Structures - Semantic Scholar · base isolation device is to attenuate the horizontal acceleration...

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 7, July 2012)

438

State Of Art Review - Base Isolation Systems For

Structures S.J.Patil

1, G.R.Reddy

2

1Heavy Water Board, Mumbai, India .

2Bhabha Atomic Research Centre, Mumbai, India .

[email protected]

[email protected]

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.

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

0.0 0.5 1.0 1.5 2.0 2.50.0

0.2

0.4

0.6

0.8

1.0

1.2

Fig.5 Regions of response control and Base Isolation

Direction of

response control

along increse in damping

Region of

Base isolation

Envelop spectra with o.2 g ZPA

1%

2%

3%

4%

5%

7%

10%

Res

ponse

Acc

eler

atio

n i

n g

unit

s

Time (Sec)

Region in which

base isolated structure lies

Fig. 1 Concepts of Base Isolation and Dampers



439

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.

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

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]

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.

Fig. 2. Laminated rubber bearing system - a) Sectional details b) Schematic diagram c) Force deformation behavior

ub

F

Top cover plate

Steel plates rubber

Bottom Cover plate

(a) (c) (b)

mb

ub



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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.

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.

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.

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 -

1. Effective isolation is achieved. It will decrease the

structural response to 1/2 -1/8 of the traditional structural

response.

2. Stable character of isolators over a long working life.

3. Recovery of the displacement after earthquakes.

4. Vertical tension capacity is good.

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.

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.

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

F

ub

(c)

mb

ub

(b)

Steel plates

Top cover plate

rubber

(a)

Lead core

Bottom cover

plate

Fig. 3. New Zealand bearing system (a) Sectional details (b) Schematic diagram (c) Force deformation behavior



441

sliding resistance and a central core of rubber which will

have beneficial effect of resilience of a rubber.

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.

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.

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.

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.

K. Pure friction system

ub

F

(c)

ub

mb

(b)

Rubber plug Top cover plate

Top connecting

plate Teflon coated steel plates

Central steel rod

Rubber core

Rubber plug

Bottom cover

plate

Bottom connecting plate

Fig. 4. Resilient – friction base isolation system a) Sectional details b) Schematic

diagram c) Force deformation behavior

ub

F

(b)

mb

ub

(a) Fig. 5. Electric De-France (EDF) system (a)

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].

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.

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

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.

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

( b)

ub

F

(a)

KL 301 rubber Bottom circular plate

Top circular plate Bolt hole

Fig. 7. High damping rubber bearing (a) Sectional details b) Force deformation

behavior

Fig. 6. Sliding resilient friction system (a) Schematic

diagram (b) Force deformation behavior

(b)

ub

F

(a)

ub

mb



443

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.

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 geo-

synthetic 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

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 steel-

plate 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 7-

storey 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).

IV. HYBRID ISOLATION SYSTEM WITH SEMI-ACTIVE

DEVICES [10]

(b)

Stainless steel

concave surface

Articulated slider Self lubricating bearing material

(a)

Fig. 8. Friction Pendulum Base Isolator (a) Friction pendulum system (b) Roller pendulum system

Roller



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Hybrid isolation system uses both passive isolation systems

and semi-active / active controlling devices. The Medical

Centre of the Italian Navy at Ancona, Italy, was selected

with the aim of analysing the behavior of a hybrid system

Fig. 11. Elasto-plastic damper

connecting lug

X- Plates

Fixed lug

Fig. 12. Lead Extrusion damper

Structure

Fig. 13. Tuned liquid damper

Cross - section

Core

Sleeve

Fig. 16. Non – buckling brace

Moving inner

cylinder

SMA wires Outer cylinder

Fixed lug

Fig. 14. SMA damper

W

W

W

Fig. 15. Tuned Mass Damper

Ground

Gap / Energy dissipating

device

Fig. 9. Use of energy dissipating devices at base level Fig. 10. Visco - Elastic Damper

Smooth synthetic

liner



445

composed by Low Damping Rubber Bearings (LDRBs)

acting as passive seismic isolators, and Magneto-

rheological (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.

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 Visco-

Elastic 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

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 Young-

dong 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.

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 –

)( bg uuMRKuuCuM (1)

0

)()((

cbbbb

bgbbg

T

ffuKuC

uuMuuRuMR

(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)



446

The force generated in the laminated bearings can be

modeled by a visco-plastic model as

x

y

pexp zukkukfx )( (3)

y

y

peypy zukkukf )( (4)

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.

The force generated in the sliding bearings can be modeled

by a visco-plastic model as

xxpx uNzukf (5)

yypy uNzukf (6)

where μ is the coefficient of friction and N is the average

normal force at the bearing.

VII. CODAL PROVISIONS FOR BASE ISOLATED

STRUCTURES – FEMA 450 [14], [15]

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.

TABLE I

NEHRP RECOMMENDATIONS

Risk Category

Earthquake Ground Motion Level

Minor (MMS

Intensity ≤ V)

Moderate

(MMS

Intensity >

V ≤ VII)

Major

(MMS

Intensity >

VII)

Life Safety a F, I F, I F, I

Structural

damage b F, I F, I I

Non structural

damage c

(contents damage)

F, I I I

a Loss of life or serious injury is not expected.

b Significant structural damage is not expected.

c Significant nonstructural (contents) damage is not expected.

F Indicates fixed base, I indicates isolated.

TABLE II

LOWER LIMIT BOUNDS FOR DIFFERENT PROPERTIES OF ISOLATORS

Design Parameter ELF procedure Dynamic properties

Response

spectrum

Response

history

Design displacement - DD DD = (g/4π2)(SDiTD/BD) - -

Total design displacement - DT DT > 1.1 D > 0.9 DT > 0.9 DT

Maximum displacement - DM DM = (g/4π2)(SMITM/BM) - -

Total Maximum displacement -

DTM

DTM > 1.1 DM > 0.8 DTM > 0.8 DTM

Design shear - Vb

(at or below the isolation system)

Vb =kDmax DD > 0.9 Vb > 0.9 Vb

Design shear Vs

(regular super structure)

Vs = kDmax DD/RI > 0.8 Vs > 0.6 Vs

Design shear Vs

(irregular super structure)

Vs = kDmax DDRI > 1.0 Vs > 0.8 Vs

Drift (Calculated using RI for Cd) 0.015hsx 0.015hsx 0.020hsx



447

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.

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.

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.

X. BASE ISOLATION ON SOFT SOILS

Soft soils produce long period waves on structures and a

structure with long period will attract lesser earthquake

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

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.

1E-4 1E-3 0.01 0.1 1 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

G/G

ma

x

Shear Strain(%)

Modulus reduction curve

Fig.17 – Modulus reduction curve



448

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]

XII. BASE ISOLATION BUILDINGS WITH SSI [9]

The influence of Soil Structure Interaction (SSI) and

possible effects of building and foundation rocking have

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

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 co-

efficient 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.

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.

TABLE III

SPRING CONSTANTS AND DAMPING COEFFICIENTS FOR FOUNDATIONS

ON HOMOGENEOUS HALF-SPACE [16]

Direction

of

Motion

Equivalent Spring

Constant for

Rectangular

Foundation

Equivalent Damping

Coefficient

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)

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



449

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

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.

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) Effective angle

of friction - degrees

Sr.

no.

Soil (Clay) Cohesion

in kN/sqm.

1. Sand, round grains, uniform 27 to 34

I. Very soft clay < 12

2. Sand, angular, well graded 33 to 45 II. Soft to medium clay 12 – 25

3. Sandy gravels 35 to 50 III. Silt clay 50 – 100

4. Silty sand 27 to 34 IV. Very stiff 100 – 200

5. Inorganic silt 27 to 35 V. Hard > 200

Fig 18b - Raft footing model Fig 18a - Isolated footing model



450

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 /

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

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.18c - RCC model with raft base

with accelerometer locations

Z Y X

Fig. 19d – Acceleration time history for A6 signal for FXRAF

and RAFWETG cases for zone III of IS 1893 excitation.

Fig. 19a – Acceleration time history for A6 signal for FXRAF

and RAFDRY cases for zone III of IS 1893 excitation.

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.



451

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.



452

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.

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.

References

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Fig. 21b – Acceleration time history for A6 signal for FXRAF and

IDSGM cases for zone V of IS 1893 excitation.

Time

Acc

eler

atio

n

Fig. 21a – Acceleration time history for A6 signal for FXRAF

and ISODRY cases for zone V of IS 1893 excitation

Acc

eler

atio

n

Time

Fig. 21c – Acceleration time history for A6 signal for FXRAF and

RAFDRY cases for zone V of IS 1893 excitaion.

Acc

eler

atio

n

Time

Fig. 21d – Acceleration time history for A6 signal for FXRAF

and ISOWETG cases for zone V of IS 1893 excitation

Acc

eler

atio

n

Time



453

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