend that all users of the software familiarise themselves with all the relevant sections on Scope and Limitations and Assumptions to be found in the Engineer's Handbooks. *UK Standing Committee On Structural Safety
The following Engineer’s Handbooks are available:
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the Building Designer Engineer’s Handbook the Wind Modeller Engineer’s Handbook the Simple Beam Engineer’s Handbook the Composite Beam Engineer’s Handbook the General Beam Engineer’s Handbook the Simple Column Engineer’s Handbook the General Column Engineer’s Handbook
In addition the following Advisory Notes are also available:
• Building Designer Advisory Note – Designing for Second-order Effects • Building Designer Advisory Note – Integrated connection design • Building Designer Advisory Note – Definition and Design of Trusses and Truss Members
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Chapter :
Chapter 1 : Modelling and Analysis
Building Designer Documentation page 17
Building Designer Engineer’s Handbook
Chapter 1
Modelling and Analysis Building Designer gives you complete flexibility to define any member with almost any properties between any two points in 3D space. While this is clearly flexible, and may sound simple, it requires you to consider the modelling of your structure in more detail than if it contained only simple (pin ended) beams and simple columns with no moment connections. This part of the documentation seeks to introduce some of the issues that you may wish to consider.
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Chapter 2
Chapter 2 : Building Effective Models in Fastrak Building Designer
Building Effective Models in Fastrak Building Designer Fastrak Building Designer is a design based structural modeller. Please remember that Fastrak Building Designer is a modelling package, which dictates the design model and which creates analysis models to accomplish this design. It is important that you recognise that you must take ownership of the creation of the model and the results that the software gives. It is possible to create complete building designs quickly and easily with Fastrak Building Designer, however as it is a design based modeller you should take account of the following issues1.
Grid Lines You can define grid lines quickly and simply in Fastrak Building Designer. Alternatively you can import them into your model from a DXF file. If you are importing grid lines from DXF files, please ensure: • that the grid lines you are using are accurate,
• that the DXF file you are importing only contains grid lines. If you are in doubt we advise you to use Building Designer’s ability to import a DXF file and create a ghost image of the structure. You can then add your Building Designer grid lines on top of the ghosted DXF image.
Attributes It is important to realise that the attribute sets are used to set up defaults for the elements (beams, columns… …) in your model. The attribute sets are not linked to the elements once they are created. You can quickly make major changes to your model, for example changing the grade of steel, quickly and easily, you need to change the appropriate attribute(s) and then apply these attribute(s) to the members. IMPORTANT — when you create any member it takes the current default attributes. The default setting for a simple beam (unless you change it) is that it is fully restrained. Please take care if creating beams that are not fully restrained.
Loading Self Weight of Concrete Slabs Building Designer automatically calculates the self weight of the structural beams/columns for you and provides automatic calculation of items like Notional Horizontal Forces (NHFs) and Wind Loads. Footnotes 1. This is particularly important if you are a new user, until you become familiar with Fastrak Building Designer.
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Please note however that Building Designer DOES NOT automatically calculate the dry weight, or the wet weight of the concrete slab. When you calculate these values you should allow for two key issues: • the slab loads you calculate should make some assessment of, and allowance for, ponding that may occur,
• when your model contains composite beams you need to create your loadcases carefully to take account of the staged construction that will actually take place. To this end Building Designer automatically creates the Slab wet and Slab Dry loadcases. It is important that you apply the relevant loads in the relevant loadcase correctly.
Loading applied to slabs Building Designer applies floor loading, area loads, line loads and point loads to the slabs in your model and distributes them in the direction of span of the slab. If you wish to apply loading directly to a beam, PARTICULARLY IF THAT BEAM SITS AT THE EDGE OF THE SLAB, then you should use element loads which apply the load directly to the member without involving the slab. To aid this Building Designer has a Create Perimeter Load facility. You can access this from the Loading menu.
Staged modelling and design Our major piece of advice when you are modelling in Building Designer is: DO NOT BUILD THE ENTIRE MODEL BEFORE YOU VALIDATE AND DESIGN IT. It is important that you build the model, validate and design it in a staged process, for example: • Validate and design ONE floor before copying it up the building,
• Resolve the gravity design before looking at the lateral design, • Resolve the sway stability before applying the wind loading. There are often many nuances to creating your model, in particular with composite design, and it is much more effective to resolve any issues once (before you copy the floor to other levels in your model) than it is to copy the floor to (say) ten other floor levels, and then address the (usually simple) issues on each copied floor (in this case ten times the work!).
Simple Construction is still the best method The most effective design for a multi storey structure is still likely to be simple beams and columns with bracing to resist the lateral forces. Simple construction in BS 5950 implies certain types of modelling and certain specific design rules (both inclusions and exclusions). We assume your familiarity with these.
Use ‘Simple1’ beams and columns where possible Building Designer will happily design moment frames or continuous beams automatically within a model, BUT, the design of these elements is much more comprehensive (and hence takes longer). For this reason you should only use such elements when your model specifically requires them.
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If Building Designer gives warnings about braces on simple beams, or intermediate floor levels on simple columns the answer is not necessarily to make the affected elements into general beams/columns. Look at the modelling and talk to CSC support if you are not sure of the route that you want to take.
NOTE — Simple Columns and Sway Stability When you use simple columns they are pinned at every floor level (except where they are connected to a braced bay) to ensure that all lateral load is transferred to the braced bay. This modelling is in line with the SCI guidelines in the Steel Designers Handbook. You may pin or fix general columns at each floor level as you wish.
Design Simple Construction for Gravity Loads only In order to speed the design process a distinction is made between those combinations consisting of gravity loads only and those which contain some components acting laterally (eg.. NHFs and wind loads). Setting simple beams, composite beams and simple columns to be designed for gravity loads only can significantly reduce the design time. General beams, general columns, trusses and braces are always designed for both gravity and lateral combinations. Engineering judgement will be required when flagging members as being ‘gravity load only’. For example:
• a simple/composite beam with an inclined braced member connected to it should be designed for both gravity and lateral loads.
• potentially, simple beams in a sloping roof would need to be designed for both gravity and lateral load Note
If a simple, or composite beam is flagged to be designed for both gravity and lateral combinations, only the component of the lateral load that acts in the plane of the strong axis of the member is considered. Any axial loads, or loads in the weak axis are ignored. A validation warning is provided if the ignored loads exceed a preset limit.
From release 7. 0 of Building Designer onwards it is now important that simple columns only receive forces from horizontal members. To ensure this a new validation check has been added. If a simple column is placed in a braced bay, or if it supports a sloping general beam or truss member a validation error will be displayed. In such circumstances it will be necessary to change the simple column to a general column.
Diaphragm Action You can switch diaphragm action on or off for a given floor as you decide. If you switch diaphragm action on, you must also then decide if this applies to the entire floor, or to part of the floor only. Footnotes 1. Pin type connections – thus in this context a composite beam is ‘simple’.
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Building Designer Documentation page 21
For futher information about the diaphragm options available refer to “Diaphragm Modelling”.
Composite Beam Design Composite design of beams is a complex procedure when done rigorously. We assume that you are familiar with the concepts of composite design before you use Building Designer. Building Designer’s composite design routines can automatically choose the optimum stud layout and automatically select an appropriate transverse shear reinforcing layout, as such the design of any composite beam may have a range of possible solutions. Example
A typical 9 m composite spine beam can be shown to be acceptable: • with studs at 190 mm cross-centres and a 457x191x67 UB, • with studs at 200mm cross-centres and a 457x191x74 UB, which of these solutions is better is up to you.
While it can sometimes be useful for Building Designer to optimise a design, you might well take the view that you would prefer to control the stud spacing and other critical design issues rather than leaving Building Designer to choose a different layout for every beam. Please consider the following when you set up the attributes for a composite beam. It is important to realise that you can define attributes which may make the design of composite beams impossible – for example if you set the stud spacing on a spine beam to 300 mm, but this does not give the minimum amount of shear interaction then the selection of a suitable beam size is not possible. You should exercise care in the use of composite beams, if in doubt design all beams as simple
beams first and then simply select those beams that you wish to be composite at a second pass.
Composite or simple beam Composite beam design is not a linear process, and some beams are simply not suitable for design as composite beams. You should take care when selecting beams for composite design, and set appropriate design attributes such as the critical ‘e’ dimension. The benefits of composite design are well known, however many beams are not suitable for composite design, including: • beams with no slab,
• very short beams, • beams with significant eccentric load (for example a beam supporting a column close to the support),
• beams with decking arrangements that will not allow effective composite action. Note
Many engineers consider edge beams to be an ineffective use of composite beams and specify simple beams to avoid significant use of transverse shear reinforcing.
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In short you should be diligent about the use of composite beams. Exercise care when determining which beams are appropriate for composite design, if in doubt design all beams as simple beams first and simply select those beams that you wish to be composite at a second pass.
Setting the appropriate level of deflection checks Building Designer provides very comprehensive deflection checks on all beams. You can set limits on the deflections for a variety of conditions (dead load only, imposed load only and/or total load). At the same time in order to allow for deflections with beams with significant web penetrations Building Designer employs a sophisticated integration based deflection check. You should take care when setting the range of deflection checks. You may consider the default deflection limits conservative for some buildings.
Building Size and Orientation The automatic calculation of NHF’s and sway checks are done on the basis that we are checking a single building as outlined in section 2 of BS 5950-1. This effectively says that each portion of the building between expansion joints should be looked at separately for sway stability. When using Building Designer we assume that you are following this logical process. Note
NHF’s and sway stability are calculated in the global X and Y direction, you should take care to input the model with this in mind.
Automatic NHF calculations If you switch the automatic calculation of NHFs on, then Building Designer follows the process below: • it automatically calculates all NHF’s based on 0.5% of the factored vertical load that passes through any beam/column intersection in the structure, Note
The values of the NHFs may vary for each load combination.
• automatically creates a sway analysis to calculation the drift in X and Y of the structure under NHF’s only,
• automatically calculates the floor to floor drift of every column in the structure and thereby establishes the worst lambda crit in both X and Y directions,
• reports this back for the critical sway stability values of lambda crit in X and Y, • if the λcrit value falls between 4 and 10, Building Designer can (if the option is selected) automatically apply the pΔ effects,
• Building Designer can automatically include the relevant NHFs in vertical load only combinations to allow for lack of fit (if required). Note
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This is not the same as sway and is a requirement of BS 5950-1.
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On a typical model, this very valuable automated process will significantly extend the design time, therefore we recommend that you leave it switched off until you require it. For instance, there is no point in switching this option on if you have no lateral load resisting system in place or if you are making small alterations to your model and you are iterating the design process.
Number of design runs The default setting for Building Designer is 3 design runs (passes) with NHFs on. This carries out: • initial load decomposition,
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initial design of all members, automatic recalculation of the self weight of the structure, automatic calculation of NHFs, redesign of the structure with NHFs and self weight, automatic recalculation of NHFs allowing for the revised self weight, a design check of the entire structure.
As a default this is a reasonable process that works for the automatic design of moment frames as well as simple structures. However if you are dealing with a large simple multi storey structure we would suggest that it would be appropriate to perform only two passes.
Number of load combinations Where you are looking at design changes, for example to rationalise an area of floor, you can switch off all the irrelevant load combinations. For example if you are looking to redesign a series of composite floor beams, then it is likely that only the construction case load case and the dead + super load case are relevant. This may allow the you to switch off all other load cases and concentrate on the gravity design issues.
Stepped Design Process If you put some of these things together you will see that there is a logical stepped design process. For example pinned beams (such as the composite beams) will mostly be unaffected by any lateral load, and hence you may design the beams looking at gravity load only. Using this ‘stepped’ process to carry out the design you should find the software provides detailed design very effectively.
Results Upon completion of the design process the Workspace presents: Model deflection results — these are not a pass/fail for the details of the model but simply an indication of the total defection of the model under all the differing loads applied. If the model suffers from excessive deflections then a warning will be shown. In this case the
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Chapter 2 : Building Effective Models in Fastrak Building Designer
remaining results could be incorrect as the overall building analysis may be indicating that the building will collapse. This may be irrelevant if you are looking at a gravity design and are happy to ignore lateral load, but it may also mean that the basic moment for which the building is being designed could alter once the building is stabilised. Sway Stability — if the NHFs option in the analysis options is on then Building Designer will carry out a full sway stability analysis. Significant failure, may mean the lack of overall stability such as the omission of diaphragm action or simply a loose piece of the structure not connected into the main bracing/diaphragm action. Use the deflection results to look at this. Lambda crit less than 4 — BS 5950-1 says that if λcrit is less than 4 a full second order analysis should be used. Methods for rigorous second order analysis are incompatible with imposed load reduction and therefore within Building Designer a λcrit less than 4 indicates that your model does not have sufficient lateral stiffness. Load in versus load out — All loads are checked in and out of the model it is essential that you check these results.
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Chapter 3 : Simple Wind Loading
Chapter 3
Building Designer Documentation page 25
Simple Wind Loading Fastrak Building Designer v6.0 has a new fully functional Wind Wizard which assesses wind loading on your building structure to BS6399-2:1997. For users who do not have access to the comprehensive Wind Wizard, there was and is the facility to load walls with a stepped horizontal pressure load - now called Simple Wind Loading. The introduction of the Wind Wizard alters the way that the Simple Wind Loading works in two respects: 1. Walls now have an inner and an outer surface. If the pressure in the direction of the wind hits the outer surface of a wall then the structure is loaded by the wind. However, if the wind strikes the inner surface of a wall then it passes through the wall and does not load the structure. The simple way to verify which way round your wall surfaces are, is to look at Show/Alter State in the structure view. 2. The Simple Wind loading strikes all outward facing walls which can be seen in the wind direction defined. There is no longer any "sheltering" of one wall by another - which was in previous versions. For many structures, this will make no difference but in some, this may alter the structure loading. Note
Please note that for existing models built in Fastrak Building Designer prior to v6.0 then the above are only relevant if you re-generate the Simple Wind Loading.
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Chapter 4
Chapter 4 : Overview of Construction Types
Overview of Construction Types Building Designer allows you to model members which are more complex than pin ended beams and simple columns. There are currently four member construction types that you can use: • Member Beams – these can be any section in any material but cannot be checked or designed by Building Designer – refer to “Member Beams and Member Columns”.
• Member Columns – these can be any section in any material but cannot be checked or designed by Building Designer – refer to “Member Beams and Member Columns”.
• General Beams – these are restricted to steel sections but such beams can then be designed by Building Designer – refer to “General Beams”.
• General Columns – these are restricted to steel sections but such columns can then be designed by Building Designer – refer to “General Columns”. In addition to the above, you can also define: • Shear Walls – these are restricted to concrete or “Other” materials but cannot be checked or designed by Building Designer –
Member Beams and Member Columns
A member can be almost anything. The view above shows member beams and member columns being used to form concrete framing to support part of the steel structure. In addition concrete shear walls are shown which provide lateral stability and support various beams. (Refer to “Sway Resistance” for more notes on the alternative methods of providing lateral stability). The procedure for defining member beams and member columns is identical to the procedure for defining other beams and columns – you set up the default attributes and then create members by clicking between any two points. The issues to give some consideration to are:
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Chapter 4 : Overview of Construction Types
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Section/Material Properties
You can set up any member properties you want, for example the dialog above is what you will see if you elect to set up a new concrete section. When analysing a concrete structure in isolation BS8110 suggests that for the purposes of establishing design forces you need to use consistent properties for all members. The point of this is that so long as everything is proportionately correct, then the design forces will be correct. However, for the purposes of deflection estimation and in any model that mixes steel/concrete/other materials, more attention needs to be paid to defining the correct properties. For concrete elements this means considering: • adjusting the gross section properties to allow for cracking, In the above dialog, if you define b and d then click Calc. Properties Building Designer calculates the gross section properties of a simple rectangular section for you. You can make whatever adjustments you wish to the calculated values to allow for cracking and/or to allow for irregular shapes, etc.)
Note
• adjusting the value of E (Young’s Modulus) to allow for load duration. When you select a concrete grade an average short term value of E is indicated for guidance. You must always define the value of E to be used for analysis.
Note
Analytical Properties (End Releases) This is common to Member Beams, Member Columns, General Beams, and General Columns, refer to “Getting the Analysis Model Right”.
General Beams General Beams are, in a sense, a more constrained subset of Member Beams: • You still have all the geometrical freedom to define the member at almost any angle/ orientation,
• General Beams are constrained to be a steel section, • The advantage is that Building Designer designs these steel sections automatically. Creating General Beams You can create General Beams in the same way as any simple- or composite-beam. Simply create a new beam attribute set and set the Construction Type on the Design tab to General. Any new beam you create using this attribute set will be a General Beam.
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You can set the end releases as part of the attribute set (the default setting is pinned).
You can create General Beams in several other ways: 1. While creating any beam (regardless of the current default attribute set), you can hold down the control key to indicate a series of points that define a continuous beam. Since simple beams and composite beams are never continuous this procedure will always convert the beam to make it a continuous general beam. Note
Continuous general beams do NOT need to be co-linear.
2. If you click on any simple beam and alter its state to make it rigid, Building Designer converts the beam to a General Beam with fixed ends. Note
Where the converted beam frames into simple columns, these columns will also automatically be converted to General Columns.
3. If you click on two simple beams with the Split/Join tool active Building Designer converts these to a continuous general beam. 4. If you insert points in simple beams by using the Modify tool and then move those points to create a non co-linear beam, then Building Designer converts the beam into a continuous General Beam.
Analytical Properties (End Releases) This is common to all Member Beams, Member Columns, General Beams, and General Columns, refer to “Getting the Analysis Model Right”.
Design Properties As with simple- and composite-beams it is best to establish the default design properties (restraint assumptions, sections for study, and such like) by setting up appropriate default attributes. If you have not set up the attributes you wanted you could of course edit the properties of any General Beam in the same way as you would for a Simple Beam.
General Columns General Columns are, in a sense, a more constrained subset of Member Columns: • You still have all the geometrical freedom to define the member at almost any angle/ orientation,
• General Columns are constrained to be a steel section, • The advantage is that Building Designer designs these steel sections automatically. Creating General Columns You can create General Columns in the same way as any Simple Column. Simply create a new column attribute set and set the Construction Type on the Design tab to General. Any new column you create using this attribute set will be a General Column.
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You can create General Columns in several other ways: 1. While working in the 3D structure view you can create columns by clicking on start and end points. While creating any column in this way (regardless of the current default attribute set), you can hold down the control key to indicate a series of points that define a continuous column. If these points are not co-linear the column cannot be a Simple Column. In this case Building Designer automatically be converts it to a continuous general column. 2. If you click on any simple column and alter its state to make it rigid, Building Designer converts the column to a General Column. On it’s own this would be a fairly pointless action, unless you have other fixed ended members with which the column can interact.
Note
3. If you insert points in simple columns by using the Modify tool and then move those points to create a non co-linear column, then Building Designer converts the column into a continuous General Column.
Analytical Properties (End Releases) This is common to all Member Beams, Member Columns, General Beams, and General Columns, refer to “Getting the Analysis Model Right”.
Design Properties As with beams it is best to establish the default design properties (restraint assumptions, sections for study, and such like) by setting up appropriate default attributes. If you have not set up the attributes you wanted you could of course edit the properties of any General Column in the same way as you would for a Simple Column.
Shear Walls Shear Walls introduce structural strength and stiffness to your structure. They are typically used to provide lateral stability to the building. (Refer to “Sway Resistance” for more notes on the alternative methods of providing lateral stability). Shear Walls do not act as a medium via which loads calculated by the Simple Wind Loading generator and Wind Wizard are applied to your structure. If this is required an additional Wind Wall panel would have to created in the same location as the shear wall. Once a shear wall has been defined, extensions can be added to the wall ends. These do not increase its strength or stiffness, but the self weight would be increased. Openings can also be placed within the wall in the form of doors or windows. These will reduce the strength, stiffness and self weight of the wall.
Creating Shear Walls To create Shear Walls you should first create an appropriate Shear Wall attribute set. The attribute set consists of the wall’s material, thickness and analysis properties.
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If the material is specified as concrete, you should select the concrete grade. The program will then display a typical short term E value for the grade chosen. You will then need to decide on an appropriate value of E to be used in the analysis, taking into account factors such as creep, cracking and shrinkage. If the material is specified as “Other” you will also be required to specify an appropriate E to be used in the analysis. You can then create the wall itself from any of the 3D or 2D views: 1. While working in the 3D structure view or a frame view you can create a shear wall by clicking on start and end points at the base of the wall, followed by a third point which can be located anywhere in the floor at the top of the wall. The wall will extend vertically upwards between the start and end point. To the height defined by the third point. 2. While working in a 2D floor view you can create a shear wall by clicking on start and end points. You then select the construction levels at which the wall starts and ends.
Shear Wall Limitations The following limitations apply: • Vertical walls only
• Rectangular walls only • Concrete or “Other” materials only • The shear walls will not be designed Analytical Properties A “mid-pier” idealisation is used for Shear Walls, this consists of: • Two horizontal elements at the bottom of the wall running between the two set out points and the mid point.
• Two horizontal elements at the top of the wall running between the two set out points and the mid point.
• Further pairs of horizontal elements for ant intermediate construction level that is flagged as a floor.
• Vertical elements joining the mid-points at the top and bottom of the wall and any intermediate floor levels.
• A fully fixed support is added at the midpoint of the wall baseline, unless it is being supported by one or more columns, another shear wall, or a transfer beam.
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The “mid-pier” analytical model can best be reviewed graphically by showing the release state of the model (pick Select/Show/Alter State, and then pick Releases from the dialog). If openings have been added to the wall the mid pier model will be modified accordingly. Additional vertical elements are introduced to the sides of the opening and a coupling beam introduced above. The strength and stiffness introduced to your structure will depend on the wall thickness and also the E value used in the analysis. Care should be taken to ensure that the E value used is realistic. The alignment (Left,Centre, or Right) of the shear wall is for cosmetic purposes only and does not affect its analytical properties.
Note
Transfer Shear Walls A shear wall may be partially or fully supported by a beam or truss member, but only if the supporting member has concrete or ‘Other’ material properties and it’s model type for shear wall modelling is set to Top Edge Beam.
Getting the Analysis Model Right Once you start using Member Beams, Member Columns, General Beams, General Columns and Shear Walls, you are no longer dealing with a simple model where all the beams have pinned ends and only resist major axis moments.
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The design forces established in frames with moment connections are all distributed according to relative member stiffnesses. Therefore, in addition to ensuring that the member properties are correct, you need to review and take control over member end releases and member orientations.
End Releases
Member end moment releases are best reviewed graphically by showing the release state of the model (pick Select/Show/Alter State, and then pick Releases from the dialog). The releases for all supports, general beams, general columns, member beams and member columns are shown. (The releases for simple beams, composite beams and simple columns are not shown purely to limit screen clutter – they are always released for major and minor axis bending.) Moment releases are indicated by an arrow with a double arrowhead. In the view above the front-left-elevation is created with General Beams and General Columns to form a moment resisting frame. The downward arrows at the nodes at the end of most of the beams therefore indicate that the beams are pinned in the minor axis moment direction. While this view is active you can select node positions (one or several at a time) and edit the releases via the Properties pane. Note
You can only select end nodes for the currently active member type.
When setting/changing moment releases for General Beams the options available include: Free — Used to indicate the free end of a cantilever. (Not really needed analytically, but needed to set effective lengths more appropriately.) Simple Connection — The connection is pinned for both major axis (Mx) and minor axis (My) bending. Moment Connection — The connection is fixed for major axis (Mx) bending but remains pinned for minor axis (My) bending. Fully Fixed — The connection is fixed for both major axis (Mx) and minor axis (My) bending.
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Continuous — This setting is automatically applied when a continuous beam is created and effectively creates an non-editable fully fixed connection between the spans of the continuous member. The connection can only be edited by splitting the beam. When setting/changing moment releases for Member Beams the options are slightly different as follows: Free — Used to indicate the free end of a cantilever. Pinned — This is the same as the Simple Connection noted above, the connection is pinned for both major axis and minor axis bending. Pinned About Local Y — For member beams the focus is on the analysis model and so the usual analytical sign conventions are applied. X is along the member, Y is the major cross section axis and Z is the minor cross section axis. Hence this setting creates a pinned connection for major axis bending but the connection remains fixed for minor axis bending. These conflicting sign conventions are a universal issue when moving between analysis and British design codes. Pinned About Local Z — This is the same as the Moment Connection noted above for general beams, the connection is fixed for major axis bending but remains pinned for minor axis bending. Due to the conflicting sign conventions noted above the minor axis is referred to as Z rather than Y. Fully Fixed — This is the same as the Fixed Connection noted above, the connection is fixed for both major axis and minor axis bending. Continuous — This setting is automatically applied when a continuous beam is created and effectively creates an non-editable fully fixed connection between the spans of the continuous member. The connection can only be edited by splitting the beam. Note
If the sign conventions seem confusing the very best way to review what you are doing is to show the graphical representation of the member releases as discussed at the start of this section.
Note
You can also edit the intermediate connections on General Columns.
Axial Releases Member end axial releases are best reviewed graphically by showing the axial release state of the model (pick Select/Show/Alter State, and then pick Axial Releases from the dialog). The axial releases for all general beams, general columns, member beams and member columns are shown. (The axial releases for simple beams and composite beams are not shown purely to limit screen clutter – they are always released axially.) In the view above the rafters have been created as General Beams and the verticals as General Columns to form a moment resisting frame. All the General Columns are fixed axially apart from two that are to act as gable posts which have been modelled with releases at the top end. While this view is active clicking on a node will toggle its state between Fixed and Released. Note
You can only select end nodes for the currently active member type.
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General Beams and Member Beams can be released axially at either end, but not both. If the beam is continuous it can only be released axially at one or other of it’s extreme ends. General Columns and Member Columns can only be released axially at the top end.
Member Orientations All member orientations are reflected in the graphical views as appropriate. By default Building Designer places beams of all types with their major axis horizontal (in the global XY) plane. For the vast majority of beams this will be the required orientation. By default Building Designer places vertical columns with their major axis in the global XZ plane. Clearly, at best, these default column orientations are only likely to be correct around 50% of the time. You can control the orientation of each newly inserted member by setting the appropriate member orientation (under the Alignment tab) of the current attribute set. You can edit the orientation of one or more selected members simultaneously by adjusting the appropriate details in the Property pane. An exception to the above is the case of inclined General Columns – as you define these Building Designer calculates the orientation angle automatically so that the web is vertical. You can not edit this angle. Caution
Since foundation shears and moments are reported relative to each column’s local axis system the automatic calculation of the member orientation for inclined general columns can initially be a little confusing.
Supports and Base Fixity A few points are worth noting on this topic: 1. The view in “End Releases” also shows that you can view and edit support releases when viewing member releases graphically. 2. Building Designer automatically creates supports as you create any columns, by default these supports are always released (pinned). 3. You cannot adjust the base fixity for Simple Columns, this is always pinned. 4. For General and Member Columns you can select the support and adjust the base fixity between different fixity settings. The settings are all based on the guidance provided in BS5950-1:2000 cl 5.1.3 as follows: a) Pinned – the default setting. b) Nominally Pinned – where a rotational spring stiffness (10 or 20% of column stiffness) is automatically calculated and applied. c) Nominally Fixed – where a rotational spring stiffness of 100% column stiffness is automatically calculated and applied. d) In cases b and c above you can also specify a user-defined base fixity.
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Nominally fixed is not the same as a fully fixed support that you might define in a typical analysis package, a nominally fixed support will rotate according to the spring stiffness and this will affect deflections. If you have a genuinely fixed support you need to use the last option above to define an increased spring stiffness.
The above options may be of particular interest where you want to achieve overall stability by frame action as opposed to diagonally braced panels. Overall you should find that, by default, members tend to be pinned in Building Designer, so if you are editing releases you will generally be adding fixity. This is the opposite of the way in which most analysis packages work (where everything is initially fixed and releases have to be added) but we consider this to be a more conservative and realistic approach.
Reviewing the Analysis Results As is noted in the above sections, once you start using general beams, general columns, member beams, member columns and shear walls it is important to review and check the analysis model you are creating. An important double check on all of this is to spend some time reviewing the analysis results. You may want to review “Initial Review of Analysis Results” for some notes/tips on this.
Practical considerations Up to this point in this chapter we have said a lot about creating members with moment connections and the care that you need to take to ensure the model you are creating is the one you intended. The automated design capabilities introduced by Building Designer in conjunction with those in General Beam and General Column allow you to consider solutions involving rigid steel framing quickly and easily in a way that may not previously have been practical from a design effort point of view. In essence you may find it more feasible and practical (within design fee limitations) to consider some new, more sophisticated, solutions. However, consider other practicalities before being tempted to venture into new styles of construction. Construction and maintenance costs will be affected, as illustrated in the two simple examples below: Connections — Moment connections will introduce fabrication and construction costs and difficulties that may offset other savings or advantages. In some cases the assumed connections/intersections may prove to be completely impractical to construct because of physical limitations. Flexibility — Simple beams operate quite independently – you can often remove a simple beam without having to worry about adjacent members. If you have continuous framing you are more likely to find that other beams need to be strengthened in order to allow one beam to be removed.
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Chapter 5
Chapter 5 : Overview of Analysis And Design Procedures
Overview of Analysis And Design Procedures
Controlling the design procedure Under the Design menu there is a Design Options… setting which shows the dialog below.
The first page of the dialog provides an option to Start from beginning of order file on each pass, this is unchecked by default. For all or the vast majority of members in most models this setting will have no effect on the results, it simply serves to speed up the iterative design procedure. However in some models checking this option may achieve a lighter weight design result. This is discussed further in “Leaving the Start from beginning of order file on each pass option un-checked”. The Perform check of fields provide a way to speed up the design process when you want to only tweak a particular part of the design of your structure. For instance if you have designed all the floors in your model, and are satisfied with the resulting beams, but you want to work with the columns, you can remove the check against the types of member with which you are satisfied, and Building Designer will ignore these during the design process. These options only affect the checking process. If a particular element needs to be designed, then this will happen irrespective of the settings you make here. A full 3D analysis may expose small forces that are normally ignored in the design of members. The options for ignore forces below on the second page of the dialog simply provide you with a way of setting negligible/nominal force levels with which you are comfortable.
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When the small forces from the 3D analysis are below the specified threshold levels they are ignored so that design can proceed automatically. If the forces are above these limits, then you will be warned during the design process.
The last page of the dialog provides similar functionality for the connections in your model. If forces below the threshold do arise, then they are ignored during the design process. Forces higher than these generate warnings during the design.
Why is it an iterative procedure? Basically the procedure is as follows: 1. For the first analysis run Building Designer assumes (guesses) section properties for the members that are to be designed (as opposed to checked). 2. Building Designer constructs and analyses this model. 3. Building Designer designs all members. 4. Building Designer compares the member sizes that result from the design (step 3) with the member sizes which were used to construct the latest analysis model (step 2) – if any of these are different then Building Designer goes back to step 2 and constructs a new analysis model based on the sections resulting from the latest design and then again proceeds to step 3. 5. If the comparison at stage 4 shows no differences then Building Designer performs a final check design on all the members in the structure.
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The need for iteration on steps 2 to 4 above occurs when structures include moment connections between members. In such circumstances no member can be operated on in isolation, as soon as the stiffness of one member is changed it can affect design forces (principally the design moments) in lots of members. Hence the analysis model must be synchronised with the latest design.
Controlling the iterative procedure Under the Design menu there is an Analysis Options… setting which shows the dialog below.
Building Designer can either perform a first-order or second-order analysis of the frame. If the sway of the frame is such that the amplification factor method for catering for sway is acceptable, then Building Designer can automatically handle this for you.
You specify the formula which you want to use. For further information see “Analysis options” in the Designing for Second-order Effects Advisory Note and “Reviewing sway” in the extended Quick Start Guide.
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The number of Passes limits the total number of times that Building Designer will loop through the iterative analysis and design procedure. Points to note on this are: • Iterative analysis and design will usually finish naturally in a few loops (repetitions of steps 2 to 4 detailed in “Why is it an iterative procedure?”). However, it is possible that it will enter an infinite cycle of loops. This setting is primarily intended to stop that from happening.
• There is no correct or wrong number to enter at this point, it is simply an upper limit. • If you set it to 10 it does not mean that the analysis and design utilises all 10 loops, you will often find that the design still converges in 2 or 3 loops in which case it would not have made any difference if you had set the limit to 5, 10 or 100.
• This setting has only been exposed because you may sometimes find it useful to set a low number to force faster design – refer to the section below.
Speeding up iterative analysis and design The iterative analysis and design procedure might be time consuming, but may be speeded up in different ways.
Limiting the iterations If you set a low number (for example 2) as the limiting number of analysis and design loops the initial design will be faster with the following implications/possibilities: 1. Setting to 2 loops means that there is one design loop and one final (faster) checking loop. 2. The analysis will therefore be correct and synchronised with the selected members. 3. This does not guarantee that all members pass (there is never such a guarantee). When the design status is checked you may find some members that fail. You could change and re-check such members interactively. 4. More difficult to spot is the possibility that the design of other members may not be as fully optimised as possible. 5. Although General Beams and General Columns are most likely to be affected by the above, any member could be affected. We anticipate that many users will prefer to set a low number such as 3 (probably leaving the start from beginning of order file option un-checked at the same time – see the next subsection) and that this will generally produce a fully optimised design for the vast majority of members (in particular all the simple beams, composite beams and simple columns) in most structures. You might then reset only the general beams and general columns to Design and rerun the design allowing more iterations. This avoids repetitive design of members such as simple and composite beams.
Leaving the Start from beginning of order file on each pass option un-checked With this option unchecked the design process for each member at the second and subsequent iterations starts with the section that was found to work in the previous iteration, if this section fails then Building Designer will start to look at bigger sections.
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When this option is checked the design process for each member at the second and subsequent iterations will start right at the beginning of the design order file working through until it finds the first section that works. By activating the start from beginning of order file feature you get the advantage of a detailed iterative design that is likely to find a reasonable minimised-weight solution, but you also get disadvantages: Slower — Each design iteration will be slower because every possible section is reconsidered for every member. Slower — Since during each design iteration some members will get bigger and some smaller the model that needs to be re-analysed will differ more greatly. This can lead to more design iterations (as well as each iteration taking longer). Non-Convergence — For the reason noted above it is more likely that the design could start oscillating between iterations where one group of members get bigger but another get smaller, and then vice versa in the next iteration. Unless you have a highly indeterminate rigid framed structure where many alternative load paths can be imagined, it is quite likely that this setting will make little difference. A point to note is that there is no single correct answer for these indeterminate structures. (If beam A is made bigger can beam B be made smaller?). There may be lots of different safe configurations and hence different solutions may be found depending on how the software is driven. The examples in “3D Analysis Effects” illustrate this principle. Overall our advice would be: • If you would prefer to see faster design we would recommend leaving this un-checked. If you intend to limit the number of design iterations (to 2 or 3) then we particularly recommend that you leave this option un-checked.
• Conversely - if you want to activate this option then we recommend that you do not apply small limits to the allowable iterations.
Initial Review of Analysis Results
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Before spending time looking at detailed design results it is always worth reviewing things at a broad-brush overview level. The most basic checks are made obvious within the workspace summaries shown above.
Maximum Nodal Deflections Maximum deflections are identified and noted – if any of these are clearly much too high then there may be mechanisms developing or the defined structure is simply not capable of dealing with the loads being applied to it. If extreme deflections are being reported then the analysis results may be suspect. If there are any issues of this nature, then you should investigate these before spending (potentially wasted) time looking at detailed design results.
Sway Sensitivity Once again, before spending too much time reviewing detailed designs, it is also advisable to review the sway results and decide on the approach you will take to designing for sway. It is conceivable that any changes you make in order to deal with sway could affect the designs of many elements. This topic is discussed in a little more detail in “Sway Resistance”.
Loading Summary This is simply a mathematical double check – does the sum of applied loads equal the sum of the base reactions? If there is a discrepancy identified by this comparison (but the maximum deflections noted above seem reasonable) you will probably need to contact your local support department for help in assessing the problem.
Review of Selected Sections It is always been worth spending a little time reviewing the results to see if they are in line with expectations. Where you have rigid framing this is even more essential. A quick review does not need to look at the design detail, simply look for things such as: • do the typical beam and column sizes look reasonable?
• • • • •
where you expect to see a hefty beam or column have you got one? are there big beams or columns where you did not expect them? where you expect similar sizes have you got similar sizes? where you expect symmetry is there symmetry? and 1
have you limited the use of composite beams to situations where composite beams are practical in reality? For further information see “Controlling Composite Beam Design” in the extended Quick Start Guide.
Footnotes 1. although not really the focus of this document
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Review Analysis Results Within Fastrak you can review the analysis results for your entire model quickly and easily. It is always worthwhile doing so as this gives you important information on how your structure is working. You can view deflected shape diagrams, axial load, shear force, bending moment and foundation load diagrams for all member types in your structure, or limit the views to just those particular member types of particular interest. In some cases this may also help you to identify issues with the analysis model. Note
The model can also be exported to S-Frame where all the same sorts of results for the static analysis can be reviewed and there is also access to more advanced analysis options, e.g. Buckling Analysis (analytical assessment of lcrit), P-Delta Analysis, Vibration and Response Spectrum Analysis, etc.
3D Analysis Effects Traditional design approaches tend to involve idealisation and simplification of the analysis model. Very often this would have meant simplification of the structure into discrete 2D planes, which could be analysed either by hand or in a simple 2D analysis. Engineers working with 3D analysis packages sometimes encounter unexpected results, which only make sense after some careful consideration. For the purposes of this document we are calling these 3D Analysis Effects. Since Building Designer allows you to model rigid frames and uses a full 3D analysis to generate design forces we anticipate that you may encounter these sorts of effects. The following two subsections illustrate two simple examples.
Continuous Beam Example
The above model is not intended to be highly realistic, it does however illustrate a 3D Analysis Effect quite clearly. Continuous beams (spanning 6 m then 9 m then 4.5 m) run from right to left of this floor area. These are supported on simple steel beams spanning front to back which are in turn supported by the columns. There are 3 internal lines of continuous beams which all receive the same loading.
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Designing by hand most engineers would probably consider that the analysis of a single 2D continuous beam line with pinned supports (as shown below) would be an adequate idealisation.
A more accurate analysis would attempt to model the spring effect at each of the supports – that is the supports are not completely fixed against vertical translation. This spring effect is inherently modelled in a full 3D analysis and the results after analysis and design in Building Designer are shown below.
Notice that different sections are chosen for the central continuous beam line on grid 3 when compared with the beam on either side of it. A first reaction to this sort of result might be to suspect that the design is wrong. However, a closer examination shows that the design is correct, and that it is correctly based on differing design forces. The results diagrams for the central continuous beam line are shown below.
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Compare these with the equivalent diagrams for one of the adjacent beams shown below.
Note
The maximum sagging moment has reduced from 255.3 kNm to 245.0 kNm.
This sort of variation is enough to force the selection of a larger beam on the central beam line where the moments are higher. Interestingly, we achieved the above result by limiting the initial design to 2 iterations and then redesigning only the continuous beams using the method covered in “Speeding up iterative analysis and design”. If we completely redesign the same model allowing up to 20 iterations and we also set the start from beginning of order file option, then the design does not converge (the design completes the maximum 20 iterations and then checks whatever sections it has at this point). The result is as shown below.
The continuous beams are actually heavier. Of the other beams, some are heavier and some are lighter. At this point all the beams pass so this is a second alternative acceptable design.
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To see a third alternative you might drive the design a different way. For example you might decide to use nothing larger than a 914 deep section, so you change the two cross beams on grids B and C to 914 305 UB 224 and put the 3 continuous beams back into design mode. This time the design converges quickly (because the conflict between the stiffness of the supporting beams and the stiffness of the continuous beams is removed). The result is shown below.
Once again, all member designs pass so this is a third alternative and completely acceptable design.
Braces Carry Gravity Loads Example This is probably a simpler example of a 3D analysis effect, however it does initially seem to fly in the face of traditionally accepted design practice. In traditional hand calculations the load chase-down puts all gravity loads into the columns. Where columns are also part of a bracing system providing sway stability, brace and column loads for the sway case are assessed in isolation and are added to the column loads for column design checks as necessary. The possibility that braces carry gravity loads is never considered in this traditional hand calculation approach.
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Consider the simple model shown above. It is braced on all four sides and in this example the initial sizes of the braces have been made very large to exacerbate the analytical effect.
When reviewing the results after design you might wonder why the column at C1 which is part of a braced panel is smaller than the column at B1 which supports the same floor area. You can review the summary design results and design forces within Building Designer, or you could export the two columns to the simple column module where you can see the results as shown below.
The top capture shows the details for the column at grid intersection C1 which is part of the braced bay. It shows that the column loses load to the brace at second floor level.
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Another good way to review these sorts of effects is through by exporting the model to S-Frame. The view below shows the axial load results for the front elevation.
The large brace forces are clear to see. If we change the brace section size to a smaller, more realistic section, then Building Designer finds that the same section size is adequate for both columns. When this model is exported to S-Frame the effect still occurs, but is less significant in this instance.
When you define braces in Building Designer you must always specify their section size, and Building Designer checks these for you (they are not designed automatically). This means that this is not an effect that can become exacerbated by an iterative analysis and design procedure. As a closing point of guidance on this topic, we suggest that you keep brace sizes to a realistic minimum during building design. This realistic minimum is likely to be driven by sway considerations. For further information see “How’s the structure working?” in the extended Quick Start Guide.
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Chapter 5 : Overview of Analysis And Design Procedures
Interactive Design You can export General Beams and General Columns to their respective stand-alone design packages. However, these members never truly stand-alone, they always interact with other members in the structure. This interaction is demonstrated in: • “Continuous Beam Example”, and
• “Braces Carry Gravity Loads Example”. When you export elements to General Beam and General Column you lock in the interaction effects. If you do not change anything you can check the same beam or column and see the same results. However, you can also run an automatic design in which case the locked in interaction effects do not change. What this can mean in practice is that you may select a different section (larger or smaller), interactively. This section may seem to work satisfactorily when designed in isolation. You can then return this amended section size to the main model (where you will have to re-analyse and check your model). It is quite possible that the section which appeared to work when designed in isolation will fail when checked after re-analysis of the full model. It is also entirely possible that other members in your model may fail (or have more capacity in hand) since the distribution of forces will be affected by the different section size which you picked.
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Chapter 6 : Construction Levels, Floors and Diaphragms
Chapter 6
Building Designer Documentation page 49
Construction Levels, Floors and Diaphragms When you define construction levels you have a number of choices/settings to control.
Is it a Floor? Construction levels are simply levels that you need to identify in order to construct your model. There will certainly be a number of levels that are clearly floor levels, but there could be many others that are not. For example you create intermediate levels in order to define: • half landing levels and stairs,
• K Bracing – you require a construction level for the intermediate bracing connection points,
• and so on. Where you define a level which is clearly not a floor, then you should not check the floor box. With this setting the options to: • activate Diaphragm action,
• set Imposed Load Reduction for column design, and • Include the level in the count of floors when calculating the imposed load reductions, are all disabled. We consider that these settings are only applicable to true floors.
Diaphragm Modelling In a typical building lateral resistance is provided at a few discrete points and it is assumed that applied lateral loads will be distributed to the lateral load resisting systems either by floor diaphragm action or by a bracing system. A diaphragm will maintain exact relative positioning of all nodes that it constrains, ie 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. You can however elect to remove General and Member beam nodes from the diaphragm, allowing axial forces to develop within those members. Note
Nodes at support positions, (either column or supplementary), are automatically excluded from all diaphragms.
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Chapter 6 : Construction Levels, Floors and Diaphragms
In Building Designer, at each floor level there are 3 options for diaphragm modelling:
• Single diaphragm (Default Setting) • Slab items defined • No diaphragm Single diaphragm.
This option switches diaphragm action on for an entire floor. Note that completely isolated areas of the floor are constrained by the same diaphragm. Hence, in the example above, if lateral load is applied to the left hand tower it will be resisted by the combined bending of both towers. The towers can not move independently at the level of the diaphragm. This would produce incorrect results. Note
The extents of a diaphragm are best reviewed graphically (pick Select/Show/ Alter State, and then pick Show Diaphragm from the dialog). Each independent diaphragm is shown in a different colour.
Slab items defined.
Using this option discrete diaphragms are created for each area of interconnected slabs.
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Chapter 6 : Construction Levels, Floors and Diaphragms
Building Designer Documentation page 51
If this option were applied to the example from the previous section a more realistic model would be created. Two separate diaphragms would exist at each floor level above the podium. As a consequence lateral load applied to the the left hand tower is not resisted by the right hand tower. Each tower can move independently.
No diaphragm.
This option switches diaphragm action off for an entire floor.
Taking slabs out of a diaphragm.
It is possible to switch diaphragm action off for one or more individual slabs within a floor. This is only possible if the diaphragm has been defined using the Slab items defined option. To demonstrate this, the example from the previous section is modified to include a link bridge between the towers. Initially, by using the Slab items defined option, a single diaphragm is created at the level of the bridge. Thisconstrains all the floor nodes within both towers at the level of the bridge, so that at this level the towers can not move independently. Providing the linking slab is substantial this may be considered to be appropriate. However, if the link becomes more slender, a point will be reached where this is no longer the case.
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Chapter 6 : Construction Levels, Floors and Diaphragms
By using the Alter Diaphragm function in the Show/Alter State dialog, diaphragm action can be switched off for the slab within the link bridge..
Because the remaining areas of slab at this level are no longer considered interconnected, two discrete diaphragms are formed and the towers act independently..
Thursday 26 April 2007 – 10:18
Chapter 7 : Rigid Framing and Gravity Loads
Chapter 7
Building Designer Documentation page 53
Rigid Framing and Gravity Loads
Backspan Beams We expect that (generally) for gravity loads you will only introduce rigid framing locally and selectively. We anticipate that one of the most common requirements for this usage will be to define Backspan Beams.
The above view shows several simple examples of backspan beams at the first floor level. Consider the left-hand-side you will see a cantilever slab area. Some of the cantilevers are on column lines, however, others extend from the side of a supporting beam and so rely on a backspan beam to restrict rotation. Along the front elevation the column line steps back between foundation and first floor levels and so the columns from first floor to roof are supported on the ends of cantilevers with backspans. This can all be done very effectively by using General Beam design. However you may find that the general points noted below still apply.
General Points to Note Pattern Loading If you are creating continuous beams you should not ignore the possibility that pattern load cases could be critical. Building Designer will NOT automatically create pattern load cases for continuous beams. However, you can create more load cases containing the appropriate loads and then create more combinations to cover the pattern load cases.
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Chapter 7 : Rigid Framing and Gravity Loads
Pattern loading is also applicable to the backspan beam, it will not affect the cantilever, but if the cantilever load is reduced/removed the sagging moments in the backspan will increase. Hence, if the cantilever moment is critical the pattern load case is less likely to be significant. Note
The Steelwork Design Guide to BS5950-1:2000 (Volume 2 - Worked Examples), includes good examples illustrating this requirement.
Transfer Beams / Levels Levels where the columns that support the floors above are discontinuous are known as transfer levels and the beams/cantilevers that support these discontinuous columns tend to be known as transfer beams. The screen shot at the start of this chapter showed transfer backspan beams. You can design all sorts of transfer beams with Building Designer. Building Designer considers imposed load reductions during the design of columns. However, these reductions are applied during the member design phase, the building analysis is always based on all loads applied simultaneously. By default therefore transfer beams will always be designed for the full (un-reduced) loading in the supported columns. If you regard this as over-conservative, then you can optimise the design of the transfer beams interactively.
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Chapter 8 : Sway Resistance
Chapter 8
Building Designer Documentation page 55
Sway Resistance
Introduction Sway resistance requirements for steel multi-storey buildings are covered in BS5959-1:2000 clause 2.4.2. Despite the fact that the 2000 amendment to BS5950 (issued in 2001) was intended to clarify the requirements for sway, it is fair to say that confusion still remains. A large proportion of support discussions undertaken by CSC relate to these requirements. An accompanying Fastrak Technical Note on the subject of Multi-Storey Sway tackles this topic in some detail. We strongly recommend that everyone using Building Designer is made aware of the design requirements imposed by the code in relation to sway. A simple overview of some of the alternative ways in which you can provide sway resistance in Building Designer is given in the following sections. For further information see “Analysis options” in the Designing for Second-order Effects Advisory Note and “Reviewing sway” in the extended Quick Start Guide.
Using Bracing This is the most traditional approach and well positioned and proportioned bracing is undoubtedly the best method of providing sway resistance. If you can include braced panels then the above mentioned technical note is totally applicable.
Note
Building Designer allows all sorts of bracing configurations including: • tension only bracing, • K bracing, and • inverted V bracing (as shown in the screen shot above). For inverted V bracing Building Designer will even automate the analytical modelling of sliding connections so that the beam does not end up being supported by the bracing.
For further information see “Add bracing” in the extended Quick Start Guide.
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Building Designer Documentation page 56
Chapter 8 : Sway Resistance
Using Steel Moment Resisting Frames If you have to provide stability using moment resisting frames, then you can do so within Building Designer using General Beams and General Columns. In such circumstances please note the following key points: 1. Once again the Fastrak Technical Note on the subject of Multi-Storey Sway is highly applicable. Moment resisting frames still need to be classified as sway or non-sway. If there are sway frames then you must take steps within the design to account for the possibility of significant P-delta effects. These steps are clearly defined in the technical note (with the possible exception noted below). 2. BS5950 provides simplified methods for estimating λcrit and for dealing with sway sensitivity where frames are clearly multi-storey frames formed with horizontal beams and vertical columns. Such frames would not tend to sway significantly under pure gravity loads. If you have more irregular framing which you find to be prone to sway when purely vertical loads are applied, then you should take extra care. 3. It is highly likely that you will want to consider the advantages and disadvantages of introducing some level of base fixity. The base fixity options are noted in “Supports and Base Fixity”.
Using Other Moment Resisting Frames You have the freedom to create moment resisting frames using any material and section that you like by using Member Beams and Member Columns. There are two particular issues of which to be wary if you attempt to provide sway resistance using other materials and framing: 1. You need to pay particular attention to the definition of appropriate section and material properties, this was touched upon in “Member Beams and Member Columns”.
Using Shear Walls
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Chapter 8 : Sway Resistance
Building Designer Documentation page 57
Stability for the very simple frame shown above is provided by shear walls. In this view the walls are rendered as if they are large solid panels, however the modelling idealisation being used is actually a “mid-pier” vertical beam element with a fixed base, and rigid cantilever arms extended out at each floor level to support any attached beams or slabs.
Swapping to the axis stick view shown above and switching off the simple beams and columns the idealisation becomes more apparent. For further information refer to the article Shear Wall Analysis - New Modelling, Same Answers - The Structural Engineer, 1st February 2005, Vol. 83 No.3, page 20 which is available on the CSC web site. This modelling idealisation of shear walls with beam elements is traditionally well accepted. The points made in “Using Other Moment Resisting Frames” regarding section and material properties are of course important. In recent years we have seen a trend towards Finite Element modelling of shear walls, this can be accomplished by exporting the Building Designer model to S-Frame and then editing it to remove the general beams and meshing up the wall panels. While this appears to be a more detailed approach that has advantages such as the ability to deal with irregular openings in wall panels, there are disadvantages. For instance, you
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Chapter 8 : Sway Resistance
do not escape from the need to consider the points made in “Using Other Moment Resisting Frames” regarding the appropriate adjustments to gross section and material properties. But, it can be done…
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Chapter 9 : Connections Integration
Chapter 9
Building Designer Documentation page 59
Connections Integration
Introduction Note
For a more comprehensive description of the engineering within Fastrak Connections see: Building Designer Advisory Note – Integrated connection design
Fastrak Connections is design software that allows you to analyse and check a wide range of connections in Fastrak Building Designer. Connections can be,
• • • • •
simple connections - transfer vertical shear only, moment connections - transfer vertical shear and major axis moment column splices - continuity splices in simple construction tubular connections - principally for use in truss work column bases - simple and moment resisting bases including soil bearing pressure and concrete base design.
The definition and check of connections is an intrinsic part of Fastrak Building Designer - all data associated with a particular connection is held within the building model. All connections can be 'opened' within Fastrak Building Designer where they can be modified and refined before saving the data back to the model. Alternatively one or many connections can be passed out to Fastrak Connections where, again, they can be modified and refined and then passed back to Fastrak Building Designer. Note that certain data cannot be modified since it would affect other parts of the building model e.g. sections size of the connecting members. As a further alternative Fastrak Connections can be run as a 'stand alone' application and the connection data entered in isolation. Whilst all data is held in the building model, the source of such data is several fold. This includes,
• attributes - certain data can be set to be used during the definition of the connections e.g. beam to beam simple connections are to be fin plates,
• derived data - the building model already holds such items as the section size and grade of the members that are to be connected and the design forces,
• default data - when the connections within the building model are set up by Fastrak, intelligent defaults are used that can establish a part or full solution to the connection configuration,
• added data - any individual connection can be edited to improve or add to the connection configuration e.g. stiffeners can be added to moment connections. Unless otherwise stated all calculations are in accordance with the relevant sections of BS 5950-1: 2000 and the design models for connections draw heavily on the series of publications from the Steel Construction Institute that cover the design of connections - the so-called 'Green Books'.
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Building Designer Documentation page 60
Chapter 9 : Connections Integration
Simple Connections Simple connections are by definition pinned connections and transfer vertical shear only. Fastrak will attempt to configure a simple connection at the end of any Simple Beam, Composite Beam or General beam that is pinned. The word "attempt" is used since there are some configurations of member and connection that are beyond the scope of the current release e.g. if the supporting beam is not an I-section or the supported beam frames in at a steep angle. Simple connections can be end plates, fin plates and (double) angle cleats. During definition of your building model a set of Connections Attributes can be established such that in preference, for example, beam to column web connections are end plates and are designed for the minimum tie force requirement of 75 kN. The defaults for these attributes are,
• • • •
beam to beam - fin plate with one line of bolts, beam to column flange - end plate, beam to column web - end plate, beam to hollow section column - fin plate.
When a particular type of connection is established by Fastrak in the building model e.g. fin plate for beam to beam connections, the default settings for bolt size and number, fin plate thickness etc. are such that the subsequent check of this connection should under normal circumstances give a Pass. This is because simple connections are more about detailing than design i.e. a well detailed simple connection will usually be adequate in design. This has been underpinned in Fastrak by careful selection of the defaults to ensure that the 'Recommended Details' and standard connections contained in the 'Green Book' on simple connections are followed. This all means that as a designer, once you have selected the type of connecting element for a particular situation e.g. fin plate for beam to beam connections, Fastrak will provide robust and well detailed simple connections for the majority of the building. It is likely then that only a few connections will not be adequate. These can be displayed to you on the main building graphic and you can then interactively adjust the connection type or configuration to establish an adequate detail. Examples might be a heavily loaded beam that might require two lines of bolts or a shallow beam where the default bolt pitch has to be decreased in order to increase the number of bolts.
General Limitations • If a simple connection exists on one or more faces and a moment connection exists on another face, they will be treated as separate connections - no influence on the strength or detailing will be taken into account.
• In a double sided connection, either side of the web you are limited to the same connection type eg. angle cleat to angle cleat, end plate to end plate or fin plate to fin plate. The one exception to this is if you have an angle cleat or an end plate on one side you are permitted a fin plate on the other.
• Any influence on a simple connection due to an incoming brace is ignored.
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Chapter 9 : Connections Integration
Building Designer Documentation page 61
Note
For a more comprehensive list of assumptions and limitations you are directed to : Building Designer Advisory Note – Integrated connection design
Moment Connections Moment connections are by definition able to transfer moment as well as vertical shear. The design is also able to deal with axial force in the beam member if present. Fastrak will attempt to configure a moment connection at the end of any General Beam that has a 'Moment Connection' or is 'Fully Fixed' at the appropriate end. The word "attempt" is used since there are some configurations of member and connection that are beyond the scope of the current release e.g. if the supporting column is not an I or H-section or the supported beam frames into another beam. Moment connections can be established at beam to I- and H-section column flanges, and at beam to beam on end e.g. apex type connections. All are formed using bolted end plates in the current release. Beam to column moment connections can be single- or double-sided. There are no Connections Attributes associated with moment connections in Fastrak Building Designer. Hence, during definition of your building model only the essential data and a number of basic defaults are set up for each moment connection. Essential data includes section size of the members joined and their design forces. Basic defaults include such items as one pair of M20 Grade 8.8 bolts top and bottom of the connection with 20 mm thick end plates. It is necessary therefore for you to 'open' each individual connection and enter such data as,
• • • •
"additional tension and shear bolts, "extensions to the end plate, "stiffeners, "haunches.
Obviously at the same time you can also adjust the default values e.g. change from 20 mm thick end plate to 25 mm thick. You may prefer to adjust the moment connections 'inside' Fastrak Building Designer or you can send one or more connections to the 'stand-alone' application. In either case the data you have added or modified is saved in Fastrak Building Designer. You can see whether your connection configuration looks sensible by right clicking on the connection in the Connections window - this displays a 3D graphic of the connection in its own window. You can adjust each of the connections individually and design them as you proceed or once you are content with the layout of all of them you can click the Check Connections icon. You can use the Show/Alter State icon to view which have passed and which have not. Fastrak checks only the strength of moment connections. Stiffness, ductility and rotation capacity can be important characteristics in some situations. Your attention is drawn to Clause 2.4.2.5 of BS 5950-1: 2000 and Section 2.5 of the Green Book on moment connections. Note
For a more comprehensive engineering description of moment connections you are directed to: Building Designer Advisory Note – Integrated connection design
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Chapter 10
Chapter 10 : Issues and Limitations
Issues and Limitations
Foundation loads There can be some differences in the base load values between the Building Designer summary table and the individual column designs (General Column and Simple Column). This occurs if there is bracing coming in at the column base – the brace loading at the base of the column is not handled in the standalone applications – it is irrelevant to the column design. The foundation values in the Building Designer summary table are correct for foundation design these should be used and not the standalone values.
Vertical cross bracing Foundation shear and vertical load When vertical cross bracings are modelled, only one member is considered active, irrespective of lateral load direction. This means that at the base, only one of the two foundations has the correct shear and vertical load. If the load direction is able to be reversed (and hence the other brace should be active) then you need to allow for the correct shear and vertical load in the corresponding foundation loads.
Column axial load When vertical cross bracings are modelled, only one member is considered active, irrespective of lateral load direction. The axial load in a column is only included into the column where the bracing is connected. This means that if the lateral load is able to reverse, one column of a braced pair will have some axial load that is not accounted for in the column design. The diagram below illustrates this. In the design model, the left-hand column will be designed without the axial compression that is actually present when the correct brace is active in tension only. Therefore great care must be taken when selecting active/inactive bracing members, and in cases where bracing loads are significant, additional checks on columns may be necessary. T
T T
C
C
Design Model
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Actual Loads
Chapter 10 : Issues and Limitations
Building Designer Documentation page 63
Notional Horizontal Force Load Calculations Gravity loads carried by braces not accounted for in NHF load calculations Any gravity load carried between floors in a brace element is not picked up in the calculation of NHF forces - in a majority of structures this is not an issue as braces tend to carry very little gravity load. However just occasionally they can be used in a model where they do carry gravity load - see brace supporting cantilever beam/slab on the right in the figure below. (actually in this case the user should use a general beam and not a brace and then the calculation of NHF forces would be correct).
A quick look at the load case summaries in the design tree will advise the user of the relative size of the error in the NHF calculation: NHF = 0.005 x grav load.
Axial load in discontinuous columns used twice in NHF load calculations If a structure has a transfer beam carrying a column, or if wall mid pier models do not align vertically in a wall (due to openings etc) then the NHFs for the axial load in the supported column are used twice in the NHF calc - once when they get onto the supported column and a second time when picked up in the level of the transfer beam. This is actually conservative as too much NHF is applied to the structure. A quick look at the load case summaries in the design tree will advise the user of the relative size of the error in the NHF calculation: NHF = 0.005 x grav load
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Chapter 11
Chapter 11 : Sign Conventions
Sign Conventions The individual objects in Building Designer use the same sign conventions as for the individual object designs. These are clearly indicated on the various design object’s analysis result graphics. Building Designer always takes account of an object’s orientation when displaying analysis results for objects in your model, rather than showing a single object’s analysis results. This means that to correlate the two sets of results you need to know which is the start and which the end of the beams in your model, and that you also need to know which is Face A of the columns. If you switch the option to show the Element Direction on, then Building Designer shows an arrow on all beams, columns and braces. This arrow points from the start to the end of beams and braces, and up columns along Face A.
General beam sign conventions The following diagrams show the sign conventions used for general beams, and are provided to help you understand the details that Building Designer presents. End 1
Action
End 2
Reaction
Action
Beam end forces
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Reaction
Chapter 11 : Sign Conventions
Building Designer Documentation page 65
End 1
End 2
Action
Action
Reaction
Reaction
Beam end moments
Simple beam sign conventions The following diagrams show the sign conventions used for simple beams, and are provided to help you understand the details that Building Designer presents. End 1
Action
End 2
Reaction
Action
Beam end forces
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Reaction
Building Designer Documentation page 66
Chapter 11 : Sign Conventions
General column foundation sign conventions The following diagrams show the sign conventions used for general columns, and are provided to help you understand the foundation loads that Building Designer presents.
Action
Reaction
Vertical force at base Face C
Face A
Action Reaction
Major axis shear at base Face D
Face B
Action Reaction
Minor axis shear at base
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Chapter 11 : Sign Conventions
Building Designer Documentation page 67
Face C
Face A
Compression side
Action Reaction
Major axis moment at base Face D
Face B
Compression side
Action Reaction
Minor axis moment at base
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Chapter 11 : Sign Conventions
Building Designer shows Foundation Loads in its graphical windows, and not base reactions. However the Base Reactions given in reports, and in the export to Excel are the reactions and not foundation loads.
Simple column foundation sign conventions The following diagrams show the sign conventions used for simple columns, and are provided to help you understand the foundation loads that Building Designer presents.
Action
Reaction
Vertical force at base Face C
Face A
Action Reaction
Major axis shear at base Face D
Face B
Action Reaction
Minor axis shear at base Building Designer shows Foundation Loads in its graphical windows, and not base reactions. However the Base Reactions given in reports, and in the export to Excel are the reactions and not foundation loads.
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Chapter 11 : Sign Conventions
Building Designer Documentation page 69
Supplementary supports sign conventions The following diagrams show the sign conventions used for supplementary supports, and are provided to help you understand the foundation loads that Building Designer presents.
Z Y X Global Axis System Action
Reaction
Vertical force at support Action Reaction
Force in X direction Action Reaction
Force in Y direction Action Reaction
Moment about X axis Action Reaction
Moment about Y axis
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Chapter 11 : Sign Conventions
Column orientation effects The foundation loads that Building Designer presents are based on those present in the column. These obviously depend on the column orientation. The diagram below shows the effects on the column action and reaction base shears as the column is rotated.
Orientation 0° Action major
Reaction major Face A
Action minor Face D
Face B Reaction minor
Face C
Orientation 90° Action minor Reaction major Face A Action major
Reaction minor Face B Face D
Face C
Effect of column orientation For General Columns there is an identical effect for moments1.
Footnotes 1. Simple columns do not generate moments at the base.
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Designing for Second-order Effects : Chapter 12 : Summary
Advisory Note for Fastrak Building Designer page 71
Building Designer Advisory Note – Designing for Second-order Effects
Chapter 12
Summary With the advent of BS 5950-1: 2000(Ref. 1) the treatment of second-order effects particularly in relation to the susceptibility of buildings to ‘sway’ has come to the fore. Many buildings, even conventionally braced structures, have been shown to be ‘sway sensitive’ when the clarified rules of BS 5950-1: 2000 are applied. In addition, the layout of multi-storey structures has changed over recent years, • longer spans,
• shallower construction (affecting connection depth), • architectural preference for little or no bracing, • minimization of heavy masonry walls that can be assumed permanent. All of these has lead to more reliance on the steel framing to resist lateral loads with much less assistance from non-primary elements that have traditionally not been taken into account and yet nevertheless significantly contributed to providing overall stability. With all these changes, the likelihood of having to take account of second-order effects has increased significantly. There are codified methods for taking these into account based on modification of first-order analysis (linear elastic) – the ‘Amplified Forces Method’ being the preferred one. There are a number of implementations of rigorous second-order analysis but these are not a panacea(Ref. 2). For most buildings it is probably better not to allow the structure to have such low sway stiffness (λcr < 4) for such techniques to be obligatory. The clearer and equally effective simple methods given in BS 5950-1: 2000 are entirely adequate for the majority of buildings. The majority of this document is devoted to an explanation of the automated process of applying the ‘Amplified Forces Method’ that is included in Fastrak Building Designer1. This process is guided by just three choices. The document first provides some reminders of the terminology with which you need to be familiar when considering second-order effects. It leads you through making the choice of whether to carry out first-order or second-order analysis and if so which formula you should adopt to calculate the ‘amplifier’ that is an intrinsic part of the ‘Amplified Forces Method’. There is also information on the ‘best’ amplification factor to use and to which of the loads this should be applied. The impact of the method on member design, base reactions, beam end reactions etc. is covered.
Footnotes 1. Portal frames have their own methods to allow for second-order effects and a comprehensive range of these is included in Fastrak Portal Frame.
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Advisory Note for Fastrak Building Designer page 72
Designing for Second-order Effects : Chapter 12 : Summary
The default settings have been sensibly chosen so that most times you will not need to make any changes but options are given so that you can make rational decisions about your particular building based on your engineering judgement. The whole process is automated, simple to understand and nothing is ‘hidden’ from you.
Please read this first
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Designing for Second-order Effects : Chapter 13 : Introduction
Chapter 13
Advisory Note for Fastrak Building Designer page 73
Introduction With the advent of BS 5950-1: 2000(Ref. 1) the treatment of second-order effects particularly in relation to the susceptibility of buildings to ‘sway’ has come to the fore. Many buildings, even conventionally braced structures, have been shown to be ‘sway sensitive’ when the clarified rules of BS 5950-1: 2000 are applied. In addition, the layout of multi-storey structures has changed over recent years, • longer spans,
• shallower construction (affecting connection depth), • architectural preference for little or no bracing, • minimization of heavy masonry walls that can be assumed permanent. All of these has lead to more reliance on the steel framing to resist lateral loads with much less assistance from non-primary elements that have traditionally not been taken into account and yet nevertheless significantly contributed to providing overall stability. Traditional methods of analysis and design sought to distribute the lateral loads back to the structure providing sway resistance in a logical, engineering manner. The structure providing sway resistance was then analysed as one or more 2D plane frames using hand or computer methods. Much of the code, BS 5950-1: 2000 and its predecessors rightly encouraged this and consequently most of the requirements are written with this in mind. There is much still to commend this approach. However, with the advent of comprehensive software tools (such as Fastrak Building Designer) the approach has become more analytical, more rigorous and based on 3D. This will tend to highlight ‘out-of-plane’ effects that were ‘unseen’ or ignored in previous 2D approaches. With all these changes, the likelihood of having to take account of second-order effects has increased significantly. There are codified methods for taking these into account based on modification of first-order analysis (linear elastic), • modified effective length method,
• amplified forces method. See Clause 2.4.2.7 of BS 5950-1: 2000. The first of these alternatives is, for a number of reasons, not recommended(Ref. 3). The second alternative is widely applicable although has some restrictions (e.g. λcr ≥ 4.0). Within these restrictions it is a valid and particularly simple method of carrying out a second-order analysis. An automated approach to the amplified forces method is provided for you in Fastrak Building Designer. This document explains how to get the best out of this feature using the various options provided. The document starts with a reminder of some of the basic concepts behind this subject matter(Ref. 3) that are embedded in BS 5950-1: 2000.
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Chapter 14
Designing for Second-order Effects : Chapter 14 : Basic concepts
Basic concepts Frame imperfections – This is a mechanism to allow for the possibility that the vertical elements in a building cannot be fabricated absolutely straight nor erected perfectly vertical. Such frame imperfections are considered when only gravity loads (dead and imposed) are applied. They are replicated by applying small horizontal loads simultaneously with the gravity loads. These are termed Notional Horizontal Forces (NHF) and are taken as 0.5% of the factored dead and imposed loads. If other lateral loads are present in the Design Combination e.g. wind loads, then the NHF need not be applied. The principle attribute of NHF’s (in this context) is that they are used in conjunction with i.e. are combined with, the gravity loads in a particular combination. They are not second-order effects and must be applied with the gravity loads whatever method is chosen to allow for second-order effects. Second-order effects – In a ‘well-behaved’ structure constructed of a material that follows Hooke’s law, the performance of the structure can be readily and accurately predicted using linear elastic analysis. This is because the deformations of the structure are proportional to the applied forces and these deformations are relatively small. This is first-order analysis and there are no significant second-order effects. Second-order analysis is required, quite simply, when the effects ignored in first-order become significant. These effects include but are not exclusively limited to, • P-Delta (P-Δ) effects – global deformations of the structure are such that the axial loads applied to members are sufficiently eccentric to the theoretical line of action that additional moments are generated within the structure,
• P-delta (P-δ) effects – local deformations of the members are such that the axial forces in members are sufficiently eccentric to the theoretical straight member that additional moments are generated internal to the member. The local deformations of the members may be due to initial imperfections in the member (bow) or due to slenderness effects. Sway/Non-sway – A categorization of a frame that indicates its propensity to be affected by second-order effects. Depending upon how stiff the frame is, P-Δ effects may be small enough to ignore (see Clause 2.4.2.5 of BS 5950-1: 2000). These are termed ‘Non-sway frames’ (see Clause 2.4.2.6 of BS 5950-1: 2000). If these P-Δ effects are too significant to ignore then the frame is termed a ‘Sway frame’ (see Clause 2.4.2.7 of BS 5950-1: 2000). In the context of BS 5950-1: 2000, the categories of ‘Sway’ and ‘Non-sway’ are established from the lowest elastic critical buckling load factor, λcr, of the frame in the sway mode – see sub-section below. If this is above a certain limit (λcr ≥ 10) then the frame is ‘Non-sway’ otherwise it is ‘Sway sensitive’. Elastic critical buckling load factor – This is the factor by which the factored loads (e.g. 1.4 Dead + 1.6 Live) would need to be multiplied in order to cause elastic buckling of the frame in the sway mode. It is usually, more succinctly, referred to as λcr (lambda_crit). See Clause 2.4.2.6 of BS 5950-1: 2000. For multi-storey buildings, it is typically established by applying some ‘arbitrary’ horizontal loads (NHF’s) and assessing the resulting deflections of the structure. The arbitrary horizontal loads are, for convenience, the same value as, but should not be confused with, those used for frame imperfections. They are (in this context) applied on their own with no other applied loading. It should be noted that this approach is not always valid (see Determination of lcr) and that portal frames have their own approach.
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Advisory Note for Fastrak Building Designer page 75
Amplification Factor – This is a factor by which the forces due to sway of the building should be multiplied to make allowance for P-Δ effects. It is usually, more succinctly referred to as kamp (kay_amp). It is determined in one of two ways depending upon whether, • the building is clad and the stiffening effect of the cladding is ignored or,
• the building is unclad or is clad and the stiffening effect of the cladding is taken into account. See Clause 2.4.2.7 of BS 5950-1: 2000. Amplified forces method – This is a way of carrying out a second-order analysis using the results from a first-order analysis and requires the amplification factor described in the sub-section above. In Clause 2.4.2.8 of BS 5950-1: 2000, the amplification factor can be applied to, • the sway effects i.e. those forces and moments that are caused by sway in the building,
• all of the forces and moments i.e. both sway and non-sway. This can be very conservative since, for example, the moments in simply supported beams due to the gravity loading will be increased and these are not sway effects,
• the horizontal loads only, providing the building is symmetric with symmetrical loading. There is no explicit requirement in BS 5950-1: 2000 to amplify the deflections – see “Serviceability Limit State”. It is the application of the amplified forces method as a solution to second-order analysis that is described and discussed in the following sections. Information on the application of rigorous second-order analysis is given elsewhere(Ref. 2).
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Advisory Note for Fastrak Building Designer page 76 Designing for Second-order Effects : Chapter 15 : Second-order options
Chapter 15
Second-order options
Analysis options The first decision to make is whether you wish to carry out a second-order analysis or not. If you open the Analysis Options dialogue off the Design menu (see below) you can select a first-order analysis or a second-order analysis.
Figure 15.1: Analysis options input dialogue You might select the former if you are confident that under no design combination is the building ‘sway sensitive’ or if you are investigating deflections only. In the former case, you can confirm your assumption of non-sway when the analysis is complete since in the results tree-view the sway results for the worst column for the worst design combination are given for each direction. Selection of the second option would be more common particularly as the amplification factors would all be 1.0 if the building were to be ‘non-sway’ under all design combinations and hence have no effect on the design. Once the second option is selected, another pair of options appears – see above. Should you wish Fastrak Building Designer to automatically apply the amplification factors appropriate to each design combination, this pair of options enables you to decide how the amplification factor is calculated. The first formula (the default) is the more likely since it refers to clad structures in which no allowance is made for the stiffening effect of cladding. See also “Amplification Factor”.
Design combinations Once you have decided to carry out a second-order analysis as described in “Analysis options” (which applies to the building as a whole), you are able to select certain parameters that determine how this is applied within each design combination.
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If you open the Details dialog for any design combination you will see that there is a tab entitled, Second-order effects as shown below.
Figure 15.2: Second-order effects input dialogue The settings here allow you to select that Fastrak Building Designer automates the amplified forces method of second-order analysis. This is set by ticking the check box for Auto-kamp (this is the default and so most times you will not need to take action). Given that Auto-kamp is switched on there are two other options that need to be set, • from which value of λcr for this design combination should the amplification factor be determined,
• to which forces should the amplifier be applied. It should be noted that if Auto-kamp is switched off then you can enter your own value of kamp but you still have to decide to which forces this should be applied. Which value of kamp? – There is only one lowest elastic critical buckling load factor for any structure. However, the BS 5950-1: 2000 method of determining λcr using NHFs results in two values – one in the global X direction and one in the global Y direction. The lowest elastic critical buckling load factor is, of course, the minimum of these two. Nevertheless, if the loading is entirely (or predominantly) in one direction and the effects of these loads are entirely (or predominantly) in that same direction then it is acceptable to apply the amplification factor for λcr in the X to the loads in the X. Similarly for the Y direction. Thus for a reasonably symmetric building subject to a design combination that contains NHFs as the lateral component, it would be reasonable to apply the kamp in the X direction to the NHFs in the X direction. It is for this reason that the second-order effects property page includes under the heading Amplifier, kamp the first two options, in global X and in global Y. For a building in which the loading or its effects are not predominantly in one direction, it would clearly be acceptable to apply the larger of the two values of kamp determined for the X and Y directions. This might be slightly conservative but is safe. Thus for an asymmetric multi-faceted building subject to a design combination containing wind loads, it would be reasonable to use the maximum value of kamp. It is for this reason that the second-order effects property page includes under the heading Amplifier, kamp the fourth option, maximum X, Y. Since it is safe and potentially the most common situation, this setting is the default.
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Advisory Note for Fastrak Building Designer page 78 Designing for Second-order Effects : Chapter 15 : Second-order options
For a building that is ‘in between’ the two cases previously described it might be more appropriate and less conservative to apply the average of the two values of kamp determined for the X and Y directions. It is for this reason that the second-order effects property page includes under the heading ‘Amplifier, kamp’ the third option, average X, Y. To which forces? – It was noted in “Amplified forces method” that there are three options in BS 5950-1: 2000 with regard to which forces the amplifier should be applied. As a reminder, these are, • the sway effects i.e. those forces and moments that are caused by sway in the building,
• all of the forces and moments i.e. both sway and non-sway. This can be very conservative since, for example, the moments in simply supported beams due to the gravity loading will be increased and these are not sway effects,
• the horizontal loads only, providing the building is symmetric with symmetrical loading. Traditionally for a braced building in simple construction, the kamp factors are applied by the designer to the lateral load component of a combination. The designer still has some choice to make between different values of kamp for each direction and for each combination (X, Y, average, maximum). Since in this type of building it is only the braced bays that resist lateral loads, it can be said that this approach satisfies the first option above. Buildings cannot be classed as braced simple construction when they, • contain moment resisting frames,
• are significantly asymmetric in terms of layout or loading, • are not multi-storey buildings in the conventional sense i.e. they are not composed of discrete, horizontal floor levels. For buildings of this type, the third option cannot be applied. Nevertheless, where the building is essentially of braced simple construction but has very minor elements of the above, the approach of applying the amplifier to the lateral loads only may be sufficiently accurate. For buildings at the other extreme i.e. where the lateral load resistance is provided entirely by moment resisting frames and the layout and loading are significantly asymmetric, the most accurate approach is achieved by applying the first option. Whilst it is the most accurate it is not efficient in terms of the analysis/design process since it requires the determination of the sway effects independent from the non-sway effects. The process for achieving this is quite complex and is not included in the Auto-kamp feature of the current version of Fastrak Building Designer. There remain then two alternatives, • apply the amplifier to the lateral loads only (as in braced simple construction),
• apply the amplifier to all loads (conservative). Which of these is adopted is a question of judgement. It cannot be made using logic built into the program and so has to be given as a designer choice. Hence, the options on the dialogue shown in “Second-order effects input dialogue”, Apply to, lateral loads only and all loads.
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That choice is dependent upon how far removed from braced simple construction the particular building is (see bulleted list above). Another factor to be considered in making this judgement is how ‘sway sensitive’ the building is i.e. the value of λcr. For cases where λcr is close to 10 i.e. only just sway sensitive the amplifier will be very small and so it is of little consequence how it is applied providing some allowance is actually made. On the other hand for λcr approaching the lower limit of 4, the second order effects will be significant and so applying them in the correct manner becomes more important. In this case the safe option is to apply kamp to all of the loads. Hence for a relatively stable structure with little or no sway forces due to gravity loads, the amplifier need only be applied to the lateral loads. On the other hand for less stable structures where the sway forces due to gravity are significant then the amplifier should be applied to all loads. It may be noted that use of the amplified forces method results in a ‘smearing’ of the second-order P-Δ effects across the structure. This is in contrast to a rigorous second-order analysis where the members most affected are those with the combination of the larger ‘P’ with the larger ‘Δ’. Indeed, using rigorous second-order analysis the forces in members can reduce since tension forces tend to reduce the propensity for buckling. Nevertheless, the amplified forces method tends to be less conservative than rigorous second-order analysis for stiffer structures since the method intrinsically allows for approximately 10% of the second-order effects to be ignored before such effects are taken into account. This is due to the allowance in the kamp formula for clad structures for other unquantifiable but helpful second order effects e.g. base stiffness, cladding stiffness.
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Chapter 16
Designing for Second-order Effects : Chapter 16 : Analysis
Analysis
General A fully integrated analysis/design process is incorporated into Fastrak Building Designer. This automatically calculates the Notional Horizontal Forces for use in the gravity load combinations to allow for frame imperfections and for use in determining the sway sensitivity of the building under each design combination. The amplified forces method of second-order analysis is carried out based on your choice of the settings described in “Second-order options”. The description of this process, given below, will help you to understand the approach taken.
Analysis model The whole of the building model is submitted for analysis. Columns used in simple multi-storey construction are usually more than one floor height in length and where spliced, the correct design of the splice should ensure full transfer of stiffness. Simple columns in braced bays are considered to be continuous from base to roof whereas the remaining simple columns are assumed pinned above each floor level. This ensures that the latter columns do not attract moments under horizontal loads and more importantly, this ensures that the braced bays (along with any other rigid moment resisting frames) have to resist all the horizontal loads. Simple beams and composite beams are (obviously) assumed pinned at each end. Similarly, vertical and horizontal braces are considered pinned at each end. In all of the above cases the end conditions cannot be edited. For all other members e.g. general columns, general beams, truss members, you can set the end conditions and Fastrak Building Designer applies the appropriate degrees of freedom in the analysis model. A ‘rigid diaphragm’ can be included at each floor and roof level. This along with any horizontal bracing systems ensures that all the lateral loading is accurately distributed between the lateral load resisting components of the building e.g. braced bays. Column bases other than those to simple columns can be set as ‘pinned’, ‘nominally pinned’ and nominally fixed’ in the ‘x’ and ‘y’ direction – see Clause 2.4.2.2 of BS 5950-1: 2000. Other (so called ‘supplementary’) supports can have all their degrees of freedom set to released or fixed and these are passed directly to the analysis model. See also Section Base stiffness.
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All loading is included in the analysis model i.e. gravity loads, wind loads and NHF’s. These are analysed as unfactored loadcases. The post processing of the analysis results uses superposition to combine these into design forces for the Ultimate Limit State. In this process the load factors appropriate to the particular design combination are used along with the value of kamp. For example, consider a brace with the following forces from unfactored loadcases, Dead Imposed Wind X
= = =
0.8 kN (due to elastic shortening of the columns) 1.1 kN (due to elastic shortening of the columns) 68.6 kN
The design force under the combination 1.2 Dead + 1.2 Imposed + 1.2 Wind X taking the kamp factor for this design combination as 1.08 applied to the lateral loads only is, 1.2 x 0.8 + 1.2 x 1.1 + 1.2 x 1.08 x 68.6
=
91.2 kN
It is interesting to note that if we had decided to apply kamp to ‘all loads’ the resulting design force would be 91.4 kN i.e. very little difference because the brace design is almost entirely lateral load dominated.
Analysis results Building results – From the analysis of each of the NHF loadcases, the deflections at the top and bottom of each stack in a column under these forces alone with no other loading can be used to calculate the storey height deflections associated with a particular design combination
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Designing for Second-order Effects : Chapter 16 : Analysis
(see “Sway X and Sway X-Y deflections” (a)). This is carried out for each of the two global directions, X and Y. Using the formula in Clause 2.4.2.6 of BS 5950-1: 2000, a value of λcr for each of the two orthogonal directions can be calculated. d x yi
dxxi
dx
NHFX
xi
FX NH
dx yi1 xi dx
NHFX
-1
FX NH
dxxi-1
(a) in-plane deflection
(b) out-of-plane deflection
Figure 16.1: Sway X and Sway X-Y deflections In the ‘Sway’ entry in the results tree view the λcr in the global X direction for the most critical stack of the most critical column is reported for each combination. Similarly for the Y direction and noting that they may not be the same column for both directions. In addition the tree view includes the value of kamp that is used for a particular combination and noting that there can be only one value of kamp per combination. The value of kamp given is either that supplied directly by you or has been calculated by the program when Auto-kamp has been selected. For the latter there are two limiting values, • if the particular λcr for the particular design combination is ≥ 10, the value of kamp will be 1.0 if you have selected the first of the two formulae described in “Amplification Factor” and given in Clause 2.4.2.7 of BS 5950-1: 20001. For example, if λcr X = 9 and λcr Y = 11 and you have decided to apply the ‘Amplifier, kamp’ ‘in global Y’ then kamp for this particular combination would be 1.0.
Footnotes 1. Note that using the second formula in Clause 2.4.2.7, kamp will always have a value irrespective of the value of λcr i.e. this type of structure e.g. unclad is always considered to be sway sensitive.
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• if λcr in either direction (X and Y) for a particular design combination is < 4, kamp will be set to 1.0. That is, no amplification of forces will take place since the amplified forces method is not valid below this limit. For example, if λcr X = 3.2 and λcr Y = 4.5 and you have decided to apply the ‘Amplifier, kamp’ as ‘maximum X, Y’ then kamp for this particular combination would not be applied and the value is displayed in the results tree view as 1.0. To draw your attention to this a warning triangle is displayed against this result. Note that if you had chosen ‘Amplifier, kamp’ ‘in global Y’ the same results would be displayed even though λcr Y is > 4. This is because the lowest λcr for the building as a whole is < 4. The detailed results for the critical columns in X and Y for a particular design combination can be investigated by double clicking the column name in the results tree view. The detailed sway results for any column can be investigated by right clicking the column on the main graphic display and selecting ‘Design Results’ off the menu that appears. These detailed results are dealt with in the next sub-section. An example of how superposition is used and is subject to your settings on the Analysis Options and Second-order effects dialogues is shown below.
Combination
Original L/F’s
kamp X/Y
Auto-kamp
Superposition
D+L
1.4D+1.6I
1.25/1.20
Auto-kamp off
1.4D+1.6I
D+L+NHFY
1.4D+1.6I+1.0NHFY
1.25/1.20
global Y/all
1.2(1.4D+1.6I+1.0NHFY)
D+L+WX
1.2D+1.2I+1.2WX
1.15/1.10
average X&Y/lateral
1.2D+1.2I+1.125(1.2WX)
D+WY
1.0D+1.4WY
1.05/1.00
max. X&Y/lateral
1.0D+1.05(1.4WY)
The table shows a number of typical design combinations with their original (first-order) load factors. The amplification factor determined by Fastrak Building Designer for the two orthogonal directions X and Y is given in the third column assuming that Second-order analysis is switched on and the first equation for the determination of kamp has been chosen on the Analysis Options dialogue. The first entry has been included without amplification and in the initial stage of the building design process is the only design combination selected for analysis and design to give very rapid sizing of all members. The mathematics of the remainder of the table entries requires no further explanation. Individual column results – In the detailed results for each column there is a Sway X and a Sway Y page (see “Sway X and Sway Y detailed results page” below). These results are per design combination and per stack and relate to the particular column selected. This may not be the critical stack for the critical column (upon which kamp for the particular design combination is based). Hence, the sway results for this particular (non-critical) column may bear no relation to the values used to determine kamp for this design combination. Thus the last line on the sway results page is provided to indicate the value of kamp that has been used for all columns for this design combination.
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You will see from “Sway X and Sway Y detailed results page” that Stack 2 of this column under a ‘dead plus wind’ design combination is ‘non-sway’. However, the most critical stack for the most critical column has a λcr of 7.5 and hence a kamp for this design combination of 1.053. It is this value that is applied to all the lateral loads in the building.
Figure 16.2: Sway X and Sway Y detailed results page The last line of the sway results page can have several configurations, • “Actual kamp applied to lateral loads for the whole structure” along with an appropriate value. This description will be present when you have selected for this design combination the ‘Apply to’, ‘lateral loads only’ on the second-order effects input dialogue (see “Second-order effects input dialogue”).
• “Actual kamp applied to all loads for the whole structure” along with an appropriate value. This description will be present when you have selected for this design combination the ‘Apply to’, ‘all loads’ on the second-order effects input dialogue (see “Second-order effects input dialogue”).
• “kamp NOT applied, second-order analysis required” along with a value of 1.0. This is
indicative that for this design combination, one (either X or Y) of the λcr values is less than 4 and hence the amplified forces method is not valid. The value of 1.0 is given to indicate that no amplification has taken place.
It should be noted that the first two messages can occur irrespective of whether the program has determined the value of kamp (‘Auto-kamp’ switched on) or whether you have provided the value of kamp (‘Auto-kamp’ switched off). The third message can only appear when ‘Auto-kamp’ is switched on. You will see from “Sway X and Sway Y detailed results page” that for each column there is a results page entitled, “Sway X-Y”. On this page, for each stack for each design combination, the out-of-plane deflections are given (see “Sway X and Sway X-Y deflections” (b)). For example, for NHF’s applied in the global X direction, the deflections in that direction are given on this page along with any deflections in the Y direction. Where the structural system that provides resistance to horizontal forces is particularly asymmetric then the value of the (out-of-plane) Y deflections can be significant. Having access to this information enables you to decide whether the out-of-plane deflections are acceptable or whether additional bracing is required. This is based on engineering judgment and not mathematics which is why there is no overall critical
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value recorded in the results tree-view. However, it was reported earlier that the most critical value of Sway X and Sway Y are reported for all the columns in the building. Adopting this conservative approach provides some allowance for the out-of-plane deflections.
4 dx
‘Storey height’, h24
4 or Flo
3 or Flo
2 or Flo
2 dx
lcr = h24 / (200 ´ (dx4 - dx2))
Figure 16.3: Column not fully restrained at one floor level It is worth noting that in calculating λcr, the storey height is taken as the height between positions on the column where restraint is provided in two orthogonal directions. Thus as an example (see “Column not fully restrained at one floor level”), consider the case where a column is restrained in both directions at Floor 2 and 4 whereas at Floor 3 there is no restraint. In this case the storey height is taken as that from Floor 2 to Floor 4 and the storey height deflection is the difference between that at Floor 4 and that at Floor 2.
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Chapter 17
Designing for Second-order Effects : Chapter 17 : Design
Design
Member design The design results for columns include information on ‘sway’ (λcr and kamp) – see “Individual column results”. For all members, the design forces (for checking moment resistance etc.) incorporate the appropriate ‘amount’ of amplified forces (second-order effects) based on your settings for both Analysis Options and Second-order effects (see “Second-order options”). The degree to which you will see second-order effects will vary, some of the influences on this variation are, • simple and composite beams are influenced very little if at all by lateral loads. Hence if your ‘amplifier’ is set to ‘lateral loads only’ then there will be negligible or no effect on the resulting design of these members.
• if you apply the amplifier to ‘all loads’ then you are likely to see member sizes increase. For simple and composite beams a kamp of 1.15, say, leads to an increase in design forces of 15% since the solution is linear. This can increase the section size by one weight.
• for columns in rigid moment resisting frames even the maximum ‘amplifier’ of 29% for
clad structures and 33% for ‘unclad’ structures (when λcr is 4) may not change the required section size. This is because such frames require large section sizes to provide adequate stiffness against sway and for within-storey deflections.
As well as the direct effect of amplifying the design forces there is also a secondary effect on the design calculations. When considering combined buckling of a member due to the application of major and minor axis moments and axial load, BS 5950-1: 2000 defines a series of ‘uniform moment factors’ (‘m factors’). Clause 4.8.3.3.4 requires that for members in ‘sway-sensitive’ frames when using the amplified forces method, the ‘m factors’ should be applied only to the ‘non-sway’ moments. The application of ‘m factors’ is then of the form, kamp * Ms + m * Mn where Ms are the ‘sway’ moments, Mn the ‘non-sway’ moments and ‘m’ is the appropriate uniform moment factor (the ‘m factor’ for lateral torsional buckling. mLT, is not affected in this way). Since no attempt is made to separate the ‘sway’ from the ‘non-sway’ moments (see Section Design combinations, To which forces?), then the one safe solution is to set all the ‘m factors’ (except mLT) to 1.0. Clearly for the members where combined buckling is the critical design check, this will lead to some conservatism.
Connection design information Base reactions – Second–order effects are ‘internal’ to the structure i.e. change the forces and moments within the members but do not affect the loading. Thus the base reactions should still sum to the applied loads although the distribution may change. Any moments at nominally fixed bases are most likely to increase. However, when using the amplified forces method of second-order analysis, the components of the forces due to lateral loads or all the loads (depending upon your settings) are increased by the amplifier. This is correct for any moments at the base but will result in the vertical and horizontal reactions at the base not summing to the applied loads. This is inevitable and although, strictly speaking, incorrect the following discussion indicates that acceptance of these amplified reactions will lead to safe foundation design.
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This amplification of the base reactions is included in the various output reports available from Fastrak Building Designer for factored values only (since the amplifier is only applied when loadcases are combined). The unfactored base reactions that are available are the first-order analysis values i.e. are not amplified. You must make some judgement regarding which set if either is given to the foundation designer. As well as second-order effects, the influence of Notional Horizontal Forces (NHF’s) on the base design needs to be considered. NHF’s are lateral loads and so from a purely analytical point of view, produce both shear and axial force in the base. The latter can add to or subtract from the axial forces due to gravity loading. There are three schools of thought regarding the treatment of base reactions resulting from NHF’s. • BS 5950-1: 2000 Clause 2.4.2.4 states that, “The notional horizontal forces applied in load combination 1 should not …. be taken to contribute to the net reactions at the foundations”. Interpreted literally this can be taken to mean that none of the forces from the NHF’s should be included in the foundation/base design. That is, neither axial, shear forces or moments.
• EC3 requires frame imperfections to be accounted for by including in the analysis model a ‘lean’ in the columns. The alternative of applying NHF’s is also given. However, it is made clear that these should be a “closed system of forces” i.e. a horizontal force at the top of the column equilibrated by one of the same value but opposite sign at the bottom of the column. In the context of BS 5950-1: 2000, this implies that the resulting shear force should be ignored but that the disturbance of the axial force distribution should be taken into account. The sum of the axial forces in the bases will always be the same.
• In a 2D plane frame without bracing the closed system of forces works well. The axials, shears and moments at the bases are very similar (even exactly the same) as those from an analysis in which an appropriate lean is built into the columns. There are two difficulties with this approach for braced buildings. Firstly, the NHF’s are applied at multiple points throughout the structure but they manifest themselves at the discrete points associated with the braces in the bottom floor. It is not possible therefore until the analysis is complete to apply the equilibrating forces at the appropriate base positions. Secondly, and more importantly, if the building and/or bracing system is significantly asymmetric then shear at the bases can appear at the braced bays that are out-of-plane to the direction in which the NHF’s are applied. These shears can be significant and could be considered as ‘real’ and not notional. Hence, they should not be ignored. Thus if you support the first school of thought, then the base reactions results tables must be edited to remove all reference to the NHF’s. Alternatively, if the Export to DXF function is being used it is a simple matter not to export the base reaction plans for the NHF loadcases. This approach is not recommended. The use of the word “net” is crucial here. The NHF’s are not real forces and so the horizontal base reactions should sum to zero since there is no other lateral loading. Hence, the wording in BS 5950-1: 2000 that the NHF’s “should not …. be taken to contribute to the net (horizontal) reactions at the foundations”. Similarly, for the second approach, the table of base reactions must be edited to remove the shear values. The DXF file will also need to be loaded into AutoCad or one of its derivatives and the shear reactions removed. This approach will ignore all horizontal base reactions but will take account of the disturbance in the vertical reactions.
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Designing for Second-order Effects : Chapter 17 : Design
Finally, the third school of thought will require you to include all forces for all the NHF loadcases. This approach will take account of the disturbance in the vertical reactions and will also include the horizontal reactions (shears). The latter may or may not be ‘real’ and are potentially large. Beam end reactions – Second–order effects are ‘internal’ to the structure i.e. change the forces and moments within the members but do not affect the loading. For beams with moment connections the second-order effects will manifest themselves at the connection and this is correct. This is one reason why the ‘amplified forces method’ is preferred over the ‘effective length method’ – see Clause 5.6.4 of BS 5950-1: 2000. The coincident shear at such connections must be in equilibrium with the applied moments (first-order and second-order components). Axial forces in such connections are generally small where the beams are horizontal. Usually it is the factored design forces that are quoted for connection design and these will include the amplification i.e. the second-order effects. Hence it is the amplified forces and moments that should be used for the connection design and these are readily available from Fastrak Building Designer. For pin ended beams with the amplifier applied to the ‘lateral loads only’ there will be little change in the design forces for the simple connections. This is correct. If the amplifier is applied to ‘all loads’ then some conservatism in the connection design forces must be accepted. Splice loads – A similar situation exists for splice loads as for the beam end reactions described above providing the splice is restrained i.e. is at or near the floor. However, splices that are away from the floor will usually be considered unrestrained and an additional second-order effect must be considered. Where a splice is not positioned close to a restraint the SCI Advisory Desk Notes, AD 243(Ref. 4) and AD 244(Ref. 5) gives appropriate advice. The text in quotation marks is from this document. "The first thing to note is that these design procedures only affect the splice. The member itself is not affected…". "Whichever type of splice is adopted, preloaded HSFG bolts should be used. … to prevent slip under ultimate limit state …" "Besides the internal forces and moments derived from equilibrium with the applied loads, allowance must be made for the following 'second-order' effects, • moments due to strut action,
• moments due to lateral torsional buckling, • moments due to amplification". These additional forces (apart from the last one) are not reported by Fastrak Building Designer and one might expect these to be anything from the same value as the 'real' moments to magnitudinally different. To meet these requirements, • the person responsible for the design of the members and the building as a whole should ensure that this information is passed to the designer of the splice if this is a different person either in-house or with another organisation,
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Designing for Second-order Effects : Chapter 17 : Design
Advisory Note for Fastrak Building Designer page 89
• the impact on the design process and the resulting size of splice (including the use of friction grip bolts) for splices that are more than the ‘traditional’ 500 mm above the floor level should be weighed against the convenience of using the column tops as hand-railing for example,
• the splice must be stiff enough and strong enough otherwise the member design will be affected possibly considerably and this affect is not easy to calculate - "Providing a splice that is more flexible than the member itself is considered to be bad practice, because it would be liable to be uneconomic and potentially unsafe. In any case it would be a complex matter to calculate the buckling resistance of a member in which the splice did not have at least the same stiffness as the member."
Serviceability Limit State Rigorous second-order analysis will show an increase in deflections due to P-Δ effects. There is no suggestion in BS 5950-1: 2000 that use of the amplified sway method of allowing for second-order effects requires similar amplification of the deflections. The inferred argument must be that for λcr as low as 4.0 (the limit of the amplified sway method) at Ultimate Limit State (ULS) then the λcr at the Serviceability Limit State will be significantly greater. Also the analysis of the structure can only give an ‘estimate’ of the deflections that are then compared with limits that experience has shown give true deflections that are not too adverse to the performance of the structure. With a ratio of ULS loads to SLS loads of 1.5, the lower limit of 4.0 for λcr infers a λcr of 6.0 for SLS. At this value, the increase in deflections due to second order effects is around 20% (from λcr/(λcr – 1)) or 11% (from λcr/(1.15 λcr – 1.5) which allows for some stiffness from cladding etc. that is not taken into account). This amount of increase can be argued both ways round i.e. it is small enough to be ignored or if taken into account, will not be too onerous. This dilemma is convenient. When the amplified sway method is used for a building with λcr of 4.0 or greater, then the deflections can be obtained by elastic analysis of the unfactored loadcases. These are summed using superposition to obtain SLS deflections for the design combinations. This conveniently maintains consistency with the amplified sway method i.e. elastic analysis of unfactored loadcases. Fastrak Building Designer reports the deflections at the Serviceability Limit State without amplification.
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Chapter 18
Designing for Second-order Effects : Chapter 18 : Other issues
Other issues
Determination of λcr The Amplified Forces method relies on establishing a reasonably accurate value of λcr. Fastrak Building Designer exclusively uses the ‘side load method’ using Notional Horizontal Forces to determine λcr. There are circumstances where a reasonably accurate determination of λcr using this method is not possible and there are specific instances where it is precluded. The explicit exceptions given in BS 5950-1: 2000 include, • single storey frames with moment-resisting connections i.e. portal frames – these have their own methods anyway,
• other frames (not necessarily single storey) that have sloping members with moment resisting connections. Given that a particular structure is not explicitly excluded, there is no other restriction on the use of the ‘side load method’. However, studies at CSC have shown that the ‘side load method’ becomes increasingly unreliable at low values of λcr. This has no practical impact since below a λcr of 4.0 the code requires the use of rigorous second-order analysis in elastically designed frames. The use of appropriate rigorous second-order analysis negates the need for an accurate determination of λcr. In addition and of more concern, the method relies on clearly identifiable floor levels that form part of a reasonably uniform layout of steelwork. There are buildings that do not fall into this classification, for example where true floors do not form the majority of the levels within the building, where floors slope or where columns run past floor levels – a classic case would be a football grandstand. In such cases, depending upon the severity of the irregularity of the layout, the side load method would not be appropriate1. Equally importantly, structures that are subject to the explicit exclusions may, nevertheless but with care, be analysed using the ‘side load method’. This typically occurs in roof structures that are sloping and have moment-resisting connections to the remainder of the underlying multi-storey building. Where there is a significant roof structure of this type, the issue is whether this will have an influence on the λcr of the building as a whole and if it does whether this, in turn, has an influence on the degree of second-order effects present in the underlying multi-storey structure. It is clear that there is potential for the lowest mode of buckling to occur in the roof structure. A tied rafter system (as opposed to a roof truss) could have a λcr significantly below that of the ‘rest of the structure’. However, it is the second-order effects that are important rather than the lowest elastic critical buckling load factor. The principal second-order effect that we are considering is P-Δ. Hence, the issue can be thought of in two halves – would ignoring the roof structure significantly affect either the axial loads (the ‘P’ part) and/or the deformations (the ‘Δ’ part).
Footnotes 1. In a recent example the ‘side load method’ showed the building to be ‘non-sway’ with λcr significantly above the limit of 10.0. A buckling analysis using CSC S-Frame showed the lowest sway mode elastic critical buckling load factor to be only 2.5.
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Designing for Second-order Effects : Chapter 18 : Other issues
Advisory Note for Fastrak Building Designer page 91
Thus, provided it is clear that the sway sensitivity of the building is determined for the underlying multi-storey structure only, you can make a judgement as to whether the possibility of a lower mode in the roof is important. It will obviously be important when it comes to designing the roof structure itself.
Connection stiffness The general assumption in an analysis/design model is that the connection between beams and columns are either pinned or fixed. In the former case (pinned), the type of connection normally adopted will generate a small moment early on in the load response history but at the ultimate limit state it has been shown that these moments dissipate and so will not normally be sufficient to affect adversely the design of the column. In the latter case (fixed), the connection must be stiff enough to not affect adversely the distribution of moments around the frame. Where there is any shortfall in stiffness i.e. the connection cannot hold the members at their original angle, there will also be an effect on the deformations of the frame – the true deflection being larger than those predicted by the analysis model. Two pieces of advice are relevant here, • BS 5950-1:2000 states in Clause 2.4.2.5, “Where moment resisting joints are used to provide sway stiffness, unless they provide full continuity of member stiffness, their flexibility should be taken into account in the analysis.”
• the SCI/BCSA Green Book on Moment Connections(Ref. 6) states in Section 2.5, “Connection rotational stiffness is inherent to the safety of this type of frame1. Flexibility in the connections adversely affects frame stability and serviceability”. The above statement from the ‘Green Book’ refers to elastic analysis. The importance of connection stiffness increases significantly for structures where P-Delta effects are significant (frames classified as ‘sway sensitive’) since the flexibility of the connections increases the deformations of the structure – the Δ in P-Δ. There is some guidance in that publication on how to deal with this and the forthcoming Eurocode for steel construction provides a method for calculating connection stiffness. In practical terms, haunched connections and extended end plate connections might be expected to have sufficient stiffness in this context but flush end plates on shallower beams may not be adequately stiff to justify discounting their influence on the analysis.
Base stiffness A modicum of base stiffness is often used when assessing the sway stability of a structure (10% of the column stiffness) or when checking the deformations at Serviceability Limit State (20% of the column stiffness). BS 5950-1: 2000 allows these figures to be used without taking into account in the foundation design, the moments that such base fixity attracts. In the former case this helps to reduce the impact of second-order effects when using the simplified methods in the Code (reduces λcr and hence the amplification factor kamp). Requirements – Most bases in braced simple multi-storey construction are assumed to be pinned. Any other fixity has no advantage in terms of strength design – bracing systems are assumed to be pin-jointed trusses and the moment profile for column design in simple Footnotes 1. “this type of frame” refers to moment resisting frames that provide lateral load resistance
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Designing for Second-order Effects : Chapter 18 : Other issues
construction is not influenced by base fixity. However, base stiffness is worth consideration when a significant amount of the lateral stability (sway stiffness) of the building is provide by rigid moment resisting frames for two reasons, • rigid moment resisting frames substituted for bracing at the lowest floor level creates a ‘soft storey’. This has a major influence on sway sensitivity, deflections and the section sizes of the members in such a frame. The use of nominally fixed bases would be a distinct asset.
• the downside of using full fixity is that this attracts moments into the foundations. Some measure of improvement in sway sensitivity and deflections but not strength or section size can be achieved by using nominally pinned bases. Clause 5.1.3.3 of BS 5950-1: 2000 allows a nominal stiffness of 10% and 20% of column stiffness to be used when calculating frame stability (sway sensitivity) and deflections respectively. Application – A requirement for base stiffness leads to a number of alternatives, • pinned bases. These are released rotationally (pinned) about the major axis and about the minor axis but are restrained rotationally (fixed) about the longitudinal axis of the column and fixed in all the translational directions.
• nominally pinned bases. These have the same release and restraints as for pinned bases but in addition the base can also be given a stiffness about the major and minor axes. The stiffness may be taken as 10% and 20% of column stiffness for frame stability and deflection calculations respectively.
• nominally fixed bases. Again these have the same release and restraints as for pinned bases but in addition the base can also be given a stiffness about the major and minor axes equal to the stiffness of the column when considering strength and frame stability. When considering deflections the base can be considered as “rigid” in accordance with Clause 5.1.3.2 (a) of BS 5950-1: 2000. These alternatives are provided for column bases (except those to simple columns) and for other (so called) supplementary supports. Fastrak Building Designer provides options to set supports as nominally pinned with a value of stiffness of either 10% (0.4EI/L), 20% (0.8EI/L) or a value of your own (in kNm/rad). Similarly for nominally fixed bases, the stiffness options are 100% (4EI/L) or a value of your own (in kNm/rad). If you wish to consider the base ‘rigid’ for deflection purposes then you can enter a very large value for stiffness. It should be noted that a practical base design is likely to have some stiffness and moment capacity in one direction about the major axis of the section. But, due to the potential for a much smaller lever arm for the HD bolts about the minor axis a different, much smaller stiffness etc. (approaching zero) could result about this other axis. Hence, the classification of pinned, nominally pinned and nominally fixed is orientation dependent. A consequence of allowing base stiffness to be specified in the above manner is the need for multiple analysis models and hence analysis passes. This is because the structure, in a typical example, can be pinned for strength, have 10% bases stiffness for frame stability and 20% bases stiffness for deflection calculations. These are not compatible and so the analysis model needs to be built and analysed separately for each of these settings. The solution is left in your hands and if you wish to take advantage of base fixity in a different manner for each limit state then you must create, solve, save and maintain separate versions of the same model.
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Designing for Second-order Effects : Chapter 19 : References
Chapter 19
Advisory Note for Fastrak Building Designer page 93
References 1. BS5950-1: 2000 Structural Use of Steelwork in Building – Part 1: Code of Practice for Design - Rolled and Welded Sections. BSI, 2000. 2. Rathbone, Alan J. Second-order analysis of 3D structures. The Structural Engineer Volume 82/No 21 2 November 2004. pp 15-16. 3. Rathbone, Alan J. Second-order effects – who needs them? The Structural Engineer Volume80/No 21 5 November 2002. pp 19-21. 4. Advisory Desk Note. Splices within unrestrained lengths. SCI AD 243. 5. Advisory Desk Note. Second order moments. SCI AD 244. 6. SCI/BCSA. Joints in Steel Construction. Moment Connections. SCI Publication P207, SCI 1995.
End of Document
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Designing for Second-order Effects : Chapter 19 : References
Integrated connection design : Chapter 20 : Introduction
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Building Designer Advisory Note – Integrated connection design
Chapter 20
Introduction This is design software that allows you to analyse and check a wide range of connections in Fastrak Building Designer. Connections can be,
• • • • • •
simple connections - transfer vertical shear only, moment connections - transfer vertical shear and major axis moment column splices - continuity splices in simple construction column -splices - moment splices1 tubular connections(1) - principally for use in truss work column bases - simple and moment resisting bases including soil bearing pressure and concrete base design.
The definition and check of connections is an intrinsic part of Fastrak Building Designer - all data associated with a particular connection is held within the building model. All connections can be 'opened' within Fastrak Building Designer where they can be modified and refined before saving the data back to the model. Alternatively one or many connections can be passed out to Fastrak Connections where, again, they can be modified and refined and then passed back to Fastrak Building Designer. Note that certain data cannot be modified since it would affect other parts of the building model e.g. sections size of the connecting members. As a further alternative Fastrak Connections can be run as a 'stand alone' application and the connection data entered in isolation. Whilst all data is held in the building model, the source of such data is several fold. This includes,
• attributes - certain data can be set to be used during the definition of the connections e.g. beam to beam simple connections are to be fin plates,
• derived data - the building model already holds such items as the section size and grade of the members that are to be connected and the design forces,
• default data - when the connections within the building model are set up by Fastrak, intelligent defaults are used that can establish a part or full solution to the connection configuration,
Footnotes 1. Not in first release
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Integrated connection design : Chapter 20 : Introduction
• added data - any individual connection can be edited to improve or add to the connection configuration e.g. stiffeners can be added to moment connections. Unless otherwise stated all calculations are in accordance with the relevant sections of BS 5950-1: 2000 and the design models for connections draw heavily on the series of publications from the Steel Construction Institute that cover the design of connections - the so-called 'Green Books'. The following advice is written principally from the point of view of operating connection design from within Fastrak Building Designer. Where necessary any important factors with regard to the operation of the stand alone application are noted.
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Integrated connection design : Chapter 21 : Practical Applications
Chapter 21
Advisory Note for Fastrak Building Designer page 97
Practical Applications
Simple Connections Simple connections are by definition pinned connections and transfer vertical shear only. Fastrak will attempt to configure a simple connection at the end of any Simple Beam, Composite Beam or General beam that is pinned. The word "attempt" is used since there are some configurations of member and connection that are beyond the scope of the current release e.g. if the supporting beam is not an I-section or the supported beam frames in at a steep angle. Simple connections can be end plates, fin plates and (double) angle cleats. During definition of your building model a set of Connections Attributes can be established such that in preference, for example, beam to column web connections are end plates and are designed for the minimum tie force requirement of 75 kN. The defaults for these attributes are,
• • • •
beam to beam - fin plate with one line of bolts, beam to column flange - end plate, beam to column web - end plate, beam to hollow section column - fin plate.1
When a particular type of connection is established by Fastrak in the building model e.g. fin plate for beam to beam connections, the default settings for bolt size and number, fin plate thickness etc. are such that the subsequent check of this connection should under normal circumstances give a Pass. This is because simple connections are more about detailing than design i.e. a well detailed simple connection will usually be adequate in design. This has been underpinned in Fastrak by careful selection of the defaults to ensure that the 'Recommended Details' and standard connections contained in the 'Green Book' on simple connections are followed. This all means that as a designer, once you have selected the type of connecting element for a particular situation e.g. fin plate for beam to beam connections, Fastrak will provide robust and well detailed simple connections for the majority of the building. It is likely then that only a few connections will not be adequate. These can be displayed to you on the main building graphic and you can then interactively adjust the connection type or configuration to establish an adequate detail. Examples might be a heavily loaded beam that might require two lines of bolts or a shallow beam where the default bolt pitch has to be decreased in order to increase the number of bolts.
Moment Connections Moment connections are by definition able to transfer moment as well as vertical shear. The design is also able to deal with axial force in the beam member if present. Fastrak will attempt to configure a moment connection at the end of any General Beam that has a 'Moment Connection' or is 'Fully Fixed' at the appropriate end. The word "attempt" is used since there Footnotes 1. In the current release end plates and angle cleats are prohibited from use with hollow sections.
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Integrated connection design : Chapter 21 : Practical Applications
are some configurations of member and connection that are beyond the scope of the current release e.g. if the supporting column is not an I or H-section or the supported beam frames into another beam. Moment connections can be established at beam to I- and H-section column flanges, and at beam to beam on end e.g. apex type connections. All are formed using bolted end plates in the current release. Beam to column moment connections can be single- or double-sided. There are no Connections Attributes associated with moment connections in Fastrak Building Designer. Hence, during definition of your building model only the essential data and a number of basic defaults are set up for each moment connection. Essential data includes section size of the members joined and their design forces. Basic defaults include such items as one pair of M20 Grade 8.8 bolts top and bottom of the connection with 20 mm thick end plates. It is necessary therefore for you to 'open' each individual connection and enter such data as,
• • • •
additional tension and shear bolts, extensions to the end plate, stiffeners, haunches.
Obviously at the same time you can also adjust the default values e.g. change from 20 mm thick end plate to 25 mm thick. You may prefer to adjust the moment connections 'inside' Fastrak Building Designer or you can send one or more connections to the 'stand-alone' application. In either case the data you have added or modified is saved in Fastrak Building Designer. You can see whether your connection configuration looks sensible by right clicking on the connection in the Connections window - this displays a 3D graphic of the connection in its own window. You can adjust each of the connections individually and design them as you proceed or once you are content with the layout of all of them you can click the Check Connections icon. You can use the Show/Alter State icon to view which have passed and which have not. Fastrak checks only the strength of moment connections. Stiffness, ductility and rotation capacity can be important characteristics in some situations. Your attention is drawn to Clause 2.4.2.5 of BS 5950-1: 2000 and Section 2.5 of the Green Book on moment connections.
Column Base Connections [To be completed]
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Integrated connection design : Chapter 22 : Scope
Chapter 22
Advisory Note for Fastrak Building Designer page 99
Scope
Simple Connections Simple connections can be one of three types - end plate, fin plate or (double) angle cleat. These are termed 'connecting elements'. In the case of fin plates and the leg of the angle cleat to the supported beam, a double line of bolts can be specified. Any of these can be provided for beam to beam, beam to column flange and beam to column web connections provided that both the supported beam and the supporting column or beam is an I- or H-section. In the current release only fin plates can be used to connect to hollow section columns. Supporting and supported members and their connecting elements are limited to S275 and S355 grade steels - there are a number of semi-empirical rules in the design models that preclude the use of S460 grade steels. For beam to beam connections a notch length can be defined that suits the width of the supporting beam flange - this must be the same on both sides of the supporting beam. For each side a notch depth at the top and bottom of the supported beam can be defined - these can be different top and bottom and, different on each side. For convenience 'standard' notch lengths and depths are defaulted and via the 'Set Standard Notches' button you can recover these if you have changed something but wish to get back to the standard values. Notch details also appear on beam to column connections but changing the default values from zero has no effect. To avoid notching in beam to beam connections a fin plate can be selected to ensure that the supported beam and the fin plate are joined outside of the flanges of the supporting beam. This will usually create a 'long fin plate' and certain additional checks are required and are carried out by Fastrak. Connecting elements can be of different types on each face of the supporting beam or column although those either side of a beam or column web have some design restrictions - see “Ultimate Limit State” on page 110. Beams can frame in at different levels or can be aligned (in beam to beam connections) with the top flange, bottom flange or centre line of the co-joined members. A wide range of bolt grades and sizes can be defined and, by default, their layout (end and edge distance, pitch and gauge) meet the 'Recommended detailing practice' given in the Green Book on simple connections (Check 1). The number of bolts is defaulted to meet the 'standard connections' contained therein. Similarly, weld sizes for use with fin plates and end plates are the standard values. For angle cleats the default bolt layout is the same in both legs but you can select different layouts in each leg if required. You are able to adjust all these defaults but you are advised to do so with care. The design forces for simple connections are shear in the plane of the web1 of the supported beam and where appropriate tie forces. A set of these can exist for each design combination and these are established from the analysis of the building model as a whole (or entered independently in the stand alone application). You can edit the design forces, delete and add
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Integrated connection design : Chapter 22 : Scope
load combinations and these changes will be reflected in the design of the connection immediately following the changes. However, all these changes will be lost if subsequently you use the 'Update Connections' function in Fastrak Building Designer. The necessity for tying is a matter of regulation and requirements are included in BS 5950-1: 2000. The April 2007 amendment to BS 5950 includes an adjustment to the tie force that is dependent on the number of storeys in the structure. Since the number of storeys in a building can vary and some ‘levels’ in a building are not ‘floors’ e.g. mezzanine. Fastrak will establish the number of storeys from the number of levels set to be ‘Included’ on the Levels dialogue of the Building menu. Note that the uppermost level will be included irrespective of the setting of the Included check box. Currently only simple connections are checked for tie forces if applicable. In the building model you are able to set simple connection attributes to allow for tying in one of three ways,
• no tie force -this member does not form part of the tying system, • minimum tie force - regulations permit only nominal tying of the building (75kN) • vertical reaction - full tying requirements are necessary and the tie force is taken to be, MAX(n x reaction, 75kN) where n is the reduction factor for number of storeys. For 5 storeys and above n = 1.0, for 4 storeys, n = 0.75, 3 storeys, n = 0.5, 2 storeys, n = 0.25 and 1 storey, n = 0.0 In the latter case if a member does not support a floor but simply acts as column tie or masonry restraint for example under 'normal' stage loading (i.e. at the Ultimate Limit State), there may be no reaction and hence the tie force will be set to the minimum. When establishing your connections, Fastrak will use these settings to determine the tie force (which may be zero if the no tie force option is selected). Results of your connection design can be viewed on the screen. The input, diagrams, and design results can be incorporated into a report by sending the connection or selection of connections out to Fastrak Connections. From there you can control exactly what you wish to see in the report. Connection components e.g. bolts, welds, stiffeners are not listed in the Material Listing report.
Moment Connections Moment connections can be one of three types - single-sided beam to column flange, double-sided beam to column flange and beam to beam. They are formed using bolted end plates that can be flush or extended top and/or bottom. Haunches can be defined below the supported beam and can be either,
• a section cutting - cut from the same size as the beam, same size as the column or from any (valid) section that you select or, Footnotes 1. Only 'positive' shear forces are used in the design. If the shear force is 'negative' the connection will be given a Beyond Scope status although the results for the 'positive' loadcase will be provided.
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Integrated connection design : Chapter 22 : Scope
Advisory Note for Fastrak Building Designer page 101
• built up from plates - a plate size (width and thickness) is required for the web and flange plates. Where a section cutting is defined, you are provided with a button to calculate 'Max Depth' of the haunch based on the length of the haunch, slope of the beam and section size selected for the haunch. In beam to column moment connections the column and the individual beams can be assigned a 'level' within the connection application. This is not reflected in the building model. Columns, beams and end plates that make up you connection can be S275, S355 and S460 grade steels. Care should be exercised when selecting S460 grade steel since the original 'Green Book' on moment connection design was written with S275 and S355 steel in mind and there may be limited availability of plate in this grade. A range of standard width and thickness of plates is provided from which the end plate can be selected. Flush end plates are given a 'projection' above and below the connection whilst for extended end plates it is necessary to define the extension (above and/or below). A wide range of bolt grades and sizes can be defined and, by default, only one pair of 'tension' bolts and one pair of 'shear' bolts are provided. You can add bolts individually or use the Generate Regular Bolt Layout button. In either case you need to go to the Combinations page and click on the appropriate Bolts column to set which of your additional bolts should be used in tension and which in shear. You need to do this for each design combination. Welds to the beam flanges, beam web and to the haunch if appropriate are all defaulted to 8 mm fillet welds. You should adjust these both in terms of size and type (a butt weld might be more appropriate) to suit the particular application. Stiffeners in moment connections are often required and a wide range is provided,
• rib - usually relatively short stiffeners that are used principally to improve column flange bending capacity,
• full depth - often required to resist compression particularly where the column is a UB section,
• shear - these can be diagonal, K or 'Morris' types and may be required to assist the web panel shear capacity,
• web plate - help both compression and shear capacity of the web panel and have the advantage that they do not interfere with any beams framing into the web,
• cap plate - can be provided for other purposes e.g. to connect parapet posts but if present will usually assist flange bending, and compression in 'reverse' bending combinations,
• flange plate - these are 'loose' flange plates i.e. they are not fully welded to the flange - they are usually tack-welded to keep them in place during transport and erection. They assist flange bending,
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Integrated connection design : Chapter 22 : Scope
• extension plate - for an extended end plate connection the extension can be relatively weak in bending and this vertical stiffener will improve the capacity. It is essential when there are two rows of bolts in the extension. Note that incorporating stiffeners is expensive in fabrication terms and most can interfere with other members framing into the same area. It is often economic to avoid stiffeners by increasing the section size (of the column and possibly the beam) or deepening the haunch. This may also avoid detailing issues and improve erection efficiency. The design forces for moment connections are shear in the plane of the web of the beam, moment in the same plane and where appropriate axial force in the beam. Tie forces are not included. For haunched connections, the moment at the 'sharp end' of the haunch can be entered - due to the length of a typical haunch this can be significantly less than that at the connection interface. This moment is used to check the capacity of the beam web at that position - (see page 112) A set of these design forces can exist for each design combination and these are established from the analysis of the building model as a whole (or entered independently in the stand alone application). You can edit the design forces, delete and add load combinations and these changes will be reflected in the design of the connection immediately following the changes. However, all these changes will be lost if subsequently you use the 'Update Connections' function in Fastrak Building Designer. Results of your connection design can be viewed on the screen. The input, diagrams, and design results can be incorporated into a report by sending the connection or selection of connections out to Fastrak Connections. From there you can control exactly what you wish to see in the report. Connection components e.g. bolts, welds, stiffeners are not listed in the Material Listing report.
Column Base Connections [To be completed]
Thursday 26 April 2007 – 10:18
Integrated connection design : Chapter 23 : Limitations and Assumptions
Chapter 23
Advisory Note for Fastrak Building Designer page
Limitations and Assumptions
Simple Connections Limitations The following limitations apply:
• single angle cleat as connecting elements are excluded, • a double line of bolts in end plates and in the leg of an angle cleat to the supporting member are excluded,
• in a beam to column web connection where the supported beam flange is wider that the internal dimension between the flanges of the supporting column, in practice a 'flange strip' would normally be called for. Such flange strips are not included in the current release and so you must judge whether they might influence the results from Fastrak,
• similarly where the bottom flange of a beam on one side of a connection might interfere with the bolts on the other (deeper) side of a connection a 'snipe' or flange strip is often used in practice. Such snipes and flange strips are not included in the current release and so you must judge whether they might influence the results from Fastrak,
• channel section and hollow section beams are excluded, • plated section beams, Westok beams and Fabsec beams are excluded, • sections with unequal flanges are excluded. This covers not only plated section beams that have unequal flanges but also Slimflor beams and asymmetric Slimflor beams,
• connections to concrete filled hollow section columns are excluded, • where more than one beam connects to the face of the supporting member, Fastrak cannot form the connection. Where a component is excluded e.g. channel section beams, Fastrak will not create a connection to that beam. In addition, where a connection would otherwise be created on one face of the supporting member but on another side a connection cannot be created then no connection at all will be created. For example, no connection will be created in the case where a channel section edge beam connects to a column on one face and an I-section connects to another face.
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Integrated connection design : Chapter 23 : Limitations and
Assumptions Essentially the assumptions in Fastrak are those inherent in the design models for simple connections given in the Green Book. However, a number of specific assumptions are made as given below. Internal forces in the supporting member from the applied loading that it carries are assumed not to influence the connection design. For example, the axial force in a column from floors above is assumed not to affect significantly the performance or behaviour of the connection. Similarly, where simple connections and moment connections are connected to the same column it is assumed that the forces imparted by the one do not influence the other. A typical example might be moment connections to the column flanges with simple connections to the column web. Not only are the designs independent but also the detailing. Hence, in this example any stiffening required by the moment connections is assumed not to interfere with the simple connections framing into the web. Any shear force out-of-plane of the beam web (minor axis shear force) is assumed not to influence the connection design. Similarly any axial force in the supported beam (other than due to tying action for structural integrity) is assumed not to be significant. In the Design Options off the Design menu in Fastrak Building Designer, you can specify limits for these forces below which you believe the design will be unaffected. If Fastrak detects forces greater than these limits, the design will still proceed but the connection will be given a Warning status and the value of these forces will be given in the results. The default values for these limits are,
• • • •
minor axis shear - 0.5 kN axial force - 1.0 kN major axis moment - 1.0 kNm minor axis moment - 0.1 kNm
The two moment limits are included even though simple connections are modelled in the analysis as pins. This is because the analysis model is a mathematical model and as such during the 'back substitution process' very small moments that are effectively zero from an engineering point of view may exist mathematically. Where a connected beam is not orthogonal to the supporting member, Fastrak takes the following approach (see also “Analysis” on page 109),
• where all angles are less than a given lower limit, then the angles are ignored and the design proceeds as if the members do connect orthogonally,
• where any angle is greater than a given upper limit, then Fastrak creates the connection but the design is not performed and the connection is given a Beyond Scope status,
• where one angle lies between the upper and lower limit, then the angle is ignored, the design proceeds as if the members do connect orthogonally and the connection is given a Warning status,
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Integrated connection design : Chapter 23 : Limitations and Assumptions
Advisory Note for Fastrak Building Designer page
• where more than one angle lies between the upper and lower limit, then Fastrak creates the connection but the design is not performed and the connection is given a Beyond Scope status. In all cases the angle can be viewed either in the design results (on the Notes page) or by editing the connection. The upper and lower limits are,
• lower limit for slope, skew and rotation - +/- 0.5 deg. • upper limit for slope and skew - +/- 10 deg., and for rotation - +/- 5 deg. The definitions of slope, skew and rotation are depicted in the figure.
Slope
Skew
Rotation
For beam to beam connections in Fastrak Building Designer, the top flanges of the supported and supporting beam are assumed to align. However, within the building model beams can be given an alignment relative to its 'cardinal points' - the default is at the centre of the top flange. Any changes that you make to the cardinal points are not reflected in the simple connection design. For example if you set the cardinal point of a supported beam to the centroid of the section and connect this to a supporting beam with the cardinal point set as the centre of the bottom flange, the simple connection design will assume, still, that the top flanges align. Similarly, in beam to column connections the centre lines of beams and the column are assumed to 'node'. However, within the building model both the column and the beams can be set 'off-grid'. Any changes you make that set beams or columns off-grid will not be reflected in the simple connection design. For example, you might offset the beams on the edge of the building towards the outer face of the column to suit cladding details. Nevertheless, the simple connection design will assume, still, that the beams and the column node on the centre lines. There are likely to be a number of positions in the building where braces connect to columns at the same position as a simple connection joining beams to the same column. The brace force is assumed to be transferred directly to the column and hence in the model has no influence on the force distribution in the connections. Similarly, the detailing of the brace end connection is assumed not to affect the configuration of the simple connections at the end of the co-joined beams.
Moment Connections Limitations The following limitations apply:
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• • • • •
Integrated connection design : Chapter 23 : Limitations and
fully welded moment connections are excluded, section types other than I- and H-sections cannot be used in moment connections, moment connections into the column web are not be permitted, the section size either side of a beam to beam moment connection must be the same, where beam to beam moment connections are formed between beams at different angles, the connection is assumed to be symmetrical - see figure,
• the projecting portion of an extended endplate both above and below is limited to have no more than two rows of bolts,
• where the projecting portion of an extended endplate either above or below has two rows of bolts, an extension stiffener must be provided,
• all components have only one value of design strength, py, taken as the lower of the two values based on flange thickness and web thickness. However, a built-up haunch will have a value of py for the haunch web and another for the haunch flange.
• when checking stiffeners in conjunction with, say, a web, the lower of the two values of py is used. This particularly affects S275 stiffeners used in conjunction with S355 sections.
• where more than one beam connects to the face of the supporting member, Fastrak cannot form the connection. Where a component is excluded e.g. channel section beams, Fastrak will not create a connection to that beam. In addition, where a connection would otherwise be created on one face of the supporting member but on another side a connection cannot be created then no connection at all will be created. For example, no connection will be created in the case where a plated section beam connects to one flange of the column and an I-section connects to another face.
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Integrated connection design : Chapter 23 : Limitations and Assumptions
Advisory Note for Fastrak Building Designer page
Assumptions Essentially the assumptions in Fastrak are those inherent in the design model for moment connections given in the Green Book. However, a number of specific assumptions are made as given below. It should be noted that, in principle, the design model in the Green Book is for moment connections with axial load and not axially loaded connections with moment. Internal forces in a column from the applied loading that it carries are assumed not to influence the connection design. For example, the axial force in a column from floors above is assumed not to affect significantly the performance or behaviour of the connection. Similarly, where moment connections and simple connections are connected to the same column it is assumed that the forces imparted by the one do not influence the other. A typical example might be moment connections to the column flanges with simple connections to the column web. Not only are the designs independent but also the detailing. Hence, in this example any stiffening required by the moment connections is assumed not to interfere with the simple connections framing into the web. Any moment or shear force out-of-plane of the beam web (minor axis moment and shear force) is assumed not to influence the connection design. In the Design Options off the Design menu in Fastrak Building Designer, you can specify limits for these forces below which you believe the design will be unaffected. If Fastrak detects forces greater than these limits, the design will still proceed but the connection will be given a Warning status and the value of these forces will be given in the results. The default values for these limits are,
• minor axis shear - 0.5 kN • minor axis moment - 0.1 kNm Where the members of a moment connection are skewed or rotated relative to each other, Fastrak takes the following approach (see also “Analysis” on page 109),
• where both the skew and rotation angles are less than a given lower limit, then the angles are ignored and the design proceeds as if the angles were zero,
• where either angle is greater than a given upper limit, then Fastrak creates the connection but the design is not performed and the connection is given a Beyond Scope status,
• where one angle lies between the upper and lower limit, then the angle is ignored, the design proceeds as if the angle is zero and the connection is given a Warning status,
• where both angles lie between the upper and lower limit, then Fastrak creates the connection but the design is not performed and the connection is given a Beyond Scope status. In cases where the angle is above the lower limit, the angle can be viewed in the design results (on the Notes page). The upper and lower limits are,
• lower limit for skew and rotation - +/- 0.5 deg. • upper limit for skew - +/- 5 deg., and for rotation - +/- 2.5 deg. For double sided beam to column moment connections and for beam to beam moment connections in Fastrak Building Designer, the top flanges of the beams are assumed to align. However, within the building model beams can be given an alignment relative to its 'cardinal points' - the default is at the centre of the top flange. Any changes that you make to the
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Integrated connection design : Chapter 23 : Limitations and
cardinal points are not reflected in the moment connection design. For example in a double sided beam to column moment connection if you set the cardinal point of one beam to the centre of the top flange and the cardinal point of the other beam as the centre of the bottom flange, the moment connection design will assume, still, that the top flanges align. The design will consider that the connection is double sided even though in this case the two connections should be treated as single sided. A similar situation occurs when the beams have the same cardinal point but the beams are defined on two separate grid lines that are close together. In this case Fastrak Building Designer will create two separate single sided connections even though the grid lines may be only a few millimetres apart! Note that for this reason in the stand alone application if you specify 'levels' for the two beams that would effectively mean that they should be treated as two separate single sided connections, Fastrak will still consider them as double sided. In beam to column moment connections the centre lines of the beams and the column are assumed to 'node'. However, within the building model both the column and the beams can be set 'off-grid'. Any changes you make that set beams or columns off-grid will not be reflected in the moment connection design. It is likely that when transferring significant moment, anything but the very smallest of offsets would invalidate the design model.
Column Base Connections [To be completed]
Thursday 26 April 2007 – 10:18
Integrated connection design : Chapter 24 : Analysis
Chapter 24
Advisory Note for Fastrak Building Designer page 109
Analysis
Global analysis Connection forces are established from a global analysis of the building as a whole. All the connection types included in Fastrak have a limited set of design forces for which the connection can be designed e.g. simple connections are designed for the appropriate beam end reaction (See “Scope” on page 99.). Non-design forces are identified and, where their value is greater than a given limit, they are displayed to you in the results along with a warning status. The 'given limits' are defined on the Force Limits - Connections page of the Design Options dialogue available from the Design menu. The forces from the global analysis are treated in the following manner for the different connection types,
• simple connections are designed for the shear in the plane of the beam web perpendicular to the centre line of the beam. For sloped connections this force is not resolved into the plane of the supporting member. For rotated connections the connecting element is assumed to be rotated by an equal amount and so no resolution of forces is necessary (although any out-of plane shear is ignored).
• moment connections are designed for the in-plane moments and forces i.e. major axis moment, shear in the plane of the beam web and axial force. Design forces for sloped connections are resolved into the plane of the supporting column. For skew and rotated beams there is no resolution of moments and forces into the plane of the supporting column.
• column bases are designed for the axial force at the base of the column, the shear in the plane of the column web and where appropriate the moment at the base of the column. Bases are assumed to be orientated to the column's major and minor axes and hence there is no requirement to resolve the force when the column is rotated. Columns can only be sloped in the pane of the web and the bottom stack axial force and shear are resolved into vertical and horizontal forces in the base. Where the global analysis includes second-order (P-Delta) effects the Ultimate Limit State design forces will include these effects also. However, for column bases the design forces for soil bearing pressure calculations are taken from an elastic global analysis of the unfactored loadcases without second-order effects.
Connection analysis The analysis of the connection itself is normally carried out by determining a rational set of load paths through the components of the connection. This is inherent in the design models adopted. The design models are based closely on those given in the series of 'Green Books' with extrapolation and addition where necessary to facilitate analysis/design of a wider range of connections.
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Chapter 25
Integrated connection design : Chapter 25 : Ultimate Limit State
Ultimate Limit State
Simple Connections The design checks for simple connections are given in the appropriate Green Book. To assist you, the results displayed in Fastrak are given the title of each check in the Green Book e.g. "Check 5 - Supported beam - capacity at notch". The design models in the Green Book are primarily based on 'standard' connections meeting orthogonally with, in the case of beam to beam connections, top of steel aligned. For such connections the design models are clearly laid out in the Green Book and so no additional explanation is given here. However, Fastrak covers a somewhat broader spectrum and so additional points of note are given below. For all connections there are 'Recommended Detailing Requirements' in the Green Book. These are recognized as 'good practice' and you should comply with these whenever possible. If you select an item or value that is beyond these standards, you will find that the particular entry appears 'orange' to give you a visual warning that you are stepping outside of good practice.1 There are strict limitations on the use of fin plates both in the Green Book and in Fastrak. These relate to edge and end distance, bolt size and grade, fin plate or web thickness, minimum 'projection' and maximum lever arm between topmost and lowermost bolts in the fin plate. These restrictions are necessary to ensure ductile behaviour and adequate rotation capacity of the connection. Both these properties are essential for a simple connection to act as a pin. Fastrak allows you to contravene these when specifying the connection but will fail the connection in design. Supported and supporting members do not always meet orthogonally - the assumption in Fastrak is that they do. For very small angles, these are likely to be less than the normal tolerance on layout and out-of-plumb and so can safely be ignored. For slightly larger angles, the affect on the design is likely to be only nominal and so, in most cases, might be ignored. For significant angles, assuming that nevertheless they are still orthogonal may not be sufficiently accurate and hence may be unconservative. More information on the treatment of non-orthogonal connections is given on page 103. Supported and supporting beams in beam to beam connection do not always align with their top flanges and across a column web beams may be at different levels. Fastrak covers such situations in one of two ways.
• Where the configuration of the connection either side of a web is not achievable in practice then you are allowed to accept the data and leave the dialogue but no design is possible until the discrepancy has been corrected (the offending detail will also appear 'red on the connection graphic). An example would be where two end plates connect either side of a column web and the 'bolt gauge' is different each side. Footnotes 1. If the item or value is not acceptable or not feasible to achieve e.g. the edge distance for the bolts is less than the minimum requirement of BS 5950-1: 2000 then the text or value appears 'red' and you cannot exit the dialogue.
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Integrated connection design : Chapter 25 : Ultimate Limit State
Advisory Note for Fastrak Building Designer page 111
• Where the configuration is achievable, Fastrak will carry out the design calculations. However as mentioned earlier, since the design models in the Green Book are based on alignment of supported beams, these calculations are extrapolations of the norm. You should therefore treat the design results of such connections with care. Supported beams either side of a beam or column web do not have to be the same type. For angle cleat one side and end plate the other side, the supporting web is checked in the normal way. However, for fin plate to one side and end plate or angle cleat the other side, there is no rigorous design model for the connection as a whole. Hence, the supporting beam web is checked for each side of the connection independently. No account is taken of any interaction between the two different connection methods on the web resistance. Fastrak reports the relevant check twice once for each connection type (Check 10 in the Green Book). The standard end plate connections in the Green Book are 8 mm and 10 mm thick and are welded to the web only. Full depth end plates up to 12 mm thick with a full profile weld can be treated as simple connections - see page 83 and Check 1 in the Green Book. You can select thicker end plates and this will result in a Warning status for the connection. In all cases the design model and Fastrak do not take account in design of any weld to the beam flange. Where a supported beam is unrestrained and is notched, this can have a significant influence on the effective length of the beam for lateral torsional buckling. This is one situation where this is a direct impact of the connection configuration on the design of the beam. This is covered by Check 7 in the Green Book. In Fastrak you can specify that the beam is unrestrained. However, in the current release the calculations for this check are not implemented. Consequently, this check is always reported with a Warning status if you specify that the beam is unrestrained. You are expected to carry out your own hand calculations. If you have responsibility for the connection design but not for the member design then it is important to ensure that the designer responsible for the beam design is informed of the impact of the connection configuration on his beam design. In the stand alone application, on the Beam Details property page there is an opportunity to enter the 'Length' (span) of the supported beam. In the building model this value is set to zero and cannot be changed. Any value specified is used only to determine whether in the case of fin plates attached to beams deeper than 610 mm that the span/depth ratio is less than 20. This is a requirement to ensure adequate rotation capacity in fin plates attached to deep beams - see Check 1 for fin plates in the Green Book. If the span/depth ratio is greater than 20 the connection will be failed in the design.
Moment Connections The design checks for bolted moment connections are given in the appropriate Green Book. The design models in the Green Book cover I- or H-section beams to the flange of I- or H-section columns with or without haunches. They can be single or double-sided. For such connections the design models are clearly laid out in the Green Book and so no general
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Integrated connection design : Chapter 25 : Ultimate Limit State
explanation is given here. You are reminded that, in principle, the design model in the Green Book is for moment connections with axial load and not axially loaded connections with moment. However, Fastrak covers a somewhat broader spectrum and so additional points of note are given below. The Green Book on moment connections was written in 1995 and so reflected the then current British Standard, BS 5950-1: 1990. However, Fastrak uses the latest British Standard, BS 5950-1: 2000 which incorporated a number of changes that affected connection design. These are, in brief,
• modification to the buckling check for unstiffened column webs in compression, • modification to the configuration of the stiffened column web and to the design force that it must resist,
• changes to the requirements for weld design - both calculation of resistance and the force for which it should be designed. The standard formulae for bearing and buckling of the unstiffened column web assume that the column flange at the point of compression is effectively held against both,
• rotation relative to the web, • lateral movement relative to the other flange. Where this is the case the effective length of the unstiffened or stiffened web is taken as 0.7L. However, this may not always be the case. Fastrak always assumes lateral restraint to the flange but you are provided with a means to select whether rotational restraint is present or not. In the latter case, you can adjust the effective lengths for the top and bottom of the connection (since they may be different) - the default value is 1.0 L. A similar provision is made for the stiffened capacity. The centre of rotation of the connection is assumed always to be the intersection of the centre-line of the compression flange with the outside face of the column flange. This position is assumed irrespective of whether some of the beam/haunch web is taken into account in resisting the compression force at that level. Similarly for a stiffened extended end plate the centre of rotation of the connection is not allowed to be within the stiff extension - no guidance exists on how far within the extension the centre of rotation can be placed and so in the meantime the conservative approach is adopted. Whilst the centre of rotation is kept constant, the centre of compression is allowed to move. This is the case when you request that the beam flange bearing check takes account of some of the beam web in its resistance calculation. In this case, the lever arm for each individual bolt is adjusted to take account of the movement of the centre of compression. When a connection contains a haunch, there is a compressive force at the level of the haunch flange/end plate intersection which can be derived from the equilibrium condition of the potential resistances of the components of the connection at that interface. In the conventional approach contained in the Green Book, all this force is assumed to 'pop out' of the flange at the sharp end of the haunch. A more accurate, more logical and less conservative approach is to use the equilibrium condition at the sharp end of the haunch under the moment at that point. Using this approach the forces at the sharp end can be resolved to give a force acting perpendicular to the web of the beam. The unstiffened or stiffened resistance of the beam web
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Integrated connection design : Chapter 25 : Ultimate Limit State
Advisory Note for Fastrak Building Designer page 113
is checked against this force. You can see from the Combinations page of the connection input dialogue that the moment used to determine the equilibrium condition at this point is termed, "Moment (Sharp End)". Note that whilst within Fastrak Building Designer, the moment, shear and axial force at the connection interface are automatically entered into the design combinations, the moment at the sharp end is not. You will have to interrogate the bending moment diagram for the individual beam members to establish the value of the moment at the sharp end and enter this directly. Also note that the effective length of the unstiffened or stiffened beam web is dependent upon the restraint conditions of the flange in a similar manner to that described above for the column web. There are a number of points associated with stiffener design, as follows,
• In positive bending, for a full depth stiffener to be effective in compression, it must be placed with its centre-line within the stiff bearing length of the haunch flange or beam bottom flange. In negative bending, for a full depth stiffener or cap plate to be effective in compression, it must be placed with its centre-line within the stiff bearing length of the beam top flange.
• It is assumed that for a tension stiffener to be effective in generating the yield lines around the bolts, the gross depth of the stiffener should be at least 75 % of the outstand of the column flange or end plate. Where this is not the case a warning is issued.
• Stiffeners may optionally be defined 'fitted to root'. This means that the corner of the stiffener is shaped to fit tightly into the root radius of the section. This facilitates the weld runs being taken around the corner of the stiffener. In the normal situation the corner of the stiffener is chamfered so that it clears the root and in this case the weld run must stop at this position. The implication for design is that in the normal situation the length of weld used to calculate its resistance is based on the dimension of the stiffener less two leg lengths of weld (see Clause 6.8.2 of BS 5950 1: 2000). However, where the stiffener is 'fitted to root' only one leg length need be deducted.
• A number of options exist for the detailing of cap plates. Normally a cap plate will sit on top of the column and be welded one side (inside) or both sides. The choice between these two weld details is given to you. The full depth stiffener at the base of a connection is usually assumed to be 'fitted'. Suitable details for the cap plate can be made such that a cap plate also can be assumed to be fitted. Hence, you are provided with the choice of assuming that a cap plate is fitted or unfitted. In the former case, in compression, the weld size will be determined whereas in the latter case only nominal welds are required. In both cases in reverse bending the weld is checked for any net tension force. The weld between the haunch flange and the beam flange is required to resist the force derived from the equilibrium condition at the 'sharp end' of the haunch (in the same manner as described earlier for the beam web in compression). Advantage is taken of the increased depth of the section at this point due to the presence of the two flanges (haunch and beam) in determining the force available to enter the haunch flange. This is resolved into the direction of the haunch flange to provide the design force for the weld. Similarly, the force that continues past the sharp end in the beam flange is used to design the haunch web to beam flange weld. For double sided connections, all checks e.g. column flange bending, beam web tension but excluding web panel shear are carried out for each side of the connection separately. Equilibrium for each side is then established but ignoring column web shear capacity. This is
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Integrated connection design : Chapter 25 : Ultimate Limit State
achieved by reducing the number of bolts or their contribution as appropriate until equilibrium is reached. The moment capacity allowing for axial load is then calculated and the net effect of the forces on the connection compared with the web panel shear capacity. This first equilibrium condition is based on bolt row capacities (termed 'potential resistance' in the Green Book). Where the applied moments would not generate these capacities as forces i.e. the applied moment is 11, then Fastrak sets the status of the check to Beyond Scope. Reference restraint axis distance, a – is measured between some reference axis on the restrained member - usually the centroid - to the axis of restraint - usually the centroid of the restraining member. The measurement is shown diagramatically in Figure G.1 of BS 5950-1: 2000. Fastrak does not attempt to determine this value automatically, since such an approach is fraught with difficulty and requires information from you which is only used for this check. Instead, by default, Fastrak uses half the depth of the restrained section, and you can specify a value to be added to, or subtracted from, this at each restraint point. You are responsible for specifying the appropriate values for each restraint position. The default value of 0 mm may be neither correct nor safe.
Compression Resistance – For most structures, all the members resisting axial compression need checking to ensure adequate resistance to buckling about both the major- and minor-axis. Since the axial force can vary throughout the member and the buckling lengths in the two planes do not necessarily coincide, both are checked. Because of the general nature of truss chords in particular, it may not always be safe to assume that the combined buckling check will always govern. Hence the compression resistance check is performed independently from the other strength and buckling checks. Effective lengths – For chords and special internals and sides of any valid section type, the value of effective length factor is entirely at your choice. The default value is 1.0. Different values can apply in the major and minor axis. For internals and sides of I- and H-sections, and hollow sections the default value is 1.0. Different values can apply in the major and minor axis. Footnotes 1. Which could happen since R is based on Z and not S.
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Definition and Design of trusses and Truss Members : Chapter 34 :
For internals and sides of channel, tee section, double angles and single angles, the settings for the effective lengths can be chosen based on the definitions given in Clause 4.7.10.2, 4.7.10.3, 4.7.10.4, 4.7.10.5 and summarized in Table 25 of BS 5950-1: 2000. The default value is not the most conservative. For double angles it is assumed that at least the minimum number of interconnections is provided and that the other relevant parts of Clause 4.7.13.1 of BS 5950-1: 2000 are satisfied. In applying Clause 4.7.10.3, the effective length, Lv, is based on this minimum. Please note that the requirements for slenderness limits (for example l/r £ 180) are no longer included in BS 5950-1: 2000. Consequently Fastrak only carries out such a check against an upper limit of l/r £ 350. If this limit is exceeded then the section will be given a Fail status. You may consider this upper limit too generous and hence, for lightly loaded members, you should ensure that the slenderness ratio is within reasonable bounds to permit handling and erection and to provide a reasonable level of robustness.
Member Buckling Resistance, Clause 4.8.3.3.1 – This check is used for channels and hollow sections. Such sections can be Class 1, 2 or 3 Plastic, Compact or Semi-compact. Two formulae are provided in Clause 4.8.3.3.1, both are checked. The second formula is calculated once for the top flange if the moments are all sagging (positive) or once for the bottom flange if all the moments are hogging (negative) or twice – once for the top flange and once for the bottom flange for double curvature bending. Only one value of the axial force, F, is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero. If this check is invoked as part of an Annex G check, and thus Mb is governed by Annex G, then mLT is taken as 1.0.
Member Buckling Resistance, Clause 4.8.3.3.2 – This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact I- and H-sections with equal flanges. Three formulae are provided in Clause 4.8.3.3.2 (c) to cover the combined effects of major and minor axis moment and axial force. These are used irrespective of whether all three forces/ moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, Clause 4.8.3.3.2 (c) can also be used in such cases by setting the axial force to zero. All three formulae in Clause 4.8.3.3.2 (c) are checked. The second formula is calculated once for the top flange if the moments are all sagging (positive) or once for the bottom flange if all the moments are hogging (negative) or twice – once for the top flange and once for the bottom flange for double curvature bending. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero.
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Definition and Design of trusses and Truss Members : Chapter 34 : Ultimate Limit State – Buckling Advisory Note for Fastrak
Member Buckling Resistance, Clause 4.8.3.3.3 – This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact hollow sections. Four formulae are provided in Clause 4.8.3.3.3 (c) to cover the combined effects of major and minor axis moment and axial force. These are used irrespective of whether all three forces/ moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, Clause 4.8.3.3.3 (c) can also be used in such cases by setting the axial force to zero. The second and third formulae are mutually exclusive – that is the second is used for CHS, SHS and for RHS when the limits contained in Table 15 are not exceeded. On the other hand the third formula is used for those RHS that exceed the limits given in Table 15. Thus only three formulae are checked; the first, second and fourth or the first, third and fourth. Either the second or third formula (as appropriate) is calculated once for the top flange if the moments are all sagging (positive) or once for the bottom flange if all the moments are hogging (negative) or twice – once for the top flange and once for the bottom flange for double curvature bending. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero.
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Chapter 35
Definition and Design of trusses and Truss Members : Chapter 35 :
Serviceability Limit State For internals and sides, no member analysis is carried out (see “Analysis” on page 125 )and hence there are no analysis results for individual members. For chords and special internals and sides, the deflection profile along the member as a whole is established based on the start slope of the member derived from the joint rotation, in the appropriate direction for the member under consideration. In the analysis results for individual chord members, the graphic will show deflection in the local z and local y for both individual loadcases and for design combinations based on unfactored loadcases. These deflections are relative to the ends of the chord member and may be positive or negative. In the design results only the positive local z and positive local y deflections are given for all Dead loads, all Imposed loads and for Total loads in a particular design combination. In all cases these are the sum of the deflections for each appropriate unfactored loadcase i.e. the load factor is taken as 1.0. For chords, it is the in-plane deflections that are of most interest. In the current release the maximum deflection for the positive local z direction is given and compared with the limits specified in the Design Wizard. No comparisons are made for negative local z deflections or for positive or negative local y deflections. This might pose a difficulty when the chord member is rotated e.g. a channel section with flange tips down. In this case the deflections used in design will be those in the local z axis of the chord even though this may lie out-of-plane of the truss as a whole.
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Definition and Design of trusses and Truss Members : Chapter 36 : Member End Fixity and Supports
Chapter 36
Advisory Note for
Member End Fixity and Supports In order to provide a robust design model, the fixity at member ends and the associated supporting structure or supports to ground must be compatible with the type of connection, base and foundation that is to be used. All internals, sides and, special internal and sides have pinned ends and these cannot be edited. The ends of chords cannot be free or continuous. Truss members are not allowed to be cantilevers – you should use a General Beam in this situation. If you require two chords to be continuous then you should specify the adjoining ends as fully fixed. From the design point of view they will be treated as two separate members. A chord connected to an existing member or to a supplementary support can have the following end fixity,
• pinned – this means pinned about the major and minor axes of the section but fixed torsionally,
• pinned about local y – this means fixed about the local z axis and fixed torsionally. This infers a rigid moment connection about the minor axis of the section,
• pinned about local z – this means fixed about the local y axis and fixed torsionally. This infers a rigid moment connection about the major axis of the section,
• fully fixed – this means fixed about local z, about local y and torsionally. This infers a rigid moment connection about both the major and minor axes. Where a chord oversails a column or beam, in a truss with an overhanging eaves detail for example, the connection to the supporting beam or column is assumed to be continuous and cannot be edited. The default for the ends of chord is pinned. Since trusses are generally planar structures then where two chords meet, the node at that position may be unstable out-of-plane of the truss. Hence, in certain situations in order to avoid instability in the analysis of the building, you may experience a validation error. Two common situations where this can occur are at the apex of a pitched roof truss and at the eaves of trusses with an overhanging eaves detail. To overcome this you have two choices,
• you can specify the ends of both chords as fully fixed. This may not be desirable since this can generate moments at that point which, depending upon the detail, the connection may not be able to resist.
• you can include some form of member, an eaves tie for example. This will supply restraint to the node out-of-plane of the truss. However, the restraint forces that this implies should be taken back to some suitable bracing system.
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Definition and Design of trusses and Truss Members : Chapter 36 :
For floor/roof trusses generated by the Truss Wizard, this is overcome using the first method since, for floor trusses especially you are unlikely to have members at that level that are out-of-plane of the truss. See figure showing floor truss example
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Definition and Design of trusses and Truss Members : Chapter 37 : MiscellaneousAdvisory Note for Fastrak Building Designer
Chapter 37
Miscellaneous Roof truss on column cap plate – If you place a parallel chord truss on top of a column and the side member is vertical, then the force in the side member is applied as an axial load to the top of the column (as expected). However, if the side member is not vertical then the load is applied as a reaction to one of the faces of the column (assuming an eccentricity of 100 mm from the face of the column web or flange as appropriate). The side to which the load is applied is determined by towards which face the side member leans. In both cases the vertical force at the end of the chord is applied as a reaction to the appropriate face, again with the standard eccentricity of 100mm from the face of the web or flange. Tubular truss design – Hollow sections are very efficient in resisting both tension and compression – the latter particularly in comparison to other section types. It is all too easy therefore to construct a tubular truss in which the members are very efficient – small size and thin-walled. Later, it may be found difficult to justify the connections between such minimum weight members and stiffening may be required. Since most of the work and hence cost is in the preparation and welding of the member ends then attempting to minimise the section size can be counter productive. Mixture of section types – A wide range of sections is available for both internals and chords. There is no restriction on the mixture of section types. Hence, for example you could use a different section type for each internal (!). More importantly you can select section types for internals and chords that would not be practical to connect. This could mean that there are no design models for such connections and/or they may very difficult (expensive) to fabricate. The choice of appropriate section sizes and types is entirely yours.
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Definition and Design of trusses and Truss Members : Chapter 37 :
Chapter 38 : Introduction
Wind Modeller Documentation page 139
Wind Modeller Engineer’s Handbook
Chapter 38
Introduction This Engineer's Handbook describes Fastrak Wind Modeller. This is design software which allows you to load a Fastrak Building Designer model for wind in accordance with BS6399-2:1997(Ref. 1). The wind loading assessment is performed on the walls and roofs which are defined in your building model. The resulting wind loads are distributed back to the members for structural analysis and design. You can use Fastrak Wind Modeller: • to determine site and effective wind speeds (standard or directional),
• • • •
to determine the zones of wind pressure on walls and roofs, to determine standard values of Cpe for each zone, to determine wind pressures on each zone, to determine wind loads and load cases for your structure.
Unless explicitly stated all calculations in Fastrak Wind Modeller are in accordance with the relevant sections of BS 6399-2:1997 incorporating Amendment 1 and corrigendum No. 1. It is essential that you have a copy of this code with you while assessing wind on any structure. In addition, you may find the following books useful to enhance your understanding of wind loading and the software: • Wind Loading - a practical guide to BS 6399-2(Ref. 2),
• Wind and Loads on buildings: Guide to Evaluating Design Wind Loads to BS6399-2:1997(Ref. 3). Unless explicitly noted otherwise, all clauses, figures and tables referred to in this document are from BS6399-2:1997. Fastrak Wind Modeller is a very powerful tool which has been developed to aid engineers in their assessment of wind loads on buildings. You will find that the determination of wind speeds, zones, pressures… is rigorous but the final wind loads adopted are your responsibility. Your attention is particularly drawn to BS6399-2:1997 – Clause 1.1. For building shapes which are not covered by the Standard you will need to seek specialist advice.
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Chapter 39
Chapter 39 : Scope
Scope Fastrak Wind Modeller works from a Fastrak Building Designer model which has been “clothed” in walls and roofs. Wind is “applied” intelligently to this building envelope within the scope below and the limitations clearly laid out in the next section. In the main, BS6399-2:1997 addresses rectilinear buildings. In order to develop a tool for engineers, we have extended this capability to address non-rectilinear buildings using the standard method. For more information, please refer to reference 2 (section 2.5.3.2.4, page 82 and 2.5.4.3.3 pages 89-90). It is assumed that the wind loads are developed to assess the overall stability of the structure and for member design. The wind loads have not been specifically developed for the design of cladding and fixings. The scope of Fastrak Wind Modeller encompasses: • Enveloping the building with walls and roofs is undertaken in Fastrak Building Designer in the normal manner. There is only limited validation of the envelope defined (for example connected walls must have consistent normal directions). The onus is on you to model the building shape as completely and as accurately as you determine necessary.
• Choice of method: • BS6399-2:1997 - Standard Method - Standard effective wind speeds with standard pressure coefficients,
• BS6399-2:1997 – Hybrid Method - Directional effective wind speeds with standard pressure coefficients.
• Basic Wind Speed or Dynamic pressure is determined using BREVe Active X Control(Ref. 4). • Having defined walls and roofs (defaults are standard wall, flat or monopitch roof depending on the slope), you are able to specify the type in more detail e.g. multi-bay, monopitch / duopitch etc.).
• The main wind parameters, are calculated for you but conservatively, (for example Crosswind Breadth, B, is determined for the enclosing rectangle of the whole building). Wherever possible other attributes are determined conservatively, but you are able to override the values should you need to.
• Given the above, zoning is semi-automatic, (not attempted for roofs with more than 4 sides which are defaulted to single conservative coefficient), with full graphical feedback. Provision is made for you to modify the zoning. For example you can define a manual zone layout, you can override the coefficients… …
• Load decomposition is fully automatic where valid, (walls and roofs need to be fully supported in the direction of span). Please note that the decomposition for walls results in nodal loads rather than UDLs / VDLs. Fastrak Wind Modeller has been developed in order to provide you with a comprehensive design tool which can assess and apply wind loading to your Fastrak Building Designer model in advance of analysis and design.
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Chapter 39 : Scope
Wind Modeller Documentation page 141
Fastrak Wind Modeller is a very flexible tool that can, should you wish, be used purely for wind assessment – by setting up a model of consisting only of walls and roofs (no members) Fastrak Wind Modeller can determine the wind loading on the building envelope.
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Chapter 40
Chapter 40 : Limitations
Limitations Wind loading is complex and its application to general structures is even more so. It is essential that you read and fully appreciate the following limitations in Fastrak Wind Modeller before using the software. It is also essential that you bear the following in mind whenever using Fastrak Wind Modeller. Throughout the development of Fastrak Wind Modeller extensive reference has been made to references 1-3 and we consider it advisable that you are fully familiar with these before using the software. Documented limitations include: • General advice
• Seek specialist advice for building shapes that are not covered by the Standard – see Clause 1.1 of BS6399-2:1997.
• Limitations of release 1.0 • Exposed members are not considered, for example lattices, trusses… … • Open sided buildings are not considered. • Free standing walls and sign boards are not considered. • Parapets and free-standing canopies are not considered. • Barrel-vault roofs are not considered. • Dominant Openings are not explicitly handled – Clause 2.6.2. However, you can use Table 17 to calculate the necessary Cpi values and manually apply them to zone loads.
• Overall Loads • Horizontal Loads – Clause 2.1.3.6 and Table 5a. No account is taken of the reduction on overall loads to allow for non simultaneous actions. This is conservative.
• Frictional Drag - Clauses 2.1.3.8, 2.4.5 and 2.5.10. Should you consider these to be significant, you will need to make a separate assessment of this load.
• Asymmetric loads Clause 2.1.3.7 – no account is made for torsional effects. If you consider these to be significant, you should create additional load cases in which to define these loads.
• Division by Parts rule for buildings, Clause 2.2.3.2, is not considered. This is conservative.
• Fastrak Wind Modeller does not check that the horizontal component of the factored wind load is greater than the factored dead loads – BS5950-1:2000 Clause 2.4.2.3.
• General items • Dynamic Augmentation Factor, (Cr), including check for applicability of Clause 1.6.1 is assumed to be undertaken by you outside the software.
• Fastrak Wind Modeller ignores the difference between the loaded area of walls and roofs defined at the centre-line rather than the sheeting dimension.
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Chapter 40 : Limitations
Wind Modeller Documentation page 143
• Asymmetric Loads for Standard Method – Clause 2.1.3.7. One interpretation of this clause is that asymmetry is handled by using both the positive and negative coefficients in the tables for roofs – reference 3, Chapter 8, page 53. Also SCI AD 273 advises engineers that they do not need to consider the downward pressures for ordinary portals with slopes less than 20° for SLS only. In Fastrak Wind Modeller, no automatic reduction is made for beneficial load. When you edit the Zone Load Data for a wind direction, having generated wind load cases, there is an option to allow for beneficial loads.
• Wind loading on walls - automatic zoning applies to all walls subject to the other limitations described below:
• Walls that are more than 15° from the vertical are outside the scope – Clause 2.4.1.5. • The inset storey clause 2.4.4.2 b) is not implemented. You can edit the zones manually according to your engineering judgement to include zone E if you consider this necessary.
• Walls of internal wells are not automatically identified – Clause 2.4.3.2a. You can manually edit the zones to apply the roof coefficient to the walls.
• Wind loading on roofs • Automatic zoning only applies to all triangular and quadrilateral roof items that are not concave, i.e. all of the internal angles < 180°.
• The inset storey clauses 2.5.1.7 a) and b) are not implemented. In clause a) the software sets Hr and H equal conservatively. You are obviously able to edit the zones manually according to your engineering judgement to include the further zones indicated in Figure 18 should you consider this necessary.
• It should be noted that in Table 8 for curved and mansard eaves, the zones start from edge of horizontal roof and not from the edge of the feature.
• Special care should be taken for winds blowing on duopitch with slopes that differ by more than 5°. If the wind is blowing on the steeper slope (that is that the less steep slope is downwind of ridge), the downwind slope should be set to be a flat roof with mansard at eaves for this wind direction.
• Mansard and multipitch roofs are not handled automatically. However, you can manually apply the relevant roof type, apex type and bay position parameters for each appropriate wind direction to match the requirements of Figure 22 and Figure 23.
• Roof overhangs are not explicitly handled. To model an overhang, you should define a roof object that has a partial overhang. You can then define Cpi values manually to either have the same coefficient as the adjacent wall, (Clause 2.5.8.2 Small Overhangs), or as an open sided building (Clause 2.6.3).
• Load decomposition and additional wind loads • All wall loads are decomposed into nodal loads on columns. • There may be situations when you perceive a need to manually define loads that can not be determined automatically. You can do this by defining additional wind load cases to contain these loads and then include these with the relevant system generated loads in design combinations in the normal way.
• Load decomposition onto shear walls
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Chapter 40 : Limitations
• As stated above, all wall loads are decomposed into nodal loads on columns. In a building that contains a shear wall, the analytical model of the shear wall consists partly of a ‘mid-pier’ vertical column at the centre of the shear wall, hence wind wall loads will be decomposed onto the mid-pier column.
• This decomposition on to the mid pier column could in certain cases result in an averaging of the wind pressure profile that removes the localised pressure increase at the corners of the building. The example below illustrates the problem and provides an alternative model as a workaround: — Physical model of shear wall — Although not shown here, wind walls are also added to all four faces of the building.
Wind zones from BS6399 Part 3 — The zones are generated on the wind wall faces.
Resulting stepped wind pressure on wind wall faces — Highest pressure occurs in Zone A, lesser pressure exists in other zones
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Chapter 40 : Limitations
Wind Modeller Documentation page 145
Wind pressure decomposed on to the shear wall — Stepped pressure gradient is averaged over the face of the shear wall and then decomposed on to the ‘mid-pier’ column at it’s centre. Hence only a single point load is applied at each floor level.
Alternative Model — Define two adjacent shear walls, making the first as wide as wind zone A. This results in a more accurate decomposition of the wind load, reflecting the stepped profile of the wind pressure.
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Chapter 41
Chapter 41 : Applying Walls and Roofs
Applying Walls and Roofs All the calculations for wind depend on the geometry and interconnectivity of the walls and roofs that envelope the building. You must therefore define the model, together with its walls and roofs before you can start to calculate the wind loading using Fastrak Wind Modeller. Whilst defining the model’s walls and roofs, it is essential that you define the largest planar surfaces possible for these if you want to get the best results from Fastrak Wind Modeller. If you ignore this advice, then the calculation of the reference height can be unconservative.
41.1
Applying Walls A single wall is determined to be a single planar surface. The outward face is vitally important for determining the wind direction relative to the wall, that is windward or leeward. It is recommended that you use Fastrak Wind Modeller’s Show/Alter State feature to check the face orientation quickly and correct any mistakes by clicking once on an item to reverse the direction. However, whenever automatic zoning is carried out, for example at the end of the Wind Wizard, the connected walls are checked to ensure that the normal directions are not inconsistent.
41.2
Applying Roofs A single roof is determined to be a single planar surface. The orientation of a roof is automatically determined when placed based upon the slope vector – the line of maximum roof slope.
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Chapter 42 : Running the Wind Wizard
Chapter 42
Wind Modeller Documentation page 147
Running the Wind Wizard Once the walls and roofs are in place, you use the Wind Wizard to define sufficient site information to calculate the effective wind speeds and dynamic pressures for the required wind directions and heights around the building, (that is the Reference Height (Hr) for each wall or roof). The BREVe ActiveX control is used for this purpose, © 2000 Building Research Establishment and Nicholas J Cook. BREVe is an aid to the use of BS6399-2:1997. BREVe automates the wind speed parts of the Standard and Directional methods of BS6399-2. The information held within BREVe is based upon the Ordnance Survey data of Great Britain. CSC take no responsibility for the accuracy of BREVe. It should be noted that BS6399-2:1997 recommends that the Standard Method requires assessment of orthogonal load cases for wind directions normal to the faces of the building. The wizard permits you to create wind load for any wind direction and thus it is up to you to create those loads for the directions most appropriate to your structure.
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Chapter 43
Chapter 43 : Creating Wind Zones on the Building
Creating Wind Zones on the Building At the end of the Effective Wind Speeds wizard, the system creates default zones for all the walls and roof items for each of the defined wind directions. Whenever this process occurs, any error and/or warning messages are written to the Output window. Where appropriate, double clicking on a message highlights the item, (although if the problem is direction specific, then you may have to switch to the relevant view).
43.1
Basic Geometry The basic building geometry is assessed as follows: • Reference Height (Hr) – is taken as the difference between highest point on wall or roof and ground level.
• Wall height (H) – is taken as the difference between highest and lowest points on the wall.
H
Hr
Ground Level
• Roof height (H) – is taken as the difference between highest point on wall or roof and ground level. This definition does not handle the upper roof of inset storey but is conservative and only affects the scaling dimension b – see Clause 2.5.1.7.
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Chapter 43 : Creating Wind Zones on the Building
Wind Modeller Documentation page 149
• The Building Breadth, B is calculated from the smallest enclosing rectangle around the whole building (considered over all roof and walls only) for the given direction. You can override the calculated value in case the Fastrak Building Designer model does not include the whole building.
B
Wind Direction
43.2
Smallest enclosing rectangle
Wall Zones
43.2.1 Wall Type We assess each wall to determine if it is a windward, leeward or side wall. We classify the type of wall dependent on θ:
Wall Normal
Wind Direction
q
j
• θ ≤ 45° – Windward, • θ ≥ 135° – Leeward, • Other walls are classed as Side. 43.2.2 Windward Walls Windward walls have a single zone and we use Table 5 with interpolation for D/H.
43.2.3 Leeward Walls Leeward walls have a single zone and we use Table 5.
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Chapter 43 : Creating Wind Zones on the Building
43.2.4 Side Walls Side walls are assessed for recesses (narrow or wide), irregular flushed faces, downwind re-entrant corners. In all cases, side walls have the relevant number of zones. Fastrak Wind Modeller uses Table 5.
43.3
Roof Zones Fastrak Wind Modeller automatically generates roof zones, where possible, for each wind direction. In essence each roof item is assessed in its own right based on its properties. The interconnectivity of touching roof items is not generally considered.
43.3.1 Direction Internally the roof slope vector (line of maximum slope) is determined from the normal vector, with its direction always giving a positive slope angle, i.e. the roof slope vector must always point up the slope. We calculate the angle between the wind direction and projection of roof slope vector onto horizontal plane (θ in range -180° to +180°).
Roof Slope Vector
Wind Direction
q
j
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Chapter 43 : Creating Wind Zones on the Building
Wind Modeller Documentation page 151
43.3.2 Automatic Zoning Automatic zoning normally only applies to all triangular roof items and quadrilateral roof items that are not concave, that is that all of the internal angles < 180°. However, additionally, it only applies to Hip Gable roofs if they are triangular, and Hip Main roofs if they are quadrilateral. Further, Downwind Slope Hip Gables must not have 2 upwind corners.
Triangular
Convex Quadrilateral
Wind Direction Downwind Hip Gable 1 upwind
Automatic Zoning Applies Concave Quadrilateral
5 sides
Wind Direction Downwind Hip Gable 2 upwind corners
No Automatic Zoning
43.3.2.1 Dimensions All zone dimensions are specified in plan. 43.3.2.2 Flat Roofs See Clause 2.5.1, Figure 16 and Table 8. 43.3.2.3 Monopitch Roofs See Clause 2.5.2.3, Figure 19 and Table 9. 43.3.2.4 Duopitch Roofs See Clause 2.5.2.4, Figure 20 and Table 10. 43.3.2.5 Hip Gable See Clause 2.5.3, Figure 21 and Table 11. 43.3.2.6 Hip Main See Clause 2.5.3, Figure 21 and Table 11. 43.3.2.7 Multi-bay Roofs We allow you to interpret Clause 2.5.5 and Figure 23 as you think appropriate and manually define the roof types and sub-types accordingly. You also have the ability to manually set the multi-bay position for each roof item for each wind direction: • Not Multi-Bay - for this wind direction (conservative default),
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Chapter 43 : Creating Wind Zones on the Building
• Upwind Bay – first bay of many for this wind direction, • Second Bay – for this wind direction, • Third or more Bay – for this wind direction. Where the reduction applies, the values of all coefficients are reduced according to Table 12.
43.3.3 Non-Automatic Zoning Where automatic zoning does not apply, the system creates a single zone covering the entire roof as follows: • Flat – B,
• • • •
43.4
Monopitch – B, Duopitch – B for upwind, F for downwind, B for side, Hip Gable – B for upwind, G for downwind, I for side, Hip Main – B for upwind, F for downwind, I for side.
User Modification of Zones Initially the expectation is that only “Expert” users may want to make changes to the actual zone layouts or other data. Whenever you edit the zones for a wall or roof item, please note that the zone layout will not be updated to reflect changes elsewhere in the model, you must make any necessary changes yourself.
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Chapter 44 : Load Decomposition
Chapter 44 44.1
Wind Modeller Documentation page 153
Load Decomposition
Roofs The direction of the one way decomposition of the wind zone loads to roof members is as specified by the span direction of the roof. All types of elements (except bracing and cold rolled members) are considered during the load decomposition.
44.2
Walls This is a two stage process. The second stage is necessary to avoid lateral loads on simple beams and distributed loads on simple columns, but is done for all element types. • The initial decomposition of wind zone loads to wall members is similar to the roof decomposition. Again all types of elements are considered except bracing and cold rolled members.
• Full/partial UDLs and VDLs on elements (lengths of beams/columns between nodes) are distributed back to nodes as if the elements were simply supported at either end.
• The final nodal load is the sum of all incoming element loads.
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Wind Modeller Documentation page 154
Chapter 45
Chapter 45 : References
References 1. British Standards Institution (2000). Loading for Buildings – Part 2: Code of practice for wind loads. BS6399-2:1997 Incorporating Amendment No. 1. 2. Cook, N.J. (1999). Wind Loading - a practical guide to BS 6399-2 Wind Loads on buildings. Thomas Telford, London. ISBN: 0 7277 2755 9. 3. Bailey, C.G. (2003). Guide to Evaluating Design Wind Loads to BS6399-2:1997.SCI Publication P286. 4. BREVe software package version 2.0.4.2. Written by Cook N.J. copyright Building Research Establishment and N.J.Cook.
Thursday 26 April 2007 – 10:18
CSC Fastrak ™
Structural steelwork analysis and design
.fastrak5950.com
HANDBOOK SIMPLE BEAM
Simple Beam Documentation page 156
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
Simple Beam Engineer’s Handbook page 157
Simple Beam Engineer’s Handbook
Chapter 1
Introduction and application This is design software which allows you to analyse and design a structural steel beam or cantilever which may have incoming beams providing restraint, and which may or may not be continuously restrained over any length between restraints You can use Simple Beam: • to determine those sections which can withstand the applied loading,
• to check a beam of known size to determine whether it is able to carry the loading. Unless explicitly stated all calculations in Simple Beam are in accordance with the relevant sections of BS 5950-1:2000. You may find the handbook and commentary to the Code of Practice published by the Steel Construction Institute useful.
Practical applications Simple Beam can be used both to design and check simple beams. You might find the following procedures useful.
Designing a beam In the typical procedure below items in brackets [] are optional.
Step
Icon
Instructions
1
Launch Simple Beam,
2
Create a new project giving the project name [and other project details],
3
Choose the type of beam as either a Simple Beam or a Cantilever Beam [and give the beam reference details],
4
Set Simple Beam into design beam mode,
5
Define the properties for the beam: • grade; • span.
6
Give the details of the beam restraints.
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Simple Beam Engineer’s Handbook page 158
Step
Icon
Chapter 1 : Introduction and application
Instructions
7
Define the loadcases that apply to the simple beam.
8
Incorporate the loadcases into a series of design combinations,
9
[Make any Design Wizard settings that you want to use to control the design.]
10
Perform the design
11
From the list of suitable sections preview the results for the more desirable sections and then choose the one that you would like to use,
12
Add in any web openings that you need to allow access for services etc.
13
Check the beam with the web openings. [Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]
14
Specify the content of the report [and print it].
15
Save the project to disk.
Checking a beam In the typical procedure below items in brackets [] are optional.
Step
Icon
Instructions
1
Launch Simple Beam,
2
Create a new project giving the project name [and other project details],
3
Choose the type of beam as either a Simple Beam or a Cantilever Beam [and give the beam reference details],
4
Set Simple Beam into check beam mode,
5
Define the properties for the beam: • section size, • grade, • span,
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
Step
Simple Beam Engineer’s Handbook page 159
Icon
Instructions
6
Add in any web openings that you need to allow access for services etc.
7
Give the details of the beam restraints.
8
Define the loadcases that apply to the simple beam.
9
Incorporate the loadcases into a series of design combinations,
10
[Make any Design Wizard settings that you want to use to control the design.]
11
Perform the check, (including any web openings),
12
[Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]
13
Specify the content of the report [and print it].
14
Save the project to disk.
Worked Example If you want to work through this example you will find the file Engineer’s Example in the \documents and settings\All Users\Application Data\CSC\Fastrak\Examples folder. You can open and use this file, but you can not save
it away unless you change its name, this is done to protect the original. Let’s take a simple example of a 9 m span spine beam with 6 m span secondary beams at third points.
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Simple Beam Engineer’s Handbook page 160
Chapter 1 : Introduction and application
The floor loading is:
Condition
Value
giving point load at 3 m and 6 m of
Dry Slab
2.0 kN/m2
36kN
Services
1.0 kN/m2
18kN
Live load
5.0 kN/m2
90kN
Design Pass 1 If you run a design you will find that Simple Beam shows a dialog of acceptable sections. If no one has tailored the sections that Simple Beam investigates, then the list will appear as below.
If you move down the list of Available files, you will see all the Section Designations that can carry the applied loading. These are only the ones that pass the design, Simple Beam has tried all the sections in each of the Available files, to determine the acceptable ones. You may have noticed the different section designations in the progress bar as the design ran. However checking all these sections comes at a price, the more sections there are to investigate, the longer the design takes. Simple Beam allows you to choose just the sections you want to include for the design through its Design Wizard.
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
Simple Beam Engineer’s Handbook page 161
Design Pass 2 Remove the tick against all the Available files whose section types you don’t want to investigate, and Simple Beam won’t look at any of these sections during the design process. If you remove the tick against all the Available files other than UBBeamOrder.Eur, and then re-perform the design you will find a significant increase in speed as Simple Beam only investigates the universal beams.
Furthermore Simple Beam investigates the sections in the order that they appear in the Section Designation list. If you scroll down many of the lists, you will find that there is a point at which larger sections give way to smaller ones again.
We have ordered the Section Designation list based on our many years experience of the industry, the sections at the top of the list are the ones we know you prefer to use, whilst those at the bottom are those which you use less frequently if at all. By default all the Section Designations are ticked, but you might want to remove the ticks against some or all of the non-preferred sections. Again this will speed the design process. You may also have other requirements specific to your own company, for instance you may never want to use sections with flanges less than 150 mm wide for erection purposes. If you remove the tick against these section sizes, then Simple Beam will never include them when it is performing a design. Thus you are controlling the design, making Simple Beam look at just the section designations you are likely to accept, and in the process speeding up the design itself.
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Simple Beam Engineer’s Handbook page 162
Chapter 1 : Introduction and application
Design Pass 3 With the tick removed against all the non-preferred sections, and all sections with flanges less than 150 wide, Simple Beam only has to check around 20 sections and the design is instantaneous.
Simple Beam maintains the Sections for Study settings that you make, until you choose to change them again. It is therefore worthwhile taking the time to tailor the list so that Simple Beam picks sections of which you are likely to approve during its designs.
Thursday 26 April 2007 – 10:18
Chapter 2 : Scope
Chapter 2
Simple Beam Engineer’s Handbook page 163
Scope This section summarises the scope of your Simple Beam application. You will find information on items such as: • basic beam details,
• • • • •
available steel sections, web opening checks. types of load that may be used, Ultimate Limit State design checks, Serviceability Limit State checks,
Simple Beam has been developed in order to provide you with a comprehensive design tool which can determine the sizes of member which can carry the forces and moments resulting from the applied loading. Alternatively you may give the size of a beam and Simple Beam will then determine whether it is able to carry the previously mentioned forces and moments and satisfy the deflection requirements. Additionally you can also use Simple Beam to check any web openings that are necessary, stiffening them where needful to attain an acceptable result.
Scope of simple beam The following sections cover each of the aspects of simple beam design, and indicate the power of the Simple Beam application.
Beam You can specify and design any simply supported simple beam with a span up to 100 metres.
Steel sections Simple Beam can handle design for an international range of steel I-sections for many different countries.
Web openings If you need to provide access for services, etc., then you can add openings to a designed beam and Simple Beam can then check these for you. You can define rectangular or circular openings and these can be stiffened on one, or on both sides. The checks that are performed are in accordance with the guidelines and design process given in the booklet Design for openings in the webs of simple beams.
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Simple Beam Engineer’s Handbook page 164
Chapter 2 : Scope
We advise you to comply with the following positional recommendations for web openings: • Web openings are designed using the bending moment and vertical shear values at the side of the opening where the moment is lower,
• Openings should preferably be positioned at the mid-height of the section. If not, the depth of the upper and lower sections of web should differ by not more than a factor of two,
• Openings should not be located closer to the support than two times the beam depth or 10% of the span whichever is the greater,
• The best location for any opening is between 1/5 and 1/3 of the span from a support in uniformly loaded beams, or in lower shear zone of beams subject to point loads,
• • • •
Openings should be not less than the beam depth, D, apart, Unstiffened openings should not generally be deeper than 0.6D or longer than 1.5D, Stiffened openings should not generally be deeper than 0.7D or longer than 2D, Point loads should not be applied at less than D from the side of the adjacent opening.
You cannot currently automatically design sections with web openings, you must perform the design first to get a section size, and then add and check the openings. This gives you complete control of the design process, since you can add appropriate and cost effective levels of stiffening if required, or can choose a different beam with a stronger web in order to reduce or remove any stiffening requirement. Note
Adjustment to deflections. The calculated deflection at both construction stage and simple stage are adjusted to allow for shear deformation in the web openings. This is carried our following the principles in Ref. 2 and AD 068.
Note
Dimensional checks. The program does not check that openings are positioned in the best position (between 1/5 and 1/3 length for udl’s and in a low shear zone for point loads). This is because for anything other than simple loading the best position becomes a question of engineering judgment.
Restraint conditions If you need to check the lateral torsional buckling of the beam you can: • define the degree of fixity that the end connections are able to provide and hence an effective length associated with the support,
• position additional restraints at any point along the beam (Simple Beam automatically uses 1.0L and 1.2L as the factors for Normal and Destabilizing loads), Help
For a definition of Destabilizing Loads see BS 5950-1:2000 clause 4.3.4.
• Simple Beam automatically takes the average of the effective length factors for differing supports, or between those for the support and the adjacent sub-beam.
• alternatively you can specify the factors that you want to use for the lengths between restraints, or you can enter the effective length of the sub-beam directly by entering a value (in m).
• specify that any length (or lengths) of the beam should be taken as being fully restrained against lateral torsional buckling, independent of the restraint conditions for the adjacent length(s).
Thursday 26 April 2007 – 10:18
Chapter 2 : Scope
Simple Beam Engineer’s Handbook page 165
Applied loading You can specify a wide range of applied loading for the simple condition: • uniform distributed loads (over the whole or part of the beam),
• point loads, • varying distributed loads (over the whole or part of the beam), • trapezoidal loads. Design checks When you use Simple Beam to design or check a beam the following conditions are examined in accordance with BS 5950-1:2000: • section classification (Clause 3.5.2),
• shear capacity (Clause 4.2.3), • moment capacity: • (Clause 4.2.5.2 for the low shear condition • Clause 4.2.5.3 for the high shear condition), • lateral torsional buckling resistance (Clause 4.3.6) This condition is only checked in those cases where the profile decking does not provide adequate restraint to the beam,
Note
• total load deflection check.
Error messages and limitations As you are defining the data for your beam Simple Beam continually checks to ensure that the data is valid. If a particular value is not valid, then it will be shown using a colour of your choice in the dialog. If a value causes a potential problem, then a different colour will be used in the dialog. If you allow the cursor to rest over the error or warning field you will see a tip telling you the acceptable range of input. Until all the information within the dialog is valid (but not free of warnings) you will not be able to save the dialog since OK will be dimmed. Although checking in this way prevents you from defining an invalid beam there are some cases where particular errors occur that cannot be trapped in this way (for instance where an error occurs due to inconsistencies that have arisen between information covered on different dialogs). In these cases when you attempt to perform a design you will see an error message indicating that data is not suitable for the design to proceed. Each message is self-explanatory. You should take a careful note of the error message and then change the beam data to correct the problem. If there are other problems with the design, then you will see a series of warning messages in the results viewer. You should take note of any such warnings and take the action that you deem appropriate. Engineering tips are also available in the results viewer which may give you useful information about the steps required to overcome a particular problem.
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Simple Beam Engineer’s Handbook page 166
Thursday 26 April 2007 – 10:18
Chapter 2 : Scope
Note
Adjustment to deflections. The calculated deflections are adjusted to allow for shear deformation in the web openings. This is carried our following the principles in Ref. 5 and AD 068.
Note
Dimensional checks. The program does not check that openings are positioned in the best position (between 1/5 and 1/3 length for udl’s and in a low shear zone for point loads). This is because for anything other than simple loading the best position becomes a question of engineering judgment.
Note
Asymmetric Slimflor beams (ASB) For all section types flange classification is only performed for the top flange, because for a simple beam this will be the flange in compression. However, in the case of a cantilever beam the bottom flange goes into compression. Hence for a cantilever beam, for the flange classification to be valid the section must be symmetric about the major axis. As a consequence ASB sections must NOT be specified for cantilever beams.
Chapter 3 : Theory and Assumptions
Chapter 3
Simple Beam Engineer’s Handbook page 167
Theory and Assumptions This section describes the theory used in the development of Simple Beam and the major assumptions that have been made, particularly with respect to interpretation of BS 5950-1:2000.
Analysis method Simple Beam uses a simple analysis of a statically determinate beam to determine the forces and moments to be resisted by the beam.
Design method The design methods employed to determine the adequacy of the section for each condition are those consistent with BS 5950-1:2000 unless specifically noted otherwise.
Section classification Cross-section classification is determined using Table 11 and Clause 3.5. The classification of the section must be Plastic (Class 1), Compact (Class 2) or Semi-compact (Class 3). Sections which are classified as Slender (Class 4) are beyond the scope of Simple Beam.
Member strength checks Member strength checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, and at each side of a web opening as well as all other points of interest along the beam. Shear capacity — is determined in accordance with Clause 4.2.3. Where the applied shear force exceeds 60% of the capacity of the section, the high shear condition applies to the bending moment capacity checks (see below). Bending moment capacity — is calculated to Clause 4.2.5.2 (low shear at point) or Clause 4.2.5.3 (high shear at point) for plastic, compact and semi-compact sections.
Lateral torsional buckling checks BS 5950-1:2000 states that lateral torsional buckling checks are required when any length is not continuously restrained. Simple Beam allows you to switch off these checks by specifying that the entire length between the supports is continuously restrained against lateral torsional buckling. If you use this option you must be able to provide justification that the beam is adequately restrained against lateral torsional buckling.
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Simple Beam Engineer’s Handbook page 168
Chapter 3 : Theory and Assumptions
When the checks are required you can position restraints at any point within the length of the main beam and can set the effective length of each sub-beam (the portion of the beam between one restraint and the next) either by giving factors to apply to the physical length of the beam, or by entering the effective length that you want to use. Each sub-beam which is not defined as being continuously restrained is checked in accordance with clause 4.3.6 and Annex B of BS 5950-1:2000.
Deflection checks The maximum deflections for dead and imposed loads are calculated separately, as is the total deflection for dead and imposed loads acting together. The loads are taken as acting on the steel beam alone. Deflection limits can be specified as a fraction of the span, or as an absolute limit, (or both).
Thursday 26 April 2007 – 10:18
Chapter 4 : References and further information
Chapter 4
Simple Beam Engineer’s Handbook page 169
References and further information
References 1. British Standards Institution. BS 5950 : Structural use of steelwork in building; Part 1. Code of practice for design in simple and continuous construction: hot rolled sections. BSI 2000. 2. The Steel Construction Institute. Design for openings in the webs of composite beams. SCI 1987.
Further information – Westok Beams For further information or technical literature on Westok Beams please contact Westok Technical Support and Design Service. Westok Structural Services Ltd. Horbury Junction Industrial Estate Horbury Junction Wakefield WF4 5ER Tel: 44-1924 264 121 Fax: 44-1924 280 030 email:
[email protected]. You can also view this information while running the program by choosing Help/About Westok Structural Services Ltd… which shows the About Westok Structural Services Ltd. dialog.
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Simple Beam Engineer’s Handbook page 170
Chapter 4 : References and further information
You can click on the email link on this dialog to create a new email message to Westok.
Thursday 26 April 2007 – 10:18
CSC Fastrak ™
Structural steelwork analysis and design
.fastrak5950.com
HANDBOOK COMPOSITE BEAM
Composite Beam Documentation page 172
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction
Composite Beam Engineer’s Handbook page 173
Composite Beam Engineer’s Handbook
Chapter 1
Introduction This Engineer’s Handbook describes your Composite Beam application. This is design software which allows you to analyse and design a structural steel beam acting compositely with a concrete slab. This slab may be created either by using profile steel decking or by using Bison precast concrete slabs. You can use Composite Beam: • to determine those sections which can withstand the applied loading when acting alone at the construction stage and when acting compositely with the concrete slab subsequently (composite stage),
• to check a beam of known size to determine whether it is able to carry the construction stage and composite stage loading. Unless explicitly stated all calculations in Composite Beam are in accordance with the relevant sections of BS 5950 : Part 3 : Section 3.1 : 1990(Ref. 1). You may find the handbook and commentary to the Code of Practice published by the Steel Construction Institute (Ref. 3 and 4) useful.
Practical applications Composite Beam can be used both to design (design beam mode) and check (check beam mode) composite beams. You might find the following procedures useful.
Designing a beam In the typical procedure below items in brackets [] are optional.
Step
Icon
Instructions
1
Launch Composite Beam,
2
Create a new project giving the project name [and other project details],
3
Choose the edge condition for the beam [and give the beam reference details],
4
Set Composite Beam into design beam mode,
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Composite Beam Engineer’s Handbook page 174
Step
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction
Icon
Instructions (Continued)
5
Define the properties for the beam: • grade, • span,
6
Give the details of the floor construction: • floor construction type: • profiled metal decking • precast concrete slabs • profiled metal deck details or Bison precast concrete slab details (including slab information) • slab details (for profiled metal deck only) • reinforcement details (transverse and any other reinforcement present in the slab for profiled metal deck, transverse to beam only for Bison precast concrete slabs), • Define the shear connector type • shear studs • Hilti connectors • Define the shear connector layout, • construction stage restraint details (for profiled metal deck spanning parallel to or at less than 45° to direction of span of beam and for precast concrete slabs at your request),
7
Define the loadcases that apply to the beam including self-weight, construction stage loadcases and composite stage loadcases.
8
Incorporate the loadcases into a series of design combinations,
9
[Make any Design Wizard settings that you want to use to control the design.]
10
Perform the design
11
From the list of suitable sections preview the results for the more desirable sections and then choose the one that you would like to use,
12
Add in any web openings that you need to allow access for services etc.
13
Check the beam with the web openings. [Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]
Chapter 1 : Introduction
Composite Beam Engineer’s Handbook page 175
Step
Icon
Instructions (Continued)
14
Specify the content of the report [and print it].
15
Save the project to disk.
Checking a beam In the typical procedure below items in brackets [] are optional.
Step
Icon
Instructions
1
Launch Composite Beam,
2
Create a new project giving the project name [and other project details],
3
Choose the edge condition for the beam [and give the beam reference details],
4
Set Composite Beam into check beam mode,
5
Define the properties for the beam: • section size, • grade, • span,
6
Add in any web openings that you need to allow access for services etc.
E:\CSC Doc\Composite Beam\Engineers handbook\Introduction\Composite Beam Engineers Handbook.fm
Composite Beam Engineer’s Handbook page 176
Step
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction
Icon
Instructions (Continued)
7
Give the details of the floor construction: • floor construction type: • profiled metal decking • precast concrete slabs • profiled metal deck details or Bison precast concrete slab details (including slab information) • slab details (for profiled metal deck only) • reinforcement details (transverse and any other reinforcement present in the slab for profiled metal deck, transverse to beam only for Bison precast concrete slabs), • Define the shear connector type • shear studs • Hilti connectors • Define the shear connector layout, • construction stage restraint details (for profiled metal deck spanning parallel to or at less than 45° to direction of span of beam and for precast concrete slabs at your request),
8
Define the loadcases that apply to the beam including self-weight, construction stage loadcases and composite stage loadcases.
9
Incorporate the loadcases into a series of design combinations,
10
[Make any Design Wizard settings that you want to use to control the design.]
11
Perform the check, (including any web openings),
12
[Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]
13
Specify the content of the report [and print it].
14
Save the project to disk.
Chapter 2 : Scope
Chapter 2
Composite Beam Engineer’s Handbook page 177
Scope This section summarises the scope of your Composite Beam application. You will find information on items such as: • basic beam details,
• available steel sections, • web opening(Ref. 5) checks. • composite construction type: • profiled steel decking, • Bison precast concrete slab, • concrete slab details, • method of shear connection: • shear studs, • Hilti™ connectors, • types of reinforcement, • Dramix Fibres • Strux Fibres • types of load that may be used, • Construction Stage design checks, • Ultimate Limit State design checks, • Serviceability Limit State checks, Composite Beam has been developed in order to provide you with a comprehensive design tool which can determine the sizes of member which: • acting alone are able to carry the forces and moments resulting from the Construction Stage,
• acting compositely with the slab using profile steel decking or with precast concrete slabs (with full or partial interaction) are able to carry the forces and moments at the Ultimate Limit State,
• acting compositely with the slab using profile steel decking or with precast concrete slabs (with full or partial interaction) are able to provide acceptable deflections, service stresses and natural frequency at the Serviceability Limit State. Alternatively you may give the size of a beam and Composite Beam will then determine whether it is able to carry the previously mentioned forces and moments and satisfy the Serviceability Limit State. Additionally you can also use Composite Beam to check any web openings that are necessary, stiffening them where needed to attain an acceptable result.
Scope of composite beam The following sections cover each of the aspects of composite beam design, and illustrate the power of the Composite Beam application.
E:\CSC Doc\Composite Beam\Engineers handbook\Introduction\Composite Beam Engineers Handbook.fm
Composite Beam Engineer’s Handbook page 178
Chapter 2 : Scope
Beam You can specify and design any simply supported composite beam with the design span taken as that defined in BS 5950-1:2000.
Westok sections You can check Westok sections in Composite Beam. The top and bottom part of the section can be formed from different beam section sizes, although they must be of the same grade. The following points are worthy of note: • Since there are so many combinations of top and bottom section you cannot automatically design composite beams with Westok sections. Help
Westok provide a design service to assist you with designs. For further information see “Westok Technical Support and Design Service” on page 179.
• If the resistance of the web post is insufficient this will yield a Fail status. To overcome this your best option is to manipulate the cell data, or to make a more effective choice of the parent sections. Caution
Although you can use stiffeners to overcome this we would encourage you not to use this option. Please refer to Westok literature which shows these requirements diagramatically. If you do choose to use stiffeners then you should be aware that there is no design, sizing or graphical representation of these.
• If you define a point load within 0.45 x Ro of the centre-line of a cell, then the maximum shear at either side of the point load is checked against the shear resistance of the net section. If the shear resistance is inadequate you will be told that a filler plate is required. The filler plate is not checked since the adjacent web post resists the same (or similar) shear and the checks on this will confirm the adequacy of the full web. Note
The program does not check the shear resistance of the minimum upper tee section alone. If you are concerned about this you will need to produce additional hand calculations. Automated calculations for this condition could easily be produced in TEDDS.
• In this release of the program you cannot define Westok sections which have different steel grades for the top and bottom section.
• When Composite Beam is calculating the properties of Westok sections the root radii are ignored.
• The size and position of the cells that you define should comply with the following limits to ensure that the design model is valid and for practical reasons (see Westok literature). The limits are generally:
• 1.08 ≤ S/Do ≤ 1.5 and 1.25 ≤ D/Do ≤ 1.75. • 1.5 < S/Do ≤ 1.8 is also allowed following the findings of the latest research program.
Thursday 26 April 2007 – 10:18
Chapter 2 : Scope
Composite Beam Engineer’s Handbook page 179
• If you do not wish to use the Autospace facility, or accept the Composite Beam defaults
you should ensure that your cells comply with the limit 0.7 ≤ Do /h ≤ 1.3. You should therefore set the value Do to be the depth of the shallower source beam, hmin, at a pitch of 1.5 x Do.
• The design model assumes that the beam spans from centre-line to centre-line (BS 5950-1:2000). However in order to check the web posts or cells Composite Beam needs to know how this relates to the physical length of the beam. You need to define this by giving offsets from the centre-line at each end of the beam to its physical end. These offsets are shown in the Analysis Results window. All shears, moments etc. can thus be taken directly from the analysis results for the span as a whole.
Design span c/c Reaction End shear for physical beam
Shear force diagram Note: the program graphics show the complete beam (design span) with the offset shown as solid.
Beam on span Beam setback
Physical beam Note
When you apply loading you need to position this based on the Design span and not on the Physical beam.
Westok Technical Support and Design Service Cellular beams are not manufactured to standard sizes in the same way as castellated beams. All sections are manufactured to the details given by the specifier to meet the requirements of each particular project. To help specifiers Westok provide a comprehensive design and advisory service completely free of charge or obligation. If you would like a Westok Engineer to provide a cellular beam design, or require technical support on any matter concerning cellular beams then please contact Westok. Help
For further details see “Further information – Westok Beams” on page 217.
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Composite Beam Engineer’s Handbook page 180
Chapter 2 : Scope
Steel sections Composite Beam can handle design for an international range of steel I-sections for many different countries and also for many specific manufacturers. If required the section can be precambered to counteract the effects of dead load on the deflection of the beam. Caution
If you want to use Bison precast concrete slabs, then you should ensure that a minimum width of flange of 133 mm is achieved to allow sufficient bearing for the slabs (50 mm) whilst still allowing a reasonable gap for stud welding and concrete infilling.
Web openings If you need to provide access for services, etc., then you can add openings to a designed beam and Composite Beam can then check these for you. You can define rectangular or circular openings and these can be stiffened on one, or on both sides. The checks that are performed are in accordance with the guidelines and design process given in the publication Design for openings in the webs of composite beams(Ref. 5). We advise you to comply with the following positional recommendations for web openings: • Web openings are designed using the bending moment and vertical shear values at the side of the opening where the moment is lower,
• Openings should preferably be positioned at the mid-height of the section. If not, the depth of the upper and lower sections of web should differ by not more than a factor of two,
• Openings should not be located closer to the support than two times the beam depth or 10% of the span whichever is the greater,
• The best location for any opening is between 1/5 and 1/3 of the span from a support in uniformly loaded beams, or in the lower shear zone of beams subject to point loads,
• • • •
Openings should be not less than the beam depth, D, apart, Unstiffened openings should not generally be deeper than 0.6D or longer than 1.5D, Stiffened openings should not generally be deeper than 0.7D or longer than 2D, Point loads should not be applied at less than D from the side of the adjacent opening.
You cannot currently automatically design sections with web openings, you must perform the design first to get a section size, and then add and check the openings. This gives you complete control of the design process, since you can add appropriate and cost effective levels of stiffening if required, or can choose a different beam with a stronger web in order to reduce or remove any stiffening requirement.
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Note
Dimensional checks. The program does not check that openings are positioned in the best position (between 1/5 and 1/3 length for udl’s and in a low shear zone for point loads). This is because for anything other than simple loading the best position becomes a question of engineering judgment or is pre-defined by the service runs.
Note
Westok beams. You cannot define other web openings when you are using Westok beams.
Adjustment to deflections The calculated deflection at both construction stage and composite stage are adjusted to allow for shear deformation in the web openings. This is carried out following the principles in Design for openings in the webs of composite beams(Ref. 5) and AD 068 but adjusted as detailed below. Design for openings in the webs of composite beams(Ref. 5) suggests a conservative allowance for increase in deflection due to openings of 3% for each individual opening. This is very conservative for the majority of circular openings, can be conservative for rectangular openings particularly if they are stiffened and can be non-conservative for some rectangular openings. The more comprehensive guidance in the SCI Advisory Desk Note AD 183 has not been implemented fully due to its complexity and so the following rules are applied.
For circular openings If If If
dwo /D ≤ 0.7 do = 0.0 0.7 < dwo /D ≤ 0.8 do = 1.0 to 2.5 (linearly interpolated) dwo /D > 0.8 do = 2.5
This may not be sufficient in the last case above and so a warning is given. In such cases you may need to refer to the Advisory Desk Note and produce hand calculations. Where dwo
= the diameter of the web opening
D
= the overall depth of the steel section
do
= the increase in the deflection per opening as a percentage
For rectangular openings An increase in deflection per opening of 3% is applied irrespective of the opening details. This may be non-conservative in some cases and so a warning is given. In such cases you may need to refer to the Advisory Desk Note and produce hand calculations. The cases when the warning is given are:
Profiled metal decking A wide range of profiled steel decking from all current UK manufacturers and some international ones is included.
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You may define the profiled metal decking to span at any angle between 0° (parallel) and 90° (perpendicular) to the direction of span of the steel beam. You can also specify the attachment of the decking for parallel, perpendicular and angled conditions giving edge distances for studs and the positions of any laps where these are known. Where you specify that the direction of span of the profiled metal decking to that of the steel beam is ≥45°, then there is no need to check the beam for lateral torsional buckling during construction stage. Where you specify that the direction of span of the profiled metal decking to that of the steel beam is 6ds
TYPICAL PLAN C30 site concrete
65 Min. Nom. gap Bison standard AN type notch
Min. Nom. 50 bearing
HOLLOW CORE
CONST RUCTION
Bond length 40 x bar dia.
125 x 19 dia. shear studs Single row (min. 65 gap) or Double row (min. 145 gap)
Slab end type ES4 [4 no. slots 500 long per 1200 slab]
Site reinforcement - for size see text
TYPICAL PLAN
ds >6ds
SOLID PLANK CONSTRUCTION
Note
See also Shear Connectors and Reinforcement for special requirements when using Bison precast concrete slabs.
Tip
In order to meet the detailing requirements for minimum bearing and a minimum gap of 65 mm when in design mode set the Beam Size Constraints – Minimum width to 133 mm.
Caution
It is advisable that the position of the plastic neutral axis is such that the majority of the stud is in compression. This is best achieved by investigating the results (Moment, Plastic moment capacity details) and ensuring that the plastic neutral axis is no higher than 50 mm above the top of the beam flange.
Concrete slab You can define concrete slabs in both normal and lightweight concrete provided that you comply with the following constraints: • the slab depth must be between 90 and 500 mm,
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Chapter 2 : Scope
• the concrete cube strength must be between 25 N/mm2 and 100 N/mm2 for normal or lightweight concrete. If you are defining an edge beam you can specify the projected distance from the centre-line of the beam to the free edge of the slab up to a maximum of 300 mm. If you use this facility then your construction details will need to justify this.
Shear connectors The shear connection between the concrete slab and the steel beam may be achieved by using normal studs or Hilti™ connectors. For Hilti™ connectors the ultimate strengths used in the design are taken from tables published by Hilti Corporation, FL-9494 Schaan, Principality of Liechtenstein, (Group Headquarters, R & D and Manufacturing). Note
To enable the full strength of a Hilti connector to be achieved the minimum distance to the edge of the sheet must be at least 15 mm (3 times the diameter of the shot fired pin). The program assumes that this dimensional constraint is met and uses the full design strength of the connector in the calculations.
For other studs you can choose whether the ultimate strengths used in the design are to be those taken from Table 5 of BS 5950 : Part 3 : Section 3.1 : 1990 or those taken from tables published by T.R.W Nelson. Alternatively, if you have another source for the appropriate ultimate strengths you can enter the information directly yourself. If you have chosen to use a precast concrete slab construction, then a particular set of stud information must be used (Diameter = 19 mm, Height = 125 mm, Ultimate Strength = 70 kN). You will not be allowed to change these values. Hilti studs can not be used with Bison precast concrete slabs. You should ensure that a minimum distance of 30 mm from the centre line of the stud to the edge of the precast concrete slab is achievable. All types of stud may be positioned in a wide range of patterns. Note
See Error Messages and Limitations and the Caution for Shear Connectors in Theory and Assumptions for essential information about the layout of shear connectors.
Reinforcement Since the profile metal decking can be perpendicular, parallel or at any other angle to the supporting beam the following assumptions have been made: • Transverse reinforcement,
• if you use single bars they are always assumed to be at 90° to the span of the beam, • if you use mesh then it is assumed to be laid so that the main bars1 are at 90° to the span of the beam. Footnotes 1. These are the bars that are referred to as longitudinal wires in BS 4483: 1998 Table 1.
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Composite Beam Engineer’s Handbook page 187
• Other reinforcement • if you use single bars they are always assumed to be laid in the direction that is parallel to the trough of the profile metal decking.
• if you use mesh then it is assumed to be laid such that the main bars(1) are always parallel to the trough. When using Bison precast concrete slabs only Transverse reinforcement can be specified. The default values are the recommended sizes and their spacing is fixed at 300 mm cross centres to coincide with the voids in the precast concrete slab.
Slab depth
Reinforcement
≤ 150
T8 @ 300
≤ 200
T10 @ 300
≤ 250
T12 @ 300
In all cases a suitable bond length should be provided to anchor the reinforcement beyond the position where it is fully utilised. Help
For further information see “Typical precast concrete slab details” on page 184.
Fibre Reinforced Concrete When using Kingspan decks an option exists to use Dramix fibre reinforcement. Similarly when using RLSD an option exists to use Strux fibre reinforcement. Note
Fibre reinforcement can not be used with any other decking manufacturer.
Note
Fibre reinforcement can't be used for edge beams, as these need traditional hooped reinf bars.
Help
For further information see the Fibre Reinforced Concrete Advisory Note.
Construction stage restraint conditions If you do need to check the lateral torsional buckling of the beam during construction (in the case where the profiled metal decking is unable to provide an acceptable level of restraint) you can: • define the degree of fixity that the end connections are able to provide and hence an effective length associated with the support,
• position additional restraints at any point along the beam (Composite Beam automatically uses 1.0L and 1.2L+2D as the factors for Normal and Destabilizing loads), Help
For a definition of Destabilizing Loads see BS 5950-1:2000 clause 4.3.4.
• Composite Beam automatically takes the average of the effective length factors for differing supports, or between those for the support and the adjacent sub-beam.
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Chapter 2 : Scope
• alternatively you can specify the factors that you want to use for the lengths between restraints, or you can enter the effective length of the sub-beam directly by entering a value (in m).
• specify that any length (or lengths) of the beam should be taken as being fully restrained against lateral torsional buckling, independent of the restraint conditions for the adjacent length(s).
Construction stage loading You may specify a wide range of applied loading at the construction stage including: • uniform distributed loads (over the whole or part of the beam),
• point loads, • varying distributed loads (over the whole or part of the beam), • trapezoidal loads. You define these loads into one or more loadcases which you then include, together with the appropriate factors in the dedicated Construction stage design combination. You can include or exclude the self-weight of the beam from this combination and you can define the load factors that apply to the self weight and to each loadcase in the combination. Note
You can not include the loads in the Slab loadcase in the Construction stage combination, since these loads relate to the slab in its dry state. The loads in the Construction stage combination should relate to the slab in its wet state and any other loads that may be imposed during construction.
Tip
If you give construction stage loadcases a suitable title you will be able to identify them easily when you are creating your construction stage design combination.
Composite stage loading You may specify a wide range of applied loading for the composite condition: • uniform distributed loads (over the whole or part of the beam),
• point loads, • varying distributed loads (over the whole or part of the beam), • trapezoidal loads. You define these loads into one or more loadcases which you then include, together with the appropriate factors in the design combinations you create. You can include or exclude the self-weight of the beam from any combination and you can define the load factors that apply to the self weight and to each loadcase in the combination. The SLAB loadcase is taken to contain Dead loads which for serviceability limit state calculations are taken as acting on the bare beam. For each other loadcase you create you specify the type of loads it contains – Dead, Imposed or Wind. For each load that you add to an Imposed loadcase you can specify the percentage of the load which is to be considered as acting long-term (and by inference that which acts only on a short-term basis).
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Composite Beam Engineer’s Handbook page 189
All loads in Dead loadcases are considered to be completely long-term while those in Wind loadcases are considered totally short-term.
Construction stage design checks When you use Composite Beam to design or check a beam for the construction stage (the beam is acting alone before composite action is achieved) the following conditions are examined in accordance with BS 5950-1:2000: • section classification (Clause 3.5.2),
• shear capacity (Clause 4.2.3), • moment capacity: • Clause 4.2.5.2 for the low shear condition, • Clause 4.2.5.3 for the high shear condition, • lateral torsional buckling resistance (Clause 4.3.6), Note
This condition is only checked in those cases where the profile decking or precast concrete slab (at your request) does not provide adequate restraint to the beam,
• web openings, • Westok checks, • Shear horizontal, • Web post buckling, • Vierendeel bending, • construction stage total load deflection check. Composite stage design checks When you use Composite Beam to design or check a beam for the composite stage (the beam and concrete act together, with shear interaction being achieved by appropriate shear connectors) the following Ultimate Limit State and Serviceability Limit State conditions are examined in accordance with BS 5950 : Part 3 : Section 3.1 : 1990 (unless specifically noted otherwise). Ultimate Limit State Checks
• section classification (Clause 4.5.2), depending on whether adequate connection is achieved between the compression flange and the slab. The section classification allows for the improvement of the classification of the section if the appropriate conditions are met,
• vertical shear capacity (BS 5950-1:2000 - Clause 4.2.3), • longitudinal shear capacity (Clause 5.6) allowing for the profiled metal decking, transverse reinforcement and other reinforcement which has been defined,
• number of shear connectors required (Clause 5.4.7) between the point of maximum moment and the end of the beam, or from and between the positions of significant point loads,
• moment capacity: • Clause 4.4.2 for the low shear condition, • Clause 5.3.4 for the high shear condition,
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Chapter 2 : Scope
Serviceability Limit State Checks • service stresses (Clause 6.2),
• concrete • steel top flange and bottom flange • deflections (Clause 6.1.2) • self-weight • SLAB loadcase, • dead load, • imposed load1, • total deflections, • natural frequency check (Clause 6.4).
Error messages and limitations As you are defining the data for your beam, Composite Beam continually checks to ensure that the data is valid. If a particular value is not valid, then it will be shown using a colour of your choice in the dialog. If a value causes a potential problem, then a different colour will be used in the dialog. If you allow the cursor to rest over the error or warning field you will see a tip telling you the acceptable range of input. Until all the information within the dialog is valid (but not free of warnings) you will not be able to save the dialog since OK will be dimmed. Although checking in this way prevents you from defining an invalid beam there are some cases where particular errors occur that cannot be trapped in this way (for instance where an error occurs due to inconsistencies that have arisen between information covered on different dialogs). In these cases you will find that: • the Design icon will be dimmed and you will need to review the input data and correct the inconsistencies before you can perform the design.
• when you attempt to perform a design you will see an error message indicating that data is not suitable for the design to proceed. Each message is self-explanatory. You should take a careful note of the error message and then change the beam data to correct the problem. Caution
During the design process Building Designer does not check some stud dimensional constraints. Once you accept a particular section Building Designer reviews the details and if any of these are in conflict it dims the design icon. In this case you should review and correct the input data.
If there are other problems with the design, then you will see a series of warning messages in the results viewer. You should take note of any such warnings and take the action that you deem appropriate. Engineering tips are also available in the results viewer which may give you useful information about the assumptions or approach adopted for the particular calculation or about a particular recommendation of good practice with which we recommend that you comply.
Footnotes 1. This is the only limit given in BS 5950 : Part 3 : Section 3.1 : 1990.
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Chapter 2 : Scope
Composite Beam Engineer’s Handbook page 191
Note
Adjustment to deflections. The calculated deflection at both construction stage and composite stage are adjusted to allow for shear deformation in the web openings.
Help
For further information see “Adjustment to deflections” on page 181.
Note
Dimensional checks. The program does not check that openings are positioned in the best position (between 1/5 and 1/3 length for udl’s and in a low shear zone for point loads). This is because for anything other than simple loading the best position becomes a question of engineering judgment.
Note
Layout of studs. The program checks for adequate shear connection between the point of maximum moment and the left hand support when the position of the maximum moment occurs at a distance of less than or equal to half the span of the beam. Otherwise the program checks for adequate shear connection between the point of maximum moment and the right hand support. This is done since the program always assumes that you will provide at least the same number of studs in the complementary length. To determine the status of the check Composite Beam applies the following rules: • If the partial interaction ratio at the point of maximum moment is less than the minimum permissible interaction ratio, then this generates a FAIL status, • If the partial interaction ratio at the point of maximum utilisation ratio occurs at a point that is not the maximum moment position is less than the minimum permissible interaction ratio, then this generates a WARNING status, • If the partial interaction ratio at any other point load is less than the minimum permissible interaction ratio, then this does not affect the status in any way.
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Chapter 3
Chapter 3 : Design Aspects
Design Aspects A substantial number of calls to CSC’s Support Department are caused by a lack of understanding of the basic design requirements for composite beams. Use of Fastrak Composite Beam does not remove your need to understand the fundamental principals of composite design. This chapter aims to explain how the program can be used effectively with regard to certain design situations:
• allow a non composite design to be carried out within Composite Beam running alone, • allow the design to automatically select the amount of transverse shear reinforcement a beam requires,
• allow you to specify the stud spacing at the start of the automatic design process if you so desire. These items are discussed further below and are followed by a short example which illustrates them.
Non-composite design within Composite Beam This feature has been provided principally because there are some beams which are simply not suitable for effective composite design. Such beams include: • very small beams,
• beams with significant offset loading, • beams where the deck is at an angle to the beam, and • beams where, for a variety of reasons, it is not possible to provide an adequate number of studs or amount of transverse reinforcement. If you find yourself in a circumstance where the Building Designer or Composite Beam is unable to find a section size which works compositely, you can ask for a non-composite design for the same loading on-the-fly. You will find that this facility is particularly useful when you are using Composite Beam as a stand-alone application, or when you extract a key beam from a Building Designer model into Composite Beam for further investigation.
To invoke non-composite design in Building Designer Edit the properties of the beam (right-click the beam and then pick the Edit option from the context menu that appears), and on the Design page ensure that Treat as non-composite is checked.
c
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Composite Beam Engineer’s Handbook page 193
To invoke non-composite design in Composite Beam Pick Beam/Non Composite from the main menu, or click the non-composite icon ( the Beam, Loading, Design toolbar.
Click on this icon to carry out a NON-COMPOSITE design of your composite beam
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) from
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Chapter 3 : Design Aspects
Automatic transverse shear reinforcement design It is possible to automatically design the amount of transverse shear reinforcement for each beam. This is achieved in Building Designer by checking the Autoselect option, which is on the Reinforcement tab of the Composite Beam Properties, (or, if running Fastrak Composite Beam directly, on the Reinforcement tab of the Floor Construction page as shown below:)
The auto-selected bars can be tied into the stud group spacing as shown above. Alternatively, the spacing can be controlled directly by the user. Irrespective of the method adopted, the user still needs to have control over the design. This is achieved in Building Designer by clicking on the Design Properties button and then the Reinforcement tab (or, if running Fastrak Composite Beam directly, via the Design Wizard). Note
Thursday 26 April 2007 – 10:18
You can only design transverse shear reinforcement automatically when you are designing a beam. If you are checking a beam, then you must specify the transverse shear reinforcement that you will provide, and then check out this arrangement.
Chapter 3 : Design Aspects
Composite Beam Engineer’s Handbook page 195
Bar spacing as a multiple of stud spacing. When the option Bar spacing as a multiple of stud spacing is checked, the Reinforcement tab provides the user with control on the bar size and the multiples of stud spacing.
These can be used to achieve a selection of say, 12mm diameter bars at 2 times the stud spacing, with a slightly greater area than a less preferable 16mm diameter bars at 4 times the stud spacing.
Controlling the bar spacing directly. When the option Bar spacing as a multiple of stud spacing is not checked, the Reinforcement tab provides the user with direct control on the bar size and the bar spacing.
Automatic transverse shear reinforcement design with Fibre Reinforced Concrete If fibre reinforced concrete (FRC) has been specified (either Dramix or Strux)and the Autoselect button is checked, if the FRC is insufficient to provide adequate longitudinal shear resistance then the transverse shear required will be calculated ignoring the presence of the FRC. Note
The resistances provided by FRC and transverse reinforcement can not be used together to give a totsl resistance.
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Specify the stud spacing at the start of automatic design In early versions Building Designer and Composite Beam would only allow you to specify a stud spacing if you were checking a beam. If you asked for the design of a beam, then this would always invoke an automatic calculation of the required number of studs, which would also optimise these to provide the ideal design. We have still retained this option (tick Auto-layout), but have also introduced the ability for you to specify a stud spacing which is to be used in the design process. For a beam which is perpendicular to the deck this spacing takes the form of specifying that a stud be placed in every trough of the deck for example. For a beam which is parallel to the deck, this takes the form of specifying the distance between the studs.
Caution
If you use this option, then it is most important to note: • that the resulting design may not be the optimal design possible for the beam, or • that composite design is not possible for the stud spacing which you have specified.
Worked Example This worked example illustrates the points covered above. If you want to work through this example you will find the file Engineer’s Example in the \documents and settings\All Users\Application Data\CSC\Fastrak\Examples folder. You can open and use this file, but you can not save
it away unless you change its name, this is done to protect the original.
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Chapter 3 : Design Aspects
Composite Beam Engineer’s Handbook page 197
Let’s take a simple example of a 9 m span spine beam with 6 m span secondary beams at third points.
For this example we shall use a Ward Building Systems, Multideck 60 deck with a 130mm slab. The floor loading is:
Condition
Value
giving point load at 3 m and 6 m of
Wet Slab
2.5 kN/m2
45kN
Dry Slab
2.0 kN/m2
36kN
Services
1.0 kN/m2
18kN
Live load
5.0 kN/m2
90kN
Wet Slab Imposed
0.5 kN/m2
9kN
The beam is designed for composite and construction stage loading.
Without transverse shear reinforcement If no transverse shear reinforcement is available then composite design of this beam is not possible. This happens since the studs on a composite beam must lie between the end of the beam and the point of maximum moment (on this beam this is the 1/3 point). This compaction of the shear studs increases the transverse shear that has to be carried from the studs to the effective concrete flange. When the studs are spaced widely enough apart so as to not need transverse shear reinforcement they do not achieve the minimum interaction requirement for the beam. (40% on a 9m span beam). When the studs are placed close enough to achieve the minimum 40% interaction transverse shear reinforcement is required. Thus with no transverse reinforcement composite design is not possible.
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Chapter 3 : Design Aspects
Design Pass 1 For the Reinforcement transverse to beam ensure that you tick Auto select and Bar spacing as a multiple of stud spacing.
and then on the Group spacing page ensure that you tick Auto-layout.
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Chapter 3 : Design Aspects
Composite Beam Engineer’s Handbook page 199
From the UbBeamOrder.Eur file this should select a 457 x 191 x 67 UB (albeit with a utilisation ratio of 1.00) with 47 shear studs at 194 mm centres and transverse shear reinforcement of T10’s at 194 mm centres.
The design has recognised that the beam needs transverse shear reinforcement to enable composite design and has consequently introduced suitable transverse reinforcement which (at your request) matches the centres of the studs required to achieve the design.
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Chapter 3 : Design Aspects
Design Pass 2 There is nothing wrong with “Design Pass 1”, however you might feel that placing studs at 194 mm centres is not desirable. If so you can set the design pass to work with a specified spacing – such as 200 mm.
With the stud spacing set to 200 mm, automatic design will select a 457 x 191 x 74 UB (one section weight heavier) with 45 studs at 200 mm centres and transverse shear reinforcement of T10’s at 200 mm centres. Whether or not this is a better design is not really the concern of this example, it is simply a different design based in different engineering input. The key issue (that should not be missed) is that if you had specified a stud spacing of 500mm, then composite design would not be possible for this beam.
Design Pass 3 Select the non-composite icon ( ) and then re-design the beam as a simple non composite beam. This time the design indicates that you need to use a 610 x 229 x 101 UB if you cannot provide the transverse shear reinforcement necessary to allow the beam to be designed for composite action.
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Chapter 4 : Theory and Assumptions
Chapter 4
Composite Beam Engineer’s Handbook page 201
Theory and Assumptions This section describes the theory used in the development of Composite Beam and the major assumptions that have been made, particularly with respect to interpretation of BS 5950 : Part 3 : Section 3.1 : 1990(Ref. 1). A basic knowledge of the design methods for composite beams in accordance with BS 5950 : Part 3 : Section 3.1 : 1990 is assumed.
Analysis method Composite Beam uses a simple analysis of a statically determinate beam to determine the forces, moments and so on, to be resisted by the beam under the Construction stage, at the Serviceability Limit State and at the Ultimate Limit State.
Design method The design methods employed to determine the adequacy of the section for each condition are those consistent with BS 5950 : Part 3 : Section 3.1 : 1990 unless specifically noted otherwise.
Construction stage Composite Beam performs all checks for this condition in accordance with BS 5950-1:2000(Ref. 2) .
Section classification Cross-section classification is determined using Table 11 and Clause 3.5. The classification of the section must be Plastic (Class 1), Compact (Class 2) or Semi-compact (Class 3). Sections which are classified as Slender (Class 4) are beyond the scope of Composite Beam.
Member strength checks Member strength checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, and at each side of a web opening as well as all other points of interest along the beam. Shear capacity — is determined in accordance with Clause 4.2.3. Where the applied shear force exceeds 60% of the capacity of the section, the high shear condition applies to the bending moment capacity checks (see below). Bending moment capacity — is calculated to Clause 4.2.5.2 (low shear at point) or Clause 4.2.5.3 (high shear at point) for plastic, compact and semi-compact sections.
Lateral torsional buckling checks BS 5950 : Part 3 : Section 3.1 : 1990 states that lateral torsional buckling checks are not required when the angle between the direction of span of the beam and that of the profile decking is greater than or equal to 45°.
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When the angle is less than this, then lateral torsional buckling checks will normally be required. Composite Beam allows you to switch off these checks by specifying that the entire length between the supports is continuously restrained against lateral torsional buckling. If you use this option you must be able to provide justification that the beam is adequately restrained against lateral torsional buckling during construction. For Bison precast concrete slabs you can specify whether or not the slabs are able to provide restraint to the beam against lateral torsional buckling. For beams with a span greater than 9 m you need to give careful consideration to the construction sequence. When the checks are required you can position restraints at any point within the length of the main beam and can set the effective length of each sub-beam (the portion of the beam between one restraint and the next) either by giving factors to apply to the physical length of the beam, or by entering the effective length that you want to use. Each sub-beam which is not defined as being continuously restrained is checked in accordance with clause 4.3.8 and Annex B of BS 5950-1:2000.
Deflection checks The maximum deflection under all loads applied in the construction stage loadcase is calculated. The loads are taken as acting on the steel beam alone. The construction stage load factors are taken as 1.0 when calculating these deflections.
Torsion for ASB and SFB beams In the design of ASB/SFB beams, torsion resulting from out of balance construction stage loading is not considered. If this condition occurs, then you will need to produce independent calculations to check this.
Composite stage Composite Beam performs all checks for the composite stage condition in accordance with BS 5950 : Part 3 : Section 3.1 : 1990 unless specifically noted otherwise.
Equivalent steel section - Ultimate limit state (ULS) An equivalent steel section is determined for use in the composite stage calculations by removing the root radii whilst maintaining the full area of the section. This approach reduces the number of change points in the calculations while maintaining optimum section properties.
Section classification (ULS) For section classification purposes the true section is used. Composite Beam classifies the section in accordance with the requirements of BS 5950-1:2000 except where specifically modified by those of BS 5950 : Part 3 : Section 3.1 : 1990. There are a small number of sections which fail to meet a classification of compact at the composite stage. Although BS 5950 : Part 3 : Section 3.1 : 1990 covers the design of such members they are not allowed in this release of Composite Beam.
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Member strength checks (ULS) Member strength checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, and at each side of a web opening as well as all other points of interest along the beam. Shear Capacity (Vertical) — is determined in accordance with Clause 4.2.3 of BS 5950-1:2000. Where the applied shear force exceeds 50% of the capacity of the section, the high shear condition applies to the bending moment capacity checks (see below). Shear Capacity (Longitudinal) — the longitudinal shear resistance of a unit length of the beam is calculated in accordance with Clause 5.6. You can set the position and attachment of the decking and details of the reinforcement that you want to provide. Composite Beam takes these into account during the calculations. The following assumptions are made: • the applied longitudinal shear force is calculated at the centre-line of the beam, and at the position of the lap (if known). If the position of the lap is not known, then the default value of 0 mm should be used (that is the lap is at the centre-line of the beam) as this is the worst case scenario. Note
The sheet cover width is shown in the Floor Construction property sheet for your information.
• the minimum concrete depth is assumed for calculating the area of concrete when the profile decking and beam spans are parallel,
• the total concrete area is used when the profile decking and beam spans are perpendicular, • the overall depth of the slab is used for precast concrete slabs. that is the topping is assumed to be structural and any voids or cores are ignored. In the calculations of the longitudinal shear resistance on the beam centre-line and at the lap, the areas used for the reinforcement are shown in the following table.
Decking angle
Reinforcement type
Area used
transverse
that of the single bars defined or for mesh the area of the main wiresa
other
that of the single bars defined or for mesh the area of the main wires(a)
transverse
that of the single bars defined or for mesh the area of the main wires(a)
perpendicular
parallel other
single bars have no contribution, for mesh the area of the minor wiresb
a. These are the bars that are referred to as longitudinal wires in BS 4483: 1998 Table 1.
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Composite Beam Engineer’s Handbook page 204
Chapter 4 : Theory and Assumptions
b. These are the bars that are referred to as transverse wires in BS 4483: 1998 Table 1.
If the decking spans at some intermediate angle (α) between these two extremes then the program calculates: • the longitudinal shear resistance as if the sheeting were perpendicular, v1,
• the longitudinal shear resistance as if the sheeting were parallel, v2 , • then the modified longitudinal shear resistance is calculated from these using the relationship, v1 x sin2(α) + v2 x cos2(α). Moment Capacity — for the low shear condition the plastic moment capacity is determined in accordance with Clause 4.4.2. For the high shear condition the approach given in Clause 5.3.4 is adopted. The overall depth of the slab is used for precast concrete slabs. that is the topping is assumed to be structural and any voids/cores are ignored. In this calculation the steel section is idealised to one without a root radius so that the position of the plastic neutral axis of the composite section can be determined correctly as it moves from the flange into the web.
Shear connectors (ULS) Composite Beam checks shear connectors to Clause 5.4.7. It calculates the stud reduction factor based on the number of studs in a group. For Bison precast concrete slabs the stud reduction factor is always 1.0. Composite Beam always uses 2*e (and not br) in the calculation of k for perpendicular profiles, and always uses br for parallel cases. For angled cases two values of k are calculated and summed in accordance with Clause 5.4.7.4. In this instance Composite Beam uses 2*e for the calculation of k1 and br for the calculation of k2. Caution
The value of e (when used) can have a very significant effect on the value of k. This can have a dramatic effect on the number of studs required for a given beam size. Alternatively for a fixed layout of studs this can have a significant effect on the required beam size.
If you choose the option to optimise the shear studs, then Composite Beam will progressively reduce the number of studs either until the minimum number of studs to resist the applied moment is found, until the minimum allowable interaction ratio (for example 40% for beams with a span less than 10 m) is reached or until the minimum spacing requirements are reached. This results in partial shear connection. Note
The value of percentage interaction is always calculated by the program as a proportion of the maximum concrete force and not simply Na/Np as in the code.
Stud optimization is a useful facility since there is often some over conservatism in a design due to the discrete changes in the size of the section.
Thursday 26 April 2007 – 10:18
Chapter 4 : Theory and Assumptions
Composite Beam Engineer’s Handbook page 205
Carefully investigate the list of sections which from your original list of Sections for study are capable of carrying the design forces. You may prefer to choose a slightly heavier beam with less studs, or a simpler layout of studs, in order to provide a more economical solution, or one that is easier to construct. Caution
Layout of studs. The program checks for adequate shear connection between the point of maximum moment and the left hand support when the position of the maximum moment occurs at a distance of less than or equal to half the span of the beam. Otherwise the program checks for adequate shear connection between the point of maximum moment and the right hand support. This is done since the program always assumes that you will provide at least the same number of studs in the complementary length. To determine the status of the check Composite Beam applies the following rules:
• If the partial interaction ratio at the point of maximum moment is less than the minimum permissible interaction ratio, then this generates a FAIL status, • If the partial interaction ratio at the point of maximum utilisation ratio occurs at a point that is not the maximum moment position is less than the minimum permissible interaction ratio, then this generates a WARNING status, • If the partial interaction ratio at any other point load is less than the minimum permissible interaction ratio, then this does not affect the status in any way.
Section properties - serviceability limit state (SLS) BS 5950 : Part 3 : Section 3.1 : 1990 indicates that the Serviceability Limit State modular ratio for all SLS calculations should be based upon an effective modular ratio derived from the proportions of long term loading in the design combination being considered. Composite beam therefore calculates the deflection for the beam based on the properties as tabulated below.
Loadcase Type
Properties used
self-weight
bare beam
Slab
bare beam
Dead
composite properties calculated using the modular ratio for long term loads
Live
composite properties calculated using the effective modular ratio appropriate to the long term load percentage for each load. The deflections for all loads in the loadcase are calculated using the principle of superposition.
Wind
composite properties calculated using the modular ratio for short term loads
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Composite Beam Engineer’s Handbook page 206
Chapter 4 : Theory and Assumptions
Loadcase Type Total loads
Properties used these are calculated from the individual loadcase loads as detailed above again using the principle of superposition
Stress checks (SLS) Composite Beam calculates the worst stresses in the extreme fibres of the steel and the concrete at serviceability limit state for each load taking into account the proportion which is long term and that which is short term. These stresses are then summed algebraically. Factors of 1.00 are used on each loadcase in the design combination (you cannot amend these). The stress checks assume that full interaction exists between the steel and the concrete at serviceability state.
Deflection checks (SLS) Composite Beam calculates the deflection in one of two ways depending upon the previous and expected future load history: • the deflections due to all loads in the SLAB loadcase and the self-weight of the beam are calculated based on the inertia of the steel beam alone (these deflections will not be modified for the effects of partial interaction). Note
It is the SLAB deflection alone which is compared with the limit, if any, specified for the SLAB loadcase deflection.
• the deflections for all loads in the other loadcases of the Design Combination will be based on the inertia of the composite section allowing for the proportions of the particular load that are long or short term (see above). When necessary these will be modified to include the effects of partial interaction in accordance with Clause 6.1.4. Note
It is the deflection due to imposed loads alone (allowing for long and short term effects) which is limited within the code. Composite Beam also gives you the deflection for the SLAB loadcase which is useful for pre-cambering the beam. The beam Self-weight, Dead and Total deflections are also given to allow you to be sure that no component of the deflection is excessive.
Natural frequency checks (SLS) Composite Beam calculates the approximate natural frequency of the beam based on the simplified formula published in the Design Guide on the vibration of floors(Ref. 6) which states that 18Natural frequency = -----δ
where δ is the maximum static instantaneous deflection that would occur under a load equivalent to the effects of self-weight, dead loading and 10% of the characteristic imposed loading, based upon the composite inertia (using the short term modular ratio) but not modified for the effects of partial interaction.
Thursday 26 April 2007 – 10:18
Chapter 5 : Theory and Assumptions – Westok beams
Chapter 5
Composite Beam Engineer’s Handbook page 207
Theory and Assumptions – Westok beams This section describes the theory used in the development of Composite Beam and the major assumptions that have been made, particularly as these relate to Westok beams and with respect to interpretation of BS 5950 : Part 3 : Section 3.1 : 1990(Ref. 1). A basic knowledge of the design methods for composite beams in accordance with BS 5950 : Part 3 : Section 3.1 : 1990 is assumed.
Construction stage Composite Beam performs all checks for this condition in accordance with BS 5950-1:2000(Ref. 2) and SCI P100(Ref. 7).
Classification The same classification rules are applied to Westok cellular beams as to normal steel beams. Although the upper and lower sections of Westok beams are welded together the combined beam is still in essence a rolled section. If a Westok beam is classified as slender, then it will be rejected, as would a normal steel beam. Note
If you define a bottom flange which is very large then it could be partly in compression. However, because of the constraining effect of its tension part, the classification is still assumed to be governed by the upper flange or the web.
Note
The depth of the web for web classification allows for the root radius of the two sections making up the cellular beam and the web thickness is taken as that of the thinner web.
Vertical shear The capacity of the Westok section is checked at: • the first web post using the shear capacity of the gross section,
• the centre-line of the first cell in from the left hand support using the shear capacity of the net section. If the check at this point fails, then you will see a warning message indicating that an infill is required at this position. The next cell is checked, and a warning message given if this also fails. This process continues until the first unfilled cell which passes the shear check is found. A similar process is adopted for the right hand web post and the cells adjacent to it.
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Composite Beam Engineer’s Handbook page 208
Chapter 5 : Theory and Assumptions – Westok beams
Horizontal Shear Horizontal shear is developed in the web post due to the change in axial forces in the tee as shown in Figure 15 of SCI Publication 100(Ref. 7) which is reproduced below. Wi Ti Vi
xe (D - Do)/2
Vh
Ti+1 Vi+1 No local Vierendeel moment acts at centre-line of opening
S - Do S
Composite Beam evaluates all web posts (including any endposts) since it is not possible to otherwise ascertain the most critical post given the almost infinite variation of moment. Note
If S1 < S/2 a half infill plate is assumed to be placed in the first cell. Similarly, if Sn < S/2 a half infill plate is assumed to be placed in the last cell. Furthermore, if S1 or Sn > S – Do /2 then this implies that a part cell is adjacent to the end of the beam. This is assumed to be fully infilled.
where S
= cell spacing
Do
= cell diameter
S1
= distance to first cell from left set back position
Sn
= distance to last cell from right set back position
Moment Capacity Composite Beam takes account of the interaction between moment and shear in Westok sections by modifying the thickness of the web of the upper and lower sections as necessary. This approach has been adopted in preference to that from BS 5950-1:2000 for a number of reasons: • It maintains consistency with designs carried out before Westok beams were included in Composite Beam. It is similar to the approach adopted for the design of the upper and lower tee sections in conventional web openings.
• It allows the shear to be distributed preferentially between the upper and lower tee sections.
• For equal flanged Class 1 or 2 Plastic or Compact sections, either approach yields the same answer. The form that this equation takes (see below) is similar to that in BS 5950-1:2000(Ref. 2).
Thursday 26 April 2007 – 10:18
Chapter 5 : Theory and Assumptions – Westok beams
Composite Beam Engineer’s Handbook page 209
For low shear the moment capacity is based on py x S ≤ 1.2 x py x Z for Class 1 and 2 sections and py x Z ≤ py x S for Class 3 sections. The calculation of S is based on the full web thickness when Fv ≤ 0.5 x Pv and on the effective web thickness when Fv > 0.5 x Pv. To ensure the correct change point, the limit above which the high shear condition exists, would need to be taken as 0.5 x Pv. In BS 5950 a limit of 0.6 x Pv is adopted (the reduction in web thickness is less than 5% at this point). However, for consistency with the reduction formula and the approach adopted for Web Openings in SCI Publication 068, the change point of 0.5 x Pv is adopted. Since it is not possible to determine the worst case of interaction of shear and moment otherwise, each cell is checked. The shear force resisted by the upper and lower sections is calculated in proportion to their respective capacities calculated as: d x t x 0.6 x py The effective web thickness of the upper and lower sections is then evaluated from: te
=
tw x (1 – (2 x Fv /Pv - 1)2)
Lateral Torsional Buckling These checks are performed in exactly the same way as for normal steel sections. The plastic and elastic section moduli and the buckling properties are based on the net section over the centre-line of the cell.
Deflection This is calculated in accordance with Section 6.3 and Figure 16 of SCI Publication 100(Ref. 7). In order to assess the deflection at any point, the method requires a unit point load to be applied at that point; Composite Beam calculates the deflection at 1/40th points along the beam. The shear and moment at the centre-line of each cell are then evaluated for this unit load. The overall deflection at the point under consideration has five contributory components. Each component is evaluated at each cell and then summed. The five contributions are due to: • bending in the tee, y1 ,
• • • •
bending in the web post, y2 , axial force in the tee, y3 , shear in the tee, y4 , shear in the web post, y5 . Note
In the calculation of deflections filled cells are treated as if they are not filled. This is conservative.
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Composite Beam Engineer’s Handbook page 210
Chapter 5 : Theory and Assumptions – Westok beams
Web Post Flexure and Buckling Composite Beam calculates the moment capacity of the web post, (section A-A in Figure 16 of SCI Publication 100(Ref. 7)). c1 , c2 and c3 are evaluated to clause 6.2.5 and the moment capacity is compared with the moment generated by the horizontal shear in the web post. Note
The end post is not checked. Web-post buckling is a lateral torsional effect local to the web post (see page 3 of Publication 100). It occurs on a diagonal from the bottom of the cell furthest away from the support up to the top of the cell closest to the support with centre of rotation being (approximately) on the weld line. At the end web-post position the length over which buckling occurs is reduced considerably. The approach detailed for internal web posts would be very conservative for the end web post. Also the type of connection used at the beam end will be significant. For instance if a partial or full depth end plate is used this check would not be valid.
Vierendeel Bending Composite Beam checks Vierendeel bending of both the upper and lower tee sections for all cells apart from any cells that have been filled to satisfy the shear condition. SCI Publication 100(Ref. 7) gives two alternative approaches to calculating the secondary bending stresses around the cell. These methods give similar results, Composite Beam uses Sahmel’s method. A plastic load distribution is used for sections which are classified as either Plastic or Compact. Both plastic and elastic load distributions are used for sections which are classified as Semi-compact. With reference to Figure 10 of SCI Publication 100(Ref. 7), Composite Beam takes the shear and axial load (from the bending moment) at the centre line of the cell from the beam analysis and proportioned between the upper and lower tee sections. At any cross section through the tee at an angle, φ, to the vertical centre-line of the cell these forces are transposed to an axial load, a shear and a moment acting on a new cross-section. Since the section properties are changing and the axial load decreases whilst the moment increases, it is unclear at what angle the interaction of bending and axial load becomes critical. An incremental approach is therefore adopted and Composite Beam increments the angle in 5° intervals. Composite Beam takes account of a reduced web thickness if high shear occurs in the tee in exactly the same way as in the moment capacity calculations.
Thursday 26 April 2007 – 10:18
Note
The upper limit on Mc for Plastic and Compact sections of 1.2 x py x Ztee is not applied.
Note
The plastic moment capacity is used for both Plastic and Compact sections. With regard this point, Section 6.2.6 of SCI Publication 100(Ref. 7) defines Mp as the plastic moment capacity for Plastic sections but as the elastic moment capacity for all other sections. This infers that Compact sections should be limited to their elastic moment capacity. There appears to be no justification for this and so the inferred requirement of SCI Publication 100(Ref. 7) is therefore ignored.
Chapter 5 : Theory and Assumptions – Westok beams
Composite Beam Engineer’s Handbook page 211
Composite Stage Composite Beam performs all checks for this condition in accordance with BS 5950 : Part 3 : Section 3.1 : 1990(Ref. 2).
Classification The same classification rules are applied to Westok cellular beams as to normal steel beams except that the lesser web thickness of the upper and lower sections is used for the web classification. If a Westok beam is classified as semi-compact or slender, then it is rejected in exactly the same way as a normal steel beam.
Vertical shear The shear capacity of the Westok section is checked at: • the first web post using the shear capacity of the gross section,
• the centre-line of the first cell in from the left hand support using the shear capacity of the net section. If the check at this point fails, then you will see a warning message indicating that an infill is required at this position. The next cell is checked, and a warning message given if this also fails. This process continues until the first unfilled cell which passes the shear check is found. A similar process is adopted for the right hand web post and the cells adjacent to it. Unlike normal beams, Section 7.2.2 of SCI Publication 100(Ref. 7) allows the contribution of the concrete to be used. It is most likely that the critical cross-section will be over the cells and therefore this accounting of the concrete resistance is compatible with the treatment of shear over standard web openings. Composite Beam makes the conservative assumption that the concrete flange is limited to the area above the profiled decking. This means that no adjustment is necessary for the various possible angles of span of the decking.
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Composite Beam Engineer’s Handbook page 212
Chapter 5 : Theory and Assumptions – Westok beams
Horizontal Shear Horizontal shear is developed in the web post due to the change in axial forces in the tee as shown in Figure 15 of SCI Publication 100(Ref. 7) which is reproduced below. Wi Ti Vi
xe (D - Do)/2
Vh
Ti+1 Vi+1 No local Vierendeel moment acts at centre-line of opening
S - Do S
Composite Beam evaluates all web posts (including any endposts) since it is not possible to otherwise ascertain the most critical post given the almost infinite variation of moment. If S1 < S/2 a half infill plate is assumed to be placed in the first cell. Similarly, if Sn < S/2 a half infill plate is assumed to be placed in the last cell. Furthermore, if S1 or Sn > S – Do /2 then this implies that a part cell is adjacent to the end of the beam. This is assumed to be fully infilled.
Note
where S
= cell spacing
Do
= cell diameter
S1
= distance to first cell from left set back position
Sn
= distance to last cell from right set back position
Longitudinal shear In the calculations of the longitudinal shear resistance on the beam centre-line and at the lap, the areas used for the reinforcement are shown in the following table.
Decking angle
Reinforcement type transverse
that of the single bars defined or for mesh the area of the main barsa
other
that of the single bars defined or for mesh the area of the main bars(a)
perpendicular
Thursday 26 April 2007 – 10:18
Area used
Chapter 5 : Theory and Assumptions – Westok beams
Decking angle
Composite Beam Engineer’s Handbook page 213
Reinforcement type transverse
parallel other
Area used that of the single bars defined or for mesh the area of the main bars(a) single bars have no contribution, for mesh the area of the minor wiresb
a. These are the bars that are referred to as longitudinal wires in BS 4483: 1998 Table 1. b. These are the bars that are referred to as transverse wires in BS 4483: 1998 Table 1.
If the decking spans at some intermediate angle (α) between these two extremes then the program calculates: • the longitudinal shear resistance as if the sheeting were perpendicular, v1 ,
• the longitudinal shear resistance as if the sheeting were parallel, v2, • then the modified longitudinal shear resistance is calculated from these using the relationship, v1 ×sin2(a) + v2 ×cos2(a).
Moment Capacity Composite Beam performs checks for both the low and high shear conditions. Low shear The design procedures are the same as used for standard beams except that the additional cases which allow the Plastic Neutral Axis to be in the upper and lower web are evaluated. High shear The effect of high shear and moment is ignored for Westok cellular beams. The calculations assume that the combination of shear and axial load is accounted for when checking Vierendeel bending in the upper and lower tee sections. Hence this protects against any failure due to reduction in moment capacity from high shear.
Web Post Flexure and Buckling Composite Beam calculates the moment capacity of the web post, (section A-A in Figure 16 of SCI Publication 100(Ref. 7)). c1 , c2 and c3 are evaluated to clause 6.2.5 and the moment capacity is compared with the moment generated by the horizontal shear in the web post.
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Composite Beam Engineer’s Handbook page 214
Note
Chapter 5 : Theory and Assumptions – Westok beams
The end post is not checked. Web-post buckling is a lateral torsional effect local to the web post (see page 3 of Publication 100). It occurs on a diagonal from the bottom of the cell furthest away from the support up to the top of the cell closest to the support with centre of rotation being (approximately) on the weld line. At the end web-post position the length over which buckling occurs is reduced considerably. The approach detailed for internal web posts would be very conservative for the end web post. Also the type of connection used at the beam end will be significant. For instance if a partial or full depth end plate is used this check would not be valid.
Vierendeel Bending Composite Beam checks Vierendeel bending of both the upper and lower tee sections for all cells apart from any cells that have been filled to satisfy the shear condition. SCI Publication 100(Ref. 7) gives two alternative approaches to calculating the secondary bending stresses around the cell. These methods give similar results, Composite Beam uses Sahmel’s method. With reference to Figure 10 of SCI Publication 100(Ref. 7), the shear and axial load (from the bending moment) at the centre line of the cell are taken from the beam analysis and proportioned between the upper and lower tee sections. At any cross section through the tee at an angle, φ, to the vertical centre-line of the cell these forces are transposed to an axial load, a shear and a moment acting on a new cross-section. Since the section properties are changing and the axial load decreases whilst the moment increases, it is unclear at what angle the interaction of bending and axial load becomes critical. An incremental approach is therefore adopted and Composite Beam increments the angle in 5° intervals. Composite Beam takes account of a reduced web thickness if high shear occurs in the tee in exactly the same way as in the moment capacity calculations. At the composite stage the concrete flange is taken to contribute to the shear capacity of the section. The shear is preferentially given to the upper tee which has the benefit that the lower tee capacity is maximised to resist the larger axial load that exists at that level. However, if, during analysis, the upper tee fails then the shear is shifted by 5% into the lower tee and the section is reanalysed. Note
The upper limit on Mc for Plastic and Compact sections of 1.2 x py x Ztee is not applied.
Note
The plastic moment capacity is used for both Plastic and Compact sections. With regard this point, Section 6.2.6 of SCI Publication 100(Ref. 7) defines Mp as the plastic moment capacity for Plastic sections but as the elastic moment capacity for all other sections. This infers that Compact sections should be limited to their elastic moment capacity. There appears to be no justification for this and so the inferred requirement of SCI Publication 100(Ref. 7) is therefore ignored.
Deflections Composite Beam calculates these at the composite stage using the properties of the gross (uncracked) composite section making allowance for the effects of partial shear connection by following the procedure given in Section 7.3 of SCI Publication 100(Ref. 7).
Thursday 26 April 2007 – 10:18
Chapter 5 : Theory and Assumptions – Westok beams
Composite Beam Engineer’s Handbook page 215
The deflections due to self-weight and SLAB loads are calculated in accordance with Section 6.3 and Figure 16 of SCI Publication 100(Ref. 7). In order to assess the deflection at any point, the method requires a unit point load to be applied at that point; Composite Beam calculates the deflection at 1/40 th points along the beam. The shear and moment at the centre-line of each cell are then evaluated for this unit load. The overall deflection at the point under consideration has five contributory components. Each component is evaluated at each cell and then summed. The five contributions are due to: • bending in the tee, y1 ,
• • • •
bending in the web post, y2 , axial force in the tee, y3 , shear in the tee, y4 , shear in the web post, y5 . Note
In the calculation of deflections filled cells are treated as if they are not filled. This is conservative.
Service Stresses Composite Beam calculates these using the properties of the gross (uncracked) composite section, and with the steel section properties appropriate to the Westok section.
Natural frequency Again Composite Beam calculates these using the properties of the gross (uncracked) composite section in a similar manner to normal steel beams, using the section properties appropriate to the Westok section. However the appropriate allowances for partial interaction are taken from Section 7.3 of SCI Publication 100(Ref. 7).
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Composite Beam Engineer’s Handbook page 216
Chapter 6
Chapter 6 : References & Further Information
References & Further Information
References 1. British Standards Institution. BS 5950 : Structural use of steelwork in building; Part 3. Code of practice for design in composite construction; Section 3.1: Design of simple and continuous composite beams. BSI 1990. 2. British Standards Institution. BS 5950 : Structural use of steelwork in building; Part 1. Code of practice for design in simple and continuous construction: hot rolled sections. BSI 2000. 3. The Steel Construction Institute. Publication 078. Commentary on BS 5950 : Part 3 : Section 3.1 : 1990. SCI 1989. 4. The Steel Construction Institute. Publication 055. Design of Composite Slabs and Beams with Steel Decking. SCI 1989. 5. The Steel Construction Institute. Publication 068. Design for openings in the webs of composite beams. SCI 1987. 6. The Steel Construction Institute. Publication 076. Design Guide on the Vibration of Floors. SCI 1989. 7. The Steel Construction Institute. Publication 100. Design of Composite and Non-Composite Cellular Beams. SCI 1990.
Further information – Bison precast concrete slabs For further information or technical literature on Bison Flooring Systems please contact the Bison Technical Department at the address shown below between the hours of 9:00 am and 5:00 pm, Monday to Friday. Bison Concrete Products Ltd. Amington House, Silica Road Tamworth Staffordshire England B77 4AZ Tel. 01827 64141 Fax. 01827 69009 For sales enquiries please contact the Sales Department during the same times.
Thursday 26 April 2007 – 10:18
Chapter 6 : References & Further Information
Composite Beam Engineer’s Handbook page 217
Further information – Westok Beams For further information or technical literature on Westok Beams please contact Westok Technical Support and Design Service. Westok Structural Services Ltd. Horbury Junction Industrial Estate Horbury Junction Wakefield WF4 5ER Tel: 44-1924 264 121 Fax: 44-1924 280 030 email:
[email protected]. You can also view this information while running the program by choosing Help/About Westok Structural Services Ltd… which shows the About Westok Structural Services Ltd. dialog.
You can click on the email link on this dialog to create a new email message to Westok.
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Composite Beam Engineer’s Handbook page 218
Thursday 26 April 2007 – 10:18
Chapter 6 : References & Further Information
CSC Fastrak ™
Structural steelwork analysis and design
.fastrak5950.com
HANDBOOK GENERAL BEAM
General Beam Documentation page 220
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
General Beam Engineer’s Handbook page 221
General Beam Engineer’s Handbook
Chapter 1
Introduction and application This is design software which allows you to analyse and design a structural steel beam or cantilever which may have incoming beams providing restraint, and which may or may not be continuously restrained over any length between restraints You can use General Beam: • to determine those sections which can withstand the applied loading,
• to check a beam of known size to determine whether it is able to carry the loading. Unless explicitly stated all calculations in General Beam are in accordance with the relevant sections of BS 5950-1:2000. You may find the handbook and commentary to the Code of Practice published by the Steel Construction Institute useful.
Practical applications General Beam can be used both to design (design beam mode) and check (check beam mode) general beams. You might find the following procedures useful.
Designing a beam In the typical procedure below items in brackets [] are optional.
Step
Icon
Instructions
1
Launch General Beam,
2
Create a new project giving the project name [and other project details],
3
Give the beam reference details,
4
Set General Beam into design beam mode,
5
Define the properties for the beam: • number of spans, span lengths, section types and section grades; • end support conditions.
6
Give the details of the beam restraints for lateral-torsional- and strut-buckling.
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General Beam Engineer’s Handbook page 222
Step
Icon
Chapter 1 : Introduction and application
Instructions
7
Define the loadcases that apply to the beam.
8
Incorporate the loadcases into a series of design combinations,
9
[Make any Design Wizard settings that you want to use to control the design.]
10
Perform the design
11
From the list of suitable sections preview the results for the more desirable sections and then choose the one that you would like to use,
12
Add in any web openings that you need to allow access for services etc.
13
Check the beam with the web openings. [Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]
14
Specify the content of the report [and print it].
15
Save the project to disk.
Checking a beam In the typical procedure below items in brackets [] are optional.
Step
Icon
Instructions
1
Launch General Beam,
2
Create a new project giving the project name [and other project details],
3
Give the beam reference details,
4
Set General Beam into check beam mode,
5
Define the properties for the beam: • number of spans, span lengths, section sizes and section grades; • end support conditions.
6
Add in any web openings that you need to allow access for services etc.
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
Step
General Beam Engineer’s Handbook page 223
Icon
Instructions
7
Give the details of the beam restraints.
8
Define the loadcases that apply to the beam.
9
Incorporate the loadcases into a series of design combinations,
10
[Make any Design Wizard settings that you want to use to control the design.]
11
Perform the check, (including any web openings),
12
[Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]
13
Specify the content of the report [and print it].
14
Save the project to disk.
Worked Example If you want to work through this example you will find the file Engineer’s Example 1 in the \documents and settings\All Users\Application Data\CSC\Fastrak\Examples folder. You can open and use this file, but you can not save it away unless you change its name, this is done to protect the original. Let’s take a simple example of a continuous 2 x 9 m span spine beam with 6 m span secondary beams at third points.
The floor loading is:
Condition Dead
giving point load at the middle support, and at 3 m and 6 m on each beam of
Value 2.0 kN/m2
36kN
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General Beam Engineer’s Handbook page 224
Chapter 1 : Introduction and application
Condition
Value
giving point load at the middle support, and at 3 m and 6 m on each beam of
Services
1.0 kN/m2
18kN
Live
5.0 kN/m2
90kN
For the purposes of this example the point load on the middle support is specified by defining half the load at the end of beam span 1 and half at the start of beam span 2. The incoming beams are such that they provide restraint against lateral-torsional-buckling to both flanges, but they don’t provide restraint against strut-buckling. The ends of the beam have simple supports onto other beams.
Design pass 1 Performing a design for UB sections only, and with all the non-preferred sections excluded from the design process (see the example in the Simple Beam Engineer’s Handbook), the first section presented is a 610 210 UB 101. If you look at the analysis results you will see that all the results are symmetric.
General Beam does not automatically consider pattern loading. If you want to do so, then you must specify the appropriate load cases and combinations yourself.
Design pass 2 If you want to work through this example you will find the file Engineer’s Example 2 in the \documents and settings\All Users\Application Data\CSC\Fastrak\Examples folder. You can open and use this file, but you can not save it away unless you change its name, this is done to protect the original. For this example you determine that the pattern that you want to consider is: • full service load on one span, with 50% service load on the other, and
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Chapter 1 : Introduction and application
General Beam Engineer’s Handbook page 225
• full live load on one span with no live load on the other. This means that you need six new loadcases: • Full Service Span 1,
• • • • •
Full Service Span 2, 50% Service Span 1, 50% Service Span 2, Live Span 1, and Live Span 2.
The loadings that these require are easy to derive from the loading given above. You also need 2 new combinations for the pattern loading: • Pattern 1 - Dead + Full Service Span 1 + 50% Service Span 2 + Live Span 1, and
• Pattern 2 - Dead + 50% Service Span 1 + Full Service Span 2 + Live Span 2. You can review the analysis results for these two combinations immediately.
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Chapter 1 : Introduction and application
If you want to review the design results, then you need to re-perform the design. This time the design is virtually instantaneous, General Beam simply checks the existing section, since after the first design it automatically changed from designing sections to checking them.
As expected it is the deflections that are affected by the pattern loading, and the shears at the ends of the beam (as they apply to the supports).
Design Pass 3 If you click the Design Beam icon again, and then perform the design and you will find that this takes significantly longer than the initial one (Design Pass 1). This is because General Beam now has to work with three combinations, rather than then initial one. The design checks have to thus run three times for each section which General Beam investigates. On completion of the design you will find that you are presented with the same list of acceptable sections as for Design Pass 1.
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Chapter 2 : Scope
Chapter 2
General Beam Engineer’s Handbook page 227
Scope In its simplest form a general beam can be a single member between supports to which it is pinned. It is distinguished from a standard simple beam primarily by the loading it has to resist. It can also be a continuous beam consisting of multiple members that do not, with the exception of the remote ends, transfer moment to the rest of the structure. General beams that share load with columns form part of a rigid moment resisting frame. The design of general beams is carried out for rolled sections only. Currently uniform and non-uniform plated sections (including Fabsec beams), and Westok beams cannot be defined as general beams. Web openings are not permitted. General beams can be connected to supports or to the supporting structure in a number of ways. For the meaning and implementation of the various choices see “Member End Fixity and Supports”. The options are subtly different depending upon whether the general beam is defined in the Building Designer1 or is defined within General Beam directly. Conditions of restraint can be defined in- and out-of-plane for strut buckling and top and bottom flange for lateral torsional buckling (LTB). It is upon these that the buckling checks are based. Where both flanges are provided with LTB restraints at the same position, they are simply considered as top and bottom flange restraints that just happen to be coincident, that is they are not treated as a torsional restraint. This means that, where a beam has one or more pairs of LTB restraints between supports, the checks are set up between supports and not between a support and an internal LTB restraint pair or between internal LTB restraint pairs. When the general beam is an object in the Building Designer the design forces for strength and buckling checks are obtained from analysis of the member using the start forces for the member. These are obtained from the solver results. There can be a difference between the start forces from the Building Designer (analysis of the entire structure) and those obtained within General Beam (analysis of a limited model). Within General Beam a full range of loading is available, from which loadcases and design combinations can be created. General beams can be transferred from Building Designer to General Beam. When a general beam has been transferred from Building Designer in this way its loads and loadcases are editable. However any changes to these will invalidate the start and end forces obtained from the building model. To cater for this, if any load or loadcase is modified, a design in General Beam will reanalyse all the beam’s loadcases. Editing of the design combinations does not require reanalysis since the start and end forces are obtained by superposition.
Footnotes 1. A general beam that is defined in Building Designer is referred to as a Building Designer general beam object.
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Chapter 2 : Scope
A full range of strength and buckling checks are available including Annex G Elastic to G.2.1. As mentioned above the buckling lengths are based on the restraints along the member. The effective lengths to use in the checks depend on: • the type of restraint particularly at supports,
• whether the loads or one component of the loads is destabilizing, • whether the frame is sway or non-sway in one or both directions – this has little effect on beam design. In all cases, General Beam sets the default effective length to 1.0L, it does not attempt to adjust the effective length (between supports for example) in any way. You are expected to adjust the effective length factor (up or down) as necessary. Any strut or LTB effective length can take the type ‘Continuous’ to indicate that it is continuously restrained over that length. Each span of a continuous beam can be of different section size, type and grade. The entire beam can be set to automatic design or check design.
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Chapter 3 : Limitations and Assumptions
Chapter 3
General Beam Engineer’s Handbook page 229
Limitations and Assumptions
Limitations The following limitations apply: • composite beams are excluded,
• continuous general beams (more than one span) must be co-linear in the plane of the web within a small tolerance (sloping in elevation is allowed),
• web openings, plated sections including Fabsec beams (with or without openings) and Westok beams are all excluded,
• sections with unequal flanges are excluded. This includes plated section beams that have unequal flanges, Slimflor beams and asymmetric Slimflor beams,
• there can be a difference in analysis results between those from the Building Designer (analysis of the entire structure) and those when run in stand-alone (analysis of a limited model),
• there is no automatic generation of pattern loads either in the stand-alone or in Building Designer.
Assumptions All supports are considered to provide torsional restraint, that is lateral restraint to both flanges. This cannot be changed. It is assumed that a beam that is continuous through the web of a supporting beam or column together with its substantial moment resisting end plate connections is able to provide such restraint. If, at the support, the beam oversails the supporting beam or column then the detail is assumed to be such that the bottom flange of the general beam is well connected to the supporting member and, as a minimum, has torsional stiffeners provided at the support to Clause 4.5.7 of BS 5950-1: 2000. In the Building Designer model, when not at supports, coincident restraints to both flanges are assumed when one or more members frame into the web of the general beam at a particular position and the cardinal point of the centre-line model of the general beam lies in the web. Otherwise, only a top flange or bottom flange restraint is assumed. Intermediate lateral restraints to the top or bottom flange are assumed to be capable of resisting the forces given in Clause 4.3.2.2 of BS 5950-1: 2000 and transferring these back to an appropriate system of bracing or suitably rigid part of the structure. Members that provide restraint to major or minor axis strut buckling are assumed to be capable of resisting 1% of the axial force in the restrained member and of transferring this to adjacent points of positional restraint as given in Clause 4.7.1.2 of BS 5905-1: 2000. It is assumed that you will make a rational and ‘correct’ choice for the effective lengths between restraints for both LTB and strut buckling. The default value for the effective length factor of 1.0 may be neither correct nor safe.
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General Beam Engineer’s Handbook page 230
Chapter 4
Chapter 4 : Analysis
Analysis
Building Modeller object The member end forces for each unfactored loadcase are obtained by submitting the whole model from the Building Designer to the solver. For a general beam in General Beam an appropriate sub-model is sent to the solver. The results from the Building Designer and those from General Beam may not be exactly the same due to (potential) differences inherent in using the full- and sub-model.
General Beam The capacity or resistance is only calculated when an applied force exists about the relevant axis that is greater than the “ignore forces below” value you have specified.
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Chapter 5 : Ultimate Limit State – Strength
Chapter 5
General Beam Engineer’s Handbook page 231
Ultimate Limit State – Strength The checks relate to doubly symmetric prismatic sections (that is rolled I- and H-sections), to singly symmetric sections i.e. channel sections and to doubly symmetric hollow sections i.e. SHS, RHS and CHS. Other section types are not currently covered. The strength checks relate to a particular point on the member and are carried out at 20th points and ‘points of interest’.
Classification General — The classification of the cross section is in accordance with BS 5950-1: 2000. General beam can be classified as: • Plastic Class = 1
• Compact • Semi-compact • Slender
Class = 2 Class = 3 Class = 4
Class 4 sections are not allowed. Sections with a Class 3 web can be taken as Class 2 sections (Effective Class 2) providing the cross section is equilibrated to that described in Clause 3.5.6 where the section is given an ‘effective’ plastic section modulus, Seff. For rolled I and H sections in the UK, this gives no advantage in pure bending since the web d/t is too small. Hence for general beams there is likely to be little advantage in using this approach since the axial loads are generally small, this classification is therefore not implemented. All unacceptable classifications are either failed in check mode or rejected in design mode. Hollow sections — The classification rules for SHS and RHS relate to “hot-finished hollow sections” only (“cold-formed hollow sections” are not included in this release).
Important Note 1. The classification used to determine Mb is based on the maximum axial compressive load in the relevant segment length. Furthermore, the Code clearly states that this classification should (only) be used to determine the moment capacity and lateral torsional buckling resistance to Clause 4.2 and 4.3 for use in the interaction equations. Thus, when carrying out the strength checks, the program determines the classification at the point at which strength is being checked.
Shear Capacity The shear check is performed according to BS 5950-1: 2000 Clause 4.2.3. for the absolute value of shear force normal to the x-x axis (Fvx) and normal to the y-y axis (Fvy), at the point under consideration.
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Chapter 5 : Ultimate Limit State – Strength
Shear buckling — When the web slenderness exceeds 70ε shear buckling can occur in rolled sections. There are very few standard rolled sections that breach this limit. General Beam will warn you if this limit is exceeded, but will not carry out any shear buckling checks.
Moment Capacity The moment capacity check is performed according to BS 5950-1: 2000 Clause 4.2.5 for the moment about the x-x axis (Mx) and about the y-y axis (My), at the point under consideration. The moment capacity can be influenced by the magnitude of the shear force (“low shear” and “high shear” conditions). The maximum absolute shear to either side of a point load is examined to determine the correct condition for the moment capacity in that direction.
Note Not all cases of high shear in two directions combined with moments in two directions along with axial load are considered thoroughly by BS 5950-1: 2000. The following approach is adopted by General Beam: • if high shear is present in one axis or both axes and axial load is also present, the cross-section capacity check is given a Beyond Scope status. The message associated with this status is “High shear and axial load are present, additional hand calculations are required for cross-section capacity to Annex H.3”. General Beam does not perform any calculations for this condition.
• if high shear and moment is present in both axes and there is no axial load (“biaxial bending”) the cross-section capacity check is given a Beyond Scope status and the associated message is, “High shear present normal to the y-y axis, no calculations are performed for this condition.”
• if high shear is present normal to the y-y axis and there is no axial load, the y-y moment check and the cross-section capacity check are each given Beyond Scope statuses. The message associated with this condition is, “High shear present normal to the y-y axis, no calculations are performed for this condition.”
Axial Capacity The axial capacity check is performed according to BS 5950-1: 2000 Clause 4.6.1 using the gross area and irrespective of whether the axial force is tensile or compressive. This check is for axial compression capacity and axial tension capacity. Compression resistance is a buckling check and as such is considered under “Compression Resistance”.
Cross-section Capacity The cross-section capacity check covers the interaction of axial load and bending to Clause 4.8.2 and 4.8.3.2 appropriate to the type (for example – doubly symmetric) and classification of the section. Since the axial tension capacity is not adjusted for the area of the net section then the formulae in Clause 4.8.2.2 and 4.8.3.2 are the same and can be applied irrespective of whether the axial load is compressive or tensile. The Note in “Moment Capacity” also applies here.
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Chapter 6 : Ultimate Limit State – Buckling
Chapter 6
General Beam Engineer’s Handbook page 233
Ultimate Limit State – Buckling
Lateral Torsional Buckling Resistance, Clause 4.3 For beams that are unrestrained over part or all of a span, a Lateral Torsional Buckling (LTB) check is required either: • in its own right, Clause 4.3 check,
• as part of an Annex G check, • as part of a combined buckling check to 4.8.3.3.1, 4.8.3.3.2 or 4.8.3.3.3, (see “Member Buckling Resistance, Clause 4.8.3.3.1”, “Member Buckling Resistance, Clause 4.8.3.3.2”, and “Member Buckling Resistance, Clause 4.8.3.3.3”, respectively) This check is not carried out under the following circumstances: • when bending exists about the minor axis only,
• when the section is a CHS or SHS, • when the section is an RHS that satisfies the limits given in Table 15 of BS 5950-1: 2000. For sections in which LTB cannot occur (the latter two cases above) the value of Mb for use in the combined buckling check is taken as the full moment capacity, Mcx, not reduced for high shear in accordance with Clause 4.8.3.3.3 (c), equation 2 (See “Member Buckling Resistance, Clause 4.8.3.3.3”). Effective lengths — The value of effective length factor is entirely at your choice. The default value is 1.0 for ‘normal’ loads and 1.2 for ‘destabilizing loads’. Different values can apply in the major and minor axis.
Lateral Torsional Buckling Resistance, Annex G This check is applicable to I- and H-sections with equal or unequal1 flanges. The definition of this check is the out-of-plane buckling resistance of a member or segment that has a laterally unrestrained compression flange and the other flange has intermediate lateral restraints at intervals. It is used normally to check the members in portal frames in which only major axis moment and axial load exist. Although not stated explicitly in BS 5950-1: 2000, it is taken that the lateral torsional buckling moment of resistance, Mb, from the Annex G check can be used in the interaction equations of Clause 4.8.3.3 (combined buckling). Since this is not explicit within BS 5950-1: 2000 a slight conservatism is introduced. In a straightforward Annex G check the axial load is combined with major axis moment. In this case both the slenderness for lateral torsional buckling and the slenderness for compression buckling are modified to allow for the improvement provided by the tension flange restraints (λLT replaced by λTB and λ replaced by λTC). When performing a combined buckling check in accordance with 4.8.3.3 the improvement is taken into account in determining the buckling Footnotes 1. Unequal flanged sections are not currently included.
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Chapter 6 : Ultimate Limit State – Buckling
resistance moment but not in determining the compression resistance. If the incoming members truly only restrain the tension flange, then you should switch off the minor axis strut restraint at these positions. The original source research work for the codified approach in Annex G used test specimens in which the tension flange was continuously restrained. When a segment is not continuously restrained but is restrained at reasonably frequent intervals it can be clearly argued that the approach holds true. With only one or two restraints present then this is less clear.BS 5950-1: 2000 is clear that there should be “at least one intermediate lateral restraint” (See Annex G.1.1). Nevertheless, you are ultimately responsible for accepting the adequacy of this approach. For this check General Beam sets mt to 1.0 and calculates nt. The calculated value of nt is based on Mmax being taken as the maximum of M1 to M5, and not the true maximum moment value where this occurs elsewhere in the length. The effect of this approach is likely to be small. If at any of points 1 - 5, R >11, then General Beam sets the status of the check to Beyond Scope. Reference restraint axis distance, a — The reference restraint axis distance is measured between some reference axis on the restrained member - usually the centroid - to the axis of restraint - usually the centroid of the restraining member. The measurement is shown diagramatically in Figure G.1 of BS 5950-1: 2000. General Beam does not attempt to determine this value automatically, since such an approach is fraught with difficulty and requires information from you which is only used for this check. Instead, by default, General Beam uses half the depth of the restrained section, and you can specify a value to be added to, or subtracted from, this at each restraint point. You are responsible for specifying the appropriate values for each restraint position. The default value of 0 mm may be neither correct nor safe.
Compression Resistance For most structures, all the members resisting axial compression need checking to ensure adequate resistance to buckling about both the major- and minor-axis. Since the axial force can vary throughout the member and the buckling lengths in the two planes do not necessarily coincide, both are checked. Because of the general nature of a general beam, it may not always be safe to assume that the combined buckling check will always govern. Hence the compression resistance check is performed independently from the other strength and buckling checks. Effective lengths — The value of effective length factor is entirely at your choice. The default value is 1.0 for ‘normal’ loads and 1.2 for ‘destabilizing loads’. Different values can apply in the major and minor axis. Beams are less affected by sway than columns but the effectiveness of the incoming members to restrain the beam in both position and direction is generally less than for columns. Hence, it is less likely that effective length factors greater than 1.0 will be required but equally factors less than 1.0 may not easily be justified. Nevertheless, it is your responsibility to adjust the value from 1.0 and to justify such a change.
Footnotes 1. Which could happen since R is based on Z and not S.
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Chapter 6 : Ultimate Limit State – Buckling
General Beam Engineer’s Handbook page 235
Of more importance in beam design is the possible existence of destabilizing loads. These can affect the effective length for lateral torsional buckling (see “Lateral Torsional Buckling Resistance, Clause 4.3”). Please note that the requirements for slenderness limits in (for example l/r ≤ 180) are no longer included in BS 5950-1: 2000. Consequently General Beam does not carry out such checks. Accordingly, for lightly loaded members you should ensure that the slenderness ratio is within reasonable bounds to permit handling and erection and to provide a reasonable level of robustness.
Member Buckling Resistance, Clause 4.8.3.3.1 This check is used for channel sections. Such sections can be Class 1, 2 or 3 Plastic, Compact or Semi-compact (Class 4 Slender sections and Effective Class 2 sections are not allowed in this release). Note that, whilst this check could be used for any section type dealt with in the subsequent sections, the results can never be any better than the alternatives but can be worse. Two formulae are provided in Clause 4.8.3.3.1, both are checked; the second is calculated twice – once for the top flange and once for the bottom flange. See also the Important Note at the end of “Member Buckling Resistance, Clause 4.8.3.3.2”. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero. If this check is invoked as part of an Annex G check, and thus Mb is governed by Annex G, then mLT is taken as 1.0.
Member Buckling Resistance, Clause 4.8.3.3.2 This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact rolled I- and H-sections with equal flanges (Class 4 Slender sections and Effective Class 2 sections are not included in this release). Three formulae are provided in Clause 4.8.3.3.2 (c) to cover the combined effects of major and minor axis moment and axial force.These are used irrespective of whether all three forces/ moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, Clause 4.8.3.3.2 (c) can also be used in such cases by setting the axial force to zero. All three formulae in Clause 4.8.3.3.2 (c) are checked; the second is calculated twice – once for each flange. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero.
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Chapter 6 : Ultimate Limit State – Buckling
Important Note 1. Clause 4.8.3.3.4 defines the various equivalent uniform moment factors. The last three paragraphs deal with modifications to these depending upon the method used to allow for the effects of sway. This requires that for sway sensitive frames the uniform moment factors, mx, my and mxy, should be applied to the non-sway moments only. In this release there is no mechanism to separate the sway and non-sway moments, General Beam adopts the only conservative approach and sets these 'm' factors equal to 1.0 if the frame is sway sensitive (in either direction). This is doubly conservative for sway-sensitive unbraced frames since it is likely that all the loads in a design combination and not just the lateral loads will be amplified. In such a case, both the sway and non-sway moments are increased by kamp and neither are reduced by the above ‘m’ factors. The calculation of mLT is unaffected by this approach, and thus if the second equation of Clause 4.8.3.3.2 (c) governs, then the results are not affected.
Member Buckling Resistance, Clause 4.8.3.3.3 This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact hollow sections (Class 4 Slender sections and Effective Class 2 sections are not included in this release). Four formulae are provided in Clause 4.8.3.3.3 (c) to cover the combined effects of major and minor axis moment and axial force. These are used irrespective of whether all three forces/ moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, Clause 4.8.3.3.3 (c) can also be used in such cases by setting the axial force to zero. The second and third formulae are mutually exclusive – that is the second is used for CHS, SHS and for RHS when the limits contained in Table 15 are not exceeded. On the other hand the third formula is used for those RHS that exceed the limits given in Table 15. Thus only three formulae are checked; the first, second and fourth or the first, third and fourth. Either the second or third (as appropriate) is calculated twice – once for each ‘flange’. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero. See also the Important Note at the end of “Member Buckling Resistance, Clause 4.8.3.3.2”.
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Chapter 7 : Serviceability Limit State
Chapter 7
General Beam Engineer’s Handbook page 237
Serviceability Limit State For general beams, the deflection profile along the member is established based on the start slope of the member derived from the joint rotation, in the appropriate direction for the member under consideration. For beams, it is the in-plane deflections that are of most interest. However, both in-plane and out-of-plane deflections are given – ‘local z’ and ‘local y’ deflections respectively. Results are given for all ‘Dead’ loads, all ‘Imposed’ loads and for ‘Total’ loads in a particular design combination. In all cases these are the sum of the deflections for each appropriate unfactored loadcase, that is the load factor is taken as 1.0. Where appropriate the maximum deflection for both the positive and negative local directions is given and compared with the limits specified in the Design Wizard. This comparison is only made for major axis deflections (local z) in the current release. In the stand-alone of General Beam the graphic will show deflection in the ‘local z’ and ‘local y’ for both individual loadcases and for design combinations based on unfactored loadcases.
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General Beam Engineer’s Handbook page 238
Chapter 8
Chapter 8 : Member End Fixity and Supports
Member End Fixity and Supports In order to provide a robust design model, the fixity at member ends and the associated supporting structure or supports to ground must be compatible with the type of connection, base and foundation that is to be used.
General Beam Stand-alone Internal supports are designated as Continuous and you cannot edit these. At the remote ends of the beam there are a number of options for the combined end fixity and support conditions. These are given below: • Free end — as in a cantilever,
• Simple connection — pinned to the support or supporting member. This means pinned about the major and minor axes of the section but fixed torsionally,
• Moment connection — major axis moment connection, and pinned about the minor axis. This option requires the size and length to the point of contraflexure of the columns above and below the connection,
• Fully fixed — encastré, all degrees of freedom fixed.
Building Designer End fixity in continuous beams — Whilst in the stand-alone program member end fixity and supports are dealt with as one entity, in the Building Designer supports are a separate issue and hence are dealt with separately below. All internal connections are considered Continuous – if a pin were to be introduced at an internal position then there would be two beams, hence you cannot edit this setting. At the remote ends of the beam there are a number of options for the end fixity depending upon to what the end of the beam is connected. These are: • If not connected to a beam or column or to a supplementary support –
• Free end (default!) • If connected to an existing member – • Simple connection (default) • Moment connection • If connected to a Supplementary Support – • Simple connection (default) • Fully fixed. The interpretation of these descriptions in relation to being pinned about a particular axis is the same as in “General Beam Stand-alone”.
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Chapter 8 : Member End Fixity and Supports
General Beam Engineer’s Handbook page 239
Moment connections to supporting beams at the remote ends of general beams are prevented. Similarly, for such connections to the web of an I/H section column or to the face of a hollow section column. If you attempt to use such a connection General Beam issues a warning message. This is to draw your attention to the difficulty and cost of making such a connection and, perhaps more importantly, to the possibility that such a joint will not behave as fully rigid.
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Chapter 9
Chapter 9 : Design Procedure
Design Procedure
Lateral torsional buckling checks The process for carrying out LTB checks is determined by whether the beam has intermediate restraints to the top or bottom flange, or both. The impact of one or more of the segment lengths being continuously restrained is also considered. The principal check is that given in Clause 4.3 of BS 5950-1: 2000 but in certain circumstances an Annex G.2 check is also carried out between torsional restraints. You have full control of whether at a particular position one or both flanges are restrained. Restraint to both flanges that are coincident will be taken as torsional1. More information on restraints and the LTB check itself are given in “Assumptions” and “Lateral Torsional Buckling Resistance, Clause 4.3” respectively. Section types that are not susceptible to LTB for example circular and square hollow sections are not processed. General Beam identifies the relevant checks and the lengths over which these checks are performed – these lengths are termed ‘segment lengths’. There is a segment length for each Clause 4.3 check and each Annex G.2 check. For each individual check the following are determined within the segment length: • maximum moment, Mx,
• uniform moment factor, mLT, based on the moment profile – Clause 4.3 only, • slenderness correction factor, nt, based on the moment ratios – Annex G.2 only. The check process generates a set of checks and their associated segment lengths in accordance with Clause 4.32. As part of the Annex G check each segment length between restraints to the top and bottom flanges is also checked to Clause 4.3 separately3. This is
Footnotes 1. In this release, such ‘torsional restraints’ are simply considered as top and bottom flange restraints that just happen to be coincident. This means that, where a beam has one or more ‘torsional restraints’ between supports, the checks are set up between supports and not between a support and an internal torsional restraint or between internal torsional restraints. 2. These are referred to as ‘proper’ 4.3 checks. 3. These are referred to as ‘Annex G’ 4.3 checks.
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Chapter 9 : Design Procedure
General Beam Engineer’s Handbook page 241
irrespective of whether these restraints are to the compression or tension flange. This can result in checks over the same length or different lengths to the ‘proper’ Clause 4.3 checks. An example is given below. ‘proper’ 4.3 ‘Annex G’ 4.3
‘proper’ 4.3
‘Annex G’ 4.3
‘Annex G’ 4.3
points of contraflexure
‘Annex G’ 4.3
‘Annex G’ 4.3 Annex G
LTB restraints and checks In this example, the ‘proper’ Clause 4.3 checks that are identified are between the torsional restraint and the first intermediate restraint that restrains the top flange when in compression, and, between this restraint and the final torsional restraint. Three checks to the top flange and two to the bottom flange are carried out as part of the Annex G check over the whole length. These in contrast are between restraints that are sometimes to the compression flange and sometimes to the tension flange.
Combined buckling checks From performing LTB and Strut buckling checks there are a series of segments over which the various checks have been carried out. For LTB, there can be a set of these between each pair of torsional restraints – typically but not exclusively between supports (only between supports in the first release). These sets can differ between the top flange and the bottom flange. at each 20th point along each span General Beam determines the segment in which it lies considering LTB of the top flange, LTB of the bottom flange, in-plane strut buckling and out-of plane strut buckling. Also, for each segment General Beam ascertains the following: • an associated effective length,
• a resistance, • the maximum axial load or moment, • and for LTB the moment profile for determining ‘m’ or ‘nt’. For LTB there can be up to three segment lengths for each point, • that associated with a ‘proper’ Clause 4.3 check,
• that associated with an Annex G check, • that associated with intermediate restraint Clause 4.3 check carried out as part of the Annex G check.
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Chapter 9 : Design Procedure
An example illustrating how the checks are applied to I- and H-sections with equal flanges (4.8.3.3.2 (c)) is given below. 1.0
3.5
1.5
points of contraflexure
2.6
3.4
LTB and strut buckling checks The beam (span) is 6.0 m long and has torsional restraints at each end. The top flange is restrained out-of-plane at 1.0 m and 4.5 m – these provide restraint to the top flange for LTB and to the beam as a whole for out-of-plane strut buckling. The bottom flange has one restraint at 2.6 m and this restrains the bottom flange for LTB and the beam as a whole for in-plane strut buckling. (This is probably difficult to achieve in practice but is useful for illustration purposes.) General Beam identifies the following lengths and checks. (in this example all the effective length factors are assumed to be 1.0 for simplicity.)
Top flange segment
Bottom flange segment
In-plane strut segment
Out-of-plan e strut segment
length
check
length
check
length
check
0 – 4.5
Proper 4.3
0 – 6.0
Annex Ga
0 – 2.6
0 – 1.0
4.5 – 6.0
Proper 4.3
0 – 2.6
Annex G 4.3
2.6 – 6.0
1.0 – 4.5
0 – 6.0
Annex Ga
2.6 – 6.0
Annex G 4.3
0 – 1.0
Annex G 4.3
1.0 – 4.5
Annex G 4.3
4.5 – 6.0
Annex G 4.3
a. Only one Annex G is reported – that with the smallest value of nt.
Thursday 26 April 2007 – 10:18
4.5 – 6.0
CSC Fastrak ™
Structural steelwork analysis and design
.fastrak5950.com
HANDBOOK SIMPLE COLUMN
Simple Column Documentation page 244
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
Simple Column Engineer’s Handbook page 245
Simple Column Engineer’s Handbook
Chapter 1
Introduction and application This is design software which allows you to analyse and design a structural steel column which complies with the requirements for simple construction. The column may have incoming simple beams which are capable of providing restraint, and may have splices along its length at which the section size may vary. You are responsible for designing the splices appropriately. You can use Simple Column: • to determine those sections which can withstand the applied loading,
• to check a column of known sizes to determine whether it is able to carry the construction stage and simple stage loading. Unless explicitly stated all calculations in Simple Column are in accordance with the relevant sections of BS 5950-1:2000. You may find the handbook and commentary to the Code of Practice published by the Steel Construction Institute useful.
Practical applications Simple Column can be used both to design and check simple columns. You might find the following procedures useful.
Designing a column In the typical procedure below items in brackets [] are optional.
Step
Icon
Instructions
1
Launch Simple Column,
2
Create a new project giving the project name [and other project details],
3
Choose the type of column as either a Steel Section or a Concrete Filled Section [and give the Column reference details],
4
Set Simple Column into design mode,
5
Define the properties for the column: • the number of floors it carries and either the column lengths between floors, or the floor levels; • the column faces into which beams trim at each level (this can vary on a level by level basis).
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Simple Column Engineer’s Handbook page 246
Step
Icon
Chapter 1 : Introduction and application
Instructions
6
Give the details of the positions of any splices and the restraint provided to the column by the incoming beams at each level.
7
[Specify any alterations to the standard eccentricities to be used to calculate the column’s eccentricity moments.]
8
[Specify the grade of steel to be investigated for each column stack (length from base–splice, splice–splice or splice–top).]
9
Define the loadcases that apply to the simple column.
10
Incorporate the loadcases into a series of design combinations,
11
[Make any Design Wizard settings that you want to use to control the design.]
12
Perform the design. Simple Column shows the first set of adequate sections that it finds for the column.
13
Specify the content of the report [and print it].
14
Save the project to disk.
Checking a column In the typical procedure below items in brackets [] are optional.
Step
Icon
Instructions
1
Launch Simple Column,
2
Create a new project giving the project name [and other project details],
3
Choose the type of column as either a Steel Section or a Concrete Filled Section [and give the Column reference details],
4
Set Simple Column into check mode,
5
Define the properties for the column: • the number of floors it carries and either the column lengths between floors, or the floor levels; • the column faces into which beams trim at each level (this can vary on a level by level basis).
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
Step
Simple Column Engineer’s Handbook page 247
Icon
Instructions
6
Give the details of the positions of any splices and the restraint provided to the column by the incoming beams at each level.
7
[Specify any alterations to the standard eccentricities to be used to calculate the column’s eccentricity moments.]
8
Specify the size of section that you want to check for each column stack (length from base–splice, splice–splice or splice–top). [Specify the grade of steel for each column stack.]
9
Define the loadcases that apply to the simple column.
10
Incorporate the loadcases into a series of design combinations,
11
[Make any Design Wizard settings that you want to use to control the check.]
12
Perform the check. Simple Column shows the results for the sizes you have specified.
13
Specify the content of the report [and print it].
14
Save the project to disk.
Worked Example If you want to work through this example you will find the file Engineer’s Example in the \documents and settings\All Users\Application Data\CSC\Fastrak\Examples folder. You can open and use this file, but you can not save
it away unless you change its name, this is done to protect the original.
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Simple Column Engineer’s Handbook page 248
Chapter 1 : Introduction and application
For an example we shall consider a corner column in a regular multi-storey structure. This has 5 regularly spaced floors so that each lift of column is 4 m as shown below.
The loading transmitted to the column by the incoming beams is:
Condition
Major Axis
Minor Axis
Dead
71 kN
44 kN
Live
44 kN
32 kN
This loading is defined in two load cases which are combined into a single combination.
Design pass 1 Using the Design Wizard to specify that the design is to look at UC sections only, with all sections available and designing the column yields a single stack of size 254 254 UC 73. Simple Column does not present a list of acceptable sections, since columns with splices already have multiple section size possibilities, which would give an inordinately large number of possibilities. Simple Column thus homes in to the first acceptable solution it finds. What would be the effect of adding splices to split the column into a series of different stacks?
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
Simple Column Engineer’s Handbook page 249
Design pass 2 Switch back to design mode, and then access the Restraints dialog. Add restraints at floor levels 2 and 4.
Perform the design again, and this time you get 3 section sizes:
• 203 203 UC 46 for the top stack, • 203 203 UC 60 for the middle stack, • 254 254 UC 73 for the bottom stack. Design Pass 3 This design is OK, but there is a difference in section depth of 44.5 mm between the 203 203 UC 60 and 254 254 UC 73. You might feel that this amount of shimming is excessive, and ask is it possible to use a heavier weight of a 203 UC to reduce this?
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Simple Column Engineer’s Handbook page 250
Chapter 1 : Introduction and application
Access the Design Wizard, and on the Size Constraints page set a maximum depth of 250 mm.
This means that Simple Column will reject any section whose depth is greater than 250 mm. If no heavier 203 UC section is adequate, then this means that Simple Column’s automatic design process will fail to find an appropriate section. Set automatic design mode and perform the design again. Simple Column now picks a 203 203 UC 86 for the bottom stack of the column. The difference in section depth is now 12.6 mm which is fine for shimming.
Thursday 26 April 2007 – 10:18
Chapter 2 : Design of concrete filled columns
Chapter 2
Simple Column Engineer’s Handbook page 251
Design of concrete filled columns The following information is drawn from A design method for concrete-filled, hollow section, composite columns(Ref. 2). The paper outlines ‘a simple design procedure’ for ‘concrete-filled composite columns suitable for manual calculations, based on the recommendations given in BS 5950 for bare steel columns’.
Proposed method The paper proposes that the properties of the bare steel section be replaced by those of the composite section. BS 5950-1:2000 states that two checks are necessary: • a local capacity check;
• an overall buckling check. For each of these a simplified approach and a more exact approach are given. For the local capacity check, Simple Column adopts the simple approach since the values of Mrx and Mry are not available for composite sections. For the overall buckling check, Simple Column adopts the more exact approach.
Points to Note 1. The partial safety factor for steel, γs, is taken as 1.0. 2. As not all hollow sections are plastic or compact, the values of B--t- and D---t are checked. If either value is greater than fifty the status of the check is set to invalid. 3. It is assumed that lateral torsional buckling does not occur. To ensure this is the case, Simple Column checks Table 15 of BS 5950-1:2000. If the section is within the specified limits then the above assumption is valid. Otherwise a warning message will be given indicating that the particular configuration is susceptible to lateral torsional buckling but that this has not been taken into account in the design. 4. The bonding strength between the hollow section and the concrete infill is not checked. 5. The concrete grade is limited to be between C25 and C50.
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Simple Column Engineer’s Handbook page 252
Chapter 3
Chapter 3 : References and further information
References and further information
References 1. British Standards Institution. BS 5950 : Structural use of steelwork in building; Part 1. Code of practice for design in simple and continuous construction: hot rolled sections. BSI 2000. 2. The Structural Engineer. Volume 75/No.21. A design method for concrete-filled, hollow section, composite columns. Y.C Wang and D.B. Moore. 1997.
Thursday 26 April 2007 – 10:18
CSC Fastrak ™
Structural steelwork analysis and design
.fastrak5950.com
HANDBOOK GENERAL COLUMN
General Column Documentation page 254
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
General Column Engineer’s Handbook page 255
General Column Engineer’s Handbook
Chapter 1
Introduction and application This is design software which allows you to analyse and design a structural steel column which can have moment or simple connections with incoming member, and which can have fixity applied at the base. The column can have incoming beams which may also be capable of providing restraint, and may have splices along its length at which the section size may vary. You are responsible for designing the splices appropriately. You can use General Column: • to determine those sections which can withstand the applied loading,
• to check a column of known sizes to determine whether it is able to carry the construction stage and simple stage loading. Unless explicitly stated all calculations in General Column are in accordance with the relevant sections of BS 5950-1:2000. You may find the handbook and commentary to the Code of Practice published by the Steel Construction Institute useful.
Practical applications General Column can be used both to design and check general columns. You might find the following procedures useful.
Designing a column In the typical procedure below items in brackets [] are optional.
Step
Icon
Instructions
1
Launch General Column,
2
Create a new project giving the project name [and other project details],
3
Give the Column reference details,
4
Set General Column into design mode,
E:\CSC Doc\General Column\General column handbook.fm
General Column Engineer’s Handbook page 256
Step
Icon
Chapter 1 : Introduction and application
Instructions
5
Define the properties for the column: • the number of levels it carries and either the column lengths between floors, or the floor levels themselves; • the positions of any splices • whether a particular level is to be counted as a supported level in the calculation of imposed load reductions, and • whether the imposed loads on the level may be decreased by the appropriate imposed load reduction factor.
6
Add construction levels to the column and for both floors and construction levels define the column faces into which members trim at each floor or level (this can vary on a floor/level by floor/level basis).
7
Define the releases to the top and bottom of each lift of the column.
8
Give the details of the lateral torsional buckling restraint provided to the column flanges by the incoming steelwork at each floor or level.
9
Give the details of the strut buckling restraint provided to the column flanges by the incoming steelwork at each floor or level.
10
[Specify the grade of steel to be investigated for each column stack (length from base–splice, splice–splice or splice–top).]
11
[Specify any alterations to the standard eccentricities to be used to calculate the column’s eccentricity moments.]
12
Define the loadcases that apply to the general column.
13
Incorporate the loadcases into a series of design combinations,
14
[Make any Design Wizard settings that you want to use to control the design.]
15
Perform the design. General Column shows the first set of adequate sections that it finds for the column.
16
Specify the content of the report [and print it].
17
Save the project to disk.
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
General Column Engineer’s Handbook page 257
Checking a column In the typical procedure below items in brackets [] are optional.
Step
Icon
Instructions
1
Launch General Column,
2
Create a new project giving the project name [and other project details],
3
Give the Column reference details,
4
Set General Column into check mode,
5
Define the properties for the column: • the number of levels it carries and either the column lengths between floors, or the floor levels themselves; • the positions of any splices • whether a particular level is to be counted as a supported level in the calculation of imposed load reductions, and • whether the imposed loads on the level may be decreased by the appropriate imposed load reduction factor.
6
Add construction levels to the column and for both floors and construction levels define the column faces into which members trim at each floor or level (this can vary on a floor/level by floor/level basis).
7
Define the releases to the top and bottom of each lift of the column.
8
Give the details of the lateral torsional buckling restraint provided to the column flanges by the incoming steelwork at each floor or level.
9
Give the details of the strut buckling restraint provided to the column flanges by the incoming steelwork at each floor or level.
10
Specify the section size and grade of steel to be investigated for each column stack (length from base–splice, splice–splice or splice–top).
11
[Specify any alterations to the standard eccentricities to be used to calculate the column’s eccentricity moments.]
12
Define the loadcases that apply to the general column.
13
Incorporate the loadcases into a series of design combinations,
14
[Make any Design Wizard settings that you want to use to control the design.]
15
Perform the design. General Column shows the first set of adequate sections that it finds for the column.
E:\CSC Doc\General Column\General column handbook.fm
General Column Engineer’s Handbook page 258
Step
Icon
Chapter 1 : Introduction and application
Instructions
16
Specify the content of the report [and print it].
17
Save the project to disk.
Worked Example If you want to work through this example you will find the file Engineer’s Example in the \documents and settings\All Users\Application Data\CSC\Fastrak\Examples folder. You can open and use this file, but you can not save
it away unless you change its name, this is done to protect the original. For an example we shall consider a corner column in a regular multi-storey structure. This has 5 regularly spaced floors supporting profiled metal decking and a concrete slab. Each lift of column is 4 m as shown below.
This general column forms part of a moment resisting frame in a structure. The forces and moments that the column has to resist are thus dependent on the other members of the frame, and to a lesser extent on the rest of the structure. These forces and moments are thus not determined by loading on the column itself directly, but are calculated by a 3D frame analysis, and applied to the column directly.
Design pass 1 Using the Design Wizard to specify that the design is to look at UC sections only, with all sections available and designing the column yields a single stack of size 305 305 UC 97.
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
General Column Engineer’s Handbook page 259
General Column does not present a list of acceptable sections, since columns with splices already have multiple section size possibilities, which would give an inordinately large number of possibilities. General Column thus homes in to the first acceptable solution it finds.
If you look at the summary of results, you will see that it is the Combined Buckling check that controls the design. (It has the highest Capacity Ratio.) If you review the restraints details for the column you will find that General Column shows strut buckling restraints applied at each floor level, but that lateral torsional buckling restraint is only provided at the top and bottom of the column.
This is intentional since Building Designer, and hence General Column does not know whether the incoming beams are capable of providing lateral torsional buckling restraint to either or both flanges of the column, since this depends on factors such as the connections you provide, the floor construction and such like. For this example the incoming beams and floor construction do provide restraint against lateral torsional buckling of both flanges at each floor.
E:\CSC Doc\General Column\General column handbook.fm
General Column Engineer’s Handbook page 260
Chapter 1 : Introduction and application
Design pass 2 Switch back to design mode, and then access the Restraints dialog. Add lateral torsional buckling restraints at all floor levels.
Perform the design again, and this time the column size drops to a 254 254 UC 89.
Design Pass 3 Again switch to automatic design and add splices at floors 2 and 4. A redesign now yields 3 section sizes: • 203 203 UC 46 for the top stack,
• 203 203 UC 71 for the middle stack, • 254 254 UC 89 for the bottom stack.
It is extremely important to note that the initial change of section size, and the latter change to a three stack column have had no effect whatsoever on the design forces that the column has been designed for. These are locked in from the Building Designer analysis. The changes you
Thursday 26 April 2007 – 10:18
Chapter 1 : Introduction and application
General Column Engineer’s Handbook page 261
have made here in General Column will affect your Building Designer model, and you will need to investigate these. In practice you would need to return your changed column to Building Designer and re-perform a design check (which includes a reanalysis of your changed structure) to determine the actual effects both on this column and on the rest of your model. This investigation is beyond the scope of this example, however in practice, after a design check in Building Designer it was found that the sections calculated above were not satisfactory.
E:\CSC Doc\General Column\General column handbook.fm
General Column Engineer’s Handbook page 262
Chapter 2
Chapter 2 : Scope
Scope In its simplest form a general column can be a single member between ‘construction levels’ that are designated as floors. It is distinguished from a standard simple column primarily by the loading it has to resist and by the use of a more rigorous approach to design than the formulae adopted for columns in simple construction. More typically a general column will be continuous past one or more floor levels, the whole forming one single entity typically from base to roof. General columns that share moments with general beams form part of a rigid moment resisting frame. The design of general columns is carried out for I-sections, H-sections and hollow sections only. Concrete filled hollow sections are not permitted. The top and bottom of each stack in the general column can be either pinned or fixed which means that the member is pinned about both the major and minor axes in the first case, or fixed about both axes in the second. The floors that define the stacks can be designated either as to be or not to be included in the determination of the imposed load reductions through a “Count as supported” check box. This feature enables what appears to be a roof to be counted as a floor, or conversely allows a mezzanine floor to be excluded from the number of floors considered for any particular general column. Also, floors can be designated to not have their imposed loads reduced, for example if they are storage or plant floors. In this case the full loading on that floor will be used in determining the reactions onto the column. The moments from fixed ended beams framing into a column are never reduced. Splices are allowed at floor levels only and must be placed at changes of angle between two adjacent stacks and at changes of section size or type. A validation error will result if this is not the case. You must detail the splice to resist the applied forces and moments. The detail should provide continuity of stiffness and strength or if designated as pinned in analysis terms be capable of acting as such i.e. has only nominal moment capacity and sufficient rotation capacity. Design forces are obtained from the Building Designer. Individual loads. loadcases and combinations can only be added through the Building Designer. The design combinations can be edited since the start and end forces are obtained by superposition. You may find this useful for re-combining the basic loadcases to account for pattern loading although the constituent loadcases would need to be separated in the building model. The (subtle) difference between a column acting as a beam-column and a beam acting as a beam-column is the predominance of axial force in the former. Thus the main design criteria are those given in Clause 4.8 of BS 5950-1: 2000 (although individual capacity and bucking checks are also carried out). Restraints to strut buckling are determined from the incoming members described within the Building Designer. The buckling checks are based on these. Restraining members framing into either Face A or C will provide restraint to major axis strut buckling. Members framing into
Thursday 26 April 2007 – 10:18
Chapter 2 : Scope
General Column Engineer’s Handbook page 263
either Face B or D will provide restraint to minor axis strut buckling. Building Designer determines the strut buckling restraints and you cannot edit these. You have complete control on the settings for the lateral torsional buckling (LTB) restraint to the flanges on Faces A and C. The default is blank so you are forced to decide whether a particular configuration of incoming members can provide LTB restraint. General Column always assumes full restraint at the base and at the roof level when carrying out LTB design checks – you are warned on validation if your LTB restraint settings do not reflect this. Restraints are considered effective on a particular plane providing they are within ±45° to the local coordinate axis system. A full range of strength and buckling checks are available including Annex G Elastic to G.2.1. As mentioned above the buckling lengths are based on the restraints along the member. The effective lengths to use in the checks depend on: • the type of restraint particularly at floor levels,
• whether the frame is sway or non-sway in one or both directions – this has a significant effect on the choice of effective length factor. Non-sway effective length factors are likely to vary from 0.7 to 1.0 whereas for sway directions the variation could be from 1.0 to infinity! (see Table 22 and Annex E of BS 5950-1: 2000). In all cases General Column sets the default effective length to 1.0L, it does not attempt to adjust the effective length in any way. You are expected to adjust the effective length factor (up or down) as necessary. Any strut or LTB effective length can take the type ‘Continuous’ to indicate that it is continuously restrained over that length. Each lift (length between splices) of a general column can be of different section size and grade. Different section types within the same column are not allowed due to the particularly complex design routines that general columns require. You are responsible for guaranteeing that the splice detail ensures that the assumptions in the analysis model are achieved and that any difference in the size of section between lifts can be accommodated practically. The entire column can be set to automatic design or check design.
E:\CSC Doc\General Column\General column handbook.fm
General Column Engineer’s Handbook page 264
Chapter 3
Chapter 3 : Limitations and Assumptions
Limitations and Assumptions
Limitations The following limitations apply: • the web of each stack of a sloping columns must lie in the same plane,
• concrete filled hollow section columns are excluded, • sections with unequal flanges are excluded. This includes plated section beams that have unequal flanges,
• there is no automatic generation of pattern loads.
Assumptions 1. The program assumes that any member framing into the major or minor axis of the column provides restraint against strut buckling in the appropriate plane. These restraints are set and cannot be changed. If you believe that a certain restraint in a particular direction is not effective then you can adjust the effective length to suit – to 2.0L for example. On the other hand, the program does not set any restraints for Lateral Torsional Buckling (LTB) although, irrespective of your setting the program assumes that the base and top of the column are restrained torsionally. 2. The default values for lateral torsional buckling restraints at floor levels and at construction levels are unchecked for both Face A and C. You are expected to make a rational decision regarding which can be checked based on the configuration of the incoming members and their connections at each level. It should be noted that torsional restraint (restraint to Face A and C) is assumed when applied at a floor level. However at a Construction level such restraints are taken as individual flange restraints that just happen to be coincident and not as a torsional restraint. 3. There are a number of practical conditions that could result in torsional restraint not being provided at floor levels. At construction levels this is even more likely given the likely type of incoming member and its associated type of connection. You must consider the type of connection between the incoming members and the column since these can have a significant influence on the ability of the member to provide restraint to one, none or both column flanges. For example, consider a long fin plate connection for beams framing into the column web where the beam stops outside the column flange tips to ease detailing. The fin plate is very slender and the beam end is remote from the column flanges such that it may not be able to provide any restraint to LTB. The fact that a slab is usually present may mitigate this. 4. Where you provide torsional restraint, it is assumed that you will also provide some system of restraint to both flanges, and that this is taken back to an independent bracing system which is capable of resisting the force couple given in Clause 4.3.3 of BS 5950-1: 2000. An example of such a system is sheeting rails defined at a construction level (they are not floors by definition) with stays to the inside flange of the column, the sheeting rails being taken back in a continuous line to a braced bay and checked for the appropriate restraint forces.
Thursday 26 April 2007 – 10:18
Chapter 3 : Limitations and Assumptions
General Column Engineer’s Handbook page 265
5. Intermediate lateral restraints to the flange on Face A or on Face C are assumed to be capable of resisting the forces given in Clause 4.3.2.2 of BS 5950-1: 2000 and transferring these back to an appropriate system of bracing or suitably rigid part of the structure. 6. Members that provide restraint to major or minor axis strut buckling are assumed to be capable of resisting 1% of the axial force in the restrained member and of transferring this to adjacent points of positional restraint as given in Clause 4.7.1.2 of BS 5905-1: 2000. 7. It is assumed that you will make a rational and ‘correct’ choice for the effective lengths between restraints for both LTB and strut buckling. The default value for the effective length factor of 1.0 may be neither correct nor safe.
E:\CSC Doc\General Column\General column handbook.fm
General Column Engineer’s Handbook page 266
Chapter 4
Chapter 4 : Analysis
Analysis
Building Modeller Object The member end forces for each unfactored loadcase are obtained by submitting the whole model from the Building Designer to the solver. There is a particular difficulty with general columns in that there may exist both ‘real’ moments and eccentricity moments from beam end reactions. The effect of the eccentricity moments can reduce those real moments from frame action that are design critical. However, there are cases where this is not true and so the eccentricity moments are included to prevent over- or under-design due to their presence. General Column checks strength against the maximum moment due to the algebraic sum of real and eccentricity moments in two directions. General Column determines the uniform moment factors for use in the buckling interaction equations only from the profile of real moments and these factors are applied only to the real moments. Note that the eccentricity moments only apply at the ends of the stack and not at intermediate positions.
Thursday 26 April 2007 – 10:18
Chapter 5 : Ultimate Limit State – Strength
Chapter 5
General Column Engineer’s Handbook page 267
Ultimate Limit State – Strength The checks relate to doubly symmetric prismatic sections i.e. rolled I- and H-sections and to doubly symmetric hot-finished hollow sections i.e. SHS, RHS and CHS. Other section types are not currently covered. The strength checks relate to a particular point on the member and are carried out at 10th points and ‘points of interest’.
Classification General — The classification of the cross section is in accordance with BS 5950-1: 2000. General columns can be classified as: • Plastic Class = 1
• Compact • Semi-compact • Slender
Class = 2 Class = 3 Class = 4
Class 4 sections are not allowed. Sections with a Class 3 web can be taken as Class 2 sections (Effective Class 2) providing the cross section is equilibrated to that described in Clause 3.5.6 where the section is given an ‘effective’ plastic section modulus, Seff. This approach is not adopted in the current version of General Column. All unacceptable classifications are either failed in check mode or rejected in design mode. Hollow sections — The classification rules for SHS and RHS relate to “hot-finished hollow sections” only (“cold-formed hollow sections” are not included in this release).
Important Note 1. The classification used to determine Mb is based on the maximum axial compressive load in the relevant segment length. Furthermore, the Code clearly states that this classification should (only) be used to determine the moment capacity and lateral torsional buckling resistance to Clause 4.2 and 4.3 for use in the interaction equations. Thus, when carrying out the strength checks, the program determines the classification at the point at which strength is being checked.
Shear Capacity The shear check is performed according to BS 5950-1: 2000 Clause 4.2.3. for the absolute value of shear force normal to the x-x axis and normal to the y-y axis, Fvx and Fvy, at the point under consideration. Shear buckling — When the web slenderness exceeds 70ε shear buckling can occur in rolled sections. There are very few standard rolled sections that breach this limit. General Column will warn you if this limit is exceeded, but will not carry out any shear buckling checks.
E:\CSC Doc\General Column\General column handbook.fm
General Column Engineer’s Handbook page 268
Chapter 5 : Ultimate Limit State – Strength
Moment Capacity The moment capacity check is performed according to BS 5950-1: 2000 Clause 4.2.5 for the moment about the x-x axis and about the y-y axis, Mx and My, at the point under consideration. The moment capacity can be influenced by the magnitude of the shear force (“low shear” and “high shear” conditions). The maximum absolute shear to either side of a point of interest is used to determine the moment capacity for that direction. High shear condition about x-x axis — The treatment of high shear is axis dependent. In this release for CHS, if high shear is present, the moment capacity about the x-x axis is not calculated, the check is given a Beyond Scope status and an associated explanatory message. High shear condition about y-y axis — For rolled and plated sections in this release, if high shear is present normal to the y-y axis then the moment capacity about the y-y axis is not calculated, the check is given a Beyond Scope status and an associated explanatory message. For hollow sections, there is greater potential for the section to be used to resist the principal moments in its minor axis. Of course for CHS and SHS there is no major or minor axis and so preventing high shear arbitrarily on one of the two principal axes does not make sense. Nevertheless, if high shear is present normal to the y-y axis then in this release the moment capacity about the y-y axis is not calculated, the check is given a Beyond Scope status and an associated explanatory message.
Note Not all cases of high shear in two directions combined with moments in two directions along with axial load are considered thoroughly by BS 5950-1: 2000. The following approach is adopted by General Column: • if high shear is present in one axis or both axes and axial load is also present, the cross-section capacity check is given a Beyond Scope status. The message associated with this status is “High shear and axial load are present, additional hand calculations are required for cross-section capacity to Annex H.3”. General Beam does not perform any calculations for this condition.
• if high shear and moment is present in both axes and there is no axial load (“biaxial bending”) the cross-section capacity check is given a Beyond Scope status and the associated message is, “High shear present normal to the y-y axis, no calculations are performed for this condition.”
• if high shear is present normal to the y-y axis and there is no axial load, the y-y moment check and the cross-section capacity check are each given Beyond Scope statuses. The message associated with this condition is, “High shear present normal to the y-y axis, no calculations are performed for this condition.”
Axial Capacity The axial capacity check is performed according to BS 5950-1: 2000 Clause 4.6.1 using the gross area and irrespective of whether the axial force is tensile or compressive. This check is for axial compression capacity and axial tension capacity. Compression resistance is a buckling check and as such is considered under “Compression Resistance”. L
Thursday 26 April 2007 – 10:18
Chapter 5 : Ultimate Limit State – Strength
General Column Engineer’s Handbook page 269
Cross-section Capacity The cross-section capacity check covers the interaction of axial load and bending to Clause 4.8.2 and 4.8.3.2 appropriate to the type (for example – doubly symmetric) and classification of the section. Since the axial tension capacity is not adjusted for the area of the net section then the formulae in Clause 4.8.2.2 and 4.8.3.2 are the same and can be applied irrespective of whether the axial load is compressive or tensile. The Note in “Moment Capacity” also applies here.
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General Column Engineer’s Handbook page 270
Chapter 6
Chapter 6 : Ultimate Limit State – Buckling
Ultimate Limit State – Buckling
Lateral Torsional Buckling Resistance, Clause 4.3 For columns that are unrestrained over part or all of a span, a Lateral Torsional Buckling (LTB) check is required either: • in its own right, Clause 4.3 check,
• as part of an Annex G check, • as part of a combined buckling check to 4.8.3.3.2 or 4.8.3.3.3, (see “Member Buckling Resistance, Clause 4.8.3.3.2”, and “Member Buckling Resistance, Clause 4.8.3.3.3”). This check is not carried out under the following circumstances: • when bending exists about the minor axis only,
• when the section is a CHS or SHS, • when the section is an RHS that satisfies the limits given in Table 15 of BS 5950-1: 2000. For sections in which LTB cannot occur (the latter two cases above) the value of Mb for use in the combined buckling check is taken as the full moment capacity, Mcx, not reduced for high shear in accordance with Clause 4.8.3.3.3 (c), equation 2 (see “Member Buckling Resistance, Clause 4.8.3.3.3”). Destabilising loads are excluded from General Column, this is justified by the rarity of the necessity to apply such loads to a column. If such loads do occur, then you can adjust the ‘normal’ effective length to take this into account although you can not achieve the code requirement to set mLT to 1.0. Effective lengths — The value of effective length factor is entirely at your choice. The default value is 1.0. Different values can apply in the major and minor axis.
Lateral Torsional Buckling Resistance, Annex G This check is applicable to I- and H-sections with equal or unequal1 flanges. The definition of this check is the out-of-plane buckling resistance of a member or segment that has a laterally unrestrained compression flange and the other flange has intermediate lateral restraints at intervals. It is used normally to check the members in portal frames in which only major axis moment and axial load exist. Although not stated explicitly in BS 5950-1: 2000, it is taken that the lateral torsional buckling moment of resistance, Mb, from the Annex G check can be used in the interaction equations of Clause 4.8.3.3 (combined buckling). Since this is not explicit within BS 5950-1: 2000 a slight conservatism is introduced. In a straightforward Annex G check the axial load is combined with major axis moment. In this case both the slenderness for lateral torsional buckling and the slenderness for compression buckling are modified to allow for the improvement provided by the tension flange restraints Footnotes 1. Unequal flanged sections are not currently included.
Thursday 26 April 2007 – 10:18
Chapter 6 : Ultimate Limit State – Buckling
General Column Engineer’s Handbook page 271
(λLT replaced by λTB and λ replaced by λTC). When performing a combined buckling check in accordance with 4.8.3.3 the improvement is taken into account in determining the buckling resistance moment but not in determining the compression resistance. If the incoming members truly only restrain the tension flange, then you should switch off the minor axis strut restraint at these positions. The original source research work for the codified approach in Annex G used test specimens in which the tension flange was continuously restrained. When a segment is not continuously restrained but is restrained at reasonably frequent intervals it can be clearly argued that the approach holds true. With only one or two restraints present then this is less clear.BS 5950-1: 2000 is clear that there should be “at least one intermediate lateral restraint” (See Annex G.1.1). Nevertheless, you are ultimately responsible for accepting the adequacy of this approach. For this check General Column sets mt to 1.0 and calculates nt. The calculated value of nt is based on Mmax being taken as the maximum of M1 to M5, and not the true maximum moment value where this occurs elsewhere in the length. The effect of this approach is likely to be small. If at any of points 1 - 5, R >11, then General Column sets the status of the check to Beyond Scope. Reference restraint axis distance, a — The reference restraint axis distance is measured between some reference axis on the restrained member - usually the centroid - to the axis of restraint - usually the centroid of the restraining member. The measurement is shown diagramatically in Figure G.1 of BS 5950-1: 2000. General Column does not attempt to determine this value automatically, since such an approach is fraught with difficulty and requires information from you which is only used for this check. Instead, by default, General Column uses half the depth of the restrained section, and you can specify a value to be added to, or subtracted from, this at each restraint point. You are responsible for specifying the appropriate values for each restraint position. The default value of 0 mm may be neither correct nor safe.
Compression Resistance For most structures, all the members resisting axial compression need checking to ensure adequate resistance to buckling about both the major- and minor-axis. Since the axial force can vary throughout the member and the buckling lengths in the two planes do not necessarily coincide, both are checked. Because of the general nature of a column, it may not always be safe to assume that the combined buckling check will always govern. Hence the compression resistance check is performed independently from all other strength and buckling checks. Effective lengths — The value of effective length factor is entirely at your choice. The default value is 1.0. Different values can apply in the major and minor axis. The minimum theoretical value is 0.5 and the maximum infinity for columns in rigid moment resisting (RMR) frames. Practical values for simple columns are in the range 0.7 to 2.0. Values less than 1.0 can be chosen for non-sway frames or for sway frames in which the effects of sway are taken into account using the amplified moments method. However, there is a caveat on the value of effective length factor even when allowance is made for sway. Footnotes 1. Which could happen since R is based on Z and not S.
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General Column Engineer’s Handbook page 272
Chapter 6 : Ultimate Limit State – Buckling
In particular for RMR frames, the principal moments due to frame action preventing sway are in one plane of the frame. There will often be little or no moment out-of-plane and so amplification of these moments has little effect. Nevertheless the stability out-of-plane can still be compromised by the lack of restraint due to sway sensitivity in that direction. In such cases a value of greater then 1.0 (or substantially greater) may be required. Similarly, in simple construction where only eccentricity moments exist, it is only the brace forces that ‘attract’ any amplification. Thus for the column themselves the reduced restraining effect of a sway sensitive structure may require effective length factors greater than 1.0 as given in Table 22 of BS 5950-1: 2000.
Member Buckling Resistance, Clause 4.8.3.3.2 This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact rolled or plated Iand H-sections with equal flanges (Class 4 Slender sections and Effective Class 2 sections are not included in this release). Three formulae are provided in Clause 4.8.3.3.2 (c) to cover the combined effects of major and minor axis moment and axial force.These are used irrespective of whether all three forces/ moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, Clause 4.8.3.3.2 (c) can also be used in such cases by setting the axial force to zero. All three formulae in Clause 4.8.3.3.2 (c) are checked; the second is calculated twice – once for Face A and once for Face C. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero.
Important Notes 1. Clause 4.8.3.3.4 defines the various equivalent uniform moment factors. The last three paragraphs deal with modifications to these depending upon the method used to allow for the effects of sway. This requires that for sway sensitive frames the uniform moment factors, mx, my and mxy, should be applied to the non-sway moments only. In this release there is no mechanism to separate the sway and non-sway moments, General Beam adopts the only conservative approach and sets these 'm' factors equal to 1.0 if the frame is sway sensitive (in either direction). This is doubly conservative for sway-sensitive unbraced frames since it is likely that all the loads in a design combination and not just the lateral loads will be amplified. In such a case, both the sway and non-sway moments are increased by kamp and neither are reduced by the above ‘m’ factors. The calculation of mLT is unaffected by this approach, and thus if the second equation of Clause 4.8.3.3.2 (c) governs, then the results are not affected.
Member Buckling Resistance, Clause 4.8.3.3.3 This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact hollow sections (Class 4 Slender sections and Effective Class 2 sections are not included in this release). Four formulae are provided in Clause 4.8.3.3.3 (c) to cover the combined effects of major and minor axis moment and axial force. These are used irrespective of whether all three forces/ moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, Clause 4.8.3.3.3 (c) can also be used in such cases by setting the axial force to zero.
Thursday 26 April 2007 – 10:18
Chapter 6 : Ultimate Limit State – Buckling
General Column Engineer’s Handbook page 273
The second and third formulae are mutually exclusive – that is the second is used for CHS, SHS and for RHS when the limits contained in Table 15 are not exceeded. On the other hand the third formula is used for those RHS that exceed the limits given in Table 15. Thus only three formulae are checked; the first, second and fourth or the first, third and fourth. Either the second or third (as appropriate) is calculated twice – once for Face C and once for Face A. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero. See also the Important Note at the end of “Member Buckling Resistance, Clause 4.8.3.3.2”.
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General Column Engineer’s Handbook page 274
Chapter 7
Chapter 7 : Serviceability limit state
Serviceability limit state There is an increased importance1 of deflections for buildings containing RMR frames and a requirement for within member deflections for general columns (in-plane and out-of-plane loading allowed). The nodal deflections are established directly from the nodal deflections in the solver and are to be presented as absolute values and relative values. The latter, storey drift, is usually the subject of codified limitations. For general columns, the deflection profile along the member must be established based on the start slope of the member derived from the joint rotation, in the appropriate direction for the member under consideration. The relative deflection of the member can be established in several ways using the start slope and the load or force profile. (Within member deflections not included in the first release.)
Footnotes 1. Compared to Simple Columns.
Thursday 26 April 2007 – 10:18
Chapter 8 : Design Procedure
Chapter 8
General Column Engineer’s Handbook page 275
Design Procedure
Lateral torsional buckling checks The process for carrying out LTB checks is determined by whether the column has intermediate restraints to Face A or Face C or both. The impact of one or more of the segment lengths being continuously restrained is also considered. The principal check is that given in Clause 4.3 of BS 5950-1: 2000 but in certain circumstances an Annex G.2 check is also carried out between torsional restraints. You have full control on whether at a particular position one or both flanges are restrained. Restraint to both flanges that are coincident and occur at a Floor level are taken as torsional, whereas at a Construction level they are simply taken as intermediate restraints to Face A and Face C that just happen to be coincident. More information on restraints and the LTB check itself are given in “Assumptions” and “Lateral Torsional Buckling Resistance, Clause 4.3” respectively. Section types that are not susceptible to LTB e.g. circular and square hollow sections are not processed. General Column identifies the relevant checks and the lengths over which these checks are performed – these lengths are termed ‘segment lengths’. There is a segment length for each Clause 4.3 check and each Annex G.2 check. For each individual check the following are determined within the segment length: • maximum moment, Mx,
• uniform moment factor, mLT, based on the moment profile – Clause 4.3 only, • slenderness correction factor, nt, based on the moment ratios – Annex G.2 only.
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General Column Engineer’s Handbook page 276
Chapter 8 : Design Procedure
The check process generates a set of ‘proper’ Clause 4.3 checks and their associated segment lengths. As part of the Annex G check each segment length between restraints to the top and bottom flanges is checked to Clause 4.3 separately. This is irrespective of whether these restraints are to the compression or tension flange. This can result in checks over the same length or different lengths to the ‘proper’ Clause 4.3 checks. An example is given below. Face A
Floor 2
Annex G
‘Annex G’ 4.3 ‘Annex G’ 4.3
‘proper’ 4.3
‘proper’ 4.3
‘Annex G’ 4.3
Face C
Floor 1
LTB restraints and checks In this example, the ‘proper’ Clause 4.3 checks that are identified are between the torsional restraint and the first intermediate restraint that restrains Face C when in compression, and, between this restraint and the final torsional restraint. Three checks to Face A are carried out as part of the Annex G check over the whole length. These in contrast are between restraints that are to the compression flange in one instance and to the tension flange in another.
Combined buckling checks From performing LTB and Strut buckling checks there are a series of segments over which the various checks have been carried out. For LTB, there can be a set of these between each pair of torsional restraints – typically between floor levels1. However, torsional restraints need not be provided at floor levels i.e. the LTB check can be across floors. The sets of LTB restraints can differ between the flange on Face A and the flange on Face C. Footnotes 1. Torsional restraint can only be applied at floors, but floors do not have to provide torsional restraint.
Thursday 26 April 2007 – 10:18
Chapter 8 : Design Procedure
General Column Engineer’s Handbook page 277
General Column uses the stack length to determine in which segments the stack lies considering LTB of the flange on Face A, LTB of the flange on Face C, minor axis strut buckling and major axis strut buckling. The stack length can be part of a segment length i.e. the segment length extends beyond the stack, can encapsulate one or more segment lengths, or both of these (for instance see Stack 5 in the example given below). For each segment General Column determines the following: • an associated effective length,
• a resistance, • the maximum axial load or moment (in the segment length not the stack length), • and for LTB the moment profile for determining ‘m’ or ‘nt’. For LTB there can be three or more segment lengths for each stack: • that associated with a ‘proper’ Clause 4.3 check,
• that associated with an Annex G check, • that associated with intermediate restraint Clause 4.3 check carried out as part of the Annex G check. An example illustrating how the checks are applied to I- and H-sections with equal flanges (4.8.3.3.2 (c)) is given below.
Floor 2 (13.7 m)
Mx C A
The column is three storeys high (including the construction level at 3.6 m) and hence has three stacks. The incoming members provide the following restraint conditions:
Floor 2
LTB torsional strut major strut minor
Floor 1
LTB Face C strut minor
Construction Level 1.1
LTB Face A strut major strut minor
Base
LTB torsional strut major strut minor
Floor 1 (7.7 m)
CL 1.1 (3.6 m)
LTB and strut buckling restraints
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General Column Engineer’s Handbook page 278
Chapter 8 : Design Procedure
The moment profiles are shown on the figure. Hence the table below shows a definitive set of LTB checks for the entire column. The uniform moment factor (mLT) for LTB checks is based only on the ‘real’ moments i.e. those due to continuity and not those due to eccentricity of the beam end reactions. All effective length factors are assumed to be 1.0 for simplicity. The checks that General Column performs are detailed below:
LTB segment
Stack
2
1
length
check
13.7 – 0.0
Annex G
13.7 – 7.7
4.3 Face C
7.7 – 0.0
4.3 Face C
13.7 – 0.0
Annex G
13.7 – 7.7
4.3 Face C
7.7 – 0.0
4.3 Face C
Minor axis strut segment
Major axis strut segment
length
length
13.7 – 7.7
13.7 – 3.6
7.7 – 3.6
13.7 – 3.6
3.6 – 0.0
3.6 – 0.0
Note: • that some checks (both LTB and strut) have a check length outside of the current stack,
• that a particular stack can incorporate more than one segment length, • that both cases can occur with the same stack. For the LTB Annex G check from 13.7 m to 0 m there will also be a set of clause 4.3 checks between each intermediate restraint for each flange separately. These will repeat some of the ‘proper’ clause 4.3 checks already carried out. Since both stacks are encompassed by the LTB length between torsional restraints, the checks for stack 1 are the same as those for stack 2. Similarly, since both stacks 1 and 2 lie in the major axis strut buckling length from 13.7 m to 3.6 m this check is repeated for both stacks.
Thursday 26 April 2007 – 10:18
Index
Building Designer Documentation page 279
A amplification factor . . . . . . . . . . . . . . . .75 amplified forces method
. . . . . . . . . . . 71, 75, 119
application Composite Beam General Beam . General Column Simple Beam . Simple Column
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
173 221 255 157 245
applying walls and roofs Fastrak Wind Modeller . . . . . . . . . . . . . . 146 Auto-k_amp . . . . . . . . . . . . . . . . . . .77
B bracing vertical cross bracing . . . . . . . . . . . . . . . .62
C check mode composite beam typical procedure general beam typical procedure . general column typical procedure . simple beam typical procedure . . simple column typical procedure .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
175 222 257 158 246
classification composite stage (ULS) . construction stage . . Westok composite stage . Westok construction stage
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
202 201 211 207
. . . .
. . . .
. . . .
member strength (ULS) . . . . natural frequency (SLS) . . . . scope - serviceability limit state (SLS) scope - ultimate limit state (ULS) . section classification (ULS) . . . section properties (SLS) . . . . shear connectors (ULS) . . . . stresses (SLS) . . . . . . .
. . . . . . . .
203 206 190 189 202 205 204 206
Connections moment connections . . . . . . . . . . . . . . . simple connections . . . . . . . . . . . . . . .
61 60
Connections Advisory Note accidental limit state . . . analysis . . . . . . . introduction. . . . . . limitations and assumptions practical applications . . . scope . . . . . . . . serviceability limit state . . ultimate limit state. . . .
. . . . . . . .
. . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
214 212 213 215 211 215 211 214 213
composite stage design checks deflections (SLS) . . . . . . . . . . . . . . . . 206 equivalent steel section (ULS) . . . . . . . . . . . . 202
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
116 109 95 103 97 99 118 110
construction stage deflection checks . . . . . . . . . lateral torsional buckling checks . . . . member strength checks . . . . . . . section classification . . . . . . . . Westok deflection checks . . . . . . . Westok horizontal shear checks . . . . . Westok lateral torsional buckling checks . . Westok moment capacity checks . . . . Westok section classification. . . . . . Westok vertical shear checks . . . . . . Westok Vierendeel bending checks . . . . Westok web post flexure and buckling checks
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
202 201 201 201 209 208 209 208 207 207 210 210
cross bracing. . . . . . . . . . . . . . . . . .
62
design mode composite beam typical procedure . general beam typical procedure. . general column typical procedure . simple beam typical procedure . . simple column typical procedure .
. . . . . . . . .
. . . . . . . .
. . . . . . . .
composite stage Westok deflection checks . . . . . . Westok horizontal shear checks . . . . Westok moment capacity checks . . . . Westok natural frequency checks . . . . Westok section classification . . . . . Westok service stress checks . . . . . . Westok vertical shear checks . . . . . Westok Vierendeel bending checks . . . . Westok web post flexure and buckling checks
. . . . . . . .
. . . . . . . .
. . . . . . . .
D
. . . . . . . .
. . . . . . . .
. . . . . . . .
Composite Beam introduce Engineer’s Handbook . . . . . . . . . . . 173 scope . . . . . . . . . . . . . . . . . . . 177 theory and assumptions . . . . . . . . . . . . . . 201 . . . . . . . . .
. . . . . . . .
deflection checks composite stage (SLS) . . . . . . . . . . . . . . . 206 construction stage . . . . . . . . . . . . . . . . 202 . . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
173 221 255 157 245
Design Options… . . . . . . . . . . . . . . . .
36
diaphragms . . . . . . . . . . . . . . . . . .
49
Dramix fibres. . . . . . . . . . . . . . . . . . 187
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Building Designer Documentation page 280
Index
E
Simple Column Engineer’s Handbook
elastic critical buckling load factor . . . . . . . . . . . 74 equivalent steel section (ULS)
. . . . . . . . . . . .202
. . . . . . . . .
245
K
error messages Composite Beam. . . . . . . . . . . . . . . . .190 Simple Beam . . . . . . . . . . . . . . . . . .165
k_amp
F
lambda crit . . . . . . . . . . . . . . . . . 71, 119
Fastrak Wind Modeller applying walls and roofs introduce . . . . . limitations. . . . . references . . . . . running the Wind Wizard scope . . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
.146 .139 .142 .154 .147 .140
fibre reinforced concrete Dramix fibres . . . . . . . . . . . . . . . . . .187 Strux fibres . . . . . . . . . . . . . . . . . .187 foundation loads . . . . . . . . . . . . . . . . . 62 frame imperfections. . . . . . . . . . . . . 71, 74, 119 further information Composite Beam. . . . . . . . . . . . . . . . .216 Simple Beam . . . . . . . . . . . . . . . . . .169
G
General Column introduce Engineer’s Handbook . . . . . . . . . . . .255 sign conventions. . . . . . . . . . . . . . . . . 66
lateral torsional buckling checks . . . . . . . . . . .
201
limitations Composite Beam . . . . . . . . . . . . . . . . Fastrak Wind Modeller . . . . . . . . . . . . . . Simple Beam . . . . . . . . . . . . . . . . .
190 142 165
M member strength checks composite stage (ULS) . . . . . . . . . . . . . . construction stage . . . . . . . . . . . . . . .
203 201
modify wind zones expert user only . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
N 206
notional horizontal force load calculations . . . . . . . . 63
O orientation effects
. . . . . . . . . . . . . . . . 70
P
H . . . . . . . . . . . .186
I introduce Composite Beam Engineer’s Handbook . Fastrak Wind Modeller . . . . . . General Beam Engineer’s Handbook . . General Column Engineer’s Handbook . Simple Beam Engineer’s Handbook . .
Thursday 26 April 2007 – 10:18
L
natural frequency checks . . . . . . . . . . . . .
General Beam introduce Engineer’s Handbook . . . . . . . . . . . .221 sign conventions. . . . . . . . . . . . . . . . . 64
Hilti connector layout minimum sheet edge distance
. . . . . . . . . . . . . . . . . . . . 77
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
.173 .139 .221 .255 .157
practical application check composite beam procedure. check general beam procedure . check general column procedure . check simple beam procedure . . check simple column procedure . Composite Beam . . . . . . design composite beam procedure design general beam procedure . design general column procedure. design simple beam procedure . design simple column procedure . General Beam . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
175 222 257 158 246 173 173 221 255 157 245 221
Index
Building Designer Documentation page 281
General Column . . . . . . . . . . . . . . . . 255 Simple Beam . . . . . . . . . . . . . . . . . 157 Simple Column . . . . . . . . . . . . . . . . 245 procedure composite beam check mode composite beam design mode general beam check mode . general beam design mode . general column check mode general column design mode simple beam check mode . simple beam design mode . simple column check mode . simple column design mode
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
175 173 222 221 257 255 158 157 246 245
R references Composite Beam . . Fastrak Wind Modeller Simple Beam . . . Simple Column . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
216 154 169 252
roof zones . . . . automatic zoning . . direction . . . . non-automatic zoning
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
150 151 150 152
second-order effects . . . . . . . . . . . . .71, 74, 119 section classification composite stage (ULS). . . . . . . . . . . . . . . 202 construction stage . . . . . . . . . . . . . . . . 201 section properties - composite stage (SLS) . . . . . . . . 205 section stresses - composite stage design (SLS) . . . . . . 206 shear connector checks (ULS) . . . . . . . . . . . . 204 shear walls . . . . . . . . . . . . . . . . . . sway resistance . . . . . . . . . . . . . . . . .
29 56
sign conventions . . General Beam . . . General Column . . Simple Beam . . . Simple Column . . . supplementary supports
64 64 66 65 68 69
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
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Simple Beam introduce Engineer’s Handbook . scope . . . . . . . . . sign conventions . . . . . theory and assumptions . . .
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. 157 . 163 . 65 . 167
Simple Column introduce Engineer’s Handbook . . . . . . . . . . . . 245 sign conventions . . . . . . . . . . . . . . . . 68 stresses - composite stage design (SLS) . . . . . . . . . 206
S scope applied stage loading . . . . . Composite Beam . . . . . . . Composite Beam – beam . . . . Composite Beam – steel section . . Composite Beam – web openings . . Composite Beam error messages . . Composite Beam limitations . . . composite stage design checks . . . composite stage loading. . . . . concrete slab . . . . . . . . construction stage design checks . . construction stage loading . . . . construction stage restraint conditions Fastrak Wind Modeller . . . . . precast concrete slabs . . . . . profiled metal decking . . . . . shear connectors . . . . . . . Simple Beam . . . . . . . . Simple Beam – beam . . . . . . Simple Beam – steel section . . . . Simple Beam – web openings . . . Simple Beam error messages . . .
Simple Beam limitations . . . . . . . . . . . . . . 165 Westok sections . . . . . . . . . . . . . . . . 178
Strux fibres . . . . . . . . . . . . . . . . . . 187 . . . . . . . . . . . . . . . . . . . . . .
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165 177 178 180 180 190 190 189 188 185 189 188 187 140 183 181 186 163 163 163 163 165
supplementary supports sign conventions . . . . . . . . . . . . . . . .
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sway/non-sway . . . . . . . . . . . . . . . . .
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T theory and assumptions analysis method – Composite Beam analysis method – Simple Beam . Composite Beam . . . . . . composite stage . . . . . . construction stage . . . . . . design method – Composite Beam . design method – Simple Beam . . Simple Beam . . . . . . . Westok composite stage . . . . Westok construction stage . . .
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201 167 201 202 201 201 167 167 211 207
W wall zones. . . . . . . . . . . . . . . . . . . 149
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leeward walls . side walls . . type. . . . windward walls
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Index
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.149 .150 .149 .149
web openings Composite Beam. . . . . . . . . . . . . . . . .180 Simple Beam . . . . . . . . . . . . . . . . . .163 Westok deflection checks composite stage . . . . . . . . . . . . . . . . .214 construction stage . . . . . . . . . . . . . . . .209 Westok horizontal shear checks composite stage . . . . . . . . . . . . . . . . .212 construction stage . . . . . . . . . . . . . . . .208 Westok lateral torsional buckling checks construction stage . . . . . . . . . . . . . . . .209 Westok moment capacity checks composite stage . . . . . . . . . . . . . . . . .213 construction stage . . . . . . . . . . . . . . . .208 Westok natural frequency checks composite stage . . . . . . . . . . . . . . . . .215 Westok section classification composite stage . . . . . . . . . . . . . . . . .211 construction stage . . . . . . . . . . . . . . . .207 Westok service stress checks composite stage . . . . . . . . . . . . . . . . .215 Westok vertical shear checks composite stage . . . . . . . . . . . . . . . . .211 construction stage . . . . . . . . . . . . . . . .207 Westok Vierendeel bending checks composite stage . . . . . . . . . . . . . . . . .214 construction stage . . . . . . . . . . . . . . . .210 Westok web post flexure and buckling checks composite stage . . . . . . . . . . . . . . . . .213 construction stage . . . . . . . . . . . . . . . .210 wind load decomposition roofs . . . . . . . . . . . . . . . . . . . .153 walls . . . . . . . . . . . . . . . . . . . .153 Wind Modeller
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Wind Wizard running . . . . . . . . . . . . . . . . . . .147 wind zones basic geometry creating . . modification . roof . . . . wall. . . .
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Thursday 26 April 2007 – 10:18
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.148 .148 .152 .150 .149
