jigme dorji thesis

8/11/2019 Jigme Dorji Thesis

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Ssc Pfanc f Bck Inld RC Fa

Stuctus n Lw and Mdu s Budns n

Bhutan

Jigme Dorji

A Thsis sbmittd for th dgr of

Mastr of Egirg

Ctr for Bilt Eviromt ad Egrig Rsarch

Qsad Uivrsity of Tcology

June 2009


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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, thepresent 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


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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.


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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


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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 Youngs 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


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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


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Chapter 6. Thesis Conclusion

6.1 Conclusion..... 128


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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

Earthquake and (c) Northridge Earthquake.

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


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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 vsEi

Figure 4.16 Maximum roof displacement vsEi..

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 withEc=15000 MPa.

Figure 4.25 Inter-storey drift ratios of models with varyingEc 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.


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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 4.39 Inter-storey drift ratios.

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.


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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.


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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 Youngs modulus of elasticity, MPa

Ec Youngs modulus of reinforced concrete, MPa

fE Youngs modulus of frame element, MPa

Ei The Youngs modulus of elasticity of infill material, MPa

mE Youngs 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


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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

Contact coefficient between infill wall and frame members

Natural frequencies, Hertz

l Length of contact between column and infill, mm

h Length of contact between beam and infill, mm

Strut angle with respect to horizontal axis, degree.

Proportionality constant between Youngs modulus and compressive strength Percentage of damping


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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


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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;


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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.


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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 eightstoreys. 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


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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.

1.2Research problem

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

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


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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 thestatic 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


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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 placesince 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


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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.

1.4Research 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


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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 Youngs modulus of elasticity of infill (Ei),

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

Youngs 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


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reviews the general response of reinforced concrete structures,

performance of the infill wall and inclusion of the infills 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.


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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.


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Chapter 2. Literature review

2.1Introduction

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 commonmethod 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.


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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


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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.


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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 providingenough 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.


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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 =

5

6

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.


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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.

2.5Analyses 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 soiland 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.


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2.5.1Static 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.

2.5.2Response 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 modesthat are very close unlike the CQC. The result from this analysis gives only the peak

structural responses at desired damping values.


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2.5.3Time 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- 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 tto 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


34/158 - 17 -

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

convergence of results.

2.5.4Viscous 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 befound by specifying damping ratios i and j for i

th and jth modes. This leads to

equations 2.9 and 2.10.

ji

ji

ia

+

=2

2.5

jijb +=

2 2.6

In which, i and j are the natural frequencies of ithand jthmodes. 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.


35/158 - 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 hasseems 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.


36/158 - 19 -

2.6.1Micro- 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 Youngs

modulus of elasticity of infill but high value of Poisons 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 Youngs

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

infilled frame. However, the influence of Poisons ratio was fond insignificant on thebehaviour 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.


37/158 - 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.

2.6.2Macro-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.


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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

=

tE

HIE

m

cf

l 2.7


39/158 - 22 -

42

2sin

4

=

tE

LIE

m

bf

h 2.8

Where;

l = 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.

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

mE = Youngs modulus of elasticity of infill masonry, MPa.

fE = Youngs modulus of frame element, MPa.

= strut angle with respect to horizontal axis, degree.

t= thickness of the infill, mm.

mmfE =

mf -compressive strength of masonry

The value of a constant 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 += 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


40/158 - 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.


41/158 - 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 underdynamic 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.


42/158 - 25 -

2.8Lateral 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 thestructural 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


43/158 - 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


44/158 - 27 -

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

reinforcement and bigger column size.

2.10Consideration 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-storeyproblem 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


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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

Youngs 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).

2.11Recent 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


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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 infills

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 infills 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 infills 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


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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


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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 infills 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 contrar

jigme dorji thesis

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