seismic performance of brick infilled rc frame...

Seismic Performance of Brick Infilled RC Frame

Structures in Low and Medium rise Buildings in

Bhutan

Jigme Dorji

A Thesis submitted for the degree of

Master of Engineering

Centre for Built Environment and Engineering Research

Queensland University of Technology

June 2009

i

Abstract

The construction of reinforced concrete buildings with unreinforced infill is common

practice even in seismically active country such as Bhutan, which is located in high

seismic region of Eastern Himalaya. All buildings constructed prior 1998 were

constructed without seismic provisions while those constructed after this period

adopted seismic codes of neighbouring country, India. However, the codes have

limited information on the design of infilled structures besides having differences in

architectural requirements which may compound the structural problems. Although

the influence of infill on the reinforced concrete framed structures is known, the

present seismic codes do not consider it due to the lack of sufficient information.

Time history analyses were performed to study the influence of infill on the

performance of concrete framed structures. Important parameters were considered and

the results presented in a manner that can be used by practitioners.

The results show that the influence of infill on the structural performance is

significant. The structural responses such as fundamental period, roof displacement,

inter-storey drift ratio, stresses in infill wall and structural member forces of beams

and column generally reduce, with incorporation of infill wall. The structures

designed and constructed with or without seismic provision perform in a similar

manner if the infills of high strength are used.

Keywords

Infilled frames, Seismic response, Influence, RC buildings, Stiffness, performance,

infill, inter-storey drift ratios, fundamental period, soft-storey

ii

Publications

Dorji,J and Thambitratnam D.P. “Seismic Response of Infilled Structures”,

Proceedings of the 20th

Australasian conference on the Mechanics of structures and

Materials, Toowoomba, Australia, 2-5 December 2008.

Dorji,J and Thambitratnam D.P. “Modelling and Analysis of Infilled frame Structures

under Seismic loads”. The Open Construction and Building Technology Journal,

Bentham Science Publisher, vol. 3, 2009.

iii

Table of Content

Abstract i

Keywords i

Publications ii

Table of Content iii

List of Figures viii

List of Tables x

Symbols vii

Acronyms viii

Statement of Original Authorship ix

Acknowledgement x

Chapter 1. Introduction

1.1 Background of this study ................................................................................... 1

1.2 Research problem ............................................................................................... 2

1.3 Significance and Innovation of the Research ..................................................... 4

1.4 Research methodology ....................................................................................... 5

1.5 Outline of the thesis ........................................................................................... 6

1.6 Conclusion………………………….................................................................8

Chapter 2. Literature review

2.1 Introduction…………………………………......................................................9

2.2 Seismic hazard exposure of Bhutan ................................................................. 10

2.3 Arcade provision .............................................................................................. 11

2.4 Seismic design principle .................................................................................. 12

2.5 Analyses types ................................................................................................. 14

2.5.1 Static analysis ....................................................................................... 15

2.5.2 Response spectrum analysis ................................................................. 15

2.5.3 Time history analysis ........................................................................... 16

2.5.4 Viscous damping .................................................................................. 17

2.6 Modelling of Infill frame ................................................................................. 18

iv

2.6.1 Micro- model ....................................................................................... 19

2.6.2 Macro-model (Equivalent diagonal strut): ........................................... 20

2.7 Strength ........................................................................................................... 24

2.8 Lateral Stiffness .............................................................................................. 25

2.9 Failure modes of infilled frames ..................................................................... 25

2.10 Consideration of infill in current codes .......................................................... 27

2.11 Recent research ............................................................................................... 28

2.12 Summary of Literature Review ....................................................................... 34

2.13 Conclusion………………………………………………………………...…35

Chapter 3. Model Development

3.1 Introduction……………………………………………………......................36

3.2 Model development ……………………………………………….................36

3.2.1 Geometry and Boundary conditions…………………………………..37

3.2.2 Material property……………………………………………………...38

3.3 Static analysis………………………………………………………………...38

3.4 Design of reinforced concrete frames……………………………………......42

3.4.1 Structures designed to IS 1893:2002……………………………….....43

3.4.2 Existing structures (referred to seismic codes)………………………...47

3.5 Gap element………………………………………………………………......48

3.6 Infilled frame models………………………………………………………...49

3.7 Ground motions……………………………………………………………....51

3.8 Conclusion.......................................................................................................52

Chapter 4. Time History Analyses

4.1 Introduction ...................................................................................................... 53

4.2 General studies .................................................................................................. 54

4.2.1 Effect of soil stiffness (Ks) ................................................................... 54

4.2.2 Frequency of Ground motion (Power spectral density analysis) ......... 55

4.3 Modal Analyses ............................................................................................... 58

4.3.1 Fundamental period ............................................................................. 58

4.4 Time History Analysis ...................................................................................... 61

4.5 Ten storey model............................................................................................... 63

v

4.5.1 Roof displacement ............................................................................... 63

4.5.2 Inter-storey drift ratios ......................................................................... 64

4.5.3 Infill stress ............................................................................................ 66

4.6 Seven storey model .......................................................................................... 67



4.6.3 Infill stresses ........................................................................................ 68

4.7 Five storey model ............................................................................................. 69

4.7.1 Roof displacements .............................................................................. 69


4.7.3 Infill stresses ........................................................................................ 71

4.8 Three storey model ......................................................................................... 71



4.8.3 Infill stresses ........................................................................................ 73

4.9 Infill strength (variation of Young’s Modulus of Elasticity Ei) ....................... 74

4.9.1 Fundamental period (T) ....................................................................... 75


4.9.3 Infill stress ............................................................................................ 78


4.10 Openings ......................................................................................................... 82

4.10.1 Fundamental periods .......................................................................... 83

4.10.2 Inter-storey drift ................................................................................... 84

4.10.3 Infill stress ............................................................................................ 85

4.10.4 Member forces ..................................................................................... 85

4.11 Strength of concrete material (Ec) .................................................................... 86

4.11.1 Fundamental period (T) ....................................................................... 87

4.11.2 Maximum roof displacements .............................................................. 88

4.11.3 Inter-storey drift ratio ........................................................................... 89

4.12 Infill thickness (t) ............................................................................................. 90


4.12.2 Inter-storey drift ratio ........................................................................... 92

4.12.3 Member forces ..................................................................................... 93

4.13 Peak ground acceleration (PGA)..................................................................... 96

vi

4.13.1 Building design without seismic provisions ........................... 96

4.13.1.1 Inter-storey drift ratios .............................................. 97

4.13.1.2 Infill stress ............................................................... 101

4.13.2 Structures constructed with seismic provision ....................... 102

4.13.2.1 Inter-storey drift ratio .............................................. 102

4.13.2.2 Infill stress……………………………………..... 105

4.15 The Arcade effect/Soft storey phenomenon ................................................. 106

4.15.1 Roof displacement ............................................................................. 106

4.15.2 Inter-storey drift ratio ......................................................................... 108

4.15.3 Column moments .............................................................................. 109

4.15.3.1 Column shears .................................................................................. 110

4.15.3.2 Beam moments................................................................................. 110

4.15.3.3 Beam shears ..................................................................................... 111

4.16 Conclusion…………………………………………………………………..112

Chapter 5. Discussion

5.1 Introduction .................................................................................................... 114

5.2 Interface element ............................................................................................ 114

5.3 Damping ......................................................................................................... 115

5.4 Parametric study results ................................................................................. 115

5.4.1 Effect of infill strength (Ei) ................................................................ 115

5.4.2 Effect of Opening ............................................................................... 116

5.4.3 Effect of infill thickness ..................................................................... 117

5.4.4 Effect of concrete strength ................................................................. 118

5.4.5 Seismic resistance capacity of infilled RC frame .............................. 119

5.4.6 Soft-storey phenomenon induced by arcade provision ...................... 121

5.5 Design guidance & recommendation ............................................................. 122

5.5.1 Fundamental period ........................................................................... 122

5.5.2 Selection of infill material ................................................................. 124

5.5.3 Inter-storey drift ratios ....................................................................... 126

5.5.4 Arcade solution .................................................................................. 126

5.6 Conclusion ................................................................................................. 127

vii

Chapter 6. Thesis Conclusion

6.1 Conclusion………….……………………………………....……… 128

viii

List of Figures

Figure 1.1 Building with Arcade provision.

Figure 2.1 Seismic hazard map of Eastern Himalaya (GSHAP, 1992).

Figure 2.2 Typical buildings structures.

Figure 2.3 Failure mechanism in reinforced concrete frames.

Figure 2.4 Rayleigh proportional damping coefficient.

Figure 3.1 Model showing the frames, infill and the gap elements.

Figure 3.2 The gap stiffness and infill stiffness.

Figure 3.3 load deflection relationship of models with and without infill.

Figure 3.4 Validation of the model.

Figure 3.5 Lateral load application to the structure.

Figure 3.6 Bare frame structures; (a) three storeys,(b) five storeys, (c) seven storeys

and (d) ten storeys.

Figure 3.7 The gap element

Figure 3.8 Infilled structures; (a) three storeys,(b) five storeys, (c) seven storeys and

(d) ten storeys.

Figure 3.9 Strong ground motions; (a) El Centro, (b) Kobe and (c) Northride

earthquakes.

Figure 4.1. Spring model representing the soil stiffness.

Figure 4.2 . Dominant frequencies of the El-Centro Earthquake.

Figure 4.3. Dominant frequencies of the Kobe Earthquake.

Figure 4.4. Dominant frequencies of the Northridge Earthquake.

Figure 4.5. Variation of Fundamental period with percent of infill present in models.

Figure 4.6. Fundamental period of vibration of infilled structures.

Figure 4.7. Roof displacement time histories; (a) El-Centro Earthquake, (b) Kobe

Earthquake and (c) Northridge Earthquake.

Figure 4.8 Inter-storey drift ratios of the ten storey structure.

Figure 4.9. Roof displacement time histories; (a) El-Centro Earthquake, (b) Kobe


Figure 4.10. Inter-storey drift ratio of a seven storey model.

ix

Figure 4.11. Roof displacement time histories of five storey model; (a) El Centro

earthquake, (b) Northridge Earthquake and (c) Kobe Earthquake.

Figure 4.12. Inter-storey drift ratio of a five storey model.

Figure 4.13. Roof displacement time histories of a three storey model; (a) El Centro

earthquake, (b) Kobe Earthquake and (c) Northridge Earthquake.

Figure 4.14. Inter-storey drift ratio of the three storey model.

Figure 4.15 Fundamental period vs Ei

Figure 4.16 Maximum roof displacement vs Ei..

Figure 4.17 Maximum stresses within the infill walls (a) Fully infilled wall and (b)

Infill with 40% opening.

Figure 4.18 Inter-storey drift ratio for un-damped models.

Figure 4.19 Inter-storey drift ratio of 3% damping.

Figure 4.20 Inter-storey drift ratio 5% damping.

Figure 4.21 Variation of fundamental period with opening.

Figure 4.22 Inter-storey drift ratios for different opening percentages.

Figure 4.23 Concrete strength vs. Fundamental period.

Figure 4.24 Roof displacement history of a model with Ec=15000 MPa.

Figure 4.25 Inter-storey drift ratios of models with varying Ec value.

Figure 4.26 Variation of fundamental period with infill thickness.

Figure 4.27 Roof displacement histories of models with different infill thickness.

Figure 4.28 Inter-storey drift ratios.

Figure 4.29 Structures without seismic provisions.

Figure 4.30 Inter-storey drift ratios of a ten storey model (a) 5% damping; (b) 0 %

damping.

Figure 4.31 Inter-storey drift ratios of a seven storey model (a) 5% damping; (b) 0 %

damping.

Figure 4.32 Inter-storey drift ratios of a five storey model (a) 5% damping; (b) 0 %

damping.

Figure 4.33 Inter-storey drift ratios of a three storey model (a) 5% damping; (b) 0 %

damping.

Figure 4.34 Inter-storey drift ratios of a ten storey model (a) 5% l damping; (b) 3 %

damping.

Figure 4.35 Inter-storey drift ratios of a ten storey model (a) 5% damping; (b) 3 %

damping.

x

Figure 4.36 Inter-storey drift ratios of a five storey model (a) 5% damping; (b) 3 %

damping.

Figure 4.37 Inter-storey drift ratios of a three storey model (a) 5% damping; (b) 3 %

damping.

Figure 4.38 (a) S-normal model and (SI) model with Arcade.


Figure 4.40 column moment.

Figure 4.41 Beam moment.

Figure 4.42 Shear in column.

Figure 4.43 Shear in beam.

Figure 5.1. Variation of fundamental period.

Figure 5.2 Variation of stresses with different parameters.

xi

List of Tables

Table 3.1 Gap stiffness corresponding to the contact coefficient.

Table 3.2 Roof displacement versus infill strength.

Table 3.3 Seismic weight calculation of three storey structure.

Table 3.4 Distribution of lateral force.

Table 3.5 Structural member sizes of models designed with seismic code.

Table 3.6 Reinforcement details of beams and columns.

Table 3.7 Structural member sizes of models designed without seismic code.

Table 4.1 Effect of lateral stiffness of soil on global structural deformation.

Table 4.2 Fundamental period of vibraton of infilled frames.

Table 4.3 Maximum principal stress in the infill.




Table 4.7 Variation of maximum infill stress in the infill wall.

Table 4.8 Variation of stress in infill with opening percentage.

Table 4.9 Variation of member forces with opening percentage.

Table 4.10 Moments in beams and columns.

Table 4.11 Shear force variation.

Table 4.12 Variation of infill stresses with PGA for non-seismic structures.

Table 4.13 Variation of infill stresses with PGA for aseismic structures.

Table 4.14 Magnification factors for structural member forces.

Table 5.1 Magnification factor.

xii

Symbols

a The mass proportional damping coefficient

Ah Horizontal seismic force coefficient

b The stiffness proportional coefficient

[C] Damping matrix

d Roof displacement of a single storey model, mm

E The Young’s modulus of elasticity, MPa

Ec Young’s modulus of reinforced concrete, MPa

fE Young’s modulus of frame element, MPa

Ei The Young’s modulus of elasticity of infill material, MPa

mE Young’s modulus of elasticity of infill masonry, MPa

F Horizontal point load, KN

fm The compressive strength of infill masonry, N/mm2

f(t) The inertial force due to earthquake, KN

g Gravitational pull force, m/s2

h Total height of the building, weight, KN

hi Height of the floor from the base, mm

I Moment of inertia of the columns, mm4

I Importance factor assigned on important structures

K Lateral stiffness of the combined RC frame and the infill N/mm

Kg Stiffness of the Gap element in N/mm

Ki Relative stiffness of the infill panel, N/mm

Kf Lateral stiffness of the RC frame system, N/mm

Ks Soil stiffness, N/mm 001197517673651

[K] Stiffness matrix

L Height of the columns, mm

[M] Mass matrix

bM The moment in the beams at the joint, KN/m

cM The moment the column at the joint, KN/m

n The number of years

rP The mean return period in years

xiii

eP The probability of exceedance in n years

Qi Horizontal seismic load at a particular storey level, F

R Response reduction factor

Sa Acceleration coefficient

t. Time, s

t Thickness of the infill wall in mm

T Fundamental period of vibration, s

Tb Fundamental period of vibration of a bare frame model, s

Ti Fundamental period of vibration of an infilled frame model, s

u Tip displacement in mm

ug Displacement at first floor level in mm

ur Horizontal displacement at roof level in mm

Vb Design base shear, KN

w Width of a diagonal strut, mm

Wi Seismic weight of a particular floor level, KN

x Displacement, mm

.

x Velocity, m/s

..

x Acceleration, m/s2

Z Seismic zone factor

µ Contact coefficient between infill wall and frame members

ω Natural frequencies, Hertz

lα Length of contact between column and infill, mm

hα Length of contact between beam and infill, mm

Φ Strut angle with respect to horizontal axis, degree.

Ω Proportionality constant between Young’s modulus and compressive strength

ξ Percentage of damping

xiv

Acronyms

FE Finite Element

IS Indian Standards

DL Dead load of the structure

LL Live load on the structure

EL Earthquake load

WL Wind load

PGA Peak ground acceleration

RC Reinforced concrete

ME Maximum earthquake

DE Design earthquake

SE Serviceable earthquake

EPA Effective peak acceleration

CQC Complete quadratic equation

FEM Finite element method

PSD Power spectral density

SRSS Square roots of the sum of squares

TVERMP Thimphu Valley Earthquake Risk Management Programme

GSHAP Seismic hazard map of Eastern Himalaya

xv

Statement of Authorship

To the best of my knowledge and belief, the content of this Thesis is not previously

submitted to meet requirements for a degree at QUT or any other institutions.

However, any information retrieved from other sources are properly cited and

acknowledged in Bibliography.

Signature;

xvi

Acknowledgement

The principal supervisor Prof. David P. Thambiratnam is highly grateful for his

guidance, continued support, encouragement and constructive suggestions throughout

the research work. It was truly a blessing in disguise to have you as the principal

supervisor who has immense knowledge in this research field, patience and an art to

give a meaning to the younger generation. Without your help and assistance, this

work would not have been what it is now. I once again thank you for everything you

contributed towards this research. The associate supervisors, Dr. Nimal Perera and Dr.

Mustafa Yosufe, are grateful for their advice and moral support during the initial stage

of this research.

The Royal Government of Bhutan is highly appreciated for awarding scholarship and

continued support during the course of research. Without your financial assistance,

the possibility of upgrading knowledge and skill is almost impossible. The faculty of

Built Environment, QUT, is highly acknowledged for giving facility support,

administrative guidance and organising numbers extracurricular activities.

At last but not the least, I sincerely thank my wife, Tshering Choden, who had come

all the way from Bhutan to Australia to support me. I also thank my parents and

family members who were in Bhutan for their indirect support.

1

1 Introduction

1.1 Background of this study

Bhutan is located in a high seismic region due to its geographical position along the

tectonic boundary of the Indian and Eurasian plates. The high seismicity in these

regions is attributed to the subduction of the Indian plate beneath the Eurasian plate,

which seems to move at an average of 23 millimetres per year (Bilham, 2001). This

movement causes elastic deformation of the plates rather than inelastic deformation

and thus strain energy has been accumulating for many years which could result in

disastrous earthquakes of greater magnitude in the future. In the last three decades the

country experienced several moderate size earthquakes.

Reinforced concrete (RC) buildings were constructed in Bhutan as early as the 1970s

and since then the infilled reinforced concrete framed building has become the

preference of clients/owners intending to construct buildings more than three storeys

high. Such buildings are mostly constructed in urban centres due to rapid growth in

urban population. The height of the structures varies from single storey to eight

storeys. Until late 1990s, there was no regulation to design and construct building

structures for seismic resistance and most building structures were designed to resist

wind and gravity loads only. However, the importance of seismic design was realized

slowly with time and thus countries such as Bhutan have come up with some rules

and regulations on aseismic structures. Currently, the Indian Seismic Standard (IS

1893: 2002) is being used for the analysis and design of new buildings in Bhutan.

Thimphu Valley Earthquake Risk Management Programme (TVERMP,2005) was

initiated to study the vulnerability of the buildings constructed without seismic

consideration, where the author was one of the working group members. The study

was conducted in two phases; (1) Preliminary study and (2) Detailed study. During

the preliminary study, buildings were randomly selected based on the age of the

structure, the material used, the codes adopted, and irregularity in plan and elevation.

The intended purpose of the building was considered for further detailed study during

2

the second phase. The detailed studies on selected buildings were performed by the

consultant, the results revealing deficits in structural resistance against earthquake

forces. However, infill walls were not considered in the analyses. The preliminary

study showed that the strength of the concrete and steel reinforcement used was

significantly lower than the present code requirements. Moreover, these structures

lacked the strength and ductility required by current seismic Standards. However,

there are no reports of damaged or collapsed buildings due to earthquakes that struck

the country in the last few decades.

The reinforced concrete frame structure with masonry is the most common type of

construction technology practised in Bhutan. Infill materials such as solid clay brick

masonry, solid or hollow concrete block masonry, adobe and stone masonry are

available. The brick masonry is the most preferred infill material in reinforced

concrete buildings because of its advantage such as durability, thermal insulation, cost

and simple construction technique. The use of adobe infill wall is rare but it has been

used in some buildings. There have been some incidences where infill walls

developed cracks after the earthquakes, especially office and residential buildings.

Moreover, the current code is silent on the use of infill material and thus the choice of

infill material is random as it is believed to be a non-structural component.

The existing seismic code (IS1893, 2002) considers the effect of infill in terms of the

fundamental period of vibration, which does not consider the extent of infill usage.

While most of the seismic codes disregard the influence of infill walls, some of the

codes do consider infill walls. Moreover, past research work has shown that there is a

considerable improvement in the lateral load resisting system by adding the walls.

The most likely reason why the influence of infill walls is ignored in seismic design

standards is due to their complicated failure mode. Infill walls fail in a brittle manner,

while the reinforced concrete can sustain lateral loads over large post-yield

deformation.

1.2 Research problem

RC framed structures with infill walls are a common form of buildings of more than

two storeys high in countries like Bhutan, where the seismicity of the region is

3

considered to be high. Infill walls, however, are treated as non-structural components

even though they provide significant improvement in lateral stiffness of the frame

structures. This leads to random selection of infill materials. The Indian Seismic

Standard (IS 1893: 2002) which is currently being used for the analysis and design of

new buildings in Bhutan does not make any specific reference to in-fill walls.

however, cracks on the infill wall do appear even under mild earthquakes and thus

there is a need to know the strength limit of infill material under the action of credible

earthquakes. Besides, some buildings are given higher importance factor even though

the use of infill material is the same irrespective of how important the structure is.

Thus, there is no information on the strength of infill wall for all categories of

structures at different performance levels.

Moreover, there are many buildings which were not designed for seismic resistance.

Although such buildings would not fulfil the requirements of the modern seismic

codes, it is important to address the seismic resistance level of these buildings and to

know the future course of action from the disaster management point of view.

Road-side buildings in commercial centres are required to provide an Arcade shown

in Figure 1.1. This provision may or not compound the structural problem under

seismic load. The inability of the static analysis on the bare frame system, generally

practised, to trace the accurate behaviour of the real structures, resulted in the use of

the empirical magnification factor which is conservative in nature. Thus, there is a

need to study the effect of Arcade on the infilled structures and study the validity of

the magnification factors of the structural member forces.

As there are many buildings which were constructed before the adoption of seismic

regulations, the construction materials used during those days were of low quality,

especially the strength of the concrete. There is a need to know the variation of

structural response with varying strength of infill and concrete material.

One of the main problems with buildings that have infill walls is that they have

different sizes of openings. The present codes are implicit in nature and it is difficult

4

to relate code prescriptions to reality. Thus, it is essential to realise the real behaviour

of typical buildings of Bhutan, under seismic loads.

Figure 1.1 Building with Arcade provision.

1.3 Significance and Innovation of the Research

Enforcement of the seismic regulation has been in place since 1998; however, the art

of construction has not changed much until today. Traditional architectural

requirements are stringent and such regulations seem to compound the intricate

behaviour of buildings under seismic excitation. Thus, there is a dire need to

understand the influence of the infill wall on the performance of the structures having

infills of varying strength. Since the existing seismic codes lacked comprehensive

5

information on the design of infilled RC frame structures, there is a need to generate

adequate information which can be used in practice and also for development of our

own codes.

The significance of this research lies in addressing the performance levels of buildings

constructed before and after the adoption of the seismic code in Bhutan. The research

achieves it by examining the influence of infill walls on the reinforced concrete

structures, and the effect of infill strength on the performance of building structures.

The current seismic code contains Empirical formulae which may or may not be

applicable to typical buildings in Bhutan. Since there is insufficient information in the

standard/ open literature on the use of infill material, the structural designer generally

considers the infill wall as a non-structural component, in-spite of its influence on

lateral strength. This research gathers information on the minimum strength of infill

required for a credible earthquake of 0.2g. Notwithstanding the above, the present

code lacks information on the selection of infill material under varying seismic loads.

The influence of infill (typically used in Bhutan) on the seismic behavior of RC

frames in low and mid rise buildings will provide adequate information on the seismic

vulnerability scenario of the building stock constructed before and after adoption of

seismic regulations. This research also generates information on infilled frame

behavior due to varying parameters which are commonly seen in the building industry

of Bhutan, and thus development of hitherto unavailable seismic design guidance for

this industry in Bhutan.

1.4 Research methodology

The finite element (FE) technique was used to carry out this research. The gap

element was used as the interface element between the infill wall and the frame

members to transfer the lateral load between columns. The stiffness of the gap

element was found by trial and error procedure and the results were validated using

results from previous research by Doudoumis (1995), in the absence of experimental

validation. The effective stiffness equation was developed for the gap element which

was then used in modelling the frame interface condition for bigger structures. Four

typical models were developed, ranging from three storeys to ten storeys. All these

6

models had openings in the centre of the infill walls which were considered to be

typical building structures in Bhutan.

The models were studied under three different earthquakes and the most severe

earthquake was chosen for a particular model to carry out parametric studies. In this

case, the Kobe earthquake was found to be dominant on a ten storey model which was

used for parametric studies. Some of the important parameters were; strength of the

infill, which was expressed in terms of Young’s modulus of elasticity of infill (Ei),

Opening size percentage, Strength of concrete, which was expressed in terms of

Young’s modulus of elasticity of concrete (Ec), thickness (t) of infill wall, Peak

Ground Acceleration (PGA) and the height of the structure. The models were

analysed under different ground motions and the results are presented for maximum

response of the structure towards a particular earthquake. This was done to study the

extent of the influence of the infill wall.

The output results were expressed in terms of fundamental periods, inter-storey drift

ratios, roof displacements, member forces and the stresses in the infill wall. These

results were used to interpret the influence of different parameters on the global

structural performances of the building models and addressing the gap that exist in

current seismic design codes. The information generated can be used to address the

lack of provision for infill wall effects in current design codes used in Bhutan.

1.5 Outline of the thesis

Chapter 1: Introduction

This chapter presents the background and introduction to the topic,

defines the research problem, states the aim and objectives and

outlines the scope and method of investigation adopted in the

research project.

Chapter 2: Literature Review

This chapter presents the review of previously published literature

in the field of infilled reinforced concrete frame structures. It also

7

reviews the general response of reinforced concrete structures,

performance of the infill wall and inclusion of the infill’s influence

in the current codes. The chapter also highlights the importance

and scope of this research.

Chapter 3: Model development and validation

This chapter presents the development of the gap stiffness and the

validation of the results on a single storey, single bay model. The

gap stiffness derived from a simple model was then used for bigger

models to perform parametric studies. The structural member sizes

of all the models were designed in accordance to appropriate codes,

assuming that the buildings were constructed in different times of

code development.

Chapter 4: Time History Analyses results

This chapter presents the results of the time history analyses of all

the models considered in this study. The results are presented

sequentially with general parameters in the beginning followed by

important parameters of the infilled framed structures. The

parametric studies were done on structural models designed for

vertical loads. The models designed with seismic provisions were

studied only for Peak ground acceleration. The results are

expressed in terms of roof displacements, inter-storey drift ratios,

member forces, fundamental periods and the stresses in the infill

wall.

Chapter 5: Discussion of analyses results

This chapter presents the discussion of the results presented in

chapters 3 and 4. This discussion covers the importance of the

results, their application, and reasons for non-coherence with the

results of research conducted by others. The discussion also covers

the short-falls in existing seismic codes (used in Bhutan) and the

contribution of the research to the development of future seismic

design codes.

8

Chapter 6: Conclusion

This chapter highlights the main contributions and outcomes of this

research. Recommendations for further research are also proposed.

1.6 Conclusion

The need for this study was envisaged and the concise methodology, the research gap

and the innovation aspired for, were presented in the current Chapter. In the next

chapter, Literature review in this particular field is presented.

- 9 -

Chapter 2. Literature review

2.1 Introduction

As early as 1960s, studies have been carried out to study the influence of infill on the

moment resisting frames under lateral loads induced by earthquakes, wind and the

blast. Numerous experimental and analytical investigations have been carried out;

nevertheless, a comprehensive conclusion has never been reached due to the complex

nature of material properties, geometrical configuration and the high cost of

computation. Though the effect of infill is widely recognised, there is no explicit

consideration in the modern codes, thus the practising/design engineers end up

designing the buildings based on judgement.

Infill is generally considered to be the non-structural elements, in-spite of its

significant contribution of lateral stiffness and strength against the lateral load

resistance of the frame structures. Conversely, there is a common misconception

among the designers that it will increase the overall lateral load carrying capacity.

This would lead to undesirable performance of moment resisting frames because the

infill which was not considered during design stage would modify the inherent

properties of RC frame members. As consequent, failure in different forms would be

the result due to additional loads on the stiffened members.

The construction of reinforced concrete structures with infill wall is a common

method of providing shelter to the ever increasing population in the developing

countries such as Bhutan, where there is seismic activity. The lack of information on

the behaviour of such structure in the existing seismic code is hence an issue. This

review presents the seismic hazard scenario of Bhutan, modeling techniques of

infilled structures, consideration of infill in current seismic codes and recent

development in this particular field.

- 10 -

2.2 Seismic hazard exposure of Bhutan

High seismicity in the Himalayan regions is attributed to the continuous movement of

the Indian plate towards the North, subducting beneath the Eurasian plate. This has

been occurring since 55 million years ago, in which the plates move at an average rate

of more than 20 mm per year (Rai, 2004; Bilham, 2001). These plates are locked and

the stresses are accumulated in elastic strain rather inelastic strain (Bilham, 2001).

Thus, the researchers speculate one or more big earthquake in the Himalayan regions.

The seismic hazard scenario of Himalaya region is shown in Figure 2.1.

Bhutan had experienced about 32 earthquakes in last seven decades (1937 to 2006).

The most powerful earthquake was on 21st January 1941, measuring the 6.75 on

Richter scale. The most recent earthquakes that stuck Bhutan were in 2003 and 2006

which had the magnitude of Mw = 5.4 and 6.4 respectively. Although, due to frequent

seismic activities in the Himalayan region, which would alleviate the formation of

large earthquakes, Bilham and Molnar (2001) reported that the big earthquakes are

overdue and could happen any time in future.

Peak Ground Acceleration in m/s2

Figure 2.1 Seismic hazard map of Eastern Himalaya (GSHAP, 1992).

Bhutan

- 11 -

The earthquake hazard is classified as three major groups (IS1893, 2002) based on the

probability of exceeding the level of PGA and the return period as follows. The

Serviceability Earthquake (SE) is the level of ground shaking that has 50 percent

chance of being exceeded in a fifty years period and has the return period of 75 years.

Design Earthquake (DE) is the level of ground shaking that has 10% of chance of

being exceeded in a fifty 50 years period and has the return period of 500 years.

Maximum Earthquake (ME) is the level of ground shaking that has 5 percent

probability of being exceeded in a 50 years period with the return period of 1000

years. The relationship between the return period and the probability of exceedance in

a fixed number of years is given by the Equation 2.1.

=rP )1ln(1

1

1

ePne

−−

2.1

Where;

rP = The mean return period in years

eP = the probability of exceedance in n years

n = the number of years

The existing seismic code (IS1893, 2002) considers DE to have the effective peak

acceleration (EPA) of 0.18g while it is 0.36g for ME earthquake level.

2.3 Typical building structures

An Arcade is a pedestrian space left in the ground floor of the building facing the road

sides as shown in Figure 2.2. It is normally provided in the commercial buildings for

smooth flow of human traffic along the narrow paths. It is also the requirement of the

City Council and the Traditional Architectural Guidelines to have Arcade in building

in commercial hub.

The arcade provision would compound the structural problems associated with

earthquake failure. Although, there is no incidence of failure occurred due to arcade

provision during moderate earthquakes that took place in last few years, the engineers

suspect that it may induce structural problems such as soft storey phenomenon during

earthquakes. Since no study has been conducted on this type of structural irregularity,

there is a lack of information on the behaviour of such structures. Moreover, the

current seismic code imposes a magnification factor for the structural member forces.

- 12 -

This topic might have been widely studied, but the relevant information is not widely

available in the literature.

Figure 2.2 Buildings with Arcade.

2.4 Seismic design principle

The fundamental principle of seismic design is to minimise the loss of lives and

property in the event of disastrous earthquake. This is being ensured by providing

enough strength, stiffness and ductility in the structures and the ductility of the

structural member could be achieved by proper reinforcing details at the appropriate

locations (IS 13920; 1993). However infill walls are treated as the non-structural

components and its significant contribution on the structure is ignored. Consequently,

this could result to unacceptable performance of the structures during earthquakes as

the presence of infill may alter the behaviour of the frame structures.

To control the sudden failure of entire structure, a concept of “strong column weak

beam” has been introduced in most seismic codes world wide, wherein the beams are

made to undergo damages before the columns during ultimate loading condition. The

displacement based design concept has gained popularity due to better understanding

of nonlinear behaviour of reinforced concrete material under the earthquake load.

Thus, failure of beams is preferred rather than the column failure. This is achieved by

introducing the strength factor of 1.3 on the column moments at the joints.

- 13 -

Accordingly, the sum of column moment at a joint should be 6/5 times the sum of

beam moments as given by Pauley (1992) in Equation 2.2.

bc MM Σ=Σ56

2.2

Where:

cM = the moment the column at the joint

bM = the moment in the beams at the joint

Both the beams and the columns are provided with the ductile detailing so that the

structure fails in desired manner as shown in figure 2.2. Allowing the beam elements

to dissipate energy prior to the columns to control the abrupt failure more realistic in

case of the bare frame system but there is no much research information available on

infilled frame designed to this concept.

Column failure mechanism Beam failure mechanism

Figure 2.3 Failure mechanism in reinforced concrete frames.

(Parducci, 1980) have reported that strong infill with weak frame undergoes

premature failure of the columns and the report was based on the test performed on a

single story infilled frame. However, there is no enough report on the infilled frame

designed to ‘strong column weak beam concept’ (CEB, 1996; CEB, 1996; Pauley,

1992). However, this concept may not be applicable to infilled RC structures as the

presence of infills alters the global behaviour of the structural system, besides

increasing the possibility of structures failing due to soft-storey mechanism during

strong earthquakes.

- 14 -

The presence of infill would alter the stiffness of the members (Pauley, 1992), thus

some members get over stressed for which they are not designed, are liable to suffer

damages. Therefore, there is a need to consider the effects of infill on framed

structures.

According to (ATC40, 1996), there are five categories of structural performance

levels such as Immediate occupancy (sp-1), Damage control (sp-2), Life safety (sp-3),

Limited safety (sp-4), and the Structural stability (sp-5). Similarly, there are four

categories of non-structural performance levels such as Operational (np-1), immediate

occupancy (np-2), Life safety (np-3) and the reduced hazard (np-4). These

performance levels are for the post earthquake assessment based on the physical

observation. Since there are many types of infill materials, performance level would

vary and thus important to study the suitability of an infill material for intended

purpose.

2.5 Analyses types

There are different types of analyses to treat the seismic forces on a structure. Most

codes specify both static and dynamic analyses, with the choice based on a number of

considerations such as the importance of the building, its height, the effect of the soil

and the seismic hazard at the location based on past events AS1170.4 (2001). The

static analysis is an indirect method of considering the effect of the ground motion on

the structure and it normally incorporates some of the dynamic features of the

problem, such as fundamental period of the building, the soil effect and the

earthquake hazard. The Time history dynamic analysis on the other hand is a direct

method in which a selected earthquake record in the form of an acceleration –time

history is used as the input. Time history analysis can be used for both linear and

nonlinear analysis. There is also a pseudo dynamic analysis method called the

response spectrum method in which the relevant periods of a building are used to

obtain the accelerations to be applied to the structure. In addition to the code specified

methods of seismic analysis, the nonlinear static pushover analysis can be used to

obtain an initial evaluation of the seismic capacity of a building.

- 15 -

2.5.1 Static analysis

This method of seismic analysis involves distribution of total base shear through out

the height of the structure. The base shear is found based on the seismic coefficient

which is based on the seismic hazard exposure of a particular location and total

weight of the structure. Although, this method is a static procedure there is an

incorporation of dynamic properties of the structure in terms of fundamental period

and response reduction factor. However, this method is limited to a regular type

structure whose maximum response is governed by the first mode of vibration.

If the infill walls are considered while modelling and analysis, most of the structures

lower than ten storeys in height would give maximum response at first mode. This is

due to an additional stiffness contributed by the infill which eventually makes the

structure stiffer and rigid.

2.5.2 Response spectrum analysis

This method uses the peak modal responses obtained from dynamic analysis on a

single-degree-of-freedom system. The peak acceleration is found for different periods

for the model and plot of spectral acceleration versus period gives the curve which is

called response spectrum curve. This curve is generally very rough but the codes

recommend the smoothened curve. The values for low range of period are kept

constant while it is varied for high period models. It is not necessary to use the code

specified spectrum if the site specific spectrum is available.

This technique is extended to the multi-degree-of-freedom system by performing

linear superimposition of modes shapes using the modal combination techniques such

as SRSS (square roots of the sum of squares) and CQC (Complete quadratic

combination). The disadvantage of SRSS is its incapability of considering the modes

that are very close unlike the CQC. The result from this analysis gives only the peak

structural responses at desired damping values.

- 16 -

2.5.3 Time history analysis

The Time history analysis involves a time-step by step integration of dynamic

equilibrium equation. The general Equation for a dynamic response of a multi-degree-

of-freedom system subjected to ground motion is given by Equation 2.6.

)()(][)(][)(][...

tFtxKtxCtxM =++ 2.3

Where;

[M] = Mass matrix,

[C] = Damping matrix,

[K] = Stiffness matrix,

..

x = Acceleration

.

x = Velocity

x = Displacement

f(t) = The inertial force due to earthquake.

The solution for this Equation can be achieved by performing numerical integration

methods such as Newmark Integration method, Wilson-θ method and Runje-Kutta

forth order method. The SAP 2000 uses Newmark Integration method in which the

acceleration is assumed to vary linearly from time t to t+∆t.

The structural responses are computed at each time step and thus Equation 2.3 is

solved. The stability criterion of the numerical method is conditional for explicit

algorithm but it is unconditionally stable for implicit algorithm. When the algorithm is

unstable (improper time step size) the higher mode shapes dominate the structural

response and it is unacceptable, especially for medium rise buildings. However, when

the time step size is too small the computation time lengthens and becomes

uneconomical. Since the present study treats only 2D analyses, the lower modes will

dominate the response. A time step of the order Tn/10 or smaller would be adequate

where Tn is the period of the nth mode and all modes up to the nth mode will then

participate in the analysis. In the present study which considered buildings with

different number of storeys, a time step of 0.005 was used. 0.005 was << fundamental

- 17 -

period T1 of the shortest building. The chosen time step provided adequate

convergence of results.

2.5.4 Viscous damping

It is a force that resists motion at all times. Ideally, there are no structures which do

not have damping because all structures are in one way or other posses force on

account of frictional force in the joints, air resistance and the frictional force within

the molecule of the material.

SAP 2000 considers the damping as the linear combination of mass matrices and the

stiffness matrices. This is given by Equation 2.4 and is also called Rayleig damping

equation.

[C] = a [M] + b [K] 2.4

Where [C], [M] and [K] were defined in the previous section. The coefficients a and b

are the mass proportional damping coefficient and stiffness proportional coefficient.

The damping is directly proportional the frequency but inversely proportional to the

mass as shown in figure 2.4. The values of proportional damping coefficients can be

found by specifying damping ratios ξi and ξj for ith

and jth

modes. This leads to

equations 2.9 and 2.10.

ji

ji

ia ωω

ωωξ +=

2

2.5

jijb ωωξ += 2 2.6

In which, iω and jω are the natural frequencies of ith

and jth

modes. Thus, the damping

value [C] can be known. In general, 5% damping ratio is considered for reinforced

structures and 3% and 7% for unreinforced and reinforced masonry structures

(Chopra, 2000). However, the damping ratios for in-filled framed structures could not

be found in any of the literature.

- 18 -

Figure 2.4 Rayleigh damping (Anil K. Chopra, 2001).

2.6 Modelling of Infill frame

Model development of any structures is crucial to achieve accurate output results.

However, it is difficult to model the as-built structures due to numerous constraints

with as it is difficult to incorporate all physical parameters associated with the

behaviour of an infilled frame structure. Even if all the physical parameters, such as

contact coefficient between the frame and infill, separation and slipping between the

two components and the orthotropic of material properties are considered, there is no

guarantee that the real structure behaves similar to the model as they also the

structural behaviour could also depend on the quality of material and construction

techniques.

However, to simulate the structural behaviour of infilled frames, two methods have

been developed such as Micro model and Macro model. The Micro model methods is

a Finite Element Method (FEM) where the frames elements, masonry work, contact

surface, slipping and separation are modelled to achieve the results. This method has

seems to be generating the better results but it has not gained popularity due to its

cumbersome nature of analysis and computation cost.

The Macro models which is also called a Simplified model or Equivalent diagonal

strut method was developed to study the global response of the infilled frames. This

method uses one or more struts to represent the infill wall. The drawback of it is to the

lack of its capability to consider the opening precisely as found in the infill wall.

- 19 -

2.6.1 Micro- model

A Finite Element (FE) method is a process of discritizing the structural components

into a smaller sizes, maintaining the constitutive laws of material, boundary

conditions, in order to improve the accuracy of results. However, this method is

mostly limited to small structures as it requires high computation equipments besides

taking comparatively longer time. Relevant research on infilled frame that were done

in past few decades were reviewed and presented in this section.

Achyutha, jagadish et al (1985) investigated the elastic behaviour of a single storey

infilled frame which had opening. The interface conditions such as slip, separation

and frictional loss at the contact surface were considered using the link element. They

were achieved by adjusting the axial, shear and tension force in the link element. The

opening was modelled by assigning very low values of infill thickness and Young’s

modulus of elasticity of infill but high value of Poison’s ratio. It was reported that the

lateral stiffness of the structure decreases with the increase in opening size. The

principal stresses were maximum at the corners of opening and the compression ends

when full contact was the condition which further increased by allowing separation at

the interface. However, the author stated that the equivalent diagonal strut mechanism

may not be applicable for structures which have openings.

The behaviour of infilled frame under an in-plane load was studied by Dhanasekar

and Page (1986). The results from biaxial tests on half scale solid brick masonry were

used to develop a material model for brick and the mortar joints which were then used

to construct non-linear finite element model. The results were that that the Young’s

modulus of elasticity of the infill has a significant influence on the behaviour of the

infilled frame. However, the influence of Poison’s ratio was fond insignificant on the

behaviour of structure. It was also reported that the infill wall failed due to shearing

along the diagonal length of the wall and hence the influence of compressive strength

of infill material was not observed. The bond strength and tensile strength of infill

masonry were found to influence the behaviour and ultimate capacity of the infilled

frame.

- 20 -

The FE model with and without a perfect contact between the infill wall and the

reinforced concrete frame was studied by Combescure and Pegon et al (1995) on a

single bay single storey structure. It was reported, under unilateral contact condition

(frictionless), the forces between the frame and fill panel are transferred through a

compression corners at the ends of diagonal strut. However, there is no transfer of

shear force from infill to frame. When a perfect contact condition was considered at

the interface, shear force transfer between the two.

Haddad (1991) studied the application of a finite element method to assess the effects

cracking and separation between the frame and infill of an infilled frame structure.

The model considered the crack size and location, relative stiffness and contact

length. It has been found that the bending and deflection decreases with the increase

in infill frame relative stiffness. Bending moment further increased with the crack

depth. The moment at the un-cracked section increased when the crack size on other

end was increasing. The magnitude and location of principal compressive and tensile

stresses were affected by crack size, contact length and infill frame relative stiffness.

However, the author recommended the good use of material and construction

techniques to reduce damages due to separation and cracking.

Similar research on the infilled structures, using FE technique, were carried out by

(Morbiducci, 2003; Saneinejad, 1990; Seah, 1998; Lourenco, 1996; Singh, 1998).

However, most of them had investigated on a single storey models under in-plane

static loads.

2.6.2 Macro-model (Equivalent diagonal strut):

The main disadvantage of performing finite element analysis for the global structural

response study is due to computation cost and the nature of complexity in model

generation. Thus, to simplify the model generation, macro-model method has been

developed based on the experimental and finite element analysis results, wherein,

diagonal struts are used to represent the infill.

- 21 -

The concept of equivalent diagonal strut method was initially introduced by Polyakov

(1960) while investigating a three storey infilled structure. The cracks along the

diagonal length of panel gave an insight of the strut behaviour of an infill panel. The

report stated that the stress from peripheral frame members to the infill was

transferred only through the compression corners of the frame-infill interface.

Benjamin and Williams (1958) investigated three different models, in which a

masonry wall, masonry wall encased with the reinforced concrete frame and the

masonry wall with steel frames. All these models were tested under an in-plane load.

The test revealed the importance of aspect ratio which influences the ultimate capacity

of the infilled frames. It was also reported that masonry has significant role in

contributing lateral strength to the frame, however the size of masonry element did

not affected the result. The importance of concrete cross-sections and steel

reinforcement was realised. Since it was the beginning of the research in this field,

dynamic loads were not considered and the thus results were conventional.

Holmes (1961) proposed the width of equivalent strut to be one third of the diagonal

length from his experimental study on a single storey single bay infilled structure

under an in-plane loads. Smith (1962) conducted a study on a infilled structure

experimentally on a small scale specimen. The specimen had steel frame and concrete

mortar as infill. The in-plane load was applied at the top corner of the infilled

specimen and was observed a compression region within the infill panel which made

the frame stiff and thus the concept of Diagonal strut method was evolved. It was also

reported that longer the contact length between the infill panel and the frame, wider

the width of strut.

Smith (1966) proposed a formula to calculate the width of strut based on the relative

stiffness of the fame and infill wall. The suggested formula was investigated by

performing numerous tests on different specimens. The theoretical relation of the

width of strut proposed by Stafford Smith is shown below.

42

sin

4

Φ= Π

tE

HIE

m

cf

lα 2.7

- 22 -

42

2sin

4

Φ= Π

tE

LIE

m

bf

hα 2.8

Where;

lα = length of contact between column and infill, mm.

H = Height of the infill wall, mm.

L = length of the infill wall, mm.

Ic = Second moment of inertia of column section, mm4.

Ib = Second moment of inertia of beam section, mm4.

hα = length of contact between beam and infill, mm.

mE = Young’s modulus of elasticity of infill masonry, MPa.

fE = Young’s modulus of frame element, MPa.

Φ = strut angle with respect to horizontal axis, degree.

t = thickness of the infill, mm.

mm fE Ω=

mf -compressive strength of masonry

The value of a constant Ω equals to 750 for concrete block and 500 for clay brick

(Pauley, 1992). Hence the width (w) of a strut element is;

22

2

1lhw αα += 2.9

Similar studies were performed by Mainstone (1971), however claimed that it is

different to previous works by not considering the aspect ratio and covering the whole

range of behaviours shown by infill in tall structures. The behaviour of infilled

structure was distinguished into two and the first one being stressing the infill wall

thoroughly assuming a perfect fit between the infill and frames. The second behaviour

assumed that the infill and the frames contact only at the compressive corners, in

which crushing of infill take place. It was also reported that the corner crushing and

the cracking along the diagonal length of the infill would take place depending on the

relative strength infill wall and the frame. Thus it was summarised that the relative

- 23 -

stiffness of the infill and frame was the important parameter of the infilled structure.

The report also includes the usefulness of the Equivalent strut method to estimate the

stiffness, strength and the ultimate strength of the system.

The effects of the location of opening on the lateral stiffness of infilled frame was

studied by Mallick and Garg (, 1971) and had recommended possible locations for

door and window. The study was conducted on a model with and without shear

connectors. It was reported that the structure with shear connector but having opening

at either ends reduces the stiffness by 85 to 90% of the fully infilled model. On the

other hand, the stiffness was reduced by 60 to 70% for the model without shear

connector. Also, it was reported that the stiffness reduces by 25 to 50% when the

opening is placed at the centre of the infill wall. Thus, the suggested position for the

door is at the centre of the lower half of the infill wall while the window can be placed

at the middle height of the infill wall at either side. However, such requirement is

stringent and not practical for general residential structures and thus reinforcement of

infill wall come into picture.

Since the opening of the infill cannot be considered using the above formula, there

are reports in which more numbers of struts can be used to accommodate the effect of

opening. Asteris (2003) developed a coefficient to reduce the width of strut element

for the infill panel which has opening. Puglisi and Uzcategui (2008) proposed a

plastic concentrator to be used with the diagonal strut element, which does the same

function as the hinges in beam and column of the reinforced concrete frames. The

advantage of using the method is to simulate the inelastic behaviour of the infilled

frame, especially in terms of stiffness degradation and low cycle fatigue.

Although the diagonal strut model have gained popularity in modelling and analysis

of infilled structures, it is only suitable for the study of global structural responses

However, the FE technique is the most preferred method for most of the researchers

as it allows to understand both local and global responses.

- 24 -

2.7 Strength

Numerous experimental and numerical investigations carried out in past have proven

that the presence of infill improves significantly the lateral strength of an infilled-

frame system. The parameters involved in increasing the strength are strength of infill

materials, strength of surrounding frame elements, relative stiffness of infill to frame

ratio, presence of opening, reinforcement of infill panel, strength of mortar and

masonry blocks, lack of initial fit between infill and the frame etc.

The presence of gap between the infill and the frame were studied by (Mainstone,

1971; Parducci, 1980; H.A., 1987; Schmidt, 1989) and reported the decrease of

strength of the infilled-frame system, however the result varied from one another

depending on the gap considered and the material used.

The openings seemed to decreases the lateral strength of the infilled frame system.

Research by (Benjamin, 1958; Liauw, 1977; Liauw, 1979) reported significant

reduction in the strength while (Dawe, 1985; Moghaddam, 1987) did not observe

any change in strength. The benefits of shear connector were presented by (Mallick,

1971; Klingner, 1976; Higashi, 1980) and their results were inconsistent. The reports

on the increase in cracking load and ultimate load of an infilled structure with the

increase in the strength of masonry block and mortar were presented (Parducci, 1980;

Mehrabi, 1994). On contrary, Moghaddam and Dowling (1987) did not find

significant increase of strength.

In experimental test by (Stylianidis, 1985), mortar strength of 2.4 MPa was used and

columns failed prior to the failure of infill. This type of failure mechanism is against

the desire of current seismic codes, yet, limitation on the strength of mortar is rarely

given in the Standards and further studies is required for high rise building under

dynamic loading. Research also found that there is a small increase in lateral strength

by providing the reinforcement in the infills, however (Zarnic, 1985) did not observed

any increase of strength due to poor bond condition between the mortar and the

reinforcement due to early cracking along bed joints.

- 25 -

2.8 Lateral Stiffness

It is known that the presence of infill in the frames increases the lateral stiffness of the

system by four to twenty times to that of bare frame system (CEB, 1996). However, it

is difficult to quantify the extra stiffness contributed by the infill in terms of absolute

figure due to numerous parameters involved in the system. Doudoumis and

Mitsopoulou (, 1995), reported that the stiffness of the infilled-frame system depend

significantly on the strength of infilling materials. However, this study was limited to

static linear procedure considering a single storey single bay model.

Numerous research were carried out by (Parducci, 1980; Mainstone, 1971;

Moghaddam, 1987; Dawson, 1972) to study on the influence of stiffness from the

infill walls, by considering a gap at the interface between the frame and the infill wall.

It was reported (Mainstone 1971) that there is a noticeable decrease in the stiffness of

the system while (Moghaddam, 1987) observed 40% decrease in stiffness.

Experimental and FE investigations were carried out for an infill frame by (Thomas,

1950; Ockleston, 1955; Benjamin, 1958; M.Sobaith, 1988; Dukuze, 2000; Anil,

2006), considering various parameters, and reported significant influence on strength

and stiffness of an assemblage. Lateral strength of building can be increased by

introducing infill panel (Anil, 2006) if the structure has problem with drift.

The lateral stiffness of retrofitted RC frame was investigated by (Erdem, 2006) on two

test specimens. The first specimen was with reinforced concrete infill while the

second specimen was with hollow concrete block with diagonally placed CFRP strip.

It was reported that the stiffness of the first specimen increased by 500%, however the

second specimen showed better strength degradation beyond the peak load. Form the

above review, it was learnt that the infill generally increases the lateral stiffness of the

structural system which could be used for resisting the lateral load from earthquakes.

2.9 Failure modes of infilled frames

The experimental as well as the numerical research performed over last few decades

showed different failure mechanism of an infilled frame structure. Most of them have

used single storey system under in-plane loads. It has been reported that the separation

takes place between the infill and the frames at the early stage of loading all around

- 26 -

the interface excepting the two compressive ends. The angular distortion of the infill

studied by Polyakov (1960) varied in its value between 31003.0/ −×=∆ h to

3107.0 −× (where ∆ is the horizontal displacement and h is the height of the storey),

depending on the relative stiffness of the infill to the frame stiffness and the external

load. The onset of separation may also depend on the quality of workmanship, lack of

fit and material quality. However, the prediction of separation is not important as it

does not considerably affect the rigidity of the infilled-frame (CEB (1996).

Stafford Smith (1966) reported that the weak frame cannot transmit the forces to the

compressive diagonal of infill and thus suffers local crushing at the ends of

compressive diagonal. On the contrary, the strong frame can transmit high forces to

the compressed diagonal which set infill to initiate cracking from the central region

and propagates towards the compressed diagonal ends (Mainstone, 1971). It was also

reported that when the weaker infill is use with stronger frame system, horizontal

sliding failure occurs along the bed joints of the masonry (Zarnic, 1985). On the

contrary, when the stronger infill was used with the weak frame, the frame underwent

premature failure of columns before the onset of frame failure (Parducci, 1980). It

means that the infilled frame does not reach to its full capacity.

Generally mortar joints are considered to be the planes of weakness due to low shear

resistance. Cracks can appear in the interface column and infill, beam and infill and

between the infill elements which give negative impression on performance of the

structures (Miranda Dias, 2007; Miranda Dias, 2007). The shearing failure of joint

was reported in research carried out by (Abdou, 2006) and (Miranda Dias, 2007)

occurred along the plane of weakness.

Merabi (1994) observed brittle shear failure of the column on windward side while

investigating the infilled frame structure which had strong infill panel and weak

frame. However, the increase in lateral load resistance was found even after the shear

failure in column, indicating some kind of ductility due to infill. On the contrary, the

formation of hinges in columns and slip in the bed joints were observed in the a weak

infill frame test specimen. The stronger frame with stronger infill had failed by

- 27 -

crushing of infill as the shear failure of columns was prevented due to enough shear

reinforcement and bigger column size.

2.10 Consideration of infill in current codes

Most of the seismic codes ignore infill due to the brittle nature of failure, varying

properties and low deformation capacity. However, the presence of infill changes the

behaviour of structural system from frame action (Murty, 2000) to truss action due to

significant contribution of initial lateral stiffness. Some of the codes which consider

the infill for seismic resistance are given below.

The IS1893 (2002), which is currently being used in Bhutan, considers the effect of

infill in terms of natural period of vibration. However, there is no proper information

on the basis of equation as it is empirically related to the height and width of a

structure. Also, the same empirical equation is used irrespective of the extent of infill

present in the structure. Moreover, there is no control over choice of infill material,

giving wide options to the builders to select material whose performance during

earthquakes is uncertain. As a result, infill wall is considered as non-structural

component of the buildings although literature revealed that there is a significant

influence on the lateral strength and stiffness of the structures. The soft-storey

problem associated with infill structures is addressed by providing a prescriptive

magnification factor on structural member forces. It is not possible to compute the

actual stiffness of the infilled structures due to absence of infill model generation in

the code. The inter-storey drift ratio is limited to 0.004 irrespective of consideration

of infill wall.

Eurocode 8 (2003) considers the effect of infill on the natural period of vibration by

taking into account the correction factor (Ct) derived based on the effective cross-

sectional area of infill wall in the first storey. It requires the frame members to resist

100% of the vertical loads and 50 to 60% of the total horizontal load on the structure.

This code allows reasonable irregularities in plan by doubling the accidental

eccentricity but recommends dynamic analysis for an unacceptable irregularity

problem. It recommends that the infill wall which has only one opening, either door or

window, has a significant influence on the frame. For other walls which have more

- 28 -

than one opening, proper measures, such as reinforcing the wall and providing

concrete member along the perimeter of the opening, are recommended. The code

also recommends the out-of plane failure of infill wall by limiting the slenderness

ration of wall to 15. It is the ratio of the length or height to thickness of the wall

whichever gives more. The stiffness of infill wall is taken into consideration by

recommending the use of diagonal strut. However, the thickness of strut is not

specified as it varies with the opening. There is no mention about the modulus of

elasticity of infill material.

Nepal code (NBC-201, 1995) considers infill by recommending the use pin-jointed

diagonal struts element as an infill wall. However, the width of strut is not

recommended and hence the consideration of opening is not realised. The distribution

of axial forces and lateral seismic loads are specific. The code also recommends the

Young’s modulus of infill material to be 2500 to 3000MPa. The walls which have

opening less than 10% of the wall area is treated as structural wall and if the opening

exceed 10%, the wall are provided with Reinforced concrete elements all around the

opening perimeter and recommends appropriate reinforcement. The out-of plane

failure is prevented by providing the concrete bands at one third and at two third of

the wall height. However, there codes which recommend the isolation of infill wall

from the frame (NZS-3101, 1995).

2.11 Recent research

A comprehensive experimental and analytical investigation into the behaviour of

infilled structures was conducted by Merabi (1994). It was reported that the infill has

significant improvement on the lateral strength and stiffness of a bare frame and also

significantly improves the energy dissipation capability of the structure. The aspect

ratio of the infill panel was found to have little influence on the behaviour of the

frame while the cyclic loadings degrade the structure faster than the monotonic

loadings. It was also reported that the increase in vertical loads significantly improves

the lateral load carrying capacity of the structure, the distribution of vertical load

between beam and column has insignificant influence. There is indication of the

increase in lateral load carrying capacity by increasing the number of storey, however

- 29 -

this may not be true for high rise structures and thus similar study has to be conducted

for higher number of storeys.

The validity of different macro-models such as 4-node shear panels, 4-node plane

stress element and the higher order 8-node plane stress element were studied by

Doudoumis and Mitsopoulou (1995) on a single storey single bay model and

comparison were made with the results of a FE model. However the macro-models

had shown inaccurate displacements and infill stresses, especially at higher infill’s

stiffness. Therefore, such macro-model does not represent the true model of real

structures.

Fardis (1996) investigated the seismic response of an infilled frame which had weak

frames with strong infill material. It was learnt that the strong infill which was

considered as non-structural is responsible for earthquake resistance of weak

reinforced concrete frames. However, since the infill’s behaviour is unpredictable,

with the likelihood of failing in brittle manner, it was recommended to treat infill as

non-structural component by isolating it from frames. On the contrary, since infill is

extensively used, it would be cost effective if infill’s positive affects are utilised.

Negro and Colombo (1997) investigated the effects of irregularity induced by non-

structural masonry wall on a full scale four storey RC structure under pseudodynamic

tests. The specimen frame was designed to Eurocode 8. The results reported that the

presence of non-structural wall can change the behaviour of framed structures

significantly. The irregular distribution of infill has been reported to impose

unacceptably high ductility demand on the frame buildings. Both numerical and

experimental investigation showed irregular behaviour of frames even if the

distribution of infill is uniform or regular.

Singh, Paul et al (1998) had developed a method to predict the formation of plastic

hinges and cracks in the infill panels under static and dynamic loads. The 3-noded

frame element, 8-noded isoparametric element and 6 noded interface element were

used to model the frame member, infill panel and the interface element. The study has

shown good agreement with the experimental results, especially in terms of failure

- 30 -

load and the strut width. The observed load factor was 12 which is significant

contribution of the infill panel and also reported the inadequacy of the Linear analysis.

Al-Chaar (1998) performed studies on the behaviour of reinforced concrete frames

with masonry infill. The test was conducted on two half-scale specimens in which one

of the frames was stronger than the other. The strong frame specimen showed

diagonal tension cracking while the weak frame failed from diagonal cracking as well

as hinging of the column at lower load. Both the frames were reported to have shown

the ductile behaviour but the extent of ductility is not specific. However, the author

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

capacity of the plane structures which are useful for structures in seismic regions.

Dominguez (2000) studied the effects of non-structural component on the

fundamental period of buildings. The models consist of five storeys, ten storeys and

15 storeys with diagonal struts as the infill (non-structural component). It was

reported that the presence of infill decreases the fundamental period of the structure.

When the models was provided with 100 mm infill thick, the fundamenta fundamental

period was decreased by 46%, 40% and 34% for five storey, ten storeys and 15

storeys. When the infill thickness was 200 mm, the fundamental period was 53%,

44% and 36% respectively. The trend of decrease in period with increase in thickness

is decreasing with the increase in height. However, the effect of thickness is not

significant. However, the effect of masonry strength was reported to be insignificant

on the fundamental period of the structure as the difference between two models

which had 8.6MPa and 15.2 MPa was 10.4%. The significant difference was observed

by increasing the number of bays. When the number of bays was increased to two, the

difference in fundamental period was 15%. However, the author did not consider the

effect of above parameters with opening in the infill panel.

Dukuze (2000) investigated the failure modes of infilled structure on a single storey

specimens with and without opening. In general, three types of failures were observed

under an in-plane load such as sliding of bed joints, tensile cracking of infill and local

crushing of compressive corners at the loaded corner. The specimen with opening at

the centre of panel had suffered shear cracks at the point of contact and severe

- 31 -

damages on the Lintel beam. It was reported that only piers (infill between opening

and column) of specimen exhibited diagonal cracking. The contact length between the

infill panel and frame had increased by increasing the stiffness of of the confining

frame. However, when the aspect ratio (H/L) was increased, the crack pattern spread

throughout the panel and the column fails in shear and bending. The failure of fully

infilled specimen was dominated with diagonal cracking along with shear slip along

mortar joints. Although, failure occurred at the loaded corners in most cases, the

specimen which had strong column, failure occurred mostly near the beam in the

loaded corner and conversely failure concentrate near the loaded region of column

when there beam is stronger than column.

Since the extent of infill’s effect on reinforced concrete frame is known to be

significant, Menari and Aliaari (2004) developed an isolation system called SIWIS

system. This system prevents the failure of column or infill walls by introducing a sub

system which is breakable after reaching the full strength and stiffness of the infill

wall. However, such system is not recommended in any of the codes yet as it would

be expensive.

The effect of masonry was studied by carrying out the pushover analysis, using the

N2 method given in the Eurocode 8 (CEN, 2004), on a four storey infilled model in

which the infill was represented with diagonal strut element (Dolsek, 2008). It was

reported that the presence of infill can totally change the distribution of damages

within the structure. However, it was also observed that the presence of infill do not

cause the failure of columns due to shear, which is contrary to literature in (Pauley,

1992). It was also reported that there is significant contribution of lateral stiffness and

strength when the seismic demand is less than the deformation capacity of the infill

wall. Conversely, the stiffness and strength degraded when seismic demand exceeded

the deformation capacity of infill. Thus there is a need to study on the requirement of

infill material.

Perera (2005) proposed a damaged model of the infilled structure, in which the infill

panel was represented with diagonal strut based on the degradation of strength and

stiffness. The damage of the structure was mainly based on the storey drift ratio. It

- 32 -

was reported that the weak frame reached the higher damage indices at failure than

the strong frame. The cracks seemed to appear when the drift ratio was within the

range of 0.17% to .46%.

A study on soft-storey issues with infilled structures was conducted on a single bay

three storey RC frame structure (Santhi, 2005). The specimen, which was 1:3 in scale,

had masonry infill in upper two stories while the ground level storey was without

infill. The results of the soft structure were compared with the bare frame structure. It

was reported that 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 forces.

Doudoumis (2006) studied the importance of contact condition between the infill and

frame members on a single storey FE model. It was reported that the interface

condition, friction coefficient, size of the mesh, relative stiffness of beam to column,

relative size of infill wall have significant influence on the response of infilled frame,

while the effect of orthotropy of infill material was reported to be insignificant. It

means the infill can be treated as homogenised material. When the mesh density was

made finer the stress pattern within the infill was also improved, with maximum

values of stresses at the compressive corners. 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. The response parameter were also

increase with stiffness of frame and infill and the relative size of frame and infill pane,

the author questions the reliability of formula established for the width of strut

element. However, this study was conducted for a single storey model under

monotonic load it is important to conduct similar studies for more number of stories

under earthquake load.

Strengthening of reinforced concrete structure with an infilled wall having an opening

within the wall was carried out by Anil (2006; Anil, 2006). It was reported that there

- 33 -

was significant increase in ultimate strength as well as the initial stiffness of the

structure. The strength and stiffness of the infilled structure increase with the aspect

ratio of the infill wall. However, when the precast infill walls were supported to both

the beams and columns, the results were found to be more accurate.

Amanat (2006) studied the rational for determining the natural period of infilled

reinforced concrete frames using the FE models. The important observation was that

the amount infill has significant influence on the fundamental period of the structure

but the distribution of infill within the structure has no significance influence. The

report also mentioned about the closeness in the fundamental period of FE model and

the code formulas, however had recommended magnification factors for the important

parameters such as span length, number of span and the amount of infill, to modify

the bare frame period given by code Equations. However, the author recommends

pursuing further study to generalise the fundamental period of the infilled frame

structures.

Kaushik (2006) conducted a comparative study of the seismic codes, especially on the

design of infilled frame structures. The study revealed that the most of the modern

seismic codes lack the important information required for the design of such

buildings. Moreover, the clauses of different codes are not consistent and vary from

country to country. Such variations were attributed to the absence of adequate

research information on important structural parameters as determination of natural

period of vibration of infilled structures, soft storey problem associated with infill’s

presence, exclusion of infill’s strength and stiffness and consideration of openings.

The main reason for not considering the beneficial affects of infill is due to variation

in material property as well as brittle nature of failure.

Similar studies were carried out by Kose MM (2008) considering the various

parameters affecting the fundamental period. The infilled frame was modelled using

the diagonal struts for the infill panels and the opening within the panels was

incorporated by reducing the width of strut using a reduction coefficient proposed in

(Asteris, 2003). The height of the structure and the amount of shear wall was reported

to be the main parameter which affect the fundamental period while the percentage of

- 34 -

opening and the number of bays were reported to have similar influence. The

important observations were that the fundamental period of the infilled frame was

5%-10% less than that of bare frame models. However, the presence of shear wall in

the infilled structure did not make any significant difference on fundamental period of

the models. Since there is no mention about the type of material for infill and shear

wall the information available is inadequate.

2.12 Summary of Literature Review

• Finite element method seems to give better results than the Equivalent

diagonal strut method. FE investigations were mostly limited to single storey

models under static loads while Equivalent diagonal technique was used for

large structures.

• The use of infill, generally considered to be non-structural, seems to provide

significant initial stiffness but limited deformation. However, there are reports

of significant improvement in energy dissipation of the system which will be

useful for seismic structures. The increase in initial stiffness of the structural

system modifies the structural behaviour from frame action to truss action.

• The infill material can be treated as homogeneous material as some of the

reports didn’t find any significant variation in the structural responses between

models of different material properties.

• An infill is not considered in most of the modern seismic codes due to brittle

nature of failure and varying properties of infill material. However, it will be

important to understand the requirement of infill material for specified levels

of earthquake in order to incorporate infill effectively. There is a lack of

adequate information in current seismic codes on the use of infill material.

• Due to limited knowledge on the dynamic behaviour of infilled frames, thus

empirical formulae are used in most of the seismic codes. Such formulae are

generally conservative.

- 35 -

• The absence of seismic code for a seismically active country like Bhutan

besides having different Architectural requirements for the building structures,

raises questions in the minds of people on the danger of bigger earthquakes.

Thus, this research was evolved.

2.13 Conclusion

Literature review presented the seismic scenario of Bhutan and the architectural

feature of the buildings which could compound the dynamic behaviour of structural

elements. Relevant research out-comes of the past research were discussed and

presented. The next Chapter presents the methodology and model development.

35

Chapter 3. Model Development

3.1 Introduction

This research studies the seismic performance of infilled reinforced concrete frame

structures designed and constructed with and without reference to seismic codes,

using dynamic computer simulation techniques. At the start a computer model of the

structure was developed and validated. Accordingly, a single storey, single bay Finite

Element (FE) model of a reinforced concrete frame with masonry infill was

developed. The Gap element, which is normally used to model the contact between

two structures, was used as the interface element connecting the infill wall to the

surrounding frame. The stiffness property of the Gap element was investigated

through a trial and error procedure to match the results of previous research,

(Doudoumis, 1995) and validated.

The Gap property developed in this study was then used in the development of multi-

storey infilled frame structures. There are two categories of multistorey models; (1)

Existing Structural models and (2) Aseismic structural models. The existing structural

elements were designed for vertical loads only while the aseismic structural models

were designed to Indian seismic code IS 1893 (2002), which is currently followed in

Bhutan. Each of these models was based on number of storeys; three storey, five

storey, seven storey and ten storey models. These models were analysed under three

typical ground motions; the El Centro earthquake, Kobe earthquake and the

Northridge earthquake. Linear Time History analyses were performed up to initiation

of failure and the results are presented in Chapter four. This Chapter will describe the

model development.

3.2 Model development

A single bay, single storey reinforced concrete frame with masonry infill, as shown in

Figure 3.1, was modelled for validation (Doudoumis, 1995). The aspect ratio of the

model is one with three meters in height and length. In the present model the interface

between the frame and the infill is provided with the Gap elements which have the

36

capability to transfer the load to the infill masonry. There are two columns, one at

each side, and a beam at the top and bottom of the system to match the existing

solution (Doudoumis, 1995). A fixed support condition was provided at the base of

the two columns while beam column joints were rigidly connected, using the FEM

software SAP 2000.

The structural members (beams and columns) were modelled as beam elements while

the infill masonry was modelled with plane stress elements as shown in Figure 3.1.

The in-fill panels were meshed with finite elements of 150 mm square in size in order

to avoid the infill behaving as a shear panel, to increase the accuracy of the result

(Doudoumis, 2007) and to provide adequate convergence of results. The size of the

finite elements used to model the beams and columns were 150mm long.

Figure 3.1 Model showing the frames, infill and the gap elements.

3.2.1 Geometry and boundary conditions

The above model was developed to match the model by Doudoumis (2007), who used

contact elements as the frame-infill interface. The size of the column section was 375

x 375 mm square and the beam section was 500 mm deep and 420 mm wide and the

infill was 200 mm thick. Frame-infill interface was modelled as a contact surface

considering the friction coefficient. Though infill masonry consists of block and

mortar, this model considers one material for the infill.

37

3.2.2 Material property

The infill masonry was considered as homogenous and the Young’s modulus of

elasticity of masonry infill was taken from the Empirical relation E = 750 fm (fm is the

compressive strength of masonry) developed by (Pauley, 1992) and the Poison’s ratio

was 0.15. The Young’s modulus of elasticity of concrete was 24000 MPa while the

Poisson’s ratio was 0.2. The densities were 19.6 KN/m3 and 24 KN/m

3 for infill

masonry and concrete members respectively.

3.3 Static analysis

In the absence of definite information on the coefficient of friction µ , Doudomis and

Mitsopoulou (1995) used two separate analyses, one with µ = 0 and the other with µ =

1 for a model with contact surface. Accordingly, a static analysis was performed to

study the stiffness of the gap element required to produce the same roof displacement.

While doing so, the gap stiffness was varied until the current result (roof

displacement) matched with the reference result and the corresponding gap stiffness

(Kg) was recorded. The same procedure was repeated for different strengths of infill

material ranging expressed in terms of young’s modulus (Ei) from 1 GPa to 15 GPa

while other parameters such as horizontal load, material property, cross section of

concrete members and the thickness of the infill were kept the same and the results are

shown in Table 3.1.

Table 3.1 Gap stiffness (Kg)in N/mm corresponding to the contact coefficient (µ = 1 and µ = 0)

Ei t

106 (N/mm)

Gap stiffness (Kg)

Corresponding to µ = 0

(N/mm)

Gap stiffness (Kg)

Corresponding to µ = 1

(N/mm )

0.2 1600 4500

0.5 12500 23000

1.0 30000 50000

1.5 50000 70000

2.0 63000 107000

2.5 70000 120000

3.0 82000 130000

38

The gap stiffness shown in Table 3.1 is for two extreme cases; frictionless contact and

100 % frictional contact at the interface. These extreme cases may not exist as

physical conditions due to the presence of mortar at the interface between the frame

and the infill. Figure 3.2 shows the average gap stiffness required to simulate different

strengths of infill material approximately corresponding to a friction coefficient µ=

0.5. From the literature review it was learnt that contact friction at the interface cannot

be quantified as it depends on many factors including the quality of workmanship.

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3 3.5

Infill stiffness (K i ) x106 N/mm

Gap s

tiff

ness (K

g)

X 1

03 N

/mm

Figure 3.2 The Gap stiffness (kg) and infill stiffness ( ki )

The linear relationship between the gap and infill stiffness, as shown in Figure 3.2,

could be used in modelling the interface element between the frame and the infill for

capturing the global structural response of an infilled frame system. In order to

simulate frame-infill interaction using a Gap element, less stiffness is required for low

strength infill and higher stiffness is required for higher strength infill. This

relationship between the infill stiffness and the gap stiffness is given by Equation 3.1.

Kg = 0.0378 Ki+347………………………………………………………………....3.1

Where

Ki = Stiffness of the infill panel

Kg = Stiffness of the gap element.

39

The analyses were repeated once again using the gap stiffness given by equation 3.1.

The load deflection curve of the infilled frame and the bare frame (without infill) were

compared as shown in Figure 3.3.

0

50

100

150

200

250

300

350

400

0 5 10 15

Deflection in mm

Load in

kN

Infilled frame

Bare frame

Figure 3.3 Load deflection relationship of models with and without infill

As shown in the above Figure, the infilled frame system has higher stiffness than the

bare frame system under lateral load. The additional stiffness of the infilled system is

due to the presence of infill. The general force displacement relationship of an infilled

frame system can therefore be written as;

F = Kd ………………………………………………...………...................... (3.2)

K = Kf+Ki ……………………………………………….….………...................... (3.3)

Ki = Eit…………………………………………………...…………….................. (3.4)

Where;

F = Horizontal load

K = Lateral stiffness of the combined RC frame and the infill.

d = Roof displacement of a system

Ki = Relative stiffness of the infill panel

Kf = Lateral stiffness of members of the RC frame system range in value from

312

33

L

EIto

L

EI

Ei = Young’s modulus of elasticity of masonry infill

L = Height of the columns

Ec =Young’s modulus of reinforced concrete

40

I = Second moment of area of the columns

t = Thickness of the infill panel

The actual extent of stiffness contribution by infill is complicated and cannot be

generalised because of numerous parameters such as relative stiffness of the beam

column ratio, relative stiffness of the frame and infill, material strength of infill and

the quality of construction.

The maximum roof displacement was observed corresponding to the lowest strength

of infill and vice versa. In general, roof displacement decreases non-linearly as the

infill strength increases. For higher strengths, the rate of reduction in displacement

becomes less as the infill strength increases. This indicates that after certain limit of

material property the stiffness of the system does not influence. The infill with 7 MPa

gave the roof displacement equal to that of the bare frame model. It was learnt from a

separate analysis which is not shown here. This indicates the material having a

Young’s modulus lower than 7 MPa will not have any affect on the frame system.

However the upper limit could not be found even by providing the same material

property to both the infill and the frame. This could be due to the presence of the gap

element. Table 3.2 shows the roof displacement of the current model, compared with

results from Literature (Doudoumis, 1995).

Table 3.2 Roof displacement verses infill strength. Infill stiffness

Ei t x610 (N/mm)

Roof displacement

(current model) mm

Roof displacement

for (u = 0 ) in mm

Roof displacement

for (u = 1) in mm

0.2 7.3 7.9 7

0.5 4.5 5.1 4.1

1 3 3.4 2.7

1.5 2.3 2.5 2.1

2 1.8 2.1 1.6

2.5 1.7 1.9 1.5

3 1.5 1.7 1.4

Figure 3.4 compares the roof displacement obtained from the present model and those

in Doudoumis and Mitsopoulou (1995) for different values of infill stiffness. As

41

shown in Figure 3.4, the strength-displacement curve of an infilled frame is almost

similar to the reference curves and thus the Gap element can be used for modelling the

interface between the frame and the infill when conducting a global response study of

the system. It is evident that the present results are in between the existing results for

u = 0, 1.

0

1

2

3

4

5

6

7

8

9

0.2 0.5 1 1.5 2 2.5 3

Relative infill stiffness (K i ) x106 N/mm

Roof dis

pla

cem

ent (m

m)

Currend model

u=0

u=1

Figure 3.4 Validation of the model

3.4 Design of reinforced concrete frames

In the previous section the stiffness of the gap elements were determined to match

different infill stiffness. These values were used in all further analyses. The reinforced

concrete structures to be treated in this Thesis were designed to IS456 (2000) and

IS1893 (2002), which are the existing codes used in the country. A Limit state design

concept has been used to proportion the structural members.

The concrete material with Young’s modulus of elasticity of 24000 MPa and Poison’s

ratio of 0.2 has been considered in this design. The structural systems under

consideration were two dimensional models with a Live load of 2 KN/m2 assumed

42

across an average tributary area of 25 m2. The density of concrete and brick masonry

was considered as 24 KN/m3 and 19.6 KN/m

3 which will be used for seismic weight

calculations.

The sources of structural mass are from structural members, infill walls, floor slabs,

finishes and the imposed load from movable objects. Here, since the structural system

under consideration is two dimensional, a tributary width of five metres is assumed to

exist and used to calculate the imposed load and the dead load from the slab. All the

loads are applied uniformly throughout the beam elements at every level of the floor

height. The uniform dead load (DL) of 37 KN/m has been calculated to act on the

beams where as the live load (LL) of 10 KN/m was applied.

3.4.1 Structures designed to IS 1893:2002

The above loads were used to find the total seismic weight which in turn was used to

obtain the total base shear of the system under static conditions. Table 3.3 shows the

seismic weight calculation of the three storey bare frame system. The total seismic

weight was 100% of dead loads and 25 % of live loads acting on the structure

according to IS1893 (2002).

The fundamental natural period of the structural system was found to be 0.34 seconds

by using the empirical expression given in the code.

75.0075.0 hT = …………………………………………………..………… (3.2)

T = Fundamental period of the reinforced concrete building structures without

infill walls

h = Height of the structure in metres.

Table 3.3 Seismic weight calculation of three storey structure.

Storey

level

Storey

height(m) Slab( KN)

Infill weight

( KN)

Column

& beam

KN)

Total

DL(KN )

Live load

( KN)

1 1.5 328.125 220.5 79.68 628.35 150

2 4.5 328.125 220.5 79.68 628.35 150

3 7.5 328.125 220.5 79.68 628.35 150

4 10.5 328.125 220.5 62.3 500.7 150

Total 2385.7 600

43

The total seismic weight of the structure given by;

LLDLW %25+=

= 2385.7 + .25 X 600

= 2535.7 KN

The design horizontal seismic coefficient is given by the following expression

(IS1893, 2002);

Rg

ZISah

A2

= ……..………………… …………………………………….. ........3.3

Where;

Z = Zone factor for the Maximum Considered Earthquake (MCE)

I = Importance factor, depending upon the importance of the structure

R = Response reduction factor

Sa/g = Average response acceleration coefficient corresponding to the period.

½ = the factor used to convert MCE to Design Basis Earthquake (DBE)

A zone factor Z= 0.36, I = 1, R = 5 and Sa/g = 2.5 (from the Figure 2, IS 1893:2002,

for medium soil condition corresponding to the fundamental structural period of 0.437

seconds).

It was assumed that the structures as residential building located in high seismic area

(seismic zone 5) of Bhutan. Based on the above assumption, the lateral seismic

coefficient was found as Ah= (0.36 x 1 x 1.5)/ (2 x5) = 0.09. The total base shear is

given by the following equation from the code;

Wh

Ab

V = ..................................................................................................................3.4

Where W is the total seismic weight of the building.

Thus, the total base shear is;

= 228.2 KN

This total base shear was distributed to storey levels using the formulae in the code

and the corresponding lateral force was applied at the storey levels as shown in Figure

3.5. Using the following formula from the code;

44

∑

=

=n

jj

hj

w

ih

iW

iQ

1

2

…………………………………..………… ………….. (8)

Where;

Wi = the seismic weight of a particular floor

hi = height of the floor from the base.

Table 3.4 Distribution of lateral force

H (m) W (KN) Whi2 Fx ( KN)

1.5 628.3 1413.6 3.2

4.5 628.3 12723.2 28

7.5 628.3 35342.2 77

10.5 500.7 55203.8 120

Total 104683 228

The primary loads cases (IS1893, 2002) considered for the design of buildings are live

load (LL), dead load (DL) and wind load (WL) or earthquake load (EL). These were

combined algebraically and the maximum analysis results were used for member

proportioning. For the three storey model, a load combination of 1.5 DL + 1.5EL

governed the member design.

Figure 3.5 Lateral load application to the structure.

45

The structural members were modelled with the aid of SAP2000 software in

compliance with (IS456, 2000; IS1893, 2002). The cross section of 340 x 340 mm

and 350 x 250 mm for columns and beams respectively, were found to be satisfactory

for these given loads for a three storey model. These sections were maintained

uniform throughout the height although small variation always exists in elevation as

well in the in-plane direction. Table 3.5 gives the reinforcement and member cross-

sectional details for the three storey model. Similarly, all other models, five storeys,

seven storeys and ten storeys, were analysed and designed to meet the current Codes

and their structural member sizes are given Table 3.6. Figure 3.6 shows all the

structural models in which the storey height is constant at 3.0m and the basement

depth is 1.5m (also mentioned in Table 3.3)

Table 3.5 Structural member sizes of models designed with seismic code.

Types of structure Column size (mm) Beam size (mm)

Three storey 340 x340 400x 300

Five storey 400 x400 400 x 300

Seven storey 450 x450 425 x 300

Ten storey 500 x500 425 x300

Table 3.6 Reinforcement detail of beams and columns.

Description Size Storey 1 Storey 2 Storey 3

Storey4

Columns size 340 x 340

340 x 340

340 x 340 340 x 340 340 x 340

Beams size 400 x 300

400 x 300

400 x 300 400 x 300 400 x 300

Main column

reinforcement

(mm2)

-

6543

6415

4921

2941

Main beam

reinforcement

(mm2)

-

1483

1670

1479

1248

These bare frame models were filled with homogeneous infill wall in their bays, and

were meshed into finite elements in order to simulate their actual behaviour. The

46

connection between the infill and the boundary concrete members were provided by

the Gap type link element which is effective only in compression.

3.4.2 Existing structure (referred seismic provisions)

The existing structural models, in this research, are existing building structures

designed and constructed without using seismic codes. Such buildings were

constructed between early 1970s and 1997. During those days reinforced concrete

structures were designed for gravity and wind loads. Thus, structural member sizes for

these models are based on the gravity and wind loads. Accordingly, all structural

members were designed for gravity loads with the aid of (IS456:2000), adopting the

Limit state method with a load factor of 1.5 for both Live and Dead load. All such

structures treated herein were modelled using the SAP2000 and the designed member

sizes are given in Table 3.7.

Table 3.7 Structural member sizes of models designed without seismic code.

Types of structure Column size (mm) Beam size (mm)

Three storey 300 x 300 300 x 250

Five storey 350 x 350 300 x 250

Seven storey 400 x 400 400 x 300

Ten storey 450 x450 400 x300

47

(a) (b)

(c) (d)

Figure 3.6 Bare frame structures; (a) Three storey, (b) Five storey, (c) Seven storey, (d) Ten

storey building frame (storey height s= 3.0m, basement depth = 1.5m)

3.5 Gap Element

The Gap element is one of the Link elements available in the SAP 2000 software

programme to augment the needs of different structural engineering applications. This

element is generally used to represent the contact between two structures and to

transmit the contact forces between them, for example, a pounding study of the two

structures under the time-history load. Both linear and non-linear options are available

but its usage has been limited to linear range in this study. Its function is limited to

compression forces only and its stiffness will be zero while in tension (SAP2000,

2007).

48

In this study, the weight of the element is considered to be zero as too many such

elements may exaggerate the total mass of the system. The effective stiffness value is

from Table 3.2 but the effective damping value is maintained as 0.05 which is the

same as for the concrete structure. Figure 3.7 shows the Gap element and its

components, in which i and j are the nodes (extreme ends) of the gap element while k

is its stiffness.

Figure 3.7 The Gap element

( Figure obtained from SAP 2000 manual)

3.6 Infilled-frame models

The Elastic modulus of infill is given by the Empirical relation E=750 fm, where fm is

the compressive strength of masonry (Pauley, 1992) with a Poison’s ratio of 0.15.

Young’s Modulus of elasticity and the Poisson’s Ratio of concrete material are

assumed as 2400 MPa and 0.2.

The storey height of all models was maintained three metres uniformly throughout the

storey levels, except the lowermost storey which is 1.5 meters in height and is

generally below the natural ground level. This restriction in storey height is analogous

to the existing general practice in Bhutan. The spacing of the columns was also

maintained uniform, which was five meters centre to centre spacing, in order to make

the structural model simple. The bare frames were analysed for loads recommended

IS1893:2002 and the members were designed to meet the requirement of IS456

(2000) with the aid of software SAP 2000.

49

The structures were considered to fall into the Indian seismic zone V, the most severe

zone, into which Bhutan is assumed to fall, in the absence of more specific seismic

hazard data. A response reduction factor (R) of 5 has been considered in the design of

all the structural systems (IS1893, 2002), consistent with the code. The Gap element

transfers the lateral load from the columns to the infill wall. However, the some of the

common phenomenon such as separation and sliding of infill wall cannot be achieved

in this method. This method is, however useful in studying the global behaviour of

infilled frame under in-plane loads. The models shown in Figure 3.8 represent the

fully infilled model, however the openings were considered in actually study.

(a) (b)

(c) (d)

Figure 3.8 Infilled frame structures; (a) Three storey, (b) Five storey, (c) Seven

storey and (d) Ten storey.

50

The infill panels were accurately fitted within the bays and their aspect ratio

(length/height) ratio varies from 1.75 to 1.8 depending on the size of the structural

members. Although the Gap element was used at the interface between the infill and

the concrete elements, there is no physical gap between the two surfaces. The

interaction between infill and the frame elements takes place through the Gap

element.

3.7 Earthquake Records

The earthquake records (ground motions) considered in this study are; the Imperial

Valley earthquake (El Centro North-South component) which had a magnitude of 6.9

on Richter scale, the Kobe Earthquake of 6.8 on Richter scale which stuck the Kobe

City on 17th

January, 1995 and the Northridge earthquake of 6.7 on Richter scale

which stuck San Fernando Valley on January 17, 1994. Though the earthquakes are

similar in terms of energy released they are different from one another in terms of

Peak Ground Acceleration record (PGA), frequency content and duration of strong

motion. The reasons to choose these motions were attributed to their nature of high

PGA, duration, frequency content and the amount of gross damage incurred due to

them.

-4

-3

-2

-1

0

1

2

3

4

0 5 10 15 20

Time (s)

Accele

ration in m

/s2

(a) El Centro Earthquake, 18th May 1940 (N / S Component).

51

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 5 10 15 20

Time (s)

Accele

ration in m

/s

2

(b) Kobe Earthquake, 17th

January 1995.

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 5 10 15 20

Time (s)

Accele

ration in m

/s2

(c) Northridge Earthquake, 17th

January 1994.

Figure 3.9 Strong Ground Motions.

The above Figures show the acceleration time history of each earthquake which was

used as the input load during the Time History Analysis. The maximum Peak ground

acceleration for El Centro, Kobe and Northridge earthquake are 0.34g, 0.83g and

0.84g respectively.

3.8 3.10 Conclusion

Computer models of the frame-infill structures were developed using gap elements at

the interface. The stiffness property of the gap element was determined for modelling

infill-frame structures to be as the interface element in modelling the full scale

structures treated in this research. The structural models were analysed under the

stated earthquakes. The influence of important parameters will be presented in the

next Chapter.

- 52 -

Chapter 4. Results of Time History Analyses

4.1 Introduction

Time history analyses were performed to study the seismic performance of the infilled

frame structures under the selected earthquake records. In doing so, parametric studies

were carried out and the influence of infill walls on the overall performance of the

structure was studied and the results are presented in this chapter. Since there is lack

of adequate information on the properties of the infill, especially for structures under

seismic loads, models were studied under elastic behaviour. The performance of the

structural models is mainly based on the onset of crack in the infill walls, to evaluate

capacity.

The parameters considered in this research are; Ground motions, (2) Strength of infill

walls expressed in terms of Young’s modulus of elasticity (Ei), (3) Percentage of

opening within the infill walls, (4) Thickness of the infill walls, (5) Strength of

concrete expressed in terms of Young’s modulus of elasticity (Ec) and (6) Peak

ground accelerations (PGA). It also covers a study on the soft-storey phenomenon

which is likely to occur in buildings whose bottom storeys are without infill walls.

Though the contribution of infill walls against lateral loads is widely recognised, there

is no explicit consideration of its effect on the concrete frame structures in most of the

contemporary seismic codes/guides and thus the buildings are designed and

constructed without proper consideration of infill. The information gathered herein

will hence be useful in developing the seismic guidance of infilled RC structures.

The out-put results are presented in terms of roof displacement, inter-storey drift ratio,

infill stresses, fundamental period of the structures and structural member forces.

- 53 -

4.2 General studies

The prerequisites of this research are the influence of soil stiffness on the lateral

deformation of the structure and the dominant frequency of the ground motions. These

are presented in the following Sections.

4.2.1 Effect of soil stiffness (Ks)

Generally for low rise buildings, simple spread footings are provided at a desired

depth below the ground level without inter-connecting beams. The soil around the

structural element (for example in basements) would normally offer some lateral

stiffness depending on the type of soil strength. This is a common practice and it may

or may not have an effect on global structural performance. There is no information in

this area other than soil foundation interaction in the literature review. However, soil-

foundation interaction is beyond the scope of the current study and the interested

readers should refer ATC-40 (1996).

To address this problem, a bare RC frame model was provided with horizontal springs

in the two exterior columns as shown in Figure 4.1. These columns were assumed to

have been founded into the soil at 1.5 m depth. The stiffness of the spring was varied

from 500 KN/m to 30000 KN/m during the Time history analyses under the El Centro

earthquake. These values depict a typical range of soil stiffness. The effect of soil was

measured in terms of displacement response at roof level (ur), ground floor level (ug)

and fundamental period.

Figure 4.1. Spring model representing the soil stiffness.

Soil stiffness

represented

by springs

- 54 -

Table 4.1 shows the results from the analyses performed under the El-Centro

earthquake. It was observed that there is negligible difference in lateral drift at both

the levels as seen from Table 4.1. When the spring stiffness was varied from 500

KN/m to 4000 KN/m, the displacements at both the levels and the period did not

change. However, when the stiffness was changed to 30,000 KN/m there was a small

change (0.2%) in roof displacement while the other response parameters remained

unchanged. Since the soil stiffness from the surrounding soil medium does not have

appreciable influence on the period and the structural displacement, it will be ignored

in the rest of the analyses.

Table 4.1 Effect of lateral stiffness of soil on global structural deformation.

Soil stiffness

(Ks) KN/m

ur

mm

ug

mm

Fundamental period

s

0 50 1.2 0.472

500 50 1.2 0.472

1000 50 1.2 0.472

2000 50 1.2 0.472

3000 50 1.2 0.472

4000 50 1.2 0.472

30000 49.9 1.2 0.472

4.2.2 Frequency of Ground motion (Power spectral density analysis)

Power spectral density (PSD) is described as the distribution (over frequency) of the

power contained in a signal, based on a finite set of data. It is measured in Watts per

unit of frequency. Since the accelogram of an earthquake motion is a time series data

of acceleration, velocity and displacement, PSD can also be used to find out the

frequency content in the series, thus allowing the dominant frequencies of a motion to

be known. The results give a fair idea of the frequency range of ground acceleration,

which is important in controlling the global structural behaviour to a certain extent,

especially to avoid resonance conditions, which will otherwise show unacceptable

behaviour.

- 55 -

Thus, the PSD analyses were carried out for the strong motions considered in this

research. Welch’s power spectrum Method (2000) was adopted to find out the

dominant frequencies of the ground motion due to its advantage over other methods.

It was carried out with the aid of Matlab. In this method, the data is divided into

segments with 50% overlap between the segments, computing the modified

periodogram of each segment. The output PSD estimate is the average of the

periodogram of each individual segment. Figures 4.2 to 4.4 show the PSD plot for all

ground motions considered in this study. The frequency range of buildings is assumed

to be from 0 Hertz to 6 Hertz and hence the following Figures show power

distribution up to 6 Hertzs.

Figure 4.2 . Dominant frequencies of the El-Centro Earthquake.

Figure 4.2 shows the dominant frequencies of the El Centro Earthquake. It ranges

from 1.15 to 2.15 Hz and 1.5 Hz being the dominant frequency of the motion. This

frequency range corresponds to the period range from 0.869 seconds to 0.465 seconds.

The response of those structures which have the fundamental period in this range are

likely to be governed by this ground motion. The dominant frequencies of the Kobe

earthquake range from 1.35 Hz to 2.8 Hz. It can be observed that the dominant

frequency is 1.35 Hertz as shown in Figure 4.3. This earthquake could affect the

response those structures whose period fall within the range of 0.35 seconds to 0.74

seconds.

- 56 -

Figure 4.3. Dominant frequencies of the Kobe Earthquake

Figure 4.4. Dominant frequencies of the Northridge Earthquake.

For the Northridge earthquake motion, the dominant frequency ranges from 0.6 Hertz

to 2.9 Hz and 0.6 Hz being the strongest. This ground motion could be damaging to

those structures having the period from 0.34 seconds to 1.66 seconds. This type of

earthquake could be disastrous to high rise structures.

PSD analysis gives only the power distribution over the frequency range, however it

does not show at which time step the maximum power is. It only gives a fair idea of

which frequency is most influential for a given range of data. From the above

observations, it can be concluded that earthquakes such as El Centro and Kobe are

dangerous for low to medium rise buildings (up to 10 storeys) while the Northridge

earthquake is more dangerous for medium to high rise buildings.

- 57 -

4.3 Modal Analyses

Modal analyses were carried out to obtain the fundamental period of the infilled

structures. The empirical formulae available in codes do not specify the extent of infill

usage in the frame system. Thus, the current study determines the periods of typical

infilled framed buildings with and without openings.

The sources of mass were from the infill walls and the frame elements. Since the

model was studied under in-plane loads the stiffness of the infill wall is considered as

well. Thus, the mass and the stiffness of the entire structure are taken into

consideration.

4.3.1 Fundamental period

Structural responses under an earthquake motion are dependent on inherent properties

(mass and stiffness) of the structure as well as the earthquake record. Infill walls,

which are usually considered as a non-structural component, have been proven to

enhance the strength and stiffness of the frame system under lateral loads. However,

very little consideration of infill has been made in the current codes, especially the

Indian code IS 1893 (2002) which is currently used in Bhutan. This code uses design

spectrum to compute the lateral seismic coefficient which is dependent on the

fundamental period of the structures. An approximate fundamental period for an

infilled structure is given by the equation;

d

ha

T 09.0= …………………………………………………………………………4-1

Where;

aT = An approximate Fundamental natural period of vibration.

h = Height of the building in meters.

d = Base dimension of the building along the considered direction of lateral force.

The code does not specify the extent of infill consideration and thus this section

presents the variation of fundamental period of vibration with opening which is

common in all the buildings. Modal analyses were conducted for all the models

designed for gravity loads only. The average density (γ ) of concrete and infill

- 58 -

material were assumed as 24 3/ mKN and 19.6 3/ mKN respectively. The fundamental

period of the bare frame model was found to be comparable to the period determined

from the code equations (IS1893, 2002) but had some differences when the models

are treated as infilled frame models, as shown in Table 4.2.

Table 4.2 Fundamental natural period of vibration of infilled frame (seconds).

Models Fully

infilled

80%

infilled

60%

infilled

40%

infilled

Bare

frame

Code

formulae

Three storey 0.165 .194 .237 0.264 0.29 0.242

Five storey .28 .307 0.389 0.436 0.53 0.383

Seven storey .37 .413 0.514 0.562 0.581 0.522

Ten storey .534 .598 0.746 0.808 0.827 0.731

There is a significant difference between the period of the bare frame models and the

infilled frame models, indicating the influence of infill on the structural performance.

The difference in fundamental period between the bare frame model and the fully

infilled model was 69.2% while the difference between the bare frame and the 40%

infilled model was 9.3%. As the opening percentage increases, the fundamental period

of the structure approaches the natural period of the bare frame as expected, and

shown in Figure 4.5. The code formula (IS 1893:2002) for the fundamental period

considers the infill, but it does not consider the infill property or the percentage

openeing in the infill. Since the fundamental period increases with the opening

percentage of the infill wall, the empirical formulae given in the codes may not be

applicable to all kinds of structures that have different opening sizes. Consequently,

the estimation of base shear, especially using the IS 1893(2002), would lead to high

values and thus the large structural member size.

In the current study, the structural model having 40% opening within the infill wall is

treated as the typical structure representing building structures in Bhutan. There will

certainly be variations in opening percentages from building to building but it is

reasonable to consider some opening rather than ignoring it or considering a fully in-

filled model. It so happens that for the particular material used for the infill in the

- 59 -

present analysis, the fundamental period of a structure with 40% opening is close to

that obtained from the code. However, as no firm conclusion can be made, a detailed

study has been done in later section on the effect of opening on the seismic response

of the structure.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9 10 11 12

Number of storeys

Fu

nd

am

en

tal p

eri

od

of

vib

ratio

n(s

) Fully infilled

80% infilled

60% infilled

40% infilled

bare

Figure 4.5. Variation of Fundamental period with percent of infill present in the models.

Figure 4.6 shows the graph (straight line) of the best fit for fundamental period versus

number of storeys for building with 40% opening within the infill walls. This can be

used to obtain the period for typical building structures in Bhutan. Relevant

information on the variation of period with opening percentages is given in later

Sections. Since the effect of infill is significant, especially, in terms of stiffness which

is directly related to the fundamental period, consideration of opening is thus

important.

- 60 -

y = 0.072x + 0.0214

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 1 2 3 4 5 6 7 8 9 10 11 12

Number of storeys

Fundam

enta

l period o

f vib

ratio

n(s

)

Figure 4.6. Fundamental period of vibration of infilled structures.

4.4 Time History Analysis

The structural models developed in Chapter three were studied under three typical

earthquakes. The frame elements were modelled as line elements with rigid joints

and provided with fixed support condition at the base. The infill panels were modelled

as plane-elements having variable properties, in terms of Ei values (Young’s

modulus), of infill wall. The panel was then linked to the boundary frames (beams and

columns) using Gap elements which transferred the load from the frames to the infill

panels, allowing the influence of infill on the RC frame to be captured. Since the

masonry infills are weak in tension, only the compressive stress could be used for

quantifying the results.

The parameters used in this study were chosen to obtain adequate information on the

influence of infill wall on the performance of structures subjected to different ground

accelerations. These parameters were: (1) Earthquake motions, wherein a dominant

earthquake was determined based on effect of frequency content and duration on the

response of structures with varying height, (2) The Young’s modulus of elasticity of

an infill (It studies the variation of structural responses to know the effects on the use

of different infill materials), (3) Opening percentage covers the rational opening sizes

typical to Bhutan buildings, (4) The Young’s modulus of concrete (It considers the

effects of concrete material on the structural responses and provides important

information on the state of structures which were constructed without seismic

- 61 -

provisions), (5) Thickness of infill wall and (6) Peak ground acceleration (It studies

the effects on the performance of buildings with or without seismic provisions and

will provide important information on the seismic resistance levels of infilled

structures).

Initially, all the models were studied under different earthquakes and the dominant

earthquake was identified for each model. The other parameters were then

investigated for a ten storey model under the dominant earthquake. Analyses were

conducted on the models with the objective of capturing the performance of existing

buildings which were designed and constructed without the provisions. The structural

models designed and constructed with seismic provisions will be studied only under

peak ground acceleration. The results consist of roof displacements, inter-storey drift

ratios, member forces and infill stresses obtained through time history analyses on the

infilled frame models. In the following sections results of the analyses are presented.

The earthquake which gives higher responses will be chosen for Time history

analysis.

This Section presents the results for structural responses of typical structures under

different earthquakes of different frequency content and duration. The heights of the

structures vary from three storeys to ten storeys. The study was carried out on all

models which had been designed for gravity loads only and having 40% opening in

the infill wall. The selection of a dominant earthquake on a particular model is

depends on the response results.

The beams and columns were provided with sizes given in Table 3.3 while the typical

thickness of the infill wall was assumed to be 250 mm for all the models. The

stiffness of the gap elements was derived from Equation 3.1. The Young’s modulus of

elasticity and the Poison’s ratio of concrete were assumed to be 24000 MPa and 0.2

respectively and similarly the infill wall was provided with (Ei) value of 5000MPa

and Poison’s ratio of 0.15.

Initially all Earthquakes records were normalised to 1g PGA for comparison. The

normalised values were then scaled down to 0.2g PGA appropriately. Thus, the

- 62 -

ground acceleration with 0.2g PGA was used as the input load for all the models. The

time step for the El Centro motion was 0.02 seconds while it was 0.005 seconds for

the Kobe and Northridge earthquakes based on the dominant period of each

earthquake. An independent study was conducted to examine the implications of the

time step size required for this analysis and convergence was established.

Rayleigh’s damping coefficients such as mass proportional coefficient (a) and

stiffness proportional coefficient (b) were assumed to be 0.05. This value was

considered to include some form of damping which is normally present in real

structures. Results are presented for damped models and un-damped models. The

dominant earthquake was decided based on the maximum roof displacement, inter-

storey drift ratios and the stresses in the infill walls under different earthquakes (of

different frequency content and duration).

4.5 Ten storey model

This model has a fundamental period of 0.745 seconds. The Kobe earthquake has the

dominant frequency of 1.35 Hz which correspond to period of 0.74 seconds. Results

are presented below for the three normalised earthquake records.

4.5.1 Roof displacement

The time histories of roof displacement of this model obtained under the three

earthquake records for a PGA of 0.2g are shown in Figure 4.7. Under the El-Centro

earthquake the maximum roof displacement is 31.5 mm at the time step of 2.2

seconds, while they were 49.1 mm and 32.6 mm at the time step of 8.690 seconds and

4.04 seconds for Kobe and North Ridge earthquakes respectively. Although, El

Centro earthquake has wide range of frequency over a long duration, the roof

displacement of the model is higher under Kobe earthquake due to the closeness in

their frequencies. The roof displacement under Northridge earthquake is the second

lowest inspite of its short duration. From this observation, it is evident that the

structural response is depended on the frequency of the model and ground motion.

However, since the frequency content as well as the duration of future earthquake is

uncertain, it is necessary to perform analysis for few ground motions.

- 63 -

(a) (b)

(c)

Figure 4.7. Roof displacement time histories in m vs. second; (a) El-Centro Earthquake, (b) Kobe


4.5.2 Inter-storey drift ratios

Building structures, under the influence of cyclic loads, sway back and forth. During

the swaying state, the structural members are deformed and member forces and

stresses are induced. If excessive deformation is allowed in the structure, there would

be problems of material failure during strong ground motion, unacceptable non-

structural damage and discomfort to the occupants, even though the structure remains

- 64 -

in a functional state. Thus, the drift limitations are set in all the Standards to avoid

problems under serviceability condition.

The inter-storey drift ratio, which is a direct measure of structural response towards

the given ground motion, was used as one of the output results in this study. It is

defined as the ratio of the difference in drifts between two adjacent levels and the

storey height, given by the formula;

u = (ui+1 - ui)/ ht……………………………………………………………………………………………………….4.1

Where;

ui = Lateral displacement at ith

storey level.

ui+1 = Lateral displacement at (i+1)th

storey level.

ht = Height of a storey between the storey level i and (i+1) levels.

Inter-storey drift ratios were found based on the maximum roof displacement response

and are presented in Figure 4.8 for the three earthquakes, all for a PGA of 0.2g. The

Kobe earthquake produced the maximum inter-storey drift ratios compared to the

other ground motions and the maximum drift was observed at level three. This was

somewhat expected as the natural period of this structure was close to the period of

the dominant mode of the Kobe earthquake. However, the drift ratios under Kobe and

Northridge earthquakes are similar as shown in Figure 4.8. Thus the frequency

content of the ground motion is an important aspect of earthquake records.

0

2

4

6

8

10

12

0.000 0.001 0.001 0.002 0.002 0.003

Inter-storey drift ratio

Sto

rey le

vels

El Centro

kobe

Northridge

Figure 4.8 Inter-storey drift ratios of the ten storey structure.

- 65 -

The inter-storey drift limit ratio of the existing code (IS1893, 2002 ) is 0.004 and the

drift ratios of the current model did not exceed the code limit. This indicates the

influence of infill on the lateral deformation of the structures.

4.5.3 Infill stress

For the models with openings the stress in the infill was found to be maximum at the

corners of the opening, unlike in the fully infilled model in which the maximum

compressive stress was at the compressive corners of the panels. This maximum

stresses were found to be the principal stress which are shown in Table 4.3. The

stresses in the infill were found to be maximum under the particular earthquake which

produced the maximum inter-storey drift and roof displacement, which in this case

was the Kobe earthquake.

For this model, the maximum stress was 4.2 N/mm2 under the Kobe earthquake while

it was 2.8 N/mm2 and 2.3 N/mm

2 for the Northridge and El Centro earthquakes. The

limiting compressive strength of the infill was 6.66 N/mm2 and this level of stress has

not reached under 0.2g PGA. However, it was observed that the stresses in the infill

wall did not correspond to the maximum inter-storey drift ratio or maximum roof

displacement. The stresses in the infill were similar under the El Centro and the Kobe

earthquakes. The results indicate that the Kobe earthquake was dominant for the ten

storey model, probably due to the closeness in their periods. Thus, the frequency

content of the ground motion is an important aspect which the design engineer must

understand. Therefore, power spectral density analysis for each earthquake is

important.

Since the maximum stresses of the infill wall is within the limits of material

compressive strength, the structure is able to resist the earthquake of 0.2g PGA

without damages in the infill wall. However, the compressive strength of infill

material should be as high as 6.66 N/mm2.


Model type Maximum Principal stress in the infill ( N/mm2)

El Centro earthquake Kobe earthquake Northridge earthquake

Ten storey 2.3 4.20 2.8

- 66 -

4.6 Seven storey model

This model has a fundamental period of 0.514 seconds and it doesn’t correspond to

the periods of any dominant modes of any of the three earthquakes.


The maximum roof displacement time histories for a seven storey model are shown

in Figure 4.9. The El-Centro earthquake produced the maximum displacement of 19.5

mm at the time step of 2.12 seconds. The Kobe and Northridge earthquakes have

resulted in roof displacement of 21.1 mm and 17.8 mm at the time step of 8.635

seconds and 3.985 seconds respectively. Since the displacements are close to one

another, further study is required to make a judgement on the selection of dominant

earthquake for this model or there may not be a single dominant earthquake.

(a) (b)

(c)

Figure 4.9. Roof displacement time histories; (a) El-Centro Earthquake, (b) Kobe Earthquake and (c)

Northridge Earthquake.

- 67 -


The inter-storey drift ratios of the seven storey model are shown in Figure 4.10. The

Kobe earthquake produced the maximum drift ratio of the three ground motions.

However, there is no significant difference in drift ratio as can be seen in the Figure

4.10 and their values vary slightly as also indicated in the roof displacement response.

The effect of duration of motion is not evident. However, the frequency content of

motion is evident as the second most dominant frequency of the Kobe earthquake is

close to the fundamental frequency of the model. The values of inter-storey drift

ratios, shown in the Figure, are within the code drift limit ratio of 0.004.

-1

1

3

5

7

9

0.000 0.001 0.001 0.002 0.002


Sto

rey le

vels

El Centro

kobe

Northridge

Figure 4.10. Inter-storey drift ratio of a seven storey model.

4.6.3 Infill stresses

The maximum stress was observed corresponding to the maximum roof displacement

response. It was observed to be 2.45 N/mm2 under the 0.2g PGA of the El-Centro and

Kobe earthquake, however was only 2.1 N/mm2 under the Northridge earthquake, as

shown in Table 4.4. Though the roof displacement and the inter-storey drift ratios

were relatively larger under the Kobe earthquake, the maximum principal stress in the

infill walls was the same for the El Centro and Kobe earthquakes.




Seven storey 2.45 2.45 2.1

- 68 -

The results presented in this Section show that the three ground motions are equally

effective for a seven storey model. However, since the results, such as roof

displacement and inter-storey drift ratios, are comparatively greater under the Kobe

earthquake, it was considered as the dominant earthquake for a seven storey model.

Since the existing code (IS1893, 2002) do not specify the requirement of infill

strength, these values cannot be compared with code values. However, the maximum

stresses shown in above Table are within the compressive stress of infill material used

in this model.

4.7 Five storey model

This model has a fundamental period of 0.39 seconds. This period is closer to the

dominant period of Northridge earthquake than to that of the other two earthquakes.

4.7.1 Roof displacements

The roof displacement time histories for a five storey model are shown in Figure

4.11. The maximum roof displacement of 10.8 mm was observed under the

Northridge earthquake at the time step of 4.330 seconds. The El Centro and Kobe

earthquakes have produced the maximum displacement of 9.7 mm and 10.7 mm at the

time step of 2.09 seconds and 8.605 seconds respectively. It is hard to decide which

one of the ground motions in dominant to this model based on the roof displacement

as both the Northridge and Kobe earthquakes have resulted in nearly the same

maximum roof displacement.

The effect of frequency content as well as the duration of the motion was not evident

from the roof displacement of a five storey model. It is possible that the fundamental

frequency of the model was not near to the frequency content of the ground motions.

The result also indicates that the frequency content of the ground motions considered

for this research is beyond the resonance condition. Therefore, it is important to

consider more number of ground motions for analysis. All ground motions were

considered for a duration of 20s and enabled the influence of frequency content and

duration of strong motion to be captured in the analyses.

- 69 -

(a) (b)

(c)

Figure 4.11. Roof displacement time histories of five storey model in m vs s; (a) El Centro earthquake,

(b) Northridge Earthquake and (c) Kobe Earthquake.


The inter-storey drift ratios for a five storey model are shown in Figure 4.10. Similar

to the roof displacement, the inter-storey drift ratios for all ground motions are almost

similar with an insignificant difference in the results. In general, all ground motions

are equally dominant to this model as they produced similar responses. The inter-

storey drift limit ratios were within the code prescribed limit of 0.004. Since the

structure becomes stiffer with the decrease in height of the structure, the inter-storey

drift ratio decreases significantly.

- 70 -

0

1

2

3

4

5

6

7

0.0000 0.0004 0.0008 0.0011 0.0015


Sto

rey le

vels

El Centro

kobe

Northridge

Figure 4.12. Inter-storey drift ratio of a five storey model.


The maximum stress was 1.68 N/mm2 under the Kobe and Northridge earthquakes

while it was 1.54 N/mm2

under El Centro earthquake, as shown in Table 4.5. In this

case, the maximum stress was observed corresponding to the maximum inter-storey

drift ratios and the roof displacement as is the case for Kobe and Northridge in this

section. The maximum stress of the infill wall obtained from the analysis was within

the compressive strength of the infill material (6.66 N/mm2). The frequency content

and duration of motion has no effect on the stresses of the infill wall.

Table 4.5 Maximum principal stress in the infill wall.



Ten storey 1.54 1.68 1.68

Although the results are comparable to one another for different earthquakes, it was

found that the Kobe earthquake was relatively more influential for a five storey

model. Thus, it shall be considered as the dominant earthquake while carrying out

parametric studies on this model.

4.8 Three storey model

This model has a fundamental period of 0.237 seconds and it is lower than the period

of dominant modes of the ground motions considered in this study.

- 71 -

4.8.1 Roof displacements

The maximum roof displacement time histories of the three storey model are shown

in Figure 4.13. The Kobe earthquake has a maximum roof displacement of 3.4 mm at

the time step of 8.595 seconds while it was 3.1 mm and 3.3 mm at the time step of

2.07 seconds and 4.270 seconds under the El Centro and the Northridge earthquakes

respectively. Since the values of the roof displacements under all earthquakes are not

significantly different from one another, any of the ground motions could be

acceptable for structures with similar dynamic properties, for studying influence of

parameter.

(a) (b)

(c)

Figure 4.13. Roof displacement time histories of a three storey model; (a) El Centro earthquake, (b)

Kobe Earthquake and (c) Northridge Earthquake.

- 72 -


The inter-storey drift ratios for the three storey model are shown in Figure 4.14. It can

be seen that the peak value of the inter-storey drift ratios are almost same. They were

well below the inter-storey drift ratio limit (0.004).

0

1

2

3

4

5

0.0000 0.0002 0.0004 0.0006


Sto

rey level

El Centro

kobe

Northridge

Figure 4.14. Inter-storey drift ratio of the three storey model.


The maximum principal stresses of the infill walls are given in Table 4.6. The stresses

are not significantly different for each earthquake, however the Kobe earthquake has

produced the maximum stress of 0.84 N/mm2

in the three storey model. The

maximum stresses of the infill are within the compressive strength of infill material.

Table 4.6 Maximum principal stress in the infill



Ten storey 0.70 0.84 0.77

With damping considered in the model, the Kobe earthquake was found to be the

most dominant ground motions on the ten storey model. When the height of the model

was decreased, all three ground motions had similar effect on the model. However the

structural responses were comparatively large under Kobe earthquake.

- 73 -

The above results show that the Kobe earthquake can be selected for analyses of ten

storey, five storey and three storey models, while Northridge earthquake can be

selected for the five storey model for parametric study. The influence of other

parameters will therefore be studied under the selected earthquakes.

4.9 Infill strength (variation of Young’s Modulus of Elasticity

Ei)

There are many different types of infill materials available in the construction

industry, and burnt clay bricks and hollow concrete block are commonly used. In

general, infill walls are treated as non-structural components of the building and

selection of material is mainly dependent on the cost and availability. On the other

hand, the performance based design concept assumes the infill wall to fail before the

frame members. Therefore, the intended performance of the buildings with high

strength infill wall under a given seismic activity is uncertain. There are many

instances where cracks in infill walls occurred during moderate earthquakes.

However, some buildings suffer from minor cracks within the infill walls even under

moderate earthquakes. Thus, it is imperative to know the minimum strength of such

infill.

To address this problem, the current study considers infill materials whose Young’s

modulus of elasticity ranges from 1000 MPa to 15000 MPa, thus representing a wide

range of infill material. The variation of global structural responses with the infill

strength, expressed in terms of the Young’s modulus of elasticity (Ei), under the

Design Basis Earthquake of 0.2g peak ground acceleration, is presented in this

Section. The results from studying the effect of this parameter will provide

information on the minimum required compressive stress of infill material to be used

in the earthquake regions.

Thus, there is a need to find out the minimum strength of the infill walls, for a given

PGA and vice versa. The performance based design principle allows cracks to develop

in the infill walls before the structure becomes unusable. This section is designed to

study the effect of strength of infill material, in terms of its Young’s modulus of

elasticity, under a Credible Earthquake, on the structural response. A review of the

- 74 -

Literature revealed that the presence of infill wall improves the lateral load resistance

of the frame system but it is ignored in the design codes.

This study was undertaken for a ten storey model with columns 450X450 mm square

and beams 400 X300 mm in depth and width. All the infill walls were 250 mm thick

and had a 40% opening at the centre of the wall panels. The Young’s modulus of

elasticity of infill material varied from 1000 MPa to 15000 MPa. This range

represents the Young’ modulus of typical infill material. The model was studied under

a peak ground acceleration of 0.2g PGA of the Kobe Earthquake since the infill walls

are allowed to fail under higher PGAs.

4.9.1 Fundamental period (T)

In general engineering practice, empirical Equations are used to estimate the period of

structures since the modelling of infill walls is complicated and costly. However, it is

important to consider the infill walls in order to increase the accuracy of the results.

Figure 4.17 gives the variation of the fundamental period of the structure with the

Young’s modulus of elasticity (Ei) of the infill material. Generally it can be seen that

the period decreases with the increase in Ei . However, at the lower range of the

Young’s modulus of infill material, the period increases due to the influence of mass

of the infill being higher than stiffness of infill at lower values of Ei. This indicates

that in general, the increase in stiffness due to increased value of Ei of infill influences

the dynamic behaviour, as expected.

In the figure below, the fundamental period of the bare frame model should be greater

than the infilled model due to the influence of the infill on the lateral stiffness of the

system. However, the models which had Ei vales less than 2500 MPa had greater

fundamental periods as the influence of mass from the infill was greater than the

stiffness from it. Thus, it is important to consider the proper infill material in

modelling the structures. This type of variations cannot be seen in the empirical

formulae given in the codes.

- 75 -

0.826

0.945

0.838

0.7460.691

0.654 0.626 0.606

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Bare 1000 2500 5000 7500 10000 12500 15000

Youngs Modulus of Elasticity(Ei ) in Mpa

Fundam

enta

l per

iod (

s )

Figure 4.15 Fundamental period vs Ei

The models with the infill walls having Ei less than 2500 MPa were found to have

longer periods than the bare frame model. The difference in the fundamental period

between the models with infill wall of Ei of 1000 MPa and 2500 MPa is 12.76%. On

other hand, the fundamental period was found to decrease by an average of 6.7% for

every 2500 MPa increase in the Young’s modulus of elasticity of infill material. This

happens when the Ei of the infill material is more than 2500 MPa. Thus, it is important

to consider the infill wall and its influence in order to understand the structural

behaviour more accurately.

4.9.1.1 Roof displacements

The roof displacement is a measure of global structural response under lateral loads

and the amount of deformation primarily depends on lateral stiffness, mass and the

magnitude of the load. The results presented show the variation in roof displacements

with the Young’s modulus of elasticity of infill material at three damping levels.

In general the roof displacement was found to decrease by increasing Ei as shown in

Figure 4.18. However, when the model is treated as an un-damped model, the

structure tends to undergo greater deformation due to the closeness in frequency

content between the ground motion and the model, resulting in the fluctuating curves

- 76 -

shown in Figure 4.15. For instance, the un-damped models with Ei value of 7500 MPa

and 2500 MPa respond strongly due to the closeness in periods or frequencies

between the model and the ground motion. Therefore, the importance of including

damping to realistically assess the structure is evident.

This also indicates that the likelihood of the structure to resonate with the ground

motion is very low as long as damping is present in the structure. This fact is proven

by the smooth roof displacement curve of the models with 3% and 5 % damping in

which neither of the models have undergone large displacements (due to resonance

conditions) across a wide range of infill materials.

The roof displacement values are much higher for un-damped models than for

damped models as expected. For a model with infill material with an Ei of 1000 MPa,

the difference in roof displacement between the un-damped and 3% damped models is

130% and it is 192% when the damping is 5%. However there was only 26%

difference in roof displacement between the 3% and 5% damped models. The results

for the un-damped model are unrealistic as there is damping in all real structures.

0

50

100

150

200

250

0 2 4 6 8 10 12 14 16

Young's modulus of elasticity (E i ) X103 MPa

Roof dis

pla

cem

ent in

mm

Undamped structure

Anticipated trend

3 % damping

5% damping

Figure 4.16 Maximum roof displacement vs Ei..

- 77 -

4.9.2 Infill stress

In the preceding sections it was found that the structural deformation generally

decreases with the increase in Ei, however stresses in infill walls were found to

increase with the increase in Ei. These stresses were obtained corresponding to the

maximum roof displacement. The maximum stresses were found in the storeys which

undergo maximum inter-storey drift. The walls in the higher storeys experience

relatively less stress than in the lower storeys.

Unlike the fully infilled frame structures where the maximum compressive stress is

generally found at the two opposite corners of the infill wall, models which have

openings within the walls have maximum stresses at the corners of the openings as

shown in Figure 4.17 for different levels of damping. The maximum principal stress

observed is shown in Table 4.8. It also shows the characteristic compressive strength

'

cf of infill found by relating it to its Ei using empirical Equations (Pauley, 1992).

(a) (b)

Figure 4.17 Maximum stresses within the infill walls (a) Fully infilled wall and (b) Infill with 40%

opening.

For the un-damped condition, stresses in the infill do not vary linearly. This is

because the model picks up the frequency of the ground motions and deforms

excessively as can be seen in some of the models which have Ei of 7500 MPa and

- 78 -

10,500 MPa. The ground motion of 0.2g PGA was found to be sufficient to form

cracks in the infill walls when damping is neglected.

However, when the damping is introduced in the model, stresses in the infill walls

decrease significantly as shown in Table 4.7. However, the stress was found to

increase with the increase in Young’s modulus of elasticity of infill material since it

attracts more lateral loads due to the increase in stiffness. Since the stress in the infill

depends on the response of the model to the ground motion, there is no direct relation

between stress and the strength of infill materials, since the response depends on the

magnitude and frequency content of future earthquakes. Generally, stress seems to

increase with the increase in Ei of infill.

On average, the maximum stresses of the models with 3% damping is 20% more than

the maximum stresses in the 5% damped models. The maximum stress of the model

with Ei of 1000 exceeds the maximum compressive strength of the infill wall however

in all other models it is found to be less than the ultimate strength of the material.

Thus, the minimum strength of the infill wall should not be less than 1.33 N/mm2 (for

0.2g PGA). The stresses which were exceeded the maximum compressive strength of

the infill are shaded, as shown in the Table.

Table 4.7 variation of maximum principal stress in the infill wall.

iE (MPa) Maximum principal stress in the infll wall

(N/mm2)

Compressive stress

'

cf (N/mm2)

ξ=0% ξ=3% ξ=5% σmax

1000 3.5 1.82 1.54 1.33

2500 9.1 3.5 2.8 3.33

5000 9.1 4.9 3.92 6.66

7500 21 5.6 4.9 10

10500 18 5.6 4.9 13.33

12500 11.2 5.6 4.9 16.66

15000 11.2 5.6 5 20

- 79 -


The inter-storey drift ratios were found corresponding to the maximum roof

displacement response of the model under 0.2g PGA of the Kobe earthquake. When

there is no damping, there is a significant reduction in the inter-storey drift ratios

when there is an increase in the Young’s modulus of elasticity of the infill walls.

Models with lowest Ei values undergo higher inter-storey drifts compared to models

with higher Ei values. However, it is difficult to quantify the percentage of drift

reduction corresponding to an increase in Ei, clearly shown in Figure 4.18, that the

curves are not uniformly spaced. This could be due to other factors such as the

fundamental period of the model, the frequency content and the duration of the

motion.

When the models are un-damped, those with an infill material having a Young’s

modulus of elasticity of less than 10,000 MPa exceed the inter-storey drift ratio limit

of 0.004. Moreover, there is no consistency in drift ratios for a regular increase in Ei

since the model picks up the frequency of the ground motion and its results in large

displacements.

0

2

4

6

8

10

12

0 0.004 0.008 0.012


sto

rey leve

l

Ei=2500 MPaEi=5000MPaEi=7500MPaEi=10000MPaEi=12500MPaEi=15000MPaEi=1000 MPa

Figure 4.18 Inter-storey drift ratio for un-damped models.

- 80 -

The inter-storey drift ratios of the models with damping ratio (ξ) of 3% are shown in

Figure 4.19. The curves are fairly smooth unlike Figure 4.20. The maximum drift

ratio was observed for the model with Ei =1000 MPa and the minimum for the model

with Ei =15000 MPa which is as expected. The average difference in drift ratio for

every 2500 MPa increase in Ei is 16.54%. The difference in drift between the models

with Ei values of 1000 MPa and 2500 MPa is 18.25% while it is only 7.5% for models

with Ei values of 12500 MPa and 15000 MPa.

Under the credible Earthquake of 0.2g and at 5% damping, the maximum inter-storey

drift ratios did not exceed the code limit of 0.004 even for a model with lowest Ei

value. Thus, the minimum compressive strength of the infill should be 1.33 N/mm2

.

0

2

4

6

8

10

12

0.000 0.001 0.002 0.003 0.004


sto

rey le

ve

l


Figure 4.19 Inter-storey drift ratio for 3% damping

The inter-storey drift ratios when the damping ratio (ξ) is 3% are given in Figure 4.20.

The average difference in drift ratio in every 2500 MPa increment in Ei is 14.91%.

For example, the difference between the models with Ei values of 1000 MPa and 2500

MPa is 20.39% while it is only 6% for models with Ei values of 12500 MPa and

15000 MPa. This level of damping also requires the minimum compressive strength

of 3.33 N/mm2

for the infill wall as shown in Table 4.8.

- 81 -

0

2

4

6

8

10

12

0.000 0.002 0.004 0.006


sto

rey le

ve

l


Figure 4.20 Inter-storey drift ratio 5% damping

It was found that the global structural responses decrease with the increase in the

Young’s modulus of elasticity of an infill wall. The fundamental period generally

decreases with increase in Ei but at lower values (Ei <1000 MPa) it was found to

increase. The inter-storey drift ratio also reduces with the increase in Ei values. At 5%

damping level the inter-storey drift ratios are within the code limit (0.004) for a wide

range of Ei values (1000 MPa -15000 MPa). The Design Basis Earthquake (0.2g

PGA) demands the compressive strength of 2N/mm2 of infill material in seismically

active regions.

4.10 Openings

Generally almost all buildings have openings in different forms such as windows,

ventilators and doors to provide comfortable living. These features of the building

cannot be considered in structural modelling by a diagonal strut method developed for

global structural behavioural studies. Thus, most of the previous research was carried

out for the fully infilled frames. Since this parameter is an inevitable part of the

building structure, its influence on the seismic resistance is presented in this Section.

The opening size, in this work, is expressed as a percentage of the the infill wall area

and assumed to be at the centre of the walls.

- 82 -

The influence of this parameter has been studied on a ten storey model under the

Kobe earthquake. Kobe earthquake was selected as it provided the maximum

responses overall, while the 10 storey model was selected as its fundamental natural

period was close to the dominant period of the earthquake. The model was analysed

for different opening percentages 20%, 40% and 60%. The opening was kept in the

centre of the panels to avoid complications due to eccentric masses. The Young’s

modulus of elasticity of the concrete and infill materials was 24000 MPa and 5000

MPa respectively. The member sizes of beams and columns are given in Table 3.5 of

Chapter three. The thickness of the walls was assumed to be 250 mm. The results are

presented for damping values of 5%.

4.10.1 Fundamental periods

The fundamental period of vibration depends on the mass and the stiffness available

in the system. By changing the amount of infill in the structure, both the stiffness and

the mass change and thus the effect of the opening is observed in the fundamental

period of vibration of a structure.

The fundamental period increases as the opening percentages increases as shown in

Figure 4.21. The opening percentage in the infill walls were varied from 0% to 100%.

The model with 100% opening size represents a fully infilled frame while the bare

frame structure was represented by a model with 100% opening percentage. As the

opening size increases, the fundamental period moves closer to the fundamental

period of the bare frame system, and vice-versa. The difference in the fundamental

period between the fully infilled frame model and the bare frame model is 54.87%

while it is 35.43%, 39.7% and 51.31% in the models with 20%, 40% and 60 %

openings, respectively. This shows that there is no linear relationship between the

fundamental period and the opening since the fundamental period changes with the

change in mass and stiffness.

Thus, it is important to consider openings in the analysis and design stage in order to

achieve accurate structural responses. In the present code IS1893 (2002) this

parameter is not taken into account. Since the opening in the infill influences the

fundamental period of the structures it should be considered in the seismic guidelines.

- 83 -

0.5340.599

0.7460.808 0.827

0

0.2

0.4

0.6

0.8

1

0 20 40 60 100

Percentage of opening

Fundam

enta

l period (

s)

Figure 4.21 Variation of fundamental period with opening.


The inter-storey drift ratio was calculated corresponding to the maximum roof

displacement response of 29.1 mm, 48.8 mm and 59.7 mm for different models

having different opening sizes. The maximum inter-storey drift ratio was at storey

level three in all the models as shown in Figure 4.22. The model with the 60%

opening showed greater drift ratio than the other models. The ratio of the maximum

inter-storey drift is 1:1.67:2.05 corresponding to models with 20%, 40% and 60%

openings, respectively. There is thus a reduction in lateral stiffness by increasing the

opening percentage, and the stiffness decreases by an average value of 38.98% for

every 20% increase in opening size.

The inter-storey drift ratios did not exceed the code limit (0.004) in all the models.

Thus it is evident that there is significant influence of infill even if there is openings

within the walls. Although, the lateral stiffness of the structure decreases with the

increase in opening size, the drift ratio requirement the structural model meets the

drift ratio requirement of the existing code. Thus, infilled structures, irrespective of

opening sizes in infill wall, the inter-storey drift ratio is within the code requirement.

- 84 -

0123456789

101112

0.000 0.001 0.002 0.003Inter-storey drift ratio

Sto

rey le

ve

l

20% opening

40% opening

60% opening

Figure 4.22 Inter-storey drift ratios for different opening percentages.

4.10.3 Infill stress

The maximum principal stresses were observed at the opposite corners of the opening.

The stress in the infill was found to increase by increasing the opening or by reducing

the amount of infill walls in the structural system as shown in Table 4.8. This clearly

indicates the contribution of infill wall to the lateral stiffness of the system. By

increasing the opening, both the mass and the lateral stiffness are decreased and the

stress in the infill tends to increase. An average increase in infill stress was found to

be 23.57% for every 20% increase in opening size.

Table 4.8 Influence of the opening percentage on the stress in infill.

Opening percentage (%) 20 40 60

Infill stress N/mm2 2.8 3.92 4.2

4.10.4 Member forces

The structural member forces were also found to vary with the opening percentages,

similar to the above results. The column moments (Mc) were found to increase by an

average of 36.591% while the beam moments (Mb) were increased by 33.88% for

every 20% increase in opening size. However, shear forces in both beams (Vb) and

columns (Vc) were generally found to decrease by increasing the opening percentages,

- 85 -

however no conclusion can be drawn as some of the shear forces, especially at the top

storeys, were found to increase with opening size.

By increasing the opening size, the moments in both columns and beams increase

since the lateral load resisting capacity seems to decrease and the forces are then

transferred to the frame members. On the other hand, shear forces decrease by

increasing the opening percentages due to a reduction in the mass of the structure.

In summary, for variation of the opening size, it was found that the fundamental

period, inter-storey drift ratios, infill stress and the moments in beams and columns all

increase by increasing the opening size. However, the shear forces in beams and

columns were generally found to decrease. These results do not conform to the

common perception that the structure would decrease its global responses by

increasing the openings. Thus, it is important to model the infill walls appropriately to

obtain accurate results.

Table 4.9 Variation of member forces with opening percentages. Storey

level

20% opening 40% opening 60% opening

Mc Mb Vc Vb Mc Mb Vc Vb Mc Mb Vc Vb

11 6.1 1.4 8.04 2.1 8.54 2.3 8.54 3.2 15.84 5.86 9.12 4.31

10 17.36 8.19 23.9 9.5 23.16 9.91 23.16 9.91 35.8 18.74 27.67 10.37

9 27.19 18.4 44.6 22.5 42.94 23.4 42.94 23.4 50.2 35.47 48.5 22.74

8 36.37 27.9 65.25 34.3 62.69 35.17 62.69 35.17 65.4 52.32 67.7 35.6

7 44.99 36.99 84.91 45.4 81.25 46.02 81.25 46.02 79.7 67.89 85.5 46.3

6 52.96 45.59 103.39 56 98.26 55.98 98.26 55.98 90.6 81.95 101.46 55.94

5 60.15 53.6 120.4 66.01 113.37 64.97 113.37 64.97 99.9 94.19 115.1 64.38

4 66.4 61.23 135.7 75.5 126.26 73.07 126.26 73.07 106.1 104.3 126.04 71.67

3 71.5 68.62 148.9 85.5 136.54 81.31 136.54 81.31 109.9 111.66 133.9 78.5

2 75.3 77.5 159.6 100.5 143.82 95.2 143.82 95.2 116.2 114.82 134.1 90.29

1 84.49 72.7 162.1 85.4 140.5 74 140.5 61.51 141.0 76.76 129.84 54.34

0 133.3

4 - 124.1 - 106.7 - 106.7 -

134.1

4 - 100 -

4.11 Strength of concrete material (Ec)

The effect of the strength of concrete material was studied by varying the Young’s

modulus of elasticity (Ec). The reason for considering this as one of the parameters is

- 86 -

because the change that has taken place in the usage of different grades of concrete

material over the years. The results would be useful for studying the response of old

buildings.

Structures designed and constructed to conform to old codes with poor material

specifications will still have to resist the same amount of lateral load as that of new

structures in a given seismic region. As concrete is the main material of a reinforced

concrete structure, this Section covers variation of structural responses with Ec under

the Kobe earthquake. This Section was studied using the ten storey model designed

for gravity loads only. The model had infill walls with a Young’s modulus of

elasticity of 2500 MPa and a Poisson’s ratio of 0.15. The thickness of the infill walls

was 250 mm throughout the storey height.

The models were analysed under the Kobe earthquake scaled to 0.2g PGA. The

models also had additional load from Dead and Live loads and their magnitudes were

22 KN/m and 10 KN/m respectively. A wide range of cE from 15000 MPa to 40000

MPa was studied. This wide range depicts structures designed and constructed with

and without seismic provisions. The results are expressed in terms of the fundamental

period, roof displacement, inter-storey drift ratio and the stresses in the infill walls.

4.11.1 Fundamental period (T)

The fundamental period of vibration that resulted from the Modal analyses indicates

that the strength of the concrete influences the period of the structure. Figure 4.23

shows the decrease in the fundamental period with the increase in cE . It was found

that the fundamental period increases by 7.8% in every 5000 MPa increment in cE .

This suggests that old buildings with low strength concrete could have a higher

fundamental period and this is not mentioned explicitly in any current codes.

- 87 -

0.9480.881

0.7840.714

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

15 20 30 40

Strength of concrete Ec (x 103MPa)

Fu

nd

am

en

tal p

eri

od

Figure 4.23 Concrete strength vs. Fundamental period.

4.11.2 Maximum roof displacements

In general, the roof displacement response was found to decrease with the increase

in cE , as anticipated. A typical roof displacement history of a model with an Ec

=15000 MPa is shown in Figure.4.24. This model has undergone the maximum roof

displacement of 38 mm at the time step of 3.36 seconds. Similarly, roof displacements

were 36.6 mm, 30.6 mm and 30.8 mm for models with cE of 20000 MPa, 30000 MPa

and 40000 MPa respectively. All these events took place at different time steps. There

is only a small difference in roof displacements of the models with cE =30000 MPa

and 40000 MPa which could be due to close proximity in frequency range between

the model and the ground motion.

Figure 4.24 Roof displacement history of a model with Ec=15000 MPa

- 88 -

4.11.3 Inter-storey drift ratio

The inter-storey drift ratios are shown in Figure 4.25. The drift ratios were found

based on the maximum roof displacement response. It was observed that the inter-

storey drift ratios vary with the Young’s modulus of elasticity of concrete; however,

the effect is not significant for a given range. For lower cE values, higher inter-storey

drift ratios were observed and vice versa. On an average, the drift ratio reduces by

4.77% when Ec increases by 5000 MPa.

0

2

4

6

8

10

12

0.000 0.001 0.002 0.003


Sto

rey le

ve

ls

Ec-15000MPa

Ec-20000MPa

Ec-30000MPa

Ec-40000MPa

Figure 4.25 Inter-storey drift ratios of models with varying Ec value.

The stresses in the infill walls were also found to correspond to the maximum roof

displacement. It was observed that the maximum principal stresses are 1.68 N/mm2

for models with cE =15000 and 20000 MPa and 1.33 N/mm2 for models with

cE =30000 MPa and 40000 MPa. Since the influence of Ec is not significant, its effect

on infill stress is not realized.

The global structural responses of the models were studied by varying the Young’s

modulus of elasticity of concrete material. It was found that the fundamental period

and the inter-storey drift ratios decreased by 7.8% and 4.77% for every 5000 MPa

increment. On the other hand, roof displacement and infill stresses are not appreciably

affected.

- 89 -

4.12 Infill thickness (t)

Burnt clay brick masonry has become a common infill material for reinforced

concrete structures in the seismically active regions, especially in developing

countries such as Bhutan. The infill thickness of 250 mm is generally used for

external walls and 125 mm for internal partitions. Often, the client chooses to use a

thickness of 125 mm for both external and internal walls throughout the building with

the intention to reducing the total seismic weight of the structure. Hence, these two

values for the effect of infill thickness, and the results are presented in following

sections.

A ten storey model was used to study the effect of the infill thickness. The analyses

were performed using a peak ground acceleration of the 0.2g Kobe earthquake on a

model with an infill thickness of 125 mm (represented by M1). The same model was

re-analysed for the same load; however the thickness of the infill wall was changed to

250 mm (represented by M2). In all analyses, a uniform load of 22 KN/m dead load

and 10KN/m live load were applied on the beams, in addition to the weight of the

structural members. These loads were assumed to exist from the tributary area of five

meters width of slab. The results are presented in terms of the fundamental period,

roof displacement, inter-storey drift ratio, member forces and the stresses in the infill

walls.

Modal analyses were performed to assess the variation of the fundamental period (T)

with the thickness of the infill walls. The period of structures should decrease with the

increase in stiffness of the model, however the current results show that there is a

small decrease followed by a small increase in period by increasing the thickness due

to the increase in mass. This indicates that the thickness has no significant influence

on the period. From these analyses a small variation was observed as shown in Figure

4.26. Since both mass and stiffness changes when the thickness increases, no linear

relationship was found to exist between fundamental period and the thickness of the

infill wall.

It was observed that there is a minimum point of fundamental period corresponding to

a particular thickness for a particular structural system, as indicated by Figure 4.26.

- 90 -

Modal analysis was separately conducted on a model by varying the thickness of the

infill wall. A detailed study of the period and infill thickness is not going to be

meaningful as they both vary from structure to structure. In this study, the M1 model

with an infill thickness of 125 mm shows that the fundamental period increases both

by increasing or decreasing the thickness. The fundamental period of the model is

0.735 second and 0.746 seconds for model M1 and M2 respectively, which is not a

significant variation. Thus, it is necessary to model the infill walls correctly to achieve

accurate result for the periods.

-

0.450

0.900

1.350

1.800

0 250 500 750 1000

Thickness of infill wall (mm)

Fu

nd

am

en

tal p

eri

od

(s)

Figure 4.26 Variation of fundamental period with infill thickness.


The roof displacement time histories for both the models are shown in Figure 4.27.

The peak roof displacement model M1 is 46.9mm at the time step of 8.688 seconds

while it is 49.1 mm at the time step of 8.690 seconds for the M2 model. Logically, the

global displacement would decrease when the thickness is increased since infill

stiffness is being increased. However, the mass of the structure predominates and thus

the period increases, and hence the global response decreases under earthquakes load.

This will hold true only if the frequency content of the ground motion does not match

the modal frequency. The difference in roof displacement between the two models is

only 4.69% which is insignificant in terms of global roof displacement response.

- 91 -

Conversely, the global roof displacements of the M1 and M2 models for un-damped

condition were 161 mm and 117 mm at the time step of 19.14 second and 10.13

second, respectively. This is a 37.72 % increase in roof displacement caused by

increasing the infill thickness. This indicates that the infill thickness has a significant

effect on resisting the lateral load when the model is considered as un-damped. The

importance of including damping is thus evident.

M1 M2

Figure 4.27 Roof displacement histories of models with different infill thickness.

4.12.1.1 Inter-storey drift ratio

The inter-storey drift ratio decreases when the infill thickness decreases from 250 mm

to 125 mm as shown in Figure 4.28, however, effect of infill thickness on the inter-

storey drift ratio is not significant. It was found that the maximum inter-storey drift

ratio of model M2 is 4.45% greater than that of the M1 model. This clearly indicates

that there is no lateral stiffness contribution from infill thickness. On the other hand,

the M1 model could have responded more due to close proximity in the frequencies

and shows different results from the expected.

When the models were treated as un-damped, it was found that the variation in

thickness significantly influences the inter-storey drift ratio. The maximum value of

inter-storey drift ratio of model M2 was 27.57% greater than that of the M1 model.

- 92 -

Since damping exists in a structure by virtue of its inherent properties, it can be

concluded that the thickness has no significant influence on the inter-storey drift ratio.

0

2

4

6

8

10

12

0.000 0.001 0.001 0.002 0.002 0.003


Sto

rey lv

el

Model M1

Model M2


4.12.1.2 Member forces

The variation of internal member forces such as moments and shear forces in the

columns and beams were studied for these models which are represented by M1 and

M2. These forces were monitored for a particular column and beam in order to

observe the consistency in variation and the maximum forces.

Table 4.10 shows the moments in a column and a beam at different storey levels of

the models. It was found that the moment decreases by decreasing the thickness in all

storeys except at roof level where it was increased by 20% and 5.861% for beams and

columns respectively. This could probably be due to the upper storey beam being

more flexible. However, this needs further investigation. The percentage difference in

moments was worked out with respect to M2 model and the negative signs, seen in

Table below, show the increase in moments.

The maximum difference in moment is 19.017% at the first storey for beams and

16.61% for the column which is at the base of the structure. This also indicates that

the percentage difference in moment between the two models does not correspond to

the maximum inter-storey drift pattern. An average difference in moments in columns

- 93 -

and beams was found to be 9.416% and 9.540%. The results signify that the thickness

has same effect on beams and columns moments. In general, moments in columns and

beams are decreased by 9.5% by reducing the thickness from 250 mm to 125 mm.

Table 4.10 Moments in beams and columns

Storey

levels

Beam moment

KNm

Column moment

KNm

% difference in

moment

M2 M1 M2 M1 Bean Column

11 3.7 4.44 11.43 12.1 -20.000 -5.8618

10 13.8 13.47 27.36 25.9 2.391 5.336

9 28.14 26.16 41.14 38.28 7.036 6.952

8 41.73 38.36 53.57 49.25 8.076 8.064

7 54.52 49.88 64.75 59.15 8.511 8.649

6 66.29 60.46 74.71 67.93 8.795 9.075

5 76.83 69.91 83.18 75.26 9.007 9.522

4 85.96 77.97 89.73 80.73 9.295 10.03

3 93.55 84.2 93.56 83.7 9.995 10.54

2 99.77 87.76 98.54 88.6 12.038 10.09

1 73.04 59.15 121.5 109.24 19.017 10.08

0 - - 134.6 112.24 - 16.61

Average increase increased in moments 9.416 9.540

Similarly, the shear forces in columns and beams were determined and are shown in

Table 4. 11. Unlike in previous section, the shear in the beam was generally decreased

by decreasing the thickness in all storeys while the column shear followed the pattern

of moment variation.

The maximum difference in shear force in beams and columns was found at storey

level one and their values are 33.312% and 27.117% respectively. However, an

average decrease in shear forces is 21.06% and 18.37% for beams and columns. The

effect of thickness is more pronounced in shear than in bending in both beams and

columns. When the thickness is increased, both the mass and the stiffness increases

but the mass predominates the stiffness and results in greater lateral loads. Thus

greater member forces are observed for thick wall models, however the contribution

- 94 -

of infill walls under lateral forces would increase and thus should result in more

member forces. Thus, it can be concluded that the shear forces in the beams and

columns are increased by increasing the thickness and vice-versa.

Table 4.11 Shear force variation

Storey

levels

Beam shear

KN

Column shear

KN

% difference

shear force

M2 M1 M2 M1 Bean Column

11 2.10 0.11 8.54 7.85 -11.000 8.080

10 9.91 8.75 23.16 22.35 11.705 3.497

9 23.4 18.3 42.94 37.66 21.966 12.296

8 35.17 27.2 62.69 52.27 22.804 16.621

7 46.02 35.4 81.25 65.91 23.120 18.880

6 55.98 42.9 98.26 78.32 23.330 20.293

5 64.97 49.7 113.37 89.2 23.503 21.320

4 73.07 55.8 126.26 98.26 23.635 22.176

3 81.31 61.9 136.54 105.19 23.872 22.960

2 95.2 72 143.82 107.99 24.380 24.913

1 61.51 41 140.5 102.4 33.312 27.117

0 - - 106.7 82.9 - 22.306

Average increase increased in moments 21.06 18.37

Similarly, the models were studied under un-damped conditions. The member forces

were also found to increase by increasing the thickness under this condition. The

moments in beams and columns decreased by 21.8% and 21.14% while the shear

forces decreased by 11.27% and 11.43. The results were similar even if the models

were treated as the un-damped models.

The maximum principal stresses in the infill walls were found to be 4.2 N/mm2 for

both the models. Since there is very small variation in roof displacement, inter-storey

drift ratio and the member forces, there is no perceptible variation in stresses of the

infill wall when there is a change in thickness.

- 95 -

Thus, the thickness of the infill wall does have an influence on the global response of

the model under seismic action, especially in terms of moments and shear forces but

its effect on inter-storey drift, stress in the infill wall, roof displacement and the

fundamental periods are insignificant.

4.13 Peak ground acceleration (PGA)

This section presents the performance of the models under varying peak ground

acceleration. The performance of an infilled frame is based on the onset of cracks

within the infill walls and the inter-storey drift ratio requirement of the existing

seismic code(IS1893, 2002). This can be achieved through the Time history analysis

under dominant earthquake determined in earlier sections.

The existing seismic code (IS1893, 2002) considers the 0.36g as an effective peak

ground acceleration for a Maximum Earthquake while it is 0.18g for a Design

Earthquake in a high seismic zone. Therefore, a PGA range from 0.1g to 0.4g is

considered for this parameter. Initially, all the models were treated as un-damped

structures and later they were treated as damped structures. Thus, the results are

presented for two damping values (ξ = 0 and 05.0 ).

4.13.1 Building design without seismic provisions

This group of structural models represents building stock constructed prior to the

adoption of seismic code i.e. before 1997 in Bhutan. The models analysed are: three

storeys, five storeys, seven storeys and ten storeys which are typical to Bhutan. All

the models were analysed in relation to the dominant earthquake determined in

Section 4.5.1. The analysis was stopped after the infill stress was equivalent to the

maximum compressive strength of the infill wall. By carrying out these analyses, one

can judge whether the infill helps in resisting strong motion and if it does, to what

extent.

The models are represented by symbols J10, J7, J5 and J3 corresponding to a ten

storey, seven storey, five storey and three storey models respectively as shown in

Figure 4.29. The infill walls were assumed to have an elastic modulus of 5000 MPa in

absence of specific material property and also due to random selection of infill

- 96 -

material. The thickness of the wall was assumed to be 250 mm. Model J10, J5 and J3

were studied under the Kobe Earthquake while J7 was studied under the Northridge

earthquakes respectively, as determined in Section 4.5.

J10 J7

J5 J3

Figure 4.29 Structures without seismic provisions.

4.13.1.1 Inter-storey drift ratios

The current code (IS1893, 2002) restricts the structural drift limit ratio to 0.004 under

an unfactored seismic load and the inter-storey drift ratios for different models with

and without damping are shown in Figure 4.30 to 4.35. This limitation is mainly to

avoid failure due to P-∆ effect and to restrict excessive sway of the structure for

human comfort.

The inter-storey drift ratios of J10 model are shown in Figure 4.30. The inter-storey

drift ratios of an un-damped model exceeded the code limit at the PGA of 0.2g while

- 97 -

the damped model exceeded after 0.3g PGA. In both cases the maximum drift ratio

occurred at storey level three as before. This shows that the structures constructed

without seismic provisions can meet the drift requirements of current code if the infill

walls are considered appropriately.

(a) (b)

Figure 4.30 Inter-storey drift ratios of a ten storey model (a) 5% damping; (b) 0 % damping.

(c) (d)

Figure 4.31 Inter-storey drift ratios of a seven storey model (a) 5% damping; (b) 0 % damping.

The maximum drift ratio of an un-damped model is three times greater than the inter-

storey drift ratio of damped model under the external load of 0.2PGA. The role of

damping in a structure significant and large amount of energy is being dissipated. On

the other hand, if the damping is not considered the seismic demand increases

0

2

4

6

8

10

12

0.000 0.002 0.004 0.006

sto

rey le

vel

Inter-story drift ratio

0.1g PGA

0.2g PGA

0.3g PGA

0.4g PGA

0

2

4

6

8

10

12

0 0.005 0.01

sto

rey le

vel

Inter-story drift ratio

0.1g

0.2g

0.3g

0

1

2

3

4

5

6

7

8

0.000 0.001 0.002 0.003 0.004


sto

rey le

vel

0.1PGA

0.2PGA

0.3PGA

0.4PGA

0

1

2

3

4

5

6

7

8

0 0.005 0.01 0.015

sto

rey le

vel

Inter-storey drif t ratio

0.1PGA

0.2PGA

0.3PGA

0.4PGA

- 98 -

alarmingly. Thus, the determination of appropriate damping level of the structure is

crucial to economise the structural design.

The inter-storey drift ratios of a seven storey model (J7) is shown in Figure 4.31. The

drift ratios of the damped model were within the code limit even under 0.4g PGA. On

the contrary, undamped model exceeded the limit limit at 0.2g PGA. Thus, this model

can survive earthquakes up to 0.4G PGA without problems in snter-storey drifts.

The inter-storey drift ratios of J5 model were within the limit (0.004) when the model

was treated as damped model under a range of PGAs (0.1g to 0.4g). However, 0.2g

PGA is enough to reach the inter-storey drift ratio limit when the model is considered

to be un-damped as shown in Figure 4.32. Therefore, the five storey model which was

designed to withstand only gravity loads can full fill the drift ratio requirement of the

code when considering the infill walls. The inter-storey drift ratios of a three model

(J3) were within the code limit under both damped and un-damped conditions. This

structure, under in-plane loads, is very stiff and their drift ratios are significantly

smaller than the code values.

The results show that the inter-storey drift demands of the structures decreases with

the height of the structure because the structure becomes stiffer by reducing the

height. For a tall structure, less seismic load is required to produce the same amount

of drift ratio of a low rise structure which requires larger load. The high rise structure

therefore could expect drift problems even under the Design Earthquake but the low

rise structures are completely free from it even at higher PGA. Thus, the presence of

infill walls significantly reduces the inter-storey ratios of the models under seismic

load.

(e) (f)

- 99 -

Figure 4.32 Inter-storey drift ratios of a five storey model (a) 5% damping; (b) 0 % damping.

(g) (h)

Figure 4.33 Inter-storey drift ratios of a three storey model (a) 5% damping; (b) 0 % damping.

By treating the models as un-damped models, the inter-storey drift ratios increased by

almost 3 times of the damped models (5%). All un-damped models have crossed the

drift ratio limit at 0.2g peak ground acceleration except the three storey model which

showed similar results to the damped model as shown in Figure 4.33. The damping of

the structures plays an important role in dissipating seismic forces.

In general, models less than ten storeys in height (approximately 30 m) do not exceed

the inter-storey drift ratio of 0.004 under the Design Earthquake of 0.2g if the infill

walls are considered. Thus, the present code limitation of inter-storey drift ratio is

conservative and may result in uneconomical design of buildings if its drift ratio limit

is used in design.

0

1

2

3

4

5

6

7

0.0000 0.0010 0.0020 0.0030

sto

rey le

vel


0.1gPGA

0.2gPGA

0.3gPGA

0.4gPGA

0

1

2

3

4

5

6

7

0 0.01

sto

rey le

ve

l


0.1gPGA

0.2gPGA

0.3gPGA

0.4gPGA

0

1

2

3

4

0.000 0.001 0.002

sto

rey le

vel


PGA=0.1g

0.2g PGA

0.3g PGA

0.4g PGA

0

1

2

3

4

0.000 0.002 0.004

sto

rey le

vel


PGA=0.1g

0.2g PGA

0.3g PGA

0.4g PGA

- 100 -


The compressive strength of the infill wall was found to be 6.66 N/mm2

corresponding to its Young’s modulus of elasticity of 5000 MPa (Pauley, 1992 #89).

The maximum principal stresses were found corresponding to the maximum roof

displacement response of the model and the values are given in Table 4.12. The

shaded values show the stress that has exceeded the compressive stress of the infill

wall.

Under un-damped condition, 0.2g PGA is sufficient to crack the infill walls in models

whose height varies from five to ten storeys. However, the stresses were within the

maximum compressive stress of the material for a three storey model.

The damped ten storey model reached the maximum stress level of the infill material

after 0.3g PGA of external load but all other models did not reach the limit even at

0.4g PGA.

Table 4.12 Variation of infill stresses with PGA for non-seismic structures.

Models Maximum principal stress in infill wall in N/mm2

0.1PGA 0.2g 0.3g 0.4g

ξ=0% ξ=5% ξ=0% ξ=5% ξ=0% ξ=5% ξ=0% ξ=5%

Ten storey 4.9 2.10 9.1 4.20 14 5.95 14 8.40

Seven storey 4.9 1.26 9.8 2.45 14 3.85 19.6 4.9

Five storey 6.3 0.84 11.9 1.68 18.2 2.8 23.8 4.62

Three storey 1.4 0.42 2.66 0.84 4.2 1.19 5.6 1.68

It can be concluded that the buildings which were constructed before 1997 (prior to

adoption of the seismic code) are not capable of withstanding a seismic acceleration

of 0.2g when the damping of the structure is assumed to be 0%, however such

condition does not exist in real buildings. On the other hand, when the damping is

introduced, the models meet the drift requirement of the code (IS1893, 2002 #88) and

infill stresses are within its compressive strength.

- 101 -

The structural model having the infill of Ei greater than 5000 MPA can resist the

seismic load of 0.4g PGA without any problems in inter-storey drift requirement and

the cracks in the infill walls. However, Ei value in this study is based on the

assumption that the infill strength may vary due to random selection material.

4.14 Structures constructed with seismic provision

This Section describes the performance of the models which are designed with

seismic provisions in developing countries like Bhutan. The objective of this

particular study is to know the level of PGA that the structure could resist without

potential cracks appearing in the infill with a given property. The Young’s modulus of

elasticity and Poisson’s ratio of infill were assumed to be 5000 MPa and 0.15. The

structural member sizes are given in Table 3.5 of Chapter three. The opening in the

infill was 40% as before. The reader is advised to refer to Section 4.5.2 for further

information on the variation of structural performance with infill strength.

Analyses were performed under the dominant ground motions but the results are

presented for damping levels of 3% and 5%. The performance of the structure is based

on the inter-storey drift ratio limit imposed in the code(IS1893, 2002) and the stresses

in the infill walls. Though the inter-storey drift ratio would not give the true

performance of the structure as this limit is set for serviceability condition, it gives a

fair idea on the use of infill materials. Similar to the previous section, the PGA of the

dominant earthquake was varied from 0.1g to 0.4g; however, if the structure has

reached the code drift ratio limit of 0.004 before 0.4g, the analysis was stopped.

Analysis beyond the 0.4g PGA was not considered because most of the current

seismic codes treat 0.4g as the effective PGA for a Maximum Earthquake(ATC-40,

1996).


The current seismic code(IS1893, 2002) restricts the inter-storey drift ratio to 0.004

for structures of more than two storeys under a serviceable earthquake load. It means

that the result on inter-storey drift ratio only give us information on the serviceable

earthquake load for an infilled frame structure.

- 102 -

Figure 4.34(a) shows the inter-storey drift ratios on a ten storey model at 5 % and 3%

damping. At 5% damping level, the inter-storey drift ratio of the model is within the

limit under the Peak ground acceleration of 0.1g, 0.2g and 0.3g but exceeds the limit

against 0.4g PGA. When the damping was 3%, the inter-storey drift ratio exceeded

the limit at 0.3g PGA as shown in Figure 4.34b.

In general, the structural responses decreased with the increase in the damping level

of the structure as expected. In this study it was found that the inter-storey drift ratio

decreased by 15.08% by increasing the damping level from 3% to 5 %. Similarly,

member forces such as shear force and moment would vary accordingly.

Figure 4.34 Inter-storey drift ratios of a ten storey model (a) 5% l damping; (b) 3 % damping.

The inter-storey drift ratio of the seven storey model did not exceed the drift limit at

5% damping at 0.4g but when the damping level was reduced to 3%, the drift ratio

limit was exceeded after 0.3g PGA. The structural responses decreased by 18.59 % by

increasing the damping level from 3% to 5%.

(a)

0

2

4

6

8

10

12

0.000 0.002 0.004 0.006


Sto

ret

level

0.1g

0.2g

0.3g

0.4g

(b)

0

2

4

6

8

10

12

0.000 0.002 0.004 0.006


Sto

rey

lev

el

0.1g

0.2g

0.3g

- 103 -

Figure 4.35 Inter-storey drift ratios of a ten storey model (a) 5% damping; (b) 3 % damping.

The inter-storey drift ratios of five storey model did not exceed the drift ratio limit

(0.004) by increasing the PGA from 0.1g to 0.4g at both damping levels. However, it

was observed that there is a significant difference of 31.24% between the inter-storey

drift ratios at two damping levels.

Figure 4.36 Inter-storey drift ratios of a five storey model (a) 5% damping; (b) 3 % damping.

Similar to the five storey model, the three storey model did not suffer from the inter-

storey drift problem when the PGA was increased from 0.1g to 0.4g PGA. However,

the difference in values at the two damping levels was 50.07%.

(b)

0

1

2

3

4

5

6

7

8

9

0.000 0.001 0.002 0.003 0.004 0.005


Sto

rey le

ve

l

0.3g

0.4g

(a)

0

1

2

3

4

5

6

7

8

9

0.000 0.001 0.002 0.003 0.004


Sto

rey

lev

el

0.1g

0.2g

0.3g

0.4g

(a)

0

1

2

3

4

5

6

7

0.000 0.001 0.002


Sto

rey

lev

el

0.1g

0.2g

0.3g

0.4g

(b)

0

1

2

3

4

5

6

7

0.000 0.001 0.002 0.003 0.004


Sto

rey

lev

el

0.3g

0.4g

- 104 -

Figure 4.37 Inter-storey drift ratios of a three storey model (a) 5% damping; (b) 3 % damping.


The maximum principal stresses are shown in Table 4.13. The ten storey model has

reached the maximum compressive stress of the material at 0.3g and 0.4g PGA

corresponding to the damping levels of 0.03% and 0.05%. All other models did not

reach the maximum stress level under a range of PGAs. The result shows that the

models with infill walls having Ei value of 5000 MPa or greater would not suffer

from cracks in the walls as the stresses obtained from the analyses are less than the

maximum compressive values. The results only give relative information in the

absence of the true properties of the infill materials.

Table 4.13 Variation of infill stresses with PGA for aseismic strctures.

Models Maximum principal stress in infill wall in N/mm2

0.1PGA 0.2g 0.3g 0.4g

ξ=3% ξ=5% ξ=3% ξ=5% ξ=3% ξ=5% ξ=3% ξ=5%

Ten storey 2.24 1.96 4.9 3.92 7.0 5.6 - 7.7

Seven storey - 1.54 - 2.3 3.5 3.5 4.9 4.9

Five storey - 0.7 - 1.4 3.08 2.1 4.2 2.8

Three storey - 0.364 - 0.77 1.19 1.12 3.08 2.8

Those building which were constructed in compliance to the seismic code do not

suffer from inter-storey drift problems if the infill is taken into account. The ten storey

(a)

0

1

2

3

4

0.000 0.001 0.002 0.003


Sto

rey

lev

el0.1g

0.2g

0.3g

0.4g

(b)

0

1

2

3

4

0.000 0.001 0.002 0.003


Sto

rey

lev

el

0.3g

.4g

.6g

- 105 -

model exceeded the drift limit at 0.3g PGA while all other models did not exceed the

limit even at the peak acceleration of 0.4g. This clearly shows the improvement of

structural performance by introducing the infill walls. The code does not specify

whether the infill is considered in imposing drift limit, but it was found from this

study that the infilled structures are well below the code limit.

The maximum stress in the infill walls were less than the maximum compressive

stress of the infill material (fi = N/mm2) under a range of seismic load. For a ten storey

model, the ground acceleration of 0.3g is sufficient to reach the strength limit of infill

material but all other models did not reach to its cracking stress level even at 0.4g

PGA. Therefore, it can be concluded that the strength requirement of the infill wall is

lower than 6.66 N/mm2 and there is no extra benefit in providing high strength infill

walls.

Thus, it can be concluded that the buildings constructed in compliance to the seismic

codes do not suffer from cracks in infill walls and inter-storey drift problems even

under 0.4g PGA when the Ei value is 5000 MPA or higher.

4.15 The Arcade effect/Soft storey phenomenon

As seen in the literature review, buildings in the commercial hub are mostly

constructed with an arcade at the ground level on one side of the building plan. There

is thus an irregularity in distribution of infill walls across the elevation and non-

uniform distribution of infill within the building structure. The existing seismic code

(IS1893, 2002) states that the storey is soft when the lateral stiffness of the storey is

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

stiffness of the three storeys above. Unless the infill walls are modelled properly, this

phenomenon, which is likely to be present in commercial structures due to the arcade

provision, is not captured in the analysis. Thus, there was a need to study whether this

type of structure suffers from soft storey or not and how realistic the scale factors

given in the code are.

- 106 -

In general, engineering practice in developing countries, infill walls are rarely

considered in modelling and analysis and the effect of an arcade at the ground storey

level is not realized. Thus, this issue is addressed in this Section and the results are

presented in terms of member forces such as moments and shear force in the beams

and columns. The seismic code in use recommends the scale factors of 2.5 if there is

storey problem in the structure.

To address this problem, a ten storey structure has been considered. There are two

models, S and SI where S model has infill walls in all the storeys (model free from

soft storey problem), while SI model does not have infill walls at the ground storey as

shown in Figure 4.38. The effect of the soil was not considered for reasons discussed

in Chapter three. The infill walls were assumed to have a typical opening of 40%

typically. Young’s Modulus of elasticity and Poisson’s ratio of the infill material were

5000 MPa and 0.15 respectively. The infill walls were 250 mm. The cross-section of

the columns and beams were 450 X 450 mm square and 400mm deep and 300 mm

wide respectively while the Young’s modulus of elasticity and Poisson’s ratio of

concrete were 24000 MPa and 0.2 respectively.

All the models were studied under the Kobe earthquake which was found to be the

dominant earthquake for this model. It was studied at 0.2g PGA and the results are

presented in the following Sections. The models were assumed to have 5% damping

to simulate the behaviour of the real structures.

S SI

Figure 4.38 (a) S-normal model and (SI) model with Arcade

- 107 -

The lateral stiffness of the models was evaluated and it was found that the SI model

has lateral stiffness of more than 70% of the storey above but it is less than 80% of the

average lateral stiffness of the three storeys above. Thus, SI model has the soft storey

problem (IS1893, 2002).


The effect of the arcade is seen even in the roof displacement response as the roof

displacement was 49.1 mm and 56.5 mm for S and SI models respectively. The

difference in roof displacement is mainly because there is a large displacement on first

floor level. The difference in roof displacement between the SI and S was 15.07%.

This value could be useful information in deciding the required separation gap

between the two structures to avoid pounding during ground vibration.


The inter-storey drift ratio gives a clear picture of structural deformation across the

height as shown in Figure 4.39. There is a significant increase in the inter-storey drift

ratio when there is no infill in the lower storeys. The maximum drift ratio was

observed at level two for the SI model and at level three in the S model. The peak

values were 0.006 and 0.008 corresponding to model S and SI respectively and the

peak inter-storey drift ratio of SI model is 1.68 times greater than the S model. This

ratio can be used as a factor to increase the member strength for structures analysed

without considering the infill walls.

The difference in drift ratio decreases along the height of the structure. In model SI,

the increase in drift ratio is up to level four and there after the values are close to the S

model. It would be reasonable to introduce the factor to structural members up to level

three for structures like the SI model. The summary of member forces escalation

factors are given at the end of the section.

- 108 -

0

2

4

6

8

10

12

0.000 0.001 0.002 0.003 0.004

Inter-storey drift ratios

Sto

rey levels

S1

S


4.15.3 Column moments

The variation of the column moment along the height of the structures is shown in

Figure 4.40. Normally the columns have maximum moment at the base of the column

under the action of lateral load due to a cantilever action created by the fixed support

condition at the base. However, it is not true in all the cases as depicted by the SI

model which does not have infill walls at the ground floor. For this particular model,

maximum moment was found at the first storey level. There is a significant increase in

column moment up to the next storey above the considered level. This means that the

columns up to the third floor level should be factored as recommended in this Section.

The S model has its maximum moment at storey level one. The ratio of maximum

column moment between these models is 1:1.76 and these ratios can be use as the

magnification factor for structures having similar problems. The ratio of moments

increases with the increase in the amount of infill in upper storey. Therefore, the

magnification factor given in the current code is not applicable for structures which

have similar opening size and thus the modelling of infill in a reinforced concrete

frame structures is important for accuracy of results.

- 109 -

0

2

4

6

8

10

12

0 50 100 150 200 250

Moments in column (KNm)

Sto

rey levels

S1

S

Figure 4.40 Column moment.

4.15.3.1 Column shears

The effect of soft storey on column shear is shown in Figure 4.41. There is a small

variation in shear force between the two models. The ratio of shear force with respect

to S model is 1:1.08.

01

2

3

4

56

78

9

10

11

12

0 25 50 75 100 125 150 175

Shear forces in columns (KN)

Sto

rey levels

S1

S

Figure 4.41 Beam moment

4.15.3.2 Beam moments

The moments in beams are also significantly increased when there are no infill walls

in the lower storeys as shown in Figure 4.42. The maximum moments were observed

in the beams of storey level two than the level one beams. However, the moments in

- 110 -

beam were increased only at a particular level which has no infill walls. The ratio of

the values of maximum moment of the model is 1:1.44. The magnification of

moments in all other storeys was not significant.

0

2

4

6

8

10

12

0 25 50 75 100 125 150

Moments in beams (KNm)

Sto

rey levels

S1

S

Figure 4.42 Shear in column

4.15.3.3 Beam shears

Shear forces of the beams in floor without infill walls are also increased, however, the

effect is not significant as the moments are. The ratio of the maximum shear force in

the beams with respect to the maximum shear force in the S model was found to be

1:1.11 as seen in Figure 4.43.

0

2

4

6

8

10

12

0 25 50 75 100 125Shear forces in beams (KN)

Sto

rey levels

S1

S

Figure 4.43 Shear in beam.

The magnification factor of member forces of columns and beams are given in Table

4.14. These factors were found with respect to the results of model S. The values in

the first column were obtained from a model which had no opening within the infill

- 111 -

wall while the values of second column were obtained for model with opening. It is

evident that the magnification factors vary with amount of infill present. The results

from current analyses are comparatively lower than that given in the existing code

irrespective of damping consideration for a typical structure which does not have infill

walls at the lower storeys. The difference between the current results and the code

value is due to conservative nature of code specification.

Similar studies were also carried out for a three storey model and observed that the

magnification factors were smaller than the ones computed for the ten storey models.

Thus, results shown in Table 4.14 are typical values for a ten storey model with 40%

opening.

Table 4.14Table 4.14 Magnification factors for structural members.

Fully infilled model Model with 40% opening

Moments in columns 2.54 1.76

Moments in beams 1.68 1.44

Shear in columns 1.43 1.08

Shear in beams 1.87 1.11

4.16 Conclusion

This Chapter presented the results from the Time History analyses on typical infilled

reinforced concrete models. It covered some of the important parameters which have

not been considered for typical building structures in Bhutan. The results were

presented covering the wide seismic hazard scenarios for a given structural property

to assess the performance level of infilled framed structures. The result are

summarised as follows;

The power spectral density (PSD) analyses revealed that the dominant frequency

content of the Kobe earthquake is 0.740 Hz while it is 0.666 Hz and 1.66 Hz for El

Centro and Northridge earthquakes. The ten storey model was found to almost

coincide it natural period with the dominant period of the Kobe earthquake. Kobe

earthquake was found to be dominant for three storeys, five storey and ten storeys

models and Northridge earthquake was dominant on seven storey model.

- 112 -

The infills strength which was expressed in terms of the Young’s modulus of

elasticity (Ei) was found to be the significant parameter for an infilled structure. It

was found that the global structural responses such as fundamental period, inter-storey

drift ratios and the stress in the infill walls increase with the increase in Ei values. The

Design Basis earthquake (0.2g PGA) requires the maximum compressive strength of

infill material to be 2 N/mm2. The performance of the model also varies with the

opening percentages. The fundamental period, inter-storey drift ratios, infill stresses

and the moments in the beams and columns were found to increases by increasing the

opening size but the shear forces in beams and columns were generally found to

decrease.

The effect of infill thickness on the global response structures under seismic

excitation in terms inter-storey drift, stress in the infill wall, roof displacement and the

fundamental periods are negligible but its influence on the member forces such as

moment and shear are significant. The performances of buildings constructed with

and without seismic provisions are almost similar if the infill walls are taken into

consideration. This is because the structural capacity is mainly depended on the

stiffness property of the model and since the infill contributes large stiffness to lateral

load, performance of structures designed with or without seismic provisions is same

as long as the infill property is same.

The damping of the structure plays an important role in dissipating the seismic forces.

It was observed that the difference in global structural response (roof displacement) of

un-damped model is almost 4 times the damped (5%) models. The soft-storey

phenomenon which is due to arcade provision in the building structures requires lesser

magnification factor than the values in the code for this typical structure. The next

Chapter will present you the discussion of the results obtained in this Chapter.

- 113 -

Chapter 5. Discussion

5.1 Introduction

This Chapter discusses the results obtained in the previous Chapters. The results are

discussed in relation to previous research results in the literature and existing code

values, and presents new information and recommendations to improve the seismic

resistance of reinforced concrete structures with the use of infill material. Also, the

Chapter presents the importance of the current results and their significance and

application to general engineering practices.

5.2 Interface element

The two methods widely recognised for analysing of infilled frame models are the

Finite element method (FE) and the Equivalent diagonal strut method. The first

method seems to give accurate results but the cost of computation is higher while the

later method cannot consider openings which are normally present in real buildings.

Moreover, the FE method uses the contact element (Doudoumis, 2006) at the frame-

infill interface whose contact coefficient varies with the quality of material and

workmanship (Moghaddam, 1987). Therefore, a simple interface element is required

which can be used to model frame-infill interaction in general engineering practices,

without compromising accuracy of the global structural responses. To address this

problem, the gap element was found to be suitable which is active in compression

only, and in which forces from frame elements are transferred to infill panels, thus

allowing the influence of the infill on the frame structure to be modelled.

Infilled structures consist of low strength mortar at the frame-infill interface. To

simulate such a condition, an effective stiffness of the gap element was found,

Equation 3.1, and the results were found comparable to the previous work carried out

by Doudoumis and Mitsopoulou (1995). This shows that, the Gap element, using the

stiffness relation developed in this study, can simplify the model generation of infill

frame structures and the computation is greatly reduced compared to a comprehensive

- 114 -

FE method. However, the effective stiffness of the gap element should be greater than

the stiffness of the surrounding elements.

Although using the FE approach with contact elements gives accurate results, as

reported in past studies, this method was found costly to carry out studies for large

structures. Thus, the practising design engineer could ignore the presence of infill,

while the macro model, which is also called diagonal strut method, fails to

accommodate the presence of openings, which are inevitably present in real building

structures. The effect on the infill panel is significantly dependent on the size of mesh.

The infill wall tends to behave as shear panel when the mesh size is large and thus the

ratio of mesh size to the panel size should be greater than 0.08.

The method developed in this study is reasonably adequate but ignores some of the

common interaction phenomenon such as separation and slipping between the frame

and the infill.

5.3 Damping

Since the exact damping property of the infilled structure can vary and generally not

very easy to predict, the results were presented for three damping levels wherever

necessary. All three damped models showed that there is little chance of having

resonance conditions in the structures. This was evident from the response of a ten

storey model which had the fundamental frequency very close to the dominant

frequency of the Kobe earthquake. However the results were almost similar under

other earthquakes whose dominant frequencies were not as close to the fundamental

frequency of the ten storey model. The inclusion of damping in realistic evaluations of

structure performance is important.

5.4 Parametric study results

5.4.1 Effect of infill strength (Ei)

Since there is no appropriate guidance on the infill material, it is randomly selected

depending on the availability and cost of the material. Thus, the use of solid concrete

- 115 -

blocks, burnt clay bricks, stone and adobe blocks were common in the past.

Consequently, some of the buildings in Bhutan have suffered from cracks in the infill

walls during the moderate seismic action, while the others survived. Thus, the effect

of infill strength was studied under a credible earthquake of 0.2g which gave the

minimum strength requirement of the infill material.

The effect of Ei is significant on the fundamental period of vibration, roof

displacement and the inter-storey drift ratios. All these responses decrease as the Ei,

increases, indicating that the Young’s modulus of material, which is empirically

related to material strength, increases the stiffness of the model, as expected.

However, the increase in fundamental period and roof displacement is significant only

at the lower range (< 2500 MPa) than in the higher range of Ei values (> 5000 MPa).

Such variation in structural response cannot be captured in general engineering

practices and thus it is important to include it in the Standards.

The stress of the infill wall was found to increase with the increase in Young’s

modulus of elasticity; however there was an insignificant increase in stress after

crossing the Ei value of 7500 MPa. This is the upper limit of the Young’s modulus of

infill material which should be used for buildings in seismic regions. The lower limit

of Ei value under a Serviceable earthquake was found to be 2000 MPa for a ten storey

structure.

5.4.2 Effect of Opening

While the consideration of the fully infilled frame is not realistic for real structures,

ignoring the openings during modelling and analysis of the infilled frame construction

would not give true results. The Equivalent diagonal strut method is quite vague as

openings are assumed to be present on either the upper or lower side of the strut, when

in reality most of the openings are present at the mid level of the floor height, typical

of buildings in Bhutan. The infill wall enhances the lateral stiffness of the framed

structures, however, the presence of openings within the infill wall would reduce the

lateral stiffness. Since the opening is a common feature of the building, consideration

of the opening should be given and its effect on the seismic resistance of the model is

important.

- 116 -

The fundamental period increases as the opening size increases, as expected, due to

reduction in stiffness of the model. Such variation of periods cannot be considered

using the Code values. The fundamental period of the fully infilled model was

54.87% higher than that of the bare frame model. However, there is no linear

relationship between the opening size and the fundamental period, and thus

consideration of opening size is important.

The roof displacement, inter-storey drift ratios and the infill stresses increase with the

increase in opening size as the frame becomes more flexible. The lateral stiffness

decreases by an average value of 38.98% for every 20% increase in opening size and

correspondingly the inter-storey drift ratios and roof displacement decreases. The

maximum infill stress which was found at the corners of the openings was found to

increase by 23.57%. This indicates that the material strength of the infill should be

increased as the opening size increases, if damage of the infill is to be prevented under

a design earthquake.

The moments in frames increase as the opening size increases, while the shear force

decreases for both beams and columns. The increase in moment could be due to

increase in flexibility, while the decrease in shear force is due to a decrease in the

mass of the structure with the larger opening size. The column and the beam moments

were increased by an average of 36.591% and 33.88% for every 20% increase in

opening size, while the shear forces in the columns and beams were generally

reduced. Overall the opening size of the infill opening affects the important response

parameters of the structure and its consideration during modelling and analysis is

important.

5.4.3 Effect of infill thickness

Since the introduction of reinforced concrete buildings in Bhutan, infill walls of

different thicknesses have been used for internal and peripheral partitions, and this

construction technique has remained unchanged. The thickness for external and

internal partitions, including the mortar finish, is generally 250mm and 125mm

respectively. However, there are buildings which have an infill thickness of 125mm

for providing similar usage. Thus, it was felt necessary to study the effect of thickness

- 117 -

under earthquake loads as the present Code does not consider the influence of infill

thickness.

The effect of thickness on the fundamental period of vibration is insignificant. From

this study, the difference in fundamental period between the models which had infill

of 125mm and 250mm thick was 1.4%. The fundamental period only slightly

increases as the infill wall thickness increases, since the increase in thickness only

increases the mass of the structure rather than its stiffness. Both the roof displacement

and the inter-storey drift ratio increase with the increase in thickness and the

percentage of increase in roof displacement and inter-storey drift ratio were 4.69%

and 4.45% respectively. Thus, it is evident that there is no improvement in the lateral

stiffness of the infill wall by increasing its thickness, for the cases treated herein.

There is however a significant increase in structural member forces, such as moments

and shear forces, due to the increase of infill thickness. The moments in the column

and beam were increased by the average percentage of 9.416 % and 9.540%

respectively, while the shear force was increased by 21.06% and 18.37% in the

column and beam respectively. Since the density of infill material was kept constant,

the increase in thickness only increases the total mass of the structure, attracting large

seismic load, resulting to a large displacement and member forces.

Since the influence of infill thickness on the global responses, particularly the natural

periods, roof displacement and the inter-storey drift ratios, were not significant; the

stresses in the infill walls were not affected by varying the thickness. The maximum

principal stress in the infill walls was found to be 4.2 N/mm2 for all models.

5.4.4 Effect of concrete strength

Over the last few decades, there have been changes in the specification of concrete

material for building construction in Bhutan. Moreover, many buildings were

constructed using the old Codes which had inferior material specification than the

modern codes. Thus, there is a need to study the effect of concrete strength as the

results will be useful in the assessment of old buildings under dynamic loads. The

- 118 -

range of the concrete strength (Ec), that was considered to study the variation of

structural responses was 15 to 40 MPa.

The global structural responses such as fundamental period, roof displacement, inter-

storey drift ratio and the infill stresses, all decrease with the increase in Ec value, as

expected. This is due to the increase in stiffness of the model as Ec increases. It was

found that the fundamental period increases by 7.8% for every 5000 MPa increase of

Ec, indicating that old buildings which have used low strength concrete could have a

longer period of vibration. This must be considered to avoid possible resonance with

seismic motions with similar dominant periods. Such variation of the period is not

considered in the Empirical formulae available in the Codes.

The effect of Ec on the roof displacement is significant only for low values of Ec. For

example, roof displacements were 36.6 mm, 30.6 mm and 30.8 mm for models with

cE of 20 GPa, 30 GPa and 40 GPa respectively. However, the effect of Ec on the

inter-storey drift ratio is not significant. The average decrease in inter-storey drift ratio

for every 5 GPa increase of Ec was just 4.77%. However, there is not much variation

in the infill stress with the concrete strength. For instance, the maximum infill stress

was 1.68 N/mm2 for the model which had an Ec of 15 GPa, while the maximum stress

in the other model which had an Ec of 40 GPa was 1.33 N/mm2.

In summary, the concrete strength is significant only in terms of its effect on the

fundamental periods. However, it does not have significant effect on the roof

displacement, inter-storey drift ratios or the infill stress, provided resonance is

averted.

5.4.5 Seismic resistance capacity of infilled RC frame

The seismic resistance of the structures described in Chapter three was studied by

varying the peak ground acceleration of the earthquake and the performance of the

structure was measured in terms of the inter-storey drift ratio and the onset of

cracking in the infill panels. The infill was assumed to crack once the stress in the

infill exceeded the ultimate compressive stress of the infill material. The Young’s

modulus of elasticity and the thickness of the infill walls were assumed as 5 GPa and

250 mm respectively (as specific material properties of infill are not available).

- 119 -

The results showed that the inter-storey drift ratio of most of the models with three

storeys to ten storeys, did not exceed the inter-storey drift ratio limit given in IS

1893(2002), even when the PGA was increased to 0.4g. An exception was the ten

storey model, which exceeded the drift ratio limit after 0.3g PGA. This shows that the

structures constructed without seismic provisions can meet the drift requirements of

the current code if the appropriate infill walls are considered. Thus, the presence of

infill walls significantly reduces the inter-storey ratios of the models under seismic

load. However, the current results could overestimate the actual capacity of real

buildings as the Young’s modulus of elasticity of the infill was considered to be 5

GPA. It also shows that the infill helps to reduce the inter-storey drift ratios,

consequently reducing the structural member forces, which indicates that the infilled

buildings have an additional strength to survive earthquake forces even if they are not

designed to resist them.

The stresses of the infill wall increases with the increase in PGA. However, all

models performed well up to 0.4g PGA of ground acceleration, except the ten storey

model whose maximum infill stress exceeded the maximum compressive stress of the

material (6.66 N/mm2). It is evident that the structure requires an Ei value of 7500

MPa if infill is to remain un-cracked at 0.4g PGA. This Ei value does vary with the

height of the structure and thus low rise structure will require lower infill strength for

the same intensity of earthquake loads.

Similar results were obtained for the models designed to conform to the seismic

requirements of the IS1893 (2002). Both the inter-storey drift ratios and the infill

stresses increased with the increase in PGA. This indicates that there is not much

influence on the storey drift and the infill stress from the structural member sizes. It

means that there is a significant stiffness contribution from the infill to overall

structural behaviour.

The above results show that the buildings which were constructed before and after

introduction of seismic Codes performed similar if the infill walls are considered.

However, the strength of the infill material Ei should be greater 5000 MPa. If the Ei,

- 120 -

values are low, structures will not be able to resist higher ground acceleration as the

lateral stiffness will be low.

5.4.6 Soft-storey phenomenon induced by arcade provision

Chapter four showed that building structures having an Arcade provision were found

to have a soft-storey problem. Such a problem cannot be determined by performing a

static analysis on the bare frame model, which is a common procedure for analysis

and design of new buildings in general engineering practices. Thus, empirical

magnification factors are used to accommodate the intended increase in the member

forces due to the soft-storey. However, these empirical values are very conservative in

nature which leads to bigger member sizes with significant cost implications.

Moreover, there is no research done for typical buildings of Bhutan which have

Arcades. The results from this research will provide useful information to the seismic

guidance of the country.

The results showed that the frame structures having an arcade are likely to suffer from

the soft-storey problem during strong ground motion. Both the moments and shear

forces of columns and beams were increased under dynamic loads. The factor by

which they increase depends on the amount of infill present in the upper storeys and

the height of the building. The low rise model (three storeys) showed a small increase

in the member forces, while the medium rise model (ten-storey) showed a significant

increase in the magnification factor. However, the magnification factors obtained

from this research are relatively lower than the values given in IS 1893 (2002). The

difference between the current results and the code values is 42% and 73.6% for

column and beam moments respectively. Such difference may have resulted due to the

difference in the amount of infill in upper storeys as the amount of infill is one of the

important parameter which significantly influences structural responses (Kose, 2008).

The current code (IS1893, 2002) prescribes one value for the moment magnification

factor for both columns and beams, and similarly for the shear forces in them. Thus,

providing the same magnification factor would be unrealistic and conservative.

However, it was found that the magnification factor is different for columns and

- 121 -

beams. Therefore, Code values over-estimate the actual behaviour, leading to cost

escalation.

5.5 Design guidance & recommendation

5.5.1 Fundamental period

Most of the existing seismic codes consider the presence of infill in terms of a

fundamental period using empirical formulae. Such formulae are general and implicit

in nature and do not mention about the extent of infill usage. Moreover, the presence

of openings in the infill walls and the use of different infill materials are not presented

in the current Code (IS1893, 2002). This research found that the Code formulae hold

for bare frames however, they significantly vary with infilled frames. This study

showed that the code Equation to determine the period of the structure holds for the

models with 40-50% openings within the infill and similarly reported by Amanad and

Ekramul (2006). Thus, the fundamental periods are related to the periods of the bare

frame model by using certain coefficients of the significant parameters.

The opening size significantly affects fundamental period, as shown in Figure 5.1(a).

The fundamental period also varies with the Young’s modulus of infill material, the

thickness of the infill wall, the strength of the concrete and the geometry of the

structural members. The variations are shown in Figure 5.1(b)-(d). The effect of the

thickness and the infill material could be expressed in the formulae. The fundamental

period of a bare frame model obtained from a modal analysis should be used in

Equation 5.1 shown below. The effect of the Young’s modulus of elasticity of

concrete and the geometric properties of structural members are not required in the

expression separately as they are the main component of the modal analysis. Thus the

fundamental period of the infilled model with varying properties of infill wall can be

related to the periods of bare frame models as given by Equation 5.1.

Ti=PγEiβ

tαTb 5.1

Where;

- 122 -

iT = Fundamental period of the infilled frame, seconds

bT = Fundamental period of bare frame model, seconds

γ =Coefficient of period variation with opening (-0.066)

β =Coefficient of period variation with infill strength (-0.012)

α =Coefficient of period variation with thickness (-0.076)

P = Opening size of the infill wall expressed as a percentage

Ei = Young’s modulus of infill wall in MPa

t = Thickness of the infill panel in mm

The coefficients γ, β and α are the average coefficients obtained from each parameter.

Figure 5.1. Variation of fundamental period

c

y = 0.6561x0.0237

0.500

0.550

0.600

0.650

0.700

0.750

0.800

0 100 200 300 400

Infill thickness (mm)

Fundam

enta

l period (

s)

a

y = 0.0031x + 0.5662

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0 25 50 75 100

Opening percentages

Fundam

enta

l period (

s)

b

y = 0.9612x-0.1667

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

Young's modulus of inf ill x 103 MPa

Fundam

enta

l period (

s)

d

y = 1.5834x-0.2389

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50

Young's modulus of concrete x103 MPa

Fundam

enta

l period (

s)

- 123 -

5.5.2 Selection of infill material

The structural responses vary with the variation of different parameters involved in

the usage of infill walls. The stresses in the walls were found to increase with the

increase in Young’s modulus of elasticity of infill material. However, it did not

increase significantly after exceeding a certain limit of Ei value under the credible

earthquake of 0.2g PGA as shown in Figure 5.2(a). For a ten storey model, an Ei of

7500 MPa (equivalent to compressive strength of 10 N/mm2) is the upper limit,

however the infill wall will not be fully stressed at this point. The minimum infill

strength (Ei) required for structural models varying from three to ten storeys in height,

under a credible earthquake of 0.2g, is shown in Figure 5.2(b). This graph shows the

requirement of different strengths of infill material as the number of storeys increase.

The graph can be easily used by the design engineers and builders as the stresses are

related to the Young’s modulus of elasticity of material by an empirical equation.

The use of the same infill material for different building categories, irrespective of the

structure’s importance, does not conform to the performance based design concept of

modern seismic engineering. This is due to the fact that the infill wall is not given any

importance, unlike frame members, and the failure of infill wall before the frame

members, would disrupt normal function of the buildings to a certain extent. Such

problems associated with the current code could be solved by referring to Figure 4.2

(c) & (d).

The requirement of infill material also depends on the opening size of the infill wall.

Although, modern seismic Codes recommend providing reinforced concrete bands all

around the opening, however such prescriptive regulations are hard to follow at

construction sites and thus majority of the constructions are without bonding beams.

Thus, as found out in this study, premature cracks in the infill, usually at the corners

of the opening, can be arrested by providing the higher strength material, using the

graph 5.2(e). However, this study did not consider the out of plane failure of the infill

walls.

Thus, the minimum compressive strength of the infill varies from 0.85 N/mm2 to 4

N/mm2

for structures less than ten storeys in height. For example, the infill strength of

- 124 -

0.85 N/mm2 can be used for a three storey structure while the infill strength of 4

N/mm2 is required for a ten storey structure, under the same earthquake intensity.

Such provision of infill could also be assessment of damages after the earthquakes.

Figure 5.2 Variation of stresses with different parameters

PGA=0.2g

(b)

0.00

1.10

2.20

3.30

4.40

0 2 4 6 8 10 12

Number of storeys

Infil

l str

ess (

N/m

m2)

0

1.1

2.2

3.3

4.4

0 25 50 75 100

Infill s

tress

in (N

/mm

2)

percentage of opening size

PGA=0.2g(e)

PGA=0.2g

(a)

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12 14 16

Young's modulus of infill x103 MPA

Infil

l str

ess in

(N

/mm

2)

(c)

0

0.1

0.2

0.3

0.4

0.5

0.00 2.00 4.00 6.00 8.00 10.00

Infill stress (N/mm2)

PG

A in

(m

/s2)

3 storey model

5 storey model7 storey model

10 storey model

(d)

0

0.1

0.2

0.3

0.4

0.5

0.00 2.00 4.00 6.00 8.00 10.00

Infill stress (N/mm2)

PG

A in

(m

/s2)

3 storey model

5 storey model

7 storey model

10 storey model

- 125 -


The study shows that the infilled frame models do not suffer from the inter-storey

drift ratio problems. The IS seismic code (2002) limits the inter-storey drift to 0.004

for a serviceable earthquake. However, the drift ratio doesn’t exceed the limit even if

the amount of opening is increased to 60%. In all the parameters, the drift ratio was

not exceeded unless the PGA was increased beyond the 0.4g. The requirement of the

drift limit results in large structural member sizes. However, the buildings that have

infill adequate infill strength will not suffer from drift problems under the serviceable

earthquakes.

5.5.4 Arcade solution

Since there are many buildings which have an Arcade, there was a need to conduct a

study on the behaviour of such typical buildings, under the influence of dynamic

earthquake loads. From this study, it was found that the model which was assumed to

have 40% opening within the infill wall, suffers from the soft-storey problem if IS

1893 (2002) is followed. However, the magnification factors for structural member

forces were found to be conservative for the buildings in Bhutan. This is due to the

difference in consideration of infill present in upper storeys, in which the

magnification factor varies with the amount of infill present in the storeys above.

Since the buildings in Bhutan have larger windows due to architectural requirements,

a small magnification factor could suffice.

The magnification factor increases with increase in the amount of the infill in upper

storeys as well as the height of the building. The low rise model (three storeys)

showed small increase in member forces while the medium rise model (ten-storey)

showed significant increase in magnification factor. However, the magnification

factors obtained from this research are relatively less than the values given the IS

1893 (2002).

Since the buildings do have openings, this research recommends the magnification

factor of structural member forces to be as shown in Table 5.1. These values were

obtained from the Time history analysis on a ten storey model which had 40%

opening size in the infill wall. However, the magnification factor for low rise structure

- 126 -

could be smaller than these values, although it should be acceptable to use for

structure lower than ten storeys as it will be conservative and safe.

Table 5.1 Magnification factor

Model with 40% opening

Moments in columns 1.76

Moments in beams 1.44

Shear in columns 1.08

Shear in beams 1.11

5.6 Conclusion

The presence of infill walls within the structural system enhances the lateral stiffness

of the structure significantly. The structural responses under seismic action decrease

with increase in the Young’s modulus of elasticity of infill material due to increase of

lateral stiffness of structure with Ei. Thus, selection of infill material and knowledge

of its Young’s modulus were found to be important depending on the performance

objectives, opening sizes and the of seismic hazard exposure.

The fundamental period, an important dynamic characteristic, of the infilled frame

was established based on the behaviour of the typical model. The magnification factor

for structural member forces was found to be relatively less than the Code values.

This Chapter presented the discussion of the important results and their applications.

Some of the discussions were covered in Chapter four along with the presentation of

research results and were thus omitted intentionally in this Chapter. The next Chapter

presents the summary of the research results and recommendations.

- 127 -

Chapter 6. Thesis conclusion

6.1 Introduction

This Chapter presents the main findings of the research carried out on infilled

reinforced concrete structures under seismic loads. Time history analyses were

performed to study the influence of infill wall on the performance of structural models

designed to represent the buildings in Bhutan. The results were presented covering the

wide seismic hazard scenarios for a given structural property to assess the

performance level of infilled framed structures.

The main findings of this research are;

• The structural member forces do vary with the different parameters associated

with the infill wall. Such variations are not considered in the current codes and

thus the guidance for design of buildings having infill wall is partly

incomplete.

• The strength of infill (Ei) has a significant role in global performance of the

structure. The structural responses such as roof displacement, inter-storey drift

ratio and the stresses in the infill wall decrease with the increase in (Ei) values.

This happens due to increase in stiffness of the model. Thus, it is important to

choose the right material for infill and consider it in the analysis and design.

• The minimum compressive strength of infill material required to remain un-

cracked under the action of a credible earthquake (0.2g PGA) varies with the

height of the building. It means that medium rise buildings should be provided

with higher strength infill material than the low rise buildings for same

exposure to seismic hazards.

• The strength requirement of infill increases with the increase in PGA,

indicating that for buildings which are considered to be important; the

importance factor should be provided for infill panels as well.

- 128 -

• Under a particular level of PGA (0.2g), the increase in infill stress is not

significant beyond infill strength of 7500 MPa. This value could be considered

to be the maximum limit of the Young’s modulus of infill material if the infill

walls are used for retrofitting an old building.

• The soft-storey phenomenon exists in buildings which have Arcade

provisions. However, the magnification factors for structural member forces

are relatively lower than the Code values. This is due to the difference in the

amount of infill considered in the model.

• Under the influence of viscous damping, the structures can engage in a

resonance condition with the ground motion frequencies. The difference

between the structural responses of un-damped and damped models is

significantly large and damping must be considered.

• The opening size of the infill openings has a significant contribution towards

the fundamental period, inter-storey drift ratios, infill stresses and the

structural member forces. Generally they increase as the opening size

increases.

• The thickness of the infill wall does have an influence on the global response

of the model under seismic action, especially in terms of moments and shear

forces. Its effect on inter-storey drift, stress in the infill wall, roof displacement

and the fundamental periods is also insignificant.

• The performance of buildings constructed with and without seismic provisions

is almost similar if the minimum value of Ei is 5000 MPa. This is because the

structural capacity of elastic system mainly depends on the stiffness property

of the model, which is greatly contributed to by the infill walls.

- 129 -

• The buildings with infill have enough redundancy to resist earthquake forces,

if proper material is used for infill walls. This means that regular buildings

could perform well even for higher intensity earthquakes.

Applications;

• The fundamental period of the infilled model recommended in Chapter four

can be used in general engineering practices.

• The magnification factors established for structural member forces for

buildings having Arcade provisions can be used in design.

• As there is no specific control over the choice of infill material in seismic

regions, the results presented in the previous Chapters can be used for

selection of infill material for both general buildings as well as for important

buildings. However, it is advisable to carry out experimental testing, if

possible, to confirm these results.

Further recommendations;

• Since the damping property of infilled structures is not yet established, there is

a scope for further research on this topic.

• For this class of building structures, recently developed seismic isolation

columns look to be a very attractive solution.

• Similar research could be done experimentally.

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