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SEISMIC RESPONSE EVALUTION OF BASE ISOLATED BUILDINGS

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 Pakistan Journal of Science (Vol. 65 No. 1 March, 2013)

SEISMIC RESPONSE EVALUTION OF BASE ISOLATED BUILDINGS

A. Hameed, M. Saleem, A. U. Qazi and H. Rizwana*

Department of Civil Engineering, University of Engineering and Technology, Lahore Pakistan

*
Graduate Program,Department of Civil Engineering, University of Engineering and Technology, Lahore Pakistan
Corresponding Author: asifhameed@uet.edu.pk

ABSTRACT: The main purpose of seismic isolation is to protect the structures against damage
from earthquake by reducing the forces transferred to the structure rather than resisting them. Non
linear dynamic analysis is carried out for buildings with and without base isolation. The non linearity is
confined at base isolator and superstructure is kept in linear elastic state. Lead rubber bearing is used as
base isolator for the study. The presented research work is divided in three phases. Comparison of
performance of fix base and base isolated building is done in phase-I. Phase-II explains the effect of
height and number of bays in base isolated buildings. In phase-III, the response of building is evaluated
by varying the properties of base isolator i.e. time period and effective damping. The results of phase-I
revealed that seismic isolation of a fix base building reduces the floor accelerations and base shear by
lengthening the natural period of vibration of the structure. Inter storey drifts are also reduced as
hysteretic damping is provided by lead core of lead rubber bearing. Relative displacements are
increased at each storey level due to lateral flexibility provided by lead rubber bearing. Phase-II
revealed that seismic isolation is more effective within the range of 8 to 10 storey for moment resisting
frame structures. Increase in number of bays is not proved to be effective as the floor accelerations,
displacements and storey drifts remains almost constant for base isolated buildings. From phase-III, it
was realized that by increasing the time period of base isolator, natural time period of vibration of
building is also increased. This increase in natural time period reduces the floor accelerations and base
shear. By increasing the time period of base isolator, the displacement at each storey level is also
increased. On the other hand by increasing the effective damping of base isolators floor acceleration,
base shear, displacements and inter storey drifts all are reduced.
Key words: Seismic Base Isolation Building, Non-linear Time History Analysis, Lead Rubber Bearings, Story Drift,
Time Period.

INTRODUCTION base isolation systems and some other are under

construction or under design process. (Farzad and Kelly,
Conventional constructions causes high floor 1999). Currently, building codes are also being applied to
accelerations and inter storey drifts due to the stiffness the newly constructed building in all seismic zones. It is
and rigidity of the buildings as compare to the laterally thought that building codes fulfill all the requirements for
flexible buildings. This endangers the safety of building the seismic design and need of seismic isolation system is
components and contents. Base isolation is to isolate the being ignored. However, it is now proved that base
building from ground by introducing a flexible layer in isolated building are much safe from earthquake damage
between building foundation and ground. It provides than a code based design building. (Farzad and Kelly,
lateral flexibility and damping to the building. Due to 1999). Now at this time a structural engineer is aware that
lateral flexibility in lateral direction the earthquake seismic isolation ensures better performance of building
movements transferred to the building reduces. This also structure during an earthquake event than design
reduces the earthquake generated effects like floor approach provided by building codes for fixed base
acceleration, base shear and inters storey drifts. In this buildings and thus proves a great technique in the seismic
way it prevents the building and its occupants from design of civil engineering structures (Jangid,2006). The
structural and non structural damage. The purpose of purpose of seismic isolation is to protect structures
seismic base isolation is to bring in flexibility at the base against damage from earthquakes by reducing the
of the structure. Damping provided by isolator dissipates earthquake forces rather than resisting them. In
energy in the form of heat and it also reduces the seismically base isolated structures, the superstructure is
amplitude of vibration of motion during an earthquake. decoupled from the ground motion by providing
(Farzad and Kelly, 1999). Base isolation has become a horizontally flexible layer between the superstructure and
widely used technique from past decades to protect the its foundation however, it is very stiff vertically. The
buildings and other structure from damaging effects of isolation system lengthened the fundamental time period
earthquake. Now various structures exist, mounting on of the structure up to a large value (Kelly, 1997). The

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 Pakistan Journal of Science (Vol. 65 No. 1 March, 2013)

damping provided by the isolation system dissipates the usually ranges from 5% to 20 % of the critical damping.
energy. This reduces the force due to earthquake that is This damping can be hysteretic or viscous in nature.
transferred to the superstructure such as inter story drift Hysteretic damping is provided using lead core or steel
and floor accelerations (Robinson, 2000). The main shims in base isolators. However, viscous damping can
objectives of the research work are to provide an be introduced by using high damping rubber isolator. By
overview of different types of base isolation systems and increasing natural period of vibration of structure, the
to evaluate the behavior, modeling and analysis of displacements are also increased while by increasing the
structures with base isolation system. Furthermore to damping the displacements are reduced. If the value of
investigate the response of symmetrical base isolated damping is greater than 20%, it may badly distort mode
buildings during earthquakes. The scope of research work shapes, and make the analysis more complicated. In
is to check the performance of regular base isolated addition, response of higher modes increases as the
building by varying its height and aspect ratio in plan. damping is increased. As a result of this substantial
Also to check the performance of base isolators by contribution of higher modes, seismic inertial forces are
varying its properties such that time period and effective also increased when compared with those produced by
damping. only the first mode (Mokha et al., 1988).
Philosophy behind seismic isolation system: Whenever
Structural Modeling and Analysis - Building
a structure is subjected to earthquake loading, the total
Description
input energy of the structure can be expressed as:
Kinetic energy (KE)+ Dissipated energy DE+
Strain energy (SE)= Input energy IE
Kinetic energy and strain energy are the
recoverable energies whereas the viscous energy and
hysteretic energy are dissipative energies. In a fixed base
structure, when input energy (earthquake loading) is so
small, the energy input to the system is dissipated in the
form of viscous energy. When earthquake loading is large
then some of the input energy dissipated in the form of
viscous energy and residual energy in the form of
hysteretic energy. If the structure has been designed to Figure 1. Plan and elevation of 10 storey building
have sufficient ductility, it may undergo plastic
deformations in certain joints, members or specially Reinforced concrete 3D intermediate moment
added components, but the phenomenon of collapse must resisting frame is used for modeling and analyses, having
be avoided. This is the ductility concept of design for the three bays in each x and y direction (Sap2000,1997). The
traditional fixed-base structures (Yang and Chang, 1999). storey height is taken 3.6m for all buildings in all three
Generally, structures having natural time period of phases. The bay width is taken as 6m in each x and y
vibration less than 0.1 to 1.0 second are more susceptible direction.
to damage during an earthquake. The resonance Elastomeric Bearing Properties
phenomena may occur as the earthquake acceleration has
dominant period of about 0.1 to 1.0 seconds. The
maximum risk is in the range of 0.2 to 0.6 seconds.
Seismic isolation introduces lateral flexibility that
lengthened the natural period of vibration of the structure.
When the time period of vibration of the structure is
increased than that of earthquake, the response of the
structure i.e. floor acceleration and storey drift are
reduced. The resonance phenomena can also be avoided Figure 2.Bilinear Model of Lead Rubber bearing
by base isolation (Robinson, 2000). By increasing the
time period of the structure and damping of the system, Bilinear modeling is used for all isolation
the floor accelerations are reduced. This shows that the systems. In fact bilinear hysteretic model can reflect the
structure above the isolation system act like a rigid body. non-linear characteristics of the lead core bearings.
On the other hand, the isolation system imparts the lateral Where Qd = Characteristic Strength, Fy = Yield
flexibility due to which the displacement of the base Force, Ku = Elastic Stiffness ,Kd = Post Yield
isolator is increased. These displacements ranges from Stiffness, Keff = Effective Stiffness , Dy = Yield
100mm to 400mm and can be controlled by imparting Displacement and Dmax= Maximum Design
damping to the isolation system. The damping value Displacement

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 Pakistan Journal of Science (Vol. 65 No. 1 March, 2013)

Step-By-Step Procedure To Calculate Mechanical 2 K eff

Ccr 
Properties Of Elastomeric Bearing  K eff 
2

Qd  
W xiii. Calculate critical damping , Where  m 
i. Select ratio which is taken as 0.12 for this
research work. ( 0.1-0.15) (Jangid, 2006)) m= mass of base isolator
ii. Select time period of base isolator Td=2.8Sec ( 2.5- xiv. Calculate effective damping , Effective
3sec) (Jangid, 2006 ) Damping  Ccr  βeff
iii. Select Damping ratio βeff =15% Effective damping ratio can also be calculated using
iv. Select post yield stiffness to elastic stiffness ratio   Qd  
 4Qd  Dd   
  9K u 
Kd  eff    
 0 .1 
 2Dd K u Dd  Qd  
Ku (Farzad and Kelly,1999)  
 
v. Calculate SM1 and SD1, For Site Class D, S1=0.6 Phase-I: Mechanical properties of lead rubber bearing
2 for phase-I are given in table 1
S M 1  FV  S1 S D1  SM1
, 3
vi. Calculate design displacement Dd , Table 1.Mechanical properties of elastomeric bearing
 g  S D1  Td  for Phase-I.
Dd   2 
 4  Bd 
2
W  2 
10 Story
K eff    
vii. Calculate effective stiffness g  Td  Where Properties
Base Isolated
W= weight of building Keff (KN/mm) 1.2027
viii. Calculate area of hysteresis loop Wd Ku (KN/mm) 2.9253
Wd  2  K e ff  Dd 2   eff Kv(KN/mm) 100000
W  Fy (KN) 308
D y  Dd   d 
Kd/Ku 0.1
ix. Calculate Dy,  4Qd 
Q  Effective damping KN-Sec/mm 0.01130
K d  K eff   d 
x. Calculate Kd,  Dd 
K u  10 K d Phase-II: Mechanical properties of lead rubber bearing
xi. Calculate Ku,
Qd
for phase-II are given in table 2 and 3
Fy 
  Kd 
1    
 K 
xii. Calculate yield force of base isolator,   u 

Table 2. Mechanical properties of elastomeric bearing for Phase-II.

5 8 10 12 15 18 20
Properties Storey Storey Storey Storey Storey Storey Storey
BI-1 BI-2 BI-3 BI-4 BI-5 BI-6 BI-7
Keff (KN/mm) 0.7833 1.2027 1.6082 1.7096 2.0595 2.3961 2.6082
Ku (KN/mm) 1.9051 2.9253 3.9117 4.0583 5.0095 5.8281 6.3410
Kv(KN/mm) 100000 100000 100000 100000 100000 100000 100000
Fy (KN) 200 308 411 437 527 613 667
Kd/Ku 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Effective damping 0.00910 0.01130 0.01309 0.01350 0.01483 0.01610 0.01670

Table 3. Mechanical properties of elastomeric bearing for Phase-II

10 Storey 10 Storey 10 Storey 10 Storey 10 Storey

Properties 3X3 Bay 3x5 Bay 3x7 Bay 3x8 Bay 3x9Bay
BI BI BI BI BI
Keff (KN/mm) 1.4946 1.6082 1.7246 1.9581 2.3173
Ku (KN/mm) 3.6355 3.9117 4.1991 4.7628 5.6364
Kv(KN/mm) 100000 100000 100000 100000 100000
Fy (KN) 382 411 442 591 593
Kd/Ku 0.1 0.1 0.1 0.1 0.1
Effective damping KN-Sec/mm 0.0126 0.01309 0.0136 0.0145 0.0157

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 Pakistan Journal of Science (Vol. 65 No. 1 March, 2013)

Phase-III: Mechanical properties of lead rubber bearing for phase-III are given in table 4 and 5

Table 4. Mechanical properties of elastomeric bearing for Phase-III (Isolator Period)

8 8 8 8 8
Properties Storey Storey Storey Storey Storey
BI-1 BI-2 BI-3 BI-4 BI-5
Isolator Time Period (Sec) 2.0 2.5 2.8 3.0 3.2
Kd/Ku 0.1 0.1 0.1 0.1 0.1
Ku (KN/mm) 10.8305 4.8927 2.9253 1.9819 1.2442
Keff(KN/mm) 2.3572 1.5086 1.2026 1.0476 0.9208
Kv (KN/mm) 100000 100000 100000 100000 100000
Fy (KN) 308 308 308 308 308
Effective damping
0.01523 0.01523 0.01523 0.01523 0.01523
(KN-Sec/mm)

Table 5. Mechanical properties of elastomeric bearing for Phase-III (Effective Damping)

8 Storey 8 Storey 8 Storey 8 Storey 8 Storey

Properties
BI-1 BI-2 BI-3 BI-4 BI-5
Effective damping
0.006984 0.009058 0.011000 0.012805 0.015230
KN-Sec/mm
Ku (KN/mm) 10.8305 10.8305 10.8305 10.8305 10.8305
Keff (KN/mm) 2.3572 2.3572 2.3572 2.3572 2.3572
Kv (KN/mm) 100000 100000 100000 100000 100000
Fy (KN) 308 308 308 308 308
Kd/Ku 0.1 0.1 0.1 0.1 0.1

Time History for Non Linear Dynamic Analyses: Non storey fix base and base isolated building. In case of fix
linear dynamic analysis has been carried out using the base building floor acceleration is much higher than that
classical El Centro earthquake recorded in Imperial of base isolated building. Maximum floor acceleration is
Valley, California, on 15 October 1940. The peak 4756 mm/sec2 in fix based building and 1590 mm/sec2 in
acceleration of the El Centro earthquake is 0.319 g, and it base isolated building.
has a total duration of 30 sec. This accelerogram is
applied since it is a typical input in most earthquake
engineering studies and also it correspond to a FLOOR ACCELERATION FOR10 STOREY
representative strong, broad-band motion.
12 Fix Base
10 Base Isolated
Storey Level

8
6
4
2
0
0 1000 2000 3000 4000 5000 6000

Floor Acceleration (mm/sec^2)

Figure 4. Floor acceleration at each storey level for fix

base and base isolated building
Figure 3 Elecento earthquake time history
Figure 5 shows the relative displacement at each
RESULTS AND DISCUSSIONS storey level for fixed base and base isolated building.
Base isolated experienced much more displacements than
Phase-I Performance of Base Isolated Building: Figure fixed base building.
4 shows floor acceleration at each storey level of 10

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 Pakistan Journal of Science (Vol. 65 No. 1 March, 2013)

maximum difference in floor acceleration is observed in

DISPLACEMENT FOR 10 STOREY case of ten storey building.
12
Base Isolated
10 Fix Base
TIME PERIOD
Storey Level

8
4.5 Fix Base
6
4

Time Period (Sec)

Base Isolated
4 3.5
3
2
2.5
0 2
0 50 100 150 200 250 300 350 400 1.5
1
Relative Displacement (mm) 0.5
0
Figure 5 Relative displacements at each storey level 0 5 10 15 20 25

for fix base and base isolated building No. of Storeys

Figure 6 shows storey drifts at each storey level Figure 7. Time period for fix base and base isolated
for base isolated building is less than that of fixed base building
building due to lateral flexibility provided by base
isolators whereas storey drift is displacement of a storey
FLOOR ACCELERATION
relative to the storey above or below. The structural and
4500
non-structural part of the buildings remains safe by Fix Base
4000

Floor Acceleration
reducing the storey drifts. 3500
Base Isolated

(mm/sec^2)
3000
2500
2000
STORY DRIFT FOR10 STOREY
1500
1000
12 Fix Base 500
10 Base Isolated 0
Storey Level

8 0 5 10 15 20 25

6 No. of Storeys
4
2 Figure 8. Floor acceleration for fix base and base
0 isolated building
0 5 10 15 20 25

Storey Drift (mm) Figure 9 represents the relative displacements at

top storey joint. It shows that as the height of the
Figure 6 Storey drift at each storey level for fix base buildings increases the relative displacements also
and base isolated building increases and it becomes constant for both fixed base and
base isolated building in case of twenty storey building.
Phase II - Effect of Height And Number of Bays of
Building RELATIVE DISPLACEMENT
Relative Displacement (mm)

Fix Base
(i) Effect of Height of Building: In this phase 700
Base Isolated
height of the building is increased by adding number of 600
500
storeys i.e.5, 8, 10, and 12,15,18,20. Figure 7 shows time
400
period of fixed base and base isolated building as the 300
height of the building increases. Due to base isolation 200
lateral flexibility increases which lengthened time period 100
of building. 0
0 5 10 15 20 25

Figure 8 shows that as the height of the No. of Storeys

buildings increases floor acceleration increases up to a

maximum value and with the further increase in height of Figure 9 Relative displacements for fix base and base
building it starts decreasing and becomes constant for isolated building
twenty storey fixed base and base isolated building. The In figure 10, base shear for both fixed base and
base isolated building is presented. As the height of the

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 Pakistan Journal of Science (Vol. 65 No. 1 March, 2013)

building increases base shear also increases and it is

TIME PERIOD
maximum up to 15 storey and then starts decreasing with
3 Fix Base
the further increase in height of the building. Due to base

Time Period (Sec)

2.5 Base Isolated
isolation, base shear reduces and the maximum difference
2
in base shear is in case of 15 storey buildings.
1.5

0.5
BASE SHEAR
10000 0
Fix Base 0 2 4 6 8 10
9000
Base Isolated
Base Shear (KN)

8000 No. of Bays in X-Direction

7000
6000
5000
Figure 12. Time period for fix base and base isolated
4000 building
3000
2000
1000 According to figure 13, the floor acceleration
0 increases slightly for both fix base and base isolated
0 5 10 15 20 25
buildings because by adding bays the mass or weight of
No. of Storeys the buildings increases which attracts earthquake forces
and increases floor acceleration but base isolation reduces
Figure 10. Base shears for fix base and base isolated this floor acceleration in an effective way.
building
FLOOR ACCELERATION
Figure 11 indicates storey drifts for 5, 8, 10,
12,15,18,20 storey fixed base and base isolated buildings. 5000 Fix Base
4500
Base isolation reduces the storey drifts for each of the Base Isolated
Floor Acceleration

4000
(mm/sec^2)

3500
building. 3000
2500
2000
BI 20 STOREY 1500
STOREY DRIFT FB 20 STOREY 1000
25 FB 18 STOREY 500
0
BI 18 STOREY
20 0 2 4 6 8 10
FB 15 STOREY
Storey Level

BI 15 STOREY No. of Bays in X-Direction

15
FB 12 STOREY
BI 12 STOREY
10
FB 10 STOREY
Figure 13. Floor acceleration for fix base and base
5
BI 10 STOREY isolated building
FB 8 STOREY
0 BI 8 STOREY
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 FB 5 STOREY
Relative displacements at top floor joint of fixed
BI 5 STOREY base and base isolated buildings are shown in figure 14 as
Storey Drift (mm)
the number of bays in x-direction increases the relative
Figure 11 Storey drift for fix base and base isolated displacements at the joints decreases due to the increase
building in stiffness of the building by adding bays in x-direction.

(ii) Effect of Number of Bays of Building: The

response of fixed base and base isolated building was RELATIVE DISPLACEMENT
Relative Displacement (mm)

evaluated by increasing number of bays in X-direction 400 Fix Base

i.e. 3x3, 5x3, 7x3, 8x3 and 9x3 bays for 10 storey 350 Base Isolated
building. ELCENTRO earthquake is also applied in x- 300
250
direction. Figure 12 shows that by adding bays in x- 200
direction the stiffness of building increases in x-direction. 150
As earthquake is also applied in x-direction so the time 100
50
period for both of fixed base and base isolated building 0
decreases considerably. 0 2 4 6 8 10
No. of Bays in X-Direction

Figure 14 Relative displacements for fix base and base

isolated building

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 Pakistan Journal of Science (Vol. 65 No. 1 March, 2013)

By increasing number of bays in x-direction, STOREY DRIFT

base shear increases for both fix base and base isolated BI 3X3
12
building. However, base isolation reduces the base shear BI 3X5
10 BI 3X7
as shown in figure 15.

Storey Level
BI 3X8
8
BI 3X9
6 FB 3X3
BASE SHEAR 4 FB3X5
FB 3X7
2
18000 FB 3X8
Fix Base FB 3X9
16000 0
Base Shear (KN)

Base Isolated 0 5 10 15 20 25 30
14000
12000
Storey Drift (mm)
10000
8000
6000 Figure 16 Storey drift for fix base and base isolated
4000
2000
building
0
0 2 4 6 8 10
Phase-III effect of Change in Properties of Base
No. of Bays in X-Direction
Isolator
Figure 15 Base shears for fix base and base isolated (i) Effect of Varying Time Period of Elastomeric
building Bearing: By varying the time period of elastomeric
bearing, lateral flexibility of the bearing also varies. This
Similar to phase-I, storey drift at each storey variation in flexibility changes the response of the base
level decreases in case of base isolated building as isolated building i.e. time period, floor acceleration, base
compared to fixed base building. Due to the lateral shear and displacements relative to the isolation system in
flexibility at base, less earthquake forces transfer to the the base isolated building. Table 5 presents the response
superstructure which causes reduction in storey drifts. of base isolated building while the time period of
elastomeric bearing is changing from 2.0 second to 3.2
second.

Table 5. Effect of change of time period of elastomeric bearing properties

8 Storey 8 Storey 8 Storey 8 Storey 8 Storey

Response of Buildings
BI-1 BI-2 BI-3 BI-4 BI-5
Time Period (Sec) 1.5205 1.7185 1.9419 2.1817 2.5699
Floor Acceleration (mm/sec2) 2357 2302 2163 1942 1591
Relative Displacement to Isolation System (mm) 190.3 250.7 300.0 321.5 326.8
Base Shear (KN) 4205 3987 3745 3550 3441

Floor acceleration decreases by increasing the time base shear decreases while base shear for fixed base
period of elastomeric bearing while it is much larger at building is much larger than that of base isolated
each storey level in case of fixed base building as shown building.
in figure 17.
BASE SHEAR FOR 8 STOREY

6000
TD= 2.0 Sec
FLOOR ACCELERATION FOR 8 STOREY TD= 2.5 Sec
4000 TD= 2.8 Sec
Base Shear (KN)

TD= 3.0 Sec

9 TD= 2.0 Sec
TD= 2.5 Sec 2000 TD= 3.2 Sec
8 FIX BASE
TD= 2.8 Sec
7 TD= 3.0 Sec
Storey Level

TD= 3.2 Sec 0

6
FIX BASE 0 2 4 6 8 10 12
5
-2000
4
3
-4000
2
1 -6000
0
0 500 1000 1500 2000 2500 3000 3500 4000 Time (Sec)
Floor Acceleration (mm/Sec^2)
Figure 18 Storey drifts for fix base and base isolated
Figure 17. Floor Acceleration at each storey level for building
fix base and base isolated building
By increasing the time period of elastomeric
Figure 18 shows the maximum and bearing the relative displacement of joints to isolation
minimum values of base shear. It is clear from the figure system at floor level increases due to larger lateral
that as time period of elastomeric bearing increases the

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 Pakistan Journal of Science (Vol. 65 No. 1 March, 2013)

flexibility whereas it is much lower in case of fixed base (ii) Effect of Varying Effective Damping of
building as shown in figure 19. Elastomeric Bearing: Table 6 shows the response of
base isolated building while increasing the damping of
elastomeric bearing.
RELATIVE DISPLACEMENT FOR 8 STOREY

9 TD= 2.0 Sec

8 TD= 2.5 Sec STOREY DRIFT FOR 8 STOREY
TD= 2.8 Sec
7 TD= 3.0 Sec
Storey Level

6 TD= 3.2 Sec 9

FIX BASE TD= 2.0 Sec
5 8 TD= 2.5 Sec
4 TD= 2.8 Sec
7 TD= 3.0 Sec

Stoery Level
3 6 TD= 3.2 Sec
2 FIX BASE
5
1
4
0
3
0 50 100 150 200 250 300 350
2
Relative Displacement (mm) 1
0
0 5 10 15 20 25
Figure 19. Relative displacements for fix base and
base isolated building Storey Drift (mm)

Figure 20 Storey drifts for fix base and base isolated

Figure 20 shows that by increasing time period
building
of elastomeric bearing storey drift reduces.

Table 6 Effect of change effective damping of elastomeric bearing properties

Response 8 Storey 8 Storey 8 Storey 8 Storey 8 Storey

of Buildings BI-1 BI-2 BI-3 BI-4 BI-5
Time Period
1.5205 1.5205 1.5205 1.5205 1.5205
(Sec)
Floor Acceleration
3712 3491 3169 3135 2995
(mm/sec2)
Relative Displacement to
Isolation System 101.5 88.37 86.57 84.95 82.82
(mm)
Base Shear
4205 4072 4035 4003 3959
(KN)

By increasing the effective damping of isolation of fix base building, floor acceleration
elastomeric bearing, an energy dissipation increase which is reduced up to 66.01%, base shear is reduced
reduces the force deformation response of the base up to 13.89% and storey drift is reduced up to
isolated building. As the effective damping of the 40.56% whereas relative displacements are
elastomeric bearing increases the floor acceleration increased up to 56.73%.
decreases because the energy that is transferred to the
Phase-II. Effect of Height and Number of Bays of
structure during an earthquake dissipates in the form of
Building
heat. Effective damping may be increased by using lead
ii. By increasing the height of the building natural
core inside the high damping or low damping rubber
time period of vibration increases causing
bearing to make better the energy dissipating properties
reduction in force response but for taller
of bearing.
buildings the natural time period of vibration is
long enough to attract low earthquake forces.
Conclusions
That is why base isolation is found to be more
Phase-I. Performance of Base Isolated Building effective for medium rise buildings. In our case
i. Base isolation of fix base building lengthened for 10 storey base isolated building, floor
the natural time period of vibration of building acceleration is reduced maximum up to 66.01%,
which also reduces the effects of force response base shear is reduced maximum up to 35.67%,
i.e. floor accelerations, base shear and inter storey drift is reduced maximum up to 75% in
storey drifts. Whereas the displacements of case of 8 storey. So, base isolation is more
superstructure increases relative to the base effective in range of 8-10 storeys.
isolators. In this research work due to base

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 Pakistan Journal of Science (Vol. 65 No. 1 March, 2013)

iii. By increasing bays the stiffness of the building reduced up to 19.3%, base shear is reduced
increases and at the same time base isolation maximum up to 5.85% and relative
imparts lateral flexibility at the base level which displacement is decreased up to 18.4%.
reduces the force response but lateral flexibility
increases the relative displacements of building REFERENCES
to the isolator system. In this case, for 9x3 bays
base isolated building the floor acceleration is Farzad, N. and J. M. Kelly. Design of Seismic Isolated
reduced maximum up to 40.6% and minimum Structures from Theory to Practice. John Wiley
up to 35.7% , storey drifts are reduced maximum & Sons Inc., USA (1999).
up to 29.8% ,base shear in reduced maximum up Jangid, R. S. Optimum Lead-Rubber Bearings for Near-
to 18.29%, relative displacement is increased up Fault Motions. Engineering Structures, Vol. 29,
to 29.5% in case of 3x3 bays. No. 10, pp.2503-2513 (2006).
Phase-III. Effect of Change in Properties of Base Kelly, J. M. Earthquake Design with Rubber. Springer-
Isolator Verlag, Inc, N.Y. (1997).
iv. By increasing the time period of base isolator Mokha, A., M. C. Constantinou and A. M. Longhorn.
the time period of building superstructure also Teflon bearings in aseismic base isolation:
increases causing reduction in force response. In Experimental studies and mathematical
this work, by increasing the time period of base modeling. Technical Report NCEER-88-0038,
isolator up to 3.2 sec, floor acceleration is SUNY Buffalo. (1988).
reduced up to 32.49%, storey drift is reduced up Sap2000. Analysis References Volume 2. Computers and
to 46.3%, and base shear is reduced up to Structures Inc., California. (1997).
18.18% whereas relative displacement is Robinson, W. H. Seismic Isolation of Civil Buildings in
increased up to 51.89%. New Zealand. John Wiley & Sons Ltd., New
v. The relative displacements are increased by Zealand. (2000).
imparting lateral flexibility, can be controlled by Yang, Y. B. and K. C. Chang. Earthquake Engineering
adding damping to the system. In this research Hand Book. Department of Civil Engineering,
work the effective damping is increased up to National Taiwan University, Taipei, Taiwan,
0.01523 KN-sec/mm, floor acceleration is R.O.C. (2003).

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