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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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“Effectiveness of Lead Rubber Base Isolators, for seismic resistance of
Buildings, supported on different soil stratas,”
1
S.K. Sabu, 2H.S.Chore, 3S.B.Patil
1,2,3
Department Of Civil Engg, Datta Meghe College Of Engg., Airoli/ University of Mumbai
Email: [email protected], [email protected], [email protected]
Abstract- Seismic Base-isolation of building is an innovative
technique used in recent years, for reducing seismic energy
transmitted to buildings, in highly seismic prone areas,. The
basic principle behind the base-isolation system is to
introduce a flexible interface between the base of a structure
and the foundation. Laminated Rubber Bearings are the
most widely used technology in seismic base isolation,
because of their technical and economic effectiveness and
reliability. Even though base isolation method is extensively
used for over 8000 structures internationally, this method is
very rarely used in India in spite of the fact that India has so
many highly seismically active zones. A case study
comprising of the seismic analysis of a sample building, in
seismic zone V of India, with lead rubber bearings at the
base is done in this paper. Linear static, and nonlinear
dynamic analysis are performed for both isolated and
non-isolated buildings under code mandated bi-directional
earth quake. A comparative seismic parametric study is also
done with conventional design. Parametric comparison is
done for the building with three types of founding soil
strata. The study reveals that for medium rise building
construction, isolation can significantly reduce seismic
response. The reduction in seismic responses are more in
harder soil founding strata. In seismic vulnerable areas
where the main concern is the mitigation of the seismic
instability with the support of critical components, the
study shows the effectiveness of the base isolation system in
terms of reduced structural responses under seismic loading.
Index Terms—Lead rubber bearings, Effective stiffness,
effective damping ratio, response spectrum.
I. INTRODUCTION
The Indian subcontinent has a history of devastating earth
quakes. The shaking
memories of high intensity
earthquakes of Bhuj(7.7) (on the anniversary of India’s
51st Republic Day, 2001) and lathur(6.2)(1993) are still
live in our minds. Third deadliest earthquake in the
history
of
the
world,
the Sumatra–Andaman
earthquake,2004(9.3), and the tsunami generated
thereafter, killed over 2,30,000 people in fourteen
countries, including15,000 people in India and inundating
coastal communities with waves up to 100 ft hight . Even
now there is frequent occurrences of earthquakes in the
Kashmir and Himalayan region. The major reason for the
high frequency and intensity of the earthquakes is that the
Indian plate is driving into Asia at a rate of approximately
47 mm/year. Geographical statistics of India show that
almost 54% of the land is vulnerable to earthquakes. A
World Bank & United Nations report shows estimates that
around 200 million city dwellers in India will be exposed
to storms and earthquakes by 2050.
The latest version of seismic zoning map of India given in
the earthquake resistant design code of India [IS 1893
(Part 1) 2002] divides India into 4 seismic zones (Zone 2,
3, 4 and 5), with Zone 5 expects the highest level of
seismicity whereas Zone 2 is associated with the lowest
level. Each zone indicates the effects of an earthquake at a
particular place based on the observations of the affected
areas and can also be described using a descriptive scale
like Modified
Mercalli
intensity
scale or
the Medvedev-Sponheuer-Karnik (MSK) scale. The
MSK intensity broadly associated with the various
seismic zones is VI (or less), VII, VIII and IX (and above)
for Zones 2, 3, 4 and 5, respectively, corresponding
to Maximum Considered Earthquake (MCE). Zone 5,
which is referred to as the Very High Damage Risk Zone
in the IS code, assigns zone factor of 0.36 to it, which is
indicative of effective (zero period) peak horizontal
ground accelerations of 0.36 g (36% of gravity) that may
be generated during MCE level earthquake in this zone.
The state of Kashmir, the western and central Himalayas,
the North-East Indian region and the Rann of Kutch fall in
this zone.
Conventional structure design approach to earthquake
resistant design of buildings, followed all over world, is
by providing building with strength, stiffness and inelastic
deformation capacity, which are great enough to
withstand a given level of earthquake-generated force of
medium intensity. Practically they all proved to be very
uneconomical and failing under high seismic activities.
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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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Seismic Base-isolation of building , developed in recent
years, is an efficient method for reducing seismic demand
to buildings, in highly seismic prone areas. This
innovative design approach aims mainly at the isolation of
a structure from the supporting ground, during earthquake
excitation. This system can separate the superstructure
and component from the earthquake ground motion, thus
reducing the structural earthquake response. Base
isolation( B I) keeps buildings, bridges, and other
structures completely operational after a major
earthquake. Seismic Isolation effectively reduces a
substantial amount of the seismic energy and
displacement transferred to the structure from the ground
by absorbing energy in base isolators. They carry vertical
load yet give lateral flexibility with damping – effectively
decoupling the structure from the ground. Many types of
structures have been designed and built with base
isolation, incorporating different concept of
base
isolation. Most of these completed buildings and those
under construction use rubber base isolation bearings,
which are found to be most efficient in base isolation
designs.
Even though base isolation method is extensively used for
various structures internationally to protect civil
engineering structures, to withstand heavy earth quakes,
this method is very rarely used in India. Base isolators are
used in India, only in five government projects and none
of buildings in private sector, due to lack of awareness
about its benefits and technical feasibility. Technoeconomic feasibility study of base isolators in Indian
conditions is the need of the hour, to popularise the
benefits of base isolators in India. An attempt is made in
this paper to analyse a sample data centre building in
seismic zone V of India, employing base isolators.
Seismic response parametric comparison is done for the
structure analysed with and without base isolators.
Parametric comparison is also done for the building, with
three types of founding soil strata. The most widely used
base isolation method of Lead Rubber Bearings are used
in the analysis and design.
II. CHOICE OF BASE ISOLATOR: RUBBER
BEARINGS VS. SLIDING BEARINGS
A variety of isolation devices including elastomeric
bearings, frictional/sliding bearings and roller bearings
have been developed and employed for aseismic design of
buildings.
economic effectiveness, inherent stability, and reliability.
Developmental studies are in progress regarding sliding
bearings on their efficiency, effectiveness and
performance. The lead rubber bearings (LRB) invented in
the 1970s, are extensively used in New Zealand, Japan
and United States. The first building isolated by the LRB
was the William Clayton building in Wellington, New
Zealand, completed in 1981. Skinner 1975; Robinson
1982; Kelly 1990 did initial developments on LRB.
Even though there are some comparative benefits with
sliding base isolator such as lengthening the period of
vibration of structure and ease to manufacture there are
certain major disadvantages as listed below;

Due to Mechanical/Friction type action, it is prone
to wear and tear and so early replacement is
required, which is practically very difficult and is
very costly

Can settle in the dish after a long period of
inactivity and is likely to tear the bowl if an
earthquake occurs, with subsequent replacement

A small earthquake could create a situation where
the pendulum gets stuck half way up the bowl due
to sticktion. The reliance on gravity to recentre the
device only works if the earthquake is big enough.

A rocking effect can occur on the isolated building
due to the nature of the bowl

Once the outer lip of the bowl is reached they may
break
Even though there is a disadvantage of large base
displacements and
excessive structural vibrations,
resulting from the increased flexibility of the rubber base
isolation system, under severe seismic excitations, they
are comparatively having so many advantages compared
to sliding base isolators, such as simplicity, economic
effectiveness, inherent stability,
reliability and
re-centring ability. It is proven in so many past
earthquakes that it is not necessary to replace the rubber
bearings during lifetime of the building and maintenance
requirement is also minimal. Even if they are pushed past
their design capacity they don’t break, but they still
continue to work at a performance level of 85%
effectiveness and they won’t fall apart like a pendular
system. Feedback in countries exposed to a high seismic
risk, confirms the efficiency of these systems Considering
all these facts Lead Rubber Base Isolation system is
selected as base isolator for this project.
The lead rubber bearings and sliding bearings are most
commonly employed base isolators for buildings of
medium height. Lot of analytical and experimental study
III. ANALYSIS AND DESIGN
had been done by various researchers for the last three
METHODOLOGY
decades, as seen in various papers reviewed, mainly on
Rubber isolators. Many rubber based base-isolation
Analysis and design of Lead rubber base isolated building
techniques such as laminated rubber bearings, lead-rubber
is done as per the procedure laid down in UBC 1997. All
bearings etc. have been implemented to full-scale
major structural components have been designed for
buildings and bridges because of their simplicity,
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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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appropriate loads and its combinations, as per relevant
clauses of IS 456:2000, IS 875 and IS 1893 where ever
applicable., for the most unfavourable effects. The
structural system is finalised with preliminary sizing of
structural members as per loading requirement and
material properties selected. The structural members are
detailed as per ductile detailing code IS 13920 as Special
Moment Resisting Frame members and shear walls.
A three dimensional structural model is developed using
the most popularly used structural analysis software
called ETABS developed by “ Computers and Structures
Incorporated ”. The structural models of the buildings
were composed of normal shell finite elements for the
shear walls. the columns were simulated by beam
elements. The isolators were modelled as spring elements.
The shear stiffness of the isolators was taken into account
as the estimated value corresponding to a 100% strain,
imposed under the action of the vertical loads. This
assumption allowed for a linear analysis of the structural
response. The critical damping factor of the isolators
normally ranges from 0.20 to 0.30. The response
acceleration spectrum method was selected for the linear
dynamic analysis; The IS code defined response spectrum
is selected and assigned for the model. It is checked for
lateral static wind load as well as static seismic loads as
per IS codes. The required data for the seismic analysis of
the sample building is identified, including the floor
layouts, loading parameters, preliminary sizing of
members along with material properties. . Technical
parameters of Base isolator also is required to be
identified. The base isolator is idealised in the
mathematical model, and the seismic analysis of the 3 D
model fitted with LRB is done to find the seismic
parameters of the structure. The seismic design
parameters of analysis files with and without base isolator
are compared. Parametric seismic analysis comparison of
the structure with base isolators is also done for various
foundation soil strata to find the performance of base
isolators in various types of founding soils.
The proposed data centre Building consists of Basement
+ Ground + 1st to 6th floors + Terrace floor. The
appropriate sized base-isolators using LRB is to be
designed for the worst combination of all possible vertical
and lateral loads.
The structure is to be analysed for all loads with and
without LRBs fitted to all columns and shear walls as per
codal requirements. Parametric comparison is to be done
for all seismic parameters for the building analysed with
and without base-isolators.
Fig. 1 Typical floor structural plan
V. DETAILS OF STRUCTURAL SYSTEM
OF DATA CENTRE BUILDING
The proposed Building is designed for Basement +
Ground + 1st to 6th floors + Terrace floor. Ground floor
houses some equipments such as chillers, first to third
floors are for computer racks whereas fourth to sixth are
office floors. Floor height of Basement floor is 4.5m and
typical floors 5.0m. The structural system supports the
vertical loads consisting of Dead and Super-Imposed
Loads as well as lateral forces of Earthquake and Wind.
The sub-structure mainly consists of raft slab foundation
and retaining walls. Columns and shear walls carry all the
gravity loads and lateral forces to the foundations. The
Structural system of typical floor is of beam slab system.
IV. CASE STUDY
To study the effect of using Lead rubber bearing on the
buildings to carry the seismic forces in highly seismic
prone areas, in Indian conditions with different soil
founding strata, Lead Rubber Bearings are designed for a
proposed medium height data centre building near Delhi
region of India. Data centre is a heavily loaded building,
which is handling very sensitive electronic information of
various corporate groups. The data stored need to be
protected and the function of data centre need to proceed
uninterrupted, during and after the occurrence of a heavy
earthquake. Even though the building is located in zone
IV, as per client’s request, it is designed for one zone
higher i.e. zone V. The structure is analysed with Soil
Structure Interaction, for Various Founding Strata Of
Hard, Medium And Soft Soils for parametric comparison
Fig 2. Section Showing Base Isolator Below Column.
The required data for the seismic analysis such as loading
parameters, preliminary sizing of members along with
material properties are worked out. Technical parameters
of Base isolator such as compression stiffness, effective
secant stiffness, equivalent damping ratio etc. are
identified from vendor specifications of Robinson
Seismic Bearings and as per UBC 97. A three dimensional
model of the structure is developed. The base isolator is
also idealised in the mathematical model, and the seismic
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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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analysis is done to find the seismic parameters of the
structure.
A. Design Loads
All partitions are of lightweight aerated blocks of 6.7
KN/m3. Live load is considered as per IS: 875
(Part-2)-1987 and as per the requirement of DATA
CENTER Loading requirements. Live load in ground
floor is 5 KN/m2 and. Office areas of fourth to sixth floors
is 5 KN/m2 . In computer rack floors of first to third floors
live load in rack areas is 18.5 KN/m2 and other areas it is 8
KN/m2. Superimposed dead load consists of Tiled Floor
finish 1.0 KN/m2, false ceiling 0.50 KN/m2 and light
weight partition of 1.0 KN/m2. Water proofing load of 3
KN/m2 is applied in terrace. Load of Water tank for
84,000 and 80,000 litres over the stairs as per functional
requirement is applied.
Fig. 3 Response spectra for 5% damping as per IS
1893-2002.
E. Structural 3d Models Of Data Centre Building
B. Wind Loads
The wind pressure as per IS: 875(Part 3)-1987, Basic
Wind Speed Vb = 47 m/sec (For Delhi region)
Risk Coefficient ,K1 = 1 Terrain Category 2,Structure
Class B,Factor K2 As per IS 875 (Part III)
Topography factor K3 = 1,Design Wind Speed Vz max =
VbK1K2K3, Design Wind Pressure,Pz = 0.6 Vz 2
Fig. 4 Typical Floor drawing
C. Earthquake Load
Static loading due to earthquake is assessed based on the
provisions of IS: 1893(Part-1):2002, for Seismic Zone
V ,Zone factor (Z)= 0.36, for Maximum Considered
earthquake(MCE) for the service life of the structure.
Importance Factor (I) = 1.0 Response Reduction Factor
(R) = 5.0, Damping = 5 % .
Design horizontal seismic coefficient Ah shall be
Dynamic analysis loading due to earthquake on the
provisions of IS: 1893(Part-1):2002 & UBC 97 Response
Reduction Factor (R) = 2.00 Damping = 25 % with Sa/g
multiplying factor of 0.55 for spectral acceleration
coefficient.
D. Materials Used
The following materials are used for the structure.
Columns = M60/50/40 , Beams and Slabs = M50/40/30,
For raft foundation = M50
The reinforcements considered is of Grade Fe500
Fig. 5 Full Model Structural System
VI. PARAMETRIC STUDY
A. Comparison Of B.I Structure With Fixed Structure.
Seismic parameters are compared, for the sample
building, by analyzing it, as a fixed base building and with
Lead Rubber Base isolators introduced at the base of the
columns and shear walls, considering medium soil
founding strata and the results are summarized in the table
1. As seen from the analysis, the time period of the
structure is increased to almost 2.5 times the
Table 1 Parametric comparison of Fixed base and B.I
Parameters
Fixed base With B.I.
Comparison
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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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Time period 1.1082
2.8612
+ 158%
of first mode
base shear 14700
8130
- 44.69%
(KN)
Overturning
3.39 x 105
1.54 x 105 - 54.57%
moment
(KNm)
Base
0.0
30.9
displacement
(mm)
Top
25.7
60.0
- 133%
displacement
(mm)
value of fixed base building, resulting in a substantial
reduction of spectral acceleration and hence reduction of
other seismic parameters of the building. Base shear and
moments are reduced by almost half. There is some base
displacement due to the introduction of base isolator but
the effective displacement of the top with respect to the
base is well within the codal limit of H/500.
B. Comparison Of B.I Structure With Different Soil
Stratas.
The building is analysed with three types of foundation
soil strata systems
A. Hard soil consisting of rocky strata
B. Medium soil consisting of hard murrum and sandy clay
C. .Soft soil consisting of soft clayey soil.
The following are the results of soil-structure interaction
analysis of the base-isolated building.
1. Base shear and moments are increased for structure
supported on softer soils,Compared to hard soil.
2. The time period of vibration of the building is not
affected
with the type of soil.
3. The response spectrum acceleration is less for buildings
supported on harder soils
4. The Base displacement and top storey drift are less for
buildings supported on harder soils
Table 2 Parametric Comparison of structure on different
soils
Parameter
s
Time
period
base shear
(KN)
BASE
moment
(KN m)
Base
displacem
ent (mm)
Top
Storey
drift w.r.t
base
(mm)
Soil
Syste
m
S1-har
d
With
B.I.
2.909
4
6060
Soil
System
S2-mediu
m
With B.I.
2.8612
8130
Soil
Syste
m
S3-sof
t
With
B.I.
2.909
4
10100
1.15 x
105
1.54 x 105
1.95 x
105
23
30.9
38.3
25.2
29.1
41.7
Comp
arison
S1-S2
Compari
son
S2-S3
-1.65
%
+
+1.68%
34.16
%
+
33.91
%
+
34.35
%
+
15.48
%
+
24.23%
+
26.62%
+
23.95%
+
43.30%
Fig. 8 Base displacement comparison for different soil
15000
10000
HARD SOIL S1
5000
MED. SOIL S2
0
SOFT SOIL S3
BASE SHEAR
Fig. 6 Base shear comparison for different soils
3
2
1
0
Fig. 9 Top storey displacement comparison for different
soil
VII.
HARD SOIL S1
SUMMARY AND CONCLUSIONS
As described above the analytical study comprising of the
seismic analysis of a sample building, in seismic zone V
MED. SOIL S2
of India, with lead rubber bearings at the base, is done
with a three dimensional structural model with response
SOFT
SOIL
S3
BASE MOMENT-KNM(x E
spectrum analysis as per code mandated bidirectional
05)
earthquake spectrum. The parametric comparison of B.I
building with fixed base model shows that for a medium
Fig. 7 Base moment comparison for different soils
rise building there is a substantial reduction in the base
shear and moments almost to the tune of 50%. Base lateral
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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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displacements, and top storey displacement were also
found to be within reasonable limits.
Parametric comparison done with three types of
founding soil strata reveals that the reduction in seismic
responses are more in harder soil founding strata. And the
responses are increased by almost 35% for structure
supported on harder to medium soil and almost 25% for
medium to soft soils. There is additional flexibility of the
structure due to reduced structural responses through
incorporation of the isolator. For strategic buildings In
seismic vulnerable areas, the main concern is the
protection of critical components from damage and
seismic instability. The study shows the effectiveness of
the LRB base isolation system in terms of reduced
structural responses under seismic loading.
Extensive further study is required both analytical and
experimental to boost the confidence of the construction
industry. Performance of structures fitted with isolators in
high seismic activity need to be observed and
documented. Shake table study of prototype models need
to be done to arrive at parameters and confirm the
effectiveness.
As the base isolators are extensively used worldwide in
high seismic areas , in near future, we will expect the same
in India also. At least in seismic zone 4 and 5 the use of
base isolators has to be encouraged, as they are
technically very effective and economically feasible. The
use of base isolators reduces inter-storey drift and
structural damages during earthquake. The building will
be ready to occupy with minor repair.
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