Performance Point Evaluation of Masonry Infill Rc Soft Storey Buildings Under Seismic Load Using...

PERFORMANCE POINT EVALUATION OF MASONRY INFILL

RC SOFT STOREY BUILDINGS UNDER SEISMIC LOAD USING

PUSHOVER ANALYSIS

Submitted by

Md. Faysal Student No: 0604038

Submitted to the

DEPARTMENT OF CIVIL ENGINEERING

BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY

In partial fulfillment of requirements for the degree of

BACHELOR OF SCIENCE IN CIVIL ENGINEERING

MARCH, 2012

ii

ABSTRACT

Due to rapid urbanization to provide sufficient car parking space, open ground story

has become a typical feature in the modern multi-storied building. This causes vertical

stiffness irregularity and develops soft story mechanism. Soft story mechanism is a

potential mode of failure of reinforced concrete structures as experienced during past

earthquake. Although some of the national codes do have certain design guide to

avoid soft story problem, the Bangladesh National Building Code (BNBC, 1993) does

not have any such provision.

This thesis has been conducted to investigate the performance of soft story structures.

For this purpose, a total of five structures range from six to twelve stories have been

considered to evaluate the performance of soft story building under seismic loading.

The performances of the structures have been evaluated using the nonlinear static

pushover analysis as per the procedure of ATC-40, 1996.

From nonlinear inelastic performance based analysis the actual behavior of the

structure during earthquake can be found. There are mainly two guidelines of this

analysis FEMA-356 and ATC-40, 1996. These analysis procedures have been

discussed in this thesis.

It has been observed that performances of these structures under seismic loading are

highly unsatisfactory. The value of base shear obtained by using Pushover analysis is

higher than any other methods like equivalent static force method or response

spectrum method. The value of displacement and inter-story drift are very high in soft

ground story. Stiffness of open ground story is significantly less than the above. The

capacity curve of soft story structure never meets seismic demand. It is seen that the

value of base shear increase with the increase in height. Investigation of buildings

with soft story shows that soft story mechanism reduces the performance of the

structure and makes them vulnerable type of construction in earthquake prone areas.

So it is vital for the engineers to provide adequate safety measures to achieve

acceptable performance in open ground story structure under seismic action. It is high

time to include provisions for soft story structure in Bangladesh National Building

Code.

iii

DECLARATION

It is hereby declared that this thesis work carried out by author and any part of it or

the thesis has not been submitted elsewhere for any other purpose.

March, 2012.

MD. FAYSAL

iv

ACKNOWLEDGEMENT

Foremost, the author would like to express his sincere gratitude to his supervisor Dr.

Khan Mahmud Amanat, Professor of Department of Civil Engineering, BUET for his

encouragement, indispensible guidance and support from the initial to final stage of

this thesis.

The author pays his deepest homage to his parents, sister and friends for their constant

inspiration.

Above all the author thanks the Almighty for all his blessings.

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TABLE OF CONTENETS

Page No.

ABSTRACT ii

ACKNOWLEDGEMENT iii

LIST OF TABLES viii- ix

LIST OF FIGURES x-xiii

CHAPTER-1 INTRODUCTION

1.1 GENERAL 1

1.2 OBJECTIVE AND SCOPE OF THE STUDY 2

1.3 METHODOLOGY 3

1.4 ORGANIZATION OF THE THESIS 4

CHAPTER-2 LITERATURE REVIEW

2.1 INTRODUCTION 5

2.2 THE SOFT GROUND STORY CONFIGURATION

2.2.1 Definition of Soft Story 6

2.2.2 Behavior of RC Frame under Lateral Load 7

2.3 BEHAVIOR OF SOFT GROUND STORY STRUCTURE 10

2.4 BUILDING DESIGN STRATEGIES IN BUILDING CODE 13

2.4.1 Indian Seismic Code, Is-1893 (2002) 14

2.5 COMPUTATIONAL MODELING AND ANALYSIS OF INFILL

FRAME 15

2.5.1 Equivalent Diagonal Strut Method 15

2.6 PAST RESEARCH ON SOFT STORY BUILDING 17

2.7 REMARKS 19

CHAPTER-3 CONCEPT OF PERFORMANCE BASED DESIGN

3.1 INTRODUCTION 20

vi

3.2 MODELING OF INFILL WALLS 20

3.2.1 Equivalent Strut Method 21

3.2.2 Equivalent Strut Width 22

3.2.1.2 Eccentricity of Equivalent Strut 23

3.2.1.3 Existing infill damage 23

3.2.3 Infill Properties 24

3.2.4 Determination of fm and Em 24

3.2.5 Calculation of Equivalent Strut Width 27

3.3 NON-LINEAR STATIC PROCEDURE

3.3.1 Reduced Demand Spectra 27

3.3.2 Development of Elastic Site Response Spectra 28

3.3.3 Seismic Zone 29

3.3.4 Seismic Source Type 30

3.3.5 Near Source Factor 30

3.3.6 Seismic Coefficients 31

3.3.7 Establishing Demand Spectra 32

3.4 CAPACITY CURVE 36

3.4.1 Capacity Spectrum Method 38

3.5 PERFORMANCE POINT 39

3.5.1 Non linear Static Procedure for Capacity evaluation

Of Structures 40

3.5.2 Structural Performance Levels and Ranges 41

3.5.3 Immediate Occupancy Structural Performance Level (S-1) 42

3.5.4 Damage Control Structural Performance Range (S-2) 43

3.5.5 Life Safety Structural Performance Level (S-3) 43

3.5.6 Limited Safety Structural Performance Range (S-4) 43

vii

3.5.7 Collapse Prevention Structural Performance Level (S-5) 43

3.6 TARGET BUILDING PERFORMANCE LEVELS 44

3.7 RESPONSE LIMITS 44

3.8 ACCEPTABILITY LIMIT 44

3.9 ELEMENT HINGE PROPERTY 46

3.9.1 Concrete Axial Hinge 47

3.9.2 Concrete Moment Hinge and Concrete P-M-M Hinge 48

3.9.3 Concrete Shear Hinge 49

3.10 CONCRETE FRAME ACCEPTABILITY LIMITS 50

3.10.1 Hinge Properties for Modeling 51

3.11 PERFORMANCE CRITERIA FOR THE STRUCTURE 51

3.11.1 Drift 51

3.11.2 Story Displacement 51

CHAPTER-4 PERFOMANCE EVALUATION OF SOFT GROUND STORY

STRUCTURE

4.1 INTRODUCTION 52

4.2 LOADING CONDITION 52

4.3 MATERIAL PROPERTIES 52

4.4 MODEL GEOMETRY 53

4.5 ASSUMPTIONS FOR PUSH OVER ANALYSIS 53

4.6 SOFTWARE USED IN THE ANALYSIS 54

4.7 DESCRIPTION OF THE STRUCTURES CONSIDERED FOR ANALYSIS 55 4.8 PERFORMANCE EVALUATION OF THE STRUCTURES 56

4.8.1 Performance Evaluation of the Structure 1 56 4.8.2 Performance Evaluation of the Structure 2 63

4.8.3 Performance Evaluation of the Structure 3 69

4.8.4 Performance Evaluation of the Structure 4 74

viii

4.8.5 Performance Evaluation of the Structure 5 79 4.9 REMARKS 82

CHAPTER-5 CONCLUSION AND RECOMMENDATIONS

5.1 GENERAL 83

5.2 FINDINGS OF THE STUDY 83

5.3 RECOMMENDATIONS FOR FUTURE STUDY 84

REFERENCES 85-86

APPENDIX 87-93

LIST OF TABLES

Table 3.1: In-plane damage reduction factor 24

ix

Table 3.2: Specified Compressive Strength of Masonry f’m (psi), based on

specifying the compressive strength of masonry units. (ACI 530, 192/

ASCE 6-92/ TMS 602-92) 25

Table 3.4: Sample Calculation of equivalent strut width 27

Table 3.5: Minimum Allowable SRA and SRv values 29

Table 3.6: Seismic Zone Factor, Z 30

Table 3.7: Seismic source type as per ATC- 40, 1996 30

Table 3.8: Seismic Source Factor 31

Table 3.9: Seismic coefficient, CA 31

Table 3.10: Seismic coefficient CV (ATC-40, 1996) 32

Table 3.11: Response quantities for 5% elastic demand 35

Table 3.12: Allowable maximum story drift ratio (ATC-40) 51

Table 4.1: Description of the Structure Considered for Analysis 55

Table 4.2: Effective damping and spectral reduction factor for structure 1 56



Table 4.5: Base Shear Comparison among different methods for structure 1 59

Table 4.6: Base shear at performance point and number and number of hinges

developed up to performance point 60




Table 4.10: Base Shear Comparison among different methods 65

Table 4.11: Base shear at performance point and number and number of

hinges developed up to performance point 66

x


Table 4.13 Effective damping and spectral reduction factor for structure 3 69

Table 4.14 Effective damping and spectral reduction factor for structure 3 70



developed up to performance point 72





Table 4.21: Base shear at performance point and number and number

of hinges developed up to performance point 77

Table 4.22: Effective damping and spectral reduction factor for structure 5 both in

long (X) and short (Y) direction 80

Table 4.23: Base shear at performance point and number and number of hinges 80

LIST OF FIGURES

Figure: 2.1 Building with soft story 5

Figure: 2.2 Soft story failure mechanisms 6

xi

Figure: 2.3 Change in lateral load transfer mechanism due to masonry in-fills 8

(Murty and Jain 2000)

Figure: 2.4.1 Interactive behavior of Frame and Infill 8

Figure: 2.4.2 Analogous Bare Frame 8

Figure: 2.5 Modes of infill failure 9

Figure: 2.6 Modes of frame failure 10

Figure: 2.8 Lateral behavior of soft story structure 11

Fig. 2.9 Effects of masonry in-fills on the first mode shape of a typical RC frame

building (a) Displacement Profile; (b) Fully in-fill frame; (c) Open ground story

frame; (Adopted from Kaushik et. Al, 2006) 12

Figure: 2.10 Effects of earthquake on buildings having soft story 13

Figure: 2.11 Material modeling of masonry infill as diagonal strut 16

Figure: 2.12.1 Masonry in-filled frame Sub-assembly in masonry infill panel

frame 16

Figure: 2.12.2 Masonry infill panel in frame structure 16

Figure 3.1: Specimen Deformation Shape 21

Figure 3.2: Strut Geometry 22

Figure 3.3: 5% Damped Elastic Response Spectrum 34

Figure 3.4: 5% Damped elastic response spectrum in ADRS format 35

Figure 3.5: Typical Capacity Curve 36

Figure: 3.6 Capacity Curve 38

Figure 3.7: Determination of performance point 40

Figure 3.8: Force deformation action and acceptance criteria 45

Figure 3.9: Concrete axial hinge properties (FEMA-356, 2000) 48

Figure 3.10: Concrete moment and P-M-M hinge property 49

xii

Figure 3.11: Concrete Shear Hinge Property 50

Figure 3.12: Generalized Load-Deformation Relationship for Components 50

Figure 4.1: Plan View Type- A & B 55

Figure 4.2: Comparison of capacity spectrum of structure 1 for different

infill condition 57

Figure 4.3: Comparison of capacity spectrum of structure 1 for different infill

condition 57


infill condition 58

Figure 4.5: Comparison of story shear observed from different method 59

Figure 4.6: Comparison of displacement (mm) in X direction 60

Figure 4.7.1: Comparison of maximum total drift ratio in X direction of structure 1 60



Figure 4.8: Deformation pattern of structure 1 at performance point 62


condition 63


infill condition 64



Figure 4.14.1: Comparison of maximum total drift ratio in X direction of

structure 2 67


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structure 2 67


structure 2 68


infill condition 69


infill condition 69


infill condition 70




structure 3 72


structure 3 72


structure 3 73

Figure 4.21: Comparison of capacity spectrum of structure 4 for

different infill condition 74





xiv




structure 4 78


structure 4 78


structure 4 79

Figure 4.26: Comparison of capacity spectrum of structure 5 for 60% infill

condition in both long and short direction 80

Figure 4.27: Comparison of displacement (mm) in both long and short direction 80

Figure 4.28: Comparison of maximum total drift ratio in both long and short

direction of structure 5 81

1

CHAPTER 1

INTRODUCTION

1.1 GENERAL

Earthquake Engineering has come a long way since its birth and still a rapid growing

subject. Numerous researches are going on this topic especially concerned with the

design of the building structures. A structure designed to higher standards to sustain

under relative frequent earthquake and being safe under destructive earthquake may

prove to be costly but preferred by owners of the buildings now-a-days.

Rapid pace of urbanization especially in big cities like Dhaka, Chittagong etc. creates

a great demand on human shelter. As a result, a lot of multistoried buildings are found

to have open ground story (soft story) in the ground floor to provide place for car

parking. It is common to have brick infill wall panels on the upper stories. This

irregularity of vertical stiffness leads to development of soft story mechanism. In

order to satisfy architectural demand, sometimes soft stories are seen in other stories.

Presence of soft story in a building makes it vulnerable especially in the earthquake

prone areas like Dhaka city situated in moderate earthquake zone. The performances

of open ground story buildings have consistently performed poorly during

earthquakes across the world. During earthquakes of Turkey 1999, Bhuj (India) 2001

etc. had collapsed due to the presence of soft story. Bangladesh has faced a number of

minor earthquakes recent years and there is a strong possibility of major earthquake.

Most of the buildings constructed without seismic resistance a moderate earthquake

may prove to be fatal for the population.

Proper seismic design can minimize the risk of fatal casualties. Seismic design has

been made mandatory in BNBC (2006). BNBC specified methods like equivalent

static force method and dynamic response spectrum method. But the seismic force

distribution depends on the distribution of story stiffness rather than its natural period.

Because of more flexibility at open ground story results in extreme deflection of story,

which in turn, leads to concentration of forces at the connections of story above

accompanied by large plastic deformations. In this process plastic hinges are formed

2

at the ends of columns of the soft story which results in an unavoidable collapse of the

total story. Therefore, the building with soft story requires special design

consideration. To understand the effect of earthquake on building structures properly

especially consist of soft story performance based analysis or Pushover analysis is

now considered most effective. In order to meet specific performance objectives

under the action of the frequent or rarer seismic events that it may experience in its

lifetime. A soft story is considered deficient to keep vertical stability under seismic

loading and to fulfill the target performance objectives. Presence of masonry infill on

upper floors making them stiffer causes rigid body movement under seismic vibration.

By implementing the diagonal strut as a replacement of brick panels the actual effect

on the building is analyzed using Pushover Method.

1.2 OBJECTIVE AND SCOPE OF THE STUDY The objectives of the present study are summarized as follows:

To develop a 3D finite element model of building with infill on upper floors

and soft ground floor.

Modeling of infill as diagonal struts and applied in different percentage

randomly on frame system.

Comparison of base shear values by the three methods for certain structural

variables.

To analyze the variation in base shear for same percentage of randomly

applied infill in different methods.

To establish the pushover curve (capacity curve) of simple structures having

soft ground story and bare frame.

To evaluate adequacy and performance of conventionally designed typical

bare frame model and soft story structure under maximum expected

earthquake

To demonstrate the effect of the soft ground story through lateral displacement

on the profile of building and base shear force.

To compare among the alternative measures to determine which measure is

suitable for the particular structure to the service requirements of the ground

story.

To investigate the failure pattern of buildings at performance point.

3

Pushover curve will present the lateral displacement as a function of the force

applied to the structure.

The capacity spectrum will be plotted that reduces the elastic spectrum to

intersect the capacity curve in spectral co-ordinates to find the performance

point.

The inter-story drift will be plotted to analyze the changes in the slope of

consecutive story at performance point.

The story displacement curve will be plotted to investigate the maximum

displacement.

1.3 METHODOLOGY Reinforced concrete moment resisting frame with open ground story and unreinforced

brick infill walls in the upper stories will be modeled using ETABS 9.7 software

package for this study. Nonlinear static pushover analysis will be performed as per as

described in ATC-40.

The investigation is based on the following assumptions:

The infill wall will be modeled as shell element as per available literature

(Stafford and Smith).

Failure of infill wall will be ignored in the study.

All dead load, live load and earthquake will be applied as described in BNBC

1993. Wind load will be applied as described in UBC 94 as it is slightly varies

from the BNBC (1993) code.

Dead load of partition wall will be applied on the beam and diagonal strut will

be considered only as active member having zero weight.

For nonlinear analysis hinge property is chosen from the provided

experimental data.

Hinges are assigned to each element to allow progressive damage of the

element to reach plastic deformation stage.

Structural behavior type is assumed to be C considering them as existing

structure.

Earthquake load will be applied incrementally in order to monitor the

formation of plastic hinges, stiffness degradation and plastic rotation.

4

1.4 ORGANIZATION OF THE THESIS The research work conducted for the achievement of the stated objective is presented

in several chapters organized in a way so that the steps involved in the study may

properly delineate the methodology. This document is organized in five chapters. A

brief description of the contents of each chapter follows:

Chapter 1 gives a statement of the basic idea of the objectives, methodology, scope

and a brief description on the organization of the thesis work being done under the

research.

Chapter 2 provides available earthquake load analysis methods, their guideline in

different codes.

Chapter 3 describes seismic analysis and design methodologies, provision for

earthquake load in building codes, various parameters of nonlinear push over analysis,

performance evaluation procedure etc. are reviewed.

Chapter 4 represents performances evaluation of different structures with variable

infill condition. Compare the value of base shear using different analysis method.

Chapter 5 depicts findings of the study, limitation of present study and

recommendations for future study.

.

5

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

Bangladesh is a developing country with a huge population. It is now facing a rapid

growth of urbanization. To improve the economic condition, this country also needs

infrastructure support. As a result a lot of buildings are built as residences or for other

purposes. The selection of the appropriate configuration of the structural system is the

key to build a safe, comfortable structure. Bangladesh is situated in active earthquake

zone. So, the threat of earthquake is inevitable although a little importance is given in

seismic design. Only few of the recent design codes underline the problem. Hence, the

problem has not been taken into sufficient consideration in the current professional

practice. To provide space for car parking most of the structures now miss the infill at

ground floor. These types of structures are known as soft story structure. These

structures are more susceptible to earthquake having irregularities in stiffness. These

irregularities occur due to random distribution of infill on the upper floors with open

ground floor. Many soft story buildings have performed poorly in the past earthquake

Figure: 2.1 Building with soft story

6

A number of cases of soft story failure have also been reported in Algeria earthquake

in 1980, San Salvador earthquake in 1986, North Ridge earthquake in 1994, Bhuj

earthquake in 2001, Bingol earthquake in 2003.This type of failure results from the

combination of several other unfavorable reasons, such as torsions, excessive mass on

upper floors, P-Δ effects and lack of ductility in the bottom story. So the soft story

structures deserve a special consideration in analysis and design. Soft story

phenomenon occurs due to inadequate bracing of structure at ground floor which is

less resistant to lateral forces of earthquake. The specific floor suffers the drift more

Fig: 2.2 Soft story failure mechanisms.

than the upper stories. Being less resistant it becomes the weakest point of the

structure and cause of the collapse of whole structure during even moderate

earthquake.

In this chapter the literature regarding structural configuration, treatment of soft story

in building codes, behavior of soft story buildings, different methods used for

evaluating the performance level of existed building and design criteria are reviewed.

2.2 THE SOFT GROUND STORY CONFIGURATION

This configuration allows a lot of space for car parking (fig 2.2). The soft story

structures are very vulnerable from a seismic point of view. As the large rotation

ductility requests concentrated alt the extreme sections of the columns of the ground

story, while the above stories behave like a quasi-rigid body. The definition of soft

story is provided by various codes. In the following section, the behavior of soft story

will be discussed.

7

2.2.1 DEFINITION OF SOFT STORY

The definition of soft story varies in different building codes. The definitions of soft

story as given by various codes are discussed below.

Uniform Building Code, 1991

A soft story is one in which the lateral stiffness is less than 70 percent of that in the

story above or less than 80 percent of the average stiffness of the three story above.

ASCE 7- Minimum Design Loads for Building and Other Structures, 2002

Soft story:

Soft story is one in which lateral stiffness is less than 70 percent of that in the story

above or less than 80 percent of the average stiffness of the three stories above.

Extreme Soft Story:

An extreme soft story is one in which the lateral stiffness is less than 60 percent of

that in the story above or less than 70 percent of the average stiffness of the three

stories above.

Bangladesh national building code (BNBC), 1993

Soft story is one in which the lateral stiffness is less than 70 percent of that in the

story above or less than 80 percent of the average stiffness of the three story above.

2.2.2 BEHAVIOR OF RC FRAME UNDER LATERAL LOAD

Basically infill is provided as partition wall, sound barrier, heat reduction etc.

According to research works of Mehrabi et al. 1996 that careful use of infill can

significantly increase the stiffness of the structure. It helps in resisting the earthquake

motion. So it is necessary to provide infill carefully to minimize the torsion and soft

story effect. But due to lack of knowledge of participation of infill during earthquake

resistance its effect remains neglected. The presence of in-filled walls on the upper

stories of a multi storied buildings make them mush stiffer than the soft ground story.

During earthquake shaking the whole upper part moves together as one system and

soft ground story takes most of the displacement.

8

Experiments like (Moghaddam and Dowling in 1987, Smith and Coul in 1991, Murty

and Jain in 2000) are done to initiate an approach to use infill to resist the lateral load

of earthquake. The present of infill in RC frames change the load transfer mechanism

from predominant frame action to predominant truss action (Fig. 2.3).

Fig. 2.3 Change in lateral load transfer mechanism due to masonry in-fills (Murty and Jain 2000)

Containing the brittle masonry ductile frame stiffens significantly because of in-plane

rigidity of the masonry wall. With its in-plane shear resistance and partly by its

behavior as diagonal bracing strut it braces the frame (Fig.2.4.1).

(1) (2)

Fig. 2.4 (1) Interactive behavior of Frame (2) Analogous braced frame.

and infill.

9

Double curvature bending of the columns and girders occur under horizontal loading.

The translation of the upper part of the column in each story and the shortening of the

leading diagonal of the frame cause the column to lean against the wall as well as to

compress the wall along its diagonal. It is roughly analogous to diagonal bracing

frame (Fig. 2.4.2)

The types of potential modes of failure of wall due to its interaction with the frame

are:

1. Shear failure stepping down through the joints of the masonry.

2. Shear stress concentration in the bed joints.

3. Diagonal cracking of the wall through the masonry along a line, lines or

parallel to the leading diagonal.

4. Tensile stress perpendicular to the leading diagonal.

5. Due to corner compressive stress infill at the end of the diagonal crushed.

The perpendicular tensile stresses are caused by the divergence of the compressive

stress trajectories on the opposite sides of the leading diagonal as they approach the

mid region of the infill. The shear failure of wall steps down through the joints of

masonry and participated by the horizontal shear stresses in the bed joints. The

diagonal cracking of the wall is through the masonry along a line or line parallel to the

Fig. 2.5 Modes of infill failure

10

loading diagonal and caused by tensile stresses perpendicular to the loading diagonal.

The perpendicular tensile stresses caused by the divergence of the compressive stress

trajectories on opposite sides of the loading diagonal as they approach the middle

region of the infill. The diagonal cracking is initiated and spreads from the middle of

the infill while the tensile stresses are at maximum tending to stop near the

compression corners, where the tension is suppressed.

Fig. 2.6 Modes of frame failure

The nature of the forces in the frame can be understood by referring to the analogous

braced frame shown in (Fig. 2.5). The windward column or the column facing the

seismic load first is in tension and the leeward column or the other side of the

building facing seismic load last is in compression. Since the infill bears on the frame

not as exactly a concentrated force at the corners, but over the short lengths of the

beam and column adjacent to each compression corner, the frame members are

subjected also to transverse shear and a small amount of bending. Consequently the

frame members or their connections are liable to fail by axial force or shear and

especially by tension at the base of the windward column (Fig. 2.6).

2.3 BEHAVIOR OF SOFT GROUND STORY STRUCTURE Soft story mechanism yields a poor frame system as there is a sudden drop in stiffness

and strength in ground floor (2.7). Bare frame is only considered during design

practice. Soft story structure shows large displacement in open story than fully infill

11

frame structure (Fig. 2.8) as most of the stress concentrated on the ground floor (Fig.

2.9).

Fig. 2.7 (1) actual building (2) Considered in design during practice.

Fig. 2.8 Lateral behavior of soft story structure

The building having soft ground story oscillates like simple pendulum during

earthquake and column of the open ground story are severely stressed (Fig. 2.9). It is

required that the column must have sufficient stiffness and also adequate ductility.

Otherwise, it will be damaged and may cause subsequent collapse of the whole

structure.

Ductility

Ductility demands refers to the extent of deformation (rotation or displacement)

beyond the elastic limit, expressed numerically as the ratio of the maximum

deformation to yield deformation at initial yield (ATC-40).

12

Thus,

µu = µm / µy Eqn. - 2.1

Here, µm = Maximum deformation prior to failure and µy = Deformation at yield

Fig. 2.9 Effects of masonry in-fills on the first mode shape of a typical RC frame

building (a) Displacement Profile; (b) Fully in-fill frame; (c) Open ground story

frame; (Adopted from Kaushik et. Al, 2006)

Ductility capacity of a structure is the ability of the structure to deform beyond the

elastic limit. Ductility demand refers to the requirements on the design of the system

should be such that exceed the ductility demand. Allowable ductility is based on the

allowable deformation that depends on the structural characteristics and materials

used. Ductility demand varies for a multi storied building varies over its height and

differs from the allowable ductility used in defining the design spectrum and

computing the story yield strength.

Effects of earthquake in soft story buildings are as follows –

13

Fig. 2.10 Effects of earthquake on buildings having soft story

The seismic force distribution and energy dissipation is dependent on the distribution

of the stiffness and mass of the structure along its height. The upper stories being very

stiff undergoes less inter-floor lateral drift while the soft ground floor being less stiff

undergoes very high lateral drift and the soft story columns dissipate most of the

seismic energy in the process of plastic hinges. Thus the possibility and risk of

collapse is very high in case of soft story under lateral loads. The feature of soft story

mechanism is not considered in the present method of analysis for earthquake load. In

static method we only consider the first mode of vibration (that’s suitable for regular

bare frames) while the behavior of infill and irregular frames are far more

complicated and uncertain. That’s why the dynamic analysis is helpful to account for

the other modes of vibration and consider the irregularity of stiffness features in

building frame caused by random distribution of infill on upper floors. If we can

consider the true dynamic features of the frame system then the design will be safer

and adequate.

2.4 BUILDING DESIGN STRATEGIES IN BUILDING CODE Most of the building codes have not yet given emphasis on designing soft story

buildings though these are more vulnerable to earthquake. Building codes such as

14

BNBC (1993), UBC (1997), ASCE (2002), ACI (2005), NZS (4203) ETC. define

analysis procedure based on structural height for irregular structure including soft

story irregularity. Due to damage of various soft story buildings in Bhuj earthquake,

2002 Indian Seismic Code IS: 1893-2002 Part-1 has included special design features

for soft story buildings. In most cases the codes state that the regular geometry

structures with properly located masonry walls perform better than irregular

geometric structures with improper masonry infill structures. As per as codes state for

low rise buildings static method and for high rise building dynamic method of

analysis are performed. Generally the natural period for in-fill RC frames are lower

than the bare RC frames. For that reason forces consider more in infill frames than in

bare frames. As per as few codes such as IS-1893 (2002), NBC-105 (1995), NSR-98

(1998) Egyptian code (1988), Venezuelan code (1988), Algerian code (1988), ESCP-

1 (1983); the empirical relationship for the time period is given by the following

equation:

Ta = d

h09.0

Eqn. – 2.2

Where, h is the height of building in meter and d is the base dimension of building in

meter at the plinth level along the considered direction of the lateral force.

French code (AFPS-90, 1990) recommends the following equation for masonry infill

buildings;

T = 0.06hd

hdh

2 Eqn. – 2.3

2.4.1 Indian Seismic Code, IS-1893 (2002)

As stated earlier, Indian code has featured some modification for soft story.

In case buildings with a flexible story, such as the ground story consisting of open

spaces for parking, that is Stilt Building. Special arrangement needs to be made to

increase lateral strength and stiffness of the soft story.

Dynamic analysis of building is carried out including the strength and stiffness effects

of infill and inelastic deformations in the members, particularly, those in the soft

story.

15

(a) The columns and beams of the soft story are to be designed for 2.5 times the

story shears and moments calculated under seismic loads specified in the other

relative clauses; or,

(b) Besides the columns designed and detailed for the calculated story shears and

moments, shear walls placed symmetrically in both directions of the building as far

away from the center of the building as feasible; to be designed extensively for 1.5

times the lateral story shear force calculated as before.

2.5 COMPUTATIONAL MODELING AND ANALYSIS OF

INFILL FRAME There are various types of modeling approach were attempted before for featuring

infill characteristics in RC frame. Holmes (1961) replaced the infill by an equivalent

pin-jointed diagonal strut. Smith (1962) conducted a series of tests on laterally loaded

square mild steel frame models infill with micro-concrete. From the model

deformation results he concluded that the wall could be replaced by an equivalent

diagonal strut connecting the loaded corners. As the elastic methods were not able to

fully feature the actual characteristics of infill frames, attention was paid to the

theories of plasticity. Wood (1958) extended the limit analysis of plasticity with the

assumption of perfect plasticity. Recently a method was developed by Saneinejad.

(1995) that allows for interface shear forces and both the elastic and plastic behavior

of material. Variations in geometrical and mechanical characteristics are used to

determine the stiffness of structural system. The analysis is carried out utilizing the

boundary element method (BEM) for the infill and dividing the frame into finite

elements, so as to transform the mutual interactions of the two subsystems into

stresses distributed along the boundary for the infill and into nodal actions for the

frame. In this analysis equivalent diagonal strut method is used which is described

below.

2.5.1 Equivalent Diagonal Strut Method

Strength predictions of infill frames are a complex, statically indeterminate problem.

The strength of a composite infill frame system is a combination of frame system and

infill. Polyakov (1960), Stafford Smith (1962, 1966, 1969), Mainstone (1971),

Klingner and Sereto (1976, 1978) Liaw and Lee (1977) are only a few of them who

has formed the basis to understand the in-plane behavior of infill frame. During

16

testing of the specimens, diagonal cracks developed in the center of the panel and

gaps formed between the frame and the infill in the non loaded diagonal corners of

the specimen, while full contact was observed in the two diagonal corners. This

behavior leads to a simplification in the in-filled frame analysis by replacing the

masonry infill by an equivalent compressive masonry strut of same mechanical

properties as the infill and the both ends must be pinned to the corner of the frame.

As per as research on modeling of infill panel as an equivalent diagonal strut method

was applied by Holmes (1961). He assumed that the infill wall acts as diagonal

compression strut as shown in (fig-2.5.a) of the same thickness and elastic modulus as

the infill with a width equal to one-third the diagonal length. He also concluded that

at the infill failure, the lateral deflection of the in-filled frame is small compared to

the deflection of the corresponding bare frame.

Saneinejad and Hobbs (1995) developed a method based on the equivalent diagonal

strut approach for the analysis and design of steel or concrete frames with concrete or

masonry infill walls subjected to in-plane forces.

Fig. 2.11 Material modeling of masonry infill as diagonal strut

Fig. 2.12.1 Masonry in-filled frame Sub-assembly in masonry infill panel frame

Fig. 2.12.2 Masonry infill panel in frame structure

17

The analytical assumptions are the contribution of the masonry infill panel (Fig.

2.12.1) to the response of the in-filled frame can be modeled by replacing the panel

by a system of two diagonal masonry compression struts (Fig: 2.12.2). However, the

combinations of both diagonal struts provide a lateral resisting mechanism for the

opposite lateral directions of loading.

2.6 PAST RESEARCH ON SOFT STORY BUILDING

Amanat and Hoque (2006) studied the fundamental periods of vibration of a series of

regular RC framed buildings using 3-D FE modeling and modal eigenvalue analysis

including the effects of infill. The time period determined from eigenvalue analysis

was remarkably close to those predicted by the code formulas. It’s also observed that

the randomness of infill application does not cause much variation of the period if the

total amount of infill panel is same. Based on the findings of the study some practical

guidelines were suggested for determining the fundamental period of RC frames

using rational approaches like modal analysis.

Costa Rican code (1986) requires that all structural resisting system must be

continuous from the foundation to the top of the building and stiffness of a story must

not be less than 50% of that of the story below.

Arlekar, Jain and Murty (1997) highlighted the importance of explicitly recognizing

the presence of the open ground story in the analysis of the building. The error

involved in modeling such buildings as complete bare frames, neglecting the presence

of in-fills in the upper story, is brought out through the study of an example building

with different analytical models.

Fardis and Panagiotakos (1997) studied through numerical analyses the effects of

masonry in-fills on the global seismic response of reinforced concrete structures.

Response spectra of elastic SDOF frames with nonlinear in-fills show that, despite

their apparent stiffening effect on the system, in-fills reduce spectral displacements

and forces mainly through their high damping in the first large post-cracking

excursion.

Mezzi (2004) illustrated soft story to be very dangerous from seismic viewpoint as the

lateral response of these buildings is characterized by a large rotation ductility

demand concentrated at the extreme sections of the columns of the ground floors,

while the superstructure behaves like a quasi-rigid body. A solution was proposed for

the preservation of a particular architectonic double soft story configuration.

18

Huang (2005) studied the structural behaviors of low-to-midrise concrete buildings of

various configurations with emphases on dynamic properties, internal energy, and the

magnitude and distribution of seismic load. Several idealized models were made to

represent different structural configurations including pure frame, frames with fully

or partially infilled panels, and frames with a soft story at the bottom level, and

Comparison was made on the fundamental periods, base shear, and strain energy

absorbed by the bottom level between these structures.

M. Helen Santhi, G. M. Samuel Knight (2005) studied two single-bay, three-story

space frames, one with brick masonry infill in the second and third floors representing

a soft-story frame and the other without infill were designed and their 1:3 scale

models were constructed according to non-seismic detailing and the similitude law.

Rodsin (2005) evaluated the potential seismic performance of building with soft story

in an area of low to moderate seismicity regions (such as Australia) by a

displacement-based method involving a push-over analysis.

Nagae (2006) studied six storied reinforced concrete building and focused on seismic

response of the soft ground floor based on the results on dynamic response analysis.

Jahid Hasnain (2009) studied this phenomenon of soft story building. He determined

the effect of randomly distributed in-fills on seismic base shear for RC buildings with

soft ground floor. In spite of providing an extensive analysis, his study is also limited

due to the following issues:

Random application of infill.

Constant beam and column size.

Application of earthquake load only along X direction.

Application of partition wall load as a constant.

Equal distribution of total number of infill along the span and bay.

Rumia Tasmim (2010) has done an extensive study of the above mentioned issues and

successfully overcome the limitations. The following modifications have been

proposed with randomly applied infill:

Variation of beam and column size with varying span or bay length. A

minimum dimension is also used.

Application of earthquake load along X and Z directions.

Variation of partition wall load along with varying percentage of infill.

19

Distribution of total number of infill as a ratio of span and bay along

the span and bay.

Mokhlesur Rahman studied the soft story phenomenon using Push over analysis. But

the study was limited to six story buildings.

In our research we provide random infill on the panel of soft story to study the

difference in performance level with increasing height and variable stiffness. It is also

one of the objectives to find the modification factor of base shear comparing among

Static Force Method, Response Spectrum Method and Pushover Analysis Method.

Data of modification factor for base shear between Static Force Method and Response

Spectrum Method is obtained from the previous research work of Ms. Rumia Tasmim.

2.7 REMARKS Like most other building codes discussed in this chapter Bangladesh National

Building Code (1993) is no different, having no provisions for building with soft

story. Irregular structures including soft story are required dynamic analysis to

understand its actual behavior. The Indian Seismic Code IS: 1893 (2002) Part-1 has

also included design provisions for soft story buildings. It is high time to incorporate

these provisions in BNBC also. In the upcoming chapters performances of soft story

building will be studied applying non-linear static analysis method or Pushover

analysis.

20

CHAPTER 3

CONCEPT OF PERFORMANCE BASED DESIGN 3.1 INTRODUCTION The most common case of soft story, especially in residential building, is the

presence of bare ground story while the upper stories are filled with masonry panel.

Although there are few other reasons of soft story formation the study here is limited

to the formation of soft story due to the existence of masonry infill. The building will

become more vulnerable to earthquake because of the presence of soft story. The

traditional approach to seismic design of a building is a force based design. The

design lateral forces on the building are determined using the response spectrum. In

this approach, there is no measure of the deformation capability of a member or of the

building. During earthquake, it is expected that the members of the building will go

beyond elastic condition which is not possible to analyze using force based design or

response spectrum method. So a performance based analysis is required to get a result

from nonlinear lateral load versus deformation analysis. It gives more choices of

performance of the building as compared to the limit states of collapse and

serviceability in a design on limit state method. In this chapter, the modeling of infill

frame by equivalent strut method and some other features of infill modeling will be

discussed. In the later portion pushover analysis procedure and performance criteria

will be discussed. In the final portion modeling parameter for hinges and site response

spectra for Dhaka zone is established.

3.2 MODELING OF INFILL WALLS

The infill walls used in the structures for partitions and other purposes can be

represented in many ways. Here in this study equivalent strut method will be used for

the simplicity.

21

3.2.1 Equivalent Strut Method

Strength predictions of in filled frames are a complex and statically indeterminate

problem. The strength of a composite in filled frame system is not only the

summation of the infill properties plus those of the frame. To better understand and

estimate the composite behavior of masonry in filled frames both analytically and

experimentally lot of efforts have been made by Polyakov (1960), Stafford-Smith

(1962, 1966, 1969), Mainstone (1971) are the few to be named. During the

experiment, as observed by Polyakov, the masonry infill can be replaced by

equivalent diagonal masonry strut because cracks developed in the center of the panel

diagonally.

Their experiments of in filled frames under lateral loads resulted in specimen

deformation shapes similar to one illustrated in figure 3.1 below-

Figure 3.1: Specimen Deformation Shape

The equivalent compressive masonry strut of width a, same thickness of masonry and

same modulus of elasticity. It is also assumed to be pinned at both ends of the

confining frame.

22

3.2.2 Equivalent Strut Width

The evaluation of the equivalent width a, varies from one reference to other. Stafford-

Smith and Carter (1969), Mainstone (1971) and others derived complex expressions

to estimate the equivalent strut width a by considering the parameters like the length

between the column/ beam and the infill as well the relative stiffness of the infill to

the frame.

Expressions used in this chapter have been adopted from Mainstone (1971) and

Stafford-Smith and Carter (1969) for their consistently accurate predictions of in

filled frame in-plane behavior when compared with experimental results of others.

a

d

h

th et a

l

Figure 3.2: Strut Geometry

The equivalent strut width a, depends on the relative flexural stiffness of the infill to

that of the columns of the confining frame. The relative infill to frame stiffness shall

be evaluated using eqn. 3.1-

λ1H = H ((Em t sin 2θ)/ (4 Ec I col hm)) 0.25 Eq. - 3.1

Where, t is the thickness of the masonry wall.

Using the expression, Mainstone (1971) considers the relative infill to frame

flexibility in the evaluation of the equivalent strut width of the panel as shown in

eqn.3.2-

a = 0.175 D (λ1H)-0.4 Eq. - 3.2

23

If there are openings present, existing infill damage or FRP overlay, however, the

equivalent width must be modified using-

amod= a (R1) (R2) ζi Eq.- 3.3

Here,

(R1)= reduction factor in plane evaluation due to presence of openings

(R2)= reduction factor for in plane evaluation due to existing infill damage

ζi = Strength increase factor due to presence of FRP overlay

Although the expression for equivalent strut width given above was derived to

represent the elastic stiffness of an infill panel, this document will extend its use to

determine the ultimate capacity of in filled structures. The strut will be assigned

strength parameters consistent with the properties of the infill if represents. A non

linear static method commonly referred to as a pushover analysis, will be used to

determine the capacity of the in filled structure.

3.2.1.2 Eccentricity of Equivalent Strut The equivalent masonry placed as shown in figure (3.2). The infill forces are assumed

to be mainly resisted by the columns and the struts are placed accordingly. The strut

should be pin-connected to the column at a distance l column from the face of the beam.

The distance is defined in equation below.

l column = a/(cosθ column) Eq.- 3.4 tanθ column = (hm - a/ cosθ column ) / l Eq.- 3.5 3.2.1.3 Existing infill damage

As the elastic limit is exceeded, the behavior of masonry infill deteriorates. So it is

important to determine exceeded limit. Existing panel damage must be classified as –

no damage, moderate damage or severe damage. If in doubt to determine the

magnitude of existing panel damage assume severe damage for a conservative

estimate. A reduction factor for existing panel damage (R2 ) must be obtained from

Table (3.1). If the slenderness ratio (hm/t) of the panel is greater than 21, (R2) is not

defined and repair is required. For panels with no existing panel damage, the

reduction factor must be taken as 1.

24

Table 3.1: In-plane damage reduction factor

Type of Damage Type of Damage

hm/t Moderate Severe

<21 0.7 0.4

>21 Repair is required. Repair is required.

3.2.3 Infill Properties

The infill masonry panel will be presented as strut member. The equivalent strut width

shall be determined according to Coul & Smith described earlier. For the modeling of

infill the following properties must be determined.

Modulus of elasticity of concrete Ec value for column and beam materials.

Sectional propertied (i.e. Depth, Width, Moment of Inertia, Center of Gravity)

of the column and beam.

Equivalent width of the masonry infill strut “a”.

fm compressive strength of the masonry assembled units.

Em modulus of elasticity of the masonry unit.

3.2.4 Determination of fm and Em

In this study masonry bricks are considered as infill material. In Bangladesh although

a variety of bricks are used as a building material but most common of them is solid

clay bricks. So, NW type brick, according to ASTM C62, 1994 is considered in this

study. Table 3.2: Specified Compressive Strength of Masonry f’m (psi), based on

specifying the compressive strength of masonry units. (ACI 530, 192/ ASCE 6-92/

TMS 602-92)

25

Compressive Strength of Masonry Units

(psi)

Specified Compressive Strength of Masonry, f'm

Type M or S Mortar (psi)

Type N mortar (psi)

14000 or more 5300 4400 12000 4700 3800 10000 4000 3300

8000 3350 2700 6000 2700 2200 4000 2000 1600

Compressive Strength of Concrete Units

(psi)

Specified Compressive Strength of Masonry, f'm

Type M or S Mortar (psi)

Type N mortar (psi)

4800 or more 3000 2800 3750 2500 2350 2800 2000 1850 1900 1500 1350

NOTE:

1. Compressive strength of solid clay masonry units is based on gross area. Compressive strength of hollow clay masonry units is based on minimum net area. Value may be interpolated.

2. Assumed assemblage. The specified compressive strength of masonry fm is based on gross area strength when using solid units or solid grouted masonry and net area strength when using ungrouted hollow units.

3. Type M, N or S mortar are according to ASTM C270 definition.

From the table for strength of clay masonry unit 4000 psi is chosen and for that fm

1600 psi for N mortar.

Em is the ratio of the stress to the strain of a material or combinations of materials as

in the case for grouted masonry.

To find out the value of Em considering the ACI/ ASCE/ TMS masonry code the

modulus of elasticity is given in the following table-

26

Compressive strength of clay Masonry (psi)

Specific Compressive Strength of

Clay Masonry Assemblage

fm( psi)

Modulus of

Elasticity (psi)

Modular Ratio, n

Modulus of

Rigidity, G = 0.40 Em (psi)

14000 or more 4400 3000000 9.7 1200000

12000 3800 2850000 10.2 1140000

10000 3300 2475000 11.7 990000

8000 2700 2025000 14.3 810000

6000 2200 1650000 17.6 660000

4000 1600 1200000 24.2 480000

Type M or S Mortar

Compressive strength of clay Masonry (psi)

Specific Compressive Strength of

Clay Masonry Assemblage

fm( psi)

Modulus of

Elasticity (psi)

Modular Ratio, n

Modulus of

Rigidity, G = 0.40 Em (psi)

14000 or more 5300 3000000 9.7 1200000

12000 4700 3000000 9.7 1200000

10000 4000 3000000 9.7 1200000

8000 3350 2512500 11.5 1005000

6000 2700 2025000 14.3 810000

4000 2000 1500000 19.3 600000

According to the table, for Em= 1270 ksi

27

3.2.5 Calculation of Equivalent Strut Width

In the following table calculation of some sample strut is shown. In the study due to

various change in geometry several equivalent strut is used. Detail sample calculation

of Equivalent strut width is given below-

Table 3.4: Sample Calculation of equivalent strut width

Column Dimension Beam Dimension θcolumn Strut

Width, a B L B H

mm mm mm mm radian inch

457 457 305 508 0.42 27

508 508 305 508 0.42 27

558 558 305 558 0.46 28

609 609 305 609 0.46 28

Eccentricity is assumed d distance below the column as per advice.

3.3 NON-LINEAR STATIC PROCEDURE

Various analysis methods- both elastic and inelastic are available for the analysis of

concrete building. Elastic analysis method available includes code static lateral force

procedure, code dynamic lateral force procedure and elastic procedures using demand

capacity ratios. The most basic inelastic analysis method is the complete non linear

time history analysis, which at this time is considered overly complex and impractical

for general use. Available simplified nonlinear method referred to as nonlinear static

analysis procedure; include the capacity spectrum method (CSM) that uses the

intersection of capacity (pushover) curve and a reduced response spectrum to estimate

maximum displacement. Simplified nonlinear static analysis procedure using

pushover method such as the capacity spectrum method and the displacement

coefficient method requires determination of three primary elements: capacity,

demand (displacement) and performance. Each of these elements is briefly discussed

below

28

Capacity

The overall capacity of a structure depends on the strength and deformation capacities

of the individual components of the structure. In order to determine capacities beyond

the elastic limits, some form of nonlinear analysis such as Pushover Analysis is

required. This procedure uses a series of elastic analysis, superimposed to

approximate a force-displacement capacity diagram of the structure. The

mathematical model of the structure is modified to account for reduced resistance of

yielding components. A force distribution is again applied until the additional

components yields through forming plastic hinges. This process is continued until the

structure becomes unstable or until the predetermined limit is reached. The push over

capacity curve approximates how the structure behaves after exceeding elastic limit.

Demand (Displacement)

Ground motion during an earthquake produce complex horizontal displacement

patterns in structures that may vary with time. Tracking this motion at every time-step

to determine structural design requirements is judged impractical. Traditional linear

analysis methods use lateral forces to represent a design condition. For nonlinear

methods it is easier and more direct to use a set of lateral displacement demand is an

estimate of the maximum expected response of the building during the ground

motion. The displacement demand is established by use of the conventional response

spectra by covering it onto Spectral Acceleration vs. Spectral Displacement.

Performance

Once a capacity curve and demand displacement is defined, a performance check can

be done. A performance check verifies that structural and nonstructural components

are not damaged beyond the acceptable limits of the performance objective for the

forces and displacement imposed by the displacement demand.

3.3.1 Reduced Demand Spectra

The capacity of a particular building and the demand imposed upon it by a given

earthquake motion are not independent. One source of this mutual dependence is

evident from the capacity curve itself. As the demand increases the structure

29

eventually yields and stiffness decreases. Conversion of the capacity curve to spectral

ordinates (ADRS) makes this concept easier to visualize. Since the seismic

acceleration depends on period, demand also changes as the structure yields. Another

source of mutual dependence between capacity and demand is effective damping. As

a building yield in response to seismic demand it dissipates energy with hysteretic

damping. Building that have large, stable hysteresis loops during cyclic yielding

dissipate more energy than those with pinched loops caused by degradation of

strength and stiffness.

Since the energy dissipated need not be stored in the structure, the effective damping

diminishes displacement demand. The reduced displacement demand is shown in

figure above. The equation for the reduced factor SRA and SRv are given by-

SRA = (3.21- 0.681ln βeff) / 2.12 ≥ value in Table Eq.- 3.6

SRv = (2.31-0.41ln βeff)/ 1.65 ≤ value in Table Eq.- 3.7

Table (3.5): Minimum Allowable SRA and SRv values

Structural Behavior Type SRA SRv

Type A 0.33 0.5

Type B 0.44 0.56

Type C 0.56 0.67

Values for SRA and SRv shall not be less than those shown in this table.

Type A, B and C is taken as defined in ATC 40.

3.3.2 Development of Elastic Site Response Spectra

Elastic response spectra for a site are based on estimate of Seismic Co-efficient, CA

which represents the effective peak acceleration (EPA) of the ground and Cv which

represents 5% damped response of a 1- second system. These coefficients for a

particular zone are dependent on the seismicity of the area, the proximity of the site to

active seismic sources and site soil profile characteristics.

3.3.3 Seismic Zone

Bangladesh is divided into three seismic zones as per BNBC (1993). The table below

shows the values of zone coefficients of Bangladesh.

30

Table 3.6: Seismic Zone Factor, Z

Zone 1 2 3

Z 0.075 0.15 0.25

3.3.4 Seismic Source Type

As per ATC- 40 (1996), three types of seismic source may be defined as a shown in

Table below.

Table 3.7: Seismic source type as per ATC- 40, 1996

Seismic Source

Seismic Source Description Maximum Moment

Slip Rate, SR (mm/ yr)

A Faults which are capable of producing large magnitude events.

M> 7 SR> 5

B All faults other than types A and C N/A N/A

C Faults that are not capable of producing large magnitude events and which have a high rate

M< 6.5 SR< 2

3.3.5 Near Source Factor

Currently data pertaining to the active faults close to Dhaka city is not available. It is

not possible to estimate the seismic source distance from a specific site being

considered in this thesis. But it may be safe to assume that all the sources are located

at distance more than 15 km and the Table(3.8) (ATC- 40, 1996) may be used to

consider the Near Source effects for the present study. The near source factor may be

used on the linear interpolation of values for distance other than those shown in the

table. The closest distance of the seismic source shall be taken as the minimum

distance between the site and the area described by the vertical projection of source on

the surface. The surface projecting need not include portions of the source a depth of

10 km or greater. The largest value of the near source factor considering all sources

shall be used in design.

31

Table 3.8: Seismic Source Factor

Seismic Source Type

Closed Distance to Known Seismic Source

< 2km 5km 10km > 15 km

NA Nv NA Nv NA Nv NA Nv

A 1.5 2 1.2 1.6 1 1.2 1 1 B 1.3 1.6 1 1.2 1 1 1 1

C 1 1 1 1 1 1 1 1

3.3.6 Seismic Coefficients

For each earthquake hazard level, the structure is assigned a seismic coefficient CA in

accordance with Table (3.9) (ATC- 40, 1996) and a seismic coefficient Cv in

accordance with Table (3.10) (ATC- 40, 1996). Seismic coefficient CA represents the

effective peak acceleration (EPA) of the ground. A factor of about 2.5 times of CA

represents the average value of peak response of a 5% damped short period system in

acceleration domain. The seismic coefficient Cv represents 5% damped response of 1

sec system. Cv divided by period (T) defines acceleration response in the velocity

domain. These coefficients are dependent on soil profile type and the product of

earthquake zoning coefficient- Z, severity of earthquake- E and near source factor

(ZEN).The soil profile types are taken from the classification of (ATC- 40, 1996).

The soil profile type SE is applicable for Dhaka City. This type includes any soil

profile with more than 10 feet or soft clay defined as a soil with PI > 20, WMC > 40

and Su< 500 psf. Table 3.9: Seismic coefficient, CA

Soil profile type Shaking Intensity, ZEN

ZEN 0.075 0.15 0.2 0.3 SB 0.08 0.15 0.2 0.3 Sc 0.09 0.18 0.24 0.33 SD 0.12 0.22 0.28 0.36 SE 0.19 0.3 0.34 0.36

SF Site Specific Geotechnical Investigation required to determine

CA

32

The value of E used to determine the product, ZEN, should be taken to be equal to 0.5 for the serviceability Earthquake, 1.0 for the design earthquake and 1.25 for serviceability earthquake.

Seismic coefficient CA should be determined by liner interpolation for values of the product ZEN other than those shown in the table.

Table 3.10: Seismic coefficient CV (ATC-40, 1996)

Soil profile type

Shaking Intensity, ZEN

ZEN 0.075 0.15 0.2 0.3

SB 0.08 0.15 0.2 0.3

Sc 0.13 0.25 0.32 0.45 SD 0.18 0.32 0.4 0.54 SE 0.26 0.5 0.64 0.84 SF Site Specific Geotechnical Investigation required to determine CA

The value of E used to determine the product, ZEN, should be taken to be equal to 0.5 for the serviceability Earthquake, 1.0 for the design earthquake and 1.25 for serviceability earthquake.

Seismic coefficient CA should be determined by liner interpolation for values of the product ZEN other than those shown in the table.

According to (ATC-40, 1996), Soil profile types for Dhaka city is SE because it

includes any soil profile with more than 10 feet or soft clay defined as a soil with

PI>20, WMC> 40 and Su < 500 psf.

3.3.7 Establishing Demand Spectra

The purpose of subsequent analysis to be made in this thesis, it is necessary to

establish an earthquake demand spectra against which building performance will be

evaluated. The following controlling parameters are considered:

Location of the site : Dhaka City

Soil profile at the site : Soil type SE as per (ATC-40), soft soil with shear wave

velocity Vs< 600 fps, N< 50 and Su < 100 psf

Earthquake source type: A – considering the events similar to the great Indian

Earthquake in Assam in 12 June, 1897

Near Source Factor : > 15 km

33

Calculation of CA

Seismic Zone Factor, Z 0.15 as per BNBC

Earthquake Hazard Level, E 1 Design Earthquake

Near Source Factor, N 1 > 15 km, Table

Shaking Intensity, ZEN 0.15

For soil type, SE 0.3 From Table

Elastic response spectra, for each earthquake hazard level of interest at a site, isbased

on the site seismic coefficients CA and CV calculated above. The coefficient CA

represents the effective peak acceleration (EPA) of the ground. A factor of about 2.5

times CA represents the average value of peak response of a 5% damped short period

system in the acceleration domain. The seismic coefficient CV represents 5% damped

response of a 1 second system and when divided by period defines acceleration

response in velocity domain.

Calculation of CV

Seismic Zone Factor, Z 0.15 as per BNBC

Earthquake Hazard

Level, E 1 Design Earthquake

Near Source Factor, N 1 > 15 km, Table

Shaking Intensity, ZEN 0.15

For soil type, SE 0.5 From Table

The following establishes 5% damped elastic response spectra as shown in figure 3.3.

Effective peak ground acceleration (EPA) = 0.3g

Average value of peak response = 0.75g

Seismic coefficient, CV = 0.5g

Ts = 0.667 sec Ts =Cv / 2.5 CA

34

Ta = 0.133 sec Ta = 0.2Ts

For seismic performance evaluation purpose, this newly constructed site specific 5%

elastic response spectra need to be converted in to ADRS format using relation-

Sd = (T2/ 4π2) Sa g Eqn. – 3.8

Figure 3.3: 5% Damped Elastic Response Spectrum

Calculated spectral acceleration, spectral displacement with respect to the period is

shown in Table 3.11. These values are used to construct the 5% Elastic response

spectrum in Sa vs. Period format (figure: 3.3) and in ADRS format (figure: 3.4).

35

Figure 3.4: 5% Damped elastic response spectrum in ADRS format

Table 3.11: Response quantities for 5% elastic demand

T (sec) Sa, g Sd, cm

0 0.3 0

0.13 0.75 0

0.667 0.75 82.99705

0.8 0.625 99.49694

1 0.5 124.3712

1.2 0.416667 149.2454

1.4 0.357143 174.1196

1.6 0.3125 198.9939

1.8 0.277778 223.8681

2 0.25 248.7423

2.2 0.227273 273.6166

2.4 0.208333 298.4908

2.6 0.192308 323.365

2.8 0.178571 348.2393

3 0.166667 373.1135

3.2 0.1563 397.9877

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 100 200 300 400 500 600

Spec

tral

Acc

eler

atio

n, S

a (g

)

Spectral Displacement, Sd

36

3.4 CAPACITY CURVE

The nonlinear pushover analysis requires development of the capacity curve. The

capacity curve is derived from a nonlinear analysis for the structure. In the process of

performing this incremental nonlinear static analysis, a capacity curve is developed

for the building. The capacity curve is simply the plot of the total lateral seismic

demand “V”, on the structure, at various increment of loading, against the lateral

deflection of the building at the roof level, under that applied lateral force. If a

building had infinite linear capacity, this capacity curve would be a straight line with

a slope equal to the global stiffness of the structure. Since real building do not have

the infinite linear capacities, the capacity curve typically consists of a series of

straight line segments with decreasing slope, representing the progressive degradation

in structural stiffness that occurs as the building is subjected to increased lateral

displacement, yielding and damage. The slope of a straight line drawn from the origin

of the plot for this curve to a point on the curve at any lateral displacement “d”

represent the secant or “effective” stiffness of the structure when pushed laterally to

that displacement. A typical capacity curve of a hypothetical structure is shown-

Figure 3.5: Typical Capacity Curve.

The discreet points indicated occurrence of important events in the lateral response

history of the structure. Such an event may be the initiation of yield in a particulars

structural element or a particular type of damage, such as spalling of cover concrete

on a column or shear failure of a spandrel element. Each point is determined by a

37

different analysis sequence. Then by evaluating the cumulative effects of damage

sustained at each of the individual events and the overall behavior of the structure is

increasing lateral displacement, it is possible to determine and indicate on the capacity

curve those total structural lateral displacements that represent limits on the various

structural performance levels, as has been done in Fig.3.5 . The Immediate Occupancy

(IO), the Life Safety level (LS) and the Structural Stability level (SS) are three

performance levels.

The point on the capacity curve at which the first element exceeds the permissible

deformation level for a structural performances level does not necessarily represent

that the structural performance level. Most structures contain many elements and have

considered redundancy. Consequently, the onset of unacceptable damage to a small

percentage of these elements may not represent an unacceptable condition with regard

to the overall performance of the building. When determining the points along the

capacity curve for the structure at which the various structural performance level may

said to be reached, the engineer must view the performance of a building as whole and

consider the importance of damage predicted for the various elements on the overall

behavior of the building.

The methodology described in ATC- 40 (1996), incorporates the concept of

“Primary” and “Secondary” elements to assist the engineers in making these

decisions. Primary elements are those that are required as part of the lateral force

resisting system for the structure. All the other elements are designated as secondary

elements. For a given performance level, secondary elements are generally permitted

to sustain more damage than primary elements since degradation of secondary

elements does not have a significant effect on the lateral load resisting capability of

building. If in the development of the capacity curve it is determined that a few

element fail to meet the acceptance criteria for a given performance level at an

increment of lateral loading and displacement, the engineer has the ability to liberal

acceptance criteria for these few elements. Care is exercised not to designate an

excessive number of elements that are effective in resisting lateral force as secondary.

38

3.4.1 Capacity Spectrum Method

The capacity spectrum method, a nonlinear static procedure, provides a graphical

representation of global force-displacement capacity curve of the structure and

compares it to the response spectra representations of the earthquake demand. This

method is a very useful tool in the evaluation and retrofit design of existing concrete

buildings. Capacity spectrum is the simple representation of the capacity curve in

ADRS domain. A capacity curve is the representation of Base shear to roof

displacement. In order to develop the capacity spectrum from a capacity curve it is

necessary to do a point by point conversion to first mode spectral co ordinates. Figure

3.6 shows a typical spectrum converted from capacity curve of a hypothetical

structure. It is seen that in the capacity spectrum that up to some displacement

corresponding to 1st point while the structure is in elastic deformation. The structure

deflects more to 2nd point. It goes to inelastic deformation. When Sa vs. Sd capacity

Figure: 3.6 Capacity Curve

curve is plotted, radial lines drawn from the origin of the plot through the curve at

various spectral displacements has a slope where ω is the radial frequency of the

effective first mode response of the structure if pushed by an earthquake to that

spectral displacement. Using the relationship T= 2π/ ω, it is possible to calculate, for

each of this radial lines, the effective period of the structure if it is pushed to a given

spectral displacements.

0

0.05

0.1

0.15

0.2

0.25

0 50 100 150 200

Spec

tral

Acc

eler

atio

n, S

a (g

)

Spectral Displacement, Sd (mm)

39

3.5 PERFORMANCE POINT

The capacity spectrum method initially characterizes seismic demand using an elastic

response spectrum. This spectrum is plotted in spectral ordinates (ADRS) format

showing the spectral acceleration as a function of spectral displacement. This format

allows the demand spectra to be “overlaid” on the capacity spectrum for the building.

The intersection of the demand and capacity spectra, if located in the linear range of

the capacity, would define the actual displacement for the structure; however this is

not normally the case as most analysis includes some inelastic behavior. To find the

point where demand and capacity are equal, a point on the capacity spectrum need to

be selected as an initial estimate. Using the spectral acceleration and displacement

defined by this point, reduction factors may be calculated to apply to the 5% elastic

spectrum to account for the hysteretic energy dissipation or effective damping,

associated with the specific point. If the reduced demand spectrum intersects the

capacity spectrum at or near the initial assumed point, then it is the solution for the

unique point where capacity equals demand. If the intersection is not reasonably close

to the initial point, then a new point somewhere between may be assumed and repeat

the process until a solution is reached. This is the performance point where the

capacity of the structure matches the demand or the specific earthquake.

Once the performance point has been determined, the acceptability of a rehabilitation

design to meet the project performance objectives can be judged by evaluating where

the performance points falls on the capacity curve. For the structure and earthquake

represented by the overlay indicated in Figure 3.7, indicating that for this earthquake

this structure would have less damage than permitted for the Life Safety level and

more than would be permitted for the Immediate Occupancy level.

40

Figure 3.7: Determination of performance point

With this information, the performance objective and/ or the effectiveness of the

particular rehabilitation strategy to achieve the project performance objectives can be

judged.

3.5.1 Non linear Static Procedure for Capacity evaluation of Structures

Instead of comparing forces, nonlinear static procedures use displacements to

compare seismic demand to the capacity of a structure. This approach included

consideration of the ductility of the structure on an element by element basis. The

inelastic capacity of a building is then a measure of its ability to dissipate earthquake

energy. The current trend in seismic analysis is toward these simplified inelastic

procedures.

The recommended central methodology is on the formulation of inelastic capacity

curve for the structure. This curve is a plot of the horizontal movement of a structure

as it is pushed to one side. Initially the plot is a straight line as the structure moves

linearly. As the parts of the structure yield the plot begins to curve as the structure

softens. This curve is generated by building a model of the entire structure from

nonlinear representation of all of its elements and components. Most often this is

accomplished with a computer and structural analysis software. The specific forces

and displacement characteristics are specified for each piece of the structure resisting

the earthquake demand. These pieces are assembled geometrically to represent the

Performance Point

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 100 200 300 400 500 600

Spec

tral

Acc

eler

atio

n, S

a (g

)


41

complete lateral load resisting system. The resulting model is subjected to increasing

increment of load in a pattern determined by its dynamic properties. The

corresponding displacements define the inelastic capacity curve of the building. The

generation of capacity curve defines the capacity of the building uniquely and

independently of any specific demand. When an earthquake displaces the building

laterally, its response is represented by a point on this curve. A point on the curve

defines a specific damage state of the building. The deformation of its entire

components can be related to the global displacement of the structure.

Building capacity and demand requires from earthquake are mutually dependent

which is depicted by capacity curve. As the demand increases the structure eventually

yields and stiffness decreases. Its period lengthens. Since the seismic accelerations

depend on period, demand also changes as the structure yields. Another source of

mutual dependence between capacity and demand is effective damping. As building

yields in response to seismic demand, it dissipates energy with yielding dissipate

more energy than those with pinched loops caused by degradation of strength and

stiffness. Since the energy that is dissipated need not be stored in the structure, the

damping has the effect of diminishing displacement demand.

3.5.2 Structural Performance Levels and Ranges

The performance of a building under any particular event is dependent on a wide

range of parameters. These parameters are defined (ATC-40, 1996; FEMA 356, 2000)

qualitatively in terms of the safety afforded by the building to the occupants during

and after the event; the cost and feasibility of restoring the building to pre- earthquake

condition; the length of time the building is removed from service to effect repairs;

and economic, architectural or historic impacts on the larger community. These

performance characteristics are directly related to the extent of damage that would be

sustained by the building.

The Federal Emergency Management Agency in its report ‘pre-standard and

Commentary for the Seismic Rehabilitation of Buildings, (FEMA-356, 2000) defines

the structural performance levels and two intermediate structural performance ranges.

42

The discrete Structural Performance Levels are –

Immediate Occupancy (S-1)

Life Safety (S-3)

Collapse Prevention (S-5) and

Not Considered (S-6)

The Intermediate Structural Performance Ranges are-

Damage Control Range (S-2) and

Limited Safety Range (S-4)

The definition of these performance ranges are given by FEMA (FEMA-356, 2000)

Acceptance criteria for performance within the Damage Control Structural

Performance Range may be obtained by interpolating the acceptance criteria provided

for the Immediate Occupancy and Life Safety Levels. The performance levels and

ranges. As per FEMA (FEMA-356, 2000), are described in the sections that follow.

3.5.3 Immediate Occupancy Structural Performance Level (S-1)

Structural Performance Level S-1- Immediate occupancy, may be defined as the post-

earthquake damage state of a structure and remains safe to occupy the pre-earthquake

design strength and stiffness of the structure and is in compliance with the acceptance

criteria specified in this standard for this Structural Performance Levels defined at

FEMA-356, 2000.

Immediate Occupancy also means the post-earthquake damage state in which only

very limited structural damage has occurred. The basic vertical and lateral force

resisting system of the building retain nearly all of their pre-earthquake strength and

stiffness. The risk of life threatening injury as a result of structural damage is very low

although some minor damage repair may be required prior to re-occupancy.

3.5.4 Damage Control Structural Performance Range (S-2)

Structural Performance Range S-2, Damage Control, may be defined as the

continuous range of damage states between the Life Safety Structural Performance

43

Level (S-3) and Immediate Occupancy Structural Performance Level (S-1) defined in

Table A1 to Table A3.

Design for the Damage Control Structural Performance Range may be desirable to

minimize repair time and operation interruption, as a partial means of protecting

valuable equipment and contents, or to preserve important historic features when the

cost of design for immediate occupancy is excessive.

3.5.5 Life Safety Structural Performance Level (S-3)

Structural Performance Level S-3, Life Safety, shall be defined as the post-earthquake

damage state that includes damage to structural components but retains a margin

against onset of partial or total collapse in compliance with the acceptance criteria

specified in FEMA for this Structural Performance Level defined at Tables A1 to A3

in FEMA-356, 2000.

Structural Performance Level S-3, Life Safety, means the post-earthquake damage

state in which significant damage to the structure has occurred but some margin

against either partial or total structural collapse remains. Some structural elements and

components are severely damaged but this has not resulted in large falling debris

hazards, either within or outside the building. Injuries mat occur during the

earthquake; however the overall risk of life threatening injury as a result of structural

damage is expected to be low. It should be possible to repair the structure; however,

for economic reasons this may not be practical. While the damaged structure is not an

imminent collapse risk, it would be prudent to implement structural repairs or install

temporary bracing prior to re-occupancy.

3.5.6 Limited Safety Structural Performance Range (S-4)

Structural Performance Range S-4, Limited Safety, may be defined as the continuous

range of damage states between the Life Safety Structural Performance at Tables A1

to A3 in FEMA-356, 2000.

3.5.7 Collapse Prevention Structural Performance Level (S-5)

Structural Performance Level S-5, Collapse Prevention, may be defined as the post-

earthquake damage state that includes damage to structural components such that the

44

structure continues to support gravity loads but retains no margin against collapse in

compliance with the acceptance criteria specified in FEMA for this Structural

Performance Level defined at Tables A1 to A3 in FEMA-356, 2000.

Structural Performance Level S-5, Collapse Prevention, means the post-earthquake

damage state in which the building is on the verge of partial or total collapse.

Substantial damage to the structure has occurred, potentially including significant

degradation in the stiffness and strength of the lateral-force resisting system, large

permanent lateral deformation of the structure and- to more limited extent-

degradation in vertical- load-carrying capacity. However, all significant components

of the gravity load resisting system must continue to carry their gravity load demands.

The fall of structural debris increases the risk significantly. The structure may not be

technically practical to repair and is not safe for re-occupancy, as aftershock activity

could induce collapse.

3.6 TARGET BUILDING PERFORMANCE LEVELS

Building performance is a combination of both structural and nonstructural

components. Tables A1 to A3 in FEMA-356, 2000 describe the approximate limiting

levels defined for a target seismic demand. These tables represent the physical states

of mathematical calculation of different performance levels.

3.7 RESPONSE LIMITS

To determine whether a building meets a specified performance objective, response

quantities from a nonlinear analysis are compared with limits given for appropriate

performance levels (ATC-40, 1996 and FEMA, 2000). The response limits fall into

two categories (i) Global Building Acceptability Limits and (ii) Element and

Component Acceptability Limits.

3.8 ACCEPTABILITY LIMIT

A given component may have a combination of both force and deformation controlled

actions. Each element must be checked to determine whether its individual

components satisfy acceptability requirements under performance point forces and

deformations. Together with the global requirements, acceptability limits for

45

individual components are the main criteria for assessing the calculated are the main

criteria for assessing the calculated building response.

Figure 3.8: Force deformation action and acceptance criteria

The figure 3.8 shows a generalized load-deformation relation appropriate for most

concrete component. The relation is described by linear response from A (unloaded

component) to an effective yield point B, linear response at reduced stiffness from B

to C, sudden reduction in lateral load resistance to D, response at reduced resistance to

E and final loss of resistance thereafter. The following main points relate to the

depicted load-deformation relation:

Point A corresponds to the unloaded condition. The analysis must recognize

that gravity loads may induce initial forces and deformations that should be

accounted for in the model. Therefore, lateral loading may commence at a

point other than the origin of the load-deformation relation.

Point B has resistance equal to the nominal yield strength. The slope from B to

C, ignoring the effects of gravity loads acting through lateral displacements, is

usually taken as between 5% and 10% of the initial slope. This strain

hardening may have an important effect on the redistribution of internal forces

among adjacent components.

The abscissa at C corresponding to the deformation at which significant

strength degradation begins.

The drop in resistance from C to D represents initial failure of the component.

A

B

C

DE

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25 30 35

Late

ral L

oad

Lateral Deformation

46

The residual resistance from D to E may be non-zero in some cases and may

be effectively zero in others. Where specific information is not available, the

residual resistance usually may be assumed to be equal to 20% of the nominal

strength.

Point E is a point defining the maximum deformation capacity. Deformation

beyond that limit is not permitted because gravity load can no longer be

sustained.

3.9 ELEMENT HINGE PROPERTY

It is known that reinforced concrete does not respond elastically to load level about

half the ultimate value. When an element is stressed beyond elastic limit, due to

inelastic deformation of the materials, the element will continue to deform

disproportionate to its load, this process is called formation of plastic hinge. Hinge

properties of RC members under different loading conditions are likely to be

different. These are discussed in the next sections.

Figure 3.8: Force deformation action and acceptance criteria

The figure 3.8 shows a generalized load-deformation relation appropriate for most

concrete component. The relation is described by linear response from A (unloaded

component) to an effective yield point B, linear response at reduced stiffness from B

to C, sudden reduction in lateral load resistance to D, response at reduced resistance to

E and final loss of resistance thereafter. The following main points relate to the

depicted load-deformation relation:

A

B

C

DE

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25 30 35

Late

ral L

oad

Lateral Deformation

47

Point A corresponds to the unloaded condition. The analysis must recognize

that gravity loads may induce initial forces and deformations that should be

accounted for in the model. Therefore, lateral loading may commence at a

point other than the origin of the load-deformation relation.

Point B has resistance equal to the nominal yield strength. The slope from B to

C, ignoring the effects of gravity loads acting through lateral displacements, is

usually taken as between 5% and 10% of the initial slope. This strain

hardening may have an important effect on the redistribution of internal forces

among adjacent components.

The abscissa at C corresponding to the deformation at which significant

strength degradation begins.

The drop in resistance from C to D represents initial failure of the component.

The residual resistance from D to E may be non-zero in some cases and may

be effectively zero in others. Where specific information is not available, the

residual resistance usually may be assumed to be equal to 20% of the nominal

strength.

Point E is a point defining the maximum deformation capacity. Deformation

beyond that limit is not permitted because gravity load can no longer be

sustained.

3.9 ELEMENT HINGE PROPERTY

It is known that reinforced concrete does not respond elastically to load level about

half the ultimate value. When an element is stressed beyond elastic limit, due to

inelastic deformation of the materials, the element will continue to deform

disproportionate to its load, this process is called formation of plastic hinge. Hinge

properties of RC members under different loading conditions are likely to be

different. These are discussed in the next sections.

3.9.1Concrete Axial Hinge

Concrete axial hinge is formed when the axial load carrying capacity of a section

exceeds its elastic limit. The elastic limit for axial capacity is different for tension and

compression. The limits are explained in figure 3.9.

48

Figure 3.9: Concrete axial hinge properties (FEMA-356, 2000)

Axial hinge features used in analysis are explained below:

Py= Asfy

Pc=0.85 Ac f’c

Slope between points B and C is taken as 10% total strain hardening for steel

Hinge length assumption for Δy is based on the full length.

Point B, C, D and E based on FEMA 273 Table 5.8, Braces in Tension

Point B’= Pc

Point E’ taken as 9Δy

3.9.2 Concrete moment hinge and concrete P-M-M hinge

Concrete moment hinge is formed when the flexural moment carrying capacity of a

section exceeds its elastic limit. The limit of flexural moment capacity and bi-axial

moment with axial load are explained in Fig. 3.10.

0

B

C

D E

0Δy

0

Fy

0

Fbuckling

<-- ----- Compression Tension ------>

49

Figure 3.10: Concrete moment and P-M-M hinge property

P-M-M hinge features used in analysis are explained below-


Θy = 0, since it is not needed.

Points C, D and E based on ATC-40, table 9.6. The four conforming

transverse reinforcing rows of the table are averaged.

My based on reinforcement provided, otherwise based on minimum allowable

reinforcement.

P-M-M curve is for M3 and is taken to be the same as the Moment curve in

conjunction with the definition of Axial-Moment interaction curves.

3.9.3 Concrete Shear Hinge

Concrete shear hinge is formed when the shear carrying capacity of a section exceeds

it elastic limit. The elastic limit for shear carrying capacity for coupling beams

controlled by the flexure and controlled by shear is explained in Fig. 3.11(ATC-40,

1996).

0

B

C

D E

0θy

0

My


50

Figure 3.11: Concrete Shear Hinge Property

Shear hinge feature used in analysis:


Vy= 2 As√fc’ + fyAsd

Points C, D and E based on ATC-40 Table 9.12, Item 2, average of the two

rows labeled “Conventional longitudinal reinforcement” and “Conforming

transverse reinforcement”

3.10 CONCRETE FRAME ACCEPTABILITY LIMITS

To determine the performance of a structure, response quantities from a nonlinear

static analysis are compared with limits for appropriate performance levels. Fig. 3.12

illustrates a generalized load-deformation relation applied in the structural component

a

b

L ate ra l D efo rm ation

Late

ral L

oad

A

B

C

D E

T Y P E - A

c

d

L ate ra l D efo rm ation

Late

ral L

oad

A

B

C

D E

T Y P E - B

F igu re 3 .12 : G enera lized load -d efo rm ation re lation fo r com ponen ts

under the present study. Curve Type A in the Fig. 3.12 has been used when the

deformation is a flexural plastic hinge. Curve type B in the Fig. 3.12 has been used

0

B

C

D E

0Δy

0

Vy

0

Vbuckling


51

when the deformation is inter-story drift, shear angle, sliding shear displacement or

beam column joint rotation.

3.10.1 Hinge Properties for Modeling

Depending upon the longitudinal reinforcement, transverse reinforcement etc.

different hinge properties may be modeled based on the modeling parameters defined

through Tables 9-6, 9-7 and 9-12 in ATC- 40. Different points A, B, C etc. are

defined in Fig. 3.4 of this chapter. For the purpose of the thesis, the ETABS’s built in

default hinge properties of concrete have been assumed. These built-in default hinge

properties are generally based on Tables 9.6, 9.7 and 9.12 in ATC-40.

3.11 PERFORMANCE CRITERIA FOR THE STRUCTURE

3.11.1 Drift

Inter-story drift ratio is defined as the ration of the inter-story drift of the story to

height of that story

Maximum drift ratio is defined as the inter-story drift ratio at the performance point

displacement.

Accepted maximum story drift ratios for different structural performance condition

are given in the table below-

Table 3.12: Allowable maximum story drift ratio (ATC-40)

Structural performance condition Maximum story drift ratio

Immediate Occupancy (IO) 0.01

Life Safety (LS) 0.02

3.11.2 Story Displacement

Story displacement at the performance point is also observed. In case of the structure

not having a performance point, Story displacement at the last step is considered since

the last step is the failure step.

52

CHAPTER 4

PERFORMANCE EVALUATION OF SOFT GROUND

STORY STRUCTURE

4.1 INTRODUCTION The procedure of the non-linear pushover analysis, nonlinear hinge property,

acceptance criteria and equivalent strut modeling for the in-filled panel have been

described in the previous chapter. In this chapter some three dimensional soft story

structure and bare frame of same configuration will be modeled and designed

following the conventional design procedure provided by finite element software

ETABS. Then the effect of infill on the frame which is designed as bare frame will be

investigated. Performance evaluation will be done by push over analysis and results of

story will be compared with conventional bare frame.

4.2 LOADING CONDITION Self weight is calculated automatically by the program. In addition to self weight

1.437×10-3 N/mm2 of floor finish and 2.395×10-3 N/mm2 of live load is applied.

Partition wall load of 0.42 kip per ft is provided uniformly on the beam.

Wind Load is automatically calculated by analysis software as per UBC 94 code.

Wind speed of 130 mph is considered for Dhaka city and exposure A is considered.

Earthquake Load automatically calculated by program as per UBC 94. As per

modification factor R= 8 has been considered. Other coefficient used is –

Seismic zone condition, Z= 0.15 for Zone 2

Structural Importance Co- efficient, I = 1.00

Site Co- efficient, S = 1.5

4.3 MATERIAL PROPERTIES Concrete properties

Modulus of elasticity 20000 N/mm2

Poisson’s ratio 0.13

53

Density 2.4×10-9 Ton/mm3

Unit weight 2.36×10-5 N/mm3

Concrete Strength 27.6 MPa

Infill properties

Density 1.92×10-9 Ton/mm3

Thickness 130mm

Modulus of Elasticity 8273.3 N/mm2

4.4 MODEL GEOMETRY

Buildings are designed as bare frame and effect of infill is investigated on a

frame that is designed as bare frame. As partition load is provided on the beam

the weight of diagonal strut is neglected.

Equivalent struts are pin connected with the column as masonry cannot take

moment.

Struts are capable of taking both tension and compression.

All supports are considered as fixed support.

All structure are assumed to have strong column-weak beam by ensuring that

flexure strength of column shall satisfy the relation ∑ Mc ≥ 1.2 ∑Mg.

Where,

∑ Mc= sum of moments at the center of the joint, corresponding to the design

flexural strength of the columns framing into joint.

∑Mg= sum of moments at the center of the joint corresponding to the design

flexural strength of the girder framing into joint.

4.5 ASSUMPTIONS FOR PUSH OVER ANALYSIS

The following assumptions relate to the pushover analysis of the structure

Moment (M3) and Shear (V2) hinges are considered at the ends of beam

members and moment (M2, M3) and axial (P) is considered at the ends of

column members. Here 2 and 3 specify the axis or directions of the loads. For

column members axis 2 is perpendicular to the line object. The direction of the

54

positive local 2 axis onto the global X-axis is in the same direction as the

positive global X axis. Axis 3 is perpendicular to the line object. The direction

of the positive local 3 axis is determined from applying the right-hand rule

using the direction of the 1 and 2 axis where 1 is along the line object. For

beam members, axis 2 is perpendicular to the line object. The positive local 2

axis points in the same direction as the global Z-axis, upward. Axis 3 is

perpendicular to the line object and is horizontal. The direction of the positive

local 3 axis is determined from applying the right hand rule using the direction

of the 1 and 2 axes where 1 is along the line object (ETABS Manual).

Push over Analysis has been done using the load pattern of equivalent static

load as calculated as per provision of BNBC, 1993.

Gravity load has been considered as the previous pushover cases for each

analysis.

Unload entire structure is selected for distribution of loads when local hinges

fail. When a hinge reaches a negative sloped portion of the stress- strain curve,

the program continues to try to increase the applied load. If the results in

increased strain (decreased stress) the analysis proceeds. If the strain tries to

reverse, the program instead reverses the load on the whole structure until the

hinge is fully unloaded to the next segment on stress- strain curve. At this

point the program reverts to increasing the load on the structure. Other parts of

the may now pick up the load that was removed from the unloading hinge.

Geometric non- linearity (p-Δ effect) is considered with full dead load and 2%

live load.

Horizontal displacement of top most corner node has been selected for

performance monitoring of the roof displacement.

4.6 SOFTWARE USED IN THE ANALYSIS

ETABS Nonlinear version 9.7, extended 3D analysis software for building systems,

product of Computers and Structures, Inc, Berkeley, California, USA is used for

modeling and performance evaluation of the structures.

55

4.7 DESCRIPTION OF THE STRUCTURES CONSIDERED FOR ANALYSIS Table 4.1: Description of the Structure Considered for Analysis Structure

No. Plan Type

No. of

Story

No. of Bays and Span in

Direction X

No. of Bays and Span in

Direction Y

Interior Column

Size (mm)

Exterior Column

Size (mm)

Size of Beam (mm)

1 A 6 4 @ 6m 4 @ 6m 457x457 457x457 305x508 2 A 8 4 @ 6m 4 @ 6m 508x508 508x508 305x508 3 A 10 4 @ 6m 4 @ 6m 558x558 558x558 305x558 4 A 12 4 @ 6m 4 @ 6m 609x609 558x558 305x609

5 B 10 4 @ 6m 3 @ 6m 558x558 558x558 305x558

4 @ 6m

4 @ 6m

PLAN- A

4 @ 6m

3 @ 6m

PLAN- B

Figure 4.1: Plan View Type- A & B

4.8 PERFORMANCE EVALUATION OF THE STRUCTURES

The performances of the structures under seismic condition are evaluated for the bare

frame and soft story condition using push over analysis. Their performances are then

compared.

56

Notifications:

RSD= Reduced Seismic Demand

CC= Capacity Curve

EDRS= Earthquake Demand Reduced Spectrum (ADRS format)

After analysis in case where structure collapses before reaching performance

point, the value of βeff is taken from the step where base shear reaches its

maximum i.e. from the collapse state.

In case of soft story structure, if fails to meet performance point, base shear

and hinge status are considered from collapse state for comparison.

4.8.1 Performance Evaluation of the Structure 1

Table 4.2: Effective damping and spectral reduction factor for structure 1

Frame Type Bare Frame Soft Story (40%

infill)

Effective Damping, βeff 10.40% 10.80%

Spectral Reduction Factor, SRA 0.762 0.749

Spectral Reduction Factor, SRv 0.818 0.809 Seismic State Co-efficient, CA 0.3 Seismic State Co-efficient, CV 0.5


condition

0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100 150 200 250 300

Spec

tral

Acc

eler

atio

n, S

a (g

)

Spectral Displacement, Sd mm

CC 40% infill

EDRS 40% infill

EDRS Bare Frame

CC Bare Frame

57



infill)

Effective Damping, βeff 10.40% 10.90% Spectral Reduction Factor, SRA 0.762 0.747 Spectral Reduction Factor, SRv 0.818 0.808 Seismic State Co-efficient, CA 0.3 Seismic State Co-efficient, CV 0.5

Figure 4.3: Comparison of capacity spectrum of structure 1 for different infill condition


Frame Type Bare Frame Soft Story

(80% infill)



Spectral Reduction Factor, SRv 0.818 0.795

Seismic State Co-efficient, CA 0.3

Seismic State Co-efficient, CV 0.5

0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100 150 200 250 300

Spec

tral

Acc

eler

atio

n, S

a (g

)

Spectral Displacement. Sd (mm)

CC 60% infill

EDRS 60% infill

CC 60% infill

EDRS Bare Frame

58


condition

Structure 1 is a six storied structure. Detailed configuration is given in section 4.7

under this chapter. The capacity spectrum of the structure is shown in figure 4.2 to 4.4

and table 4.2 to 4.4. From evaluation, it has been found that capacity of bare frame

meets demand but structure has to deform a considerable amount to meet the demand

curve. As a result some of its elements are stressed above their elastic limit and

elements become nonlinear. Fig. 4.2 to 4.4 describes the fact more clearly. It has also

been observed in case of soft storied structure with the increase in infill the value of

βeff increases but capacity spectrum never meets the demand curve. Columns of the

ground floor collapse before reaching the demand result the failure of the structure.

Table 4.5: Base Shear Comparison among different methods for structure 1

Base Shear (KN) Bare Frame 40% infill 60% infill 80% infill

ESFM Method 2104 2104 2104 2104

RSM Method 3299 3760 4234

Push Over Method 3797 5570 5789 5958

Table above shows, the more the presence of infill above the ground story the more is

the value of base shear because story above are stiffer than the ground floor.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100 150 200 250 300

Spec

tral

Acc

eler

atio

n, S

a (g

)

Spectral Displacement. Sd mm

CC 80% infill

EDRS 80% infill

CC Bare Frame

EDRS Bare Frame

59

Figure 4.5: Comparison of story shear observed from different method

From table 4.6, it is found that for bare frame structure develops limited number of

hinges beyond life safety performance level at performance point. The base shear

developed at performance point is less than that of the soft story structure because of

no active member in form of diagonal strut.


developed up to performance point

In-fill Condition of Frame

Base Shear (KN)

Status of Hinge Formation at Different Performance Stages

A-B B-IO IO-LS LS-CP CP-C C-D

D-E

>E Total

Bare Frame

3797 1020 205 68 175 0

2 0 1470

Soft story (40% infill)

5570 1049 300 35 85 0 0 1 0 1470


5788 1072 292 21 81 0 0 4 0 1470


5958 1105 261 19 79 0 2 4 0 1470

0

1000

2000

3000

4000

5000

6000

7000

Base

She

ar, K

N

No infill 40% 60% 80%

ESFM

RSM Method

Push Over Method

60

The following figure 4.6 shows the displacement of structure 1 at different percentage

of infill present. The deformation of soft story is not uniform and also maximum.

Figure 4.6: Comparison of displacement (mm) in X direction

The following figure 4.7 shows the drift at different story level of structure 1 having

different infill percentage.

Figure 4.7.1: Comparison of maximum total drift ratio in X direction of structure 1

0

1

2

3

4

5

6

7

0 50 100 150 200 250 300

No

of s

tory

Displacement (mm)

60% infill

80% infill

40% infill

Bare Frame

0

1

2

3

4

5

6

7

0 0.005 0.01 0.015 0.02 0.025

No.

of S

tory

Drift in X

40% infill

Bare Frame

IO

LS

61



Figure 4.7 shows that the drift of the soft ground story is above the level of immediate

occupancy and also non uniform along both directions. Difference of performance

level in bare and soft story is due to stiffness irregularity.

0

1

2

3

4

5

6

7

0 0.005 0.01 0.015 0.02 0.025

No.

of S

tory

Drift , X

60% infill

Bare Frame

IO

LS

0

1

2

3

4

5

6

7

0 0.005 0.01 0.015 0.02 0.025

No

of S

tory

Drift in X

80% infill

bare frame

IO

LS

62

a) Exterior Frame (Bare) b) Interior Frame (Bare)

c) Exterior Frame (Bare) d) Interior Frame (Bare)

Figure 4.8: Deformation pattern of structure 1 at performance point

4.8.2 Performance Evaluation of the Structure 2 Table 4.7: Effective damping and spectral reduction factor for structure 2

Frame Type Bare Frame Soft Story (40% infill)

Effective Damping, βeff 11.50% 11.90% Spectral Reduction Factor, SRA 0.727 0.718 Spectral Reduction Factor, SRv 0.791 0.785 Seismic State Co-efficient, CA 0.3 Seismic State Co-efficient, CV 0.5

63



Frame Type Bare Frame Soft Story

(60% infill)







0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100 150 200 250 300

Spec

tral

Acc

eler

atio

n, S

a (g

)


SS 40% infill

EDRS SS 40% infill

EDRS Bare Frame

CC Bare Frame

0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100 150 200 250 300

Spec

tral

Acc

eler

atio

n, S

a (g

)


CC SS 60% infill

EDRS 60% infill

EDRS Bare Frame

CC Bare Frame

64



infill)







condition

Structure 2 is an eight storied structure. Detailed configuration is given in section 4.7

under this chapter. The capacity spectrum of the structure is shown in figure 4.9 to

4.11 and table 4.7 to 4.9. From evaluation, it has been found that capacity of bare

frame meets demand but structure has to deform a considerable amount to meet the

demand curve. As a result some of its elements are stressed above their elastic limit

and elements become nonlinear. Fig. 4.9 to 4.11 describes the fact more clearly. It has

also been observed in case of soft storied structure with the increase in infill the value

of βeff increases but capacity spectrum never meets the demand curve. Columns of the

ground floor collapse before reaching the demand result the failure of the structure.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100 150 200 250 300

Spec

tral

Acc

eler

atio

n, S

a (g

)


SS 80% infill

EDRS 80% infill

EDRS Bare Frame

CC Bare Frame

65

Table 4.10: Base Shear Comparison among different methods


ESFM Method 1823 1823 1823 1823

RSM Method 4494 5021 5566

Push Over Method 5570 7420 7816 8136

Table above shows, the more the presence of infill above the ground story the more is the value of base shear because story above are stiffer than the ground floor.






0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Base

She

ar (K

N)

NO infill 40% 60% 80%

ESFM

RSM

Push over

66



Infill Condition of Frame

Base Shear (KN)


A-B B-IO

IO-LS LS-CP CP-C C-D

D-E

>E Total

Bare Frame

5570 1491 94 54 246 0 0 5 0 1890

Soft Story (40% infill)

7420 1544 216 33 96 0 1 0 0 1890


7816 1548 234 18 88 0 0 2 0 1890


8136 1578 207 22 79 0 2 2 0 1890

The following figure 4.13 shows the displacement of structure 2 at different

percentage of infill present. The deformation of soft story is not uniform and also

maximum.


0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250 300 350 400

No.

of S

tore

y

Displacement (mm)

SS 40% infill

SS 80% infill

SS 60% infill

Bare Frame

67

The following figure 4.14.1 - 3 shows the drift at different story level of structure 2

having different infill percentage.



0

1

2

3

4

5

6

7

8

9

0 0.005 0.01 0.015 0.02 0.025

No.

of S

tory

Drift in X direction

SS 40% infill

Bare Frame

IO

LS

0

1

2

3

4

5

6

7

8

9

0 0.005 0.01 0.015 0.02 0.025

No.

Sto

ry

Drift in X

SS 60% infill

Bare Frame

IO

LS

68


Figure 4.14.1- 3, shows that the drift of the soft ground story is above the level of

immediate occupancy and also non uniform along both directions. Difference of

performance level in bare and soft story is due to stiffness irregularity.









0

1

2

3

4

5

6

7

8

9

0 0.005 0.01 0.015 0.02 0.025

No.

of S

tory

Drift in X

SS 80% infill

Bare Frame

IO

LS

Linear (LS)

69


condition

Table 4.13 Effective damping and spectral reduction factor for structure 3








condition

0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400

Spec

tral

Acc

elar

atio

n, S

a (g

)


CC SS 40% infill

EDRS 40% infill

EDRS Bare Frame

CC Bare Frame

0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400

Spec

tral

Acc

elar

atio

n, S

a (m

m)

Spectral Displacement, mm

CC SS 60% infill

EDRS 60% infill

CC Bare Frame

EDRS Bare Frame

70

Table 4.14 Effective damping and spectral reduction factor for structure 3


Effective Damping, βeff 11.20% 11.7



Seismic State Co-efficient, CA 0.3 Seismic State Co-efficient, CV 0.5


Structure 3 is a ten storied structure. Detailed configuration is given in section 4.7





and elements become nonlinear. Fig. 4.15 to 4.17 describes the fact more clearly. It

has also been observed in case of soft storied structure with the increase in infill the

value of βeff increases but capacity spectrum never meets the demand curve. Columns

of the ground floor collapse before reaching the demand result the failure of the

structure.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400

Spec

tral

Acc

elar

atio

n, S

a

Spectral Displacement, Sd

SS CC 80% infill

EDRS 80% infill

EDRS Bare Frame

CC Bare Frame

71



ESFM Method 2104 2104 2104 2104

RSM Method

3299 3760 4234

Push Over 5033 7828 9402 9543










In-fill Condition of

Frame

Base Shear (KN)



D-E

>E Total

Bare Frame 5033 1815 138 117 230 0 5 5 0 2310

Soft Story (40% infill) 7828 1829 213 136 128 0 1 3 0 2310


9402 1892 299 24 93 0 0 2 0 2310

Soft Story (80% infill) 9543 1949 254 18 87 0 1 1 0 2310

0

2000

4000

6000

8000

10000

12000

Base

She

ar (K

N)

NO infill, 40%, 60%, 80%

ESFM

RSM

Push over

72



maximum





0

2

4

6

8

10

12

0 50 100 150 200 250 300 350

No

of S

tory

Displacement, mm

60% infill

Bare Frame

40% infill

80% infill

0

2

4

6

8

10

12

0 0.005 0.01 0.015 0.02 0.025

No

of S

tory

Drift in X

40% infill

Bare Frame

IO

LS

73



Figure 4.20.1- 3, shows that the drift of the soft ground story is above the level of

immediate occupancy and also non uniform along both directions. Difference of

performance level in bare and soft story is due to stiffness irregularity.

0

2

4

6

8

10

12

0 0.005 0.01 0.015 0.02 0.025

No

of S

tory

Drift in X

60% infill

Bare Frame

IO

LS

0

2

4

6

8

10

12

0 0.005 0.01 0.015 0.02 0.025

No

of S

tory

Drift in X

Bare Frame

80% infill

IO

LS

74




infill)







condition



infill)






0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400

Spec

tral

Acc

eler

atio

n, S

a (g

)


CC 40% infill

EDRS Bare Frame

CC Bare Frame

EDRS SS 40% infill

75


condition



infill)







condition

0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400

Spec

tral

Acc

eler

atio

n, S

a (g

)


Series1

EDRS 60% infill

EDRS Bare Frame

CC Bare Frame

0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400

Spec

tral

Acc

eler

atio

n, S

a m

m


EDRS 80% infill

CC SS 80% infill

CC Bare Frame

EDRS Bare Frame

76

Structure 4 is a twelve storied structure. Detailed configuration is given in section 4.7





and elements become nonlinear. Fig. 4.21 to 4.23 describes the fact more clearly. It

has also been observed in case of soft storied structure with the increase in infill the

value of βeff increases but capacity spectrum never meets the demand curve. Columns

of the ground floor collapse before reaching the demand result the failure of the

structure. It is also observed that hinges formed for soft story is more and some of

them reach damage state. As a result the structure fails ultimately. The structure also

fails before developing base shear.



ESFM Method 2255 2255 2255 2255

RSM Method

4494 5021 5566

Push Over Method (KN) 4745 9111 9381 9572








0

2000

4000

6000

8000

10000

12000

Base

She

ar (K

N)

NO infill, 40%, 60%, 80%

ESFM

RSM

Push over

77




Base Shear (KN)


A-B B-IO

IO-LS

LS-CP

CP-C

C-D

D-E

>E Total

Bare Frame 4915 2200 125 124 276 0 2 3 0 2730 Soft Story

(40% infill) 9111 2231 324 70 96 0 1 8 0 2730


9381 2248 329 48 103 0 0 2 0 2730


9572 2274 345 16 86 0 3 6 0 2730



maximum.




0

2

4

6

8

10

12

14

0 100 200 300 400 500

No

of S

tory

Displacement (mm)

Bare Frame

SS 80% infill

SS 60% infill

SS 40% infill

78



0

2

4

6

8

10

12

14

0 0.005 0.01 0.015 0.02 0.025

No

of S

tory

Drift in X

40% infill

Bare Frame

IO

LS

0

2

4

6

8

10

12

14

0 0.005 0.01 0.015 0.02 0.025

No

of S

tory

Drift in X

SS 60% infill Drift

Bare Frame Drift

IO

LS

79



All of the structures investigated above are square in shape. To see the effect of soft

story on building of rectangular shape next investigation has been done. Usually most

of buildings consist of 40% to 60% infill so this investigation has been done on a ten

storied soft story structure containing 60% infill.

Table 4.22: Effective damping and spectral reduction factor for structure 5 in long (X)

and short (Y) direction


infill)






0

2

4

6

8

10

12

14

0 0.005 0.01 0.015 0.02 0.025

No.

of S

tory

Drift in X

Bare Frame

SS 80% infill

IO

LS

80

Figure 4.26: Comparison of capacity spectrum of structure 5 for 60% infill condition

in both long and short direction

From figure 4.26 it is found that capacity curve does not meet demand curve in long

direction but it meets demand curve in short direction. The reason must be the lack of

stiffness in short direction as number of brick infill panel is less.


From the table 4.24, it is observed that number of hinges developed at performance

point have reached in damage state. Hinges form in damage state more in short

direction.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 100 200 300 400

Spec

tral

acel

erat

ion,

Sa

(g)


CC 60% infill (long)

CC 60% infill (Short)

EDRS 60% infill (long)

EDRS 60% infill (short)


Base Shear (KN)



D-E >E Total

Soft Ground Story in

X

7720 1571 143 35 53 0 0 2 0 1804

Soft Ground Story in

Y

7656 1651 60 43 46 0 1 3 0 1804

81

Figure 4.27: Comparison of displacement (mm) in both long and short direction

From the figure above, it is seen that drift of soft story is slightly more in short

direction than in long direction.

Figure 4.28: Comparison of maximum total drift ratio in both long and short direction of structure 5

The value of drift in short direction is slightly more than in long direction although capacity curve in short direction meet the demand curve.

0

2

4

6

8

10

12

0 20 40 60 80 100 120

No

of s

tory

Displacement (mm)

Displacement (long)

Displacement (short)

0

2

4

6

8

10

12

0 0.005 0.01 0.015 0.02

No.

of S

tory

Drift

Drift (long)

Drift (Short)

82

4.9 REMARKS

The results obtained from the evaluations of the structures are presented in previous

sections. The observation results are as follows-

The value of base shear obtain from pushover method is much more than the

value obtained from equivalent static force method or response spectrum

method.

Open ground story buildings are extremely vulnerable to earthquake loading.

In case of seismic loading the capacity curve does not meet the demand curve.

Some structural members face displacement of higher value.

The number of hinges form is more and some of them reach at damage state.

As a result structure suffer collapse before reaching their require demand.

Inter-story drift are very high in soft ground story.

The amount of displacement is very high in soft story. The demand of ductility

is also very high. Stiffness of the above stories is much higher than ground

story.

So it must be a vital concern for the design engineer to provide adequate measures

to achieve acceptable performance in open ground story structure which can be

done by adding strength, stiffness, ductility or any combination of these or

alternatively the demand from the earthquake load can be reduced by adding

damping.

83

CHAPTER 5

CONCLUSION AND RECOMMENDATIONS

5.1 GENERAL

Open ground story has now become a common feature in the modern day multi-

storied apartment buildings to fulfill the demand of parking spaces. As a result

vertical stiffness irregularity develops the soft story mechanism. It is most common

failure mode of RC structures during earthquake. This thesis has been conducted to

find out the actual behavior of the soft story under the seismic activity and to compare

the value of base shear found from different analysis techniques. For this reason a

total of five structures range from six to twelve stories has been used to perform the

pushover analysis. The performances of the structures have been evaluated as per the

procedure of ATC-40, 1996 and FEMA 356, 2002.

5.2 FINDINGS OF THE STUDY The findings of the study can be summarized as follows-

The values of base shear found from different analysis method are compared.

Base shear value found from push over method is very much higher than

equivalent static force method or response spectrum method.

The value of base shear increases with the increase in story height and

percentage of infill.

The seismic performances of buildings having soft ground stories are very

poor during earthquake shaking. In most of the cases structure collapses due

to failure of ground story columns before reaching performance point.

The performance of soft ground story signifies that these are the most

vulnerable type of structure in the event of earthquake. So alternative

measures must be taken to prevent the damage of the structures.

The top and bottom parts of the column are most susceptible point because

strut action of infill imposes a concentrated load at these joints. So special

seismic detailing as recommended in the seismic code should be adopted.

84

The value of inter story drift, displacement and ductility demand are very high

in soft story.

The lateral stiffness of soft story is very less than that of the stories above.

Although the seismic performance of bare frame buildings are better than soft

story structure but these type structures are not present.

The capacity curve of bare frame structure meets the demand curve but it has

to displace a lot more than their elastic limit.

The displacement and inter story drift of bare frame are uniform throughout

the structure because no change in stiffness throughout the story.

5.3 RECOMMENDATIONS FOR FUTURE STUDY

This research has been carried out over reinforced concrete structure of regular

geometry. Number of bays and span remain same from six to twelve stories. The

condition of soft story was considered only at the bottom story. No remedial measures

have been discussed. Considering the limitations, the following recommendations for

future study may be suggested:

The thesis is concentrated only on the reinforced concrete structure. Further

analysis can be done on steel structure.

Effect of the presence of soft story on different level of structure can be

investigated.

Studies can be performed on existing old reinforced concrete structure to

improve their performance against earthquake by adding shear wall, bracing

etc.

Base isolation is an effective technique to reduce earthquake demand. Further

studies can be performed to see the effect of it at the performance point.

The research can be extended considered structures higher than twelve stories

for finding out the appropriate remedial measures for taller structures.

Retrofit design of existing structure can be investigated.

Finding out remedial measures and verification of the selected measures can

be done to extend the study.

85

REFERENCES

ACI Committee 318, (2002) Building Code Requirement for Structural Concrete,

American Concrete Institute, Detroit.

ATC (1996) Seismic Evaluation and and Retrofit of Concrete Building (Volume 1).

Report (ATC-40), Applied Technology Council, California, USA.

BNBC (1993) Bangladesh National Building Code, Housing and Building Research

Institute, Mirpur, Dhaka, Bangladesh.

IS-1893 (2002) Bureau of Indian Standards, Indian Standard Criteria for Earthquake

Resistant Design of Structures-Part-1: General Provisions and Buildings (Fifth

revision), New Delhi, India

Chopra, A.K (2003) Dyanmics of structure- Theory and application of Earthquake

Engineering, Second Edition, Prentice-Hall of India, New Delhi, India.

Computer and Structures Inc. ETABS Nonlinear Version 9.7, California, USA.

FEMA 356, 273. (2002) Pre-standard and Commentary for the Seismic Rehabilitation

of Buildings, Federal Emergency Management Agency, Washington D.C., USA.

Stafford-Smith, B., “ Lateral Stiffness of In-filled Frames” Journal of the structural

Division, ASCE, vol. 92, December 1962.

Stafford-Smith, B., “ Behavior of square In-filled Frames” Journal of the structural

Division, ASCE, vol. 92, December 1966.

Stafford-Smith, B. and C.Carter, “A Method of Analysis for Infill Frames.”

Proceedings of the institution of Civil Engineers, vol. 44, 1969.

86

Amanat, K. M. and Hoque, E. (2006) “A Rationale for Determining the natural

period of RC Building FramesHaving Infill” Engineering Structures, Vol.28, pp. 495-

502.

Rahman, “S.M. Enhancement of Seismic Performance of Soft Ground Story RCC

Structures.” .” B.Sc Engg. Thesis, Department of Civil Engineering, Bangladesh

University of Engineering and Technology.

Shahrin R. (2009) “Seismic Performance Evaluation of RC Buildings by Pushover

Analysis.” B.Sc Engg. Thesis, Department of Civil Engineering, Bangladesh

University of Engineering and Technology.

Tasmim R. (2011), “Effect on Base Shear Under Seismic Load for Masonry Infilled

RC Soft Story Buildings.” .” B.Sc Engg. Thesis, Department of Civil Engineering,

Bangladesh University of Engineering and Technology.

87

APPENDIX

88

89

90

91

92

Manual Calculation

Calculation of Equivalent Strut Width:

Sample calculations of equivalent strut width are shown below-

A frame having 3m vertical height from floor to floor, beam thickness of 508 mm or

20inch is considered. In horizontal direction the center to center distance is 6m and

the column size is 457mm x 457 mm or 18” x 18” in cross section.

Icol= (18x 18^3)/ 12 = 8748 inch^4

H= 120 inch

hm= Clear height from floor top to beam bottom

= 120-20 = 100 inch

Ec = 2900 ksi

Em= 1260ksi

l = 20*12- 20 = 220 inch

t = thickness of the infill wall = 5 inch

D = √ (H^2 + l^2) = 241.66 inch

Tanθ = (100/220) => θ = 24.44 deg.

λ1H = H ((Em t sin 2θ)/ (4 Ec I col hm)) 0.25

λ1H = 3.144

a = 0.175 D (λ1H)-0.4 => a = 26.44 inch

tanθ column = (hm - a/ cosθ column ) / l => θcol = 18.2deg

l column = a/(cosθ column) => l column = 27.8inch A typical calculation of reduced demand spectrum (ADRS format) is done below-

93

Calculation is done for 12 storied structure (Plan A) with 60% infill. βeff = 11.2% (from ETABS)

SRA = (3.21- 0.681ln βeff) / 2.12 = 0.738

SRv = (2.31-0.41ln βeff)/ 1.65 = 0.799

As shown in table 4.18.

CA= SRA* 0.7/2.5 = 0.207

Cv = SRv * 0.5/0.3 = 0.344

Peak, SA= 2.5 CA = 0.525 (g)

Ts = Cv/ (2.5 CA) = 0.665 sec.

Ta= 0.2 Ts = 0.133 sec.

At T = 1 sec , Spectral Acceleration, Sa (g)= Cv/ T = 0.344 (g)s

Spectral Displacement, Sd = T^2* Sa * 9810 /(4* 3.14 ^2) = 85.48 mm

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