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CHAPTER 1
INTRODUCTION
1.1 GENERAL
Severity of ground shaking at a given location during an earthquake
can be minor, moderate and strong. Relatively speaking, minor shaking
occurs frequently; moderate shaking occasionally and strong shaking rarely.
The engineers do not attempt to make an earthquake proof building
that will not get damaged even during the rare but strong earthquake; such
buildings will be too robust and also expensive. Instead, the engineering
intention is to make buildings earthquake resistant; such buildings resist the
effects of ground shaking, although they may get damaged severely but would
not collapse during the strong earthquake. Thus, safety of people and contents
is assured in earthquake-resistant buildings, and thereby a disaster is avoided.
This is a major objective of seismic design codes throughout the world.
1.2 EARTHQUAKE DESIGN PHILOSOPHY
The earthquake design philosophy may be summarized as follows.
a) Under minor but frequent shaking, the main members of the
building that carry vertical and horizontal forces should not be
damaged; however building parts that do not carry load may
sustain repairable damage.
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b) Under moderate but occasional shaking, the main members
may sustain repairable damage, while the other parts of the
building may be damaged such that they may even have to be
replaced after the earthquake.
c) Under strong but rare shaking, the main members may sustain
severe (even irreparable) damage, but the building should not
collapse. Thus, after minor shaking, the building will be fully
operational within a short time and the repair costs will be
low. And, after moderate shaking, the building will be
operational once the repair and strengthening of the damaged
main members is completed. But, after a strong earthquake,
the building may become not functional for further use, but
will stand so that people can be evacuated and their property
recovered.
The consequences of damage have to be kept in view in the design
philosophy. For example, important buildings, like hospitals and fire stations,
play a crucial role in post-earthquake activities and must remain functional
immediately after the earthquake. These structures must sustain very little
damage and should be designed for a higher level of earthquake protection.
The collapse of dams during earthquakes can cause flooding in the
downstream reaches, which itself could lead to a secondary disaster.
Therefore, dams (and similarly, nuclear power plants) should be designed for
a still higher level of earthquake motion. The performance of the building
during earthquake is shown in Figure 1.1.
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Figure 1.1 Performance of the Building (Murthy 2002a)
1.3 EARTHQUAKE OCCURANCE IN COIMBATORE
Until 1967, most of the engineers and scientists in the country were
under the impression that peninsular India was free from seismic activity. But
the earthquakes at Koyna (1967) and Latur (1993) changed their perception.
Tamil Nadu is thought to be mostly free from seismic activity. But an
earthquake rocked Coimbatore as far back as 1900. A study of the earthquake
will help in understanding its nature and drawing lessons that would be of
help in the event of its recurrence.
An earthquake of moderate magnitude (6 on the Richter scale)
occurred near Coimbatore at 3.11a.m. (Indian Standard Time) on February 8,
1900. Its maximum intensity was VII on the Modified Mercalli Intensity
Scale. It caused the largest extent of damage at two locations - Coimbatore
and Coonoor - and its impact was felt in the areas that lie between Udipi in
the north and Thiruvananthapuram in the south and Kozhikode, Bangalore,
Chennai, Nagapattinam and Madurai in the east-west direction. The epicenter
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of the earthquake was located at 10° 45' North Latitude and at 76° 45' East
Longitude.
At Coimbatore (Intensity VII), several buildings were seriously
damaged. Roof tiles collapsed. The central prison building suffered the most.
A Roman Catholic Church near the railway station gave in. A boy who was
trapped in a collapsed mud house was rescued. At Coonoor (Intensity VII),
the railway refreshment room and the Commissioner's bungalow developed
cracks. The Nilgiri Railway (Intensity VII) suffered losses owing to the fall of
boulders on the track. The shock was felt at Kuppam (Intensity VI) and
Perundurai (Intensity VI). Records from Mysore (Intensity VI) said that the
resting cattle stood up. The roof tiles of most of the houses were damaged and
walls developed cracks.
A statistical analysis indicates that such seismological events might
recur once in 100 years, plus or minus about 30 years. Although such
statistical forecasts are probabilistic and somewhat gross, this gives a
reasonable idea about the seismogenic potential of the region. It is recorded
that seismic activity in this region has been on the rise for the past 15 years.
There were two earthquakes, each with a magnitude of around 5.0 on the
Richter scale, in Idukki and Coimbatore districts on Kerala-Tamil Nadu
border in December 2000 and January 2001 respectively. It is also recorded
that seismic activity began sometime in 1988 when the Idukki area was
shaken by an earthquake of a magnitude of 4.8 on the Richter scale.
Incidentally, the Idukki dam happens to be located in this seismically
vulnerable area, the dam area has a seismic network.
Modern seismological instruments record earthquakes of a very
low magnitude - up to -2.0 or so. As a result of the tremendous increase in
their detection potential, instruments record thousands of seismic events of
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small magnitudes, known as micro-earthquakes or ultra-micro-earthquakes
(of negative magnitudes of up to -3.0). Seismologically speaking, it is not
correct to describe them as earthquakes. These are minor geological
movements that occur routinely.
The Governments of Tamil Nadu and Kerala should use this
historical information as inputs to protect the people of their states from any
seismic event in future. Earthquakes do not kill people. It is the collapse of
man-made structures that kills.
Studies indicate that any seismic event in the Coimbatore region
could reach a maximum magnitude of 5.5 to 5.75 on the Richter scale. The
attempt here is to awaken the people and the local body administration so that
the people are trained and prepared to face any such eventuality. This is a
probabilistic assessment, done with a view to increasing the people's
preparedness.
Earthquakes are part of the dynamic movement of the earth. All the
advancements in Science and Technology cannot prevent an earthquake.
People should learn to live with this reality. Compared to other disasters, an
earthquake lasts for the least duration and the possible response time is very
low. Even if one gets only 10 seconds, one should come out of his or her
house when an earthquake occurs.
Recently on 11th April 2012, a mild earthquake shook Coimbatore
city during afternoon around 2.30 p.m. following earthquakes in Indonesia to
a magnitude of 8.9 on Richter scale. People living near Saravanmpatti of
Coimbatore district felt the tremors and shake of building and ran to the
safety zones to safeguard their lives. Such tremors and shakes were felt in
Chennai, some part of Tamilnadu and East coast of India on the same day.
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During earthquake many people were killed and badly injured
because of
1) Poorly constructed buildings either totally or partially
collapsed.
2) Walls collapsing within narrow streets, burying people
escaping into them.
3) Untied roofs and cantilevers falling onto the people.
4) Free standing high boundary walls, parapets and balconies
falling due to severe shaking.
5) The failure of modern reinforced structures with large open
spaces at ground to first floor level,
a. For example garage or shop spaces, collapsing and
burying occupants (soft storey collapses)
6) Inhabitants not knowing how to respond to the shaking and
collapse of walls around them.
1.4 PRESENT SCENARIO OF STRUCTURES IN
COIMBATORE
In Coimbatore many of the existing buildings were built with the
masonry infill as nonstructural element and the analysis as well as design is
carried out by using only the mass but neglecting the strength and stiffness
contribution of infill. Therefore, the entire lateral load is assumed to be
resisted by the frame only. One of the disadvantages of neglecting the effect
of infill is that, the building can have both horizontal as well as vertical
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irregularities due to uncertain position of infill and openings in them. Also,
the infill walls are sometimes rearranged to suit the changing functional needs
of the occupants. The changes are carried out without considering their
adverse effects on the overall structural behavior.
In Coimbatore, the structural designer’s during past years used to
ignore the stiffness and strength of infill in the design process and treated the
infill as non-structural elements. This is mainly due to lack of generally
accepted seismic design methodology in the present Indian Code that
incorporated structural effects of infill. Hence, there is a vital need to create
awareness to develop a design methodology for seismic design of masonry
infill Reinforced Concrete structure.
1.5 METHODS OF ANALYSIS
For seismic performance evaluation, a structural analysis of the
mathematical model of the structure is required to determine force and
displacement demands in various components of the structure. Several
analysis methods, both elastic and inelastic, are available to predict the
seismic performance of the structures.
1.5.1 Elastic Methods of Analysis
Sermin Oguz (2005), the force demand on each component of the
structure is obtained and compared with available capacities by performing an
elastic analysis. Elastic analysis methods include code static lateral force
procedure, code dynamic procedure and elastic procedure using demand-
capacity ratios. These methods are also known as force-based procedures
which assume that structures respond elastically to earthquakes.
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In code static lateral force procedure, a static analysis is performed
by subjecting the structure to lateral forces obtained by scaling down the
smoothened soil-dependent elastic response spectrum by a structural system
dependent on force reduction factor, "R". In this approach, it is assumed that
the actual strength of structure is higher than the design strength and the
structure is able to dissipate energy through yielding.
In code dynamic procedure, the force demands on various
components are determined by an elastic dynamic analysis. The dynamic
analysis may be either a response spectrum analysis or an elastic time history
analysis. Sufficient number of modes must be considered to have a mass
participation of at least 90% for response spectrum analysis. Any effect of
higher modes is automatically included in time history analysis.
In Demand and Capacity Ratio (DCR) procedure, the force actions
are compared to corresponding capacities as demand and capacity ratios.
Demands for DCR calculations must include gravity effects. While code
static lateral force and code dynamic procedures reduce the full earthquake
demand by an R-factor, the DCR approach takes the full earthquake demand
without reduction and adds it to the gravity demands. DCRs approaching 1.0
(or higher) may indicate potential deficiencies.
Although force-based procedures are well known to engineering
profession and easy to apply, they have certain drawbacks. Structural
components are evaluated for serviceability in the elastic range of strength
and deformation. Post-elastic behavior of structures could not be identified by
an elastic analysis. However, the post-elastic behavior should be considered,
as almost all structures are expected to deform in inelastic range during a
strong earthquake. The seismic force reduction factor "R" is utilized to
account for inelastic behavior indirectly by reducing elastic forces to inelastic.
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Force reduction factor, "R", is assigned considering only the type of lateral
system in most codes, but it has been shown that this factor is a function of
the period and ductility ratio of the structure as well.
Elastic methods can predict elastic capacity of the structure and
indicate where the first yielding will occur, however they don’t predict failure
mechanisms and account for the redistribution of forces that will take place as
the yielding progresses. Real deficiencies present in the structure could be
missed. Moreover, force-based methods primarily provide life safety but they
can’t provide damage limitation and easy repair.
The drawbacks of force-based procedures and the dependence of
damage on deformation may have led the researchers to develop
displacement-based procedures for seismic performance evaluation.
Displacement-based procedures are mainly based on inelastic deformations
rather than elastic forces and use nonlinear analysis procedures considering
seismic demands and available capacities explicitly.
1.5.2 Inelastic Methods of Analysis
Structures suffer significant inelastic deformation under a strong
earthquake and dynamic characteristics of the structure change with time. So
investigation of the performance of a structure requires inelastic analytical
procedures accounting for these features. Inelastic analytical procedures help
to understand the actual behavior of structures by identifying failure modes
and the potential for progressive collapse. Inelastic analysis procedures
basically include inelastic time history analysis and inelastic static analysis
which is also known as pushover analysis.
The inelastic time history analysis is the most accurate method to
predict the force and deformation demands at various components of the
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structure. However, the use of inelastic time history analysis is limited
because dynamic response is very sensitive to modeling and ground motion
characteristics. It requires proper modeling of cyclic load deformation
characteristics considering deterioration properties of all important
components. Also, it requires availability of a set of representative ground
motion records that accounts for uncertainties and differences in severity,
frequency and duration characteristics. Moreover, computation time, time
required for input preparation and interpreting voluminous output make the
use of inelastic time history analysis impractical for seismic performance
evaluation.
Inelastic static analysis or pushover analysis is the preferred
method for seismic performance evaluation due to its simplicity. It is a static
analysis that directly incorporates nonlinear material characteristics.
1.6 OBJECTIVES, SCOPE AND METHODOLOGY OF THE
STUDY
Objectives
This study aims to investigate the effect of brick masonry infill wall
on a reinforced concrete moment resisting frame conventionally designed as a
bare frame, using available macro model .The specific objectives of the study
are:
i. To study the effect of brick masonry infill wall on existing
reinforced concrete moment resisting frame, subjected to
earthquake induced by the lateral load.
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ii. To study the effect of an existing reinforced concrete moment
resisting bare frame, subjected to earthquake induced by the
lateral load.
iii. To study the performance of the structural beam element
retrofitted with Glass Fiber Reinforced Polymer (GFRP)
composites.
Scope
i. This study deals with the reinforced concrete moment
resisting frame with full unreinforced brick masonry infill
frame and bare frame subjected to earthquake forces.
ii. Infill and bare frame are analysed by the Nonlinear Analysis
(Pushover Analysis) method using SAP 2000 software
version11.
iii. The performance levels of various components of buildings,
behavior of the components and failure mechanism in
buildings, hinge formation, performance point, base shear
versus roof displacement and capacity curve will be studied in
detail.
iv. The weak Reinforced Cement Concrete (RCC) structural
elements are identified and retrofitted with Glass Fiber
Reinforced Polymer (GFRP) composites subjected to static
loading.
v. The performances of the existing buildings are improved by
local retrofitting technique based on SAP 2000 results.
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vi. To suggest a suitable retrofitting technique so that the strength
and performance level of deficient structure could be
improved during the earthquakes.
Methodology
i. To identify the typical deficiency existing in reinforced
concrete building and collection of relevant structural data.
ii. To evaluate the structural diagnosis using Nondestructive
technique facilities.
iii. Modeling of the structural system and Analysis of the model
for seismic vulnerability by using SAP 2000 version 11.
iv. Modeling and experimental studies on the retrofitted deficient
members.
v. The performances of the existing buildings that are improved
by local retrofitting technique based on SAP 2000 results.
vi. To suggest a suitable retrofitting techniques so that the
strength of structure could be enhanced and performance of
structure during an earthquake could be improved.
1.7 ORGANIZATION OF THE THESIS
This thesis consists of eight chapters. The chapters are arranged in
the following order so as to present the study carried out in a coherent manner
in this thesis.
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Chapter 1 deals with the general introduction about the earthquake
concepts, objectives, scope and methodology of the present investigation.
Also includes the impact of this study in the present scenario in India.
Chapter 2 reviews the research works carried out during the last
few decades on analytical and experimental studies on pushover analysis and
retrofitting of the reinforced concrete structures.
Chapter 3 discusses the details of the existing building under
study, modeling aspects were considered and the procedure for seismic
evaluation of buildings using pushover analysis by SAP 2000 version 11.
Chapter 4 presents the results and discussions based on the
analytical results and its evaluation on existing RCC building through
pushover curve, capacity curve and hinge formation.
Chapter 5 presents the seismic retrofitting method on existing
RCC bare frame building, based on experimental data pertaining to identified
GFRP composite beams.
Chapter 6 presents the analysis of bare frame with strengthened
beams by SAP 2000. The improved performance level of existing RCC
building is discussed in detail. It is compared with and without retrofitting of
the RCC structures.
Chapter 7 outlines of the General conclusions and specific
conclusions based on analytical and experimental work.
Chapter 8 gives suggestions based on the research work and
recommendations for future research are discussed.