modeling of shear walls for nonlinear and push over...
TRANSCRIPT
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Modeling of Shear Walls for Nonlinear and
Pushover Analysis of Tall Buildings
Naveed Anwar, D. EngNaveed Anwar, D. EngNaveed Anwar, D. EngNaveed Anwar, D. Eng
Asian Center for Engineering Computations and Software, AIT
Asian Institute of Technology, Thailand
14th ASEP International Convention, Philippines, May 2009
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Some of the Questions Related to Shear Walls
� What is a Shear Wall
� How does a Shear Wall behave
� What is the normal role of Shear Wall
� What role a Shear Wall can play
� How to Model and Design the Shear Walls for the intended role
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What is a Shear Wall?
� How can we “tell” when a member is a shear wall
� Is the definition based on ?
� Intended Use
� Shape in Cross-section
� Geometry in Elevation
� Loading Type and Intensity
� Behavior and Theory
� Location, Direction, Orientation
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Shear Wall or Column
Wall Column
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Shear Wall or Frame
Shear Wall FrameShear Wall or Frame ?
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Shear Wall or Truss?
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Conventional Role of Shear Walls
� Provide lateral stiffness to buildings
� Reduce Drift Ratio
� Reduce wind-induced acceleration
� Provide strength against lateral loads
� Shift moment and shear away from frame members
� Change the deformation and mode of the building
� Interact with frames to convert shear and moment to axial
forces through outriggers etc.
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Tall Shear Wall Design
� Primarily governed by Flexural Strength, can be allowed to yield at
well defined locations
� Shear is generally not
the critical factor for tall shear walls
� All walls must and can remain elastic in shear without failure
H=100 m
H=50 m
W=1
W=1
V
M
25 50 (2 times)
833 3,333 (4 times)
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Seismic Code Development
Actual Elastic Demand Vs Code-mandated Design ForcesThe difference is expected be handled by
yielding, ductility, energy dissipation and reduction of demand
Typically handled by “Response reduction Factor”
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New Design Approaches
� Current building codes do not adequately address many
critical aspects in seismic design of tall buildings
� Performance based design provides a desirable
alternative
� Reinforced concrete walls are effective to resist lateral
loads while providing good performance
� Various approaches exist to predict the reliable nonlinear and inelastic response of RC walls
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Performance Based Design in International Context
� Explicitly stated by local authorities in some countries
such as Japan and China
� UBC, IBC and other codes provide little-to-no specific
guidance
� Eurocode 8 is not performance based
� Much framework for performance based design is in
Vision 2000, ATC40 and FEMA 356
� Recently, performance based design of high-rise
buildings issued in LATBSDC 2008 and SEAONC 2007
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Basic Vertical Seismic Systems
Moment Resisting Frame
Braced Frame Shear Walls
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Typical Multi-Story Structural Systems
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Nonlinear Performance Comparisons
� Six alternative structural systems compared by pushover plots for specific four-story building
�BF – Braced Frame
�SW – Shear Wall
�EBF – Eccentric Braced Frame
�MF – Steel Moment Frame
�MF+Dampers – Steel Moment Frame with Passive Dampers
�BI – Base Isolation
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Nonlinear Performance Comparisons
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Role of Shear Walls, Outriggers, Dampers.
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Ductile Core Wall Structural System
� Offer lower costs, faster construction and flexible architecture
� Seismic forces are resisted by reinforced
core surrounded by elevator banks
� For buildings 100 m or taller, core has a minimum dimension of 10 m in plan
and 50cm 90cm thick
Concrete Core Wall Building under Construction, the Washington Mutual/Seattle Art Museum
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Ductile Core Wall System Projects
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Ductile Core Wall Structural System
One Rincon Hill in San Francisco, California (57-story, 625 feet)
3D View Lateral Force Resisting System
Plan View
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Modeling and Analysis Goals
� For static push-over analysis, overall strength should be calculated correctly and the stiffness along the curve should be essentially accurate.
� For a dynamic analysis, the cyclic behavior and energy dissipation should be essentially correct
� Meaningful deformation demand-capacity values and usage ratios should be calculated for assessing
performance
� The demand-capacity values and deflected shape
should show any concentrations of damage
� The Goal is to get results that can be useful for design, not to get an exact simulation of the behavior
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Distinct Parts in a Wall
Yielding of horizontal ties and crushing of struts
Shear yield, severe diagonal cracking or concrete crushing
Shear yield or vertical crushing of concreteUndesirable
Yielding of vertical tiesYielding of longitudinal steelYielding of the vertical steelDesirable
Staggered openingsWell-defined vertical and horizontal segments
Vertical cantileverType
Strut and Tie Action in Right Part
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Very Large Openings
may convert the Wall
to Frame
Very Small Openings
may not alter wall
behavior
Medium Openings
may convert shear
wall to Pier and
Spandrel System
Pier Pier
Spandrel
Column
Beam
Wall
Openings in Shear Walls
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Main Aspects of behavior for Planner walls
In-Plane Behavior : Key Aspects
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Unsymmetrical Bending Behavior
� As a cantilever bends and concrete cracks, the neutral axis shifts towards the compression side.
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� If a beam element is connected to a shear wall, a beam element must be imbedded in the wall
Connecting a Beam to a Shear Wall
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� Elastic Behavior
�Curvature varies linearly along length
�There may be significant local deformation in the pier
� Actual Behavior
�Plastic zone may form near end
�Crack may open because of bond slip
Coupling Beam Behavior - Bending
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� Elastic Behavior
� Compression diagonal shortens
� Tension diagonal extends
� Beam as a whole does not extend
� Actual Behavior with Conventional Reinforcement
� Vertical steel yields
� Horizontal steel does not yield
� Beam as a whole does not extend
� Actual Behavior with Diagonal Reinforcement
� Tension diagonal yields
� Compression diagonal has a much smaller deformation
� Beam as a whole must increase in length
Coupling Beam Behavior - Shear
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Handling Nonlinearity in Shear Walls
� Hinging is expected in shear walls near the base
� Difficult to convert a large shear wall core into an equivalent column and beam system
� The question remains on how to effectively models
� Another major question is the length of the hinge zone
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� Paulay and Priestly (“Seismic Design of Reinforced Concrete and Masonry Buildings”, Wiley, 1992)
Lp = 0.2 Dw + 0.044 he
Lp = hinge length
Dw = depth of wall cross section
he = effective wall height (height of cantilever wall with a single load at
the top and the same moment and shear at the hinge as in the actual
wall
A larger shear (i.e., a larger bending moment gradient) gives a smaller
hinge length
� FEMA 356 recommends a hinge length equal to smaller of (a) one half the cross section depth (b) the story height.
Hinge Length for a Wall
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Nonlinear Modeling of Shear Walls
� For Elastic Model
� Shell or Membrane model is common
� Normal shell model can not handle Nonlinearity or hinging
� A Study carried out to compare various methods in an attempt to answer the questions
1. Single Column model
2. Fiber or Frame model
3. Strut and Tie model
4. Nonlinear Layered Shell model
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The Main Comparative Parameter
� The Moment Curvature of the Wall Section is used as
the reference for comparison of the
wall model response
� This is reasonable,
as the wall is tall enough to deform in flexure
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Single Column Model
� Simplest model
� Equivalent column at the center line of wall section
� Rigid links are required to make deformation compatibility
� Non-linear axial-flexural hinges at the top and bottom
� Optional shear hinges at the mid height
� Requires predefined hinge length
� Suitable for walls of small proportions
� Difficult to handle cellular core walls or walls with openings
� Disregards the wall rocking and effect of neutral axis shift
� Used as reference model and quick assessment of
performance
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Column Model for Planer Walls
Rigid Zones
• Specially Suitable when H/B is more than 5
• The shear wall is represented by a column of
section “B x t”
• The beam up to the edge of the wall is modeled as
normal beam
• The “column” is connected to
beam by rigid zones or very large cross-section
BB
HH
tt
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Column Models for Cellular Walls
� Difficult to extend the concept to
Non-planer walls
� Core Wall must be converted to
“equivalent” column and
appropriate “rigid” elements
� Can be used in 2D analysis but
more complicated for 3D analysis
� After the core wall is converted to
planer wall, the simplified procedure can used for modeling
BB
HH
tt
BB
HH
2t2t
tt
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Single Column Model
� Disregards the neutral axis shift on vertical displacements
� Disregards the rocking of wall
� Computes Response assuming plane -Section remain plane
� Not suitable for short/squat walls
� Can not capture geometric changes, openings,
Single Column
Model Behavior
Experimentally
Observed Behavior
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Single Column Model
Frame ElementShear Wall
Moment Hinge, directly using the Moment Curvature
of the Wall Section, multiplied by Hinge Length
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Axial Load-Deformation Hinge Property
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Fiber or Frame Model
� Wall section is discretized by closely spaced columns
� Nonlinear axial load-deformation hinges are used
� Different ductility shall be used for unconfined and confined portion of the wall
� Eliminate the predefined hinge length which is needed in single element models
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Fiber or Frame Model
Discretized into Frame Elements. Each
column acts as a “Fiber” representing
part of the wall
Shear Wall Section
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Diaphragm constraint and
Beam constraint
Diaphragm constraint and
Beam constraint
Diaphragm constraint and
Beam constraint
Axial hinge
Shear link
Release moment in both ends of fiber
element
Fiber or Frame Model
� Shear link
element is
used to provide shear
stiffness
Fiber or Frame Model
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Axial Hinges
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Hinges for Nonlinear Modeling
� Upper Portion is assumed or designed to be Elastic
� Axial Hinges for Column Fibers
� Moment Hinges for the Spandrel
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Moment Hinges
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Strut and Tie Model
� Extensively used for deep beams and shear walls
� Nonlinear axial load-deformation hinges are used
� Difficult to determine the size and reinforcement in diagonal elements
� Hinges in diagonal struts should be force control to
detect shear failure or may or the diagonals may be forced to remain elastic
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Strut and Tie Model
C
tB
t x 2t
t x t
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Strut and Tie Model
Full WallWall with Opening
Opening
OpeningDisplacement controlled Axial Hinges
Force Controlled Axial Hinges
matching shear capacity
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Nonlinear Layered Shell
� This element is not available in many software yet
� Nonlinear stress-strain relationship is sampled at Gauss points
� Integration is performed by standard 2x2 Gauss points
� Equivalent to having two fibers in each local 1 & 2 directions
� Stresses at locations other than Gauss points are interpolated or extrapolated
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Nonlinear Layered Shell
Layered Shell Nonlinear stress-strain curve of concrete
Nonlinear stress-strain curve of steel
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Practical Shear Wall Model
� Membrane behavior of vertical stress in concrete S22 and rebar stress S11 is taken to be nonlinear
� Horizontal rebar is neglected
� Out of plane behavior is assumed liner, single concrete plate layer is used
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Shear Wall Model using Shell Elements
Nonlinear behavior
in vertical rebar
Nonlinear behavior in S22
component of concrete
S22
S11S12
N = Nonlinear, L = Linear
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Comparative Study
� Two walls are selected to compare the non linear pushover curves generated by various modeling technique
� Pushover analysis is performed by displacement control (top displacement of 5% drift)
� Inverted triangular loading is used
� Axial hinges are assigned in the mid length of the member for fiber or frame model and strut and tie models
� For the cracked section models, 50% bending stiffness and 40% shear stiffness of gross section are used
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Comparative Study
Wall -01: Planner Wall
20 Stories @ 3.2 m = 64 m
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Comparative Study
20 Stories @ 3.2 m = 64 m
Wall -02: Core Wall with Opening
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Time Period Comparison
ModeSingle Column
(Cracked)
sec
Full Shell(Gross)
sec
Full Shell (Cracked)
sec
Fiber/ Framesec
Strut and Tiesec
1 2.24 1.58 1.59 1.49 1.42
2 0.37 0.26 0.27 0.25 0.25
3 0.14 0.10 0.10 0.09 0.12
4 0.08 0.08 0.08 0.08 0.10
Planner Wall
ModeSingle Column
(Cracked)sec
Full Shell(Gross)
sec
Full Shell (Cracked)
sec
Fiber or Framesec
Strut and Tiesec
1 2.85 1.85 1.87 1.83 2.16
2 2.06 1.43 1.45 1.41 1.61
3 0.47 0.31 0.32 0.30 0.38
4 0.35 0.25 0.27 0.24 0.31
Core Wall
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Time Period Comparison
� The Elastic stiffness should be represented realistically. This can be checked through time period comparison.
� It is difficult to estimate the level of cracking or the size of members for Fiber or Strut-Tie models.
� Shell Models tend to stiffer than others due to shear strain contribution and higher in-plane stiffness
� Loss of mass in Fiber and Strut and Tie model and overlapping mass in Column model should be considered
� Time is effected by nonlinear response due to reduction in stiffness
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Moment-Curvature Relationship
Planner Wall
Moment-Curvature (Planner Wall)
0
500
1000
1500
2000
2500
3000
3500
4000
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
Curvature
Mo
me
nt
(To
n-m
)
Single Column
Strut and Tie
Fiber or Frame
Nonlinear Shell
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Moment-Curvature Relationship
Core Wall
Moment-Curvature (Core Wall)
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045
Curvature
Mo
me
nt
(To
n-m
)
Single Column
Fiber or Frame
Strut and Tie
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Base Shear Vs. Top Displacement (Ton, m)
Planner Wall
Core Wall
Fiber or Frame Model Strut and Tie Model
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Hinge Formation
Fiber/Frame Strut and Tie Fiber/Frame Strut and Tie
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Limitations of Pushover Analysis
� Static pushover analysis is typically unidirectional, single pattern load analysis, in which most hinges will deform monotonically
� Higher mode contributions are not considered
� Material and section hysterics can not be considered directly
� The hinge properties typically will be based on the envelop curve from the expected hysteresis curves
� The material or section degradation due to cyclic response is not explicitly considered
� The Dynamic effects are not considered
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Nonlinear Time History Analysis
� For a detailed nonlinear time history analysis, the effective of material as well as section level hysterics and degradation for cyclic response needs to be
considered
� Although the basic modeling approaches presented
for the static pushover analysis are also suitable for the NLTH, the hinge properties as well as modeling
should represent the hysteric behavior
� The NLTH takes considerably more effort and understanding, specially for selection and scaling of
Time
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Conclusions
� The objective of this study was to investigate the various
approaches of nonlinear modeling of shear walls to predict their nonlinear response by Pushover Analysis
� Refined fiber or frame model has the capability to represent
the nonlinear flexural behavior more reliable than strut and tie
model
� The fiber model can be used to estimate the extent of yielding
in the shear walls and can be used to determine the hinge
length more realistically than based on single or double story
concept
� Both models lack the proper representation nonlinear shear
behavior and shear flexural interaction behavior