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IJSRD - International Journal for Scientific Research & Development| Vol. 2, Issue 09, 2014 | ISSN (online): 2321-0613
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Analysis of RC Framed Structure Using Base Isolation Techniques by Use
of Elastomeric Bearing Fazil ali K
Department of Civil Engineering
KMEA Engineering CollegeAbstract— Earthquake is most extreme condition of any
building may be required to survive during its lifetime.In
this paper we are evaluating the base isolation method using
Elastomeric bearing for preventing the super structure from
damaging components of earthquake. We are comparing the
seismic Response of normal structure and base isolated
structure from the results obtained in Etab software.
Key words: Base isolation method, Rubber bearings.Static
Analysis, Model analysis, Time History Analysis
I. INTRODUCTION
Earthquake is most extreme condition of any building may
be required to survive during its lifetime. An Earthquake
would Structurally damage the building and incase the
damage is above threshold level there would be no option
but to demolish and reconstruct .While building collapse is
the primary cause of loss of life in most earth-quakes, other
contributors to earthquake loss include equipment and
contents damage, business interruption, and damage to
lifelines, such as water, power, gas, communications, and
transportation. The field of Earthquake Engineering has
existed in our country for over 35 years now. Indian
earthquake engineers have made significant contributions to
the seismic safety of several important structures in the
country. However, as the recent earthquakes have shown,
the performance of normal structures during past Indian
earthquakes has been less satisfactory. This is mainly due to
the lack of awareness amongst most practicing engineers of
the special provisions that need to be followed in earthquake
resistant design and thereafter in construction. Earthquake
motion causes horizontal and vertical ground motions.
Vertical ground motion having a much smaller magnitude is
the most usual. In general, all structures are conventionally
designed to carry the gravity loads. The vertical ground
motion due to earthquakes can be resisted by the factor of
safety provided in the design of structures. The structures
which are designed to carry only the gravity loads will not
able to resist the horizontal ground motion. The horizontal
ground motion causes the most significant effect on the
structure by shaking the foundation. Hence, it is necessary to
check the adequacy of the structures to withstand the
horizontal ground motion.
A. Base isolation
A series of devastating during the last few decades has
created considerable interest in the minds of all concerned
about the subject of earthquake resistant design. Modern
seismic design relies upon the inelastic response of
structural members and systems to dissipate the energy
imparted to a structure by an earthquake. In recent years,
base isolation technique has become an increasingly applied
structural design technique for buildings and bridges in
highly seismic areas. The objective of seismic isolation
systems is to decouple the building structure from the
damaging components of the earthquake input motion that is
to prevent the superstructure of the building from absorbing
the earthquake energy. The entire superstructure must be
supported on discrete isolators whose dynamic
characteristics are chosen to uncouple the ground motion.
Displacement and yielding are concentrated at the level of
the of the isolation devices and the superstructure behaves
very much like a rigid body. The main feature of the base
isolation technology is that it introduces flexibility in the
structure. This helps in further reducing the seismic response
of the building. Another approach for controlling seismic
damage in building and improving their seismic
performance is by installing seismic dampers in place of
structural elements, such as diagonal braces. These dampers
act like the hydraulic shock absorbers in cars much of the
sudden jerks are absorbed in the hydraulic fluids and only a
little is transmitted above the chassis of the car.Seismic
energy is transmitted though them, dampers absorb part of
it, and thus damp the motion of the building. Thus the
dampers reduce the energy available for shaking the
building. This means that the building deforms less, so the
chance of damage is reduced. The concepts of base isolation
systemand its performance during strong ground motion and
also the various applications in base isolation system and
also used various types of dampers.
B. Concept of base isolation
One of the most widely implemented and accepted seismic
protection systems is base isolation systems. Seismic base
isolation is a technique that mitigates those effects
of an earthquake by essentially isolating the structure and its
contents from potentially dangerous ground motion,
especially in the frequency range where the building is most
affected. The objective is to simultaneously reduce inter-
storey drifts and floor accelerations to limit or avoid
damage, not only to the structure but also to its contents, in a
cost-effective manner. Seismic base isolation is emerging as
on alternative approach for earth-quake protection of
structures. The basic concept in this approach is to uncouple
a structure from the ground by interposing a flexible
element/bearing between the structure and foundation. Many
buildings have been constructed on some type of rubber
bearings, and such structures have shown superior
performance in earthquakes.
C. Aim of base isolation
The aim of base isolation is to minimize the energy that is
transferred from ground motion to the structure by bu
ering it with a bearing layer at the foundation which has
relatively low stiffness. The bearing level has a longer
period than the superstructure, which reduce the force and
displacement demands on the superstructure, allowing it to
remain elastic and generally undamaged. As the result of
flexibilization, the natural period of the past fixed-base
structures undergo a jump and the new base isolation
structure has a new natural period. The flexibility of the
Analysis of RC Framed Structure Using Base Isolation Techniques by Use of Elastomeric Bearing
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interposing layers between structure and its foundation lead
to a bigger fundamental period for structural ensemble.
D. Mechanism of base isolation
The isolation reduces the fundamental lateral frequency of
the structure from its fixed base frequency (or increases the
time period of the structure) and thus shifts the position of
structure in the spectrum from the peak-plateau region to the
lower regions. Also it brings forth additional damping due to
the increased damping introduced at the base level and thus
further reduction in the spectral acceleration is achieved.
………………(1.1)
………………(1.2)
…………(1.3) Source [8]
Tb, , are those of isolated system.
= base slab of mass -lateral stiffness.
Elements of Seismic Base Isolation System
Seismic base isolation consists of
Energy dissipation core (lead plug).
Vulcanized rubber layers.
Steel reinforcing plates.
Bottom mounting plate
Cover rubber.
Energy dissipation core reduces earthquake forces
and displacements by energy dissipation and also it provides
wind resistance. Rubber layers provide lateral flexibility to
the system. steel reinforcing plates provide vertical load
capacity and also it confines lead core. Bottom mounting
plate is integrated with the isolator and it is used to connect
the structure below and above isolator. Rubber is used to
protect the steel plate.
Fig. 1: Schematic diagram of Base Isolator
E. Response of buildings
As a result of an earthquake, ground beneath each building
begins to move, say first to left, building responds with
movement, which tends towards the right that is the
buildings undergoes displacement towards right. The
buildings displacement in the direction opposite to ground
motion is actually due to inertia. The inertial forces acting
on a buildings are the most important of all those generated
during an earthquake. But when the ground shakes, isolated
buildings do not move. No inertia force is transferred to the
buildings due to shaking of ground. The inertia force acting
on buildings have been reduced. Acceleration is decreased
because base isolation system lengthens a buildings period
of vibration. Structure with longer periods of vibration tends
to reduce acceleration, while those with shorter periods tend
to increase or amplify acceleration. The inertial forces which
the buildings undergoes are proportional to the buildings
acceleration during ground motion
Fig. 2: Comparison of fixed base with isolated base
F. Application of base isolation system
Base isolation systems are found useful for short period
structures, sayless than 0.7s including soil-structure
interaction. Base isolation provides an alternative to the
conventional,fixed-base design of structures and may be
cost-effective for some new buildings in locations where
very strong ground shaking. Base isolation is an alternative
for buildings that must remain functional after a major
earthquake(e.g.hospital,emergency, communications centre,
computer processing centre, etc.) Several new buildings
have been isolated using rubber or elastomeric bearings
.several commercial brands of base isolators are now
available in the market especially in foreign countries. Care
should be taken to identify the most suitable type of device
for a particular building. The first base-isolated building in
the United States is the foothill communities law and
justices centre at Los Angeles completed in 1985, the
building is four stories high with a full basement and sub-
basement for the isolation system, which consists of 98
isolators of multilayered natural rubber bearings reinforced
with steel plates.
Fig. 3: Diagram showing Rubber bearing in building
II. MODELLING AND ANALYSIS
A Institutional building with 10 storey was selected. The
number of bay is assumed as 3 in X direction with a spacing
of 5m. The number of bay is assumed as 3 in y direction
Analysis of RC Framed Structure Using Base Isolation Techniques by Use of Elastomeric Bearing
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with a spacing of 4m. The bottom storey is of 3.5 height and
all other typical floors are of 3m height. The plan of the
building is as shown in the below figure.
Fig. 4: Plan of the building
The institutional building constructed using base
isolation techniques at Calicut .At ground floor, slabs are
not provided and the floor will directly rest on ground. The
beams rest centrally on columns to avoid local eccentricity.
For all structural elements, M20 grade concrete will be used.
The building rest on fixed base and base isolation. Centre-
line dimensions are followed for analysis and design.
Preliminary sizes of structural components are assumed by
experience. Rubber bearings base isolation is used The
modeling was done using Etab Software. The two building
was modeled with same properties of the members and the
support condition was fixed. The first model is the building
with fixed base condition and the second model with base
isolated condition.
The properties of the model are as follows:
Support condition - Fixed Support
Location of the building – Calicut ,Seismic Zone
factor (Z) - 0.16
Type of soil – Medium
Type of Building – Institutional
Type of Frame – SMRF
Damping - 0.05
Cross section of Column - 300mmx500mm
Cross section of Beam - 300mmx600mm
Thickness of Slab - 150mm
Width of masonry Infill wall - 230mm
Height of ground Floor - 3500mm
Height of other Floor - 3000mm
Number of storey - ten (G+9)
Spacing in x direction - 5m
Spacing in y direction - 4m
A. Rubber bearing properties:
The seismic isolators are link component 0.5 m length
placed between the fixed base and the coloumns. The
parameters selected to define the seismicisolators in Etab
program are as follows
Link property:
Link type : Rubber
Linear effective stiffness:
U1 = 2500000 ( kN)m
U2 =1600( kN)m
U3 =1600( kN)m
Nonlinear stiffness :
U1 = 5000 (kN)m
U2 =160( kN)m
U3 =160( kN)m
Post yield stiffness Ratio:
U2 = 0.1
U3 = 0.1
source[25]
B. Modelling Details:
The three dimensional view of the two models are as shown
in the below figures
Fig. 5: building with fixed base condition
Fig. 6: building with base isolated condition
Analysis of RC Framed Structure Using Base Isolation Techniques by Use of Elastomeric Bearing
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C. Static Analysis:
The static analysis was done and the storey displacement
storey drift and storey shear were obtained and tabulated and
plotted graphs obtained from software are as shown below.
1) Storey Displacement:
Fig. 7: building with fixed base condition
Fig. 8: building with base isolated condition
Storey no Story displacement in x direction in mm
fixed base base isolated
10 12.4 13.78
9 11.8 13.6
8 11 13.4
7 9.9 13.1
6 8.7 12.21
5 7.3 11.8
4 5.8 10.6
3 4.3 10.3
2 2.7 9.1
1 1.2 8.21
Base 0 6.15
Table 5.1: Story displacement in X-direction for both Fixed
and base isolated
From the above table and graph we can see that the storey
displacement is decreased for the base isolated compared to
fixed base. In base isolated structures the story displacement
is decreasing for upper stories
2) Storey Drift:
Fig. 9: building with fixed base condition
Fig. 10: building with base isolated condition
Storey no Storey drift in x direction
Fixed base Base isolated
10 0.000191 0.000179
9 0.000277 0.000217
8 0.000359 0.000297
7 0.000424 0.000368
6 0.000470 0.00391
5 0.000504 0.000498
4 0.000529 0.000515
3 0.000562 0.000519
2 0.000696 0.000503
1 0.001769 0.000347
Analysis of RC Framed Structure Using Base Isolation Techniques by Use of Elastomeric Bearing
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Base 0 0
Table 5.2: Story drift in X-direction for both Fixed and base
isolated
From the above table and graph we can see that the storey
drift is decreased for the base isolated compared to fixed
base. In base isolated structures the base story cause a high
drift and on going to upper stories the storey drift is reduced.
3) Storey Shear:
Fig. 11: building with fixed base condition
Fig. 12: building with base isolated condition
Storey no Storey shear in x direction
Fixed base Base isolated
10 -69.192 -67.19
9 -130.949 -124.95
8 -179.96 -171.92
7 -217.65 -210.716
6 -245.335 -235.67
5 -265.28 -257.284
4 -278.34 -271.04
3 -285.42 -276.42
2 -288.87 -280.86
1 -289.78 -265.16
Base 0 0
Table 5.3: Story shear in X-direction for both Fixed and
base isolated
From the above table and graph we can see that the storey
shear is decreased for the base isolated compared to fixed
base. The storey shear for the bottom story is reduced in
base isolated compared to fixed base.
D. Model Analysis
The elastic properties and mass of building causes to
develop a vibratory motion when they are subjected to
dynamic action. The dynamic properties of the structure
such as natural period, Damping, and mode shape play a
crucial role in determining response of building. The
building with higher natural frequency and short natural
period will suffer higher accelerations but smaller
displacement. The period and mode shape of the building
for both the cases are compared below.
Fig. 13: Fixed base Mode 1
Period = 1.377
Fig. 14: Base isolated Mode 1
Period = 2.274
Analysis of RC Framed Structure Using Base Isolation Techniques by Use of Elastomeric Bearing
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Fig. 15: Fixed base Mode 4
Period = 0.456
Fig. 16: Base isolated Mode 4
Period = 0.597
Fig. 17: Fixed base Mode 10
Period = 0.188
Fig. 18: Base isolated Mode 10
Period = 0.205
From the mode shape obtained for the both models. we can
clearly say that the the period is increasing for base isolated
structures compared to fixed base. Here the mode 1 show a
65% increase in period for base isolated and that of mode 10
it shows 20 % increase in period
E. Dynamic Analysis -Time History Analysis
The time history analysis method technique represents the
most sophisticated method of dynamic analysis for
buildings. In this method the mathematical model of the
building is subjected to acceleration from earthquake
records that represents the expected earthquake at the base
of the structure. It consist of step by step direct integration
over a time interval. The Earthquake motions are applied
directly to the base of model of the structure. Instantaneous
stresses throughout the structure are calculated at small
intervals of time for the duration of earthquake. The time
history analysis were done and the time history analysis
results are as follows
1) Time-Acceleration plot
Fig 19: Fixed base
Analysis of RC Framed Structure Using Base Isolation Techniques by Use of Elastomeric Bearing
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Fig 20: Base isolated
2) Time-Velocity plot
Fig 21: Fixed base
Fig 22: Base isolated
3) Time-displacement plot
Fig 23: Fixed base
Fig 24: Base isolated
From the time History Analysis the above graphs indicate
that the Acceleration ,Velocity, Displacement are reduced
for a structure with base isolation compared to Fixed base
III. CONCLUSIONS
From the comparison of fixed base and base isolation
methods the following conclusions are made:
Base isolation is very promising technology to
protect different structures from seismic excitation
It is observed that when increasing the number of
stories maxium storey displacement is become
considerable
Story drift and storey shear are also reduced in base
isolated building making the super structure
flexible
From time history Analysis the Acceleration,
velocity, displacement are low for base isolated
structures. It make the structure rigid and stiffer
It can be concluded that the performance of base
isolated structure is efficient in seismic prone areas
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Analysis of RC Framed Structure Using Base Isolation Techniques by Use of Elastomeric Bearing
(IJSRD/Vol. 2/Issue 09/2014/185)
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