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ABSTRACT
Earthquakes present a threat to public safety and welfare in a significant
portion everywhere. Recent earthquakes in many parts of the world have
demonstrated the vulnerability of existing reinforced concrete structures to
moderate and strong ground motions. There are many existing buildings which
have been designed according to earlier codes. In these codes, either design for
seismic loads was not a requirement or design was for lower levels of seismic
forces. Many existing buildings do not meet the seismic strength requirements of
present day earthquake codes due to structural inadequacies. Structures
adequately designed for usual loads like dead loads, live loads, wind loads, etc are
not necessarily safe for earthquake forces. For normal loads, the structure remains
within elastic range of the material during service stage. It is neither practical nor
economically viable to design structures to remain within elastic limits during
earthquakes. The evaluation based on nonlinear static by pushover analysis is
necessary to check the adequacy of existing building. The objective of the
present study is to evaluate the adequacy of seismic effect by means of Pushover
analysis using SAP 2000 and retrofit the building for the deficiency found in the
analysis. The building considered for analysis is an existing multistory
(G+11)designed for earlier code of IS 456- 1964 without considering earthquake
effect. The building is modeled as 3-D and slab as Diaphragm and earthquake
load for zone III is adopted to check the adequacy. In Pushover analysis, most of
the hinges were formed in the two rows of lower levels of bottom most storey in
PUSHOVER-X and all the rows of bottom most storey in PUSHOVER-Y are
occurred. Hence the Ground floor column has to be strengthened to withstand
the earthquake forces for Zone III for the building located at Chennai. Theperformance point was obtained in the zone III for the pushover analysis along
X and Y direction are enclosed in the report for evaluation. The inter storey drift
as obtained in the analysis and permissible were compared. Therefore, the
building was found to be inadequate for design earthquake in Ground storey
needs to be retrofitted by using FRB composites or any other suitable
strengthening methods to suit the site requirement.
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CHAPTER 1
INTRODUCTION
1.1 GENERAL
Recent earthquakes in India and in different parts of the world occurred,
resulting losses, especially human lives, have highlighted the structural
inadequacy of buildings to carry seismic loads. There is an urgent need for
assessment of existing buildings in terms of seismic resistance. No one can
predict where and when earthquake will appear and what intensity they will
strike the ground motion. Many existing buildings are designed according to
earlier codes. In these codes, either design for seismic loads was not a
requirement or design was for lower levels of seismic forces. Structuresadequately designed for usual loads like dead loads, live loads, wind loads,
etc are not necessarily safe for earthquake forces. For normal loads, the
structure remains within elastic range of the material during service stage. It
is neither practical nor economically viable to design structures to remain
within elastic limits during earthquakes. As per IS1893 (part 1)- 2002
Criteria for Earthquake Resistant Design of Structures is to ensure that
structures possess at least a minimum strength to withstand minor
earthquakes which occur frequently, without damage; resist moderate
earthquakes without significant structural damage though some non-
structural damage may occur; and aims that structures withstand major
earthquakes without collapse. One of the major challenges that faced by
structural engineers is to determine the seismic capacity of an existing
building and to rehabilitate these buildings to upgrade their seismic capacity
if needed.
Significant amount of research have been reported towards the mitigation of
seismic hazard by devising suitable structural system, improving
conventional design and construction procedure, proposing careful detailing
of structural systems and improving many new materials and energy devices
conductive to the dissipation of energy imparted to the structure during
seismic excitation.
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Many existing multi-storey buildings in earthquake prone regions of India
are vulnerable to severe damage under earthquakes. These buildings do not
meet the requirements of seismic design. The buildings, which appeared to
be strong enough, were crumbled like houses of cards during Bhuj
earthquake. The following are the reasons for retrofitting an existingbuilding:-
(i) The building was not designed as per the codes.
(ii) Subsequent revision of codes and design practice.
(iii) Subsequent upgrading of the seismic zone.
(iv) Deterioration of strength due to aging of the building.
(v) Modification of the building.(vi) Change in use of the building.
(vii) Wrong construction practices and
(viii) Lack of knowledge for earthquake resistant design.
It is uneconomical to demolish these buildings and rebuild them as per
the prescribed codes. Therefore studying seismic response to evaluate
the existing buildings for their seismic performance using pushover
analysis (Nonlinear static analysis) and retrofit it if there is any deficiency
in the design/ strength requirement for survival during earthquake forces.
As the world move toward the implementation of Performance Based
Engineering philosophies in seismic design of civil structures, new
seismic design provisions will require structural engineers to perform
nonlinear analysis of the structures they are designing.
Nonlinear static analysis, or pushover analysis, has been developed
over the past twenty years and has become the preferred analysis
procedure for design and seismic performance evaluation purposes as
the procedure is relatively simple and considers post-elastic behavior.
However, the procedure involves certain approximations and
simplifications that some amount of variation is always expected to exist
in seismic demand prediction of pushover analysis.
Although, in literature, pushover analysis has been shown to capture
essential structural response characteristics under seismic action, the
accuracy and the reliability of pushover analysis in predicting global and
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local seismic demands for all structures have been a subject of
discussion and improved pushover procedures have been proposed to
over come certain limitations of traditional pushover procedures.
However, the improved procedures are mostly computationally
demanding and conceptually complex that use of such procedures areimpractical in engineering profession and codes.
As traditional pushover analysis is widely used for design and seismic
performance evaluation purposes, its limitations, weaknesses and the
accuracy of its predictions in routine application should be identified by
studying the factors affecting the pushover predictions. In other words,
the applicability of pushover analysis in predicting seismic demands
should be investigated for low, mid and high-rise structures byidentifying certain issues such as modeling nonlinear member behavior,
computational scheme of the procedure, variations in the predictions of
various lateral load patterns utilized in traditional pushover analysis,
efficiency of invariant lateral load patterns in representing higher mode
effects and accurate estimation of target displacement at which seismic
demand prediction of pushover procedure is performed.
1.2 NEED FOR NON LINEAR ANALYSIS
For seismic performance evaluation, a structural analysis of the
mathematical model of the structure is required to determine force and
displacement demands in various components of the structure. Several
analysis methods, both elastic and inelastic, are available to predict the
seismic performance of the structures.
Most of the codes including Indian codes are based on linear analysis
and limit state/ultimate design procedure .The earthquake codes
calculated for a linear structure is reduced by a reduction factor based
on available ductility ratio and over strength in the structure. It has been
found in the fast earthquakes that this criterion is generally adequate for
the normal type of structures. In respect of structures with some
irregularity or structures required to satisfy a particular performance
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level, this criterion is not sufficient and a nonlinear analysis of the
building is required. The most basic nonlinear analysis procedure is the
complete nonlinear time history analysis. However, this method hasdifficulty in selection of design time history, as the codes give design
response spectrum and not the design time history. Further this method
is considered to be too complex and impractical for general users. FEMA
273 and ATC 40 present some simplified nonlinear analysis method,
which can be used easily, and provide valuable insight in to the behavior
of the structure during earthquake. The pushover method uses
intersection of capacity (pushover) curve and the reduced responsespectrum to determine maximum displacement. Hence this method is
used for analysis purpose to evaluate the performance of the existing
building using SAP 2000.
1.3 DESCRIPTION OF PUSHOVER ANALYSIS
Pushover 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 applied to the structure, in the presence of full gravity
dead, until a predetermined level of roof displacement is approached.
The magnitude of lateral loads at floor levels do not affect the response
of the structure in displacement-controlled pushover analysis, but the
ratio in which they are applied at each floor level alters response of the
structures.
Pushover analysis is an efficient way to analyse the behavior of the
structure, highlighting the sequence of member cracking and yielding as
the base shear value increases. This can be used for the evaluation of
the performance of the structure and the locations with inelastic
deformation. The primary use of pushover analysis is to obtain a
measure of over strength and to obtain a sense of the general capacity
of the structure to sustain inelastic deformation. Push-over analysis can
provide a significant insight into the weak links in seismic performance of
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a structure. A serious 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 performances
criteria. The performance criteria for pushover analysis is generallyestablished as the desired state of the building given a roof-top or
spectral displacement amplitude.
The loads acting on the structure are contributed from slabs, beams,
columns, walls, ceiling finishes. They are calculated by conventional
methods according to IS 456-2000 and are applied as gravity loads
along with live loads as per IS 875(Part II) in the model created. The
lateral loads and their vertical distribution on each floor level aredetermined as per IS 1893-2002. These loads are then applied in
PUSH-Analysis case during the analysis.
Pushover analysis can be performed as force-controlled or
displacement controlled. In force-controlled pushover procedure, full load
combination is applied as specified, i.e, force-controlled procedure should
be used when the load is known (such as gravity loading). Also, in force-
controlled pushover procedure some numerical problems that affect the
accuracy of results occur since target displacement may be associated
with a very small positive or even a negative lateral stiffness because of
the development of mechanisms and P-delta effects.
Generally, pushover analysis is performed as displacement-controlled
proposed by Allahabadi to overcome these problems. In displacement-
controlled procedure, specified drifts are sought (as in seismic loading)
where the magnitude of applied load is not known in advance. The
magnitude of load combination is increased or decreased as necessary
until the control displacement reaches a specified value. Generally, roof
displacement at the center of mass of structure is chosen as the control
displacement. The internal forces and deformations computed at the
target displacement are used as estimates of inelastic strength and
deformation demands that have to be compared with available capacities
for a performance check.
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Structures are expected to deform inelastically when subjected to
severe earthquakes, so seismic performance evaluation of structures
should be conducted considering post-elastic behavior. Therefore, a
nonlinear analysis procedure must be used for evaluation purpose as
post-elastic behavior can not be determined directly by an elasticanalysis. Moreover, maximum inelastic displacement demand of
structures should be determined to adequately estimate the seismically
induced demands on structures that exhibit inelastic behaviour. Various
simplified nonlinear analysis procedures and approximate methods to
estimate maximum inelastic displacement demand of structures are
proposed in literature. The widely used simplified nonlinear analysis
procedure, pushover analysis, has also been an attractive subject ofstudy.
1.4 PAST STUDIES OF NONLINEAR ANALYSIS
Rosenblieth and Herera (1964) proposed a procedure in which the
maximum deformation of inelastic SDOF system is estimated as the
maximum deformation of a linear elastic SDOF system with lower lateral
stiffness (higher period of vibration, Teq) and higher damping coefficient
(.eq) than those of inelastic system.
Gulkan and Sozen (1974) noted that most of the time the displacement
would be significantly smaller than the maximum response under
earthquake loading. Gulkan and Sozen developed an empirical equation
for equivalent damping ratio using secant stiffness. Only 2D models of
stuctures in regular plan and elevation can be analysed by the
procedure.
Iwan and Kowalsky (1980) developed empirical equations to define the
period shift and equivalent viscous damping ratio to estimate maximum
displacement demand of inelastic SDOF system from its linear
representation.
Fajfar and Fischinger (1987) proposed the N2 method as a simple
nonlinear procedure for seismic damage analysis of reinforced concrete
buildings.
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Kunnath et al (1990) developed an analytical modeling scheme to
assess the damageability of reinforced concrete buildings experiencing
inelastic behaviour under earthquake loads.
Jain and Mir (1991) presented the inelastic response of six-storey
reinforced concrete frames. These frames were subjected to the El-
Centro earthquake. It was shown that the ductility requirements in
columns were quite high and they were unsafe.
Vasseva (1994) carried out a seismic analysis of frames taking intoaccount the geometrical non-linearities. Considerable displacement
appeared in the columns on the first storeys when geometric non-
linearities were taken into account.
Soroushian et al (1998) developed an empirical hysteretic model for
masonry shear walls using the results of cyclic tests performed on thirty
seven single -storey walls.
Dymiotis et al (1999) studied the seismic reliability of reinforced
concrete frames with uncertain drift and member capacity. A statistical
description of the critical inter-storey drift was derived using existing
experimental results mainly from shaking tables of small scale bare
frames.
Elnashai (2001) analyzed the dynamic response of structure using static
pushover analysis. The significance of pushover analysis as an
alternative to inelastic dynamic analysis in seismic design and
assessment were discussed.
1.5. USE OF PUSHOVER RESULTS
Pushover analysis has been the preferred method for seismic
performance evaluation of structures by the major rehabilitation guidelines
and codes because it is conceptually and computationally simple. Pushover
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analysis allows tracing the sequence of yielding and failure on member and
structural level as well as the progress of overall capacity curve of the
structure.
The expectation from pushover analysis is to estimate critical responseparameters imposed on structural system and its components as close as
possible to those predicted by nonlinear dynamic analysis. Pushover
analysis provide information on many response characteristics that can
not be obtained from an elastic static or elastic dynamic analysis. These are
estimates of inter-storey drifts and its distribution along the height
determination of force demands on brittle members, such as axial
force demands on columns, moment demands on beam-columnconnections
determination of deformation demands for ductile members
Identification of location of weak points in the structure (or potential
failure modes)
consequences of strength deterioration of individual members on the
behavior of structural system
identification of strength discontinuities in plan or elevation that will lead
to changes in dynamic characteristics in the inelastic range
verification of the completeness and adequacy of load path
Pushover analysis also expose design weaknesses that may remain
hidden in an elastic analysis. These are story mechanisms, excessive
deformation demands, strength irregularities and overloads on potentially
brittle members.
Although pushover analysis has advantages over elastic analysis
procedures, underlying assumptions, the accuracy of pushover
predictions and limitations of current pushover procedures must be
identified. The estimate of target displacement, selection of lateral load
patterns and identification of failure mechanisms due to higher modes of
vibration are important issues that affect the accuracy of pushover
results.
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Target displacement is the global displacement expected in a design
earthquake. The roof displacement at mass center of the structure is
used as target displacement. The accurate estimation of target
displacement associated with specific performance objective affect the
accuracy of seismic demand predictions of pushover analysis.
In pushover analysis, the target displacement for a multi degree of
freedom (MDOF) system is usually estimated as the displacement
demand for the corresponding equivalent single degree of freedom
(SDOF) system. The basic properties of an equivalent SDOF system
are obtained by using a shape vector which represents the deflected
shape of the MDOF system.
However, in pushover analysis, generally an invariant lateral load
pattern is used that the distribution of inertia forces is assumed to be
constant during earthquake and the deformed configuration of structure
under the action of invariant lateral load pattern is expected to be similar
to that experienced in design earthquake. As the response of structure,
thus the capacity curve is very sensitive to the choice of lateral load
distribution , selection of lateral load pattern is more critical than the
accurate estimation of target displacement.
The lateral load patterns used in pushover analysis are proportional to
product of story mass and displacement associated with a shape vector
at the story under consideration. Commonly used lateral force patterns
are uniform, elastic first mode, "code" distributions and a single
concentrated horizontal force at the top of structure. Multi-modal load
pattern derived from Square Root of Sum of Squares (SRSS) story
shears is also used to consider at least elastic higher mode effects for
long period structures. These loading patterns usually favor certain
deformation modes that are triggered by the load pattern and miss others
that are initiated and propagated by the ground motion and inelastic
dynamic response characteristics of the structure . Moreover, invariant
lateral load patterns could not predict potential failure modes due to
middle or upper story
mechanisms caused by higher mode effects. Invariant load patterns can
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provide adequate predictions if the structural response is not severely
affected by higher modes and the structure has only a single load yielding
mechanism that can be captured by an invariant load pattern.
FEMA-273 recommends utilising at least two fixed load patterns that formupper and lower bounds for inertia force distributions to predict likely
variations on overall structural behavior and local demands. The first pattern
should be uniform load distribution and the other should be "code" profile or
multi-modal load pattern. The 'Code' lateral load pattern is allowed if more
than 75% of the total mass participates in the fundamental load.
The invariant load patterns can not account for the redistribution of inertiaforces due to progressive yielding and resulting changes in dynamic
properties of the structure. Also, fixed load patterns have limited capability to
predict higher mode effects in post-elastic range. These limitations have led
many researchers to propose adaptive load patterns which consider the
changes in inertia forces with the level of inelasticity. The underlying
approach of this technique is to redistribute the lateral load shape with the
extent of inelastic deformations. Although some improved predictions have
been obtained from adaptive load patterns , they make pushover analysis
computationally demanding and conceptually complicated. The scale of
improvement has been a subject of discussion that simple invariant load
patterns are widely preferred at the expense of accuracy.
Whether lateral loading is invariant or adaptive, it is applied to the
structure statically that a static loading can not represent inelastic dynamic
response with a large degree of accuracy.
The above discussion on target displacement and lateral load pattern
reveals that pushover analysis assumes that response of structure can be
related to that of an equivalent SDOF system. In other words, the response
is controlled by fundamental mode which remains constant throughout the
response history without considering progressive yielding. Although this
assumption is incorrect, some researchers obtained satisfactory local and
global pushover predictions on low to mid-rise structures in which response
is dominated by fundamental mode and inelasticity is distributed throughout
the height of the structure .
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1.6 LIMITATIONS OF PUSHOVER ANALYSIS
It must be emphasized that the pushover analysis is approximate in nature
and is based on static loading. As such it cannot represent dynamic
phenomena with a large degree of accuracy. It may not detect someimportant deformation modes that may occur in a structure subjected to
severe earthquakes, and it may exaggerate others. Inelastic dynamic
response may differ significantly from predictions based on invariant or
adaptive static load patterns, particularly if higher mode effects become
important.
Limitations are imposed also by the load pattern choices. Whatever load
pattern is chosen, it is likely to favor certain deformation modes that aretriggered by the load pattern and miss others that are initiated and
propagated by the ground motion and inelastic dynamic response
characteristics of the structure. The simplest example is a structure with a
weak top story. Any invariant load pattern will lead to a concentration of
inelastic deformations in the top story and may never initiate inelastic
deformations in any of the other stories. Thus, good judgment needs to be
employed in selecting load patterns and in interpreting the results obtained
from selected load patterns.
1.7 RETROFIT METHODS
The purpose of seismic retrofitting a building is to enhance the structural
capacities (lateral strength, lateral stiffness, ductility, stability and integrity)
so that the building can withstand the design level of earthquake. After
analysis, a decision on whether or not to retrofit an unsafe building
depends on many factors. Lifeline buildings must necessarily be retrofitted,
in view of their extreme importance otherwise, they may meet the tragic fate
of the Bhuj District Hospital Complex. Important buildings should also be
retrofitted.
If the seismic strength of an existing building (or structural
component) is only 33% of that required by the current standard for a new
building, the risk involved is as high as about 20 times that of the new
building. If the strength is two-thirds that required by current standard, the
risk reduced to 3 times the standard risk ; this level of risk is generally
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considered as the limit of acceptable risk. Hence, it is recommended that
seismic retrofit be necessarily undertaken when the strength of an existing
building drops 70% of the capacity required by the current standard.
RCC Columns are the key elements of concrete structures designed toresist vertical as well as lateral loads. Majority of the structures that were
built in India during 20th century are seismically deficient. Seismic retrofit of
these older structures, particularly columns has been an important issue.
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CHAPTER 2
AIM AND SCOPE OF THE PRESENT INVESTIGATION
2.1 AIM AND OBJECTIVE OF STUDY
The aim of the present study is to check the adequacy of seismic effect an
existing multistory RCC building during earthquake by Pushover analysis. The
study focuses on the following for detailed evaluation. -
1) To check the adequacy of existing building for present seismic condition as
per IS: 1893-2002 by Pushover analysis using SAP 2000.
2) To find whether building is capable to withstand seismic load for present
conditions and to determine the maximum critical load at which building will
fail. and what load building will be failed.
3) To suggest suitable retrofit measure at affordable cost.
2.2 NEED FOR STUDY
(i) The existing building is designed for gravity load of Dead load, Live load
and Lateral load of Wind load based on Working Stress method using old
code IS:456- 1964 without considering the seismic loads.
(ii) It is a aged building used for Public purpose. (year of construction 1976).
(iii) It is high rise massive structure, hence attract higher seismic forces.
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CHAPTER 3
ANALYSIS OF EXISTING MULTISTOREY RCC BUILDING AND
RETROFIT
3.1 INTRODUCTION
Earthquakes produce the most severe loading on structures. Code of Practice
for earthquake engineering has been designed with aim that human lives are
protected, damage is limited and service structures repair operational. Earthquake
causes shaking of the ground. So, a building resting on the latter will experiencemotion at its base. The earthquake resistant structure must include a complete
seismic and gravity force resisting system capable of providing adequate strength,
stiffness and energy dissipation capacity to withstand the seismic ground motion
within the prescribed limits of deformation and strength demand. Earthquake
ground motion causes shaking of the structures leading to inertia forces. The
ground motion is quite random in magnitude and direction. At any instant the
ground motion can be resolved into horizontal and vertical components. Since the
existing structure is designed against gravity loads and lateral load of wind loads,
structure need to be designed and checked to resist horizontal components of the
inertia forces.
3.2 DESCRIPTION OF STRUCTURE
(i) Building Frame System : RC OMRF
(ii) Usage : Office purpose
(iii) Built in the year : 1976
(iv) Seismic Zone : III (Chennai)
(v)No. of Storey : G+11
(vi) Foundation : Multiple Piles
(vii) Materials used : M15 & Fe415
(viii) Plan Dimension : 33.0m x 16.6m
(ix) Height of Building : 40.8 m
(x) Soil type assumed : Type II (Medium soil)
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Fig 3.1 Structural Plan
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Fig 3.2 RCC Details of Beams
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Fig 3.3 RCC Details of Columns
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3.3 SEISMIC LOAD CALCULATION
To ensure safety of building of structure under earthquake IS:1893 -2002 is
used to calculate total design lateral force of design seismic base shear (V B) along
X and Y direction shall be calculated.
Gravity loads
At Floor levelsDeadload
Liveload
Totalload
Self weight 0.12 25 3.00 3.00Floor finishes 0.05 20 1.00 1.00
Partition wallWeight 1.00 1.00Live load (OfficePurpose) 4.00 4.00
5.00 4.00 9.00kN/m
At Roof level
Self weight 0.12 25 3.00 3.00Weatheringcourse 2.25 2.25Live load (Roof withaccess) 1.50 1.50
5.25 1.50 6.75kN/m
ComponentWeightBeam Weight at each floorlevel 0.30 0.48 165.00 25.00 584
0.23 0.18 87 25 90
684 KN
Column Weight at eachfloor level 0.45 0.9 3.4 11 25 377
0.4 0.8 3.4 22 25 598977 KN
Wall Weight at eachfloor 0.23 3.1 50 19.2 684
0.23 2.80 30.00 19.20 3711055 KN
Parapet wall weight 0.23 0.90 99.20 19.20 394 KN
Slab Floor area 33.00 16.60 547.8
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slab load at roof level 547.80 5.25 0.0 1.50 2876slab load at floor level 547.80 5.00 0.5 4 3834.6
Equivalent load at rooflevel 2875.95 684.00 488.5 527.5 394 4969.95 4970
Equivalent load at each floorlevel 3834.60 684.00 977 1055 0 6550.60 6551No. of Storeys 12Total Seismic weight ofbuilding = (4970+6551x11)= 77031
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Seismic load calculation - with brick infill loads
Equivalent Static load Method is adopted as per IS 1893 -2002
Seismic base shear Vb Ah W
Ah Z I Sa
2 R g
Site location Chennai
Zone factor Z 0.16 Zone 3
Importance factor I 1.00 Table 6
Response Reduction factor R 3 Table 7
Force along width direction - X direction
Seismic base shear Vb Ah W
Ah Z I Sa
2 R gHeight of the Building h 40.8 m
Width of the building d 33 m
For RC frame building Ta 0.09 x h / dwith brick infill
0.64For Medium soil site & T = 0.64Sa/g 2.13
Ah 0.057Force along width direction - Y directionResponse Reduction factor R 3
Height of the Building h 40.8 m
Width of the building d 16.6 m
For RC frame building Ta 0.09 x h / d
with brick infill0.901
For medium soil site & T =0.901Sa/g = 1.51
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TABLE 3.1 LATERAL FORCE ON NODAL POINTS
Floo
r
level
Wi
(KN)
h
(m)
Wi hi2
(106)
Wi hi2
Wi
hi2
Forces in
(KN)
Force/Node
(KN)
X Y X Y
12 4970 40.8 8.273 0.178 776 551.2 70.55 183.73
11 6551 37.4 9.163 0.197 859.6 610.5 78.14 203.47
10 6551 34 7.573 0.163 710.4 504.5 64.58 168.17
9 6551 30.6 6.134 0.132 575.4 408.7 52.31 136.22
8 6551 27.2 4.847 0.104 454.6 322.9 41.33 107.63
7 6551 23.8 3.711 0.080 348.1 247.2 31.64 82.41
6 6551 20.4 2.726 0.059 255.7 181.6 23.25 60.54
5 6551 17.0 1.893 0.041 177.6 126.1 16.14 42.04
4 6551 13.6 1.212 0.026 113.7 80.7 10.33 26.91
3 6551 10.2 0.682 0.015 63.9 45.4 5.81 15.14
2 6551 6.8 0.303 0.007 28.4 20.2 2.58 6.73
1 6551 3.40 0.076 0.002 7.10 5.0 0.65 1.68
77031 46.592
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Fig3.4 Dead Load along X direction
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Fig3.5 Dead Load along Y direction
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Fig3.6 Live Load
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Fig3.7 Seismic load along Y direction
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Fig3.8 Seismic load along X direction
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3.4 PUSHOVER ANALYSIS
Pushover analysis is a static non-linear procedure in which the magnitude of the
lateral forces are incrementally increased, maintaining the predefined distribution
pattern along the height of the building. With the increase in magnitude of the
loads, weak links and failure of the building are found.
Pushover analysis can determine the behaviour of a building including the
ultimate load and the maximum inelastic deflection. Local non-linear effects are
modeled and the structure is pushed until a collapse mechanism developed. At
each step, the base shear and the roof displacement can be plotted to generate
the pushover curve. It gives an idea of the maximum base shear that the structure
is capable of resisting. For regular buildings, it can also give a rough idea about
the global stiffness of the building.
Instead of plotting the base shear versus roof displacement, the base
acceleration can be plotted with the roof displacement (Capacity spectrum). The
spectral acceleration and spectral displacement, as calculated from the linear
elastic response spectrum for a certain damping (initial damping 5%) is plotted in
the acceleration displacement response spectrum (ADRS) format. The locus of the
demand points in the ADRS plot is referred to as the demand spectrum. The
demand spectrum corresponds to the inelastic deformation of the building.
The seismic performance of a building can be evaluated in terms of
pushover curve, performance point, displacement ductility, plastic hinge formation
etc. The base shear Vs roof displacement curve is obtained from the pushover
analysis from which the maximum base shear capacity of structure can be
obtained, The curve is transformed into capacity spectrum by SAP 2000 as per
ATC 40 and demand or response spectrum is also determined for the structure
depending upon the seismic zone, soil conditions and required building
performance level. The performance point is point where the capacity curve
crosses the demand curve. The intersection of demand and capacity spectrum at
5% damping gives the performance point of the structure analysed. If the
performance point exists and the damage state at this point is acceptable, the
structure satisfies the target performance level. At the performance point, the
resulting responses of the building should be checked using certain acceptability
criteria. It must be emphasized that the pushover analysis is approximately innature and is based on the statically applied load. It estimates an envelope curve
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of the behaviour under the dynamic load.
3.4.1 ASSUMPTIONS
(1) Seismic Zone III is considered as the building is located at Chennai.
(2) As the building is constructed very long back, the age factor in the analysisis not considered.
(3) Building considered to be noncompliant with IS 13920:1993 (R=3).
(4) As the foundation rest with multiple pile with pile cap, Fixity is considered
at pile cap. Soil-structure interaction neglected.
(5) Elevator walls not considered as lateral load resisting elements.
3.4.2 METHODOLOGY OF PUSHOVER ANALYSIS
The following sub sections provide procedure for determining capacity, demandand Performance using capacity spectrum method.
3.4.3 STRUCTURAL MODELLING
A computer model was created and non linear analysis was performed using
SAP 2000. For the analysis, 3-D modeling of the existing building reinforced
concrete frame was developed. The beams and columns were modeled as frame
elements considering the flexural properties to be assigned to beams and columns
were cross sectional dimensions, material properties, etc. The stiffness for
columns and beams were taken as 0.7E I g,0.5EIg according for the cracking in the
members and the contribution of flanges in the beams.
The beam-column joints were modeled by giving end offsets at the joints. This is
intended to get the bending moments at the face of the beams and columns . A
rigid zone factor of one was taken to entire rigid connection of the components.
Floor slabs were assumed to act as diaphragms, which ensure integral action of
all the vertical lateral load-resisting elements. The weight of the slab was
distributed as triangular and trapezoidal load for two way slab and uniformly
distributed load for one way slab to the surrounding beams as per IS:456-2000.
The brick infill load was assigned on the beams. The seismic mass at each floor
was calculated and applied at front nodes at each direction as nodal forces. The
effect of soil-structure interaction was ignored in the analysis. The ends of the
columns are assumed to be fixed at the bottom.
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3.5 PUSHOVER ANALYSIS
The lateral force distribution along the height of the building according to
IS:1893-2002 was used in the pushover analysis. Pushover analyses were
performed independently in the two orthogonal X and Y directions using SAP
2000. The target displacement at the roof of the building was taken as 0.004 timesbuilding height to comply with Clause 7.11.1 of IS:1893-2002. Beams and columns
were modeled with concentrated plastic hinges at the column and beam faces
respectively. Beams have moment (M3) hinges, whereas columns have axial load
and biaxial moment (PMM) hinges. Geometric non-linearity of the structure was
considered in the lateral pushover analyses. The results of pushover analysis
both in X and Y direction are depicted in the figures shown. The Green color
indicates pushover curve, the red color indicates demand curve and the yellowcolor indicates damping curve. The intersection of pushover and demand curve
shows the performance point. If the performance point is reached in both the
direction, the building will be seismic resistant and there is no need to retrofit.
There are three pushover cases for the evaluation of buildings. Gravity push is
used to apply gravity load. Push X is the lateral push in X direction starting at the
end of gravity push. Push Y is the lateral push in Y direction starting at the end of
gravity push.
The pushover analysis was conducted for the frame considering P-
effect. The pushover analysis involves application of monotonically increase lateral
deformation patterns and monitoring inelastic behaviour within the structure. The
relationship of base shear and roof displacement (capacity curve) and 5% damped
elastic design response spectrum (demand curve) of the model was established.
The capacity and demand curves converted into a spectral displacement and
spectral acceleration format to obtain the performance. The performance point is
the intersection point of the capacity and the demand curves.
The output of the capacity curve gives the coordinates of the pushover
curve and summarizes the number of inches in each state. The coordinates of
capacity curve and demand curve were transform into spectral acceleration versus
spectral displacement coordinates. The curve slotted in this format are called as
capacity spectrum and demand spectrum respectively. Applied Technology
Council(ATC 40) provides three different procedures(procedures A,B and C) to
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establish the earthquake induced deformation demands in this study, procedure
B was adopted.
The number of hinges formed in the beams and columns at the
performance point (or at the point of termination of the pushover analysis) and
their performance levels can be used to study the vulnerability of the building. Thevulnerability can be quantified using the concept of vulnerability index.
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Fig3.9 Isometric View of model
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Fig3.10 Slab Modeling (Diaphragm)
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Fig3.11 Beam Column Joint (End length offset)
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Fig 3.12 Deformed Shape (PUSH-X)
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Fig 3.13 DEFORMATION (PUSH-X)
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Fig 3.14 HINGE FORMATION (PUSH-X)
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Fig 3.15 Hinge Formation (PUSH-Y)
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Fig3.16 Capacity Curve (PUSH-Y)
Fig3.17 Capacity Curve (PUSH-X)
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Fig3.18 Base Vs Displacement Curve (PUSH-X)
Fig3.19 Base Vs Displacement Curve (PUSH-Y)
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CHAPTER 4
RESULTS AND DISCUSSION
4.1 INTRODUCTION
A Twelve- storeyed Reinforced concrete 3-D space frame of an existing
building was taken as a case study for evaluating the adequacy of seismic effect
by PUSHOVER analysis using SAP 2000. The frame was subjected to specified
seismic forces for Zone III as per IS: 1893 (Part 1)-2002 in addition to Gravity
loads with P- effect. The various results of the building on Pushover curve,
Displacements Vs Storey Drifts, Location of Hinges formed are indicated in the
above figures shown.
4.2 PUSHOVER CURVE
The Pushover curves for the building in X direction and Y direction of the
model are indicated in the Figures 3.18 and 3.19 shown. These curves depict
the global behaviour of the model in terms of its stiffness and ductility. The
stiffness and ductility ratios of the frame along Y direction is 1.20 times greater
than that along X direction.
4.3 CAPACITY SPECTRUM, DEMAND SPECTRUM AND PERFORMANCE
POINT
The demand and capacity spectra for the lateral push along the two orthogonal
direction for the Zone III are shown in Figures 3.16 and 3.17 . Performance point
was obtained for the model in Zone III along both the directions. The pushover
analysis indicates the performance in X direction is stronger than performance in
Y direction.
4.4 DISPLACEMENTS AND STOREY DRIFTS
The displacements at ultimate load are plotted in Figures 3.16 and 3.17. Theinter-storey drifts corresponding to the displacements profiles are shown in Figures
3.20 and 3.21. It can be seen that the inter-storey drift at the lower floor levels is
more than the permissible limit of 0.4%.
4.5 LOCATION OF HINGES
The location of hinges formed in the building model during earthquake forces
along both the directions are shown in figures 3.12 and 3.15. The hinges are
formed in the lower most storey in first two rows of column in X direction and threerows of lower most columns in Y direction.
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4.6 BASE SHEAR
From the result it is observed that maximum base shear was 4036 KN which is
about 20% of seismic weight of frame and the maximum displacement
corresponding to this base shear is 0.53m. The frame is pushed to a maximum
displacement of 4% of its height.4.7 VULNERABILITY INDEX
The vulnerability indix of the building in both X and Y directions are given in the
Table 4.1. It can be seen that the vulnerability index of the building is high along X
and Y directions which suggests the retrofitting of the building. It can be
considered that the building in lower most storey has to be strengthened so as to
fulfill the requirement of safety limit of earthquake for the present zone III.
4.7 RETROFIT MEASUREAs the hinges are formed in the lower most storey of columns, the size of the
column or reinforcement of column has to be enhanced suitably to stiffen the
columns. Retrofit may be divided into Global and local strategies. Introducing
walls or braces in an open ground storey are example of global strategies. The
local strategies include jacketing of columns and beams by concrete or steel and
use of carbon fibre sheet and fibre reinforced polymer wraps. Since only
strengthening is required in Ground storey,
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CHAPTER 5
SUMMARY AND CONCLUSION
5.1 SUMMARY
The seismic behaviour of an existing multistory RCC building was
investigated for Zone III as per latest IS Codal provision and Pushover analysis
using SAP 2000. The following parameters are observed:
Pushover Curve
Capacity Spectrum, Demand Spectrum and Performance Point
Displacement Vs Storey Drifts Location of Hinges
Base Shear
Inter Storey drift
Vulnerability index
Based on the results obtained from the software, the following conclusions are
made:
5.2 CONCLUSION
The shear capacity of the frame is observed to be little higher than the demand in
the zone III. The pushover curves in X direction and Y direction gives the
performance of the structure.
The inter-storey drift at the lower floor levels exceed permissible code limit of
0.4%. The inter-storey drift profile of the frame illustrates the soft-storey
mechanism which is undesirable in the seismic regions.
The hinges are concentrated at the lower most floor level of columns in both X
and Y direction Pushover cases which demonstrates the inadequacy of some of
the columns in the ground storey.
The vulnerability index of the frame is high and therefore, the frame is found to
be unsafe for the design earthquakes. When a column is subjected to earthquake
loading, its energy absorption capacity is the main concern rather than its load
carrying capacity. Even though various alternatives are available, it is preferable to
use FRB composites because they possess high strength to weight ratio andresistance to corrosion.
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5.3 SCOPE FOR FUTURE STUDY
Non-linear behaviour of the similar multi storey existing steel structure can be
studied and compared with RCC building behaviour. It can also be suggested to
compare the results of two similar software SAP 2000 and ETAB for evaluvating
the seismic effect for Zone III with Zone IV for an existing multi-storey RCCbuilding and the results are validated.
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REFERENCES
1. ATC 40 (1996) Seismic evaluation and retrofit of concrete buildings,California seismic safety commission.
2. FEMA 273 (1997) NEHRP Guidelines for the seismic rehabilitation of
buildings, Building Seismic safety council, Washingtgon D.C.
3. Earthquake Analysis and Design of structures proceedings of NationalConference during Feb. 2006 Coimbatore.
4. Arlekar, J.N. and Murty, C.V.R (2000), Vibration Survey of RC framebuildings having Brick Masonry Infill Walls, The Indian Concrete Journal,Vol.74, No.10, PP 581-586.
5. Chopra, A.K. and Goel, R.K. (2002), A Modal Pushover Analysis procedurefor estimating Seismic Demands for Buildings, Journal of Earthquake
Engineering and Structural Dynamics, Vol.31, No.3, PP 561-582.
6. Dymiotis, C., Kappos, A.J. and Chryssanthopoulos, M.K. (1999), SeismicReliability of RC Frames with Uncertain Drift and Member Capacity, Journalof Structural Engineering. ASCE, Vol.125, No.9, pp 1038-1047.
7. Elnashai, A.S. (2001), Advanced Inelastic Static (PUSHOVER) analysis forEarthquake Application. Journal of Structural Engineering and Mechanics,Vol.12 No.1, pp 51-69.
8. IS:1893 (part-I)- 2002,Criteria for Earthquake Resistant Design of
Structures.
9. Jain AK and Mir, RA (1991), Inelastic Response of reinforced concreteframes under Earthquakes. The Indian Concrete Journal, Vol.65, No.4 PP175-180.
10. Kanitkar, R. and Kanitkar, V. (2004), Seismic Performance of ConventionalMulti-storey Buildings with open ground floors for vehicular parking. TheIndian Concrete Journal, Vol.78, No.2, pp 99-104.
11. Kunnath S.K. Hoffmann, G.Reinhorn, A.M. and Mander JB (1995), GravityLoad-Designed reinforced concrete buildings. Part.II. Evaluation ofDetailing Enhancements, ACI Structural Journal, Vol.92, No.4, pp 470-478.
12. Kunnath S.K. Reinhorn, A.M. and Park, Y.J. (1990), Analytical modeling ofInelastic Seismic Response of RC Structures, Journal of StructuralEngineering, ASCE, Vol116, No.4, pp 996-1017.
13. Soroushian, P., Obaeki, K. and Choi, K.B. (1998), Nonlinear Modeling andSeismic Analysis of Masonry Shear Walls, Journal of StructuralEngineering, ASCE, Vol.114, No.5, pp 1106-1119.
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