Integrated Design Process

Overall Design Process
• Conception
• Modeling
• Analysis
• Design
• Detailing
• Drafting
• Costing
Integrated
Design
Process

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

Beams, Columns, Two-way Slabs, Flat Slabs, Pile caps
Shear Walls, Deep Beams, Isolated Footings, Combined Footings
Sub-structure and Member Design
Frame and Shear Walls
Lateral Load Resisting System Floor Slab System
Gravity Load Resisting System
Building Structure
Floor Diaphragm
The Building Structural System - Physical

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

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

The Simplified Structural System
STRUCTURE
pv
EXCITATION
Loads
Vibrations
Settlements
Thermal Changes
RESPONSES
Displacements
Strains
Stress
Stress Resultants

Analysis of Structures
pv






xx yy zz
vx
x y z
p
    0
Real Structure is governed by “Partial
Differential Equations” of various order
Direct solution is only possible for:
• Simple geometry
• Simple Boundary
• Simple Loading.

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

Structural
Model
The Need for Structural Model
EXCITATION
Loads
Vibrations
Settlements
Thermal Changes
RESPONSES
Displacements
Strains
Stress
Stress Resultants
STRUCTURE
pv

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”

Continuum to Discrete Model
pv
(Governed by partial
differential equations)
CONTINUOUS MODEL
OF STRUCTURE
(Governed by either
partial or total differential
equations)
DISCRETE MODEL
OF STRUCTURE
(Governed by algebraic
equations)
3D-CONTINUM
MODEL

From Classical to FEM Solution






xx yy zz
vx
x y z
p
    0
 
t
v
t
s
t
v
dV p udV p uds
_ _ _
  


Assumptions
Equilibrium
Compatibility
Stress-Strain Law
(Principle of Virtual Work)
“Partial Differential
Equations”
Classical
Actual Structure
Kr R

“Algebraic
Equations”
K = Stiffness
r = Response
R = Loads
FEM
Structural Model

Simplified Structural System
Loads (F) Deformations (D)
Fv
F = K D
F
K
D

The Structural System
EXCITATION RESPONSES
STRUCTURE
pv
• Static
• Dynamic
• Elastic
• Inelastic
• Linear
• Nonlinear

The Equilibrium Equations
1. Linear-Static Elastic OR Inelastic
2. Linear-Dynamic Elastic
3. Nonlinear - Static Elastic OR Inelastic
4. Nonlinear-Dynamic Elastic OR Inelastic
F
Ku 
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(
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(
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(
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( t
F
t
Ku
t
u
C
t
u
M 
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)
(
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(
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(
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(
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( t
F
t
F
t
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t
u
C
t
u
M NL 
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F
F
Ku NL 


Basic Steps in FEA
Evaluate Real Structure
Create Structural Model
Discretize Model in FE
Solve FE Model
Interpret FEA Results
Physical significance of Results
Engineer
Engineer + Software
Software

X
Z
Y
Membrane/ Panel
In-Plane, Only Axial
Shell
In-Plane and Bending
Plate/ Slab
Out of Plane, Only Bending
General Solid
Regular Solid
Plate/ Shell
( T small compared to Lengths )
( Orthogonal dimensions)
Discretization of Continuums
Beam Element
Solid Element
H, B much less than L

Global Modeling of Structural Geometry
(b) Solid Model (c) 3D Plate-Frame (d) 3D Frame
(a) Real Structure
(e) 2D Frame
Fig. 1 Various Ways to M odel a Real Struture
(f) Grid-Plate

Dimensions of Elements
• 1 D Elements (Beam type)
– Can be used in 1D, 2D and 2D
– 2-3 Nodes. A, I etc.
• 2 D Elements (Plate type)
– Can be used in 2D and 3D Model
– 3-9 nodes. Thickness
• 3 D Elements (Brick type)
– Can be used in 3D Model
– 6-20 Nodes.
Truss and Beam Elements (1D,2D,3D)
Plane Stress, Plane Strain, Axisymmetric, Plate and Shell Elements (2D,3D)
Brick Elements

DOF for 1D Elements
Dx
Dy
Dx
Dz
Dy
Dx
Dy
Rz
Dy
Rx
Rz Dx
Dz
Dy
Rx
Rz
Ry
2D Truss 2D Beam 3D Truss
2D Frame 2D Grid 3D Frame
Dy
Rz

DOF for 2D Elements
Dx
Dy
Dy
Ry ?
Rz
Rx
Dz
Dy
Rx
Rz
Ry ?
Dx
Membrane Plate Shell

DOF for 3D Elements
Dx
Dz
Dy
Solid/ Brick

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

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

1 unit









x1
x3
x2
3D Problem
2D Problem
Plain-Strain
Assumptions
Plane Stress and Plane



x
x
Plane Stress Problem
Plane Strain Problem

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

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

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

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

Connecting Different Types of Elements
Truss Frame Membrane Plate Shell Solid
Truss
OK OK Dz OK OK OK
Frame
Rx, Ry, Rz OK
Rx, Ry, Rz,
Dz
Rx ?
Dx, Dy
Rx ? Rx, Ry, Rz
Membrane
OK OK OK Dx, Dy OK OK
Plate
Rx, Rz OK Rx, Rz OK OK Rx, Rz
Shell
Rx, Ry, Rz OK
Rx, Ry, Rz,
Dz
Dx, Dz OK Rx, Rz
Solid
OK OK Dz Dx, Dz OK OK
0
Orphan Degrees Of Freedom:
1 2 3 4

What Type of
Analysis should be
Carried Out?

Analysis Type
– The Type of Excitation (Loads)
– The Type Structure (Material and Geometry)
– The Type Response
The type of Analysis to be carried out
depends on the Structural System

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

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

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”

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.

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

Elasticity and Linearity
Action
Deformation
Action
Deformation
Action
Deformation Action Deformation
Linear-Elastic Linear-Inelastic
Nonlinear-Elastic Nonlinear-Inelastic

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

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

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

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

Structural Members
Dimensional Hierarchy of Structural Members
Continuum
Regular Solid
(3D)
Beam (1D)
b h
L>>(b,h)

b
h
t
z
Plate/Shell (2D)
x z
t<<(x,z)
 x
z
y
x L

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

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

Graphic Object Representation
Object
Line
Area
Volume
Point Load
Concentrated Load
Beam Load
Wall Load
Slab Load
Slab Load
Wind Load
Seismic Load
Liquid Load
Node
Beam / Truss
Connection Element
Spring Element
Plate Element
Shell Element
Panel/ Plane
Solid Element
Point Support
Column Support
Line Support
Wall Support
Beam Support
Soil Support
Soil Support
Point
Load
Geometry
Medium
Support
Boundary
ETABS uses graphic object modeling concept

Load Transfer Path is difficult to Determine
• Complexity of Load Transfer
Mechanism depend on:
– Complexity of Load
– Complexity of Medium
– Complexity of Boundary
Point Line Area Volume
Line
Area
Vol.
Line
Area
Volume
Load
Medium
Boundary

Load Transfer Path is difficult to Determine
Transfer of a Point Load to Point Supports Through Various Mediums
Point Line Area Volume

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

The Frame Element
• The Actions Corresponding to Six DOF at Both Ends, in
Local Coordinate System
1
3
2
3
2
+P
+V2
+V3
+V3
+V2
+P
1
3
2
3
2
+T
+M2
+M3
+M3
+M2
+T

Shell Element
General
•Total DOF per Node = 6 (or 5)
•Total Displacements per Node = 3
•Total Rotations per Node = 3
•Used for curved surfaces
Application
•For Modeling surface elements carrying
general loads
Building Specific Application
•May be used for modeling of general slabs
systems. But not used generally
1
2
3
U1, R1
Node 3
U3, R3
U2, R2
U1, R1
Node 1
U3, R3 U2, R2
U1, R1
Node 4
U3, R3
U2, R2
U1, R1
Node 2
U3, R3
U2, R2
Shell

Plate Element
General
•Total DOF per Node = 3
•Total Rotations per Node = 2
•Plates are for flat surfaces
Application
out of plane loads
•For representing floor slabs for Vertical
Load Analysis
•Model slabs
R1
Node 1
U3
R2
1
2
3
R1
Node 2
U3
R2
R1
Node 3
U3
R2
R1
Node 4
U3
R2
Plate

Membrane Element
General
•Total DOF per Node = 3 (or 2)
•Total Rotations per Node = 1 (or 0)
•Membranes are modeled for flat surfaces
Application
in-plane loads
•For representing floor slabs for Lateral
Load Analysis.
• Model Shear walls, Floor Diaphragm etc
Membrane
U1
Node 1
R3
U2
U1
Node 3
R3
U2
U1
Node 4
R3
U2
U1
Node 2
U2
3 2
1

Meshing Slabs and Walls
In general the mesh in the slab
should match with mesh in the wall
to establish connection
Some software automatically
establishes connectivity by using
constraints or “Zipper” elements
“Zipper”

Selection Of Structural Systems
Basic Concepts and Considerations

Knowledge Model for System Selection
Structural
System Selection
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Systems Engineering
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Value Engineering Econom
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Construction
Engineering
K
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Artificial Intelligence
Software Engineering
Bu
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Stru
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• Architecture
• Building Services
• Construction Eng.
• Value Eng.
• Aesthetics
• Ergonomics Eng.
• Structural Eng.
• Knowledge Eng.
• Economics
• Artificial Intelligence
• System Eng.
• Common Sense

Determining System Suitability










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
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p
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The Analytical Hierarchy Approach
A weighted importance and suitability value analysis to
determine the comparative value of a system or option
Value of
an Option
Global
Importance
Weights and
Scores
Sub
Importance
Weights and
Scores
Suitability
Value and
Score

Evaluating System Suitability
Slab Systems Criteria Weights and Scores System
Value
(V)
Main Criteria Ai Am
Sub Criteria Bij Sub Criteria Bin Bmn
Item k Item p Item k Item p Item p
Wt Score Wt Score Wt Score Wt Score Score
System – 1
System – l Cijkl Sijkl Cijnl Sijpl Cinkl Sinkl Cinnl Sinpl Smnpl
System - q






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

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 
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

ijk
p
k
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ij
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C
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The Suitability Equation
Using the Suitability Equation

Assigning Suitability Values
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
Score or Weight Representation of Suitability
0 Definitely not required, definitely not suitable, ignore

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

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

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

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

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

Vertical Load
Behavior and Response

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

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

Popular Gravity Load Resting Systems

Gravity Load Transfer Paths
Single Path
Slab On Walls
Single Path
Slab on Columns
Dual Path
Slab On Beams,
Beams on Columns

Gravity Load Transfer Paths
Mixed Path
Slab On Walls
Slab On Beams
Beams on Walls
Complex Path
Slab on Beams
Slab on Walls
Beams on Beams
Beams on Columns
Three Step Path
Slab On Ribs
Ribs On Beams
Beams on Columns

Simplified Load Transfer
Transfer of Area Load
To Lines To Points To Lines and Points

Load Transfer Through Slab and Beam

Slab System Behavior
Slab T = 200 mm
Beam Width, B = 300 mm
Beam Depth, D
a) 300 mm
b) 500 mm
c) 1000 mm
D
B

Moment Distribution in Beam-Slab
Effect of Beam Size on
Moment Distribution
a) Beam Depth = 300 mm
b) Beam Depth = 500 mm
c) Beam Depth = 1000 mm

Moment Distribution in Slabs Only
Effect of Beam Size on Moment Distribution
a) Beam Depth = 300 mm b) Beam Depth = 500 mm c) Beam Depth = 1000 mm

Modeling and Analysis for
Vertical Loads

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

Column Strip
Middle Strip
Design
Strip
Middle Strip
Design
Strip
The Design Strip Concept

Using Equivalent Frame Method – Design Strip
Column Strip
½ Middle Strip
½ Middle Strip
Design Strip
L2
L2
L1
Longitudinal Beams
Transverse Beams
Drop Panels

Lateral Load
Resisting Systems
The Components Needed to
Complete the Load-Transfer Path
for Lateral Loads

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
Lateral Load Bearing Systems

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

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)

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

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

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

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

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

Braced Tube Systems
• Diagonal Braces are added to
the basic tubular structure
• This modification of the
Tubular System reduces shear
lag between opposite faces

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 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

Create a Simple 2D Model
1. Consider the Structure
Plan and 3D View
2. Select and
isolate Typical
2D Structure
4. Obtain results
3. Discretize
the Model,
apply loads

Using Linked Frames
Plan
Modeling
Shear Wall
Typical Frame Elevation
Linked Elements
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
Link Element act as a member which links the forces of
one frame to another frame, representing the effect of
Rigid Floor.
F3
F2
F1
F1
F2 F3

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 in-
plane stiffness
– Shear walls may be modeled by plate or panel or
plane stress element. The out of plane bending is
not significant

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

Use Plate
Elements
Modeling of Floor Diaphragm
Use Diagonal
Bracing
• Use Plate Elements
– Panels, Plane Stress
• Use Diagonals
– In 3D Frame Models
• Use Conceptual Rigid
Diaphragm
– Link Frames in 2D
– Master DOF in 3D
– Use Approximately

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

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

How RFD Concept Works
UL
UL1
UL2
UL3
X
Y
F3 , 2
F1 , 1
F3 , 3
Building d.o.f.’s
F2 , 1
r x
r q
rY
Local Frame DOF

When Single Rigid Floor Cannot be Used

Automatic Floor Meshing
and Auto Load Transfer
(In ETABS)

Area Objects: Slab
By default uses two-way load transfer
mechanism
Simple RC solid slab
Can also be used to model one way slabs

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

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

Automatic Floor Meshing
First step to Auto Load Transfer

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

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

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.

Automatic Meshing of Line Objects
Girder A
Girder B
Beam
1
Beam
2
Piece 1 Piece 2 Piece 3
Beam 1 Beam 2
b) Girders A and B As Modeled in
the ETABS Analysis Model
a) Floor Plan
Example showing how beams are automatically divided (meshed) where they
support other beams for the ETABS analysis model

Automatic Meshing of Area Objects
– ETABS automatically meshes a floor-type area object up into four-
sided (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

Girder A
Girder B
Beam
1
Beam
2
Beam
3
Girder A
Girder B
Beam
1
Beam
2
Beam
3
c) ETABS Automatic Floor Meshing
b) ETABS Imaginary Beams Shown Dashed
a) Floor Plan
Example of ETABS automatically generated mesh for floor-type area objects

d) ETABS Automatic Floor Meshing
b) ETABS Imaginary Beams Connecting
Columns Shown Dashed
a) Floor Plan (No Beams)
c) ETABS Imaginary Beams Extended to
Edge of Floor Shown Dashed
Example of ETABS
automatically generated mesh
for floor-type 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

c)
b)
a)
f)
e)
d)
i)
h)
g)
C1 C2
C3
C4
C1 C2
C3
C4
C1 C2
C3
C4
C1 C2
C3
C4
C1 C2
C3
C4
C1 C2
C3
C4
C1 C2
C3
C4
C1 C2
C3
C4
C1 C2
C3
C4
Illustration of how ETABS
creates the distribution of
imaginary beams

Automatic Transformation and
Transfer of Floor Loads to
Appropriate Elements
(Using the Auto Meshed Geometry)

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

Load Transformation
Area Objects
– load transformation occurs after any
automatic meshing into the analysis
model
– ETABS normalizes the coordinates of
the four corner points of the area object
– The normalization is the key
assumption in this method
– It is a perfectly valid assumption if the
quadrilateral is a square, rectangular or
a parallelogram

a) Quadrilateral Element
Edge 1
E
d
g
e
4
1
2
4
3
E
d
g
e
2
Edge 3
b) The r and s Axes
Edge 1
E
d
g
e
4
1
2
4
3
r
s
E
d
g
e
2
Edge 3
(1, 1)
(-1, 1)
(1, -1)
(-1, -1)
c) Corner Point r-s Coordinates
1
2
4
3
r
s
(r, s)
P
(1, 1)
(-1, 1)
(1, -1)
(-1, -1)
d) Point Load, P
1
2
4
3
r
s
Example of transfer of out-of-plane loads
for other area objects

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.

Load Transformation
Rectangular Interior Meshed Element with Uniform Load
Edge 1
Edge 3
Edge
2
Edge
4
x
Edge 1
Edge 3
Edge
2
Edge
4
x / 2 x / 2
Uniform load = w
Direction of deck span
a)Rectangular Interior Element
of Meshed Floor
b)Distribution of Uniform Load
wx / 2
c) Loading on Edges 2 and 4
Example of rectangular interior meshed
element with a uniform load
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

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
Edge 1
Edge 3
Edge
2
Edge
4
x1 x2
Point load, P
a) Rectangular Interior Element
of Meshed Floor
b)Distribution of Point Load
x1 x2
Edge 4 Edge 2
P
P * x2
x1 + x2
P * x1
x1 + x2
c) Loading on Edge 2
P * x1
x1 + x2
d) Loading on Edge 4
P * x2
x1 + x2
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

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.

General Interior Meshed Element
d)
Edge 1
Edge 3
E
d
g
e
2
E
d
g
e
4
Edge 1
Edge 3
E
d
g
e
2
E
d
g
e
4
e) Transformation of Uniform Load
Edge 1
Edge 3
E
d
g
e
2
E
d
g
e
4
Uniform load
a) General Interior Element of
Meshed Floor Deck
b)
Edge 1
Edge 3
E
d
g
e
2
E
d
g
e
4
Edge 1
Edge 3
E
d
g
e
2
E
d
g
e
4
c)
g) Loading on Edge 2
f) Loading on Edge 1
h) Loading on Edge 3 i) Loading on Edge 4
Midpoint
Midpoint
Example of general interior meshed element with a
uniform load
a) General Interior Element of
Meshed Floor Deck
Edge 1
Edge 3
E
d
g
e
2
E
d
g
e
4
P1
P2
P3
b)
Edge 1
Edge 3
E
d
g
e
2
E
d
g
e
4
P1
P2
P3
Line 1
Line 2
Line 3
Example of general interior meshed
element with a point load

Exterior Meshed Element
Edge of deck is at
center of spandrel
beam, typical in this
example
Beam 1a
B C
A
E F
D
a) Floor Plan b) Deck Meshing
Beam 1b Beam 1b
Beam
2a
Beam
2b
Beam
2a
Beam
2b
Example of exterior meshed
elements with real beams on all sides
Beam 3a
B C
A
E
D
Beam 3b
Beam
1a
Beam
1b
Imaginary
Beam
5
Beam
2b
Beam 3a Beam 3b
Beam
1a
Beam
1b
Beam
2a
Beam
2b
Beam 4a Beam 4b
Imaginary
Beam
6
No beam at
edge of deck
No beam at
edge of deck
Example of exterior meshed elements
with cantilever beams extending to
edge of deck

Imaginary Beam 8
B C
A
E
D
Imaginary
Beam
5
ImaginaryBeam
6
Beam 3a Beam 3b
Beam
1a
Beam
1b
Beam
2a
Beam
2b
Beam 3a Beam 3b
Beam
1a
Beam
1b
Beam
2a
Beam
2b
Imaginary Beam 7
Imaginary Beam 8
E1
ImaginaryBeam
6
Beam 3b
Beam
2b
E2
c) Condition at Skewed Deck
Edge (Areas D and E)
Imaginary Beam 7
D
D
Beam 3a
Beam
1b
No beam at
edge of deck
No beam at
edge of deck
Example of exterior
meshed elements
with cantilever
beams extending to
edge of a skewed
deck

Beam 1
B C
A
E
D
Beam
2
Beam 1
Beam
2
Column 1 Column 1
Edge of deck
Example of exterior meshed elements with overhanging slab

Beam 1a Beam 1a
B C
A
E F
D
Beam 1b Beam 1b
Beam
2a
Beam
2b
Beam
2a
Beam
2b
G H I
J
K
Beam
3a
Beam
3b
Example of exterior meshed elements with overhanging slab

Effect of Deck Openings
a) Floor Plan with Unframed Opening
Beam 1
4' 6' 14'
6'
4'
2'
b) Floor Plan with Framed Opening
(Beams on all Sides)
Beam 1
4' 6' 14'
6'
4'
2'
c) Unframed, unloaded opening
4' 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.
d) Unframed, loaded opening
e) Framed, unloaded opening
f) Framed, loaded opening
0.7k
0.6 klf
0.2 klf
0.6 klf 0.6 klf
0.6 klf 0.6 klf
0.1 klf
0.1 klf
0.7k
1.5k 1.5k
Example of effect of openings
on distribution of load over
deck sections

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

Floors with Membrane Slab Properties
i) Real beam on one side
plus two vertical
support elements at
corner points
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
1
1
1
1
1
3
1
3
l) Vertical support
elements at two
adjacent corner points
(no real beams)
j) Vertical support
elements at all corner
points (no real beams)
1
1
3
3
3
k) Vertical support
elements at three
corner points (no real
beams)
4
2
2
4
1 2
1
2
3
1 2
1 2
m)Vertical support
elements at two
opposite corner points
(no real beams)
1
1
Legend
Real beam at shell edge
No beam at shell edge
Tributary area dividing line
Vertical support element
midpoints
n) Vertical support
elements at one
corner point (no
real beams)
1
1
2
2
f) Real beam on one side
e)Real beams on two
opposite sides
d)Real beams on two
adjacent sides
c)Case 2 of real beams on
three sides
b)Case 1 of real beams on
three sides
a)Real beams on all sides
1
3
2
4
1
3
2
4
1
2
3
1
2
3
1
2
3
1
2
3
1
2
1
2
1
1
1
1
2
2
i) Real beam on one side
plus two vertical
support elements at
corner points
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
1
1
1
1
1
3
1
3
2
2
2
2
midpoint
2
2
3
3
3
3
3
4
4
3
midpoints
Tributary areas for various
conditions of a membrane slab

Floors with Membrane Slab Properties
a)Full uniform load
transformation
b)Partial uniform load
transformation
c)Line load transformation d)Point load transformation
1
3
2
4
3
2
4
1
1
3
2
4
3
2
4
1
1
3
2
4
3
2
4
1
1
3
2
4
3
2
4
1
Example of load distribution on a
membrane slab

The 5-Story Walkup Flats
4.0 4.0 5.5 5.5 4.0 4.0
6.0
6.0
2.8
2.8
Column Layout Plan
1
2
3
5
6
A C
B D E F G
4

4.0 4.0 5.5 5.5 4.0 4.0
6.0
6.0
2.8
2.8
Slab and Beam Layout
1
2
3
5
6
A C
B D E F G
4
C1= 0.3 x 0.8
C2 = 0.3 x 0.4
B1 = 0.25 x 0.4
B2 = 0.25 x 0.5
S1 = 0.15
B1
B2
C1
C2

1
2
3
5
6 4
3.0
3.0
3.0
3.0
3.5
2.0
Section

35 Story Office Building
6.0 6.0 8.0 8.0 6.0 6.0
8.0
8.0
1
2
4
5
A C
B D E F G
3
7.0
7.0 Plan
Typical Floor
(B1, B2, 4-35)

6.0 6.0 8.0 8.0 6.0 6.0
8.0
8.0
1
2
4
5
A C
B D E F G
3
7.0
7.0 Plan
Floor 1-2

6.0 6.0 8.0 8.0 6.0 6.0
8.0
8.0
1
2
4
5
A C
B D E F G
3
7.0
7.0 Plan
Floor 3

1
2
4
5 3
2 @ 5.0
2 @ 2.8
32 @ 3.5
Section at
C and D

1
2
4
5 3
2 @ 5.0
2 @ 2.8
32 @ 3.5
Section at
B and E

1
2
4
5 3
2 @ 5.0
2 @ 2.8
32 @ 3.5
Section at
A and G

Integrated Design Process

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