Jigme Dorji Thesis
Transcript of 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
4.6.1 Roof displacement ............................................................................... 67
4.6.2 Inter-storey drift ratios ......................................................................... 68
4.6.3 Infill stresses ........................................................................................ 68
4.7 Five storey model ............................................................................................. 69
4.7.1 Roof displacements .............................................................................. 69
4.7.2 Inter-storey drift ratios ......................................................................... 70
4.7.3 Infill stresses ........................................................................................ 71
4.8 Three storey model ......................................................................................... 71
4.8.1 Roof displacements .............................................................................. 72
4.8.2 Inter-storey drift ratios ......................................................................... 73
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
4.9.2 Roof displacements .............................................................................. 76
4.9.3 Infill stress ............................................................................................ 78
4.9.4 Inter-storey drift ratios ......................................................................... 80
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
4.12.1 Roof displacement ............................................................................... 91
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 4.4 Maximum principal stress in the infill.
Table 4.5 Maximum principal stress in the infill.
Table 4.6 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
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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.
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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.
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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.
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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
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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
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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.
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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.
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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
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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
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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