Aug 23-24, Kuala Lumpur, Malaysia
International Seminar on
Computer Aided Analysis and Design
Of Building Structures
•Institute of Engineers Malaysia
•Computers and Structures Inc., USA
•Asian Center for Engineering Computations and Software
Asian Institute of Technology, Thailand
Building Structures
Modeling and Analysis Concepts
Naveed Anwar
Asian Center for Engineering Computations and Software, ACECOMS, AIT
Overall Design Process
•
•
•
•
•
•
•
Conception
Modeling
Analysis
Design
Detailing
Drafting
Costing
Modeling, Analysis and Design of Buildings
Integrated
Design
Process
AIT - Thailand
ACECOMS
Building Systems
• Building is an assemblage of various Systems
–
–
–
–
–
–
–
Basic Functional System
Structural System
HVAC System
Plumbing and Drainage System
Electrical, Electronic and Communication System
Security System
Other specialized systems
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The Building Structural System - Physical
Building Structure
Floor Diaphragm
Frame and Shear Walls
Lateral Load Resisting System
Floor Slab System
Gravity Load Resisting System
Sub-structure and Member Design
Beams, Columns, Two-way Slabs, Flat Slabs, Pile caps
Shear Walls, Deep Beams, Isolated Footings, Combined Footings
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The Building Structural System - Conceptual
• The Gravity Load Resisting System (GLRS)
– The structural system (beams, slab, girders, columns, etc)
that act primarily to support the gravity or vertical loads
• The Lateral Load Resisting System (LLRS)
– The structural system (columns, shear walls, bracing, etc)
that primarily acts to resist the lateral loads
• The Floor Diaphragm (FD)
– The structural system that transfers lateral loads to the
lateral load resisting system and provides in-plane floor
stiffness
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Building Response
• Objective: To determine the load path gravity and lateral loads
• For Gravity Loads - How Gravity Loads are Distributed
– Analysis of Gravity Load Resisting System for:
• Dead Load, Live Live Load, Pattern Loads, temperature, shrinkage
– Important Elements: Floor slabs, beams, openings, Joists, etc.
• For Lateral Loads – How Lateral Loads are Distributed
– Analysis of Lateral Load Resisting System for:
• Wind Loads, Seismic Loads, Structural Un-symmetry
– Important elements: Columns, shear walls, bracing , beams
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Structural Response
To Loads
The Simplified Structural System
STRUCTURE
RESPONSES
EXCITATION
Loads
Vibrations
Settlements
Thermal Changes
Modeling, Analysis and Design of Buildings
pv
Displacements
Strains
Stress
Stress Resultants
AIT - Thailand
ACECOMS
Analysis of Structures
 xx  yy  zz


 pvx  0
x
y
z
pv
Real Structure is governed by “Partial
Differential Equations” of various order
Direct solution is only possible for:
• Simple geometry
• Simple Boundary
• Simple Loading.
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The Need for Modeling
A - Real Structure cannot be Analyzed:
It can only be “Load Tested” to determine response
B - We can only analyze a
“Model” of the Structure
C - We therefore need tools to Model the
Structure and to Analyze the Model
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The Need for Structural Model
STRUCTURE
RESPONSES
EXCITATION
Loads
Vibrations
Settlements
Thermal Changes
Displacements
Strains
Stress
Stress Resultants
pv
Structural
Model
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Finite Element Method: The Analysis Tool
• Finite Element Analysis (FEA)
“A discretized solution to a continuum
problem using FEM”
• Finite Element Method (FEM)
“A numerical procedure for solving (partial)
differential equations associated with field
problems, with an accuracy acceptable to
engineers”
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Continuum to Discrete Model
pv
3D-CONTINUM
MODEL
(Governed by partial
differential equations)
Modeling, Analysis and Design of Buildings
CONTINUOUS MODEL
OF STRUCTURE
(Governed by either
partial or total differential
equations)
DISCRETE MODEL
OF STRUCTURE
(Governed by algebraic
equations)
AIT - Thailand
ACECOMS
From Classical to FEM Solution
Equilibrium
Actual Structure
 xx  yy  zz


 pvx  0
x
y
z
“Partial Differential
Equations”
FEM
Assumptions
Classical
Structural Model
Kr  R
Stress-Strain Law
Compatibility

t
_
_
“Algebraic
Equations”
_
 dV   p u dV   p u ds
t
v
t
s
v
(Principle of Virtual Work)
Modeling, Analysis and Design of Buildings
K = Stiffness
r = Response
R = Loads
AIT - Thailand
ACECOMS
Simplified Structural System
Loads (F)
Deformations (D)
Fv
D
K
F
F=KD
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The Structural System
STRUCTURE
RESPONSES
EXCITATION
pv
• Static
• Dynamic
Modeling, Analysis and Design of Buildings
• Elastic
• Inelastic
• Linear
• Nonlinear
AIT - Thailand
ACECOMS
The Equilibrium Equations
1. Linear-Static
Elastic OR Inelastic
Ku  F
2. Linear-Dynamic Elastic
Mu(t )  Cu (t )  Ku(t )  F (t )
3. Nonlinear - Static
Elastic OR Inelastic
Ku  FNL  F
4. Nonlinear-Dynamic
Elastic OR Inelastic
Mu(t )  Cu(t )  Ku(t )  F (t ) NL  F (t )
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Basic Steps in FEA
Evaluate Real Structure
Create Structural Model
Discretize Model in FE
Solve FE Model
Engineer
Interpret FEA Results
Engineer + Software
Software
Physical significance of Results
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Discretization of Continuums
General Solid
( Orthogonal dimensions)
Z
H, B much less than L
Regular Solid
X
( T small compared to Lengths )
Y
Beam Element
Solid Element
Plate/ Shell
Membrane/ Panel
In-Plane, Only Axial
Modeling, Analysis and Design of Buildings
Plate/ Slab
Out of Plane, Only Bending
Shell
In-Plane and Bending
AIT - Thailand
ACECOMS
Global Modeling of Structural Geometry
(a) Real Structure
(b) Solid Model
(c) 3D Plate-Frame
(e) 2D Frame
(d) 3D Frame
(f) Grid-Plate
Fig. 1 Various Ways to Model a Real Struture
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Dimensions of Elements
• 1 D Elements (Beam type)
– Can be used in 1D, 2D and 2D
– 2-3 Nodes. A, I etc.
Truss and Beam Elements (1D,2D,3D)
• 2 D Elements (Plate type)
– Can be used in 2D and 3D Model
– 3-9 nodes. Thickness
Plane Stress, Plane Strain, Axisymmetric, Plate and Shell Elements (2D,3D)
• 3 D Elements (Brick type)
– Can be used in 3D Model
– 6-20 Nodes.
Brick Elements
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
DOF for 1D Elements
Dy
Dy
Dy
Dx
2D Truss
Rz
Dz
Dx
3D Truss
2D Beam
Ry
Dy
Rz
Dy
Dx
Rz
Dy
Dz
Rx
Dx
Rx
Rz
2D Frame
Modeling, Analysis and Design of Buildings
2D Grid
3D Frame
AIT - Thailand
ACECOMS
DOF for 2D Elements
Ry ?
Ry ?
Dy
Dy
Dy
Rz
Rx
Dx
Membrane
Modeling, Analysis and Design of Buildings
Plate
Dz
Dx
Rx
Rz
Shell
AIT - Thailand
ACECOMS
DOF for 3D Elements
Dy
Dz
Dx
Solid/ Brick
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Frame and Grid Model
• The structure represented by rod or
bar type elements
• Does not model the cross-section
dimensions
• Suitable for skeletal structures
• Sometimes surface type structures
can also be represented by frame
model
• The simplest and easiest model to
construct, analyze and interpret
• Can be in 2D or in 3D space
3D Frame
2D Grid
2D Frame
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Membrane Model
•
•
•
•
•
•
Ignore bending stiffness
Tension / Compression
In- plane Shear
For in plane loads
Principle Stresses
suitable for very thin structures
/ members
• Thin Walled Shells,
• Specially Suitable for Ferro
Cement Structure
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Plane Stress and Plane
Plain-Strain
Assumptions
x
1 unit
  
  
  
 
 
x2
x1
x3
3D Problem
 
 
 
 
x
2D Problem
Plane Strain Problem
Modeling, Analysis and Design of Buildings
Plane Stress Problem
AIT - Thailand
ACECOMS
Plate Bending Model
• Primarily Bending mode
• Moment and Shear are
predominant
• Suitable for moderately thick
slabs and plates
• For Out-of-plane loads only
• Can be used in 3D or 2D models
• Suitable for planks and
relatively flat structures
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
General Plate-Shell Model
• Combined Membrane and Plate
• Suitable for general application
to surface structures
• Suitable for curved structures
• Thick shell and thin shell
implementations available
• Membrane thickness and plate
thickness can be specified
separately
• Numerous results generated.
Difficult to design the section for
combined actions
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Solid Model
•
•
•
•
Shear Axial deformation mode in 3D
Suitable for micro-models
Suitable for very thick plates / solids
May not be applicable much to
ferocement structures
• Use 6 to 20 node
elements
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Soil-Structure Interaction
• Simple Supports
• Fix, Pin, Roller etc.
• Support Settlement
• Elastic Supports
• Spring to represent soil
• Using Modulus of Sub-grade reaction
• Full Structure-Soil Model
• Use 2D plane stress elements
• Use 3D Solid Elements
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Connecting Different Types of Elements
Truss
Truss
Frame
Membrane
Plate
Shell
Solid
OK
OK
Dz
OK
OK
OK
Rx, Ry, Rz
OK
Rx, Ry, Rz,
Dz
Rx ?
Dx, Dy
Rx ?
Rx, Ry, Rz
OK
OK
OK
Dx, Dy
OK
OK
Rx, Rz
OK
Rx, Rz
OK
OK
Rx, Rz
Rx, Ry, Rz
OK
Rx, Ry, Rz,
Dz
Dx, Dz
OK
Rx, Rz
OK
OK
Dz
Dx, Dz
OK
OK
Membrane
Plate
Shell
Frame
Solid
Orphan Degrees Of Freedom:
0
Modeling, Analysis and Design of Buildings
1
2
3
4
AIT - Thailand
ACECOMS
What Type of
Analysis should be
Carried Out?
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Analysis Type
The type of Analysis to be carried out
depends on the Structural System
– The Type of Excitation (Loads)
– The Type Structure (Material and Geometry)
– The Type Response
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Basic Analysis Types
Excitation Structure Response
Basic Analysis Type
Static
Elastic
Linear
Linear-Elastic-Static Analysis
Static
Elastic
Nonlinear
Nonlinear-Elastic-Static Analysis
Static
Inelastic
Linear
Linear-Inelastic-Static Analysis
Static
Inelastic
Nonlinear
Nonlinear-Inelastic-Static Analysis
Dynamic
Elastic
Linear
Linear-Elastic-Dynamic Analysis
Dynamic
Elastic
Nonlinear
Nonlinear-Elastic-Dynamic Analysis
Dynamic
Inelastic
Linear
Linear-Inelastic-Dynamic Analysis
Dynamic
Inelastic
Nonlinear
Nonlinear-Inelastic-Dynamic Analysis
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Some More Solution Types
• Non-linear Analysis
–
–
–
–
–
P-Delta Analysis
Buckling Analysis
Static Pushover Analysis
Fast Non-Linear Analysis (FNA)
Large Displacement Analysis
• Dynamic Analysis
– Free Vibration and Modal Analysis
– Response Spectrum Analysis
– Steady State Dynamic Analysis
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Static Vs Dynamic
• Static Excitation
– When the Excitation (Load) does not vary rapidly with Time
– When the Load can be assumed to be applied “Slowly”
• Dynamic Excitation
– When the Excitation varies rapidly with Time
– When the “Inertial Force” becomes significant
• Most Real Excitation are Dynamic but are considered
“Quasi Static”
• Most Dynamic Excitation can be converted to
“Equivalent Static Loads”
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Elastic Vs Inelastic
• Elastic Material
– Follows the same path during loading and unloading and returns to initial
state of deformation, stress, strain etc. after removal of load/ excitation
• Inelastic Material
– Does not follow the same path during loading and unloading and may not
returns to initial state of deformation, stress, strain etc. after removal of
load/ excitation
• Most materials exhibit both, elastic and inelastic behavior
depending upon level of loading.
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Linear Vs Nonlinear
• Linearity
– The response is directly proportional to excitation
– (Deflection doubles if load is doubled)
• Non-Linearity
– The response is not directly proportional to excitation
– (deflection may become 4 times if load is doubled)
• Non-linear response may be produced by:
– Geometric Effects (Geometric non-linearity)
– Material Effects (Material non-linearity)
– Both
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Linear-Elastic
Action
Action
Elasticity and Linearity
Deformation
Action
Action
Deformation
Linear-Inelastic
Nonlinear-Elastic
Deformation
Modeling, Analysis and Design of Buildings
Nonlinear-Inelastic
Deformation
AIT - Thailand
ACECOMS
Physical Object Based
Modeling, Analysis and Design
Continuum Vs Structure
• A continuum extends in all direction, has infinite
particles, with continuous variation of material
properties, deformation characteristics and stress state
• A Structure is of finite size and is made up of an
assemblage of substructures, components and members
• Dicretization process is used to convert Structure to
Finite Element Models for determining response
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Physical Categorization of Structures
• Structures can be categorized in many ways.
• For modeling and analysis purposes, the overall physical
behavior can be used as basis of categorization
–
–
–
–
–
Cable or Tension Structures
Skeletal or Framed Structures
Surface or Spatial Structures
Solid Structures
Mixed Structures
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Structure Types
• Cable Structures
• Cable Nets
• Cable Stayed
• Bar Structures
• 2D/3D Trusses
• 2D/3D Frames, Grids
• Surface Structures
• Plate, Shell
• In-Plane, Plane Stress
• Solid Structures
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Structure, Member, Element
• Structure can considered as an assemblage of “Physical
Components” called Members
– Slabs, Beams, Columns, Footings, etc.
• Physical Members can be modeled by using one or more
“Conceptual Components” called Elements
– 1D elements, 2D element, 3D elements
– Frame element, plate element, shell element, solid element, etc.
• Modeling in terms Graphical Objects to represent Physical
Components relieves the engineers from intricacies and
idiosyncrasy of finite element discretization
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Structural Members
Continuum
Regular Solid
(3D)
y
Plate/Shell (2D)
x z
t<<(x,z)
z
x
Beam (1D)
b h
L>>(b,h)
h
t
z
x
L
b
Dimensional Hierarchy of Structural Members
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Load Transfer Path For Gravity Loads
• Most loads are basically “Volume Loads” generated due to
mass contained in a volume
• Mechanism and path must be found to transfer these loads to
the “Supports” through a Medium
• All types of Static Loads can be represented as:
– Point Loads
– Line Loads
– Area Loads
– Volume Loads
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The Load Transfer Path
• The Load is transferred through a
medium which may be:
–
–
–
–
–
A Point
A Line
An Area
A Volume
A system consisting of combination of
several mediums
• The supports may be represented as:
–
–
–
–
Point Supports
Line Supports
Area Supports
Volume Supports
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Graphic Object Representation
Object
Load
Geometry
Medium
Support
Boundary
Point
Point Load
Concentrated Load
Node
Point Support
Column Support
Line
Beam Load
Wall Load
Slab Load
Beam / Truss
Connection Element
Spring Element
Line Support
Wall Support
Beam Support
Area
Slab Load
Wind Load
Plate Element
Shell Element
Panel/ Plane
Soil Support
Volume
Seismic Load
Liquid Load
Solid Element
Soil Support
ETABS uses graphic object modeling concept
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Load Transfer Path is difficult to Determine
• Complexity of Load Transfer
Mechanism depend on:
Load
Vol.
– Complexity of Load
– Complexity of Medium
– Complexity of Boundary
Area
Line
Point Line
Area
Volume
Medium
Line
Area
Volume
Boundary
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Load Transfer Path is difficult to Determine
Point
Line
Area
Volume
Transfer of a Point Load to Point Supports Through Various Mediums
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Objects in ETABS
• Building Object Specific Classification
–
–
–
–
–
Plank – One way slabs
Slab – One way or Two way slabs
Deck – Special one way slabs
Wall – Shear Walls, Deep Beams, In-Fill Panel
Frame – Column, Beam or Brace
• Finite Elements
–
–
–
–
–
Shell
Plate
Membrane
Beam
Node
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The Frame Element
• The Actions Corresponding to Six DOF at Both Ends, in
Local Coordinate System
2
2
1
1
+V2
+M2
+P
2
2
3
3
+V3
3
+V3
+P
+V2
Modeling, Analysis and Design of Buildings
+T
+M3
3
+M3
+T
+M2
AIT - Thailand
ACECOMS
Shell Element
General
•Total DOF per Node = 6 (or 5)
•Total Displacements per Node = 3
•Total Rotations per Node = 3
•Used for curved surfaces
U3, R3
U3, R3
U2, R2
Node 3
U2, R2
Node 4
U1, R1
Application
•For Modeling surface elements carrying
general loads
3
•May be used for modeling of general slabs
systems. But not used generally
Modeling, Analysis and Design of Buildings
U3, R3
1
U3, R3
U2, R2
Node 1
Building Specific Application
2
U1, R1
U2, R2
Node 2
U1, R1
U1, R1
Shell
AIT - Thailand
ACECOMS
Plate Element
General
•Total DOF per Node = 3
•Total Displacements per Node = 1
•Total Rotations per Node = 2
•Plates are for flat surfaces
U3
U3
R2
Node 3
R1
Application
•For Modeling surface elements carrying
out of plane loads
3
•For representing floor slabs for Vertical
Load Analysis
•Model slabs
Modeling, Analysis and Design of Buildings
R1
2
1
U3
R2
Node 1
Building Specific Application
R2
Node 4
U3
R2
Node 2
R1
R1
Plate
AIT - Thailand
ACECOMS
Membrane Element
General
•Total DOF per Node = 3 (or 2)
•Total Displacements per Node = 2
•Total Rotations per Node = 1 (or 0)
•Membranes are modeled for flat surfaces
Application
•For Modeling surface elements carrying
in-plane loads
Building Specific Application
•For representing floor slabs for Lateral
Load Analysis.
• Model Shear walls, Floor Diaphragm etc
Modeling, Analysis and Design of Buildings
R3
U2
U2
Node 4
Node 3
U1
3
U1
2
1
R3
U2
Node 1
R3
U2
Node 2
U1
U1
Membrane
AIT - Thailand
ACECOMS
Meshing Slabs and Walls
“Zipper”
In general the mesh in the slab
should match with mesh in the wall
to establish connection
Modeling, Analysis and Design of Buildings
Some software automatically
establishes connectivity by using
constraints or “Zipper” elements
AIT - Thailand
ACECOMS
Selection Of Structural Systems
Basic Concepts and Considerations
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Knowledge Model for System Selection
En
Ju gine
Co dgem ering
mm en
on t an
Se d
ns
e
re
g
rin
e
e
gin
n
E
ms
e
t
s
Sy
En
g
ine
eri
ng
Structural
System Selection
Artificial Intelligence
Structural
Engineering
Ec
on
om
ics
Ergo
n
Eng omics
inee
ring
ng
eri
e
gin
En
e
lu
Va
Ae
sth
etic
s
Construction
Engineering
dge
wle ing
Kno ineer
Eng
Modeling, Analysis and Design of Buildings
So
ftw
a
Building
Services
Engineering
Architecture
Building Services
Construction Eng.
Value Eng.
Aesthetics
Ergonomics Eng.
Structural Eng.
Knowledge Eng.
Economics
Artificial Intelligence
System Eng.
Common Sense
re
ctu
ite
ch
Ar
•
•
•
•
•
•
•
•
•
•
•
•
AIT - Thailand
ACECOMS
Determining System Suitability
The Analytical Hierarchy Approach
A weighted importance and suitability value analysis to
determine the comparative value of a system or option
 n
 p

Vl   Ai Si   Bij Sij   Cijkl Sijk  
i 1
 k 1

 j 1
m
Value of
an Option
Global
Importance
Weights and
Scores
Modeling, Analysis and Design of Buildings
Sub
Importance
Weights and
Scores
Suitability
Value and
Score
AIT - Thailand
ACECOMS
Evaluating System Suitability
The Suitability Equation
 n
 p

Vl   Ai Si   Bij Sij   Cijkl Sijk  
i 1
 k 1

 j 1
m
Using the Suitability Equation
Slab Systems
Criteria Weights and Scores
Main Criteria Ai
Sub Criteria Bij
Item k
Am
Sub Criteria Bin
Item p
Item k
System
Value
(V)
Bmn
Item p
Item p
Wt
Score
Wt
Score
Wt
Score
Wt
Score
Score
Cijkl
Sijkl
Cijnl
Sijpl
Cinkl
Sinkl
Cinnl
Sinpl
Smnpl
System – 1
System – l
System - q
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Assigning Suitability Values
Score or Weight
Representation of Suitability
10
Most important, most suitable, most desirable, essential
8,9
Very important, very suitable, very desirable
6,7
Important, suitable or desirable
5
May be or could be important, suitable or desirable
4,3
May not be important, suitable or desirable
1,2
Not important, not suitable, not desirable
0
Definitely not required, definitely not suitable, ignore
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Selection of Structural System
Function has considerable effect on the selection
of structural system
Based on Function/Occupancy of Tall Buildings:
• Residential Buildings
– Apartments
– Hotels
– Dormitories
• Office and Commercial Buildings
• Mixed Occupancy – Commercial + Residential
• Industrial Buildings and Parking Garages
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Typical Characteristics of Residential Bldg
•
•
•
•
•
•
•
Known location of partitions and their load
Column lines generally matches architectural layout
Typical spans 15-22 ft
Tall buildings economy in achieved using the thinnest slab
One way pre-cast or flat slab – popular
Lateral load resistance provided by frame or shear walls
More or less fixed M/E system layouts
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Typical Characteristics of Office and Commercial Bldg
•
•
•
•
Unknown location of partitions and their load
Typical spans 20-35 ft
Need for flexible M/E layouts
Post-tension or ribbed and flat slab with drop panel –
popular
• Ideal balance between vertical and lateral load resisting
systems: sufficient shear walls to limit the resultant
tension under gravity plus wind
• Lateral load resistance varies significantly
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Vertical Load
Resisting Systems
The Components Needed to
Complete the Load-Transfer Path
for Vertical Gravity Loads
Gravity Load Resisting Systems
Purpose
“ To Transfer Gravity Loads Applied at the Floor Levels
down to the Foundation Level”
•
Direct Path Systems
• Slab Supported on Load Bearing Walls
• Slab Supported on Columns
•
Indirect Multi Path Systems
• Slab Supported on Beams
• Beams Supported on Other Beams
• Beams Supported on Walls or Columns
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Vertical Load Resisting Systems
1. Slabs supported on Long Rigid Supports
–
–
–
Supported on stiff Beams or Walls
One-way and Two-way Slabs
Main consideration is flexural reinforcement
2. Slab-System supported on Small Rigid Supports
–
–
–
Supported on Columns directly
Flat Slab Floor systems
Main consideration is shear transfer, moment distribution in various
parts, lateral load resistance
3. Slabs supported on soil
–
–
Slabs on Grade: Light, uniformly distributed loads
Footings, Mat etc. Heavy concentrated loads
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Vertical Load
Behavior and Response
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Popular Gravity Load Resting Systems
• Direct Load Transfer Systems (Single load transfer path)
–
–
–
–
Flat Slab and Flat Plate
Beam-Slab
Waffle Slab
Wall Joist
• Indirect Load Transfer System (Multi step load transfer path)
– Beam, Slab
– Girder, Beam, Slab
– Girder, Joist
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Conventional Approach
• For Wall Supported Slabs
– Assume load transfer in One-Way or Two-Way manner
– Uniform, Triangular or Trapezoidal Load on Walls
• For Beam Supported Slabs
– Assume beams to support the slabs in similar ways as walls
– Design slabs as edge supported on beams
– Transfer load to beams and design beams for slab load
• For Flat-Slabs or Columns Supported Slabs
– Assume load transfer in strips directly to columns
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Popular Gravity Load Resting Systems
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Gravity Load Transfer Paths
Single Path
Single Path
Dual Path
Slab On Walls
Slab on Columns
Slab On Beams,
Beams on Columns
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Gravity Load Transfer Paths
Mixed Path
Complex Path
Three Step Path
Slab On Walls
Slab On Beams
Beams on Walls
Slab on Beams
Slab on Walls
Beams on Beams
Beams on Columns
Slab On Ribs
Ribs On Beams
Beams on Columns
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Simplified Load Transfer
To Lines
To Points
To Lines and Points
Transfer of Area Load
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Load Transfer Through Slab and Beam
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Slab Deformation and Beams
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Slab System Behavior
D
B
Slab T = 200 mm
Beam Width, B = 300 mm
Beam Depth, D
a) 300 mm
b) 500 mm
c) 1000 mm
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Moment Distribution in Beam-Slab
Effect of Beam Size on
Moment Distribution
a) Beam Depth = 300 mm
c) Beam Depth = 1000 mm
Modeling, Analysis and Design of Buildings
b) Beam Depth = 500 mm
AIT - Thailand
ACECOMS
Moment Distribution in Slabs Only
Effect of Beam Size on Moment Distribution
a) Beam Depth = 300 mm
Modeling, Analysis and Design of Buildings
b) Beam Depth = 500 mm
c) Beam Depth = 1000 mm
AIT - Thailand
ACECOMS
Modeling and Analysis for
Vertical Loads
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Modeling for Gravity Loads
• Must be carried out for several load cases/ patterns
• Does not change much for different floors
1. Use “Direct Design” Methods
–
–
–
Model, analyze and design “Floor by Floor, Without columns”
Slab analysis and design by using Coefficients
Beam analysis as continuous beams
2. Use Sub-Frame Concept
–
–
Model slab/ beam for in-plane loads
Model, analyze and design “Floor by Floor, With columns”
3. Use Grid, Plate Model for the Floor
–
–
Model slab and beams for out-of plane loads
Analyze un-symmetrical loads, geometry, openings etc.
4. Use full 3D Modeling
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The Design Strip Concept
Column Strip
Middle Strip
Design Strip
Design Strip
Middle Strip
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Using Equivalent Frame Method – Design Strip
Design Strip
½ Middle Strip
L2
Column Strip
½ Middle Strip
Drop Panels
L2
Longitudinal Beams
Transverse Beams
Modeling, Analysis and Design of Buildings
L1
AIT - Thailand
ACECOMS
Lateral Load
Resisting Systems
The Components Needed to
Complete the Load-Transfer Path
for Lateral Loads
Lateral Load Bearing Systems
Purpose
“ To Transfer Lateral Loads Applied at any location in the
structure down to the Foundation Level”
•
Single System
•
•
•
•
•
Moment Resisting Frames
Braced Frames
Shear Walls
Tubular Systems
Dual System
• Shear Wall - Frames
• Tube + Frame + Shear Wall
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Lateral Loads
• Primary Lateral Loads
– Load generated by Wind Pressure
– Load generated due to Seismic Excitation
• Other Lateral Loads
– Load generated due to horizontal component of Gravity
Loads in Inclined Systems and in Un-symmetrical
structures
– Load due to lateral soil pressure, liquid and material
retention
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Sample Lateral Load Resistance Systems
• Bearing wall system
– Light frames with shear panels
– Load bearing shear walls
• Fully Braced System (FBS)
– Shear Walls (SW)
– Diagonal Bracing (DB)
• Moment Resisting Frames (MRF)
– Special Moment-Resisting Frames (SMRF)
– Concrete Intermediate Moment-Resisting Frame (IMRF)
– Ordinary Moment-Resisting Frame (OMRF)
• Dual Systems (DS)
– Shear Walls + Frames (SWF)
– Ordinary Braced Frame (OBF)
– Special Braced Frame (SBF)
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Moment Resisting Frame
• The Load is transferred by
shear in columns, that
produces moment in
columns and in beams
• The Beam-Column
connection is crucial for the
system to work
• The moments and shear
from later loads must be
added to those from gravity
loads
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Shear Wall and Frame
• The lateral loads is
primarily resisted by the
shear in the walls, in turn
producing bending moment
• The openings in wall
become areas of high stress
concentration and need to
be handled carefully
• Partial loads is resisted by
the frames
• Traditionally 75/25
distribution haws been used
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Shear Wall - Frame
• The Walls are part of the
frame and act together with
the frame members
• The lateral loads is
primarily resisted by the
shear in the walls, in turn
producing bending moment.
• Partial loads is resisted by
the frame members in
moment and shear
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Braced Frame
• The lateral loads is primarily
resisted by the Axial Force in
the braces, columns and
beams in the braced zone.
• The frame away from the
braced zone does not have
significant moments
• Bracing does not have to be
provided in every bay, but
should be provided in every
story
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Tubular Structure
• The system is formed by using
closely spaced columns and deep
spandrel beams
• The lateral loads is primarily
resisted by the entire building
acting as a big cantilever with a
tubular/ box cross-section
• There is a “shear lag” problem
between opposite faces of the tube
due to in-efficiency of column
beam connection
• The height to width ratio should
be more than 5
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Braced Tube Systems
• Diagonal Braces are added to
the basic tubular structure
• This modification of the
Tubular System reduces shear
lag between opposite faces
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Lateral Load
Resisting
System
Behavior, Response
and Modeling
Modeling for Lateral Loads
1. 2D Frame Models
–
–
Convert building in to several 2D frames in each direction
Suitable for symmetrical loads and geometry
2. 3D Frame Model
–
–
Make a 3D frame model of entire building structure
Can be “open floor” model or “braced floor” model
3. Full 3D Finite Element Model
–
A full 3D Finite Element Model using plate and beam elements
4. Rigid Diaphragm Model
–
A special model suitable for buildings that uses the concept of Rigid
Floor Diaphragm
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Modeling as 2D Frame(s)
• Convert 3D Building to an assemblage of 2D Frames
– Using Independent Frames
– Using Linked Frames
– Using Sub-Structuring Concept
• Advantages
– Easier to model, analyze and interpret
– Fairly accurate for Gravity Load Analysis
• Main Problems:
–
–
–
–
Center of Stiffness and Center of Forces my not coincide
Difficult to consider building torsional effects
Several Frames may need to be modeled in each direction
Difficult to model non-rectangular framing system
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Create a Simple 2D Model
2. Select and
isolate Typical
2D Structure
1. Consider the Structure
Plan and 3D View
3. Discretize
the Model,
apply loads
4. Obtain results
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Using Linked Frames
F1
Linked Elements
Shear Wall
F2
F3
Modeling
Plan
F1
F2
F3
Link Element can allow only to transmit the shear and
axial force from one end to other end. It has moment
discontinuity at both ends
Typical Frame Elevation
Modeling, Analysis and Design of Buildings
Link Element act as a member which links the forces of
one frame to another frame, representing the effect of
Rigid Floor.
AIT - Thailand
ACECOMS
Full 3D Finite Element Model
• The columns and beams are modeled by using
beam elements
• The slabs and shear walls are modeled by using
plate elements
– At least 9 or 16 elements in each slab panel must be
used if gravity loads are applied to the slabs
– If the model is only for lateral analysis, one element
per slab panel may be sufficient to model the inplane stiffness
– Shear walls may be modeled by plate or panel or
plane stress element. The out of plane bending is
not significant
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Full 3D Finite Element Model
Example:
– Uses more than 4000
beam and plate elements
– Suitable for analysis for
gravity and lateral loads
– Results can be used for
design of columns and
beams
– Slab reinforcement
difficult to determine
from plate results
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Modeling of Floor Diaphragm
• Use Plate Elements
– Panels, Plane Stress
Use Diagonal
Bracing
• Use Diagonals
– In 3D Frame Models
• Use Conceptual Rigid
Diaphragm
Use Plate
Elements
– Link Frames in 2D
– Master DOF in 3D
– Use Approximately
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The Rigid Floor Diaphragm
• Combines the simplicity and advantages of the 2D Frame
models with the accuracy of the 3D models
• Basic Concept:
– The building structure is represented by vertical units (2D Frames,
3D Frames and Shear Walls), connected by the invisible rigid
diaphragm
– The lateral movement of all vertical units are connected to three
master degree of freedom
– This takes into account the building rotation and its effect on the
vertical units.
– The modeling and analysis is greatly simplified and made efficient
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Rigid Floor Diaphragm Concept
• Modeled as Rigid Horizontal Plane of infinite
in-plane stiffness (in X-Y plane)
• Assumed to have a hinge connection with
frame member or shear wall, so flexural
influence of all floors to lateral stiff ness is
neglected
• All column lines of all frames at particular
level can not deform independent of each
other
• The floor levels of all frames must be at the
same elevation and base line, but they need
not have same number of stories
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
How RFD Concept Works
Y
Building d.o.f.’s
F1 , 1
UL
r q
UL3
rY
X
F3 , 3
UL2
rx
UL1
F3 , 2
F2 , 1
Local Frame DOF
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
When Single Rigid Floor Cannot be Used
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Automatic Floor Meshing
and Auto Load Transfer
(In ETABS)
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Area Objects: Slab
By default uses two-way load transfer
mechanism
Simple RC solid slab
Can also be used to model one way slabs
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Area Object: Deck
Use one-way load transfer mechanism
Metallic Composite Slabs
Includes shear studs
Generally used in association with
composite beams
Deck slabs may be
o Filled Deck
o Unfilled Deck
o Solid Slab Deck
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Area Object: Plank
By default use one-way load transfer
mechanism
Generally used to model pre-cast slabs
Can also be simple RC solid slab
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Automatic Floor Meshing
First step to Auto Load Transfer
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Basic Floor Modeling Object
• Points
– Columns
– Load Points
– Boundary Point
• Lines
– Beams
• Areas
–
–
–
–
Deck:
Represents a Steel Metal Deck, One way Load Transfer
Plank : Represents clearly on-way slab portion
Slab:
Represents one-way or two-way slab portion
Opening: Represents Openings in Floor
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Automatic Meshing
• ETABS automatically meshes all line objects with frame
section properties into the analysis model
• ETABS meshes all floor type (horizontal) area objects (deck
or slab) into the analysis model
• Meshing does not change the number of objects in the
model
• To mesh line objects with section properties use Edit menu
> Divide Lines
• To mesh area objects with section properties use Edit menu
> Mesh Areas
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Automatic Meshing
• Automatic Meshing of Line Objects
– Frame elements are meshed at locations where other frame
elements attach to or cross them and at locations where point
objects lie on them.
– Line objects assigned link properties are never automatically
meshed into the analysis model by ETABS
– ETABS automatically meshes (divides) the braces at the point
where they cross in the analysis model
– No end releases are introduced.
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Automatic Meshing of Line Objects
Beam 1
Girder A
Beam 2
Beam 1
Piece 1
Piece 2
Beam 2
Piece 3
b) Girders A and B As Modeled in
the ETABS Analysis Model
Girder B
a) Floor Plan
Example showing how beams are automatically divided (meshed) where they
support other beams for the ETABS analysis model
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Automatic Meshing of Area Objects
– ETABS automatically meshes a floor-type area object up into foursided (quadrilateral) elements
– Each side of each element of the mesh has a beam (Real or Imaginary)
or wall running along it
– ETABS treats a wall like two columns and a beam where the columns
are located at the ends of the wall and the beam connects the columns.
– Each column is assumed to have four beams connecting to it
– The floor is broken up at all walls and all real and imaginary beams to
create a mesh of four-sided elements
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Automatic Meshing of Area Objects
Girder B
a) Floor Plan
Beam 3
Beam 2
Beam 1
Girder A
Beam 3
Beam 2
Beam 1
Girder A
Girder B
b) ETABS Imaginary Beams Shown Dashed c) ETABS Automatic Floor Meshing
Example of ETABS automatically generated mesh for floor-type area objects
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Automatic Meshing of Area Objects
Example of ETABS
automatically generated mesh
for floor-type area objects
Modeling, Analysis and Design of Buildings
a) Floor Plan (No Beams)
b) ETABS Imaginary Beams Connecting
Columns Shown Dashed
c) ETABS Imaginary Beams Extended to
Edge of Floor Shown Dashed
d) ETABS Automatic Floor Meshing
AIT - Thailand
ACECOMS
Automatic Meshing of Area Objects
– For floors that are automatically meshed by ETABS it is
recommended that model beams (or at least null-type line objects)
are connecting columns rather than no beams (or line objects)
– This makes the automatic meshing for the analysis model cleaner,
faster and more predictable
– Including beams and/or null-type line objects between all
columns in your model makes automatic floor meshing more
predictable
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Automatic Meshing of Area Objects
C4
C3
C4
C3
C4
C3
C1
a)
C2
C1
b)
C2
C1
c)
C2
C4
C3
C4
C3
C4
C3
C1
d)
C2
C1
e)
C2
C1
f)
C2
C4
C3
C4
C3
C4
C3
C1
g)
C2
C1
h)
C2
C1
i)
C2
Illustration of how ETABS
creates the distribution of
imaginary beams
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Automatic Transformation and
Transfer of Floor Loads to
Appropriate Elements
(Using the Auto Meshed Geometry)
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Load Transformation
The main issue:
How point loads, line loads and area loads that lie on an area
object in your object-based ETABS model are represented in
the analysis model
There are four distinct types of load transformation in
ETABS for out-of-plane load transformation for floor-type
area objects
•
•
•
•
with deck section properties
with slab section properties that have membrane behavior only
all other types of area objects
In-plane load transformation for all types of area objects
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Load Transformation
Area Objects
e2
Edge 3
4
3
a) Quadrilateral Element
b) The r and s Axes
(-1, -1)
Modeling, Analysis and Design of Buildings
3
(-1, 1)
2
r
4
(1, -1)
c) Corner Point r-s Coordinates
(-1, -1)
3
e4
Ed g
r
4
s
(1, 1)
1
2
– The normalization is the key
assumption in this method

Edge 3
3
s
1
Edge 1
Ed g
e2
Edge 1
(-1, 1)
– It is a perfectly valid assumption if the
quadrilateral is a square, rectangular or
a parallelogram
2
e4
2
Ed g
– ETABS normalizes the coordinates of
the four corner points of the area object
s
1
Ed g
– load transformation occurs after any
automatic meshing into the analysis
model
(1, 1)
(r, s)
1
r
P
4
(1, -1)
d) Point Load, P
Example of transfer of out-of-plane loads
for other area objects
AIT - Thailand
ACECOMS
Load Transformation
• The load distribution for deck sections is one way, in
contrast to slab sections which are assumed to span in two
directions
• ETABS first automatically meshes the deck into
quadrilateral elements
• Once the meshing is complete ETABS determines the
meshed shell elements that have real beams along them and
those that have imaginary beams
• It also determines which edges of the meshed shell elements
are also edges of the deck.
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Load Transformation
Rectangular Interior Meshed Element with Uniform Load
x
x/2
Edge 3
x/2
wx / 2
Edge 3
Edge 2
Edge 4
Edge 2
Direction of deck span
Edge 4
If the supporting member
at the end point of an
imaginary beam is itself
imaginary, then the load
from the imaginary beam
tributary to that end point
is lost, that is, it is
ignored by ETABS
c) Loading on Edges 2 and 4
Uniform load = w
Edge 1
a) Rectangular Interior Element
of Meshed Floor
Edge 1
b) Distribution of Uniform Load
Example of rectangular interior meshed
element with a uniform load
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Load Transformation
Rectangular Interior Meshed Element with Point Load
– ETABS distributes the point load to the appropriate edge beams
(based on the direction of the deck span)
– If the beams along edges are real beams ETABS transfers the load onto
adjacent beams
x1
x2
Point load, P
Edge 1
Edge 2
Direction of deck span
a) Rectangular Interior Element
of Meshed Floor
Modeling, Analysis and Design of Buildings
P
P * x1
x1 + x2
Edge 3
Edge 4
If the supporting
member at the end point
of an imaginary beam is
itself imaginary, then the
load from the imaginary
beam tributary to that
end point is lost, that is,
it is ignored by ETABS
Edge 4
x1
Edge 2
x2
P * x2
x1 + x2
c) Loading on Edge 2
P * x2
x1 + x2
P * x1
x1 + x2
b) Distribution of Point Load
d) Loading on Edge 4
AIT - Thailand
ACECOMS
Load Transformation
Rectangular Interior Meshed Element with Line Load
– A line load is transformed in a similar fashion to that for a point load
using a numerical integration technique
– The line load is discredited as a series of point loads which are
transformed to surrounding beams
– The series of point loads is then converted back to a line load on the
surrounding beams
– An area load that does not cover the entire element is also transformed in
a similar fashion to that for a point load using a numerical integration
technique.
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
General Interior Meshed Element
Edge
4
Direction of deck span
3
Edge
Line 3 P3
Midpoint
Edge 1
b)
c)
b)
Edge 1
d)
2
Edge
Edge 1
3
4
Edge 1
Line 1
Edge
Edge
P1
3
Edge
2
Edge
Line 2
4
P2
P1
a) General Interior Element of
Meshed Floor Deck
Edge 1
a) General Interior Element of
Meshed Floor Deck
Edge
Edge
4
2
P2
Edge
Edge
4
P3
Edge 1
Midpoint
2
3
Edge
Edge
3
2
2
Edge
E dge
4
2
3
Edge
Edge
E dge
4
Uniform load
Edge
E dge
3
Edge
Edge 1
e) Transformation of Uniform Load
f) Loading on Edge 1
Example of general interior meshed
element with a point load
g) Loading on Edge 2
h) Loading on Edge 3
i) Loading on Edge 4
Example of general interior meshed element with a
uniform load
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Exterior Meshed Element
D
E
Beam 2b
F
A
B
Beam 2a
C
Beam 3b
D
Beam 3a
A
B
Beam 4a
a) Floor Plan
C
Imaginary Beam 5
Beam 1a
Beam 2a
Beam 1a
Beam 3b
No beam at
edge of deck
Modeling, Analysis and Design of Buildings
E
Imaginary
Beam 6
Beam 3a
Beam 1b
No beam at
edge of deck
b) Deck Meshing
Beam 2b
Beam 1b
a) Floor Plan
Example of exterior meshed elements
with cantilever beams extending to
edge of deck
Beam 1b
Beam 2b
Edge of deck is at
center of spandrel
beam, typical in this
example
Beam 1a
Beam 2a
Beam 1b
Beam 2b
Example of exterior meshed
elements with real beams on all sides
Beam 4b
b) Deck Meshing
AIT - Thailand
ACECOMS
Exterior Meshed Element
am 8
ary Be
am 7
ImaginaryBeam 6
Imagin
E
B
A
Imaginary Beam 5
Beam 3b
Beam 3a
Beam 2a
C
No beam at
edge of deck
b) Deck Meshing
a) Floor Plan
D
Modeling, Analysis and Design of Buildings
am 8 E2
ary Be
m7
Imagin
ry Bea
D
Beam 2b
a
Imagin
Beam 1b
Example of exterior
meshed elements
with cantilever
beams extending to
edge of a skewed
deck
Beam 3a
E1
ImaginaryBeam 6
Beam 2a
Beam 1a
Beam 3b
D
Beam 1b
Beam 2b
Beam 3a
Beam 1a
Beam 1b
No beam at
edge of deck
Beam 2b
ary Be
Imagin
Beam 3b
c) Condition at Skewed Deck
Edge (Areas D and E)
AIT - Thailand
ACECOMS
Exterior Meshed Element
Edge of deck
E
D
Beam 1
Beam 1
a) Floor Plan
Column 1
A
B
Beam 2
Beam 2
Column 1
C
b) Deck Meshing
Example of exterior meshed elements with overhanging slab
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Exterior Meshed Element
a) Floor Plan
E
A
B
Beam 1b
F
C
Beam 3b
D
I
J
Beam 3a
Beam 1a
Beam 2b
Beam 1b
H
Beam 2a
Beam 2a
Beam 1a
Beam 2b
G
K
b) Deck Meshing
Example of exterior meshed elements with overhanging slab
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Effect of Deck Openings
6'
14'
Note: Assume floor loading is 100
psf. Opening is either loaded or
unloaded as noted in c, d, e and f
which are loading diagrams for
Beam 1.
Example of effect of openings
on distribution of load over
deck sections
6'
4'
6'
14'
4'
4'
0.6 klf
2'
0.2 klf
Beam 1
c) Unframed, unloaded opening
a) Floor Plan with Unframed Opening
4'
6'
14'
d) Unframed, loaded opening
0.7k
0.7k
0.1 klf
0.6 klf
6'
0.6 klf
2'
4'
e) Framed, unloaded opening
Beam 1
b) Floor Plan with Framed Opening
(Beams on all Sides)
Modeling, Analysis and Design of Buildings
0.6 klf
1.5k
1.5k
0.1 klf
0.6 klf
f) Framed, loaded opening
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ACECOMS
Load Transformation
Vertical Load Transformation for Floors with Membrane
Slab Properties
– only applies to floor-type area objects with slab section
properties that have membrane behavior only
– The load distribution for membrane slab sections is two way
– The actual distribution of loads on these elements is quite
complex
– ETABS uses the concept of tributary loads as a simplifying
assumption for transforming the loads
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
1
1
3 3
1
1
midpoints
Floors with Membrane Slab Properties
1
h) Real beams on two
adjacent sides plus
one vertical support
element at corner point
g) Real beam on one side
plus one vertical
support element at
corner point
3
2
3
2
4 4
2 2
1
1
a) Real beams on all sides
3 3
2 2
1
1
b) Case 1 of real beams on
three sides
4
3
4
3 3
1
1
c) Case 2 of real beams on
three sides
2
2
1
d) Real beams on two
adjacent sides
1
e) Real beams on two
opposite sides
2
midpoint
1
1
2
2
2
3
2
3
2
1
m)Vertical support
elements at two
opposite corner points
(no real beams)
2
2
1
2
k) Vertical support
elements at three
corner points (no real
beams)
1
2
l) Vertical support
elements at two
adjacent corner points
(no real beams)
1
1
1
f) Real beam on one side
Real beam at shell edge
1
n) Vertical support
elements at one
corner point (no
real beams)
No beam at shell edge
Tributary area dividing line
Vertical support element
Legend
3 3
1
1
1
h) Real beams on two
adjacent sides plus
one vertical support
element at corner point
g) Real beam on one side
plus one vertical
support element at
corner point
4
2
1
1
1
1
2
j) Vertical support
elements at all corner
points (no real beams)
1
1
3
2
2
2 2
3
3
1
1
i) Real beam on one side
plus two vertical
support elements at
corner points
3
midpoints
1
i) Real beam on one side
plus two vertical
support elements at
corner points
Tributary areas for various
conditions of a membrane slab
3
3
4 Analysis
3 and Design of Buildings
Modeling,
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ACECOMS
Floors with Membrane Slab Properties
3
3
3
3
4 4
2 2
4 4
1
2 2
1
1
a) Full uniform load
transformation
1
b) Partial uniform load
transformation
3
3
3
3
4 4
Example of load distribution on a
membrane slab
2 2
4 4
2 2
1
1
1
c) Line load transformation
1
d) Point load transformation
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
Type of Slab Systems in SAFE
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The 5-Story Walkup Flats
A
B
C
D
E
F
G
6
5
6.0
4
6.0
3
2
1
2.8
2.8
4.0
4.0
5.5
5.5
4.0
4.0
Column Layout Plan
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The 5-Story Walkup Flats
A
B
C
D
E
F
G
6
5
C2
C1
C1= 0.3 x 0.8
C2 = 0.3 x 0.4
6.0
4
B1 = 0.25 x 0.4
B2 = 0.25 x 0.5
B1
6.0
B2
S1 = 0.15
3
2
1
2.8
2.8
4.0
4.0
5.5
5.5
4.0
4.0
Slab and Beam Layout
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
The 5-Story Walkup Flats
3.0
3.0
3.0
3.0
3.5
2.0
6
5
3
4
2
1
Section
Modeling, Analysis and Design of Buildings
AIT - Thailand
ACECOMS
35 Story Office Building
5
7.0
4
8.0
3
8.0
2
Plan
Typical Floor
(B1, B2, 4-35)
7.0
1
A
6.0
B
6.0
C
Modeling, Analysis and Design of Buildings
8.0
D
8.0
E
6.0
F
6.0
G
AIT - Thailand
ACECOMS
35 Story Office Building
5
7.0
4
8.0
3
8.0
2
Plan
Floor 1-2
7.0
1
A
6.0
B
6.0
C
Modeling, Analysis and Design of Buildings
8.0
D
8.0
E
6.0
F
6.0
G
AIT - Thailand
ACECOMS
35 Story Office Building
5
7.0
4
8.0
3
8.0
2
Plan
Floor 3
7.0
1
A
6.0
B
6.0
C
Modeling, Analysis and Design of Buildings
8.0
D
8.0
E
6.0
F
6.0
G
AIT - Thailand
ACECOMS
35 Story Office Building
32 @ 3.5
2 @ 5.0
2 @ 2.8
Section at
C and D
5
Modeling, Analysis and Design of Buildings
4
3
2
1
AIT - Thailand
ACECOMS
35 Story Office Building
32 @ 3.5
2 @ 5.0
2 @ 2.8
Section at
B and E
5
Modeling, Analysis and Design of Buildings
4
3
2
1
AIT - Thailand
ACECOMS
35 Story Office Building
32 @ 3.5
2 @ 5.0
2 @ 2.8
Section at
A and G
5
Modeling, Analysis and Design of Buildings
4
3
2
1
AIT - Thailand
ACECOMS