EARTHQUAKE VULNERABILITY ASSESSMENT OF … VULNERABILITY ASSESSMENT OF RC FRAMED BUILDINGS Submitted...

EARTHQUAKE VULNERABILITY ASSESSMENT OF RC

FRAMED BUILDINGS

Submitted In the partial fulfillment for the Award of degree of

Masters of Technology in Structural Engineering

Submitted By

Anand Paul (2011 PST 5111)

Supervisors

DEPARTMENT OF CIVIL ENGINEERING

MALAVIYA NATIONAL INSTITUTE OF TECHNOLOGY JAIPUR

June 2013

Dr. S. D. BHARTI Dr. M.K. SHRIMALI

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ii

EARTHQUAKE VULNERABILITY ASSESSMENT OF RC

FRAMED BUILDINGS

Submitted In the partial fulfillment for the Award of degree of

Masters of Technology in Structural Engineering

Submitted By

Anand Paul (2011 PST 5111)

Supervisors

DEPARTMENT OF CIVIL ENGINEERING

MALAVIYA NATIONAL INSTITUTE OF TECHNOLOGY JAIPUR

June 2013

Dr. S. D. BHARTI Dr. M.K. SHRIMALI


iii

CERTIFICATE

This is to certify that the dissertation work entitled “Earthquake Vulnerability Assessment of

RC Framed Buildings” which is being submitted by Mr. Anand Paul in partial fulfillment for

the award of the degree of Master of Technology in Structural Engineering, MNIT, JAIPUR is a

bonafied work done by him under our guidance and supervision.

Date:

Place:

(Dr. M.K. Shrimali)

Department of Civil Engineering

MNIT Jaipur

(Dr. S. D. Bharti)

Department of Civil Engineering

MNIT Jaipur


iv

ACKNOWLEDGEMENT

I express my deep gratitude to my guides Dr. M.K. Shrimali and Dr. S.D. Bharti for their

valuable guidance and instructions during the course of this dissertation. This dissertation report

would not have been possible without solemn guidance & co-operation of people who are involved

directly or indirectly.

(Anand Paul)


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ABSTRACT

Past earthquakes in India and round the globe have shown that soft ground story in the building

causes a serious risk to their integrity and stability. Substantial damages or collapses of numerous

buildings with soft ground story irregularity in recent earthquakes compelled an intense research

effort, employing linear and nonlinear analysis methods to assess the performance and capacity of

such buildings under seismic actions.

Objective of this study is to demarcate the nonlinear behavior of the structures by nonlinear static

pushover to evaluate the limits and efficacy of current design philosophies of analyzing and

designing buildings and also to validate the adaptability and adoptability of open ground story

structures in Indian seismic scenario. A set of three dimensional analytical models with various

structural arrangements, viz. bare frame and infilled frame models are investigated with nonlinear

static pushover, considering various site parameters. The investigation methodology consists of

(a) Creating analytical finite element models representing the building in India

(b) Determining the nonlinear member parameters

(c) Performing the nonlinear pushover analyses on these models

(d) Assessing codal provisions by evaluating the results obtained.

Using these analysis results, the open ground story behavior is scrutinized for studying the scope

and significances of this irregularity in detail. In addition, Indian code is evaluated considering the

soft ground story irregularity and the provisions suggested by code are ascertained.

Keywords: Open ground story, Pushover Analysis, Equivalent Diagonal Strut Model


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Contents CHAPTER 1. .......................................................................................................................................... 1

1.1 Introduction ............................................................................................................................. 1

1.2 Objectives and Scope ............................................................................................................... 2

1.3 Thesis Organization ................................................................................................................. 3

CHAPTER 2. .......................................................................................................................................... 4

REVIEW OF LITERATURE .................................................................................................................. 4

2.1 Soft Storey ............................................................................................................................... 4

2.2 Masonry Infill in RC Frames .................................................................................................... 6

2.2.1 Infill Panels with Openings ............................................................................................. 11

2.3 Nonlinear Static Analysis Methods ......................................................................................... 12

CHAPTER 3. ........................................................................................................................................ 18

SOFT STORY IRREGULARITY.......................................................................................................... 18

3.1 General .................................................................................................................................. 18

3.2 Soft Story Behavior ................................................................................................................ 18

3.3 Indian Earthquake Code ......................................................................................................... 20

CHAPTER 4. ........................................................................................................................................ 21

NONLINEAR PUSHOVER ANALYSIS .............................................................................................. 21

4.1 Introduction ........................................................................................................................... 21

4.2 Nonlinear Behavior & Plastic Hinge formations in Structural Elements .................................. 21

4.3 Analysis ................................................................................................................................. 23

CHAPTER 5. ........................................................................................................................................ 25

EQUIVALENT BRACED FRAME METHOD ..................................................................................... 25

CHAPTER 6. ........................................................................................................................................ 29

ANALYTICAL MODELS .................................................................................................................... 29

6.1 General .................................................................................................................................. 29

6.2 Description of Analytical Models ........................................................................................... 29

6.3 Modeling of Plastic Hinges .................................................................................................... 35

6.3.1 RCC Frame Members ..................................................................................................... 35

6.3.2 Infill Wall Panels ............................................................................................................ 38

CHAPTER 7. ........................................................................................................................................ 41

RESULTS AND DISCUSSIONS ...................................................................................................... 41

7.1 Bare Frame ............................................................................................................................ 41


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7.2 Open First-Story Frame .......................................................................................................... 42

7.3 Fully Infilled Frame ............................................................................................................... 44

7.4 Effect of Codal Provisions ...................................................................................................... 46

7.5 Discussions ............................................................................................................................ 49

7.6 Failure Pattern ........................................................................................................................ 49

7.7 Summary and Conclusions ..................................................................................................... 50

CHAPTER 8. ........................................................................................................................................ 52

REFERENCE ........................................................................................................................................ 52


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Figures Figure 1 : Open ground story building that has been collapsed in an earthquake due to soft story

behavior........................................................................................................................................... 2

Figure 2 Excessive drift in soft ground story under lateral loading .............................................. 19

Figure 3 Collapse mechanism of a building structure having a soft story .................................... 19 Figure 4: The generalized load deformation relation for nonlinear response ............................... 21

Figure 5: Acceptance criteria on a force versus deformation diagram. ........................................ 22

Figure 6 : A pushover curve of a building structure. .................................................................... 23

Figure 7 : Equivalent Strut Model (a) Masonry Infill Panel (b) Strength Envelope of Masonry

Panel .............................................................................................................................................. 25

Figure 8 : Idealized Pin jointed compression strut model............................................................. 27

Figure 9: Compressive stress-strain curves for masonry .............................................................. 28

Figure 10 Plan of the model representing point of application of pushover loads ....................... 31

Figure 11 : Bare frame model ....................................................................................................... 32

Figure 12 Open Ground story model with diagonal struts ............................................................ 33

Figure 13 : Fully infilled structure with equivalent single struts .................................................. 34

Figure 14 : Idealized M-θ Curves for Hinge Definitions .............................................................. 36

Figure 15: Idealized P-M interaction curve for columns .............................................................. 36

Figure 16: P-M Interaction curve of RC column .......................................................................... 37

Figure 17 : Load Displacement plot for defining Axial Hinges in diagonal struts ....................... 38

Figure 18: Calculation of lateral strength of masonry-infill walls ................................................ 39

Figure 19 : Plastic Hinge Assignments ......................................................................................... 39


ix

Figure 20 : Section Assignments .................................................................................................. 40

Figure 21 : Pushover curve for Model 1 ....................................................................................... 41

Figure 22 : Hinge formation pattern at Ultimate load. .................................................................. 42

Figure 23: Pushover curve for Model 2 ........................................................................................ 43

Figure 24: Hinge formation pattern at Ultimate load.................................................................... 44

Figure 25: Pushover curve for Model 3 ........................................................................................ 45

Figure 26: Hinge formation pattern at Ultimate load.................................................................... 46

Figure 27 : Ground storey members designed for code-specified factors. ................................... 46

Figure 28: Pushover curve for Strengthened Model ..................................................................... 47

Figure 29: Hinge formation pattern at ultimate load .................................................................... 48

Figure 30 : Ground storey Columns designed for code-specified factors .................................... 48

Tables

Table 1 : Moment – Rotation values used for defining Plastic Hinges ........................................ 37

Table 2 : Pushover Summary ........................................................................................................ 49


x

Notations

Notations Description

Vy Lateral force corresponding to yield in Masonry

Vm Maximum post-yield lateral force of Masonry

t The thickness or out of plane dimension of the infill panel

Inclination of Strut at base to horizontal

'mf Masonry prism strength

'm Strain corresponding to prism strength

s Shear strength or cohesion of masonry

dA Area of equivalent strut

dL Length of equivalent strut

c Upper bound or failure normal contact stresses at the column-

infill interface

cf The compressive strength of the masonry

af Compressive stress of infill in its central region

b Contact shear stresses at the beam-infill interface

'h

Height of infill panel

'l Length of the infill panel

We Width of equivalent strut

dm The length of the infill diagonal

Vb Total design base shear

W Seismic weight of the whole building

Ah The design horizontal acceleration spectrum

Z The seismic zone factor

I Importance factor

2R The response reduction factor

aS

g The average response acceleration coefficient


xi

ifT Fundamental natural period

h The total height of the main structure

d Maximum base dimensions of the building along the considered

direction of seismic force

c Neutral axis depth at ultimate moment

θy Yield rotations

θu Ultimate rotations

E Modulus of elasticity

I Moment of inertia

u & y Idealized ultimate and yield curvatures of the section respectively

pl Length of the plastic hinge

Py Idealized yield force in the axial direction

Pm Idealized maximum force in the axial direction

vy Displacement corresponding to Py

vm Displacement corresponding to Pm


1

CHAPTER 1.

1.1 Introduction

Open first story is a typical feature in the modern multistory constructions in urban India. The

parking and commercial areas with higher story heights and less infill walls reduces the stiffness

of the lateral load resisting system at ground story and progressive collapse becomes obvious in a

severe earthquake for such buildings. Though functional planners applaud open ground storey

models in order to facilitate the necessity of parking space in residential and commercial buildings,

such features are critically undesirable in buildings built in seismically active areas. Open ground

floor with increased column height outcomes soft-ground storey, which has been proved to be

highly vulnerable during past major ground motions, Kobe (1994), Bhuj (2001). Jabalpur

earthquake of 22 May 1997 (Jain, et al, 1997) also illustrated the handicap of Indian buildings with

soft first storey. This earthquake was the first one in an urban neighborhood in India and provided

a cause to assess the performance of engineered structures in the country during ground shaking.

The susceptibility of open storey buildings to earthquake induced damages on one hand and the

functional necessity on the other hand marks open ground storey an unavoidable, moreover a

challenging task for structural engineers.

Masonry infill in RC frames acts as a diaphragm in vertical plane that imparts significant lateral

strength and stiffness to RC frames under lateral loads. Infilled frames are also substantially

stronger, but less deformable, than identical bare frames. In symmetric structures with continuous

infilled frames along height, the better stiffness and strength can protect a building from damage

due to excessive lateral drift or insufficient strength. Due to higher stiffness, infill panels could

attract considerably greater forces that can lead to premature failure of panels, and probably of the

whole building. Hence, it is necessary for designers to take into account the effects of infills

masonry in the design of RC structures. The amplified displacement, force and ductility demands

in the ground storey columns in such constructions are neglected when analyzed and designed as

bare frames. Hence, it is of utmost concern to validate the adaptability and performance of open

ground story under the possible action of earthquakes. In order to prevent soft story collapse

mechanisms in the building structures, seismic demands must be determined accurately.


2

Figure 1 : Open ground story building that has been collapsed in an earthquake due to soft story behavior.

1.2 Objectives and Scope

Inelastic behavior of a structure during an earthquake is quite important, which can be achieved

by determining the displacement and ductility demands of a structure. If these demands are not

assessed accurately during the design or assessment phase of the building structure it may lead to

concentrated or progressive collapse in a severe earthquake. In the evaluation of irregular

structures, especially building with open ground stories becomes more important as they have been

seriously damaged or collapsed in the earthquakes due to their special collapse mechanisms. The

soft story irregularity, which is one of the most hazardous vertical irregularities, is investigated in

this study. The main objectives of this study can be listed as follows:

Determination of the nonlinear behavior of building structures with open ground story

utilizing nonlinear static pushover techniques, for various deformation levels.

Evaluation of accuracy and efficacy of current design methodologies involving the design

of open ground story structures, idealized as bare frame structures.

Validation of provisions for magnification factor of 2.5 defined in IS1893 for the design of

columns and beams of open ground story.


3

Three-dimensional RC analytical models are formed and designed conforming to IS 456. These

models are then assessed by exploiting nonlinear static pushover analyses techniques with various

lateral load patterns. The results obtained from analyses are evaluated, considering performance at

various deformation levels, which define the performance of the structures.

1.3 Thesis Organization

This thesis is organized into seven chapters and six appendices. Chapter 1 includes the general

summary of the study. Chapter 2 elucidates previous research on the nonlinear pushover analysis,

analysis of structures with infill panels and soft story behavior in open ground story buildings. In

Chapter 3, the soft story irregularity is explained in the context of open ground story buildings and

code provisions are also described. Detailed explanations on nonlinear member behavior used in

the study and nonlinear pushover method are explicated in Chapter 4. Advanced modeling of infills

using equivalent strut is explained in chapter 5. In Chapter 6, the detailed information about

The analytical models adopted in the analyses

Pushover lateral load patterns

Hinge definitions and

Details of infill idealization,

Which are performed on the model buildings is specified. The assessment and comparison of the

analyses results are given in Chapter 7. Finally, Chapter 8 contains references.


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CHAPTER 2.

REVIEW OF LITERATURE

The study investigates a full scale multi storey structure with soft ground storey considering the

structural action of infill walls. The literature review has been divided into three parts, specifically,

current advancement in studies on soft storey building and theories on structural behavior of infill

wall panels and their analysis and finally nonlinear push over analysis techniques used for analysis

of structure.

2.1 Soft Storey Structures

Open first storey is an unavoidable feature in many urban multi storey buildings in India. Several

earthquakes in the past, e.g., San Fernando 1971, Northridge 1994, Kobe 1995, have demonstrated

the potential hazard associated with Soft Storey buildings. Major damages in reinforced concrete

and steel buildings in the Hyogoken-Nanbu earthquake, 1995 (AIJ, 1995) [1], and critical hospital

facilities in the San Fernando earthquake of 1971, were attributed to the soft first storey. Alarming

amount of damage to the buildings with open basements for parking has been reported during the

Northridge earthquake of January 17, 1994 [2]

In a developing country like India open ground storey serves increased parking and aesthetic

demands in residential structures. The Indian Standard Code IS1893 (Part I 2010) defines “Soft Storey” as “one in which the lateral stiffness is less than seventy percent of that in the storey above or less than eighty percent of the average lateral stiffness of the stories above”. Indian Standard

also prescribes that “the columns and beam of soft storey are to be designed for 2.5 times the storey shear and moments under seismic loads. As a matter of fact, most Indian buildings, with no

masonry infill walls in the first storey, belong to “buildings with soft first storey.” Though soft

storey is considered as a separation between the ground and the super structure, it reduces stiffness

and increases the ductility demand of the elements under seismic loading and reversal of stresses.

The dynamic response of soft ground storey under earthquake loading is still a topic discussion

and debate among researchers.

The total seismic base shear as experienced by a building during an earthquake is dependent on its

natural period; the seismic force distribution is dependent on the distribution of stiffness and mass

along the height. In structures with open first storey and higher storeys being stiff undergo lesser

inter-storey drifts. Yet, the inter-storey drift in the ground first storey is large. Strength demands

on these columns are also large, as the shear in the first storey is concentrated. For the higher

storeys, the forces in the columns are effectively reduced. Due to the presence of the structures

with abrupt variations in storey stiffness, have irregular lateral force distribution along the height,

which causes locally induced stress concentration, which can adversely affect the performance of


5

structures during ground shaking. Such buildings are required to be analyzed by the dynamic

analysis and designed carefully. Enhanced design of weak/soft-story frame members is done in

different national codes based on empirical or semi-empirical relations. Inadequate literature is

available in support of these relations, demanding an urgent need for more research in this area.

Flexible first storey concept was first proposed by Marta (1929) and was studied further by Green

(1935) and Jacobsen (1938), and commented that soft first storey columns are to be designed to

yield during an earthquake, such that shear forces transmitted to the rest of the structure are

reduced. In such structures, stiff ground storey attracts the seismic energy and upper stories are

less vulnerable to shear forces. This increases the ductility demand strength demand of the ground

storey. Recently Ari Wibowo et. al. [3] explored many features about soft-storey buildings and

their collapse mechanisms, from experimental field study of a five storey building in Melbourne,

concluded that column of the soft-storey are susceptible to large lateral displacements. They also

claimed that there is a significant contribution from the ground storey slab to the lateral stiffness

of the column. Their seismic performance evaluation using a displacement based assessment

showed that the precast soft storey structure would perform satisfactorily in regions of lower to

moderate seismicity, whilst the performance would be compromised in regions of high seismicity,

where catastrophic collapse through p-Δ effects could be expected due to the excessive

displacement demands. In addition, an in situ reinforced concrete system would perform

significantly worse due to the reduced drift capacity associated with column flexure deformations

rather than rigid body rocking of the precast system.

Y.L Mo and Y.F. Chang (1993) found that soft storey system of base isolation have drawbacks,

unless used with sliding bearing system during ground motions. Arlekar, Jain and Murty [4]carried

out a study to evaluate error involved in modeling such buildings as complete bare frames, ignoring

the influence of infills in the upper storeys. An example building with different analytical models

were studied and stiffness balancing methods were proposed and it was concluded that stiffness

demand of open ground storey can be fulfilled by providing stiffer columns in the open ground

storey or by providing a concrete service core in the building, to serve the lateral drift demand. Fardis [5]carried out study on the design of infilled RC structures, following conclusions were

made; Overall influence of infills are found to be beneficial when infills with regular distribution

is adopted; provisions of EC8 were found to be too conservative while designing the structure as

bare suffices (their main effect is on energy dissipation and not response period); The only adverse

effect for regular structures is a tendency for drift concentration in the bottom storey, though

deformations in that storey are well below that required for a soft storey mechanism. For structures

with an open bottom storey, the concentration of drift and structural damage of the columns

become significant. In structures with infilled panels along two adjacent sides, the response can be

quasi -rotational. As a result the far corner columns may need to be designed for the simultaneous

peaks for the two directional components. In all other respects the RC frame may be designed as

bare.


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2.2 Masonry Infill in RC Frames

RC framed buildings are usually analyzed and designed as bare frames, neglecting the strength

and stiffness contribution of the infill walls. The behavior of infill frames and bare frames are

different under the action of seismic lateral loading. Understanding the response and behavior of

infills under earthquake makes it possible to exploit the benefits of infills in a rational manner. In

addition, construction of infill is cheaper because it uses locally available material. It has good

sound & heat insulation and waterproofing properties, ensuing in better occupant comforts and

economy. It has been realized by many researchers that the infill panels have significant effects on

the structural and mechanical behavior of RC frames; hence, it’s been considered that infill

changes the stiffness, ultimate capacity and failure mode of RC framed structures to a considerable

extent. Masonry infill walls confined by reinforced concrete RC frames on all four sides play a

vital role in resisting the lateral seismic loads on buildings.

Since masonry infill increases the strength, lateral stiffness and dead weight of a framed structure

it will also change the dynamic characteristics of the structure. It has been proven that infill causes

considerable increase in global stiffness. On the other hand, potentially negative effects may occur

such as torsional effects induced by in plan-irregularities, soft-storey effects caused by

irregularities and short column effects, Nollet and Smith, [6]. Even though the effect of infill is

ignored by structural engineers, it cannot always end up with a conservative design because

1. Infill panels changes the structural natural period, and

2. Extensive increase of axial force in columns and beams due to diagonal strut behavior

leading to lack of ductility capacity in structure.

Simplified macro models (equivalent strut models) are formed, capable of replacing commonest

failure modes of failure in masonry panels, which should be the most practical concern of an

engineer.

An early contribution to understanding the complex nature of masonry infill frames was introduced

in 1956 by Polyakov [7] with a concept of equivalent struts in which infill panels was considered

to behave as a braced frame with diagonal compression struts. He proposed that stresses from

frame to the infill are only transferred in the compression zone of the infill-frame interface, with a

distribution more like a diagonally braced system than a homogenous shear wall. B.Srinivas and

B.K.Raghu Prasad [8]studied response of a five story reinforced masonry infill and bare frame

building model, designed according to IS 1893 codal provisions under three strong motion records

from Imperial Valley (1979), Northridge (1994) and San Fernando (1971) earthquakes. The

building the infill walls are modeled by equivalent strut approach and the bottom storey of the

building kept openly for considering the realistic behavior of the presently existing buildings in

India. Nonlinear static and nonlinear dynamic analyses were performed to study the response

behavior of the buildings. Results showed that presence of infill walls greatly contributes the

stiffness to lateral loads and the deviation in storey response quantities (displacement, storey shear)


7

are decreasing due to the infill masonry walls but the response quantities at the soft storey level is

significantly large. These effects, however, were not found to be significant in the bare frame

model. It was also found that the location of plastic hinges concentrated at bottom stories causes

severe structural damage in infilled frame structure at first storey but in the case of bare frame

model hinges spread throughout the height and influence of infill walls on static and dynamic

behavior of structures causes a decrease in storey shear and inter storey drifts.

Hossein Mostafaei and Toshimi Kabeyasawa [9]studied the Effect of Infill Masonry Walls on the

Seismic Response of Reinforced Concrete Buildings subjected to the Bam Earthquake. The

response simulations were performed for different categories of bare frame and infilled frame. A

method was developed to model infill walls with or without openings. The outcomes of the

analyses were compared to damage and residual cracks observed on the masonry infill walls and

reasonable correlations were obtained between analytical and observed results. It was concluded

that the presence of masonry infill walls is the main reason for the nearly linear responses of the

building during the earthquake.

Tso-Chien Pan et. al [10]carried out full-scale ambient vibration test on a typical high-rise

residential building and it was found that adding the brick partition walls increases the natural

frequencies of a building, from 0.71 to 1.75 Hz when brick walls were aligned along the

longitudinal direction, since the stiffness in the longitudinal direction was increased dramatically.

Thus, the longitudinal direction becomes the stiffer direction, and the fundamental mode of the

building switches to the transverse direction, which was consistent with the observation made

during the ambient vibration tests. The modal frequencies and the mode shapes of the model with

the brick infill walls match well with some of the building dynamic characteristics measured.

Therefore, in order to achieve a good correlation between the numerical and the experimental

results, the non-structural brick infill walls need to be included in the model. It was also suggested

that that plane stress elements can model the brick infill walls well in the small strain situation.

Kaushik [11] made comparative study among the different seismic codes and found inconsistency

in the consideration of infill and reported that most codes do not consider infill due to its brittle

nature of failure and lack of adequate information. The validity of different macro-models

consisting of 4- node shear panels, 4-node plane stress element and the higher order 8-node plane

stress element were studied by Doudoumis and Mitsopoulou [12] and reported inaccuracy in

results of macro models. Singh, Paul et al. [13] had developed a method to predict the formation

of plastic hinges and cracks in the infill panels under static and dynamic loads by using 3-noded

frame element, 8-noded isoparametric element and 6 noded interface element for frame member,

infill panel and the interface element respectively.

The study showed good agreement with the experimental results, especially in terms of failure load

and the strut width. Doudoumis [14]studied the importance of contact condition between the infill

and frame members on a single storey Finite element model. It was reported that the friction


8

coefficient, interface condition, size of the mesh, relative size of infill wall, relative stiffness of

beam to column have significant influence on the response of infilled frame, while the effect of

orthotropy of infill material was insignificant. The stress pattern within the infill also improved,

with maximum values of stresses at the compressive corners when the mesh density was made

finer. The existence of friction coefficient at the interface was reported to increase the lateral

stiffness of the system. However, friction coefficient is dependent on the quality of material and

the workmanship CEB 1996 which is difficult to define accurately, hence codes do not provide

any guidance. Moghaddam and Dowling [15] reported the high initial stiffness and low

deformation capacity of infill. Merabi [16] reported significant improvement of lateral stiffness,

strength and energy dissipation capability of infilled structures from the analytical and

experimental studies.

Kappo and Ellul (2000) carried out a study evaluating the effect of applying EC8 to RC structures

with infill panels. The study determines that EC8 is over conservative by disregarding the

contribution to strength of the infills. It is proposed that design of frames be based on models

which include infill elements using two different stiffness assumptions. Base shear should be

calculated assuming the secant stiffness at peak load for the infill panels. Element actions should

then be found assuming a lower stiffness of infills (approx. one third). Combescure and Pegon

(2000) carried out numerical studies and a testing programme on infilled frame structures. Both

micro (panel element) and macro (strut element) models were considered. The modeling showed

the validity of the diagonal strut model and highlighted the importance of identifying appropriate

strut parameters. The study established that an effective strut width of approximately 25% of the

diagonal length was appropriate for the cracked stiffness and stiffness at maximum strength.

Though a concentric strut was used, micro-modeling indicated a concentration of shear at the end

of the columns, indicating that an eccentric strut model would be required for the detailed

evaluation of member actions. Bruno et al [17] carried out a study on the seismic performance of

pre-code RC buildings, including the effects of infill panels. Masonry infills were modeled using

concentric equivalent struts. The study indicated that the presence of continuous infill panels

significantly enhances the performance of the pre-code buildings.

On account of high initial stiffness, the change in structural behavior from frame action to truss

action was studied (2000). The study found the influence of masonry infills to be beneficial. Infill

panels increase strength, stiffness, energy dissipation and overall ductility of the building. Further,

they dramatically decrease the deformation and ductility demand on RC frame members. These

buildings therefore perform well in moderate earthquakes. Detrimental effects of infills, such as

short column effect, soft-storey effect, and torsion, are however a concern. Consequently,

structural member forces in the beams and columns of an infilled structure are reduced.

D. K. Bell and B. J. Davidson [18] attempted to evaluate the earthquake risk in reinforced concrete

frame building with brick infill panels on the exterior walls. An eccentric strut infill model was

analyzed on ETABS and seismic performance of the building was studied complying with NZSEE


9

and FEMA-273 guidelines and it was found that the performance of the building to be satisfactory

for the design earthquake. The ratio of elastic shear force in a panel to the expected shear strength

was typically in the range of 1 to 2, indicating that moderate damage would be likely. They also

suggested that infill panels, present in regular arrangement, have a significant beneficial influence

on the behavior of RC buildings and infill masonry panels have a detrimental influence on the

behavior of buildings due to soft storey effects but due to stiffness, strength, and damping effects

of infill panels, deformations are below that required for a soft storey mechanism.

Fardis [19] investigated the seismic response of an infilled frame which had weak frames with

strong infill material and reported the strong infill is responsible for earthquake resistance of weak

reinforced concrete frames. Negro and Colombo [20] investigated the effects of irregularity

induced by non-structural masonry wall on a full scale four storey RC structure under pseudo-

dynamic loads and observed changes in the behavior of frame due to infill. The irregular

distribution of infill has been reported to impose unacceptably high ductility demand on the frame

buildings. Al-Chaar [21] performed studies on the behavior of infilled RC frames. The frames were

reported to have shown the ductile behavior but the extent of ductility is not specified. However,

he concluded that the infill wall improves the stiffness, strength and energy absorption capacity of

structures which will be useful for seismic structures. Dolsek and Fajfar [22] carried out pushover

analysis on a four storey structure and reported total change in distribution of damages within the

structure. However, the presence of infill did not cause the shear failure of columns, which is

contrary to literature suggested by Pauley & Priestley [23]. Amanad [24] reported that the amount

of infill has significant influence on the fundamental period of the structure; however

recommended pursuing further study in this field. Kose MM [25] conducted a study on the

parameters affecting the natural period of the infilled frames. The Equivalent diagonal strut was

used as the infill panels and opening was considered by varying the width of struts proposed in

separate study (Asteris [26]). The height of the structure and the amount of shear wall were

reported to be the main influencing parameters. A soft-storey issues associated with infilled

structures was studied (Santhi, Knight et al. 2005) on a single bay three storey RC frame which

had no opening in infill panels. The natural frequency of the soft structure was decreased by 30%

while the shear demand was increased by 2.5 times of the bare frame. The bare frame structures

behaved in flexure mode while the soft structure behaved in shear mode. However, the author has

not considered the opening as the presence of it may reduce shear force. Most of the past research

has considered simple single storey systems or diagonal strut models for the infill, ignoring

openings which are normally present. The possibility of the infill having a wide range of properties

has also been treated. It is thus evident that there is inadequate research on infills.

An experimental program was carried out by Al-Chaar [27] to evaluate the behavior of five half-

scale, laboratory models with single-story and different numbers of bays and results indicated that

infilled RC frames exhibit significantly higher residual strength, ultimate strength, and initial

stiffness than bare frames without reducing any ductility in the force–deflection response.

Additionally, the number of bays seems to be influential with respect to the failure mode, peak and


10

residual capacity and shear stress distribution. Moghaddam and Dowling [15] experimentally

showed that masonry infill walls have a very high initial lateral stiffness and low deformability.

Murty and Jain [28] established that introduction of infills in RC frames changes the lateral load

transfer mechanism of the structure from predominant frame action to predominant truss action,

which in turn reduces bending moments and increase the axial forces in the frame members.

Kasim Armagan et. al (2007) studied, a 3-story R/C frame structure with different amount of

masonry infill walls to investigate the effects of infill walls on earthquake response of these type

of structures. Diagonal strut approach was assumed for modeling masonry infill walls. Pushover

curves are found for the structures using nonlinear and established that the stability and integrity

of reinforced concrete frames are enhanced with masonry infill walls and presence of masonry

infill wall also alters displacements and base shear of the frame. Uneven distributions of masonry

infill walls in elevation can cause unacceptably elastic displacement in the soft storey frame. The

performance of buildings with infilled walls can be predicted by simplified diagonal models.

Comparatively simple and accurate method can be obtained by using these models for including

the effects of the infill walls.

Damage of masonry infilled RC frames subjected to blasting induced ground excitations was

assessed by Hong Hao et. al [29] and suggested that the influence of masonry infill on frame

response depends on the physical properties as well as the geometry of the wall. The stability and

integrity of RC frames are enhanced with a masonry infill wall. Besides the response level, the

presence of masonry infill also alters the damage pattern of the RC frame. Neglecting such effects

in the damage assessment of the masonry infilled frame structures will lead to unreliable results.

In the research work of Dorji and Thambiratnam [30] it was found that the strength of the infill

and the therefore the Youngs modulus has a significant influence on the overall performance of

structure. The structural response of the building such as roof displacements, inter storey drift ratio

and stress in the infill decreases as Youngs modulus of infill material increases. They suggested

that the opening size of the infill has vital effect on the fundamental period hence on the member

forces. Finally they found that there won’t be much difference in the performance of the structure if the infill elastic modulus is less than 5000 N/mm2.

Recently, L. Su & J. Shi [31], researchers from china proposed a new nonlinear methodology for

analyzing RC framed structures with infill, DBELA (Displacement Based Loss Assessment

Methodology) considering both micro and macro models. They investigated yield displacement,

period-height relationships and hysteretic characteristics of infilled RC frames and concluded that

masonry infill can increase earthquake energy dissipation and can reduce yield displacement and

corresponding yield period, compared to traditional beam sway structure; it was found that the

new methodology can precisely predict maximum displacement for both yield and post-yield

limits.


11

2.2.1 Infill Panels with Openings

Window and door openings are inevitable parts of infill walls due to functional causes.

Publications like FEMA-273 and ATC-40 contain provisions for the calculation of stiffness of

solid infilled frames mainly by modeling infill panels as a “diagonal strut.” But provisions are not

provided for infilled frames with openings. Various analytical models developed to estimate lateral

stiffness and strength of the infilled frames with openings is equivalent frame model, single

diagonal strut model finally multi-diagonal strut model. Equivalent frame model is based on the

theory of equivalent frame in which members have properties of composite sections of the actual

structure (Liauw [32], Kodur et al. 1998). The equivalent diagonal strut model is the most

simplified yet practically accurate macro-model, usually done by modeling the infill panel as a

single diagonal strut connecting two compressive diagonal corners. Key to this method lies in

calculation of effective width of equivalent diagonal strut. Several attempts have been made to

establish the effective width of diagonal strut for infilled frames without opening (Smith and Carter

1969, Holmes 1961, Mainstone 1971, Liauw and Kwan 1984 and Priestley 1992). They noticed

that frames without infill fail in flexure, with simple four-hinge mechanism, while frames with

infill fail in shear or in the tension column.

Model work by Fiorato et al. in [33] clearly demonstrated that infill substantially augments a

frame’s post peak behavior. Quasistatic, cyclic work on multiple story models concluded that

lateral strength and energy dissipation capacity were greatly improved, as well as lateral stiffness.

In attempts to try to quantify these phenomena, it was found that Stafford-Smith and Carter’s [34]

diagonal-strut approach reasonably predicted stiffness, but not peak strength. A study by Mehrabi

et al [35] on half scale frames subjected to in-plane loading also demonstrated substantial strength

and stiffness gains, as well as improved energy dissipation. Their research concentrated on various

levels of infill and boundary frame strength as predictors for damage onset at different levels of

story drift. Gulan and Sozen [36] proposed a seismic vulnerability ranking method for existing

infill RC structures based on panel or column geometry. It was found that valuation of the filler

contribution to the frame stiffness should be based on the compression/tension strength of the

mortar.

The effective width of diagonal strut for infilled frame without opening may be reduced by a

reduction factor to simulate the presence of openings of various aspect ratios in the infilled frame

was studied by Durrani and Luo [37], Al-Chaar [38]. Multi-strut models were proposed to

represent the local effects due to the presence of openings (Thiruvengadam 1985, White et. al

1999, Al-Chaar 2002). The published researches on infilled frames point to a need for suitable

quantitative design provisions to account for effect of openings. The relation for strut-width

proposed by Durrani and Luo [37] can be used to obtain the lateral stiffness of infilled frames due

to presence of central opening, developed numerically on the basis of Finite Element (FE) analyses

only and was not verified with experimental results. The expression established by Durrani and

Luo are too complex for use in design office because these account for beam stiffness, column

stiffness and infill panel aspect ratios. However, strut-width proposed by Al-Chaar [38]is simpler


12

than that proposed by Durrani and Luo [37]. Pushover analysis of infilled frame with openings by

Al-Chaar’s width of strut predicts in-plane strength but underestimates the initial lateral stiffness.

Hence, modification factors were proposed by Al-Chaar to obtain a reasonable value of initial

stiffness directly from the pushover curve.

It was proposed by Holmes [39] that the presence of central opening can be considered by reducing

the effective width through a reduction factor, Rw=1−2.6αco, where αco=ratio of the area of opening

to the area of the infill. Reduction factor for effective width of diagonal strut over that of the solid

reinforced concrete infilled frame to calculate its initial lateral stiffness was proposed by studying

seven specimens of infilled frame, when a central window opening is present, by Goutam Mondal

(2006). It was established that the effect of opening on the initial lateral stiffness of infilled frames

should be neglected if the area of opening is less than 5% of the area of the infill panel and strut-

width reduction factor should be set to one, the frame is to be examined as a solid infilled frame.

Influence of infill on the initial lateral stiffness of infilled frame can be neglected if the area of

opening exceeds 40% of the area of the infill panel and hence the strut-width reduction factor be

set to zero, in other words the frame is to be analyzed as a bare frame.

2.3 Nonlinear Static Analysis Methods

Nonlinear analyses are realized to sketch pushover curves and results are presented in comparison

and the effects of irregular configuration of masonry infill wall on the performance of the structure

are established. From the pushover curves, relative story displacements, story displacements,

maximum plastic rotations are calculated. Regarding with the analysis results, effects of

irregularities are obtained in the structural behavior during ground motions. Nonlinear Structural

analyses are to be used to determine the earthquake behavior of structures with infill walls.

Nonlinear analyses are getting improved and so many methods are developed in nonlinear

structural analyses (Atımtay, 2000; 2001). The aim of the nonlinear structural analyses is to

determine and control the performance of the structure under earthquake. Benefit of pushover

analysis is that it is capable of locating the most vulnerable part of the structure under lateral

loading. It provides details that cannot be obtained from elastic analyses, strength and ductility of

structure. In the analysis of the inelastic behavior of the building structures, there are two common methods

that are based on the nonlinear static pushover analysis. Capacity Spectrum method, which is also

referred in [40], is one of the most widespread methods used for the analysis of buildings. Which

was developed by freeman et.al, in this method, the structural capacity curve is calculated and

compared with the demand spectrum. A performance point lies on the capacity spectrum and

therefore the demand spectrum is obtained for performance analysis of the structure. The second

method, that is named Displacement coefficient method that is delineated in FEMA-356 [41], is

based on the displacement modification factors used for modifying the elastic spectral

displacement of the same SDOF system.


13

Since higher mode effects and invariant load patterns used in these methods leads to inefficiency

to reproduce the real behavior of the structures, Numerous researches are being done to eliminate

these drawbacks. Fajfar and Fischinger [42] proposed a loading pattern, invariant story forces

which are proportional to the deflected shape of the structure. Whereas, Eberhard and Sozen [43]

offered load patterns established from mode shapes derived from secant stiffness at each load

increments. In the research work of Park and Eom [44], they suggested a new design method using

secant stiffness and stated that the method directly estimates the inelastic strength and deformation

demands more effectively. In their study, they highlighted that the soft-story can only be vetoed

by energy dissipation within the structure which can be achieved only by spreading the plastic

hinges along height of the building.

A series of pushover analyses were performed on the buildings by Moghaddam [45], to determine

the higher mode effects in tall buildings, in which the elastic mode shapes are used as load patterns.

Sasaki, Freeman and Paret [46] proposed a multimodal method effective in predicting higher mode

effects though it does not provide exact seismic response of such structures. Unlike above-

mentioned methods, Chopra and Goel [47]designed a procedure for pushover analysis, titled as

Modal Pushover Analysis (MPA). Comparing the results achieved from this procedure with

various other load patterns, showed that the MPA is more accurate than all pushover analysis

methods in assessing floor displacements, plastic hinge rotations story drifts and plastic hinge

locations whereas other pushover methods underestimate the story drift demands and lead to large

slips in plastic hinge rotations. Additionally, it was stated that MPA results are comparable to the

time history analysis results.

The exactness of MPA procedure is evaluated by Chintanapakdee and Chopra [48] and it was

identified that the MPA outcomes were analogous with nonlinear dynamic analyses. In the study,

MPA method was also used to evaluate seismic demand of inelastic systems, defined by an elastic

design spectrum. Modal pushover analysis procedure was found to be more reliable than FEMA-

356 for irregular frames. It is also stated that if sufficient modes are taken into account, MPA

provides very close results to the time history analysis results compared to other load distributions.

In addition, the irregularities influence the variation of story drifts, leading to strength irregularity

larger than stiffness irregularity, and their combination has the largest effect. Attard and Fafitis

[49] developed an improved method of MPA in which an alternate load pattern was obtained from

a mode shape at yielding point. Study stated that, after iteration on the parameters achieved from

time history analysis, the proposed method offers comparable results.

The impact of higher mode effects in pushover analysis was investigated by Chopra and Goel [50],

and it was found that the higher mode pushover curves causes plastic hinge mechanisms which are

left undetected by effective first mode load pattern and other force distributions given by FEMA-

356. Plastic Hinge mechanisms do not develop during ground motion in a regular building without

a soft and/or weak story due to higher mode pushover curves. Study stated that reversals in a higher


14

mode pushover curve arises after formation of a mechanism, provided, resultant force is in such a

direction, which displaces the roof in a direction opposite prior to formation of the mechanism.

This occurs only in higher mode pushover analyses but not in the pushover analyses for the first

mode or FEMA-273 force distributions. It was stated that story drift demands in the modified and

neighboring stories are amplified and whereas in other stories is decreased, in case of soft or weak

story. Conversely, a stiff or strong story lessens the drift demand in the modified and neighboring

stories and surges the drift demands in other stories. Though the roof displacement is generally

insensitive to vertical irregularity, but it is considerably different for frames that have irregular

stiffness or irregular strength in their lower half. The base story irregularity has vital influence on

the floor-wise distribution of displacements.

Adaptive Pushover Procedure (APM) was proposed by Gupta and Kunnath [51] that can account

for the higher mode effects and overcome the shortcomings of the FEMA-356 procedure. It is

noted that the FEMA 356 procedure fails in accurate evaluation of ductility demands, and APM is

flawless in determining seismic demands. Jan et al. [52]developed a new form of pushover analysis

procedure, considering higher mode effects, named Upper Bound Pushover Analysis. Though

Triangular load patterns & MPA procedure predicts the seismic demands in low rise structures

than the proposed method, these procedures underestimate the responses in high and mid-rise

structures. The proposed method is capable of predicting responses in high and mid-rise structures.

Undesirably, proposed method overestimates the demands in upper stories and undervalues the

demands in lower ones. Kalkan and Kunnath [53] predicted seismic demands of structures and the

results of time history analysis results are compared with various nonlinear pushover static

loadings. The pushover methods were point out as an improvement over traditional elastic force-

based procedures and provide critical information on potential collapse mechanisms and the

vulnerability for soft stories. For structures responding primarily in the first mode, it was suggested

that, nonlinear static methods would be reliable choice to estimate inelastic demands, but this can

also be misleading in the estimating seismic demands of upper stories in mid-rise buildings.

Various load patterns have also been studied by researchers, other than those mentioned above.

Mwafy and Elnashai [54]examined the applicability and adoptability of inelastic static pushover

analysis in forecasting the seismic response of reinforced concrete buildings. It is stated that, if the

load pattern is chosen cautiously, the model can represent the inelastic response of the low and

mid-rise buildings. Whereas in high-rise buildings, it was recommended to use more load patterns

due to the higher mode effects. Furthermore, the uniform load pattern can establish very

conservative prediction of seismic demands. Krawinkler and Seneviratna [55] briefed basic

concepts in pushover analysis. Moreover, they assessed the accuracy of pushover expectations and

identified the conditions under which the pushover will provide adequate results. Inadequate or

even misleading pushover predictions were also identified and suggested that carefully performed

pushover analysis can provide insight into structural aspects that governs performance during

severe seismic activity. They reconfirmed that the structures with primary mode of vibration as the


15

fundamental mode, ductile demands will be obtained better with pushover analysis. Faults such as

story mechanisms, excessive deformation demands, strength anomalies and overloads on columns

& connections that may remain hidden in an elastic analysis will be made observable with this

analysis.

Inel M. et. al. [56] assessed different load patterns used in pushover analysis which covered

buildings with a soft-story. They found out that simplified inelastic techniques provide very good

predictions of peak displacement response for both regular and weak-story buildings. It was

concluded that results of inter-story drift & story shear were improved when multiple modes are

interpreted. Results also showed that simplifications in the first mode lateral load pattern can easily

be applied with a insignificant loss of accuracy.

Korkmaz and Sarı [57] evaluated the response of the framed structures under various load patterns

by performing pushover & nonlinear dynamic time history analysis. In high-rise frame structures,

the first yielding and shear failure of the columns is experienced at the larger displacements story

and uniform distribution always lead to higher base shear-weight ratio compared to other load

distributions for the conforming story displacement. It was also found for long period high-rise

reinforced concrete frame structures, results of nonlinear static pushover analysis does not comply

nonlinear dynamic time history analysis results. Additionally pushover analyses results for

uniform load distribution estimates peak seismic demands during the given earthquakes more

reasonable than the other load distributions. Pushover analysis method under various loading

patterns and procedures was studied by Oguz [58] and concluded that the variation in results of all

the modal loading patterns & the triangle load patterns is negligible for mid-rise & low-rise

structures. Triangular loading patterns predict displacements and inter-story drift ratios between

the results of Modal-PA and Elastic First Mode load patterns in low and mid-rise structures. None

of the load patterns could capture the exact demands & hinge locations attained by time history

analysis but the exactness of the results may be practical, subjected on the load patterns for low

and mid-rise structures and accuracy decreases in high-rise buildings. Moreover, no enhancement

was observed by using FEMA-273 and MPA procedures, consenting for higher mode effects. She

recommended use of elastic first mode load pattern in the pushover analyses and avoiding use of

uniform load pattern in view of the results on actual demands & accuracy. Bayülke et. al. [59]

investigated earthquake damaged and undamaged RC buildings by non-linear pushover analysis

technique, to determine lateral force displacement relationships and to compare limit lateral forces

with the lateral load level which were calculated from elastic acceleration spectrums for calculated

R factors, concluded that the structures with symmetric shear walls in plan does not lose their

lateral stiffness’ in a risky way during ground shaking like the ones without shear walls after the

limit lateral force level in addition to this formation of the failure mechanism is found to be very

quick and progressive for the buildings without shear walls.


16

Polat et.al. [60], did a case study on the conservative retrofitting with linear analysis by assessing

the seismic demands and cost requirements attained by linear analysis was found to be irrational

and the usages of more realistic analysis methods was strongly suggested in such cases. In a similar

study, Hasgür et al. [61] pointed out expected damages due to destructive earthquakes &

determined the relationships and properties of seismic damage indices by non-linear analysis for

RC building structures with elements of various bending shear, yield capacities and corresponding

pre and post strengthening curvatures. He concluded that retrofitting by using the results of the

nonlinear analysis approaches were more accurate & better in cost concerns.

Türker et. al. [62] studied a set of models considering the effects of the in-fills. Considering in-

fills in the nonlinear pushover analysis of structures showed better performances. He

recommended that Turkish Code should be revised detailing such analysis methods. The low-rise

structures met the performance standards but mid and high-rise buildings were stated to be

insufficient in meeting the performance demands of the code. Inel et.al [63] evaluated existing

construction practice studying models with a soft story. It is concluded in that study that the

increase in the confinement level upsurges the sustained level of damage and the effect of infills

are substantial in low rise structures consisting weaker members. The main reason for collapse was

found to be weak columns & strong beams and structural irregularities viz. short column, soft story

and heavy overhangs are quite dangerous but the soft story irregularity with a heavy overhang is

the most unsafe. Additionally, the irregularity effect was found to be more predominant in mid-

rise structures than the low-rise structures and soft story irregularity aroused due to the absence of

infills at the ground story is found to be more vital than the stiffness based ones.

Athanassiadou [64] studied multi-story investigative models irregular in vertical and compared the

ductility levels and pushover analysis results. High ductility and normal ductility demands were

concluded to be cost ineffective and their seismic performance was found to be equally acceptable.

Even though the beams of normal ductile buildings are said to have some weakness in shear

capacity, the over strength of the both ductility levels are found to be analogous. Also the inelastic

pushover procedures are found to be in accurate in demand predictions as they ignore higher mode

effects. In the research of Ruiz and Diederich [65] on soft story behaviour and irregularities in the

building structures, local ductility demands of a set of analytical models with a weak story was

investigated and it was found that the performances of the frames rest on the resistance factors and

closeness of the dominant response period and dominant period of earthquake. In addition, P-Δ

effects considered were found to be higher. The nonlinear response of structures with excessive

stiffness & strength above the first story was studied by Esteva [66] and it was stated that the

response of a building is quite sensitive to the stiffness variation along the height of the building

and the p-Δ effects are significant on response. He suggested use of a safety factor to normalize

local ductility demands in a soft story which is reliant on natural period of a structure.


17

Chang and Kim [67] scrutinized a 20-story structure with a soft story by nonlinear time-history

and nonlinear pushover analysis and concluded that low strength reduction factor with perfectly

yielding mechanism is mandatory for effective protection and advised that an amplification factor

shall be applied to soft stories for which displacements might be reduced due to this effect.

Chopra et al. [68] studied the yielding point of a soft first story for the sufficient protection of

upper stories from substantial yielding. They concluded that to limit the force transmitted to the

adjacent story above, an elastic-perfectly plastic mechanism is desirable as any residual stiffness

increase the shear force conducted. Even if the first story confines the forces transmitted to upper

stories, the resultant shear wave propagates; hence, any weakness of strength in an upper story

may lead to collapse. It was also stated that the first soft story mechanisms must be designed for

very large displacements. Mezzi [69] studied the retrofitting methods for buildings with a soft

story and concluded that even though passive control systems is an effective solution for

retrofitting, base isolation is the most economic one.


18

CHAPTER 3.

SOFT STORY IRREGULARITY

3.1 General

Irregularities in the building lead to unpredictable behavior of the structure during seismic ground

motions even if quality of construction is maintained or elements satisfy the requirements of the

code. The effects of irregularity cannot be accessed using conventional analysis methodologies. A

structure with vertical irregularity is found to be having non uniform drift demands. Moreover,

open ground storey structures are susceptible to concentrated ground story drifts due to very low

stiffness with respect to over lying stories. Current building codes emphasis on formation of plastic

hinges in the structural members especially due to the lateral loading in order to increase the energy

dissipation level of the structural system. Concentration of plastic hinges in definite locations

occurs due to irregularities in the structure during an earthquake, which is undesirable.

The vertical irregularities in building structures may be categorized as weak story, soft story,

discontinuity of vertical elements and mass irregularity. Depending on the soft story criteria in the

Indian codes, it is obvious that the mass irregularity is also considered within the soft story

irregularity definition. Although weak and soft story irregularities may cause analogous structural

damages in an earthquake, these two irregularity types are quite different in definition. A weak

story is defined by comparing effective shear areas of lateral force resisting systems of the adjacent

stories; on the other hand, the soft story irregularity is defined by relating the stiffness of the lateral

force resisting systems of adjacent stories. Basically the difference between the soft and weak story

irregularity can be explained by considering the difference in stiffness and strength. Moreover, the

changes in the element sizes will affect both. The behavior of the structures having soft stories has

been presented in the following section.

3.2 Soft Story Behavior

Structures having weak ground stories suffered major structural damage and collapsed in the recent

earthquakes. Large open spaces with less infill, exterior walls and higher floor heights at the

ground level result in soft ground story. In such structures, the stiffness of the lateral load resisting

systems at that level is quite less than the stories above or below, leading to high lateral

displacement in that floor. Lateral displacement diagram of a building with a soft ground story

under lateral loading is shown in Figure 1.


19

Figure 2 Excessive drift in soft ground story under lateral loading

Non uniform lateral force occurs along the height of the structure during an earthquake if irregular

inter-story drifts take place between adjacent stories. This focuses lateral forces on the story (or

stories) having large displacement(s). Hence, if the ductility demands are not met in the design of

such building for that story and the inter-story drifts cannot be limited leading to a local failure

mechanism or even a story failure mechanism, which can lead to the collapse of the entire system,

arose due to the high level of load-deformation (P-Δ) effects. Figure 2 shows the collapse

mechanism of such a building structure with a soft ground story under both earthquake and gravity

loads.

Figure 3 Collapse mechanism of a building structure having a soft story


20

Lateral drift of a story depends on stiffness, mass and lateral force distributed on that story. Also,

the lateral force distribution along the height of a structure is directly associated to mass and

stiffness of each story. If P-Δ effect is referred to be the main cause of dynamic collapse of building

during earthquakes, precisely determined lateral displacements from elastic design procedure can

provide very significant information regarding structural behavior of the system. Therefore

dynamic analysis technique is required in design codes for accurate distribution of the earthquake

forces along the building height, comprehending modal effects and local ductility demands

efficiently. Even though some of the current codes define a soft story irregularity in contrast of

stiffness between adjacent floors, displacement based criteria will be more appropriate for such

irregularity determination, since it can quantify the mass, stiffness and force distribution concepts.

Next section will elaborate the requirements in the design of building structures with a soft story

in Indian codes.

3.3 Indian Earthquake Code

As described earlier as per Indian code, the stiffness of a story should not be less than 60% of the

adjacent story above or should not be less than 70% of the average stiffness of the three stories

above. Additionally, the Indian Earthquake Code requires relative displacements in adjacent

stories to be greater than 1.3, in order to define the irregularity as a soft story.

The Indian Earthquake Code also suggests pushover analysis confirming to ATC-40 for the

determination of ductility demands. However, accepting that this method may not be very

appropriate, code suggests an amplification factor of 2.5, for amplifying the member forces, to be

used for the design of the soft story’s columns and beams. Then again, an amplification factor of

1.5 is suggested if symmetric shear walls are implemented in plan of such buildings.


21

CHAPTER 4.

NONLINEAR PUSHOVER ANALYSIS

4.1 Introduction

In order to explore the nonlinear behavior of the building structures nonlinear static pushover are

performed on the analytical models. In this section, the nonlinear material properties used in this

study and the underlying principles on the nonlinear static pushover analysis methods is explained.

4.2 Nonlinear Behavior & Plastic Hinge formations in Structural Elements

The nonlinear performance of a structure depends on the nonlinear responses of the structural

elements that contribute to the lateral force resisting system. Therefore, it is necessary to describe

and evaluate nonlinear behavior of any such elements before applying nonlinear analysis method

on a structure.

The generalized load deformation relation of a structural member while exhibiting nonlinear

behavior as per FEMA-356 is shown in Figure. When the member yields (applied load/yield load

proportion (Q/Qy) equal to 1), subsequent strain hardening accommodates the strain hardening in

the load-deformation relation since the member deforms toward the expected strength. Horizontal

axis of this diagram can either express curvature or strain.

Figure 4: The generalized load deformation relation for nonlinear response (ATC 40)

The load-deformation relation is defined by an elastic response until point B. After point B, the

member yields and again a linear response is observed with a reduced stiffness between points B

and C. At point C, sudden reduction in load resistance of the element occurs and graph drops to

point D. The residual resistance is observed until point E, where the ultimate loss of resistance


22

takes place. The initial slope, between points A and B defines the elastic stiffness of the structure.

The second slope between points B and C is taken as 10% of the initial slope for analyses in the

study. Point C in this diagram signifies the ultimate strength of the element where the substantial

stiffness degradation begins. This nonlinear response of the structural member is called hinge

property which is defined symmetrically case of columns and beams in order to include the

reversals to the calculations. In order to model nonlinear response of an element, ATC-40 and

FEMA-356 express the parameters A, B and C in Figure by defining plastic rotation angles.

Figure 5: Acceptance criteria on a force versus deformation diagram (ATC 40)

Acceptance criteria are also defined in ATC-40 and FEMA-356 codes depending on the plastic

hinge rotations by considering various performance levels. In Figure 5 the acceptance criteria on

a force versus deformation diagram are given. In this diagram, points marked as IO, LS and CP

represent immediate occupancy, life safety and collapse prevention, respectively.


23

4.3 Analysis

Nonlinear static pushover analysis has become most commonly used technique to interpret the

nonlinear behavior of the structures in the recent years. In simplified procedure, a capacity curve

is achieved which depicts the relation of base shear and roof displacement. The curve also

represents the performance of the building structure under increasing base shear forces. As the

capacities of the members of the lateral force resisting system surpass their yield limits during the

increasing of the base shear forces, slope of the force-deformation curve will alter, and hence the

nonlinear behavior can be represented.

Figure 6 : A pushover curve of a building structure (ATC 40)

Depending on the initial load pattern, in the analysis, applied lateral forces to a model are increased

in a consistent manner. Member forces are obtained for each step. In the next step of the analysis

stiffness of the members are changed when capacity is exceeded, according to the hinge properties.

This progression ends when the structure becomes unstable. Figure shows a typical pushover

curve. Pushover analysis can either be performed considering the control over the force or the

displacement. Force control option is beneficial when the magnitude of the load is known clearly,

and the structure is predicted to support that load. The displacement control is useful when the

extent of the load is unknown and displacements are investigated.

SAP2000, computer software is utilized in the study, due to its simplicity and its computation

power to carry out the pushover analyses. Following steps are performed in the pushover analysis

of a structure in SAP2000:

The model representing the structure is generated and vertical loads (dead load and live

load); member properties and member nonlinear behaviors are defined and assigned to the

model.

Hinge properties are defined and are assigned to the member ends considering end-offsets.


24

Lateral load patterns to be used in the pushover analyses are assigned.

Force controlled pushover loading is used initially for the lateral load increment analyses,

applied to the model as a pushover case. Initial pushover load case is comprises of the dead

loads and reduced live loads.

Now a new displacement controlled pushover case is assigned considering the lateral load

pattern which was used above for the incremental pushover analysis, starting from the

initial pushover case.

In this study, the hinge properties are determined according to ATC-40 and FEMA-356 for beams

and columns. The tabulated forms of the moment-rotation relationships of the hinges at the beams

and at the columns that are used in the analytical models are given in Table.


25

CHAPTER 5.

EQUIVALENT BRACED FRAME METHOD In order to model structures with infilled walls, equivalent diagonal strut approach for analysis and

design of infilled frames has been adopted. The method considers the inelastic and plastic behavior

of infilled frames bearing in mind the limited ductility of infilled materials. It offers a rational basis

for assessing the lateral strength and stiffness of the infilled frames and infill diagonal cracking

load. Analytical macro-model was used which is based on equivalent strut approach, combined

with a smooth hysteretic model to replicate masonry infills in nonlinear analyses (monotonic

pushover analysis). The envelope properties of the strut such as stiffness and the control points of

force-deformation relations was obtained from mechanics of infill-frame interaction proposed by

Saneinejad [70].

Figure 7 : Equivalent Strut Model (a) Masonry Infill Panel (b) Strength Envelope of Masonry Panel

Monotonic lateral force-deformation relation is bilinear curves having an initial elastic stiffness

until the yield force, Vy, and post-yield degraded stiffness till the maximum force, Vm. The

maximum lateral force, Vm, and the corresponding displacement, um, in the infill masonry panel.


26

'

'

'

'

'

cos

1 0.45 tan

1.25

cos

d m

s

m

m d

A f

tl

VMPa tl

L

1

'

cosm d

m

Lu

2

Where,

t = the thickness or out of plane dimension of the infill panel

= 1tan /h l

'mf = masonry prism strength

'm = corresponding strain

s = shear strength or cohesion of masonry

The area, dA , and the length, dL , of the equivalent diagonal strut were obtained from :

' ' '1 0.5

cos cos

c b ac c b

c c cd

fth tl th

f f fA

3

2 '2 '21d cL h l

4

Where the quantities c , b , c , b , af and cf depend on the geometric and material properties of

the frame and the infill panel.

c = Upper bound or failure normal contact stresses at the column-infill interface

cf = the compressive strength of the masonry

af = compressive stress of infill in its central region

b = contact shear stresses at the beam-infill interface


27

'h = height

'l = length of the infill panel

Different analytical macro-models based on overall behavior of an infill panel are developed to

imitate the behavior of infilled frames. Single strut model is the widely used even though multi-

strut models are found to give better results. Though single strut models are the simplest one, they

are incapable in capturing the local effects occurring in the frame members. It is evidently the most

ideal for the analysis of large structures. In this study, RC frames with unreinforced masonry infill

walls are idealized as equivalent braced frames (EBF) with infill panels replaced by "equivalent

struts" [71]. State-of-the art entitles that the constitutive relationship of the strut elements has been

established only for the single strut models, it has been advised that currently only single strut

idealization may be used in rigorous non-linear pushover analyses of RC frames with infill walls.

It is found that infill walls reduces inter-storey drifts and enhance stiffness and strength of a

structure. Resulting in low ductility of infilled structures compared to bare frame structures.

Workmanship, Quality of infill material and frame-infill interface significantly affects the

performance of infilled frames. A pin-jointed strut with its width taken as one-third the infill the

strut as the input and found its immediate acceptance in elastic foundation, a non-dimensional

parameter was also defined as the relative lateral stiffness of the infill, in earlier models equivalent

single strut. This study uses a method that is further extended to predict the lateral stiffness and

the width of diagonal strut, derived in terms of a relative infill/frame stiffness parameter.

Figure 8 : Idealized Pin jointed compression strut model

A simple and conservative expression of the width of equivalent strut was proposed as:

We = 0.25dm 5

Where,

dm = the length of the infill diagonal.


28

Figure 9: Compressive stress-strain curves for masonry


29

CHAPTER 6.

ANALYTICAL MODELS

6.1 General

In order to investigate significance of infills in soft ground story behavior of the structures, several

three- dimensional analytical models are considered in this study. Analytical models are explained

in this section. In addition to this, various pushover parameters and strut properties adopted are

detailed.

6.2 Description of Analytical Models

A typical five-storey residential building, with five bays in the longitudinal direction and three in

the transverse direction, is considered. Four full scale models were modeled in SAP2000. All

models were analyzed as per IS 456 and IS 1893. The frame members are designed by the limit

state method given in IS 456.The grade of concrete used is M25 and that of steel is Fe415. The

modulus of elasticity of concrete is taken as 5700√𝑓𝑐𝑘 N/mm2 where fck is 28-day characteristic

cube strength, recommended by IS 456. The Poisson’s ratio and unit weight are taken as 0.2, 25

kN/m3, respectively, for concrete. For models with infill masonry panels, modulus of elasticity and

Poisson’s ratio are taken as 6,300 N/mm2 and 0.15 respectively. The floor and the roof slabs are

taken as 130 mm thick. The floor finish on floors and the weathering course on the roof are taken

as 1.0 kN/m and 2.25kN/m, respectively. The live load on floors and that on the roof are taken as

2.0 kN/m and 0.75 kN/m, respectively. Wall loads were calculated for respective the wall thickness

and were applied as UDL on beams assuming the unit weight of masonry as 21KN/m3. Beam self-

weights are calculated and assigned to the beams as uniformly distributed dead loads in related

beams. Additionally, column self-weights are calculated and assigned as joint loads on the

columns.

In case of models with infill panels, the wall panel sizes complies to the Indian masonry code for

partition walls with adequate restraint at both ends and at the top. The infilled frame in this model

was idealized as an equivalent diagonally-braced frame with the diagonal compression struts pin-

connected to the frame corners. The arrangement of brick walls are shown in specific model

descriptions as equivalent single struts. The masses of brick panels are lumped to act at the floor

levels. The external and the internal brick walls are taken to be 230 mm and 115 mm thick,

respectively; larger thicknesses than these are provided if required from design considerations.

The following load combinations given in IS 1893 are considered for design.

1. 1.5(DL + LL)


30

2. 1.2(DL + LL* + ELx)

3. 1.2(DL + LL* +ELy)

4. 1.5(DL ± ELx)

5. 1.5(DL ± ELy)

6. 0.9DL ± 1.5ELx

7. 0.9DL+ 1.5Ely

Where, LL equals 25 percent of the full design live load LL on the floors and is zero on the roof.

When the earthquake load is considered, the seismic weight is obtained by considering 25 percent

of LL. The total design base shear, Vb , on the building is calculated as per the IS 1893, and given

by

Vb = AhW, 6

Where, W is the seismic weight of the whole building and Ah the design horizontal acceleration

spectrum given by

2a

h

SZIA

R g

7

Where,

Z = the seismic zone factor taken as 0.36 for seismic zone V

I = the importance factor taken as 1.0 for the ordinary residential building

2R = the response reduction factor taken as

6.0 for the ordinary RC MRFs detailed as per IS 456 and as

10.0 for special RC MRFs detailed as per the Indian seismic detailing code.

aS

g = the average response acceleration coefficient

The fundamental natural period, T, (seconds) of the bare and infilled frames are calculated using

the empirical expressions given in IS 1893.

0.09if

hT

d

8

Where,


31

h= the total height of the main structure, m

d= the maximum base dimensions of the building along the considered direction

of seismic force, m

The lateral seismic forces at each floor Qi are applied at a design eccentricity of 0.05bi where bi

is the floor plan dimension of floor i perpendicular to the direction of lateral seismic force. The

structure is discretized into three-dimensional frame elements. The nodes at each floor are

constrained by rigid diaphragms.

2

2

1

i ii B N

j j

j

W hQ V

W h

9

Where,

iW = the seismic weight of floor i.

ih = the height of floor I measured from the base.

𝑁 = the total number of floors in the building

(Number of levels at which the masses are lumped).

Figure 10 Plan of the model representing point of application of pushover loads

Geometrical characteristics and the structural arrangement of each model have been described in

detailed below.


32

Model 1 – Bare Frame Model

This model was designed bare frame force and moment which is currently practiced in India. The

wall loads are applied as UDLs on the beam elements.

Figure 11 : Bare frame model

The member reinforcements are calculated by this software and the other displacement

requirements, which are defined in the IS 456, are taken into account.


33

Model 2 – Infilled Model with open ground story

The infill panels in higher stories were idealized as equivalent struts. The dimensions of the

equivalent struts were obtained from the method explained above. Axial hinges were assigned to

struts.

Figure 12 Open Ground story model with diagonal struts


34

Model 3 – Fully infilled Model

The model was designed considering axial forces in columns and beams by resolving forces from

the equivalent struts. Hinge definitions are calculated and assigned as in model 2.

Figure 13 : Fully infilled structure with equivalent single struts


35

6.3 Modeling of Plastic Hinges

6.3.1 RCC Frame Members

After obtaining the dimensions and the reinforcements of the members, nonlinear hinge properties

are calculated and matched with the SAP2000’s default hinge properties.

6.3.1.1 Plastic Hinge Length

The plastic hinges in members are assumed to form at a distance equal to half the average plastic

hinge length, lp, where lp is given by Baker’s formula

1 30.8p

zl k k c

d

10

Where,

z =distance of critical section to the point of contraflexure

d = effective depth of the member

c = neutral axis depth at ultimate moment

1k = 0.7(for mild steel)

3k = 0.9(for cold-worked steel)

'3

0.6

0.6 0.0128 11.7

0.9

ck f

11

Where,

'cf = 0.8 times the cube strength

The axial force vs. moment interaction diagram must be calculated and plotted in order to

determine nonlinear hinge properties of a column. The column exhibits nonlinear behavior when

an axial force and corresponding moment value of loading is formed outside plotted interaction

diagram.


36

Figure 14 : Idealized M-θ Curves for Hinge Definitions

For determining the nonlinear hinge properties of a beam, the moment capacity values must be

calculated. When the moment value at that beam exceeds the calculated capacity moment, this

beam exhibits nonlinear behavior.

Figure 15: Idealized P-M interaction curve for columns

The moment-curvature (M- Φ) curves of columns are calculated for axial load corresponding to full DL and 25% Live Load. These curves are idealized as bilinear curves using initial tangent and

ultimate moment. The bending moment diagram of a member under lateral forces varies linearly.

The moment-rotation (M− θ) relationship for this distribution of moments is obtained using the

M- Φ relationship. A fixed-end member can be replaced by an equivalent cantilever member of

half span, with a concentrated load at its tip, Fig 3, if the point of contraflexure is at the midspan.

The yield and ultimate rotations θy and θu respectively, are obtained by the following relations:

2

4

y

y

LM

EI

12

u y p u yl

13

Where,


37

L , E and I = length, modulus of elasticity and moment of inertia of the member

respectively

u & y = idealized ultimate and yield curvatures of the section respectively

pl = length of the plastic hinge

Figure 16: P-M Interaction curve of RC column

Table 1 : Moment – Rotation values used for defining Plastic Hinges


38

6.3.2 Infill Wall Panels

The contribution of masonry infill panels to the response of infilled frame is modeled by replacing

the infill panels with equivalent diagonal struts. The lateral force-deformation relationship of these

struts is obtained using relations specified above and following graph is obtained.

Figure 17 : Load Displacement plot for defining Axial Hinges in diagonal struts

Value of f’m and ε’

m is taken as 4N/mm2 and 0.00007 respectively from brick prism test results.

The shear strength of masonry is take as 2.5 time the shear stress permissible as per IS1905:1987

and coefficient of friction µ f as 0.45. For small deformation, the idealized yield force, Py, and

idealized maximum force, Pm, in the axial direction and the corresponding displacements, namely

vy and vm , are obtained from the corresponding quantities in the lateral direction obtained from

equations

cosm

m

VP

14

cosm mv u 15

cosy

y

VP

16

cosy yv u

17


39

Figure 18: Calculation of lateral strength of masonry-infill walls

The figure below demonstrates assigned hinge locations when infill panels are replace by

equivalent strut members. In case of bare frames P M hinges are assigned to columns and moment

hinges are assigned to beam members.

Figure 19 : Plastic Hinge Assignments

The geometrical properties of the model are indicated in the figure. The size and reinforcement

details changes as the model changes.


40

Figure 20 : Section Assignments


41

CHAPTER 7.

RESULTS AND DISCUSSIONS

7.1 Bare Frame

The present analysis and design philosophies can be represented by the bare frame model, which

neglects the effects and contribution of infill panels in the building, thereby ignoring their stiffness

and strength contributions. Figure shows the pushover curve for the model.

The pushover obtained by the pushover analysis of the frame is shown in Figure 21. Performance

was observed to be Linear in different members of the frame up to a base shear of about 10%

seismic weight and up to a top floor displacement corresponding to 0.33% drift ratio. Nonlinearity

was observed to be well scattered along the height of the frame and failure of frame was found to

take place due to the flexural failure of first story columns at the ultimate lateral load corresponding

to 17% seismic weight and lateral drift of 1%.

Figure 21 : Pushover curve for Model 1


42

Figure 22 : Hinge formation pattern at Ultimate load.

Failure of the first-story columns (Figure 22) was observed because the frame was designed as a

weak column-strong beam frame system to reflect the current design practice adopted by designers

in India as well as in many other countries.

7.2 Open First-Story Frame

In this model, stiffness and contribution of masonry infills were taken into account in the upper

stories; frames in the first story were kept open, without diagonal struts.


43

Figure 23: Pushover curve for Model 2

The push curve shows linear behavior up to base shear corresponding to 16% seismic weight and

up to lateral drift of 0.3% Figure 23. The lateral strength of the open first-story frame was found

to be that corresponding to about 20% seismic weight at about 3.5% lateral drift.

Most of the lateral deformations were found to be collected by the soft and weak first story due to

heavy mass contribution of higher stories and lack of stiffness aroused due to the absence of infills

in the first story. This lead to excessive force demand on first-story columns and beams under

increasing lateral deformation, causing failure of the frame by flexural and shear failure of ground-

story beams and columns.


44

Figure 24: Hinge formation pattern at Ultimate load.

7.3 Fully Infilled Frame

Infill walls in frames provide main energy dissipation mechanism in structures when subjected to

earthquake, provided weak stories are avoided. In this model stiffness and strength of masonry

infills was considered in all stories using equivalent strut design method; hence, a large increase

in lateral strength and stiffness of the frame was observed.


45

Figure 25: Pushover curve for Model 3

However, the lateral strength of the frame reduced drastically after failure of infills in the ground

story. A significant variation was observed in the capacity curves when different mortar grades

were used in masonry.

First, inelastic activity was observed at a very high lateral load corresponding to 35% seismic

weight; however, the corresponding drift level was only 0.06%, indicating a very strong and stiff

system. The failure of some of the first-story infills took place at a base shear corresponding to

44% seismic weight and only 0.3% drift, and therefore, a large drop was observed in lateral

strength. This indicates the brittleness associated with the fully infilled frame systems. After the

failure of a significant amount of infill walls in the frame, lateral load behavior was observed to

be quite similar to that of the bare frame. It was noted that the presence of infills in the first story

prevents premature failure of first story columns.


46

Figure 26: Hinge formation pattern at Ultimate load.

7.4 Effect of Codal Provisions

Figure 27 : Ground storey members designed for code-specified factors.


47

Indian seismic code advices that the members of the open ground story are to be designed for 2.5

times design seismic forces. The efficiency of these strengthening provisions, in which the lateral

strength of the columns and beams of the open ground story required to be increased using

predetermined multiplying factors, was evaluated considering a multiplying factor of 2.5.

Reinforcement details and plastic hinge properties were improved for sections.

Figure 28: Pushover curve for Strengthened Model

Elastic behavior was observed up to a base shear corresponding to about 40% of seismic weight

and 0.15% drift. The lateral strength of the frame was found to increase significantly to about 50%

seismic weights; however, corresponding lateral drift reduced to only 1.6%. The collapse

mechanism of the strengthened frame did not alter when compared with the non-strengthened open

first story frame. Plastic hinges were observed to be concentrated in only first-story columns due

to their increased stiffness and the frame failed by flexural failure of these columns. Plastic hinges

were not observed in first-story beams; therefore, it is necessary to ascertain if there is any

advantage to increasing the design forces for open first-story beams.


48

Figure 29: Hinge formation pattern at ultimate load

Further, only columns designed for 2.5 times seismic loads and it was observed that plastic hinges

developed in only the first-story columns. Increasing the strength of first-story beams exerts

additional force demands on the first-story columns. This was also observed by Fardis and

Panagiotakos studying a similar clause in older version of Eurocode 8 for buildings with severe

vertical anomalies. But the new version of Eurocode 8 necessitates that lateral strength of only the

first-story columns has to be increased.

Figure 30 : Ground storey Columns designed for code-specified factors


49

The frame members continued under elastic limit up to a base shear corresponding to about 40%

seismic weight and lateral drift of 0.15%. Plasticity developed in most RC members of the open

first story and infills in the second story at a base shear of about 43% seismic weight and 0.2%

lateral drift. Yet, lateral deformability was reduced significantly due to shear failure of the first-

story beams at 1.1% lateral drift. Thus, another scheme was studied in which the ground story

beams were provided with confining shear reinforcement to increase their ductility. Shear strength

of the beams in the open first story was also amplified by 2.5 times for this purpose. Though lateral

strength of this frame was not enhanced significantly, considerable improvement was observed in

its lateral deformability 1.7% drift. Finally, failure of the frame was induced by flexural failure of

the first-story columns.

7.5 Discussions

Model Yield Roof

Displacement

(in m)

Yield Base

Shear (in kN)

Ultimate Roof

Displacement

(in m)

Ultimate Base

Shear (in kN)

Bare 0.056582 2481.393 0.142139 4655.558 Open Ground Story 0.02099 4094.768 0.060727 6443.687 Fully Infilled 0.011079 8805.309 0.017076 11599.06 Strengthened Frame 0.006554 7026.229 0.041662 8853.736

Table 2 : Pushover Summary

The lateral strength of open ground story was increased by 2.5 times by designing bottom story

columns and beams for 2.5 times seismic design forces. On the contrary lateral drift at ultimate

load was considerably reduced, to a value less than one and half the values observed for open

ground story. The pushover curves were similar for models improved with both strengthening

techniques. The lateral drift for model with column alone redesigned with magnified forces was

higher than that of the model designed with codal provisions, whereas the lateral capacity of the

former one was slightly higher. Therefore, it can be concluded that there is no significant advantage

in designing the ground story beams for modified forces. Yet it is suggested that sufficient

confining shear reinforcement is necessary in these beams to improve their ductility and shear

strength to prevent their premature shear failure.

7.6 Failure Pattern

As expected, the performance of the bare frame was flexural in nature due to the absence of brittle

infill panels. Mixed response was observed in the case of the open first-story frame compared to


50

bare frames, failure took place due to failure of RC members in flexural and shear. Lateral load

carrying capacity of fully infilled frame was found to decrease drastically after the failure of

masonry infills in the first and higher stories, followed by the lateral load behavior of a bare frame.

In strengthened frames using codal method, nonlinearity was found to be focused in the first-story

columns due to increased stiffness, were failure of the frame took place by flexural failure of the

first-story columns.

When only first-story columns were designed for magnified forces, failure took place at a very low

lateral drift by shear failure of first-story beam, when the first-story beams were provided with

normal shear reinforcement. It was noticed that when sufficient confining shear reinforcement was

provided in these beams, flexural failure of first-story columns took place at significantly higher

lateral deformation. In comparison to codal models, plasticity was observed to be dispersed also

in the second story of the frame as well in modified code model.

7.7 Summary and Conclusions

In the present study, acceptability and adoptability of open ground story structures along with

current analysis and design practices in India was studied. The effectiveness of several

strengthening schemes in improving the performance of open first-story RC frames was evaluated

using pushover analyses of typical 3D frame models.

The strengthening schemes recommended in IS1893-2002 were found to be ineffective in

improving the lateral deformability of such structures exploiting predetermined multiplying factors

for increasing the lateral strength. Additionally, it was concluded that beams in the open first story

are not required to be designed for higher seismic forces. However, sufficient confining

reinforcement is to be provided in these beams to improve their ductility and shear strength. Even

though the lateral strength of the open ground story buildings are comparatively improved by codal

methods the overall ductility and deformation capacity of the building is reduced.

An abrupt reduction in lateral strength was observed in fully infilled structures immediately after

the failure of first story infills. Hence, the ductility of these frames was found to be significantly

lower. Infill panels improve the energy dissipation and global stiffness of the building, thereby

improving the comparative performance during severe earthquakes. The drift and the strength

demands in the ground storey columns are very large for structures with soft ground storeys. It

may not be easy to provide such capacities in the columns of the first storey. Hence, it is clear that

such structures will exhibit poor performance during a strong shaking.

Major conclusions of the present study are:

Performance of open ground story under lateral loadings, especially ductility of RC frames

cannot be improved by using codal strengthening provisions, by designing the ground-story

members for magnified forces.


51

It was found that the current analysis practice of idealizing such as bare frame structures

overestimates the ductility and drift performance during design earthquakes.

Open first story structures with concentrated vertical irregularity are least advised in

seismically active regions due its susceptibility to failure under severe earthquakes and lack

of adequate energy dissipation efficiency, which has been proved in past earth quakes.

Appropriate strengthening systems are to be designed and tested for such buildings for

which further research is necessary.


52

CHAPTER 8.

REFERENCES

[1] AIJ, "Preliminary Reconnaissance Report of the 1995 Hyogoken-Nanbu Earthquake,"

Architectural Institute of Japan, Tokyo, Japan. , 1995.

[2] EQEI, "The January 17, 1994 Northridge, California Earthquake - An EQE Report,," EQE

International, San Francisco, USA, 1994.

[3] J. L. W. ,. N. T. L. E. F. G. Ari Wibowo, "Collapse modelling analysis of a precast soft

storey building in Australia," Elsevier Ltd., 2010.

[4] S. K. J. a. C. M. Jaswant N. Arlekar, "Seismic Response of RC Frame Buildings with Soft

First Storeys," 1997.

[5] M. N. a. P. T. B. Fardis, "Seismic design and response of bare and masonry-infilled

reinforced concrete buildings.Part II: Infilled structures.”," vol. 475–503., 1997.

[6] M. S. B. Nollet, "Stiffened-Story Wall-Frame Tall Building Structure," Vols. Computers

and Structure, Vol. 66, No. 2-3,, 1998.

[7] S. V. Polyakov, "On the interactions between masonry filler walls and enclosing frame

when loaded in the plane of the wall," 1956.

[8] B. a. B. Prasad, "The Influence of Masonry Infill In Reinforced Concrete Multi-Story

Buildings to Near-Fault Ground Motions," 2006.

[9] H. M. a. T. Kabeyasawa, "Effect of Infill Masonry Walls on the Seismic Responseof

Reinforced Concrete Buildings Subjected to the , Bam Earthquake Strong Motion :A Case

Study of Bam Telephone Center," 2004.

[10] X. Y. a. J. M. W. B. Tso-Chien Pan, "Effects of infill walls and floor diaphragms on the

dynamic characteristics of a narrow-rectangle building," 2006.

[11] D. C. R. K. J. Hemant B. Kaushik, "Code Approaches to Seismic Design of Masonry-

Infilled Reinforced Concrete Frames: A State-of-the-Art Review," vol. Volume 22, 2006.


53

[12] I. N. D. a. E. N. Mitsopoulou, "The deficiency of the macromodels with fixed support

interface conditions in the analytical modelling of infill panels," 1995.

[13] D. K. P. a. V. V. S. H. Singh, "Inelastic dynamic response of reinforced concrete infilled

frames," vol. 62, 1998.

[14] I. N. Doudoumis, "Finite technique modelling and investigation of the behaviour of elastic

infilled frames under monotonic loading," 2006.

[15] H. A. a. D. Moghaddam, "The State of the Art in Infilled Frames," 1987.

[16] A. B. Mehrabi, "Behavior of Masonry Infilled Reinforced Concrete Frames Subjected to

Lateral Loadings," University of Colorado, 1994.

[17] L. D. D. A. F. M. Silvia BRUNO, "SEISMIC PERFORMANCE OF PRE-CODE

REINFORCED CONCRETE BUILDINGS," 2000.

[18] B. D.K. Bell, "Evaluation of Earthquake Risk Buildings with Masonry Infill Panels," 2001.

[19] M. N. Fardis, "Experimental and Numerical Investigation on the seismic Response of R.C

Infilled frames and Recommendations for code Provisions," European Consortium of

Earthquake Shaking Tables and Prenormative Research in Support of Eurocode 8, 1996.

[20] P. N. a. A. Colombo, "Irregularities induced by nonstructural masonry panels in framed

buildings",," vol. 19, 1997.

[21] G. Al-Chaar, "Non-ductile behavior of reinforced concrete frames with masonry infill

panels subjected to in-plane loading.’’," 1998.

[22] M. D. a. P. Fajfar, "The effect of masonry infills on the seismic response of a four-storey

reinforced concrete frame -- a deterministic assessment," vol. 30, 2008.

[23] T. a. P. Paulay, "Seismic Design of Reinforced Concrete and Masonry Buildings," 1992.

[24] K. M. A. a. H. Kramul, "A rationale for determining the natural period of RC building

frames having infill," 2006.

[25] M. M. Kose, "Parameters affecting the fundamental period of RC buildings with infill

walls," vol. 31, 2008.

[26] M. P. G. Asteris, "Lateral Stiffness of Brick Masonry Infilled Plane Frames," 2003.


54

[27] M. I. S. S. Ghassan Al-Chaar, "Behavior of Masonry-Infilled Nonductile Reinforced

Concrete Frames," 2002.

[28] M. C. &. J. S.K., "Beneficial influence of masonry infill walls on seismic performance of

RC frame buildings.," 2000.

[29] G.-W. M. ,. Y. L. Hong Hao, "Damage assessment of masonry infilled RC frames

subjected to blasting induced ground excitations," 2002.

[30] J. D. a. D. Thambiratnam, "Modelling and Analysis of Infilled Frame Structures Under

Seismic Loads," School of Urban Development, Queensland University of Technology,

2009.

[31] J. S. Liang Su, "Displacement-based earthquake loss assessment methodology for RC

frames infilled with masonry panels," Elsevier Ltd., 2012.

[32] T. C. Liauw, "An approximate method of analysis for infilled frames with or without

opening,," 1972.

[33] A. E. S. M. A. Fiorato, "An investigation of the interaction of reinforced concrete frames

with masonry walls.," 1970.

[34] B. S. S. A. H. KHAN, "A Simple Method of Analysis for Deflection and Stresses in Wall-

Frame Structures," 1976.

[35] P. B. S. Armin B. Mehrabi, "Finite Element Modelling of Masonry Infilled RC Frames,"

1997.

[36] P. a. S. M. A. Gulan, "Procedure for determining seismic vulnerability of building

structures.," 1999.

[37] A. J. a. L. Durrani, "Seismic Retrofit of Flat-Slab Buildings with Masonry Infills," 1994.

[38] G. L. G. E. a. A. Al-Chaar, "Effects of openings on structural performance of unreinforced

masonry infilled frames," 2003.

[39] M. Holmes, "Steel frames with brickwork and concrete infilling," 1961.

[40] A. T. Council, "ATC 40,Seismic Evaluation and Retrofit of Concrete Buildings," 1996.

[41] F. E. M. Agency, "Prestandard and Commentary for the Rehabilitation of Buildings,"

FEMA 356,, 2000.


55

[42] P. a. F. M. Fajfar, "Nonlinear Seismic Analysis of R/C Buildings," European Earthquake

Engineering, 1997.

[43] E. M. a. S. M.A, "Behavior-Based Method to Determine Design Shear in Earthquake

Resistant Walls,," Journal of the Structural Division, American Society of Civil Engineers,

Vols. Vol.119, No.2, 1992.

[44] P. H. a. E. T, "Direct Inelastic Earthquake Design Using Secant Stiffness," ANCER

Networking of Young Earthquake Engineering Researchers and Professionals, 2004.

[45] M. A.S., "A Pushover Procedure for Tall Buildings," 12th European Conference on

Earthquake Engineering, 2002.

[46] F. S. a. P. T. Sasaki F., "Multi-Mode Pushover Procedure-A Method to Identify the Effect

of Higher Modes in a Pushover Analysis Proc," 6th U.S. National Conference on

Earthquake Engineering, Seattle, , 1998..

[47] C. A. a. G. R.K, "Modal Pushover Analysis Procedure to Estimate Seismic Demands for

Buildings: Theory and Preliminary Evaluation," National Science Foundation;U.S.-Japan

Cooperative Research in Urban Earthquake Disaster Mitigation, 2001.

[48] C. C. a. C. A.K, "Evaluation of Modal Pushover Analysis Using Generic Frames,,"

Earthquake Engineering and Structural Dynamics, 2003.

[49] A. T. a. F. A, "Modeling of Higher-Mode Effects Using an Optimal Multi-Modal Pushover

Analysis,," Earthquake Resistant Engineering Structures, 2005.

[50] C. A. a. G. R.K, "Role of Higher-"Mode" Pushover Analyses in Seismic Analysis of

Buildings,," Earthquake Spectra, , vol. Vol.21 No.4, 2005.

[51] G. B. a. K. K, "Adaptive Spectra Based Pushover Procedure for Seismic Evaluation of

Structures," Earthquake Spectra, 2000.

[52] L. M. a. K. Y. Jan T.S., "An Upper-Bound Pushover Analysis Procedure for Estimating the

Seismic Demands of High-Rise Buildings,," Engineering Structures 26, 2004.

[53] K. E. a. K. S, "Assessment of Current Nonlinear Static Procedures for Seismic Evaluation

of Buildings," Engineering Structures, 2007.

[54] M. A. a. E. A.S., "Static Pushover versus Dynamic Analysis of R/C Buildings,,"

Engineering Structures,, 2001.


56

[55] K. H. a. S. K, "Pros and Cons of a Pushover Analysis of Seismic Performance Evaluation,,"

Engineering Structures, 1998.

[56] T. T. a. A. M. Inel M., "The Significance of Lateral Load Pattern in Pushover Analysis,,"

2003.

[57] S. A. a. A. B. Korkmaz A., "An Evaluation of Pushover Analysis for Various Load

Distributions," in 5th National Conference on Earthquake Engineering, 2003.

[58] O. S, "Evaluation of Pushover Analysis Procedures for Frame Structures," Master of

Science Thesis, 2005.

[59] K. F. D. A. K. C. M. H. a. S. L. Bayülke N., "Nonlinear Pushover Analysis of Reinforced

Concrete Structures and Comparison with Earthquake Damage," in 5th National

Conference on Earthquake Engineering, 2003.

[60] K. M. a. H. B. Polat Z., "Performance Evaluation of a Conventionally Retrofitted Building

by Nonlinear Static Analysis," in 5th National Conference on Earthquake Engineering,

2003.

[61] T. B. a. N. C. Hasgür Z., "Evaluation of the Propriety of Non-linear Analyses Results with

Seismic Damage Indicators for R.C. Building Structures," in 5th National Conference on

Earthquake Engineering, 2003.

[62] E. E. a. H. U. Türker K., "Türk Deprem Yönetmeligine Göre Tasarlanmıs Betonarme Yapıların Performanslarının Degerlendirilmesi,," 2004.

[63] Ö. H. B. H. Inel M., "Re-evaluation of Building Damage During Recent Earthquakes in

Turkey," in Engineering Structures, 2008.

[64] A. C.J., "Seismic Performance of RC Plane Frames Irregular in Elevation,," Engineering

Structures, 2007.

[65] R. E. a. D. R, "The Seismic Performance of Buildings with Weak First Story," Earthquake,

1989.

[66] E. L, "Nonlinear Seismic Response of Soft First Story Buildings Subjected to Narrow Band

Accelograms," in 10th World Conference of Earthquake Engineering, 1992.

[67] C. S. a. K. S, "Structural Behaviour of Soft Story Buildings," in National Earthquake

Engineering Congress, 1994.


57

[68] C. D. a. C. R. Chopra A., "Earthquake Resistance of Buildings with a Soft First Story,,"

Earthquake Engineering and Structural dynamics, 1973.

[69] M. M, "Enhancing the Seismic Performance of Existing "Pilotis" Configurations," 2006.

[70] A. S. a. B. Hobbs, "INELASTIC DESIGN OF INFILLED FRAMES," JOURNAL OF

STRUCTURAL ENGINEERING, no. 121:634-650, 1995.

[71] D. D. a. C. Murty, "Brick masonry infills in seismic design of RC frame buildings," Indian

Concrete Journal Part 1 & 2, 2004.


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