[email protected] 9036098037 Slab in ETABS, shell or membrane Regardless of the software you use, the issue is that there is a wide difference between shell behavior and a membrane one. Structural engineer should know that a shell has six components of stresses possible within it, 4 in the plane of the shell and two in the perpendicular direction to that plane. The shell has a flexural stiffness that allows creating such perpendicular stresses to its plane. Meanwhile a membrane shall have only four components within the plane of the membrane. There is no perpendicular stress to the membrane because there is no flexural stiffness in that direction. Hence a concrete plate is similar to shell in design in light of the presence of flexural stiffness in both. FAIR ENOUGH? For the case of modeling your problem if it is a flat slab there is no hustle. Represent it by a shell with the same thickness as the slab. If there are beams involved, you should draw your shell all-over the beams, then ask the software to subdivide the whole plane model by choosing beams and the slab at the same time. Do I get a like on this illustration?
Shell Vs membrane is a very commonly debated topic among Etabs users.I am sharing response I got from E-tabs tech support Please bear with long description: The rigid diaphragm assumes infinite in-plane stiffness of floors and therefore reduce the stiffness matrix. The semi-rigid diaphragm uses the inplane stiffness of slab and does not condense the stiffness matrix. For most of the concrete structure where slab is sufficiently thick and we do not expect any deformation, results for semi-rigid diaphragm will be same as rigid diaphragm. Can use membrane definition. However, if you are expecting the slab deformations then modeling a semi--rigid diaphragm will be the correct way to handle such cases. A semi-rigid diaphragm is same as assigning no diaphragm except it allows you to assign the accidental ECC due to Wind or Seismic load cases. At the same time , wind loads can be applied at the center of masses and you do not need to use Area Exposure method to define Wind load case for semi-rigid diaphragm cases.Need to use shell definition. There are three levels of modeling for floors in ETABS. 1. The simplest level is that the floor is modeled as one big polygon and is used only to define the extent of the floor. It is assigned meshing type - "For Defining Rigid Diaphragm and Mass only (No stiffness - No vertical load transfer)" . It is assigned a rigid diaphragm so it connects all elements falling within laterally but is unable to transfer any vertical load. This model can be used for a quick
study of the lateral load resisting system. 2. The second level is a step higher in that the floor is meshed coarsely and given only membrane or deck properties. One could assign a rigid diaphragm if needed, otherwise connectivity is still provided through the coarse mesh. The vertical load is transferred to edges of coarse mesh and is either supported directly by columns or by beams and walls on the edges. This is commonly used with composite floors and can be used for thinner concrete floors where beams are designed for full gravity loads and slab bending stiffness is not important to the lateral analysis. 3. The third level is to have a decent mesh of the floor either done externally or internally that connects the major structural elements and also models the correct bending stiffness of the floor for frame action with columns. This obviously is the correct model and the reason to go to the previous two levels is to avoid getting a huge model that takes too much time solving or cannot even be solved on current hardware. Depend on the length of span and thickness of the slab itself. The rule of thumb is if ratio span/thickness >40 we can use membrane but if the ratio between 10 and 40 shell can help you much better.thx You should use a Plate or Shell Element for correct slab behaviour. In LUSAS you have a choice of surface elements and if your model is just 2D slab then plates give the best results for a given number of elements. Always do a mesh sensitivity check however. For a concrete slab I would use a thick plate or a thick shell which models shear deformation (thin ones do not). you can of course model beams under the slab where appropriate using correct eccentricities.
when you use software you must define your slab as shell because in reality you structure work with the six component, but when you design you only use the three component if it's slab use only your bending Mx, My and Mxy you can neglect the normal forces Fx, Fy and Fxy
If the engineer is concerened with seismic load and distribution of seismic loads to the lateral load resisting system, then the memberane action would be enough to represent the slab behavior (as a diaphgram subjected to in-plane load). The lose of accuracy due to not considering the flexural behavior of the slab is minimal in this case. However, if the purpose is to design the slab itself due to gravity loads (transverse loads to slab plane), then shell action (memberane+flexural) must be used, meaning using shell elements with 6 DOF's. If the slab is too thick, then shell element with shear deformation shall be considered.
Great discussion, now I enjoy using ETABS 2013. To crosscheck with manual calculations, the distribution of loads on Frame objects can be viewed from
Display Menu. How can we look at the load transferred from the slab to beams : Show Frame/Line Loads Click the Display menu > Show Loads > Frame/Line command to access the Show Frame/Line Loads form, which has the following options: Load Case Choose the static load case whose frame/line loads you want to display from the drop-down box. Note that static load cases are defined using the Define menu > Static Load Cases command and assigned using the Assign menu > Frame/Line Loads command. Load Type Choose the type of load that you want to display. Note that you can only display one of these types of loads at a time. Also, if a load type is unavailable (grayed out), that type of load has not been assigned (see the Assign menu > Frame/Line Loads commands). The choices are: Span Loading Applied Directly to the Line Object (Forces): Includes all of the point, uniform and trapezoidal force loads (not moment loads) applied to the line object. Span Loading Applied Directly to the Line Object (Moments): Includes all of the point, uniform and trapezoidal moment loads applied to the line object. All Loading that is Tributary to the Line Object (Forces): Displays the calculated point, uniform and trapezoidal loads tributary to the line object. All Loading that is Tributary to the Line Object (moments): Displays the calculated point, uniform and trapezoidal moment loads tributary to the line object. among above four option the last two option will be highlighted after getting analyse the model once.
with all what our enabled Engineers have elaborated with in depth knowledge, can we make the assumptions below; Membrane: used for conventional slab with beam. Shell: is used for flat slab and wall. Plate thin: is used for raft foundation where bending is predominant. Plate thick: is used where shear is predominant.
Membrane is simply like a truss member in line member analogy where as plate is like a beam and shell is an arch. The behavior between these analogy are same. I hope every one is okay with the knowledge of truss member, beam and an arch member. ETABS-2013-Video Training
Chapter-1 Introduction/Kick-Start
Introduction
Advanced capabilities of ETABS 2013
Brief history
ETABS 2013 Levels
Modeling Process
Modeling Features
Analysis Features
Design Feature
Detailing Feature
Modeling Terminology & details
Load case
Load Combinations
Design Settings and scope
Output and Display settings
Model Initialization
New model quick template
GUI after setting template
Chapter-2
Foundation Concept startup
The ETABS graphical user interface
File Operations & Different Extn
Time Saving Options
Templates and Defaults
Basic Process
Standard Tool bars for Doc Managing
Standard Tool bars for Doc Zoom
Standard Tool bars for Doc View
Standard Tool bars for Graphics & Load Display
Model Explorer
Draw Tools
Draw beam-Plan/Elev/3D
Draw Column-Plan/Elev/3D
Draw Brace-Plan/Elev/3D
Quick Draw beam-Plan/Elev/3D
Quick Draw Column-Plan/Elev/3D
Quick Draw Secondary Beam-Plan/3D
Quick Draw Braces-Plan/Elev/3D
Draw Floor/Wall-Plan/Elev/3D
Draw Rectangular Floor/Wall-Plan/Elev
Quick Draw Floor/Wall-Plan/Elev
Draw Wall-Plan
Quick Draw Wall-Plan
Draw Wall opening-Plan/Elev/3D
Draw Links--Plan/Elev/3D
Draw Wall stacks-Plan/Elev/3D
Wall Meshing techniques
How to apply conditioning to walls/slabs/floor for EQ Analysis
Understanding of Edge constraints for Shear wall/slabs
Example Case study mesh
Units
Display Units
Units Setting
Project-1
Project-2
Project-3
Exercise-1
Exercise-2
Exercise-3
Chapter-3 Modeling & Properties Options
Quick Template settings and Structural Objects
Properties of Steel deck
Properties of Staggered truss
Properties of Flat Slab
Properties of Flat Slab with Perimeter Beams
Properties of Waffle Slab
Properties of 2 Way or Ribbed Slab
Grid System
Uniform Grid Spec
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Properties Add
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Properties Modify
Frame section Reinforcement-Rebar
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Story data
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Project-1
Project-2
Project-3
Project-4
Exercise-1
Exercise-2
Chapter-4 Advanced Modeling Options
Replicate Options
Extrude Joints & Frames
Add model from Template-2D Frame
Add model from Template-2D Truss
Add model from Template-2D Wall
Add model from Template-3D-Steel Deck
Add model from Template-3D-Staggered Truss
Add model from Template-3D-Flat Slab
Add model from Template-3D-Flat Slab with Perimeter Beams
Add model from Template-3D-Waffle Slab
Add model from Template-3D-2 Way or Ribbed Slab
Move Joints/Frames/Shell
Define Material
Material Property Data
Frame Properties
Snap Options
Cladding non-structural member for wind load
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Project-2
Project-3
Project-4
Project-5
Project-6
Chapter-5 Advanced Boundary Conditions
Joint Loads-Force Parameters
Load Pattern & options
Joint Loads-Ground Displacement Parameters
Joint Loads-Temperature
Frame Loads-Point based load Parameters
Frame Loads-Distributed load Parameters
Frame Loads-Temperature
Frame Loads-Structure Wind Parameters
Shell Loads-Uniform Load Sets Parameters
Shell Loads-Uniform Parameters
Joint Restraints
DOF-For Pin,Roller,Fixed & free etc
How to reconfigure Supports
IS 875- Part 1-Dead Loads & ETABS
IS 875- Part 2-Imposed Load & ETABS
IS 875- Part 3-Wind Loads & ETABS
IS.15498.2004
Explanation about EQ Load and Basic Orientation on Project
International Other country Load cases i.e ASCE,AS,EURO,Italian,NBCC,Turkish,AS/NZS,UBC
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Project-2
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Project-4
Exercise-1
Exercise-2
Exercise-3
Chapter-6 Detailing–ETABS-2013
Detailing scope
Detailing Preference
Concrete Detailing Preference-Slab/Beam/Col/Wall
Steel Detailing Preference
Rebar Section Rules
Drawing Sheet Setup
Start Detailing
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Detailing Reports and configuration/Visualization
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Export Drawings
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Exercise-1
Exercise-2
Chapter-7 Concrete Frame Design-RCC Design
RCC Design Overview
Capability RCC Design
What To do check list before running RCC Analysis
RCC Design Preference
IS 456:2000 & Design Guide Lines
International RCC Design codes for other countries & Implementation
RCC Design Member
Start Design/Check
Display Design info
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Rebar % and other important Output discussion
Project-1
Exercise-1
Exercise-2
Exercise-3
Chapter-8 Shear Wall Design
Shear Wall Design Preference
IS 456:2000 & Design Guide Lines
Shear Wall Design –PIERS & SPANDRELS
Shear Wall Optimization for Thickness & Target Drift
Supported Codes
Design Combination
Start Design/Check
Display Design info
Display Design Results
Shear Wall pier summary
Shear wall Design Details
Design Output Complete detail discussion
Longitudinal reinforcement and other important Output discussion
Project-1
Chapter-9
Modal Frequency Analysis
What is modal Analysis?
The Natural Frequency of a Building
Structural Matrix & System Response
Benefits of modal analysis
How Natural Frequency emanates?
Resonance
Case study-1
Case Study-2& Examples
Modal case Data config
Modal Result Table config
Modal Periods & Frequencies
Modal Participation Mass Ratio
Modal Participation Factors
Load Participation Ratio
Modal Direction Factors
Modal Report Export to Excel
Mode Shapes Settings
Project-1
Chapter-10 Import/Export-Concept
Importing Fundamentals and Unit systems
Importing from Old ETABS Versions
Importing from STAAD.Pro
Importing from Revit Structure
Importing DWG
Importing DXF
Importing Architectural Options setting
Layer control
ARCH Layer Auto create wall/column/beam/area options
Properties configuration while auto create
Exporting DWG/DXF
Project-1-dxf
Project-2-dwg
Project-3-STAAD
Chapter-11 Selection Quick Tricks
Pointer/Window Selection
Poly selection
Intersection poly
Interesting Line
Co-ordinate specification
Object type selection i.e. column/beam/wall etc.
Properties selection types
Material properties selection
Frame sections
Slab sections
Deck sections
Wall sections
Link/support properties
Point spring
Line spring
Area spring
Panel zones
Labels
Group
Stories
All
Deselect strata in detail
Invert selection
Clear selection
- See more at: http://store.smartlearningindia.com/Single-Software-Training-DVD/ETABS-2013-VideoTutorials-Training-DVD-id-274196.html#sthash.9stD8H0e.dpuf SIZE OF RCC BEAM FROM CENTERING POINT OF VIEW The posting on the size of beams and columns as pointed out by you should be a practical as well as economical point of view. While designing the depth of beam 1" per one foot span is a thumb rule used while in the good old days when the working stress is adopted that is span/12 but now it can be span/15 to span/20 depending on span, loading and other considerations and make use of LIMIT STATE METHOD OF DESIGNING R.C.C. MEMBERS. The idea of span/12 can be taken as a preliminary calculation and design but practically it can be reduced. The other point is if you need to reduce the steel to some extend the depth may be taken as span/12 as the cost of steel/Kg is more than a few centimeter increase in depth of beam concrete. Wise decision has to be exercised while designing considering the practical aspect and avoiding the material wastage. In this respect use of 13" will be good enough for the use of timber shuttering rather than having odd sizes to match with the available timer sizes which will avoid wastage and lead not only economy but also save time.
