SEISMIC ANALYSIS OF RCC FRAMED BUILDING

ABSTRACT

Masonry infill are normally considered as non-structural elements and their stiffness

contributions are generally ignored in practice, such an approach can lead to an unsafe design.

Until now infill wall system has been studied by many authors like P.G.Asteris (2003); and

Depts. Das and C.V.R.Murty (2004); B.Srinivas and B.K.Raghu Prasad (2009); Mulgund G.V

(2011).

It is proposed to develop an infill wall system in edges of the building to reduces the seismic

effects on buildings. In the present studies RCC framed buildings are analyzed by ETABS

software. In modeling the masonry infill panels the Equivalent diagonal strut method is used for

of the braces of the buildings. This work includes four kinds of 15 story rectangular RCC framed

building with height to depth ratio (H/D) as 7.02 which are describe here.

The building with shear wall at the corners and dimensions as 35 m x 25 m; building with shear

wall at the corners and also center core of shear walls and dimensions as 35 m x 25 m; the

building with shear wall at the corners and dimensions as 25 m x 25 m; and building with shear

wall at the corners and also center core of shear walls and dimensions as 25 m x 25 m.

In each building after design infill walls are modeled in different cases. Infill walls are used on

the exterior beams as dead load (Case 1); infill walls are used on the exterior beams as strut for

all storey's (Case 2) and infill walls are used on the exterior beams as strut for all storey's except

the first story (Case 3).

CHAPTER- 1

INTRODUCTION

Masonry infill are normally considered as non-structural elements and their stiffness

contributions are generally ignored in practice, such an approach can lead to an unsafe design.

The masonry infill walls though constructed as secondary elements behaves as a constituent

part of the structural system and determine the overall behavior of the structure especially

when it is subjected to seismic loads. In this paper seismic analysis has been performed using

Equivalent Lateral Force Method for different reinforced concrete (RC) frame building models

that include bare frame, unfilled frame and open first storey frame. The results of bare frame,

in filled frame and open first storey frame are discussed and conclusions are made. In modeling

the masonry infill panels the Equivalent diagonal Strut method is used and the software ETABS

is used for the analysis of all the frame models. Mostly two common structural damages

observed caused by masonry infill walls in earthquakes i.e soft stories and short columns. In

office or residential building outer side central opening are used. In this case central opening

are provided in periphery wall with different percentage i.e. 15% and 25% and brick

compressive strength are used as per IS : 1905-1987 i.e. 5.0 and 12.5 N/mm2 and Brick

Masonry strength is 0.50 and 1.06N/mm2.

Reinforced concrete (RC) frame buildings with masonry infill walls have been widely

constructed for commercial, industrial and multi storey residential uses in seismic regions.

Masonry infill typically consists of bricks or concrete blocks constructed between beams and

columns of a reinforced concrete frame. The masonry infill panels are generally not considered

in the design process and treated as architectural (non-structural) components. Nevertheless,

the presence of masonry infill walls has a significant impact on the seismic response of a

reinforced concrete frame building, increasing structural strength and stiffness (relative to a

bare frame) [1]. Properly designed infill can increase the overall strength, lateral resistance and

energy dissipation of the structure. An infill wall reduces the lateral deflections and bending

moments in the frame, thereby decreasing the probability of collapse. Hence, accounting for

the infill in the analysis and design leads to slender frame members, reducing the overall cost of

the structural system. The total base shear experienced by a building during an earthquake is

dependent on its time period. The seismic force distribution is dependent on the stiffness and

mass of the building along the height. The structural contribution of infill wall results into stiffer

structure thereby reducing the storey drifts (lateral displacement at floor level). This improved

performance makes the structural design more realistic to consider infill walls as a structural

element in the earthquake resistant design of structures.

1.2 Organization of the Dissertation

The dissertation is divided into six chapters as follows.

First chapter is introduction of the work.

Second chapter entitled Review of Literature describes in detail the various works

conducted by the researchers to understand the behavior of masonry infill and concrete shear

wall frames and their effect on strength requirements, for different type of buildings by the

seismic analysis and summary of literatures need for the present investigation, and describes

the objectives and scope of the present study are organized in the project. This chapter also

describes the importance of the study.

Third chapter includes different seismic analysis procedures such as linear and non

linear both static and dynamic analysis. It also gives introduction to hinges and to their

properties. It includes detailed procedure for pushover analysis and graphical representation of

pushover curves. Advantages of inelastic procedure over elastic procedure and brief details of

earthquake.

Fourth chapter provide complete details of different models which has used in this

dissertation and modeled in etabs software with their elevation and 3D views. Manually

calculations of natural time periods, base shears and distribution of lateral seismic shear forces

of the different models.

Fifth chapter is the discussion of results by considering the different parameters of the

building model.

The chapter sixth gives summary, conclusions and further scope of the study, and at last references.

1.3 OBJECTIVES OF THE STUDY

The current study is concerned with the application of recent techniques of analyses proposed by the international seismic codes and guidelines to determine the elastic and inelastic responses of typical multi-storey buildings made of reinforced concrete due to earthquake ground motions. It aims to encourage the inclusion of performance-based concepts in local seismic codes of design and evaluation. Basically, the objectives of this thesis are:

To assess the implementation of recent linear and nonlinear analyses, within the current practice of seismic design codes, for estimating seismic demands of multi-storey buildings.

To evaluate the seismic demands estimated using the pushover technique by comparison with the dynamic results determined by nonlinear time-history analysis using two natural records of ground motions: Elcentro and Bhuj.

PARAMETER OF STUDY

To Study the behavior of building on influence of masonry infill on the overall behavior of structure when subjected to lateral seismic forces.

To verify the effect of vertical irregularity on the fundamental natural period of the building and its effect on performance of the structure during earthquake for different building models selected.

Finding out the deflections and storey drifts at each storey using linear and non-linear analyses.

To study the ductility factor for different building models with and without infill and the performance level of the structures during earthquake.

CHAPTER-2

LITERATURE REVIEW

2.1 GENERAL

It has always been a human aspiration to create taller and taller structures.

Development of metro cities in India there is increasing demand in High Rise Building. The

reinforced cement concrete moment resisting frames in filled with unreinforced brick masonry

walls are very common in India and in other developing countries. Masonry is a commonly used

construction material in the world for reason that includes accessibility, functionality, and cost.

The primary function of masonry is either to protect inside of the structure from the

environment or to divide inside spaces. Normally considered as architectural elements.

Engineer’s often neglect their presence. Because of complexity of the problem, their interaction

with the bounding frame is often neglected in the analysis of building structures. When

masonry in fills are considered to interact with their surrounding frames, the lateral load

capacity of the structure largely increases. This assumption may lead to an important

inaccuracy in predicting the response of the structure. This occurs especially when subjected to

lateral loading. Role of infill’s in altering the behavior of moment resisting frames and their

participation in the transfer of loads has been established by decades of research. A review of

the developments in the seismic analysis, design, and experimental investigations are

presented along with the codal provisions of various countries.

2.2 STRUCTURAL ANALYSIS

Multi-storied-framed building construction commenced in the late 19th century even

though the analytical methods were not developed until the early 20th century. The earlier

structures were analyzed using approximate methods such as cantilever method developed by

A. C. Wilson in 1908 and portal method by Albert Smith in 1915. Iterative methods, namely

Hardy Cross method of 1932 and Kani’s method of 1947 were developed, and widely adopted

for the analysis of building frames in preference to the direct solutions of the equations formed

by the slope-displacement method. The behavior of framed buildings under seismic loads was

investigated extensively because of the wide spread damage in various countries. Sano in 1916

introduced the term ‘seismic coefficient’, which is widely adopted in determining the lateral

forces on multi-storied structures. The behavior of the industrial bank building in Japan, which

withstood the 1923 Great Kanto earthquake without significant damage, was invaluable in

comprehending the term seismic coefficient. In the years that followed, several building codes

were modified to include the clause that “the structure shall be designed to withstand a

horizontal seismic coefficient exceeding 0.1”. This may be the first specification for earthquake

resistance and seismic load considerations.

Various research works and experiments have been carried out since a long time all over the

globe to understand or to evaluate the effect of seismic forces on existing RC building in high

seismic zones and in hilly terrain. The concepts of modeling and analysis techniques used for

this purpose are also getting improved with advancement of engineering and technology from

past experience,

2.3 REVIEW

M C Griffith and A V Pinto [1] have investigated the specific details of a 4-story, 3-bay

reinforced concrete frame test structure with unreinforced brick masonry (URM) infill walls

with attention to their weaknesses with regards to seismic loading. The concrete frame was

shown to be a “weak-column strong-beam frame” which is likely to exhibit poor post yield

hysteretic behavior. The building was expected to have maximum lateral deformation

capacities corresponding to about 2% lateral drift. The unreinforced masonry infill walls were

likely to begin cracking at much smaller lateral drifts, of the order of 0.3%, and completely lost

their load carrying ability by drifts of between 1% and 2%.

Shunsuke Otani [2] studied the development of earthquake resistant design of RCC Buildings

(Past and Future). The measurement of ground acceleration started in 1930’s, and the response

calculation was made possible in 1940’s. Design response spectra were formulated in the late

1950’s to 1960’s. Non-linear response was introduced in seismic design in 1960’s and the

capacity design concept was introduced in 1970’s for collapse safety. The damage statistics of

RCC buildings in 1995 Kobe disaster demonstrated the improvement of building performance

with the development of design methodology. Buildings designed and constructed using

outdated methodology should be upgraded. Performance basis engineering should be

emphasized, especially for the protection of building functions following frequent earthquakes.

Ciro Faella, Enzo Martinelli, Emidio Nigro [3] proposed an assessment procedure in terms of

displacement capacity and demand. The sample application of the proposed procedure to a

typical building emphasized how easy and quick can be its application. As a brief parametrical

investigation, the influence of subsoil stiffness on the seismic vulnerability of the building was

analyzed pointing out that vulnerability was much larger as subsoil was less stiff. A rational

design procedure for choosing the retrofitting system was proposed with the aim of

determining the key mechanical characteristics of a bracing system working in parallel with the

existing structure for complying the safety requirement provided by Euro code 8 – Part 3

entirely devoted to existing structures. In the proposed design procedure, according to a

displacement-based approach, the strengthening substructure was designed in terms of lateral

stiffness, because Page12 displacement demand is strictly controlled by the displacement

capacity of the existing structure. For this reason, usual force-based design procedures suitable

for new structures, in which displacement capacity is only imposed by the new structure itself,

are not directly applicable for bracing system utilized for retrofitting existing structures.

Oğuz, Sermin [4] ascertained the effects and the accuracy of invariant lateral load patterns

utilized in pushover analysis to predict the behavior imposed on the structure due to randomly

selected individual ground motions causing elastic deformation by studying various levels of

nonlinear response. For this purpose, pushover analyses using various invariant lateral load

patterns and Modal Pushover Analysis were performed on reinforced concrete and steel

moment resisting frames covering a broad range of fundamental periods. The accuracy of

approximate procedures utilized to estimate target displacement was also studied on frame

structures. Pushover analyses were performed by both DRAIN-2DX and SAP2000. The primary

observations from the study showed that the accuracy of the pushover results depended

strongly on the load path, the characteristics of the ground motion and the properties of the

structure.

Durgesh C. Rai [5] gave the guidelines for seismic evaluation and strengthening of buildings.

This document was developed as part of project entitled ―Review of Building Codes and

Preparation of Commentary and Handbooks, awarded to Indian Institute of Technology Kanpur

by the Gujarat State Disaster Management Authority (GSDMA), Gandhinagar through World

Bank finances. This document was particularly concerned with the seismic evaluation and

strengthening of existing buildings and it was intended to be used as a guide.

G E Thermou and A S Elnashai [6] made a global assessment of the effect of repair methods on

ductility, strength and stiffness, the three most important seismic response parameters, to

assist researchers and practitioners in decision-making to satisfy their respective intervention

aims. Also the term ‘rehabilitation’ was used as a comprehensive term to include all types of

retrofitting, repair and strengthening that leads to reduced earthquake vulnerability. The term

‘repair’ was defined as reinstatement of the original characteristics of a damaged section or

element and was confined to dealing with the as-built system. The term ‘strengthening’ was

defined as intervention that lead to enhancement of one or more seismic response parameters

(ductility, strength, stiffness, etc.), depending on the desired performance.

A.Kadid and A. Boumrkik [7] proposed use of Pushover Analysis as a viable method to assess

damage vulnerability of a building designed according to Algerian code. Pushover analysis was a

series of incremental static analysis carried out to develop a capacity curve for the building.

Based on the capacity curve, a target displacement which was an estimate of the displacement

that the design earthquake would produce on the building was determined. The extent of

damage experienced by the structure at this target displacement is considered representative

of the damage experienced by the building when subjected to design level ground shaking.

Since the behavior of reinforced concrete structures might be highly inelastic under seismic

loads, the global inelastic performance of RC structures would be dominated by plastic yielding

effects and consequently the accuracy of the pushover analysis would be influenced by the

ability of the analytical models to capture these effects.

R.K. Goel [8] evaluated the nonlinear static procedures specified in the FEMA-356, ASCE/SEI 41-

06, ATC-40, and FEMA-440 documents for seismic analysis and evaluation of building structures

using strong-motion records of RC buildings. The maximum roof displacement predicted from

the nonlinear static procedure was compared with the value derived directly from recorded

motions for this purpose. It was shown that: (i) the nonlinear static procedures either

overestimates or underestimates the peak roof displacement for several of the buildings

considered in the investigation; (ii) the ASCE/SEI 41-06 Coefficient Method (CM), which was

based on recent improvements to the FEMA-356 Coefficient Method suggested in the FEMA-

440 document, does not necessarily provide better estimate of the roof displacement; and (iii)

the improved FEMA-440 Capacity Spectrum Method (CSM) provided better estimates of the

roof displacement compared to the ATC-40 CSM.

Saptadip Sarkar [9] studied the Design of Earthquake resistant multi stories RCC building on a

sloping ground that involves the analysis of simple 2-D frames of different floor heights and

varying number of bays using a software tool named STAAD Pro. Using the analysis results

various graphs were drawn between the maximum compressive stress, maximum bending

moment, maximum shear force, maximum tensile force and maximum axial force being

developed for the frames on plane ground and sloping ground. The graphs were used to draw

comparisons between the two cases and the detailed study of Short Column Effect failure. In

Page14 addition to that, the feasibility of the software tool to be used was also checked and the

detailed study of seismology was undertaken.

Siamak Sattar and Abbie B. Liel [10] quantified the effect of the presence and configuration of

masonry infill walls on seismic collapse risk. Infill panels are modeled by two nonlinear strut

elements, which have compressive strength only. Nonlinear models of the frame-wall system

were subjected to incremental dynamic analysis in order to assess seismic performance. There

was an increase observed in initial strength, stiffness, and energy dissipation of the in filled

frame, when compared to the bare frame, even after the wall’s brittle failure modes. Dynamic

analysis results indicated that fully-in filled frame had the lowest collapse risk and the bare

frames were found to be the most vulnerable to earthquake-induced collapse. The better

collapse performance of fully-unfilled frames was associated with the larger strength and

energy dissipation of the system, associated with the added walls.

Benyamin Monavari, Ali Massumi & Alireza Kazem [12] used nonlinear static analysis and five

locals and overall yields and failure criteria to estimate seismic demands of buildings. The

failure is directed towards losing structure’s performance during the earthquake or subsequent

effects. Because of the consequent excitations of an earthquake or lateral imposed loads on a

structure, the stiffness of some elements of structure reduced and the structure started to fail

and lose its performance; although failure happened either in small parts of structure or at the

whole. In this study thirteen reinforced concrete (RC) frame buildings with 2, 3, 4, 5, 6, 7, 8, 9,

10, 11, 12, 16 and 20 stories, having 3 and 4 bays were designed using seismic force levels

obtained from the Iranian Seismic Code 2005 and proportioned using the ACI318-99 Building

Code and then were modeled by IDARC. Pushover analysis with increasing triangular loading

was used.

Haroon Rasheed Tamboli & Umesh N. Karadi [13] performed seismic analysis using Equivalent

Lateral Force Method for different reinforced concrete (RC) frame building models that

included bare frame, in filled frame and open first story frame. In modeling of the masonry infill

panels the Equivalent diagonal Strut method was used and the software ETABS was used for the

analysis of all the frame models. In filled frames should be preferred in seismic regions than the

open first story frame, because the story drift of first story of open first story frame is very large

than the upper stories, which might probably cause the collapse of structure. The infill Page15

wall increases the strength and stiffness of the structure. The seismic analysis of RC (Bare

frame) structure lead to under estimation of base shear. Therefore other response quantities

such as time period, natural frequency, and story drift were not significant. The

underestimation of base shear might lead to the collapse of structure during earthquake

shaking

2.4 SUMMARY

RC frames with unreinforced masonry infill walls are common in developing countries with

regions of high seismicity. Often, engineers do not consider masonry infill walls in the design

process because the final distribution of these elements may be unknown to them, or because

masonry walls are regarded as non-structural elements. Separation between masonry walls and

frames is often not provided and, as a consequence, walls and frames interact during strong

ground motion. This leads to structural response deviating radically from what is expected in

the design.

In this the Behavior of masonry in filled concrete frames under the lateral load is studied, RCC

buildings are generally analyzed and designed as bare frame. But after the provision of infill

walls, mass of the building increases and this will result in the increase of the stiffness of the

structure. During the seismic activities, response of the structure with infill walls is quite

different for the structure without infill walls. Infill walls changes the dynamic behavior of the

structure.

2.5 NEED FOR THE PRESENT INVESTIGATION

A review of literature reveals that there is no standard procedure to model multistoried-framed

buildings, particularly in the context of Indian construction practices. Even the current Indian

code of practice IS 1893 (Part 1): 2002 does not specifically refer to stilt type buildings. Since

stilt type framed structures are widely adopted in India, there is a need to study the seismic

behavior of such structures.

In such a situation, an investigation has been performed to study the behavior of such

buildings with masonry infill walls subjected to earthquake loads. So, that the stability of the

structure could be achieved at the time of earthquake.

Alternative measures need to be adopted for this specific situation. The under-lying

principle of any solution to this problem is in (a) increasing the stiffness of the first storey such

that the first storey is at least 50% as stiff as the second storey, i.e., soft first storey's are to be

avoided, and (b) providing adequate lateral strength in the first storey. The possible schemes to

achieve the above are (i) provision of stiffer columns in the first storey, and (ii) provision of a

concrete service core in the building. The former is effective only in reducing the lateral drift

demand on the first storey columns and the latter is effective in reducing the drift as well as the

strength demands on the first storey columns.

CHAPTER -3

SEISMIC ANALYSIS PROCEDURES

3.1 INTRODUCTION

Being an engineer we must choose the best seismic analysis method (based on the

complexity of the structure) in order to obtain the best estimate of the seismic displacement

demands.. The analysis procedures can be divided into linear procedures (linear static & linear

dynamic) and non linear procedures (nonlinear static and nonlinear dynamic)

3.2 LINEAR STATIC ANALYSIS

When loads are applied to a body the body deforms and the effects of loads are transmitted

throughout the body The external forces induce internal forces and reactions to render the

body into a state of equilibrium. In linear static procedures the building is modeled as an

equivalent single-degree of freedom (SDOF) system with a linear static stiffness and an

equivalent viscous damping. The seismic input is modeled by an equivalent lateral force with

the objective to produce the same stresses and strains as the earthquake it represents. Based

on an estimate of the first fundamental frequency of the building using empirical relationships

or Rayleigh’s method, the spectral acceleration Sa is determined from the appropriate response

spectrum, which, multiplied, by mass of the building M, results in the equivalent

lateral force V:

The coefficient Ci takes into account issues like order effects, stiffness degradation, but also

force reduction due to anticipated inelastic behavior. The lateral force is then distributed over

the height of the building and the corresponding internal forces and displacements are

determined using linear elastic analysis.

These linear static procedures are used primarily for design purposes and are incorporated in

most codes. Their expenditure is rather small. However, their applicability is restricted to

regular buildings for which the first mode of vibration is prominent.

3.4 NONLINEAR STATIC ANALYSIS

3.4.1 Introduction

Pushover Analysis is a nonlinear static method of analysis. This analysis technique, also

known as sequential yield analysis or simply “Pushover” analysis has gained significant

popularity during past few years. It is one of the three analysis techniques recommended by

FEMA 273/274 and a main component of Capacity Spectrum Analysis method (ATC-40). The

following are the definitions which are most commonly used in Pushover Analysis.

3.5 NONLINEAR DYNAMIC ANALYSIS

In nonlinear dynamic procedure the building model is similar to the one used in non-

linear static procedures incorporating directly the inelastic material response using in general

finite elements. The main difference is that seismic input is modelled using a time history

analysis, which involves time-step-by-time-step evaluation of the building response.

This is the most sophisticated analysis procedure for predicting forces and

displacements under seismic input. However, the calculated response can be very sensitive to

the characteristics of the individual ground motion used as seismic input; therefore several

time-history analyses are required using different ground motion records. This most basic

inelastic method at this time is considered overly complex and impractical for general use.

3.6 ADVANTAGES OF INELASTIC PROCEDURE OVER ELASTIC PROCEDURES.

Although an elastic analysis gives a good understanding of the elastic capacity of

structures and indicates where first yielding will occur, it cannot predict failure mechanisms and

account for redistribution of forces during progressive yielding. Inelastic analyses procedures

help demonstrate how buildings really work by identifying modes of failure and the potential

for progressive collapse. The use of inelastic procedures for design and evaluation is an

attempt to help engineers better understand how structures will behave when subjected to

major earthquakes, where it is assumed that the elastic capacity of the structure will be

exceeded. This resolves some of the uncertainties associated with code and elastic procedures

3.7 SAFETY EVALUATION OF REINFORCED CONCRETE BUILDINGS

3.7.1 Introduction

Safety against collapse of reinforced concrete is usually defined in terms of its ductility

ratios. The design of reinforced concrete structures is performed by using resistance smaller

than the one required for the system to remain elastic under intense ground shaking. Then, the

seismic codes implicitly cause structural damages during strong earthquake motions and the

design relies on the capacity of the structures to undergo large inelastic deformations and to

dissipate energy without collapse. This design methodology is used by all design standards

including IS 1893.

3.7.2 Ductility Ratio

Ductility ratio is the capacity of a member to sustain inelastic deformations without

failure. There are three ductility ratios that are used to evaluate capacity of reinforced

concrete structures. They are as below

1. Displacement ductility ratio

2. Curvature ductility ratio

3. Rotational ductility ratio

3.7.2.1 Displacement Ductility (Global Ductility) Ratio

Displacement ductility ratio of building is defined as the ratio between maximum top

displacements of building to the yield top displacement of that building.

d=

dmaxd y …………………….. (1)

in which dmax and dy are maximum displacements and yield displacement respectively.

The above ductility ratio is called global ductility demand ratio of building. Similarly ductility

supply ratio of building is defined as ratio between ultimate top displacements of building to

the yield top displacement

s=

dud y …………………….. (2)

where du is the top displacement at collapse of the building.

3.7.3 Response Reduction Factor (R)

The response reduction factors takes into account the ductility of the structural

system and over strength so that the structure can be designed to the level of yield force of the

structure and rely on non –linear response of the structure in the case of severe earth quake. It

is, therefore, obvious that structure having low over strength or low ductility should be

designed for higher seismic coefficients. This means designed for higher design seismic

coefficients than that for the buildings.

Extensive research has been devoted in the past couple of decades towards the

developments of inelastic design spectra based on displacement rather than force criteria.

Such research was primarily motivated by the mount of performance-based seismic design,

which launched the development of new analysis and design tools based on displacement

rather than forces.

This family of methods aims at the calculation of the reduction factor R commensurate

with the achievement of a target ultimate displacement du. The target displacement often

referred to in an indirect manner through non-dimensional parameters, which are either the

ductility or displacement coefficient.

In this context, inelastic design response spectra can be expressed in many formats, the

most widely recognized of which are:

The conventional constant ductility plot, which depicts the reduced spectral

acceleration Sa/R values as a function of period for different ductility levels.

The capacity spectrum format, in which the reduced spectral acceleration values are

drawn as a function of peak displacement (elastic or ultimate) for different ductility

levels.

The yield point spectra format, which is plot of the strength coefficient

C=mSa/R/W=Sa/g/R as a function of the yield displacement dy.

The earliest R- -T relationship was developed by Newmark and Hall (1982) based on

observations conducted on the seismic response of a number of undamped SDOF systems,

which lead the following relations.

For long-period systems, the obtained ultimate displacement was not much greater

than the elastic spectral displacement therefore: R=

For short-period systems, the reduction the force led the system to exhibit large

ductility demands. Therefore, it was considered safer to keep the response in the elastic

range, i.e. R=1

For intermediate-period system, an equal energy rule proved to be useful. That is, the

energy absorbed by the inelastic system should be equal to that absorbed by elastic

one: R=√2μ−1

3.7.4 SEISMIC VULNERABILITY

The vulnerability of a building subjected to an earthquake is dependent on seismic

deficiency of that building relative to a required performance objective. The seismic deficiency

is defined as a condition that will prevent a building from meeting the required performance

objective. Thus, a building evaluated to provide full occupancy immediately after an event may

have significantly more deficiencies than the same building evaluated to prevent collapse.

Depending on the vulnerability assessment, a building can be condemned and

demolished, rehabilitated to increase its capacity, or modified so that the seismic demand on

the building can be reduced. Thus, structural rehabilitation of a building can be accomplished in

a variety of ways, each with specific merits and limitations related to improving seismic

deficiencies.

3.7.5 HOW DO BUILDINGS RESIST EARTHQUAKE FORCES

As a building responds to ground motions produced by an earthquake, the bottom of

the structure moves immediately, but the upper portions do not because of their mass and

inertia. Figure-3.4 shows the base of a building moving while the upper part lags behind.

The horizontal force, or base shear, created by ground motion resulting from an

earthquake must be resisted by the building. The more the ground moves, or the greater the

weight of the building, the more force must be resisted by the building. When an architect or

engineer designs a building, he or she must determine the maximum force a building might

have to resist in the future. Buildings are always designed to handle normal vertical and lateral

forces. However, once you introduce the possibility of an earthquake, a building must be

designed for extraordinary horizontal or lateral forces. The horizontal (lateral) forces associated

with an earthquake can be thought of as a lateral force applied to each floor and to the roof of

a building. Figure 3.5 shows the vertical and horizontal forces on a building during an

earthquake. Panel (a) shows the direction of gravitational forces on a building, panel (b) shows

the horizontal force of seismic waves, and panel (c) shows the combined forces of gravity and

an earthquake applied to the floors and roof of a building.

Fig -3.4 behaviour of building in ground acceleration

Horizontal forces accumulate along the floors and roof and then are distributed through

the vertical supports into the foundation. A structural engineer must design a building so that

lateral forces are distributed throughout the building without a break. Several structural

systems, such as floors, walls, and columns, may be used in new buildings to reduce the effects

of earthquakes and associated natural disasters.

Fig –3.5 forces acting on the building during ground excitation

3.7.6 STIFFNESS:

A building is made up of both rigid and flexible elements. For example, beams and

columns may be more flexible than stiff concrete walls or panels. Less rigid building elements

have a greater capacity to absorb several cycles of ground motion before failure, in contrast to

stiff elements, which may fail abruptly and shatter suddenly during an earthquake. Earthquake

forces automatically focus on the stiffer, rigid elements of a building. For this reason, buildings

must be constructed of parts that have the same level of flexibility, so that one element does

not bend too much and transfer the energy of the earthquake to less ductile When the

earthquake struck, the longer, more flexible columns at the front of the building passed the

earthquake forces on to the short, stiffer columns in the back instead of distributing the forces

equally among all of the columns. Deflection the extent to which a structural element moves or

bends under pressure, played a major role. The longer columns simply deflected or bent

without cracking. The short columns, therefore, were overwhelmed and cracked.

Fig3.6 showing long and short columns

3.7.7 EFFECT OF INFILL

The presence of the infill walls increases the lateral stiffness considerably. Due to the

change in stiffness and mass of the structural system, the dynamic characteristics change as

well. infill walls have an important effect on the resistance and stiffness of buildings. However,

the effect of the infill walls on the building response under seismic loading is very complex and

math intensive.

Exterior masonry walls and/or interior partitions built as an infill between a reinforced

concrete frame’s beams and columns are usually considered to be non-structural elements in

design. The interaction between the frame and infill is often ignored. However, the actual

behavior of such structures observed during past earthquakes shows that their response is

often wrongly predicted during the design stage .Infill-frames have been used in many parts of

the world over a long time. In these structures, exterior masonry walls and/or interior

partitions, usually regarded as non-structural architectural elements, are built as an infill

between the frame members. However, the usual practice in the structural design of infill-

frames is to ignore the structural interaction between the frame and infill. This implies that the

infill has no influence on the structural behavior of the building except for its mass. This would

be appropriate if the frame and infill panel were separated by providing a sufficient gap

between them. However, gaps are not usually specified and the actual behavior of infill frames

observed during past earthquakes shows that their response is sometimes wrongly predicted.

Infill-frames have often demonstrated good earthquake-resistant behavior, at least for

serviceability level earthquakes in which the masonry infill can provide enhanced stiffness and

strength. It is expected that this structural system will continue to be used in many countries

because the masonry infill panels are often cost-effective and suitable for temperature and

sound insulation purposes. Hence, further investigation of the actual behavior of these frames

is warranted, with a goal towards developing a displacement-based approach to their design.

Behavior of masonry in filled concrete frames under the lateral load is studied.

Investigations showed that, one of the most appropriate ways of analyzing the masonry in filled

concrete frames is to use the diagonally braced frame analogy. RCC buildings are generally

analyzed and designed as bare frame. But after the provision of infill walls, mass of the building

increases and this will result in the increase of the stiffness of the structure. During the seismic

activities, response of the structure with infill walls is quite different for the structure without

infill walls. Infill walls changes the dynamic behavior of the structure

3.7.8 SOFT STOREY

Recent trend of urbanization of cities of the developing countries, especially in South

Asia region, is witnessing construction of multi-storeyed buildings with open ground floor

reserved for car parking or other utility services. Though multi-storeyed buildings with open

(soft) ground floor are inherently vulnerable to collapse due to earthquake load, their

construction is still widespread in the developing nations. Social and functional need to provide

car parking space at ground level far out-weighs the warning against such buildings from

engineering community. These buildings are generally designed as RC framed structures

without regards to the structural action of the masonry infill (MI) walls present in the upper

floors. However, in reality, masonry infill (MI) walls in the upper floors make those floors much

stiffer against lateral load (e.g. earthquake) compared to ground floor rendering these buildings

into soft story buildings. Experience of different nations with the poor and devastating

performance of such buildings during earthquakes always seriously discouraged construction of

such a building with a soft ground floor. Typical examples of soft story (ground floor) failures

are shown in Fig-4.7. However, construction of such a building with isolated MI wall requires

high construction skill and may not be appropriate for the developing nations.

Some national codes like the Indian seismic code [9] requires members of the soft story (story

stiffness less than 70% of that in the story above or less than 80% of the average lateral stiffness

of the (three stories above) to be designed for 2.5 times the seismic story shears and moments,

obtained without considering the effects of MI in any story.

Fig-3.7: Soft story failure

Diaphragms:

The floor and roof systems that distribute an earthquake’s lateral forces are referred to

as diaphragms. Diaphragms support the gravitational and lateral forces on a building and

transfer them to vertical structural elements like shear walls, braced frames, and moment-

resistant frames. These vertical elements help resist lateral forces and are therefore called

horizontal (or lateral) bracing systems

3.7.10 STOREY DRIFTS:

Drift is the extent to which a building bends or sways. Limits are often imposed on drift

so a building is not designed to be so flexible that the resulting drift or swaying during an

earthquake causes excessive damage. Figure shows how a building can be affected by drift in an

earthquake. If the level of drift is too high, a building may pound into the one next to it. Or the

building may be structurally safe but non-structural components, such as ceilings and walls,

could be damaged as the building bends and the ceilings and walls are ripped away from their

attachments. Of course, people in the building could be killed or injured from falling debris.

Fig3.8 show the effect of high story drift

3.7.11 EFFECT OF SHEAR WALL

Reinforced concrete walls are strength and portent elements frequently used in

constructions in seismic areas because they have a high lateral stiffness and resistance to

external horizontal loads, these shear walls may be added solely to resist horizontal forces or

concrete walls enclosing stairways elevated shafts and utility cores may serve as shear walls.

shear walls not only have a very large enplane stiffness and therefore resist lateral load and

control deflection very efficiently but they also helps in reductions of structural & non-

structural damage. The building incorporated with shear wall sufficiently ductile will be much

away from seismic vulnerability and building failure in the earthquake sensitive zones thus

resulting in increased life safety & low property loss.

3.7.10 BEHAVIOUR OF SHEARWALL:

Shear wall constructed in the high rise buildings, generally behave as vertical cantilever

beam with their Strength controlled by flexure as shown in fig(1) rather than by shear such

walls are subjected to bending moments and Shears originating from lateral loads ,and to axial

compression caused by gravity these may therefore be designed in same manner as regular

flexural element .when acting as a vertical cantilever beam the behavior of a shear wall which is

properly reinforced for shear, will be governed by the yielding of the tension reinforcement

located at the vertical edge of the wall and, to some degree, by the vertical reinforcement

distributed along the central portion of wall. It is thus evident that the shear is critical for the

Wall with relatively low height-to-length ratio, and tall shear walls are controlled mainly by

flexural Requirements. Since the ductility of flexural member such as tall shear wall can be

significantly affected by the maximum usable strain in compression zone of concrete.

Confinement of concrete that the ends of Shear wall section would improve the performance of

such shear wall. Tall shear walls in multi-storey buildings the shear walls are slender enough

and are idealized as cantilever fixed at base Their seismic response is dominated by flexure.

Because of load reversals, shear walls sections necessarily contains substantial quantity of

compression reinforcement. The fig below shows the diagonal tension cracks in tall shear wall

and the formation of plastic hinges in the axial compression :

Fig-3.9 Behaviour of shear wall under flexure & formation of plastic hinges

Shear walls are the main vertical structural elements with a dual role of resisting both

the gravity and lateral loads. Wall thickness varies from 150 mm to 500 mm, depending on the

number of stories, building age, and thermal insulation requirements. In general, these walls

are continuous throughout the building height a shear wall may be tall shear wall or low shear

wall also known as squat walls characterized by relatively small height-to-length ratio.

Chapter - 4

ANALYTICAL MODELLING AND NUMERICAL STUDIES