m.e. thesis push over analysis

7/30/2019 M.E. thesis Push over analysis

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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.

m.e. thesis push over analysis

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