International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 7, July 2012)
State Of Art Review - Base Isolation Systems For
Structures
S.J.Patil1, G.R.Reddy2
1
Heavy Water Board, Mumbai, India .
Bhabha Atomic Research Centre, Mumbai, India .
2
1
[email protected]
2
[email protected]
This paper describes the development of base isolation
techniques and other techniques developed around the
world.
As of now, in India, the use of base isolation techniques
in public or residential buildings and structures is in its
inception and except few buildings like hospital building at
Bhuj, experimental building at IIT, Guwahati, the general
structures are built without base isolation techniques.
National level guidelines and codes are not available
presently for the reference of engineers and builders.
Engineers and scientists have to accelerate the pace of their
research work in the direction of developing and
constructing base isolated structures and come out with
solutions which are simple in design, easy to construct and
cost effective as well.
Many significant advantages can be drawn from
buildings provided with seismic isolation. The isolated
buildings will be safe even in strong earthquakes. The
response of an isolated structure can be ½ to 1/8 of the
Abstract — This paper presents an overview of the present
state of base isolation techniques with special emphasis and a
brief on other techniques developed world over for mitigating
earthquake forces on the structures. The dynamic analysis
procedure for isolated structures is briefly explained. The
provisions of FEMA 450 for base isolated structures are
highlighted. The effects of base isolation on structures located
on soft soils and near active faults are given in brief. Simple
case study on natural base isolation using naturally available
soils is presented. Also, the future areas of research are
indicated.
Keywords—State of Art, base isolation, modulus reduction,
FEMA 450, isolated footing, raft footing.
I. INTRODUCTION
The structures constructed with good techniques and
machines in the recent past have fallen prey to earthquakes
leading to enormous loss of life and property and untold
sufferings to the survivors of the earthquake hit area, which
has compelled the engineers and scientists to think of
innovative techniques and methods to save the buildings
and structures from the destructive forces of earthquake.
The earthquakes in the recent past have provided enough
evidence of performance of different type of structures
under different earthquake conditions and at different
foundation conditions as a food for thought to the engineers
and scientists. This has given birth to different type of
techniques to save the structures from the earthquakes.
Base isolation concept was coined by engineers and
scientists as early as in the year 1923 and thereafter
different methods of isolating the buildings and structures
from earthquake forces have been developed world over.
Countries like US, New Zealand, Japan, China and
European countries have adopted these techniques as their
normal routine for many public buildings and residential
buildings as well. Hundreds of buildings are being built
every year with base isolation technique in these countries.
Response Acceleration in g units
1.2
1.0
Direction of
response control
along increse in damping
0.8
1%
2%
3%
4%
5%
7%
10%
0.6
0.4
Region in which
Region
of
base isolated
Base
isolation
structure lies
0.2
0.0
0.0
0.5
1.0
1.5
2.0
2.5
Time (Sec)
Envelop spectra with o.2 g ZPA
Fig. 1 Concepts of Base Isolation and Dampers
Fig.5
Regions of response control and Base Isolation
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Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 7, July 2012)
traditional structure. Since the super structure will be
subjected to lesser earthquake forces, the cost of isolated
structure compared with the cost of traditional structure for
the same earthquake conditions will be cheaper. The
seismic isolation can be provided to new as well as existing
structures. The buildings with provision of isolators can be
planned as regular or irregular in their plan or elevations.
[1].
Researchers are also working on techniques like tuned
mass dampers, dampers using shape memory alloys etc.
Tuned mass dampers are additional mass on the structure
provided in such way that the oscillations of the structure
are reduced to the considerable extent. The mass may be a
mass of a solid or a mass of a liquid. Dampers using shape
memory alloys are being tried as remedy to earthquake
forces. In this system, super elastic properties of the alloy is
utilized and there by consuming the energy in deformation
at the same time the structure is put back to its original
shape after the earthquake.
acceleration transmitted to the superstructure. All the base
isolation systems have certain features in common. They
have flexibility and energy absorbing capacity. The main
concept of base isolation is to shift the fundamental period
of the structure out of the range of dominant earthquake
energy frequencies and increasing the energy absorbing
capability. The concept is explained in Fig. 1.
Presently base isolation techniques are mainly
categorized into three types viz. 1) Passive base isolation
techniques 2) Hybrid isolation with semi-active devices 3)
Hybrid base isolation with passive energy dissipaters.
These different techniques are discussed in short below –
III. PASSIVE BASE ISOLATION TECHNIQUES
A. Mud layer below the structure
Frank Lloyed Wright was the first person who
implemented the idea of base isolation technique for
isolating Imperial Hotel structure in Tokyo, by providing
closely spaced short length piles in 8 feet thick soil layer
underlain by a thickness of mud layer over hard strata. The
building survived an earthquake in 1923. [2], [3]
II. BASE ISOLATION TECHNIQUES
In traditional seismic design approach, strength of the
structure is suitably adjusted to resist the earthquake forces.
In base isolation technique approach, the structure is
essentially decoupled from earthquake ground motions by
providing separate isolation devices between the base of
the structure and its foundation. The main purpose of the
base isolation device is to attenuate the horizontal
B. Flexible first storey
The flexible first storey concept was first proposed by
Martel in 1929 and was further studied by Green in 1935
and Jecobson in 1938 thereby reduce the loading on upper
storey members. However, further studies by Chopra et. al.
ub
mb
Top cover plate
F
ub
Bottom Cover plate
Steel plates
(a)
(b)
rubber
(c)
Fig. 2. Laminated rubber bearing system - a) Sectional details b) Schematic diagram c) Force deformation behavior
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with the aid of computers showed that the concept is
impractical [3]. Also the recent earthquakes at Bhuj in
India and Kobe in Japan have revealed that most of the
buildings with soft storey have suffered extensive damage.
response.
C. Roller bearings in foundations
Roller bearing systems proposed for isolation of the
structures were having serious drawback as the rollers were
having to and fro motion in particular direction and
earthquake has three directions motion due to which
earthquake forces could not be isolated effectively. Also
the main problem was that the device needed maintenance
for keeping in good operation throughout its working life
period. The system was further modified with ball bearing
system.
4. Vertical tension capacity is good.
D. Rubber layer as foundation support
School building in Skopje, Yugoslavia constructed on
rubber foundations in 1969 [3], used to bounce and rock
forward and backward during earthquake due to uniform
stiffness of rubber in all directions. Also the rubber
foundation bulged under the weight of the building.
F. New Zealand bearing system
The system (ref. Fig. 3), invented in NewZealand in the
year 1975, [4] is improved version of laminated rubber
bearing wherein a centrally located lead core is introduced,
which has energy dissipating capacity. The presence of lead
core reduces displacement of the isolator and isolator
essentially works as hysteretic damper device. The device
has been extensively used in New Zealand, Japan and USA.
Buildings isolated with these devices performed well
during the 1994 North ridge earthquake and 1995 Kobe
earthquake.
2. Stable character of isolators over a long working life.
3. Recovery of the displacement after earthquakes.
5. Isolators are insensitive for foundation settlement, which
are generally small in magnitude. It could adjust the
structure force by deformation of rubber bearings when
foundation settlement of building happens before or after
earthquakes.
6. Decreasing the temperature stress in structures by free
horizontal deformation of bearings during large change of
temperature around the structure.
E. Laminated rubber bearing system
Laminated rubber bearings (LRB) (ref. Fig. 2), which
are made of thin layers of steel plates and rubber built in
layers one over the other, have horizontal flexibility, high
vertical stiffness and they can be characterized by natural
frequency and damping constant.
The main advantages [1] of rubber bearing system are -
G. Resilient – friction base isolation system
Resilient – Friction Base Isolation (R-FBI) system (ref. Fig.
4) proposed by Mostaghel and Khodaverdian consists of
concentric layers of Teflon coated plates which will have
1. Effective isolation is achieved. It will decrease the
structural response to 1/2 -1/8 of the traditional structural
Top cover plate
ub
F
mb
Bottom cover
plate
Steel plates
rubber
Lead core
(a)
ub
(b)
(c)
Fig. 3. New Zealand bearing system (a) Sectional details (b) Schematic diagram (c) Force deformation behavior
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sliding resistance and a central core of rubber which will
have beneficial effect of resilience of a rubber.
The upper surface of the R-FBI system is replaced with
friction plates. As a result the structure can slide on its
foundation in a manner similar to that of the EDF base
isolator system. For a low level of seismic excitation, the
system behaves as an R-FBI system. The sliding in the top
plates occurs only during high level of ground acceleration,
which provides additional safety against unexpected severe
ground motion.
H. Electric de-France system
Electric De-France (EDF) (ref. Fig. 5) system is friction
type base isolation system developed under the auspices of
Electric de France in the year 1970. The system is
standardized for Nuclear power plants in the region of high
seismicity. The system consists of laminated Neoprene pad
topped by a lead bronze plate, which is in frictional contact
with steel plate anchored to the base raft of the structure.
Therefore its cross section is similar to the LRB system.
The neoprene pad has very low displacement capacity (5
cm approx.) and when this capacity is exceeded, the sliding
element provides the needed movement. The system does
not include any restoring device and hence permanent
displacement could occur. The system has been
implemented in nuclear power plant at Koeberg in South
Africa.
J. High damping rubber bearing
A blend of high damping rubber is used in these bearings
(ref. Fig. 7). The compound, a high damping elastomer, is
called KL301 and is manufactured by the Bridgestone
Corporation Limited, Japan. KL301 has a shear modulus of
about 4300 kPa at very small strains, which decreases to
650 kPa at 50% strain, to 430 kPa at 100% strain and 340
kPa at 150% strain. The typical bearing made of this
rubber, consists of 20 layers of 2.2 mm thick rubber at 176
mm dia, nineteen 1mm steel shims, and 12 mm top and
bottom plates. The design axial pressure is 3.23 MPa. The
bearings were designed with flange type end plates to
provide bolted structure and foundation connection.
I. Sliding resilient- friction system
The design of sliding resilient- friction base isolator
(refer fig. 6) was proposed by Su et. al. This isolator is
combination of good features of EDF and R-FBI systems.
K. Pure friction system
ub
F
Rubber plug
mb
Top cover plate
Top connecting
plate
Teflon coated steel plates
ub
ub
Central steel rod
Rubber core
Rubber plug
Bottom cover
plate
F
(b)
(b)
mb
Bottom connecting plate
ub
(a)
(c)
Fig. 4. Resilient – friction base isolation system a) Sectional details b) Schematic
Fig. 5. Electric De-France (EDF) system (a)
diagram c) Force deformation behavior
Schematic diagram (b) Force deformation
behavior
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A pure friction type base isolator consists of developing
frictional force by providing a sand layer or rollers at the
base, which will dissipate the energy of earthquake force.
The system is developed in China for low-rise structures.
The system is useful for wide range of frequency input.
The main advantage of this isolation device is that it is
very cheap. The main problem with the system is that it is
unable to recover the displacement after earthquakes and
sand layer is very sensitive for foundation settlement. [1].
name of GERB was developed with large helical steel
springs having flexibility both in horizontal and vertical
direction. The vertical frequency of the system was 3 - 5
times the horizontal frequency. The steel springs were used
with GERB visco damper.
The system has been used in two steel framed houses in
santa Monica, California. These houses were strongly
affected by the 1994 Northridge earthquake. The response
of these buildings was monitored and it was not effective in
reducing the accelerations in these buildings due to rocking
motion.
L. Friction pendulum system
Friction pendulum system (ref. Fig. 8) uses geometry
and gravity to achieve the desired seismic isolation. It is
based on well-known engineering principles of pendulum
motion. The structure supported by the FPS responds to the
earthquake motions with small pendulum motions. The
friction damping absorbs the earthquake energy. There are
variety of friction pendulum system developed by various
researchers such as, variable frequency pendulum isolators
by pranesh & sinha, 2000, variable curvature pendulum
systems by Tsai et al, 2003, sliding concave foundation by
Hamidi et al., 2003, double concave friction pendulum
system by Fenz and Constantinou, 2006, Triple friction
pendulum bearing, Fenz and Constantinou 2008. Friction
pendulum system is very efficient and cost effective
seismic protection device, which simply alter the force
response characteristics of the structure at base isolation
level.
N. Sleeved pile isolation system [4]
Where foundation soil is very soft up to large depths
and provision of pile foundation is necessary, sleeved pile
isolation system is useful from earthquake considerations.
The system consists of providing a casing around the pile
and a gap is maintained between the pile and the casing to
accommodate the sway of the pile under earthquake load.
The pile is passed through the soft soil and is supported and
anchored in the rock below.
This system was implemented in the Union house in
Auckland, New Zealand in the year 1983. The building is
12 storeys tall and is supported on piles through soft soil
for depth of 10m enclosed in steel casing. The period of the
building on the sleeved pile system is 4 seconds.
O. Rocking systems [4]
Tall slender structures, having heavy mass at the top,
will invariably develop overturning moments which will
lead to development of tensions in the foundations. It is
extremely difficult to provide tension capacity in the
foundations when foundations are in weak soil and
providing anchors is a costly affair. As a remedy to this
M. Spring type systems [4]
Elastomeric and sliding isolation systems are effective
in isolating the structure from horizontal forces. When
three dimensional isolation is required, spring type systems
have been used. The spring type system under the brand
ub
F
Top circular plate
Bolt hole
mb
F
ub
ub
Bottom circular plate KL 301 rubber
(a)
(b)
Fig. 6. Sliding resilient friction system (a) Schematic
diagram (b) Force deformation behavior
(a)
( b)
Fig. 7. High damping rubber bearing (a) Sectional details b) Force deformation
behavior
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problem, it is possible to allow lifting of columns or piers
from the foundation. This type of partial isolation will
reduce the earthquake loads throughout the structure.
This concept was implemented in a railway bridge on
south Rangitikei river in New Zealand in the year 1972. It
has 69m long pier, which has been designed to lift under
the earthquake load. Two large energy dissipating devices
that are based on the elastic-plastic torsion of mild steel
bars have been provided inside each pier. The method is
not used again probably due to complexities involved in
analysis and design of the system.
0.15. The authors have suggested arrangements as shown in
fig 9 except the energy dissipating devices.
Q. BS cushion [6]
In 1999 a new kind of base isolator called BS cushion
was invented (Chinese Patent Number ZL99202381.5) in
Hangzhou, China. It is Treated Asphalt-Fiber Seismic Base
Isolation Cushion”. The advantage of this kind of isolator is
its low cost and safety while its isolation effect is moderate.
The invention of BS cushion reminds laminated steelplate rubber bearing. Fiber and treated asphalt in BS
cushion play similar role as of steel-plate and rubber in
laminated rubber bearing respectively. Before 2001 two 7storey masonry-concrete residential buildings isolated with
BS cushion were built in Hangzhou, China. One is isolated
by replacing some depth of base soil under mattress
foundation with alternative setting of 4 layers of BS
cushion and 4 layers of sand. The fundamental period of
this building is elongated from 0.3 second to 1 second (0.3s
is tested from a similar building and 1s is tested from this
building).
P. Base isolation using Geo- Synthetic materials [5]
M.K. Yegian and U. Kadakal have developed a
technique of isolating the base of the structures using geosynthetic material. They have used high strength, non
woven geotextaile placed over an ultra high molecular
weight polyethelene (UHMWPE) liner. These two
materials have a static friction co-efficient of 0.1 and a
dynamic friction coefficient of 0.07. Thus a geo-synthetic
material placed underneath a foundation of a structure and
over a liner will allow the dissipation of earthquake energy
in sliding friction. They suggested that the sliding friction
between the two materials should be in the range of 0.05 to
Self lubricating bearing material
IV. HYBRID ISOLATION SYSTEM WITH SEMI-ACTIVE
DEVICES [10]
Articulated slider
Stainless steel
concave surface
Roller
(a)
(b)
Fig. 8. Friction Pendulum Base Isolator (a) Friction pendulum system (b) Roller pendulum system
443
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Hybrid isolation system uses both passive isolation systems
and semi-active / active controlling devices. The Medical
Smooth synthetic
liner
Centre of the Italian Navy at Ancona, Italy, was selected
with the aim of analysing the behavior of a hybrid system
Gap / Energy dissipating
device
Ground
Fig. 10. Visco - Elastic Damper
Fig. 9. Use of energy dissipating devices at base level
Structure
X- Plates
Fixed lug
connecting lug
Fig. 11. Elasto-plastic damper
SMA wires
Fig. 12. Lead Extrusion damper
Fig. 13. Tuned liquid damper
Outer cylinder
W
W
W
Moving inner
cylinder
Fixed lug
Fig. 14. SMA damper
Fig. 15. Tuned Mass Damper
Sleeve
Core
Cross - section
Fig. 16. Non – buckling brace
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composed by Low Damping Rubber Bearings (LDRBs)
acting as passive seismic isolators, and Magnetorheological (MR) dampers, acting as semi-active
controlling devices. The analyses showed that significant
reduction of the building accelerations (up to 50%) can be
achieved with the hybrid system.
steel collar and surrounding concrete. This protects the
brace from buckling and allows proper dissipation of
energy in the brace through stable hysteresis loop. A
buckling restrained or core loaded or non-buckling brace
developed in IIT, Madras also works on the similar lines
and dissipates the earthquake energy. [10], [11]
Tuned Mass Damper (TMD) [2] is a spring – mass
damper device generally connected to the structure at its
top. It has been used as a passive control device for
response reduction of tall buildings.
Examples Of Isolated Structures In Different Countries –
Few examples of isolated structures are William Clayton
building, New Zealand [8], Medical Centre of the Italian
Navy (Sarvesh K. Jain And Shashi K. Thakkar, 2004,
LRB+MRD system), Nam-Han River bridge on the Youngdong expressway Seoul, Korea (Sun Young Lee, et al.,
2004, LDRB+MRD system), Experimental building at IIT,
Guwahati, India [8] etc.
The number of seismically isolated buildings in Japan,
Russia, China, USA, Italy, Armenia, New Zealand were
1600, 500, 458, 100, 27, 14 and 11 respectively up to
December 2002, 2003 [13] and every year the number of
isolated structures are increasing.
V. HYBRID BASE ISOLATION WITH PASSIVE ENERGY
DISSIPATERS
The energy dissipating devices (ref. Fig. 9 to 16) mainly
dissipate the earthquake energy and thereby reduce the
effect of the earthquake on the structure. These devices can
be used at the base of the structure or in superstructure at
appropriate locations. They can be used in combination
with passive base isolation techniques. The different
devices developed world over are shown in Fig. 9 to 16.
Structure responses can be controlled by using ViscoElastic dampers (VEDs) [2], which are made of linear
springs and dash pots provided in parallel and are generally
used in bracings of building frame or at ground level.
Elasto-Plastic Dampers (EPDs) [8],[18], [19] are made
of number of small ‘X’ shaped plates, which yield at small
deformation thereby dissipate high amount of energy.
Lead Extrusion dampers (LEDs) [8] work on the
principle of extrusion of lead. It absorbs vibration energy
by plastic deformation of the lead, during which
mechanical energy is converted into heat, lead gets heated
up and on being extruded, lead re-crystallizes immediately
and recovers its original mechanical properties before next
extrusion
Tuned Liquid dampers (TLDs) [8] are rigid wall
containers filled up to required height with a liquid
(generally water) to match the sloshing frequency of the
liquid with that of the structure. These containers are
generally placed on the top of the structure. The vibration
energy is dissipated in the sloshing action of the liquid.
Shape Memory Alloy Dampers (SMADs) [8], [18], [19]
made of nickel-titanium (Ni-Ti) alloy wires has an
interesting pseudo-elastic property by which the alloy
regains its initial shape when external load is removed.
This property is useful in putting back the structure to its
original shape. Also it can sustain large amount of inelastic
deformation.
An un-bonded brace, a technique developed in Japan,
consists of developing a brace which is prevented from
buckling by way of providing a metal collar filled with
concrete at the center of brace and a thin layer of viscous
material which allows slip and Poisson’s ratio expansion at
the slip surface provide relative movement between the
VI. DYNAMIC ANALYSIS OF BASE ISOLATED STRUCTURES
[9]
Generally the base isolated buildings are designed such
that the superstructure above isolators and base structure
below isolators remain elastic and non-linearity is
contained within the isolators. The equations of motion
used are as follows –
Mu  Cu  Ku  MR(ug  ub )
(1)
R T M (u  R(ug  ub )  M b (ug  ub )
 Cb u b  K b u b  f  f c  0
(2)
Where M, C and K are super structure mass, damping and
stiffness matrices respectively in the fixed base condition,
R – influence matrix ; ü, ů and u represent the floor
acceleration,
velocity
and
displacement
vectors
respectively, relative to the base; üb – vector of base
acceleration relative to the ground; üg – vector of absolute
ground acceleration; Mb – Diagonal mass matrix of the
rigid base; Cb – Resultant damping matrix of linear elastic
isolation elements and f – vector containing the forces
mobilised in the isolation bearings and devices and fc –
control forces (null vector in case of passive control)
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TABLE I
where ke = pre-yield stiffness, kp = post-yield stiffness, uy is
the yield displacement, zx and zy are dimensionless
hysteretic variables defined by Park et al.
NEHRP RECOMMENDATIONS
Earthquake Ground Motion Level
Risk Category
Minor (MMS
Intensity ≤ V)
Moderate
(MMS
Intensity >
V ≤ VII)
Major
(MMS
Intensity >
VII)
Life Safety a
F, I
F, I
F, I
F, I
F, I
I
F, I
I
I
Structural
damage b
The force generated in the sliding bearings can be modeled
by a visco-plastic model as
Non structural
damage c
(contents
damage)
a
b
c
f x  k p u x  uNz x
(5)
f y  k p u y  uNz y
(6)
where μ is the coefficient of friction and N is the average
normal force at the bearing.
Loss of life or serious injury is not expected.
VII. CODAL PROVISIONS FOR BASE ISOLATED
STRUCTURES – FEMA 450 [14], [15]
Significant structural damage is not expected.
Significant nonstructural (contents) damage is not expected.
As of now codes of many countries do not have
provision of guidelines for design of base isolated
structures. However countries like US have developed
guidelines for the design of base isolated structures which
contain in FEMA (Federal Emergency Management
Agency) 450. Some of the provisions are given in Table 1
which shows Protection Provided by NEHRP
Recommended Provisions for Minor, Moderate and Major
Levels of Earthquake Ground Motion.Lower limit bounds
for different properties of isolators are given in table 2.
F Indicates fixed base, I indicates isolated.
The force generated in the laminated bearings can be
modeled by a visco-plastic model as
fx  k p u x  (k e  k p )u y z x
(3)
f y  k p u y  (k e  k p )u y z y
(4)
TABLE II
LOWER LIMIT BOUNDS FOR DIFFERENT PROPERTIES OF ISOLATORS
Design Parameter
ELF procedure
Design displacement - DD
DD = (g/4π2)(SDiTD/BD)
Total design displacement - DT
DT > 1.1 D
Maximum displacement - DM
DM = (g/4π2)(SMITM/BM)
Total Maximum displacement - DTM > 1.1 DM
DTM
Design shear - Vb
Vb =kDmax DD
(at or below the isolation system)
Design shear Vs
Vs = kDmax DD/RI
(regular super structure)
Design shear Vs
Vs = kDmax DDRI
(irregular super structure)
Drift (Calculated using RI for Cd)
0.015hsx
446
Dynamic properties
Response
Response
spectrum
history
> 0.9 DT
> 0.9 DT
-
-
> 0.8 DTM
> 0.8 DTM
> 0.9 Vb
> 0.9 Vb
> 0.8 Vs
> 0.6 Vs
> 1.0 Vs
> 0.8 Vs
0.015hsx
0.020hsx
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Where –
DD = The design displacement at the center of rigidity of
the isolation system
DM = Maximum displacement at the centre of rigidity of
the isolation system
g = acceleration due to gravity
SDI = Design 5% damped spectral acceleration parameter
at one Second period as determined in Chapter 3
SMI = Design 5% damped spectral acceleration
parameter at one Second period as determined in Chapter 3
T = Effective period of seismically isolated structure at
the design displacement in the direction under
consideration
BD & BM = Numerical co-efficient related to the
effective damping of the isolation system at the design and
maximum displacements.
W = Seismic weight above the isolation interface
kDmax = Maximum effective stiffness of the isolation
system at the design displacement in the horizontal
direction under consideration.
RI = Numerical co-efficient related to the type of seismic
force resisting system above isolation system which takes
the values of 1.0 ≤ RI = 3/8 R ≤ 2.0 as defined in the
code.
forces. Hence a question is generally raised on the
effectiveness of base isolation in case of soft soil
foundation. Kelly and others after performing some
experiments have concluded that the base isolation on soft
soils can be effective in case the load on the isolator is
large and the sizes of the isolation system are sufficiently
large to accommodate the large displacements.
XI. BASE ISOLATION FOR NEAR FAULT MOTION
A number of accelerograms recorded in the recent past
at near fault locations raise a question about the
effectiveness of isolation system due to mainly two reasons
–
The ground motion normal to fault trace is richer in long
period spectral components than that parallel to the fault.
The fault normal and fault parallel motions are more or less
un-correlated. The fault parallel motions often exhibit
higher spectral acceleration components at short period
than the fault normal motion. The resultant maximum
displacement is due to the normal component of the near
fault motion. The contributions from the parallel
components in the near fault motion may be ignored.
The second aspect of the near field ground motion that
strongly impacts the isolation system is the presence of
long duration pulses. The ground motion may have one or
more displacement pulses, which have peak velocities of
the order of 0.5m/sec and the duration in the range of 1- 3
VIII. TESTING OF ISOLATORS Code requires that at least two full sized specimens of
each type of isolator be tested. The tests required are a
specified sequence of horizontal cycles under DL + 0.5LL
from small horizontal displacements up to DTM. In addition,
tests are also carried out for the maximum vertical load
1.2DL + 0.5LL + Emax and for the minimum load 0.8DL –
Emin where Emax and Emin are the maximum downward
and upward load on the isolator that can be generated by an
earthquake.
1.2
1.0
G/Gmax
0.8
IX. SUITABILITY OF SEISMIC BASE ISOLATION [9]
Implementations of base isolation techniques are
generally effective in conditions – a) The soil strata on
which structure is standing does not produce long period
ground motion. b) The structure is fairly squat with
sufficiently high column loads. c) The site permits
horizontal displacement at base of the structure of the order
of 200mm or more. d) Lateral loads due to wind are less
than approximately 10% of the weight of the structure.
0.6
0.4
Modulus reduction curve
0.2
0.0
1E-4
1E-3
0.01
0.1
1
10
Shear Strain(%)
Fig.17 – Modulus reduction curve
secs. The pulses will have a large impact on isolators and
may lead to large displacements of isolator. The large
isplacements can be accommodated by providing large
isolators.
X. BASE ISOLATION ON SOFT SOILS
Soft soils produce long period waves on structures and a
structure with long period will attract lesser earthquake
447
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A comparative study of performance of various isolation
systems has been carried out by Jangid and Kelly and they
have concluded that EDF type isolation system is optimum
choice of design for site located near faults. [9]
the soil is comparable, then soil may contribute to the
building behavior.
XIII. FUTURE TRENDS IN BASE ISOLATION
Many of the base isolation techniques described above
involve the materials, which are susceptible to deterioration
with time. . Regular inspection and maintenance of the
system is required. Special measures need to be taken for
fire protection. As such it is desirable to develop such an
isolator, which has a life span equal to the life of a
structure, free from effects of environment and fire. Also it
should be free from maintenance. Hence it will be an ideal
case if researchers develop an isolator using materials
which are unaffected by environment or affected by it to
very low extent like natural earth of specific qualities
having inherent properties of spring action and friction.
The equivalent spring constants and damping coefficient for foundations resting on soil can be worked out
using equations given in the table 3. Equations for damping
accounts for material as well as radiation effects.
XII. BASE ISOLATION BUILDINGS WITH SSI [9]
The influence of Soil Structure Interaction (SSI) and
possible effects of building and foundation rocking have
TABLE III
SPRING CONSTANTS AND DAMPING COEFFICIENTS FOR FOUNDATIONS
ON HOMOGENEOUS HALF-SPACE [16]
Direction
of
Equivalent Spring
Constant for
Rectangular
Foundation
Equivalent Damping
Horizontal
KH = 2(1+ ν)G
βX√(BL)
CH =C1KHR√( ρ/G)
Rocking
KR = (G/(I- ν)) βψ
BL2
CR =C2KRR√( ρ/G)
Vertical
Kν=(G/(I-ν))βν√(BL)
CV =C3KVR√( ρ/G)
Torsion
-
CT = √(KTIT )/(1+2IT
/ΡR5)
Motion
Coefficient
From the above equations which are valid for low
strains, it can be seen that the spring stiffness is more for
large size foundations and for greater value of shear
modulus G. Also it is dependent on Poisson’s ratio of soil.
Thus the spring values of the soil can be altered by varying
the above parameters by choosing soil of appropriate
properties. In similar way the damping properties of the
soil medium can be altered. Also the effect of damping and
isolation can be obtained by allowing the structure to slide
on a soil medium to required extent. However, in the case
of using soft geological materials such as soil, sand,
pulversied granite etc, the material will see large strains.
Typically for a soil the variation of G with strain is shown
in the fig 17. It is evident from the graph that during the
non-linear behavior of the soil, the G value decreases for
larger strains which in turn will have lesser soil stiffness
and increased damping leading to enhanced isolation effect.
The increased damping with strains as shown in the figure
will allow to control the displacements.
Where Ρ = mass density of soil
Vs = Shear wave velocity of soil medium
G = ρ Vs2
ν = Poisson’s ratio of soil medium
R = equivalent radius for rectangular foundation (R= √BL/π for
translation and R= 4√4BL3 / 3π for rocking).
B = width of the foundation perpendicular to the direction of
horizontal excitation
L = length of the foundation in the direction of horizontal
excitation
I0 = total mass moment of inertia of structure and foundation
about the rocking axis at the base
IT = polar mass moment of inertia of structure and foundation
βx, βψ, and βv are constants depending on ratio L/B
C1 = 0.5 ; C 2 =0.30/(1+ βψ); C3 = 0.8
been examined by Novak M. and Henderson P. The effect
of soil structure interaction on the modal properties and
seismic forces is small when isolators are much more
flexible than soil. However if the flexibility of isolator and
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conditions varies largely. The properties of the soil can be
altered by providing geo-membrane in the soil [5]. The cost
of soil is meager and needs no maintenance cost. Knowing
these facts, clean river sand was tried as base isolation
material and experiments were conducted on a RCC frame
model having raft foundation and isolated footing as
shown in Fig.18.
River sand, having engineering properties shown in Table 5
[20], is used in the experiments.
Properties of different types of soils occurring in the
nature are given in table 4.
XIV. CASE STUDY
In the past, experiments have been performed on
structures resting on sand layer [9], Such systems dissipate
earthquake energy before reaching to the structure and are
useful for wide range of frequencies. Varieties of soil are
available in abundance in nature and each type of soil
exhibit different properties. The same soil exhibits different
properties in dry and wet conditions or static and dynamic
conditions. The shear modulus in static and that in dynamic
The size of the model is 1.2 x 1.2 x 1.5 m and the raft
model is provided with a base raft of size 1.5 x 1.5 x 0.1 m
and isolated footing model is provided with footings of size
0.35 x 0.35 x 0.1 m. The size of the cross section of beams
TABLE IV
REPRESENTATIVE VALUES OF ANGLE OF FRICTION FOR SANDS AND SILTS AND REPRESENTATIVE VALUES OF COHESION FOR CLAYS.
Sr.
no.
Soil (sands and silts)
1.
2.
3.
4.
5.
Sand, round grains, uniform
Sand, angular, well graded
Sandy gravels
Silty sand
Inorganic silt
Effective angle
of friction - degrees
27 to 34
33 to 45
35 to 50
27 to 34
27 to 35
Sr.
no.
I.
II.
III.
IV.
V.
Soil (Clay)
Very soft clay
Soft to medium clay
Silt clay
Very stiff
Hard
Fig 18b - Raft footing model
Fig 18a - Isolated footing model
449
Cohesion
in kN/sqm.
< 12
12 – 25
50 – 100
100 – 200
> 200
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sand layer. The locations of the accelerometers fixed on the
TABLE V
RIVER SAND PROPERTIES
Property
Value
Property
Value
Gravel
5.6%
Cu
7.37
Coarse sand
19.4%
Cc
1.12
Medium Sand
53%
γdmax
1.71g/cc
Fine Sand
17.6%
γdmin
1.40g/cc
Silt + Clay
4.4%
Sp.gravity
2.66
D60
1.18mm
emax
0.77
D10
0.16mm
emin
0.48
D30
0.46mm
Fig. 19a – Acceleration time history for A6 signal for FXRAF
and RAFDRY cases for zone III of IS 1893 excitation.
and columns is 0.1 x 0.1 m. and roof slab thickness is 0.05
m. A metallic box of size 2.0 m x 2.0 m x 1.2 m height is
used and is filled with 300 mm thick layer of the dry
sand(RAFDRY/ISODRY) / wet sand (RAFWET /
Fig. 19b – Acceleration time history for A6 signal for FXRAF
and RDSGM cases for zone III of IS 1893 excitation.
Fig. 19c – Acceleration time history for A6 signal for FXRAF
and RAFWET cases for zone III of IS 1893 excitation.
Z
Y
X
Fig.18c - RCC model with raft base
with accelerometer locations
ISOWET)
/
dry
sand
with
geo-membrane
(RDSGM/IDSGM)
/ wet sand with geo-membrane
(RAFWETG / ISOWETG) in separate cases. The box is
fixed on tri-axial shake table and RCC model is kept on the
Fig. 19d – Acceleration time history for A6 signal for FXRAF
and RAFWETG cases for zone III of IS 1893 excitation.
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model are indicated in Fig. 18c. Different tests were
conducted on the model.
The initial research studies show encouraging results.
The comparison of acceleration responses recorded at roof
level accelerometer A6 for raft model FXRAF and
RAFDRY, RDSGM, RAFWET and RAFWETG cases for
zone III excitation level of IS 1893 are shown in Fig. 19a to
19d. Also responses for isolated footing model FXISO are
compared with ISODRY, IDSGM, ISOWET and
ISOWETG cases for zone V excitation of IS 1893 and are
shown in Fig.21a to 21d.
The response acceleration
comparison shows that the peak accelerations are less in
case of models on sand layer as compared to fixed base
case.
The FFTs for raft cases (Fig. 20a) as well isolated
footing (Fig 20b) cases clearly indicates that there is a
reduction in the natural frequency of the model
XV. DISCUSSIONS AND CONCLUSIONS
The above studies reveal that the provision of sand layer
below base of structure reduces frequency of the structure
and reduction in frequency is more with more excitation
Fig. 20a - FFTs showing comparison of frequencies of
RAFDRY and R DSGM cases with FXRAF case for
zone III of IS 1893 excitation.
Fig. 20b - FFTs showing comparison of frequencies of
RAFWET and RAFWETG cases with FXRAF case for
zone III of IS 1893 excitation.
.
Fig 22a - FFTs showing comparison of frequencies of
ISODRY and IDSGM cases with FXISO case for zone V
of IS 1893 excitation.
Fig. 21b - FFTs showing comparison of frequencies of
ISOWETG case with FXISO case for zone V of IS 1893
excitation.
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force. As the frequencies are reduced, the response of the
structure also reduces. The part of the excitation energy
gets dissipated in the movement and friction of the sand
layer, thereby the energy reaching to the structure reduces
giving lesser accelerations to the structure, which is
observed from acceleration response comparisons of fixed
base structure and structure on sand layer.
Acceleration
The present study is based on sand layer of 300 mm
thickness. However, the experiments can be performed by
varying the thickness of layer and different types of soil.
ACKNOWLEDGEMENTS
The authors are grateful to the Management of Heavy
Water Board, Bhabha Atomic Research Center, Central
Power Research Institute and National Institute of
Technology, Suratkal, India for permitting and supporting
the above study.
Time
Fig. 21a – Acceleration time history for A6 signal for FXRAF
and ISODRY cases for zone V of IS 1893 excitation
Acceleration
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