Prof. A. Meher Prasad · Overview What is pushover analysis? What are its fundamental techniques?...
Transcript of Prof. A. Meher Prasad · Overview What is pushover analysis? What are its fundamental techniques?...
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Prof. A. Meher PrasadProf. A. Meher PrasadDepartment of Civil EngineeringDepartment of Civil Engineering
Indian Institute of Technology MadrasIndian Institute of Technology Madras
email: [email protected]
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Dynamic Dynamic -- Loads change with time Loads change with time Nonlinear Nonlinear -- Loaded beyond Elastic LimitLoaded beyond Elastic Limit
Type Usual Name Dynamic Effects Material Nonlinearity
Linear Static Equivalent Static No No
Linear Dynamic Response Spectrum Yes No
Nonlinear Static
Pushover Analysis No Yes
Nonlinear Dynamic
Time History Yes Yes
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OverviewOverview
What is pushover analysis?What is pushover analysis?
What are its fundamental techniques?What are its fundamental techniques?
What tools can be used?What tools can be used?
Common pitfalls in pushover analysisCommon pitfalls in pushover analysis
Example of pushover analysis applicationExample of pushover analysis application
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Why PushWhy Push--Over Analysis?Over Analysis?
Static Nonlinear Analysis technique, also known as sequential Static Nonlinear Analysis technique, also known as sequential yield analysis, or simply "pushyield analysis, or simply "push--over" analysis has gained over" analysis has gained significant importance during the past few years. significant importance during the past few years.
It is one of the three analysis techniques recommended by FEMA It is one of the three analysis techniques recommended by FEMA 273/274 and a main component of the Capacity Spectrum Method 273/274 and a main component of the Capacity Spectrum Method (ATC(ATC--40). 40).
Proper application can provide valuable insights into the Proper application can provide valuable insights into the expected performance of structural systems and components expected performance of structural systems and components
Misuse can lead to an erroneous understanding of the Misuse can lead to an erroneous understanding of the performance characteristics.performance characteristics.
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What is PushWhat is Push--Over Analysis? Over Analysis?
Push-over analysis is a technique by which a computer model of the building is subjected to a lateral load of a certain shape (i.e., parabolic, inverted triangular or uniform).
The intensity of the lateral load is slowly increased and the sequence of cracks, yielding, plastic hinge formations, and failure of various structural components is recorded.
Push-over analysis can provide a significant insight into the weak links in seismic performance of a structure.
Push-over analysis is a technique by which a computer model of the building is subjected to a lateral load of a certain shape (i.e., parabolic, inverted triangular or uniform).
The intensity of the lateral load is slowly increased and the sequence of cracks, yielding, plastic hinge formations, and failure of various structural components is recorded.
Push-over analysis can provide a significant insight into the weak links in seismic performance of a structure.
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What is PushWhat is Push--Over Analysis? Over Analysis?
A series of iterations are usually required during which, the structural deficiencies observed in one iteration, are rectified and followed by another.
This iterative analysis and design process continues until the design satisfies a pre-established performance criteria.
The performance criteria for push-over analysis is generally established as the desired state of the building given a roof-top or spectral displacement amplitude.
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Objectives of PushObjectives of Push--Over Analysis Over Analysis
To obtain the maximum shear strength of the structure, Vb, and the mechanism of collapse.
To evaluate if the structure can achieve the collapse mechanism without exhausting the plastic rotation capacity of the members.
To obtain the monotonic displacement and global ductility capacity of the structure.To estimate the concentration of damage and IDI (Interstorey Drift Index) that can be expected during the nonlinear seismic response.
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PushPush--over Curve or Capacity Spectrumover Curve or Capacity Spectrum
RoofRoof--top Displacementtop Displacement
Using simple modal analysis Using simple modal analysis equations spectral displacement equations spectral displacement and roofand roof--top displacement may be top displacement may be converted to each other.converted to each other.
V/W
(Acc
eler
atio
n)V/
W (A
ccel
erat
ion)
LowLow--Strength; LowStrength; Low--Stiffness; BrittleStiffness; Brittle
Moderate Strength and Stiffness; DuctileModerate Strength and Stiffness; Ductile
HighHigh--Strength; HighStrength; High--Stiffness; BrittleStiffness; Brittle
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Design Spectra RepresentationDesign Spectra Representation
PushPush--Over Analysis Over Analysis -- Composite or ADRS Plot Composite or ADRS Plot
Ordinary DesignOrdinary Design
Period
V/W (Acceleration)
DESIGN SPECTRUM
Spectral or Roof-top Displacement
V/W (Acceleration)Constant Period Lines
ELASTIC DEMAND SPECTRUM
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What Tools Can Be Used? What Tools Can Be Used?
Nonlinear Analysis software with built-in push-over analysis capabilities
DRAIN Spread PlasticityIDARC Spread and Point PlasticitySAP2000NL Point PlasticityETABS Point PlasticityANSYS Spread Plasticity SAVE Point Plasticity (Public version)
Spread Plasticity (Research version)
Sequential application of linear analysis software
Nonlinear Analysis software with built-in push-over analysis capabilities
DRAIN Spread PlasticityIDARC Spread and Point PlasticitySAP2000NL Point PlasticityETABS Point PlasticityANSYS Spread Plasticity SAVE Point Plasticity (Public version)
Spread Plasticity (Research version)
Sequential application of linear analysis software
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Spread and Point Plasticity
1. Nonlinearity is assumed to be distributed along the length of the plastic hinge.
2. It provides a more accurate representation of the actual non-linear behaviour of the element
1. Plasticity is assumed to be concentrated at the critical locations. In addition to usual ‘moment hinges’, there can be ‘axial hinges’ and ‘shear hinges’.
2. Plastification of the section is assumed to occur suddenly, and not gradually or fibre-by-fibre.
Length of plastic hinge
Plastic Hinge
Curvature diagram along the length of the member
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Establishing the Performance Point
• No building can be pushed to infinity without failure.
• Performance point is where the Seismic Capacity and the Seismic Demand curves meet.
• If the performance point exists and damage state at that point is acceptable, we have a building that satisfies the push-over criterion.
• No building can be pushed to infinity without failure.
• Performance point is where the Seismic Capacity and the Seismic Demand curves meet.
• If the performance point exists and damage state at that point is acceptable, we have a building that satisfies the push-over criterion.
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ATC-40 MethodATC-40 Method
This is an iterative procedure involving This is an iterative procedure involving several analyses. several analyses.
For each analysis an effective period for For each analysis an effective period for an equivalent elastic system and a an equivalent elastic system and a corresponding elastic displacement are corresponding elastic displacement are calculated.calculated.This displacement is then divided by a This displacement is then divided by a damping factor to obtain an estimate of real damping factor to obtain an estimate of real displacement at that step of analysis.displacement at that step of analysis.
V/W (Acceleration)
Roof-top Displacement
5% damped elastic spectrum
δe
T eff
T0
∆e/B
0 0.05
3.21 0.68ln( )2.12
2.31 0.4 ln( )1.65
eff
effA
effV
SR
SR
β κβ
β
β
= +
−=
−=
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ATCATC--40 Nonlinear Static Procedure40 Nonlinear Static Procedure
1.1. Develop the Pushover CurveDevelop the Pushover Curve
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ATCATC--40 Nonlinear Static Procedure40 Nonlinear Static Procedure
2.2. Convert Pushover Curve to capacity diagramConvert Pushover Curve to capacity diagram
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ATCATC--40 Nonlinear Static Procedure40 Nonlinear Static Procedure
3.3. Plot elastic design spectrum in APlot elastic design spectrum in A--D formatD format
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ATCATC--40 Nonlinear Static Procedure40 Nonlinear Static Procedure
4.4. Plot the demand diagram and capacity diagram togetherPlot the demand diagram and capacity diagram together
Intersection point gives the displacement demandIntersection point gives the displacement demand
Avoids nonlinear RHA; instead analyse equivalent linear Avoids nonlinear RHA; instead analyse equivalent linear systemssystems
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ATCATC--40 Nonlinear Static Procedure40 Nonlinear Static Procedure
5.5. Convert displacement demand to roof displacement and Convert displacement demand to roof displacement and component deformation.component deformation.
6.6. Compare to limiting values for specified performance goals.Compare to limiting values for specified performance goals.
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Points to be taken care..Points to be taken care..
1. Do not underestimate the importance of the loading or 1. Do not underestimate the importance of the loading or displacement shape function. displacement shape function.
2.2. Know your performance objectives before you push the building. Know your performance objectives before you push the building. 3. If it is not designed, it cannot be pushed. 3. If it is not designed, it cannot be pushed. 4.4. Do not ignore gravity loads. Do not ignore gravity loads. 5.5. Do not push beyond failure unless otherwise you can model failurDo not push beyond failure unless otherwise you can model failure.e.6.6. Pay attention to rebar development and lap lengths.Pay attention to rebar development and lap lengths.7.7. Do not ignore shear failure mechanisms Do not ignore shear failure mechanisms 8. P8. P--Delta effects may be more important than you think. Delta effects may be more important than you think. 9.9. Do not confuse the PushDo not confuse the Push--over with the real earthquake loading. over with the real earthquake loading. 10. Three10. Three--dimensional buildings may require more than a planar push. dimensional buildings may require more than a planar push.
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1. Do not underestimate the importance of the loading shape function.
1. 1. Do not underestimate the importance of the loading shape function.
The loading or deformation shape function is selected to represent the predominant dynamic mode shape of the building.
It is most common to keep the load shape constant during the push.
Loading shape importance increases for tall buildings whose earthquake response is not dominated by a single mode shape.
For these buildings, a loading shape function based on the firstmode shape may seriously underestimate the seismic demand on the intermediate floor levels.
The loading or deformation shape function is selected to represent the predominant dynamic mode shape of the building.
It is most common to keep the load shape constant during the push.
Loading shape importance increases for tall buildings whose earthquake response is not dominated by a single mode shape.
For these buildings, a loading shape function based on the firstmode shape may seriously underestimate the seismic demand on the intermediate floor levels.
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1. Do not underestimate the importance of the loading shape function.
1. 1. Do not underestimate the importance of the loading shape function.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 0.2 0.4 0.6 0.8
/H(%)
Vb/
WInverted TriangleUniformparabola
∆
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Adapting Load PatternsAdapting Load Patterns
• So called “higher mode effects” as the load distribution changes
• Limit base moment increases adapts for maximum shear force
• Limit base shear increases adapts for maximum bending moment
• Not apparent from linear analysis
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2. Know your performance objectives before you push the building.
2. Know your performance objectives before you push the building.
No building can be displaced to infinity without damage.
It is of paramount importance to understand the specific performance objectives desired for the building.
Performance objectives such as collapse prevention, life safety, or immediate occupancy have to be translated into technical terms such as: (a) a given set of design spectra, and (b) specific limit states acceptable for various structural components
A push-over analysis without a clearly defined performance objectives is of little use.
No building can be displaced to infinity without damage.
It is of paramount importance to understand the specific performance objectives desired for the building.
Performance objectives such as collapse prevention, life safety, or immediate occupancy have to be translated into technical terms such as: (a) a given set of design spectra, and (b) specific limit states acceptable for various structural components
A push-over analysis without a clearly defined performance objectives is of little use.
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BUILDINGBUILDING PERFORMANCE LEVELSPERFORMANCE LEVELS
NoNorehabilitationrehabilitation
55--EECollapse Collapse
PreventionPrevention44--EE33--EENRNRNRNRNN--EE
Not ConsideredNot Considered
66--DD55--DD44--DD33--DD22--DDNRNRNN--DDHazards ReducedHazards Reduced
66--CC55--CC44--CC33--CCLife SafetyLife Safety22--CC11--CCNN--CC
Life SafetyLife Safety
NRNRNRNRNRNR33--BB22--BB11--BB
Immediate Immediate OccupancyOccupancy
NN--BBImmediate OccupancyImmediate Occupancy
NRNRNRNRNRNRNRNR22--AA11--AAOperationalOperational
NN--AAOperationalOperational
SS--66Not ConsideredNot Considered
SS--55Collapse Collapse
PreventionPrevention
SS--44Limited SafetyLimited Safety
SS--33Life SafetyLife Safety
SS--22Damage ControlDamage Control
SS--11Immediate Immediate OccupancyOccupancy
Structural Performance Levels and RangesStructural Performance Levels and RangesNonstructuralNonstructuralPerformancePerformance
LevelsLevels
Ref: FEMA 356
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Earthquake Levels (FEMA356)Earthquake Levels (FEMA356)
pp tt NN Approximate Approximate NN
yearsyears yearsyears yearsyears
Serviceability earthquake Serviceability earthquake -- 11 50%50% 5050 7272 7575 FrequentFrequent
Serviceability earthquake Serviceability earthquake -- 22 20%20% 5050 224224 225225 OccasionalOccasional
Design basis earthquake (DBE)Design basis earthquake (DBE) 10%10% 5050 475475 500500 RareRare
5%5% 5050 975975
10%10% 100100 949949
2%2% 5050 24752475
10%10% 250250 2373237325002500 Extremely Extremely
rarerareMaximum considered (MCE)Maximum considered (MCE) earthquake earthquake --2 2
(alternate)(alternate)
10001000 Very rareVery rareMaximum considered (MCE) earthquake Maximum considered (MCE) earthquake --1 1
(alternate)(alternate)
RemarksRemarksEarthquake levelsEarthquake levels
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Performance Objectives (FEMA 356)Performance Objectives (FEMA 356)
ppoonnmm2% in 50 2% in 50
yearsyearsMaximum considered Maximum considered Earthquake (MCE)Earthquake (MCE)
llkkjjii10% in 50 years10% in 50 yearsDesign basis earthquake Design basis earthquake (DBE)(DBE)
hhggffee20% in 50 years20% in 50 yearsServiceability Serviceability earthquake earthquake -- 22
ddccbbaa50% in 50 years50% in 50 yearsServiceability Serviceability earthquake earthquake -- 11
Collapse Collapse PreventionPrevention
Life Life SafetySafety
ImmediateImmediateOccupancyOccupancy
OperaOpera--tionaltional
Target building performance levelTarget building performance levelProbability of Probability of ExceedanceExceedancein a periodin a period
Earthquake levelsEarthquake levels
Basic Safety Objective
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3. If it is not designed, it cannot be pushed.3. If it is not designed, it cannot be pushed.
E, I, and A are not sufficient.
Push-over characteristics are strong functions of force-displacement characteristics of individual members and their connections.
If detailed characteristics are not known, the push-over analysis will be an exercise in futility.
E, I, and A are not sufficient.
Push-over characteristics are strong functions of force-displacement characteristics of individual members and their connections.
If detailed characteristics are not known, the push-over analysis will be an exercise in futility.
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4. Do not ignore gravity loads.4. Do not ignore gravity loads.
Inclusion or exclusion of the gravity loads can have a pronounced effect on the shape of the push-over curve and the member yielding and failure sequence.
Example:
Due to the unsymmetric distribution of + and - reinforcements in R/C beams, gravity load delays the onset of yielding and cracking in the beams, resulting in a stiffer structure at lowermagnitudes of base shear.
The ultimate capacity of the structure, is usually reduced withincreasing gravity load.
Inclusion or exclusion of the gravity loads can have a pronounced effect on the shape of the push-over curve and the member yielding and failure sequence.
Example:
Due to the unsymmetric distribution of + and - reinforcements in R/C beams, gravity load delays the onset of yielding and cracking in the beams, resulting in a stiffer structure at lowermagnitudes of base shear.
The ultimate capacity of the structure, is usually reduced withincreasing gravity load.
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5. Do not push beyond failure unless otherwise you can model failure
5. Do not push beyond failure unless otherwise you can model failure
Displacement or CurvatureFo
rce
or M
omen
t
Ultimate Capacity
Displacement
Late
ral F
orce
Actual
Modeled withfailures ignored
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6. Pay attention to rebar development and lap lengths.
6. Pay attention to rebar development and lap lengths.
For R/C members of existing structures, it is very important to note the development lengths when calculating member capacities.
If inadequate development lengths are present, as they are in most of the older buildings, the contributing steel area should be reduced to account for this inadequacy.
Failure to do so will result in overestimating the actual capacity of the members and results in an inaccurate push-over curve.
For R/C members of existing structures, it is very important to note the development lengths when calculating member capacities.
If inadequate development lengths are present, as they are in most of the older buildings, the contributing steel area should be reduced to account for this inadequacy.
Failure to do so will result in overestimating the actual capacity of the members and results in an inaccurate push-over curve.
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Joint DetailingJoint Detailing
Such reinforcement detailingSuch reinforcement detailingshould not be usedshould not be used
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7. Do not ignore shear failure mechanisms7. Do not ignore shear failure mechanisms
If the shear capacity of structural members is not sufficient topermit the formation of flexural plastic hinges, shear failure will precede the formation of plastic hinges at the end of the member.
In R/C members, even if the shear capacity is sufficient, but lateral reinforcement is not spaced close enough at the plastic hinge zones, the concrete may crush in the absence of sufficient confinement.
If this happens, the plastic capacity is suddenly dropped to what can be provided by the longitudinal steel alone.
If the shear capacity of structural members is not sufficient topermit the formation of flexural plastic hinges, shear failure will precede the formation of plastic hinges at the end of the member.
In R/C members, even if the shear capacity is sufficient, but lateral reinforcement is not spaced close enough at the plastic hinge zones, the concrete may crush in the absence of sufficient confinement.
If this happens, the plastic capacity is suddenly dropped to what can be provided by the longitudinal steel alone.
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Shear FailureShear Failure
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Short Column FailureShort Column Failure
• This failure can be avoided by providing special confining reinforcement over entire column length
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8. P-∆ effects may be more important than you think.
8. P-∆ effects may be more important than you think.
The P-∆ effects become increasingly significant with larger lateral displacements and larger axial column forces.
Strong column - weak beam design strategy commonly deals with the moment capacity of columns in the undeformed state.
In a substantially deformed state, the moment capacity of columns may be sufficiently reduced to counteract the strong column - weak beam behaviour envisioned by the design.
Cases of plastic hinge formations during a push-over analysis in columns "designed" to be stronger than the beams are not rare.
The P-∆ effects become increasingly significant with larger lateral displacements and larger axial column forces.
Strong column - weak beam design strategy commonly deals with the moment capacity of columns in the undeformed state.
In a substantially deformed state, the moment capacity of columns may be sufficiently reduced to counteract the strong column - weak beam behaviour envisioned by the design.
Cases of plastic hinge formations during a push-over analysis in columns "designed" to be stronger than the beams are not rare.
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9. Do not confuse the Push-over with the real earthquake loading.
9. Do not confuse the Push-over with the real earthquake loading.
The push-over load is monotonically increased
The earthquake generated forces continually change in amplitude and direction during the duration of earthquake ground motion.
Push-over loads and structural response are in phase
Earthquake excitations and building response are not necessarily in phase.
This is particularly true for near-fault ground motions which tend to concentrate the damage on the lower floors, an effect which is difficult to model by the push-over loads.
The push-over load is monotonically increased
The earthquake generated forces continually change in amplitude and direction during the duration of earthquake ground motion.
Push-over loads and structural response are in phase
Earthquake excitations and building response are not necessarily in phase.
This is particularly true for near-fault ground motions which tend to concentrate the damage on the lower floors, an effect which is difficult to model by the push-over loads.
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9. Do not confuse the Push-over with the real earthquake loading.
9. Do not confuse the Push-over with the real earthquake loading.
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
-0.008 -0.006 -0.004 -0.002 0 0.002 0.004 0.006 0.008
/H
Vb/
W
IDARC
SAP
0.16g
0.25g
0.3g
0.35g
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10. Three-dimensional buildings may require more than a planar push.
10. Three-dimensional buildings may require more than a planar push.
For building with strong asymmetry in plan, or with numerous non-orthogonal elements, a planar (two dimensional) push-over analysis may not suffice.
For such cases a 3D model of the building must be constructed and subjected to push-over analysis.
Three dimensional buildings may be pushed in the principal directions independently, or pushed simultaneously in orthogonal directions.
For building with strong asymmetry in plan, or with numerous non-orthogonal elements, a planar (two dimensional) push-over analysis may not suffice.
For such cases a 3D model of the building must be constructed and subjected to push-over analysis.
Three dimensional buildings may be pushed in the principal directions independently, or pushed simultaneously in orthogonal directions.
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Analysis ProcedureAnalysis Procedure
SAP2000 NLSAP2000 NL
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Pushover Analysis ProcedurePushover Analysis Procedure
Create 3D Model
Assign end offsets
Define Hinge properties
Assign Hinge propertiesBeams – Default M3Columns – Default PMM
Define Static Pushover Cases
Gravity Pushover (Force controlled) DL+0.25LLLateral Pushover(Displacement controlled)
Define Load case(Lateral Load at centre of mass)
Run Static analysis
Run static pushover analysis
Establish Performance point
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Material PropertiesMaterial Properties
Concrete PropertiesConcrete Properties
•• Cube compressive strength, Cube compressive strength, ffckck
•• Modulus of Elasticity of concrete ( )Modulus of Elasticity of concrete ( )
Reinforcing Steel PropertiesReinforcing Steel Properties
•• Yield strength of steel Yield strength of steel
•• Modulus of Elasticity of steel EModulus of Elasticity of steel Ess
5000c ckE f=
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Modification FactorsModification FactorsFactors to estimate the expected strengthFactors to estimate the expected strength
1.5 times the Concrete compressive strength (1.5 times the Concrete compressive strength (ffckck))Steel yield stress (Steel yield stress (ffyy) (Factor of 1.25 used for capacity ) (Factor of 1.25 used for capacity estimation considering strain hardening of steel)estimation considering strain hardening of steel)
No Description of available information mk
1 Original construction documents, including material testing report
1.0
2 Documentation as in (1) but no material testing undertaken 0.93 Documentation as in (2) and minor deteriorations of original
condition0.8
4 Incomplete but usable original construction documents 0.75 Documentation as in (4) and limited inspection and material
test results with large variation.0.6
6 Little knowledge about the details of components 0.5
Knowledge Factors, mk
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Material PropertiesMaterial Properties
Frame ElementsFrame Elements
Infill (struts)Infill (struts)
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Modeling of Structural elementsModeling of Structural elements
Beams and columns 3D Frame elements
Slab Diaphragm action(ignore the out of plane stiffness)
Flat slabs Plate elements
Beam column joints End offsets (Rigid zone factor 1)
Asymmetric Structures Centre of mass(add non structural mass to corresponding beams)Centre of stiffness
Inclusion of appendages Include water tanks, cantilever slabs
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Modeling of Structural elementsModeling of Structural elements
Stairway slabs Equivalent frame elements
Shear Walls Wide Column Elements
FoundationIsolated footings
Single pile
Multiple piles
Plinth beams
Hinged at the bottom of foundation
Fixed at five times the diameter of pile
Fixity of columns at top of pile cap
Frame elements
Infill walls Equivalent strut method
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Modeling of Beams and ColumnsModeling of Beams and Columns
3D Frame Elements3D Frame Elements
Cross Sectional dimensions, reinforcement details, material typCross Sectional dimensions, reinforcement details, material typee
Effective moment of inertiaEffective moment of inertia
BeamsBeams RectangularRectangular 0.5 0.5 IIgg
TT--BeamBeam 0.7 0.7 IIgg
LL--BeamBeam 0.6 0.6 IIgg
ColumnsColumns 0.7 0.7 IIgg
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Modeling of BeamsModeling of Beams
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Modeling of ColumnsModeling of Columns
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Modeling of Beam Column JointsModeling of Beam Column Joints
End offsets (Rigid zone factor 1)End offsets (Rigid zone factor 1)
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Modeling of SlabModeling of Slab
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Modeling of Infill Modeling of Infill Equivalent Strut ApproachEquivalent Strut Approach
Step 1.Step 1. Equivalent Strut Properties Equivalent Strut Properties –– Smith and Carter ModelSmith and Carter Model
a) Strength of infill, Pa) Strength of infill, P
b)b) Initial modulus of elasticity of infill, Initial modulus of elasticity of infill, EEii
c) Equivalent strut width (when force in the strut = R), wc) Equivalent strut width (when force in the strut = R), w
d)d) Thickness of infill, t Thickness of infill, t
Step 2.Step 2. Stress Stress –– Strain ValuesStrain Values
Stress = P/Stress = P/AEAEii ,where A = wt,where A = wt
Strain = P/AStrain = P/A
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MASTER NODE
BEAM
L
t
Modeling of Shear Wall (Lift Core )Modeling of Shear Wall (Lift Core )
y
x
Equivalent Wide Column Elements connected to the frame Equivalent Wide Column Elements connected to the frame through rigid linksthrough rigid links
Type I Model Type I Model -- Single Lift Core ColumnSingle Lift Core Column
Beam elements with rigid ends
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Type II Model Type II Model -- Single Lift Core ColumnSingle Lift Core Column
The lift core can be treated as a single column with master nodeThe lift core can be treated as a single column with master nodedefined at the centroid and the beams connected by rigid linksdefined at the centroid and the beams connected by rigid links
CORE MASTER NODE
BEAM
SLAVE NODE
x
y
Modeling of Shear Wall (Lift Core )Modeling of Shear Wall (Lift Core )
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For axial and torsional rigidity, the full crossFor axial and torsional rigidity, the full cross--sectional area should be sectional area should be usedused
CORE MASTER NODE
BEAM
SLAVE NODE
x
y
FORA, J
Modeling of Shear Wall Modeling of Shear Wall (Lift Core Column Properties)(Lift Core Column Properties)
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For shear along y axis and bending about xFor shear along y axis and bending about x--axis (ground motion axis (ground motion along yalong y--axis), the walls in the direction of ground motion should be axis), the walls in the direction of ground motion should be considered as two parallel elementsconsidered as two parallel elements
CORE MASTER NODE
BEAM
SLAVE NODE
x
y
FORAy, Ixx
Modeling of Shear Wall Modeling of Shear Wall (Lift Core Column Properties)(Lift Core Column Properties)
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For shear along x axis and bending about yFor shear along x axis and bending about y--axis (ground motion axis (ground motion along xalong x--axis), the walls in the direction of ground motion should be axis), the walls in the direction of ground motion should be considered as three parallel elementsconsidered as three parallel elements
CORE MASTER NODE
BEAM
SLAVE NODE
x
y
FORAx, Iyy
Modeling of Shear Wall Modeling of Shear Wall (Lift Core Column Properties)(Lift Core Column Properties)
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Beam Hinge Properties Beam Hinge Properties -- Flexural hinge (MFlexural hinge (M33))
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Hinge Properties for Beams Hinge Properties for Beams
A
B C
D E
1.0
c
ab
∆y ∆
Late
ral L
oad
Lateral Deformation
Generalized Load Deformation Relations
* ATC 40 Volume 1
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Beam Hinge Properties Beam Hinge Properties -- Shear hingeShear hinge
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Beam Hinge Properties Beam Hinge Properties -- Shear hingeShear hinge
Shear capacityShear capacity
0 .6sy y svv
dV f As
=
Total Shear Capacity, Total Shear Capacity, VVyy = = VVcc + + VVsysy
Vy
0.2 Vy
∆y ∆m=15∆y
Shear deformation (∆)
Shear strength (V)
Residual Shear Strength
Vu = 1.05Vy
1.5∆y
= 0
Refer Clause 6.3.3 of IS13920
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Column Hinge PropertiesColumn Hinge Properties-- Flexural hinge Flexural hinge (PM(PM22MM33))
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Hinge Properties for ColumnsHinge Properties for Columns
A
B C
D E
1.0
c
ab
∆y ∆
Late
ral L
oad
Lateral Deformation
* ATC 40 Volume 1
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Column Hinge PropertiesColumn Hinge Properties-- Shear hingeShear hinge
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Column Hinge PropertiesColumn Hinge Properties-- Shear hingeShear hinge
Shear capacityShear capacity
0.8 ( 1 5 1)6
0.116 1.0100
ckc
ck
st
f
f bdwhereA
βτ
β
β
+ −=
= ≥
31 1.5u
g ck
PA f
δ = + ≤
3 0 . 5u
g c k
PA f
δ = ≤
Note:
For moderate and high ductility of the column section
is taken in calculation (ATC 40)
c cV bdδτ=
0 .6sy y svv
dV f As
=
Total Shear Capacity, Total Shear Capacity, VVyy= = VVcc + + VVsysy
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Yield deformation (∆y) is to be calculated using the following formula.
Where G = Shear modulus of the reinforced concrete section
Ag = Gross area of the section
l = Length of member
geffy AG
lR
lGA
RstiffnessShear
strengthshearYield75.0××
=
⎟⎟⎠
⎞⎜⎜⎝
⎛==∆
Column Hinge PropertiesColumn Hinge Properties-- Shear hingeShear hinge
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Column Hinge PropertiesColumn Hinge Properties-- Shear hingeShear hingeThe ultimate shear strength (Vu) is taken as 5% more than yield shear strength (Vy) and residual shear strength is taken as 20% of the yield shear strength for modelling of the shear hinges as shown in Figure.
Similarly maximum shear deformation is taken as 15 times the yield deformation. The values were taken as per SAP 2000 manual recommendations.
Vy
0.2 Vy
∆y ∆m=15∆y
Shear deformation (∆)
Shear strength (V)
Residual Shear Strength
Vu = 1.05Vy
1.5∆y
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Infill Properties Infill Properties -- Axial hinge (P)Axial hinge (P)
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Static Pushover Case Data(Gravity Pushover – Force Controlled)
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Lateral Load PatternLateral Load Pattern
Determination of the Load pattern: (IS 1893 (part 1) : 2002 )
Q3
Q2
Q1
Fundamental natural period
Design Base Shear
Design Lateral Force
dh.T a
090=
WAV hB =
∑= 2
2
jj
iiBi hW
hWVQ
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Static Pushover Case Data(Lateral pushover – Displacement controlled)
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Seismic Evaluation of a Seismic Evaluation of a Typical RC BuildingTypical RC Building
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Building DataBuilding Data
Building frame system RC OMRFUsage ResidentialBuilt in 1999Zone V
G+4Multiple PilesAbout Y-axisM15 & Fe 415
25.2m X 13.95mBuilding height 15.7m Soil Type (assumed) Type-II (Medium)
Number of storiesFootingSymmetryMaterial usedPlan dimensions
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PlanPlan-- Beam Locations Beam Locations
n Storey number
Beams (only in 1 to 4 floor)
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Plan Plan -- Column and Equivalent Strut Column and Equivalent Strut LocationsLocations
Infill wall Location
Storey number n
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CommentsComments
Visual inspection did not reveal concrete deterioration.Visual inspection did not reveal concrete deterioration.Knowledge factor was not applied.Knowledge factor was not applied.
Architectural drawings were not available. Location of infill wArchitectural drawings were not available. Location of infill walls alls was postulated.was postulated.
Geotechnical data was not available.Geotechnical data was not available.
Rebar detailing was not complete in the available structural Rebar detailing was not complete in the available structural drawings.drawings.
Building considered to be noncompliant with IS 13920: 1993 Building considered to be noncompliant with IS 13920: 1993 (R = 3).(R = 3).
Fixity considered at pile cap. SoilFixity considered at pile cap. Soil--structure interaction neglected.structure interaction neglected.
Elevator walls not considered as lateral load resisting elementsElevator walls not considered as lateral load resisting elements..
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Plan Plan –– Frames along XFrames along X--directiondirection
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Plan Plan –– Frames along YFrames along Y--directiondirection
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Elevation along line AElevation along line A--AA
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Typical Beam Section (Ground Floor)Typical Beam Section (Ground Floor)
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Typical Column Sections (Ground Floor)Typical Column Sections (Ground Floor)
Tie spacing 100 mm c/c near beam-to-column joints
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Detailed Structural AnalysisDetailed Structural Analysis
Gravity Load AnalysisGravity Load Analysis
Lateral Load AnalysisLateral Load Analysis
Linear static analysis Linear static analysis
(Equivalent Static Method, IS 1893 (Part 1): 2002)(Equivalent Static Method, IS 1893 (Part 1): 2002)
Response Spectrum Method Response Spectrum Method
(IS 1893 (Part 1): 2002)(IS 1893 (Part 1): 2002)
NonNon--linear Static Analysis linear Static Analysis
(Pushover Analysis, ATC 40)(Pushover Analysis, ATC 40)
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Structural ParametersStructural Parameters
Center of Mass (m)
Center of rigidity (m)
Static Eccentricity, esi
( m)
X-direction
X-direction
Y-direction
0.05 0.33
0.08
0.08
0.08
0.08
0.05
0.05
0.05
0.05
12.60
12.60
12.60
12.60
12.60
Design Eccentricity, edi
(m)
X-direction
Y-direction
X-direction
Y-direction
7.23
7.23
7.23
7.23
1.34
7.23
1.34
1.20
0.82
0.82
0.82
1.34
1.34
0.821.34
12.55
12.55
12.55
12.55
12.55
FloorSeismic Weight
(kN)
Lumped Mass (Ton) Y-
direction
5 3550 255 6.90
7.15
7.15
7.15
7.15
4 4175 306
3 4175 306
2 4175 306
32001 222
0.05di si ie e b= −1.5 0.05di si ie e b= +
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Location of Centre of MassLocation of Centre of Mass
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Calculation of Base Shear Calculation of Base Shear IS 1893(Part 1):2002IS 1893(Part 1):2002
Base shear, VB = AhW W = Total seismic weight of the buildingZ = 0.36 (for Zone V)I = 1 (for normal building)R = 3 (for OMRF)Sa/g = 2.5 corresponding to both the time
period in with-infill case.Ah = 0.15
ah
SZIA2R g
⎛ ⎞= ⎜ ⎟
⎝ ⎠
VB = 0.15 × 20270 kN
= 3039 kN
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Comparison of Base ShearComparison of Base Shear
Without infill stiffness With infill stiffness
Analysis methodsVx (kN) Vy (kN) Vx (kN) Vy (kN)
EQX 2796 - 3039 -
EQ 1773 1851 2092 2170
EQY - 2796 - 3039
Equivalent Static Method
Response Spectrum Analysis
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Comparison of Fundamental PeriodsComparison of Fundamental Periods
Empirical Formulae Computational Model
Time Period (s)
Sa/g
With infill stiffness
Without infill
stiffness
With infill stiffness
Without infill stiffness
Tax= 0.28 0.59 0.73 0.83
2.50
Tay= 0.38
2.50 2.30 1.87 1.64
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First five modes and their participationFirst five modes and their participation
Without infill With infillMass Participation
(%)Mass Participation
(%)UX Uy UX Uy
T (s)T (s)Mode
1 0.83 88.34 1.95 0.73 92.29 1.102 0.78 2.22 86.71 0.69 1.26 90.233 0.42 1.23 0.47 0.38 0.72 0.594 0.25 6.05 0.16 0.22 4.44 0.135 0.24 0.14 8.02 0.21 0.11 6.33
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Mode ShapesMode Shapes
Second Mode T=0.76s (UY=90.51%)
First Mode T=0.83s (UX=92.91%)
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Mode ShapesMode Shapes
Fourth Mode T=0.25s(UX=5.39% UY=0.04%)
Third Mode T=0.39s (RZ) (UX=0.11% UY=0.52%)
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Mode ShapesMode Shapes
Fifth Mode T=0.24s(UX=0.03% UY=7.07%)
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Demand and Capacity for Columns Demand and Capacity for Columns -- Moment Moment (Equivalent static method)(Equivalent static method)
Absolute CapacitiesAbsolute Demand
(Without Infill stiffness)Absolute Demand(With Infill stiffness)
P (kN)
M2 (kNm)
M3 (kNm)
DCRP
(kN)M2
(kNm)M3
(kNm)
342 338
354
354
368
365
220
159
310
435
445
285
1712
1860
2400
2506
1546
207
2029
2.30
2.60
1.81
1.84
2.20
1.57
218
242
277
253
277
236
280
250
263
296
263
2871
3102
3070
3241
3301
3241
Section
P (kN)
M2 (kNm)
M3 (kNm)
1C1 1744 323 311 2.49
1C2 1534 433 334 2.72
DCR
1C3 2266 288 335 1.95
1C4 2614 414 350 1.92
1C5 1422 420 346 2.36
2C4 2355 416 270 1.04
2 2uR ux uyM M M= +
Y
Y
X X
ex ey
Pu
Pu
Puz
MuR,y
Mux1
PuR
Muy1
Muy = Pu ey
load contour
MuR,xθ
Mux = Pu ex
x
z
y
θ
0
A
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Demand and Capacity for Columns Demand and Capacity for Columns –– Shear Shear (Equivalent Static Method)(Equivalent Static Method)
Absolute Capacities
Absolute Demand
(With infill stiffness)
Sections
Vu (kN)
DCRAbsolute Demand
(Without infill stiffness)
Vd (kN)
DCR
0.74 161
206
177
209
212
231
0.87
0.64
0.80
0.64
0.74
0.74
0.69
0.80
0.81
0.55 0.82
250
259
275
282
285
282
Vd (kN)
184
226
189
227
231
154
1C1
1C2
1C3
1C4
1C5
2C5
VVuu is higher of the shear from analysis and the shear is higher of the shear from analysis and the shear corresponding to the flexural capacity corresponding to the flexural capacity MMuu (V(Vuu = = MMuu / L/ Lss))
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Maximum displacement response in Maximum displacement response in XX--direction (Equivalent Static Method)direction (Equivalent Static Method)
0
1
2
3
4
5
0 20 40 60 80 100
Displacement (mm)
Stor
ey L
evel
0
1
2
3
4
5
0 20 40 60 80 100
Displacement (mm)St
orey
Lev
el
Without InfillWith Infill
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InterInter--storey Drift in Xstorey Drift in X--direction direction Equivalent Static MethodEquivalent Static Method
0
2
4
6
8
10
12
14
16
18
0 20 40 60Inte r-store y dri ft ( X 10-2%)
Stor
ey le
vel (
m)
0
2
4
6
8
10
12
14
16
18
0 20 40 60Inter-storey drift ( X 10-2%)
Stor
ey le
vel (
m)
With infill Without infill
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Performance ObjectivePerformance Objective
1.1. Design Basis Earthquake + Life Safety Design Basis Earthquake + Life Safety (2% total drift)(2% total drift)
2.2. Maximum Considered Earthquake + Collapse Prevention Maximum Considered Earthquake + Collapse Prevention (4% total drift)(4% total drift)
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Distribution of Lateral Force Distribution of Lateral Force at each Storey Level for Lateral Pushover at each Storey Level for Lateral Pushover
Q1= 1.00
Q2= 3.25
Q3= 6.83
Q4= 11.70
Q5= 15.22
1
2
3
4
5
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Moment Rotation Curve for a Typical Moment Rotation Curve for a Typical ElementElement
Hinge Property
0
0.2
0.4
0.6
0.8
1
1.2
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
Rotation/SF
Mom
ent/S
F
A
BC
DE
IO LS CP
B Yield state
IO Immediate Occupancy
LS Life Safety
CP Collapse Prevention
C Ultimate state
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Demand SpectrumDemand Spectrum
Seismic Coefficient, CA
SoilZone II(0.10)
Zone III(0.16)
Zone IV(0.24)
Zone V(0.36)
Type I 0.10 0.16 0.24 0.36Type II 0.10 0.16 0.24 0.36Type III 0.10 0.16 0.24 0.36
Seismic Coefficient, CV
Type I 0.10 0.16 0.24 0.36Type II 0.14 0.22 0.33 0.49Type III 0.17 0.27 0.40 0.60
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0
500
1000
1500
2000
2500
3000
3500
4000
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
Roof Displacement (m)
Bas
e Sh
ear (
kN)
Without infillstiffnessWith infillstiffness
Base Shear Vs. Roof Displacement Base Shear Vs. Roof Displacement ––Push XPush X
µ = 2.41∆/h = 0.49%
µ = 1.46∆/h = 0.34%
1.5 BV
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0
500
1000
1500
2000
2500
3000
3500
4000
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
Roof Displacement (m)
Bas
e Sh
ear (
kN)
Without infillstiffnessWith infillstiffness
Base Shear Vs. Roof Displacement Base Shear Vs. Roof Displacement ––Push YPush Y
1.5 BV
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Capacity and Demand SpectraCapacity and Demand Spectra(With infill stiffness)(With infill stiffness)
0.00.10.20.30.40.50.60.70.80.91.0
0.0 0.2 0.4
Spectral Displacement (m)
Spec
tral
Acc
elar
atio
n C
oeffi
cien
t (S
a/g)
0.0
0.2
0.4
0.6
0.8
1.0
0.00 0.10 0.20 0.30
Spectral Displacement (m)Sp
ectr
al A
ccel
arat
ion
Coe
ffici
ent
(Sa/
g)Lateral Push along X Lateral Push along Y
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Capacity and Demand SpectraCapacity and Demand Spectra(Without infill stiffness)(Without infill stiffness)
0.0
0.2
0.4
0.6
0.8
1.0
0.00 0.10 0.20 0.30Spectral Displacement (m)
Spec
tral
Acc
elar
atio
n C
oeffi
cien
t (S
a/g)
0.00.10.20.30.40.50.60.70.80.91.0
0.0 0.2 0.4Spectral Displacement (m)
Spec
tral
Acc
elar
atio
n C
oeffi
cien
t (S
a/g)
Lateral Push along X Lateral Push along Y
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Retrofitting SchemeRetrofitting Scheme
1. Continuing infill walls only at a few locations.
2. Strengthening of the ground floor columns.
Ground Floor Plan
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Capacity Curve Capacity Curve –– Push XPush X
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20
Roof Displacement (m)
Bas
e Sh
ear (
kN)
∆/h = 1 %
BV
CB
A
∆/h=0.28%
∆/h=0.48%
D
∆/h=0.75%
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State of the Hinge at A and B in Lateral loadState of the Hinge at A and B in Lateral load
A, ∆/h=0.28% B, ∆/h=0.48%
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State of the Hinge at C and D in Lateral loadState of the Hinge at C and D in Lateral load
C, ∆/h=0.75% D, ∆/h=1%
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Performance Point ( Demand spectrumPerformance Point ( Demand spectrum-- Z )Z )
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.1 0.2 0.3 0.4
Spectral Displacement (m)
Spe
ctra
l Acc
elar
atio
n C
oeffi
cien
t (S
a/g) 5%
15%
17.3%
TTeffeff = 1.224s= 1.224s
ββeffeff = 24.9%= 24.9%
V = 7682 V = 7682 kNkN
D = 0.167 m = 0.93% of HD = 0.167 m = 0.93% of H
SSaa = 0.29 m/s= 0.29 m/s22
SSdd = 0.11 = 0.11 m/s
Performance PointPerformance Point
Demand SpectrumDemand Spectrum
Capacity SpectrumCapacity Spectrum
Effective PeriodEffective Period
m/s
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Storey DisplacementsStorey Displacements
0
3
6
9
12
15
18
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20
Displacement (m)
H(m
)
A B C D
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IDIIDI
0
1
2
3
4
5
0.000 0.005 0.010 0.015 0.020IDI
H(m
)
AB
C
D
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What if Performance Point Does Not Exist?What if Performance Point Does Not Exist?
FE
FI Inelastic demand spectrum
V/W (Acceleration)
5% damped elastic spectrumcapacity spectrum
ADD STRENGTH OR STIFFNESS OR BOTH
Roof-top Displacement
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What if Performance Point Does Not Exist?What if Performance Point Does Not Exist?
FE
FI Inelastic demand spectrum
V/W (Acceleration)
5% damped elastic spectrumcapacity spectrum
ENHANCE SYSTEM ENHANCE SYSTEM DUCTILITY DUCTILITY
Roof-top Displacement
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What if Performance Point Does Not Exist?What if Performance Point Does Not Exist?
FE
V/W (Acceleration)
5% damped elastic spectrumNew demand spectrum
REDUCE SEISMIC DEMAND BY:REDUCE SEISMIC DEMAND BY:ADDING DAMPING OR ISOLATIONADDING DAMPING OR ISOLATION
FI
Roof-top Displacement