Parametric study of base isolated building

International Journal of Civil Engineering Research and Development (IJCERD), ISSN 2248-
9428 (Print), ISSN- 2248-9436 (Online) Volume 4, Number 2, April - June (2014)
30
PARAMETRIC STUDY OF BASE ISOLATED BUILDING WITH
ECCENTRICITY IN SUPER STRUCTURE WITH VARIOUS
LOCATION OF BASE ISOLATORS
Sonal Thakkar1
, Jay Assudani2
1
Asst. Professor, Civil Engineering Department, Institute of Technology,
Nirma University
2
Structrual Engineer, DuCone Consultant.
ABSTRACT
An attempt is made to understand the behavior of base isolated building with
varying percentage of mass eccentricity. Different variants of lead rubber bearing base
isolated three storey was taken and various distribution of isolators was studied. The
asymmetry was obtained by use of different percentage of stiffness of base isolators and
four asymmetric distribution of isolators (CI = ±CM and CI = ±0.5 CM) were considered
for 5 %, 10% and 15% of mass eccentricity in super structure. The paper tries to analyze
best possible placement of isolators so that balance is achieved between torsional moment
and displacement.
Key words: Base isolated structure, Centre of Mass, Centre of isolators.
1. INTRODUCTION
Structures are generally build to satisfy the condition that its capacity is also more
than the demand which will be imposed on the structure. Vibration control theory in civil
structures, is not as old compared to application in aerospace and machines. Generally
earthquake and wind are two major contributors to structural vibrations and their vibrations
can be controlled by changing mass, rigidity, stiffness, damping, shape or by applying
passive or active control devices [1,2].In traditional earthquake design the mitigation of
earthquake is done by either increasing the capacity of building against lateral load or by
providing high ductility. In recent years, another alternate technique to mitigate the seismic
effect on the building has evolved and that is base isolation technique. Base isolation not
only reduces the seismic forces transmitted to the superstructure but also reduces floor
acceleration induced by the earthquake. In isolated structures, due to insertion of a flexible
layer between foundation and superstructure, the fundamental time-period of the system
IJCERD
© PRJ PUBLICATION
International Journal of Civil Engineering Research and
Development (IJCERD)
ISSN 2248 – 9428(Print)
ISSN 2248 – 9436(Online)
Volume 4, Number 2, April - June (2014), pp. 30-40
© PRJ Publication, http://www.prjpublication.com/IJCERD.asp

31
increases to a value higher than the predominant energy containing time-periods of
earthquake ground motions[1,2,3].
2. PRINCIPLE OF BASE ISOLATION
Base isolation is today most widely accepted earthquake resistant design
philosophy[4]. The term base isolation means the structure i.e. a building, bridge or piece of
equipment is separated from its foundation. Generally since in earthquake the ground
shakes and hence the structure needs to be separated from ground.The concept behind base
isolation is to decouple the building from the ground in such a way that the earthquake
motions are not transmitted up through the building, or are at least greatly reduced. Thus in
principle, seismic isolation introduces flexibility at the base of a structure in the horizontal
plane, while at the same time introducing damping elements to restrict the amplitude of the
motion caused by the earthquake. Mounting buildings on an isolation system will prevent
most of the horizontal movement of the ground from being transmitted to the buildings.
These results in a significant reduction in floor accelerations and inter story drifts. Since a
base-isolated structure has fundamental frequency lower than both its fixed base frequency
and the dominant frequencies of ground motion, the first mode of vibration of isolated
structure involves deformation only in the isolation system whereas superstructure remains
almost rigid. In this way, the isolation becomes an attractive approach where protection of
expensive sensitive equipment’s and internal non-structural components is needed [1, 3].
3. TYPES OF BASE ISOLATORS
The most common type of base isolators used in buildings are:
1. Laminated Rubber (Elastomeric) Bearing.
2. High Damping Rubber Bearing
3. Flat Slider Bearing
4. Lead Rubber Bearing
5. Friction Pendulum System.
Elastomeric bearings / Laminated Rubber Bearing System: have been used widely in
bridges as bearings pads between the girder and the supporting structure for many years. It
consists of electrometric bearings have multiple layers of steel shims and rubber laminated
together under high pressure and heat in a mould. Steel shims prevent lateral bulging of the
rubber when axial loaded as they do not resist shear forces and do not prevent the horizontal
deformation of the layered rubbers. Steel shims increase the vertical stiffness of isolator but
do not increase the lateral stiffness of electrometric bearings. Generally, electrometric
bearings have low critical damping resistance, approximately 2% to 3% of critical viscous
damping and have minimum resistance under service loads [3,4,5].
High Damping Rubber Bearings: are similar in shape to the electrometric bearing, except
that it is made of specially compounded rubber layers that are usually made of materials that
are highly nonlinear in terms of shear strains. In HDR, the effective damping is a function
of strain and effective stiffness and damping depend on elastomeric properties, fillers,
contact pressure, velocity of loading, load history, temperature.

32
Flat Slider Bearings: This bearings provide hysteresis shape with no strain hardening after
the applied force exceeds the coefficient of friction times the applied vertical load. This is
attractive from a structural design perspective as the total base shear on the structure is
limited to the sliding force. In this system displacements are unconstrained and structure
continues to move in the same direction.
Lead Rubber Bearings: The lead rubber bearing is formed of a lead plug force-fitted into a
pre-formed hole in an elastomeric bearing. The lead core provides rigidity under service
loads and energy dissipation under high lateral loads. Top and bottom steel plates, thicker
than the internal shims, are used to accommodate mounting hardware. The entire bearing is
encased in cover rubber to provide environmental protection, as shown in figure1.When
subjected to low lateral loads such as earthquake & wind, the lead rubber bearing is stiff
both laterally and vertically. The lateral stiffness results from the high elastic stiffness of the
lead plug and the vertical rigidity results from the steel-rubber construction of the bearing.
At higher load levels the lead yields and the lateral stiffness of the bearing is significantly
reduced. This produces the period shift effect characteristic of base isolation. As the bearing
is cycled at large displacements such as during moderate and large earthquakes, the plastic
deformation of the lead absorbs energy as hysteretic damping. The equivalent viscous
damping produced by this hysteresis is a function of displacement and usually ranges from
15% to 35%.
Fig. 1: Lead Rubber Bearing
Friction Pendulum System: This approach for increasing flexibility in a structure, is to
provide a sliding or friction surface between the foundation and the base of the structure.
The isolator provides a resistance to service load by the coefficient of friction, as for flat
slider. Once the coefficient of friction is overcome the articulated slider moves and because
of the accompanied with a vertical movement of the mass provides a restoring force.
4. MODELLING AND STUDY OF 3 STOREY BUILDING FOR FIXED AND BASE
ISOLATION SYSTEM WITH VARIOUS ECCENTRICITIES
4.1 Introduction
It is a known fact that though regular and symmetric building offer maximum
resistance during Earthquake, buildings with asymmetric plan and elevation are choice of
architects, builders and also investors. Due to this asymmetry torsion will be induced in this
building [6-8]. Hence an attempt is made to study the effect of eccentricity both in
superstructure and base isolators. Finite Element analysis was of one fixed base building
and four base isolated buildings of 5%, 10% and 15% of mass eccentricities in super
structure and various distribution of isolators in base isolation system such that eccentricity
is produced between Centre of Mass (CM), and Centre of isolator system (CI) for four

33
cases.Twenty isolators are placed under all the columns and distribution of isolators is done
in such a way that torsional effects on the isolator system is minimized.
4.2 Fixed base building & Base isolated building frame
The fixed base building is reinforced concrete frame structure of 3 storey building
with symmetric plan. The slab was 120 mm thick and dead load including self-weightwas 4
kN/m2
, while it was subjected to a Live load of 3 kN/m2
on all floors. Concrete of M 25
grade was used while reinforcement consisted on Fe 415 grade of steel. Beam sizes were
kept as 300 mm x 450 mm while columns were designed as 500 mm x 500mm for all
storey. Kobe response spectrum was applied in X direction and both fixed and base isolated
building were analyzed. Figure 2 shows the plan of building. The same plan is adopted for
base isolated building.
Fig. 2: Plan of fixed and base isolated building
Table 1: Properties of Isolators
4.3 Properties of Isolators
In case of base isolated building total 24 isolators were used having different vertical
and horizontal stiffness. The yieldstrength of isolator is taken as the 5% weight of the
structure. The post to yield stiffness ratio (K2=K1) is 0.10. The effective damping of isolator
is taken as 10% for this study. The properties of isolators, which consists of nonlinear
behavior, are shown in table 1. These properties are used to model the structure.
4.4 Distribution of Isolators
To study the effect of placement of isolators in the base isolated building four
models were considered and compared with fixed base building. In all these models centre
of mass (CM) and Centre of Stiffness (CS) was kept fixed while centre of isolators (CI) was
shifted.There are two eccentricities in the structure, one is mass eccentricity denoted by

34
emand second is stiffness eccentricity denoted by eb. In first case of building, isolators were
positioned in such a manner that its centre coincides with the centre of mass of the building.
Therefore eccentricity is induced only between centre of stiffness and centre of isolators
(CI = CM). Second asymmetric distribution of isolators was done by shifting centre of mass
and centre of isolators on opposite sides of centre of stiffness. This is referred as CI = -CM.
In the third case centre of isolators is kept exactly in the middle of centre of stiffness and
centre of mass in such a manner that eb = 0.5 em. This case is called CI = 0.5 CM. In fourth
case, centre of stiffness is kept exactly in the middle of centre of isolators and centre of
mass producing CI=-CM. Figure3 shows different position of isolators.
Fig. 3: Position of Centre of Isolators
5. VARIATION IN MASS ECCENTRICITY
To achieve 5 % of mass eccentricity in superstructure, centre of isolators and
stiffness of isolators had to be varied for all four cases i.e. CI = CM, CI= -CM,
CI = + 0.5 CM and CI = - 0.5 CM. Figure 4 shows the stiffness of isolators and eccentricity
for CI = CM case. Similar calculations were done for other three cases to find stiffness of
isolators. The percentage reduction or increase in stiffness of isolators on the right and left
side is shown in Table 2.Eccentricity in base isolation system, calculated for case of CI =
CM for 5 % of mass eccentricity with original stiffness of isolator as 4213.20 kN/m is as
follows.
Taking moment at the left face of the building,
= 4 (0.83k × 4 + 0.83k × 8 + 1.17k × 11 + 1.17k × 15 + 1.17k × 19) /24 × k
= 4 (0.83 × 4213.20 × 4 + 0.83 × 4213.20 × 8 + 1.17 × 4213.20 × 11 + 1.17 × 4213.20 × 15
+1.17 × 4213.20 × 19) /24 × 4213.30 = 10.45 m
So, eccentricity in the base isolation e = 10.45 - 9.50= 0.95
TABLE 2: Stiffness of Isolators for 5 % mass eccentricity
Eccentricity Model Name Percentage reduction or
increase in stiffness of
isolator on the left side of
CS
Percentage reduction or
increase in stiffness of isolator
on the right side of CS
5% CI= CM -17% 17%
5% CI= -CM 17% -17%
5% CI= 0.5CM -8% 8%
5% CI= -0.5CM 8% -8%

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Fig. 4: CI= CM model with 5% eccentricity of mass
Similarly to achieve 10 % of eccentricity in mass, the percentage reduction or
increase in stiffness of isolators was done as shown in Table 3 and eccentricity achieved for
CI = CM was 1.90 m and to shift centre of isolators ,stiffness of isolators on the left and
right side were differed as shown in Fig. 5, for CI=CM. Similar calculations were done for
other three cases.
CS
isolator on the right side of
CS
10% CI= CM -35% 35%
10% CI= -CM 35% -35%
10% CI= 0.5CM -17% 17%
10% CI= -0.5CM 17% -17%
To achieve 15% of mass eccentricity for all four cases, the percentage increase or
decrease in stiffness is indicated in Table 4, while for CI = CM case the eccentricity was
found to be 12.35 m. Figure 6 shows the value of stiffness of isolators to the right and left
side of centre of stiffness to shift the centre of isolators.

36
CS
isolator on the right side of
CS
15% CI= CM -50% 50%
15% CI= -CM 50% -50%
15% CI= 0.5CM -25% 25%
15% CI= -0.5CM 25% -25%
7. BEHAVIOUR OF FIXED AND BASE ISOLATED BUILDING
There are many parameters that can be used to assess the seismic response of the
building, but generally computation of base shear and storey drift gives reasonable
assessment to study overall seismic effect. Building was analyzed for Kobe Response
Spectra. Various parameters like time period, base shear, displacement and rotation were
considered and evaluated.
7.1 Time Period
When models of different eccentricity were compared, it could be concluded that
model CI = - CM has maximum time period. If one eccentricity between CS and CM is
( em ) which is equal for all distribution of isolators and another eccentricity is the
eccentricity between CS and CI i.e eb then both eccentricities together will form the total
eccentricity which in the case of CI = - CM , already amounts to 2.em and causes longer
structural periods.
It can also be noted that the time period for the first mode of the base isolated
building with 5% of mass eccentricity in super structure and various distribution of isolators
in base isolation system such that CI = CM, CI = - CM, CI = 0.5 CM, CI = - 0.5 CM was
increased by 2.298, 2.375, 2.294 and 2.334 times respectively compared to the fixed base

37
building. While in the case when eccentricity was increased by 10% the increase was 2.438,
2.687, 2.423 and 2.556 times respectively, compared to the fixed base building. For 15 % of
mass eccentricity and various distributions of isolators the increase was 2.610, 3.065, 2.603
and 2.805 times respectively. Thus in can be said that there is substantial increase in time
period for base isolated buildings compared to fix base building. Figure 7 shows the
graphical representation of various time periods with eccentricity variation.
Fig. 7: Comparison of Mode v/s Time Period for 5 %, 10%, and 15% of eccentricity.
7.2 Base Shear
The graphical representation of comparison of base shear for base isolated buildings
with 5%, 10% and 15% of mass eccentricities in super structure and various distribution of
isolators in base isolation system such that CI = CM, CI = - CM, CI = 0.5, CM, CI = - 0.5
CM are presented in Figure8. Compared to fixed base building, the base shear for the base
isolated building with 5% of mass eccentricity in super structure and various distribution of
isolators in base isolation system like CI= CM and CI = - CM the decrease was 29.8 %
while for CI = 0.5 CM and CI = - 0.5 CM decrease was 29.73%. When eccentricity of 10%
of mass in super structure was made for different base isolator distribution, the decrease was
27 % approximately compared to fix base building. While for 15% of mass eccentricity in
super structure the decrease was 23.5 % approximately when compared with fixed base
building. From results, it can be said that by use of base isolators substantial reduction in
base shear was achieved.
7.3 Displacement
From results, it can be said that the best protection is not reached in the case of CI =
CM i.e where the eccentricity of the isolation system is similar to the eccentricity of the
superstructure as is commonly perceived while when CI = 0.5 CM bearing distribution does
not give so drastically unfavorable results. Much better protection was reached in case
where CI was positioned further away from the CM (eg. original, CI = - CM and CI = -
0.5CM distribution). The minimum displacement for base isolated buildings was obtained
for the case of CI = - CM. Results are graphically shown in figure 9.

38
Fig. 8: Base shear comparison for various eccentricity
Fig. 9: Displacement comparison for various eccentricity
7.4 Rotation
From results, it can be said that the CI = - CM and CI = - 0.5 CM distributions of
isolators gave more torsion response. The minimum rotation for base isolated buildings was
obtained for the case of CI = CM. Figure 10 shows graphical representation of various
eccentricity
Fig. 10: Comparison of rotational response for various eccentricity

39
8. CONCLUSION
Nonlinear response spectrum analysis of one fixed base building and four base
isolated buildings with 5%, 10% and 15% of mass eccentricity in super structure and
various distribution of isolators in base isolation system such that CI = CM, CI = - CM, CI=
0.5 CM, CI = - 0.5 CM is carried out.
• Results of Kobe Response spectrum analysis shows that time period of the building
increases with the eccentricity, but varies with different isolator position when is
compared with fixed base building. The increase in time period for first mode of
base isolated building with 5%, 10% and 15 % of mass eccentricities in super
structure and various distribution of isolators in base isolation system for CI =CM is
2.298,2.438 to 2.61 while for CI =-CM it is 2.375, 2.687 and 3.065 compared to
fixed base building.
• There is reduction in the base shear of base isolated building compared to fixed base
building, but as the eccentricity increases from 5 % to 15 %, the reduction decreases
from 30 % to 24 % for different location of isolators.
• The displacement, when considered for different eccentricities was minimum when
centre of isolator and centre of mass are on opposite sides of centre of stiffness,
while rotation is minimum when both of centre of mass and centre of isolator are
equal.
• The rotation of the base isolation system for base isolated buildings with 5%, 10%
and 15% of mass eccentricities in super structure and various distribution of
isolatorsin base isolation system such CI = CM, CI = - CM, CI = 0.5 CM,
CI = - 0.5CM, the minimum rotation is obtained for the case of CI = CM.
Thus, it can be said that when centre of mass and centre of isolators are equal,
torsional amplification can be reduced even when there was eccentricity in superstructure,
but displacement increased with increase in eccentricity. This conclusions were contradicted
when centre of isolators and centre of mass are on opposite sides. Therefore, optimum
solution was to place centre of isolator CI = ±CM/2, which will help not only to reduce the
top displacement of superstructure but will also reduce the rotation of building.
REFERENCES
[1] R.I.Skinner, W.H.Robinson, G.H.McVerry, "An Introduction to Seismic Isolation,"
John Wiley & Sons, DSIR Physical Sciences, Wellington, New Zealand.
[2] Farzzad Naeim and James M Kelly, “ Design of Seismic Isolated structures: from
theory to Practice, 199, John Wiley & Sons, Inc.
[3] Aung Chan Win, “ Analysis and Design of Base Isolation for Multi-storeyed
Building”, GMSARN International Conference on Sustainable Development: Issues
and Prospects for the GMS, November 2008
[4] Trevor E Kelly, S.E, "Holmes Consulting Group Ltd," Design Guidelines, Revision
0:July 2001, Wellington, New Zealand.
[5] Masahiko Higashino, Shin Okamoto, "Response Control & Seismic Isolation of
Buildings."

40
[6] Arturo Tena-Colunga, Jose Luis Escamilla-Cruz, "Torsional amplifications in
asymmetric base-isolated structure," Journal of Engineering Structures, 28 March
2006.
[7] Arturo Tena-Colunga, Christian Zambrana-Rojas, "Dynamic torsional amplifications
of base-isolated structures with an eccentric isolation system", Journal of
Engineering Structures, July 2005.
[8] Vojko Kilar, David Koren, "Seismic behaviour of asymmetric base isolated
structures with various distributions of isolators," Engineering Structures December
2008.

Parametric study of base isolated building

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