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SEISMIC ANALYSIS OF RCC FRAMED BUILDING
ABSTRACT
Masonry infill are normally considered as non-structural elements and their stiffness
contributions are generally ignored in practice, such an approach can lead to an unsafe design.
Until now infill wall system has been studied by many authors like P.G.Asteris (2003); and
Depts. Das and C.V.R.Murty (2004); B.Srinivas and B.K.Raghu Prasad (2009); Mulgund G.V
(2011).
It is proposed to develop an infill wall system in edges of the building to reduces the seismic
effects on buildings. In the present studies RCC framed buildings are analyzed by ETABS
software. In modeling the masonry infill panels the Equivalent diagonal strut method is used for
of the braces of the buildings. This work includes four kinds of 15 story rectangular RCC framed
building with height to depth ratio (H/D) as 7.02 which are describe here.
The building with shear wall at the corners and dimensions as 35 m x 25 m; building with shear
wall at the corners and also center core of shear walls and dimensions as 35 m x 25 m; the
building with shear wall at the corners and dimensions as 25 m x 25 m; and building with shear
wall at the corners and also center core of shear walls and dimensions as 25 m x 25 m.
In each building after design infill walls are modeled in different cases. Infill walls are used on
the exterior beams as dead load (Case 1); infill walls are used on the exterior beams as strut for
all storey's (Case 2) and infill walls are used on the exterior beams as strut for all storey's except
the first story (Case 3).
CHAPTER- 1
INTRODUCTION
Masonry infill are normally considered as non-structural elements and their stiffness
contributions are generally ignored in practice, such an approach can lead to an unsafe design.
The masonry infill walls though constructed as secondary elements behaves as a constituent
part of the structural system and determine the overall behavior of the structure especially
when it is subjected to seismic loads. In this paper seismic analysis has been performed using
Equivalent Lateral Force Method for different reinforced concrete (RC) frame building models
that include bare frame, unfilled frame and open first storey frame. The results of bare frame,
in filled frame and open first storey frame are discussed and conclusions are made. In modeling
the masonry infill panels the Equivalent diagonal Strut method is used and the software ETABS
is used for the analysis of all the frame models. Mostly two common structural damages
observed caused by masonry infill walls in earthquakes i.e soft stories and short columns. In
office or residential building outer side central opening are used. In this case central opening
are provided in periphery wall with different percentage i.e. 15% and 25% and brick
compressive strength are used as per IS : 1905-1987 i.e. 5.0 and 12.5 N/mm2 and Brick
Masonry strength is 0.50 and 1.06N/mm2.
Reinforced concrete (RC) frame buildings with masonry infill walls have been widely
constructed for commercial, industrial and multi storey residential uses in seismic regions.
Masonry infill typically consists of bricks or concrete blocks constructed between beams and
columns of a reinforced concrete frame. The masonry infill panels are generally not considered
in the design process and treated as architectural (non-structural) components. Nevertheless,
the presence of masonry infill walls has a significant impact on the seismic response of a
reinforced concrete frame building, increasing structural strength and stiffness (relative to a
bare frame) [1]. Properly designed infill can increase the overall strength, lateral resistance and
energy dissipation of the structure. An infill wall reduces the lateral deflections and bending
moments in the frame, thereby decreasing the probability of collapse. Hence, accounting for
the infill in the analysis and design leads to slender frame members, reducing the overall cost of
the structural system. The total base shear experienced by a building during an earthquake is
dependent on its time period. The seismic force distribution is dependent on the stiffness and
mass of the building along the height. The structural contribution of infill wall results into stiffer
structure thereby reducing the storey drifts (lateral displacement at floor level). This improved
performance makes the structural design more realistic to consider infill walls as a structural
element in the earthquake resistant design of structures.
1.2 Organization of the Dissertation
The dissertation is divided into six chapters as follows.
First chapter is introduction of the work.
Second chapter entitled Review of Literature describes in detail the various works
conducted by the researchers to understand the behavior of masonry infill and concrete shear
wall frames and their effect on strength requirements, for different type of buildings by the
seismic analysis and summary of literatures need for the present investigation, and describes
the objectives and scope of the present study are organized in the project. This chapter also
describes the importance of the study.
Third chapter includes different seismic analysis procedures such as linear and non
linear both static and dynamic analysis. It also gives introduction to hinges and to their
properties. It includes detailed procedure for pushover analysis and graphical representation of
pushover curves. Advantages of inelastic procedure over elastic procedure and brief details of
earthquake.
Fourth chapter provide complete details of different models which has used in this
dissertation and modeled in etabs software with their elevation and 3D views. Manually
calculations of natural time periods, base shears and distribution of lateral seismic shear forces
of the different models.
Fifth chapter is the discussion of results by considering the different parameters of the
building model.
The chapter sixth gives summary, conclusions and further scope of the study, and at last references.
1.3 OBJECTIVES OF THE STUDY
The current study is concerned with the application of recent techniques of analyses proposed by the international seismic codes and guidelines to determine the elastic and inelastic responses of typical multi-storey buildings made of reinforced concrete due to earthquake ground motions. It aims to encourage the inclusion of performance-based concepts in local seismic codes of design and evaluation. Basically, the objectives of this thesis are:
To assess the implementation of recent linear and nonlinear analyses, within the current practice of seismic design codes, for estimating seismic demands of multi-storey buildings.
To evaluate the seismic demands estimated using the pushover technique by comparison with the dynamic results determined by nonlinear time-history analysis using two natural records of ground motions: Elcentro and Bhuj.
PARAMETER OF STUDY
To Study the behavior of building on influence of masonry infill on the overall behavior of structure when subjected to lateral seismic forces.
To verify the effect of vertical irregularity on the fundamental natural period of the building and its effect on performance of the structure during earthquake for different building models selected.
Finding out the deflections and storey drifts at each storey using linear and non-linear analyses.
To study the ductility factor for different building models with and without infill and the performance level of the structures during earthquake.
CHAPTER-2
LITERATURE REVIEW
2.1 GENERAL
It has always been a human aspiration to create taller and taller structures.
Development of metro cities in India there is increasing demand in High Rise Building. The
reinforced cement concrete moment resisting frames in filled with unreinforced brick masonry
walls are very common in India and in other developing countries. Masonry is a commonly used
construction material in the world for reason that includes accessibility, functionality, and cost.
The primary function of masonry is either to protect inside of the structure from the
environment or to divide inside spaces. Normally considered as architectural elements.
Engineer’s often neglect their presence. Because of complexity of the problem, their interaction
with the bounding frame is often neglected in the analysis of building structures. When
masonry in fills are considered to interact with their surrounding frames, the lateral load
capacity of the structure largely increases. This assumption may lead to an important
inaccuracy in predicting the response of the structure. This occurs especially when subjected to
lateral loading. Role of infill’s in altering the behavior of moment resisting frames and their
participation in the transfer of loads has been established by decades of research. A review of
the developments in the seismic analysis, design, and experimental investigations are
presented along with the codal provisions of various countries.
2.2 STRUCTURAL ANALYSIS
Multi-storied-framed building construction commenced in the late 19th century even
though the analytical methods were not developed until the early 20th century. The earlier
structures were analyzed using approximate methods such as cantilever method developed by
A. C. Wilson in 1908 and portal method by Albert Smith in 1915. Iterative methods, namely
Hardy Cross method of 1932 and Kani’s method of 1947 were developed, and widely adopted
for the analysis of building frames in preference to the direct solutions of the equations formed
by the slope-displacement method. The behavior of framed buildings under seismic loads was
investigated extensively because of the wide spread damage in various countries. Sano in 1916
introduced the term ‘seismic coefficient’, which is widely adopted in determining the lateral
forces on multi-storied structures. The behavior of the industrial bank building in Japan, which
withstood the 1923 Great Kanto earthquake without significant damage, was invaluable in
comprehending the term seismic coefficient. In the years that followed, several building codes
were modified to include the clause that “the structure shall be designed to withstand a
horizontal seismic coefficient exceeding 0.1”. This may be the first specification for earthquake
resistance and seismic load considerations.
Various research works and experiments have been carried out since a long time all over the
globe to understand or to evaluate the effect of seismic forces on existing RC building in high
seismic zones and in hilly terrain. The concepts of modeling and analysis techniques used for
this purpose are also getting improved with advancement of engineering and technology from
past experience,
2.3 REVIEW
M C Griffith and A V Pinto [1] have investigated the specific details of a 4-story, 3-bay
reinforced concrete frame test structure with unreinforced brick masonry (URM) infill walls
with attention to their weaknesses with regards to seismic loading. The concrete frame was
shown to be a “weak-column strong-beam frame” which is likely to exhibit poor post yield
hysteretic behavior. The building was expected to have maximum lateral deformation
capacities corresponding to about 2% lateral drift. The unreinforced masonry infill walls were
likely to begin cracking at much smaller lateral drifts, of the order of 0.3%, and completely lost
their load carrying ability by drifts of between 1% and 2%.
Shunsuke Otani [2] studied the development of earthquake resistant design of RCC Buildings
(Past and Future). The measurement of ground acceleration started in 1930’s, and the response
calculation was made possible in 1940’s. Design response spectra were formulated in the late
1950’s to 1960’s. Non-linear response was introduced in seismic design in 1960’s and the
capacity design concept was introduced in 1970’s for collapse safety. The damage statistics of
RCC buildings in 1995 Kobe disaster demonstrated the improvement of building performance
with the development of design methodology. Buildings designed and constructed using
outdated methodology should be upgraded. Performance basis engineering should be
emphasized, especially for the protection of building functions following frequent earthquakes.
Ciro Faella, Enzo Martinelli, Emidio Nigro [3] proposed an assessment procedure in terms of
displacement capacity and demand. The sample application of the proposed procedure to a
typical building emphasized how easy and quick can be its application. As a brief parametrical
investigation, the influence of subsoil stiffness on the seismic vulnerability of the building was
analyzed pointing out that vulnerability was much larger as subsoil was less stiff. A rational
design procedure for choosing the retrofitting system was proposed with the aim of
determining the key mechanical characteristics of a bracing system working in parallel with the
existing structure for complying the safety requirement provided by Euro code 8 – Part 3
entirely devoted to existing structures. In the proposed design procedure, according to a
displacement-based approach, the strengthening substructure was designed in terms of lateral
stiffness, because Page12 displacement demand is strictly controlled by the displacement
capacity of the existing structure. For this reason, usual force-based design procedures suitable
for new structures, in which displacement capacity is only imposed by the new structure itself,
are not directly applicable for bracing system utilized for retrofitting existing structures.
Oğuz, Sermin [4] ascertained the effects and the accuracy of invariant lateral load patterns
utilized in pushover analysis to predict the behavior imposed on the structure due to randomly
selected individual ground motions causing elastic deformation by studying various levels of
nonlinear response. For this purpose, pushover analyses using various invariant lateral load
patterns and Modal Pushover Analysis were performed on reinforced concrete and steel
moment resisting frames covering a broad range of fundamental periods. The accuracy of
approximate procedures utilized to estimate target displacement was also studied on frame
structures. Pushover analyses were performed by both DRAIN-2DX and SAP2000. The primary
observations from the study showed that the accuracy of the pushover results depended
strongly on the load path, the characteristics of the ground motion and the properties of the
structure.
Durgesh C. Rai [5] gave the guidelines for seismic evaluation and strengthening of buildings.
This document was developed as part of project entitled ―Review of Building Codes and
Preparation of Commentary and Handbooks, awarded to Indian Institute of Technology Kanpur
by the Gujarat State Disaster Management Authority (GSDMA), Gandhinagar through World
Bank finances. This document was particularly concerned with the seismic evaluation and
strengthening of existing buildings and it was intended to be used as a guide.
G E Thermou and A S Elnashai [6] made a global assessment of the effect of repair methods on
ductility, strength and stiffness, the three most important seismic response parameters, to
assist researchers and practitioners in decision-making to satisfy their respective intervention
aims. Also the term ‘rehabilitation’ was used as a comprehensive term to include all types of
retrofitting, repair and strengthening that leads to reduced earthquake vulnerability. The term
‘repair’ was defined as reinstatement of the original characteristics of a damaged section or
element and was confined to dealing with the as-built system. The term ‘strengthening’ was
defined as intervention that lead to enhancement of one or more seismic response parameters
(ductility, strength, stiffness, etc.), depending on the desired performance.
A.Kadid and A. Boumrkik [7] proposed use of Pushover Analysis as a viable method to assess
damage vulnerability of a building designed according to Algerian code. Pushover analysis was a
series of incremental static analysis carried out to develop a capacity curve for the building.
Based on the capacity curve, a target displacement which was an estimate of the displacement
that the design earthquake would produce on the building was determined. The extent of
damage experienced by the structure at this target displacement is considered representative
of the damage experienced by the building when subjected to design level ground shaking.
Since the behavior of reinforced concrete structures might be highly inelastic under seismic
loads, the global inelastic performance of RC structures would be dominated by plastic yielding
effects and consequently the accuracy of the pushover analysis would be influenced by the
ability of the analytical models to capture these effects.
R.K. Goel [8] evaluated the nonlinear static procedures specified in the FEMA-356, ASCE/SEI 41-
06, ATC-40, and FEMA-440 documents for seismic analysis and evaluation of building structures
using strong-motion records of RC buildings. The maximum roof displacement predicted from
the nonlinear static procedure was compared with the value derived directly from recorded
motions for this purpose. It was shown that: (i) the nonlinear static procedures either
overestimates or underestimates the peak roof displacement for several of the buildings
considered in the investigation; (ii) the ASCE/SEI 41-06 Coefficient Method (CM), which was
based on recent improvements to the FEMA-356 Coefficient Method suggested in the FEMA-
440 document, does not necessarily provide better estimate of the roof displacement; and (iii)
the improved FEMA-440 Capacity Spectrum Method (CSM) provided better estimates of the
roof displacement compared to the ATC-40 CSM.
Saptadip Sarkar [9] studied the Design of Earthquake resistant multi stories RCC building on a
sloping ground that involves the analysis of simple 2-D frames of different floor heights and
varying number of bays using a software tool named STAAD Pro. Using the analysis results
various graphs were drawn between the maximum compressive stress, maximum bending
moment, maximum shear force, maximum tensile force and maximum axial force being
developed for the frames on plane ground and sloping ground. The graphs were used to draw
comparisons between the two cases and the detailed study of Short Column Effect failure. In
Page14 addition to that, the feasibility of the software tool to be used was also checked and the
detailed study of seismology was undertaken.
Siamak Sattar and Abbie B. Liel [10] quantified the effect of the presence and configuration of
masonry infill walls on seismic collapse risk. Infill panels are modeled by two nonlinear strut
elements, which have compressive strength only. Nonlinear models of the frame-wall system
were subjected to incremental dynamic analysis in order to assess seismic performance. There
was an increase observed in initial strength, stiffness, and energy dissipation of the in filled
frame, when compared to the bare frame, even after the wall’s brittle failure modes. Dynamic
analysis results indicated that fully-in filled frame had the lowest collapse risk and the bare
frames were found to be the most vulnerable to earthquake-induced collapse. The better
collapse performance of fully-unfilled frames was associated with the larger strength and
energy dissipation of the system, associated with the added walls.
Benyamin Monavari, Ali Massumi & Alireza Kazem [12] used nonlinear static analysis and five
locals and overall yields and failure criteria to estimate seismic demands of buildings. The
failure is directed towards losing structure’s performance during the earthquake or subsequent
effects. Because of the consequent excitations of an earthquake or lateral imposed loads on a
structure, the stiffness of some elements of structure reduced and the structure started to fail
and lose its performance; although failure happened either in small parts of structure or at the
whole. In this study thirteen reinforced concrete (RC) frame buildings with 2, 3, 4, 5, 6, 7, 8, 9,
10, 11, 12, 16 and 20 stories, having 3 and 4 bays were designed using seismic force levels
obtained from the Iranian Seismic Code 2005 and proportioned using the ACI318-99 Building
Code and then were modeled by IDARC. Pushover analysis with increasing triangular loading
was used.
Haroon Rasheed Tamboli & Umesh N. Karadi [13] performed seismic analysis using Equivalent
Lateral Force Method for different reinforced concrete (RC) frame building models that
included bare frame, in filled frame and open first story frame. In modeling of the masonry infill
panels the Equivalent diagonal Strut method was used and the software ETABS was used for the
analysis of all the frame models. In filled frames should be preferred in seismic regions than the
open first story frame, because the story drift of first story of open first story frame is very large
than the upper stories, which might probably cause the collapse of structure. The infill Page15
wall increases the strength and stiffness of the structure. The seismic analysis of RC (Bare
frame) structure lead to under estimation of base shear. Therefore other response quantities
such as time period, natural frequency, and story drift were not significant. The
underestimation of base shear might lead to the collapse of structure during earthquake
shaking
2.4 SUMMARY
RC frames with unreinforced masonry infill walls are common in developing countries with
regions of high seismicity. Often, engineers do not consider masonry infill walls in the design
process because the final distribution of these elements may be unknown to them, or because
masonry walls are regarded as non-structural elements. Separation between masonry walls and
frames is often not provided and, as a consequence, walls and frames interact during strong
ground motion. This leads to structural response deviating radically from what is expected in
the design.
In this the Behavior of masonry in filled concrete frames under the lateral load is studied, RCC
buildings are generally analyzed and designed as bare frame. But after the provision of infill
walls, mass of the building increases and this will result in the increase of the stiffness of the
structure. During the seismic activities, response of the structure with infill walls is quite
different for the structure without infill walls. Infill walls changes the dynamic behavior of the
structure.
2.5 NEED FOR THE PRESENT INVESTIGATION
A review of literature reveals that there is no standard procedure to model multistoried-framed
buildings, particularly in the context of Indian construction practices. Even the current Indian
code of practice IS 1893 (Part 1): 2002 does not specifically refer to stilt type buildings. Since
stilt type framed structures are widely adopted in India, there is a need to study the seismic
behavior of such structures.
In such a situation, an investigation has been performed to study the behavior of such
buildings with masonry infill walls subjected to earthquake loads. So, that the stability of the
structure could be achieved at the time of earthquake.
Alternative measures need to be adopted for this specific situation. The under-lying
principle of any solution to this problem is in (a) increasing the stiffness of the first storey such
that the first storey is at least 50% as stiff as the second storey, i.e., soft first storey's are to be
avoided, and (b) providing adequate lateral strength in the first storey. The possible schemes to
achieve the above are (i) provision of stiffer columns in the first storey, and (ii) provision of a
concrete service core in the building. The former is effective only in reducing the lateral drift
demand on the first storey columns and the latter is effective in reducing the drift as well as the
strength demands on the first storey columns.
CHAPTER -3
SEISMIC ANALYSIS PROCEDURES
3.1 INTRODUCTION
Being an engineer we must choose the best seismic analysis method (based on the
complexity of the structure) in order to obtain the best estimate of the seismic displacement
demands.. The analysis procedures can be divided into linear procedures (linear static & linear
dynamic) and non linear procedures (nonlinear static and nonlinear dynamic)
3.2 LINEAR STATIC ANALYSIS
When loads are applied to a body the body deforms and the effects of loads are transmitted
throughout the body The external forces induce internal forces and reactions to render the
body into a state of equilibrium. In linear static procedures the building is modeled as an
equivalent single-degree of freedom (SDOF) system with a linear static stiffness and an
equivalent viscous damping. The seismic input is modeled by an equivalent lateral force with
the objective to produce the same stresses and strains as the earthquake it represents. Based
on an estimate of the first fundamental frequency of the building using empirical relationships
or Rayleigh’s method, the spectral acceleration Sa is determined from the appropriate response
spectrum, which, multiplied, by mass of the building M, results in the equivalent
lateral force V:
The coefficient Ci takes into account issues like order effects, stiffness degradation, but also
force reduction due to anticipated inelastic behavior. The lateral force is then distributed over
the height of the building and the corresponding internal forces and displacements are
determined using linear elastic analysis.
These linear static procedures are used primarily for design purposes and are incorporated in
most codes. Their expenditure is rather small. However, their applicability is restricted to
regular buildings for which the first mode of vibration is prominent.
3.4 NONLINEAR STATIC ANALYSIS
3.4.1 Introduction
Pushover Analysis is a nonlinear static method of analysis. This analysis technique, also
known as sequential yield analysis or simply “Pushover” analysis has gained significant
popularity during past few years. It is one of the three analysis techniques recommended by
FEMA 273/274 and a main component of Capacity Spectrum Analysis method (ATC-40). The
following are the definitions which are most commonly used in Pushover Analysis.
3.5 NONLINEAR DYNAMIC ANALYSIS
In nonlinear dynamic procedure the building model is similar to the one used in non-
linear static procedures incorporating directly the inelastic material response using in general
finite elements. The main difference is that seismic input is modelled using a time history
analysis, which involves time-step-by-time-step evaluation of the building response.
This is the most sophisticated analysis procedure for predicting forces and
displacements under seismic input. However, the calculated response can be very sensitive to
the characteristics of the individual ground motion used as seismic input; therefore several
time-history analyses are required using different ground motion records. This most basic
inelastic method at this time is considered overly complex and impractical for general use.
3.6 ADVANTAGES OF INELASTIC PROCEDURE OVER ELASTIC PROCEDURES.
Although an elastic analysis gives a good understanding of the elastic capacity of
structures and indicates where first yielding will occur, it cannot predict failure mechanisms and
account for redistribution of forces during progressive yielding. Inelastic analyses procedures
help demonstrate how buildings really work by identifying modes of failure and the potential
for progressive collapse. The use of inelastic procedures for design and evaluation is an
attempt to help engineers better understand how structures will behave when subjected to
major earthquakes, where it is assumed that the elastic capacity of the structure will be
exceeded. This resolves some of the uncertainties associated with code and elastic procedures
3.7 SAFETY EVALUATION OF REINFORCED CONCRETE BUILDINGS
3.7.1 Introduction
Safety against collapse of reinforced concrete is usually defined in terms of its ductility
ratios. The design of reinforced concrete structures is performed by using resistance smaller
than the one required for the system to remain elastic under intense ground shaking. Then, the
seismic codes implicitly cause structural damages during strong earthquake motions and the
design relies on the capacity of the structures to undergo large inelastic deformations and to
dissipate energy without collapse. This design methodology is used by all design standards
including IS 1893.
3.7.2 Ductility Ratio
Ductility ratio is the capacity of a member to sustain inelastic deformations without
failure. There are three ductility ratios that are used to evaluate capacity of reinforced
concrete structures. They are as below
1. Displacement ductility ratio
2. Curvature ductility ratio
3. Rotational ductility ratio
3.7.2.1 Displacement Ductility (Global Ductility) Ratio
Displacement ductility ratio of building is defined as the ratio between maximum top
displacements of building to the yield top displacement of that building.
d=
dmaxd y …………………….. (1)
in which dmax and dy are maximum displacements and yield displacement respectively.
The above ductility ratio is called global ductility demand ratio of building. Similarly ductility
supply ratio of building is defined as ratio between ultimate top displacements of building to
the yield top displacement
s=
dud y …………………….. (2)
where du is the top displacement at collapse of the building.
3.7.3 Response Reduction Factor (R)
The response reduction factors takes into account the ductility of the structural
system and over strength so that the structure can be designed to the level of yield force of the
structure and rely on non –linear response of the structure in the case of severe earth quake. It
is, therefore, obvious that structure having low over strength or low ductility should be
designed for higher seismic coefficients. This means designed for higher design seismic
coefficients than that for the buildings.
Extensive research has been devoted in the past couple of decades towards the
developments of inelastic design spectra based on displacement rather than force criteria.
Such research was primarily motivated by the mount of performance-based seismic design,
which launched the development of new analysis and design tools based on displacement
rather than forces.
This family of methods aims at the calculation of the reduction factor R commensurate
with the achievement of a target ultimate displacement du. The target displacement often
referred to in an indirect manner through non-dimensional parameters, which are either the
ductility or displacement coefficient.
In this context, inelastic design response spectra can be expressed in many formats, the
most widely recognized of which are:
The conventional constant ductility plot, which depicts the reduced spectral
acceleration Sa/R values as a function of period for different ductility levels.
The capacity spectrum format, in which the reduced spectral acceleration values are
drawn as a function of peak displacement (elastic or ultimate) for different ductility
levels.
The yield point spectra format, which is plot of the strength coefficient
C=mSa/R/W=Sa/g/R as a function of the yield displacement dy.
The earliest R- -T relationship was developed by Newmark and Hall (1982) based on
observations conducted on the seismic response of a number of undamped SDOF systems,
which lead the following relations.
For long-period systems, the obtained ultimate displacement was not much greater
than the elastic spectral displacement therefore: R=
For short-period systems, the reduction the force led the system to exhibit large
ductility demands. Therefore, it was considered safer to keep the response in the elastic
range, i.e. R=1
For intermediate-period system, an equal energy rule proved to be useful. That is, the
energy absorbed by the inelastic system should be equal to that absorbed by elastic
one: R=√2μ−1
3.7.4 SEISMIC VULNERABILITY
The vulnerability of a building subjected to an earthquake is dependent on seismic
deficiency of that building relative to a required performance objective. The seismic deficiency
is defined as a condition that will prevent a building from meeting the required performance
objective. Thus, a building evaluated to provide full occupancy immediately after an event may
have significantly more deficiencies than the same building evaluated to prevent collapse.
Depending on the vulnerability assessment, a building can be condemned and
demolished, rehabilitated to increase its capacity, or modified so that the seismic demand on
the building can be reduced. Thus, structural rehabilitation of a building can be accomplished in
a variety of ways, each with specific merits and limitations related to improving seismic
deficiencies.
3.7.5 HOW DO BUILDINGS RESIST EARTHQUAKE FORCES
As a building responds to ground motions produced by an earthquake, the bottom of
the structure moves immediately, but the upper portions do not because of their mass and
inertia. Figure-3.4 shows the base of a building moving while the upper part lags behind.
The horizontal force, or base shear, created by ground motion resulting from an
earthquake must be resisted by the building. The more the ground moves, or the greater the
weight of the building, the more force must be resisted by the building. When an architect or
engineer designs a building, he or she must determine the maximum force a building might
have to resist in the future. Buildings are always designed to handle normal vertical and lateral
forces. However, once you introduce the possibility of an earthquake, a building must be
designed for extraordinary horizontal or lateral forces. The horizontal (lateral) forces associated
with an earthquake can be thought of as a lateral force applied to each floor and to the roof of
a building. Figure 3.5 shows the vertical and horizontal forces on a building during an
earthquake. Panel (a) shows the direction of gravitational forces on a building, panel (b) shows
the horizontal force of seismic waves, and panel (c) shows the combined forces of gravity and
an earthquake applied to the floors and roof of a building.
Fig -3.4 behaviour of building in ground acceleration
Horizontal forces accumulate along the floors and roof and then are distributed through
the vertical supports into the foundation. A structural engineer must design a building so that
lateral forces are distributed throughout the building without a break. Several structural
systems, such as floors, walls, and columns, may be used in new buildings to reduce the effects
of earthquakes and associated natural disasters.
Fig –3.5 forces acting on the building during ground excitation
3.7.6 STIFFNESS:
A building is made up of both rigid and flexible elements. For example, beams and
columns may be more flexible than stiff concrete walls or panels. Less rigid building elements
have a greater capacity to absorb several cycles of ground motion before failure, in contrast to
stiff elements, which may fail abruptly and shatter suddenly during an earthquake. Earthquake
forces automatically focus on the stiffer, rigid elements of a building. For this reason, buildings
must be constructed of parts that have the same level of flexibility, so that one element does
not bend too much and transfer the energy of the earthquake to less ductile When the
earthquake struck, the longer, more flexible columns at the front of the building passed the
earthquake forces on to the short, stiffer columns in the back instead of distributing the forces
equally among all of the columns. Deflection the extent to which a structural element moves or
bends under pressure, played a major role. The longer columns simply deflected or bent
without cracking. The short columns, therefore, were overwhelmed and cracked.
Fig3.6 showing long and short columns
3.7.7 EFFECT OF INFILL
The presence of the infill walls increases the lateral stiffness considerably. Due to the
change in stiffness and mass of the structural system, the dynamic characteristics change as
well. infill walls have an important effect on the resistance and stiffness of buildings. However,
the effect of the infill walls on the building response under seismic loading is very complex and
math intensive.
Exterior masonry walls and/or interior partitions built as an infill between a reinforced
concrete frame’s beams and columns are usually considered to be non-structural elements in
design. The interaction between the frame and infill is often ignored. However, the actual
behavior of such structures observed during past earthquakes shows that their response is
often wrongly predicted during the design stage .Infill-frames have been used in many parts of
the world over a long time. In these structures, exterior masonry walls and/or interior
partitions, usually regarded as non-structural architectural elements, are built as an infill
between the frame members. However, the usual practice in the structural design of infill-
frames is to ignore the structural interaction between the frame and infill. This implies that the
infill has no influence on the structural behavior of the building except for its mass. This would
be appropriate if the frame and infill panel were separated by providing a sufficient gap
between them. However, gaps are not usually specified and the actual behavior of infill frames
observed during past earthquakes shows that their response is sometimes wrongly predicted.
Infill-frames have often demonstrated good earthquake-resistant behavior, at least for
serviceability level earthquakes in which the masonry infill can provide enhanced stiffness and
strength. It is expected that this structural system will continue to be used in many countries
because the masonry infill panels are often cost-effective and suitable for temperature and
sound insulation purposes. Hence, further investigation of the actual behavior of these frames
is warranted, with a goal towards developing a displacement-based approach to their design.
Behavior of masonry in filled concrete frames under the lateral load is studied.
Investigations showed that, one of the most appropriate ways of analyzing the masonry in filled
concrete frames is to use the diagonally braced frame analogy. RCC buildings are generally
analyzed and designed as bare frame. But after the provision of infill walls, mass of the building
increases and this will result in the increase of the stiffness of the structure. During the seismic
activities, response of the structure with infill walls is quite different for the structure without
infill walls. Infill walls changes the dynamic behavior of the structure
3.7.8 SOFT STOREY
Recent trend of urbanization of cities of the developing countries, especially in South
Asia region, is witnessing construction of multi-storeyed buildings with open ground floor
reserved for car parking or other utility services. Though multi-storeyed buildings with open
(soft) ground floor are inherently vulnerable to collapse due to earthquake load, their
construction is still widespread in the developing nations. Social and functional need to provide
car parking space at ground level far out-weighs the warning against such buildings from
engineering community. These buildings are generally designed as RC framed structures
without regards to the structural action of the masonry infill (MI) walls present in the upper
floors. However, in reality, masonry infill (MI) walls in the upper floors make those floors much
stiffer against lateral load (e.g. earthquake) compared to ground floor rendering these buildings
into soft story buildings. Experience of different nations with the poor and devastating
performance of such buildings during earthquakes always seriously discouraged construction of
such a building with a soft ground floor. Typical examples of soft story (ground floor) failures
are shown in Fig-4.7. However, construction of such a building with isolated MI wall requires
high construction skill and may not be appropriate for the developing nations.
Some national codes like the Indian seismic code [9] requires members of the soft story (story
stiffness less than 70% of that in the story above or less than 80% of the average lateral stiffness
of the (three stories above) to be designed for 2.5 times the seismic story shears and moments,
obtained without considering the effects of MI in any story.
Fig-3.7: Soft story failure
Diaphragms:
The floor and roof systems that distribute an earthquake’s lateral forces are referred to
as diaphragms. Diaphragms support the gravitational and lateral forces on a building and
transfer them to vertical structural elements like shear walls, braced frames, and moment-
resistant frames. These vertical elements help resist lateral forces and are therefore called
horizontal (or lateral) bracing systems
3.7.10 STOREY DRIFTS:
Drift is the extent to which a building bends or sways. Limits are often imposed on drift
so a building is not designed to be so flexible that the resulting drift or swaying during an
earthquake causes excessive damage. Figure shows how a building can be affected by drift in an
earthquake. If the level of drift is too high, a building may pound into the one next to it. Or the
building may be structurally safe but non-structural components, such as ceilings and walls,
could be damaged as the building bends and the ceilings and walls are ripped away from their
attachments. Of course, people in the building could be killed or injured from falling debris.
Fig3.8 show the effect of high story drift
3.7.11 EFFECT OF SHEAR WALL
Reinforced concrete walls are strength and portent elements frequently used in
constructions in seismic areas because they have a high lateral stiffness and resistance to
external horizontal loads, these shear walls may be added solely to resist horizontal forces or
concrete walls enclosing stairways elevated shafts and utility cores may serve as shear walls.
shear walls not only have a very large enplane stiffness and therefore resist lateral load and
control deflection very efficiently but they also helps in reductions of structural & non-
structural damage. The building incorporated with shear wall sufficiently ductile will be much
away from seismic vulnerability and building failure in the earthquake sensitive zones thus
resulting in increased life safety & low property loss.
3.7.10 BEHAVIOUR OF SHEARWALL:
Shear wall constructed in the high rise buildings, generally behave as vertical cantilever
beam with their Strength controlled by flexure as shown in fig(1) rather than by shear such
walls are subjected to bending moments and Shears originating from lateral loads ,and to axial
compression caused by gravity these may therefore be designed in same manner as regular
flexural element .when acting as a vertical cantilever beam the behavior of a shear wall which is
properly reinforced for shear, will be governed by the yielding of the tension reinforcement
located at the vertical edge of the wall and, to some degree, by the vertical reinforcement
distributed along the central portion of wall. It is thus evident that the shear is critical for the
Wall with relatively low height-to-length ratio, and tall shear walls are controlled mainly by
flexural Requirements. Since the ductility of flexural member such as tall shear wall can be
significantly affected by the maximum usable strain in compression zone of concrete.
Confinement of concrete that the ends of Shear wall section would improve the performance of
such shear wall. Tall shear walls in multi-storey buildings the shear walls are slender enough
and are idealized as cantilever fixed at base Their seismic response is dominated by flexure.
Because of load reversals, shear walls sections necessarily contains substantial quantity of
compression reinforcement. The fig below shows the diagonal tension cracks in tall shear wall
and the formation of plastic hinges in the axial compression :
Fig-3.9 Behaviour of shear wall under flexure & formation of plastic hinges
Shear walls are the main vertical structural elements with a dual role of resisting both
the gravity and lateral loads. Wall thickness varies from 150 mm to 500 mm, depending on the
number of stories, building age, and thermal insulation requirements. In general, these walls
are continuous throughout the building height a shear wall may be tall shear wall or low shear
wall also known as squat walls characterized by relatively small height-to-length ratio.
Chapter - 4
ANALYTICAL MODELLING AND NUMERICAL STUDIES