rubber used in seismic technology

8/22/2019 Rubber used in Seismic Technology

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Rubber Used In Seismic Technology

INTRODUCTION

Vibration is what one feels upon putting ones hand on the hook of a car with

the engine running or on the base of a running motor. Vibration is a magnitude that

oscillates about a mean reference.

Shock is defined as a motion in which there is a sharp sudden change in

velocity. Shock usually entails a single impulse of energy of short duration and large

acceleration. Examples are blows of a hammer or a box falling to the ground from

certain height. Vibration needs isolation but shocks need damping.

We come across vibration in everyday life like that produced by passing by

trains, high velocity wind on large buildings and recently we also experienced the

havoc of the mightiest vibrations nature can produce THE EARTHQUAKE

TREMORS. Mankind has faced and has learnt, to survive against the odds of

nature, for generations.

Usually isolators and shock absorbers (dampers) are used to provide

protection against various vibrations and shocks that we come across in everyday life

and industries like shocks in machine mounts and vibration protection of tape and

CD player in the cars.

Rubber is found to be of profound utility in this area due to its internal

structural properties. Especially natural rubber has proved to be excellent in the field

of damping both technically and economically. Recent developments in seismic

technology has larger role for seismic natural rubber loading and bridge bearings.

The designs are improving day by day as the basic mechanism of how rubber works

C.O.E. & T., Akola 1


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comes to light and by the use of other polymeric fibres like aramide fibres, glass

fibres, carbon fibres..etc.

There are now 400 structures by seismic isolation in 17 countries. 115 are in

U.S. In new six story building at the Martin Luther King/Drew Medical center in Los

Angeles, California, Damper introduced structure (DIS) fabricated the largest High

Damping Rubber (HDR) isolators used to date, in U.S. at 40 in diameter and 20

high, each unit weights nearly two tons with attachments plates. Seventy isolators

make this one of the largest installed systems in U.S.

HOW EARTHQUACK OCCURE?

Seismic Waves

When you throw a stone into a quiet body of water waves are created which

move out from the point of impact. Energy is being propagated along these paths and

as it moves some of the energy is lost. The further the wave travels the lower its

energy. Earthquake waves (or the waves produced when an artificial quake is

created) can be subdivided into two types:

Body Waves

There are two types of waves which travel within the body of the Earth.

1) P waves are sometimes called compressional waves orprimary waves orpush-

pull waves and they are transmitted (propagated by movements of the material in the

Earth parallel to the direction in which the wave is moving.



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(Compressional Waves)

Imagine a line of stationary cars waiting to enter a freeway. Each car is

connected to posts on the side of the road by very strongrubber bands. A speeding

car strikes the last car in line. That car moves forward striking the car in front of it,

which moves forward and so on. The rubber band allows each struck car to move a

short distance forward before the carsnaps back. Another speeding car impacts the

end of the line. If you were to stand off to one side and observe the process you

would see the individual cars oscillating about an average position where the

direction of movement of the cars is parallel to the direction of the wave.

The following movie is quite large and may not be practical with a relatively

slow modem. Another representation of the propagation of a P wave

2) S waves are also called shear waves orsecondary waves and they propagate by

movements of the Earth perpendicular to the direction in which the wave is moving.

(Shear Waves)

Take a piece of rope 12 feet long. Color red foot long section in the near one

end of the rope. Tie one end of the rope to a fixed object. Standing about 10 feet

from the object, shake the end of the rope with an up and down motion. A standing

wave with peaks and valleys is created. If you stand off to one side, you will see the



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red section move up and down whereas the wave itself moves toward the fixed

object.

Another representation of the propagation of an S wave

3) The velocity of a P or S wave is a function of the physical properties of the rock

the wave is traveling through. Different rocks have different physical characteristics

(such as how compressible they are and how the respond to shear stresses) and

therefore different P and S wave velocities. In fact, if the velocity of the wave can be

measured, it may be possible to predict the type of rock the wave traveled through -

indirect detection of rock type!

The velocity of a P wave is proportional to:

a) Velocity P wave ~ ((B + G)/Density), where:

B = the bulk modulus - the resistance to change in volume

G = the Shear modulus - the resistance to change in shape

Density = mass/volume

b) Velocity S wave = (G/Density)

From equations (a) and (b) we can formulate some important generalizations about

these earthquake waves.



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a) Note that Density appears in the denominator of both (1) and (2). If the

numerators of these expressions are held constant, we would predict that the

velocities of both waves would decrease with increasing depth of penetration since

we know that pressure and density increase with increasing depth. However,

observations show that both waves speed up with increasing depth. Therefore, the

numerators must be increasing at a greater rate than density; rocks get stronger with

increasing depth of burial. The comments above assume that both waves are

traveling in the same material.

b) The velocity of a P wave is greater than that of an S wave as the numerator of (1)

is larger than the numerator of (2).

c) If a wave passes from one material to another, the wave may speed up, slow down

or not change its velocity, depending on the contrast in properties of the two types of

material.

d) As a special case, consider a liquid. In general, liquids offer no resistance to

changes in shape so that G = 0. Therefore, a liquid will not support the transmission

(propagation) of an S wave and a P wave will slow down in a liquid.

e) When P and S waves encounter the surface of the Earth new waves are created.

These surface waves are responsible for much of the damage associated with an

earthquake. The motion of these surface waves is quite complicated. Watch the

motion associated with the propagation of a Rayleigh wave



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f) The arrival of a P wave is recorded prior to the arrival of an S wave as the P wave

has a higher velocity than that of the S wave that traveled along the same path. The

further the recording station is from the epicenter (the location on the surface of the

Earth that is closest to the focus), the longer the time between the arrival of the P

wave and the arrival of the S wave.

Behaviour of P waves , S waves and surface waves

arrival of arrival of arrival of surface wave

p wave s - waveNoise

In the animation above you are watching a seismograph located an unknown

distance from the epicenter of an earthquake. In the absence of the arrival of

earthquake vibrations - Noise - reach the recording station. These could be due to

traffic, the wind, or other natural or man-made sources of vibrations. Time is being

recorded on the X-axis (horizontal) of the plot shown in the animation and the

displacement of the seismograph recording is recorded on the Y-axis (vertical). Note

that the P wave is recorded first. At a later time the first S wave is recorded and at a

still later time the surface waves begin arriving at the recording station. The further



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away the station is from the epicenter, the greater the difference in arrival times

between the P and S waves.

GENERAL REQUIREMENTS FOR DESIGNING

DAMPERS

Natural Frequency :

All mounting systems have a natural frequency (fn) the frequency at which

the system will oscillate if it is displaced from its static position and released.

Damping :

Controlling the natural frequency provides one means to control vibration.

Damping provides another. Damping is the dissipation of energy, usually by

releasing it in the form of low-grade heat

The loss factor is used to quantify the level of hysteretic damping of a

material. The loss factor ( ) is the ratio of energy dissipated from the system to the

energy stored in the system for every oscillation. A loss factor of 0.1 is generally

considered a minimum value for significant damping.

Material Approximate Loss Factor

Aluminum 0.007 - 0.005

Steel 0.05 - 0.10

Neoprene 0.1

Butyl Rubber 0.4



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Vibration Isolation

The performance of an isolation system is determined by the transmissibility

of the system the ratio of the energy going into the system to the energy coming from

the system.

Following graph indicates dissipation of energy into isolation resion by low damping

and high damping rubber seismic isolator.

Transmissibility Vs Frequency Ratio

Isolation Efficiency Vs Crossover frequency Ratio



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Why rubber is best suited?

It is always worth pointing out again that rubber has no magic properties

enabling it to prevent the transmission of noise and vibration. Though this delusion

seems to be still as widespread as ever. The results obtained with rubber spring gear

units are due to the elastic and damping moduli and unexpected results generally

follow from the ways in which these moduli change with frequency. ,amplitude ,

temperature and other parameters. When the rubber spring promotes silence, or

reduces the transmission of high frequencies by comparison with steel springs, the

reason is generally that its properties do not favour the generations of sanding waves

such as are very usual in helical springs.

Rubber is preferred to steel and fluid springs because it is a more convenient,

more economical or more reliable means of achieving the result; in other words

because it is about 100,000 times more flexible than steel, has a tensile stress only

about 30 times smaller and because it has a high bulk modulus.

Following graph Indicates comparison of Low

Damping rubber & High Damping rubber (HD-MRB)



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Temperature Dependency of the Normalised Modulus

PROPERTIES OF VARIOUS TYPES OF RUBBERS

C.O.E. & T., Akola

PROPERTIES:Natural

Rubber

Styrene

Butadiene

Ethylene

PropyleneNeoprene Nitrile Silicone

Urethane

RubberFluorocarbon

Hardness Range

(Shore A)20 - 90 40 - 90 30 - 90 10 - 95 20 - 95 10 - 85 10A to 80D 50 - 95

Tear Resistance Good Fair Good Good Fair Poor Excellent Fair

Abrasive Resistance Excellent Good Good Excellent Good Poor Outstanding Good

Compression Set Good Excellent Good Fair to Good Good Fair Good Very Good

Permability to

Gases

Fair Fair Fair to Poor Low Fair Fair Fair Very Low

DiluteFair to

GoodFair to Good Excellent Very Good Good Fair Poor Excellent

ConcentratedFair to

GoodFair to Good Good Good Good Fair Poor Excellent

Aliphatic

HydrocarbonsPoor Poor Poor Good Excellent Poor Excellent Excellent

Aromatic

HydrocarbonsPoor Poor Poor Fair Good Poor Fair to Good Excellent

Oxygenated

(ketones,etc.)Good Poor Good Poor Poor Fair Poor Poor

Lacquer Solvents Poor Poor Good Poor Fair Poor Poor Poor

Swelling in

Lubricating Oil

Poor Poor Poor GoodVery

Good

Fair Excellent Excellent

Oil and Gasoline Poor Poor Poor Good Excellent Fair Excellent Excellent

Water Absorption Very Good Very Good Excellent GoodFair to

GoodGood

Good @ 70F

Poor @ 212 FVery Good

Ozone Fair Fair Excellent Excellent Fair Excellent Outstanding Excellent

Sunlight Poor Fair Very Good Very Good Poor Excellent Good Very Good

Temperature

Range (c)-55 - +90 -50 - +100 -50 - +150 -40 - +100 -40 - +100 -60 - +200 -25 - +100 -20 - =200

Major Attributes ResilienceGeneral

Purpose

General

Purpose

Oil & Gas

Resistance,

Weatherability

Oil

Resistance

Heat

Resistance

Abrasion

Resistance,

Load Bearing

Characteristics

Heat Resistant,

Excellent Solvent

& Chemical

Resistance10


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BASIC MECHANISM OF ELASTOMER DAMPERS

Damping Generation Mechanism in High-damping Rubber Seismic Isolator

We can understand the mechanism of damping generation in a high-damping

rubber as follows. There are two types of cross-link real rubber vulcanizates. Few

chemical cross-links and a large number of physical cross-links, which consist of the

molecule-molecule interaction junctions weakly, adhered to each other by molecular

attraction forces. When an external force is given to rubber vulcanizates. All

molecules begin to move to the extension direction. Although chemical cross-links

move as tightly fixed cross-line k points in the system. In the case of physical cross-

links one molecule slides on the other one and another cross-link point is reproduced

at different position on a molecule. This sliding or rubbing of molecules generates

friction heat, which is dissipated outside of the system. In unfilled rubber

vulcanizates, since the molecular accretion force in physical cross-links is very weak.

Then friction heat is negligibly small. This is the reason why hysterics energy, i.e. a

damping capacity, is small in untilled rubbers. On the other hand, when we mix

special Fillers in rubber, a tremendous number of' physical cross-links are newly

produced in the system. In which new physical cross-links have much stronger

interaction between fillers and rubber molecules compared with the molecule-



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molecule interaction . If we follow such a system, since a molecule adhering to the

surface of filler must slide on the surface against its strong attraction force, a vast

amount of friction heat will be generated. This is the mechanism of high-damping

rubbers. This can be easily understood if one considers the energy in consumed

energy in pulling up a silk thread from its bundle and u sticky thread form a mixture

of coal tar and threads.

Laminated Lead-Rubber Seismic isolator

Seismic isolation bearings isolate a structure from the ground motion

produced by an earthquake. The energy absorption devices are designed to absorb

the energy associated with an earthquake. This seismic (Base) Isolator consists of

alternate layers of rubber and steel bonded together, with a cylinder of pure lead

tightly inserted through a hole in the middle. The rubber layers allow the isolator to

easily displace sideways, reducing the earthquake loads felt by the building and its

occupants. They also act as a spring, ensuring that the structure returns to its original

position after the shaking has stopped.


Seismic isolation systemillustrated here is one ofgeneral designs of seismic

isolations. Vulcanisedrubber layers that can movein rotational direction.

Cover Rubber-Protects steel plates

Bottom Moulding plate- Integral with isolator- Connects to structure above

and below isolator

Steel Reinforcing Plates- Provides vertical load capacity- Confiness load core

Energy Dissipation core-Reducess earthquake forces anddisplacements by energy dissipation

-Provides wind resistance.Internal Rubber layersProvides lateral flexibility


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By bonding the rubber to thin layers of steel, the isolator becomes much

stiffer under vertical loads so that the structure will not move up and down during

day-to-day use.

Thick steel plates are bonded to the top and bottom surfaces to allow the

isolator to be solidly bolted to the structure above and to the foundation below. The

isolator lead core stops the structure from moving sideways under wind and other

non-seismic loads. During earthquake events, the lead is pushed sideways by the

rubber and steel layers absorbing a portion of the earthquake energy. This dampening

affect helps to further reduce the earthquake forces and help control the lateral

displacement of the structure.

Energy Dissipation vs. Without Energy Dissipation


With Energy Dissipation Inter-story driftis reduced by a factor of approximately two with

the installation of energy dissipaters.Without Energy Dissipation


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Seismic Isolation vs. Without Seismic Isolation

COST REDUCTION OF SEISMIC BASE ISOLATORS

An individual isolator can weight around 1 ton and often more. To extend this

valuable earthquake-resistant strategy to housing and commercial buildings, it is

necessary to reduce the cost and weight of the isolators.


Seismically Isolated Structure

The deformation pattern of an isolatedstructure during an earthquake. Movement

takes place at the level of the isolators. Flooraccelerations are low; the building, itsoccupants and loose contents are safe.

Conventional Structure

The deformation pattern of a conventional structureduring an earthquake. Accelerations of the ground

are amplified on the higher floors and the loosecontents are damaged. Building deformations and

distortions could be permanent


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Seismic Isolator in Foundation

The primary weight in an isolator is in the reinforcing steel plates used to

provide the vertical stiffness of the rubber-steel composite element. A typical rubber

isolator has two large end-plates [around 1 in. (25 mm) thick] and 20 thin reinforcing

plates [around 3 mm (l/8 in.) thick]. The high cost of producing the isolators results

from the labor involved in preparing the steel plates and laying-up the rubber sheets

and steel plates for vulcanization bonding in a mold. The steel plates have to be cut,

sand-blasted, acid cleaned, and coated with bonding compound. Next, the

compounded rubber sheets with the interleaved steel plates are put into a mold and

heated under pressure for several hours to complete the manufacturing process.

reduction in weight is possible as fiber materials are available with an elastic

stiffness that is of the same order as steel. Some of the fibres to be enlisted are

aramide fibres, carbon fibres , glass fibres, etC. Thus the reinforcement needed to

provide the vertical stiffness can be obtained by a similar volume of very much

lighter material. The reduction in cost may be possible if the use of fiber allows a

simpler, less labor-intensive manufacturing process. It is also possible that the

current approach of vulcanization under pressure in a mold with steam heating can

be replaced by microwave heating in an autoclave.

Another advantage to using fiber reinforcement is that it would then be

possible to build the isolators in long rectangular strips, whereby individual isolators

could be cut to the required size. In current use all isolators are either circular or



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square in the mistaken belief that for the isolation system for a building to be

isotropic, it needs to be made of isolators with a symmetrical shape. Rectangular

isolators in the form of long strips would have distinct advantages over square or

circular isolators when applied to buildings for which the lateral resisting system is

made up of walls. When isolation is applied to buildings with structural walls,

additional wall beams are needed to carry the wall from isolator to isolator. A strip

isolator would have a distinct advantage for residential housing constructed from

concrete . In modeling the isolator reinforced with steel plates, the plates are

assumed to be in extensional and rigid in flexure. The fiber reinforcement is made up

of many individual fibers grouped in strands and coiled into a cord of sub millimeter

diameter. The cords are more flexible in tension than the individual fibers, so that it

is possible that they may stretch when the isolator is loaded by the weight of a

building. On the other hand, they are completely flexible in bending, so that the

assumption that is made when modeling current isolators--that plane sections remain

plane--no longer holds. In fact, when a fiber reinforced isolator is loaded in shear, a

plane cross section becomes curved. This leads to an unexpected advantage in the

use of fiber reinforcement. When the bearing is displaced in shear, the tension in the

fiber bundle, acting on the curvature of the reinforcing sheet caused by the shear,

produces a frictional damping due to individual strands in the fiber bundle slipping

against each other. This energy dissipation in the reinforcement adds to that of the

elastomer. In fact, recent tests show that this energy dissipation is larger than that of

the elastomer. Therefore, when designing a fiber reinforced isolator for which a



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specified level of damping is required, it is not necessary to use elaborate

compounding to provide the damping, but to use the additional damping from the

fiber.

2.Vertical Stiffness of Fiber Reinforced isolator

The essential characteristic of the elastomeric isolator is the very large ratio

of the vertical stiffness relative to the horizontal stiffness. This is produced by the

reinforcing plates, which in current industry standard are thin steel plates. These

plates prevent lateral bulging of the rubber, but allow the rubber to shear freely. The

vertical stiffness can be several hundred times the horizontal stiffness. The steel

reinforcement has a similar effect on the resistance of the isolator to bending

moments, usually referred to as the "tilting stiffness". This makes the isolator stable

against large vertical loads and is an important.



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ADVANTAGES AND DISADVANTAGES OF SEISMIC

ISOLATORS

Advantages

Long Lasting

Cheaper and more effective than conventional isolator

Capacity to absorb vibrational forces including seismic forces helps to protect

buildings and machinery etc.

High loss factor

It dissipate more amount of seismic energy.

Disadvantages

High application cost

Due to high application cost cannot be applied for local building purpose.

Very high amount of Earthquake can damage the structure

PRACTICAL APPLICATION OF RUBBER SEISMIC ISOLATORS

U.S. Applications

The first base-isolated building in the United

States is the Foothill Communities Law and

Justice Center, a $30 million legal services

center in Rancho Cucamonga San Bernardino

County, about 97 km (60 miles) east of


Foothill Communities Law andJustice Center

(1985)


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downtown Los Angeles. Completed in 1985, the building is four stories high with a

full basement and sub-basement for the isolation system, which consists of 98

isolators of multilayered natural rubber bearings reinforced with steel plates. The

superstructure of the building has a structural steel frame stiffened by braced frames

in some bays.

The building is located 20 km (12 miles) from the San Andreas fault. San

Bernardino County, the first in the U.S. to have a thorough earthquake preparedness

program, asked that the building be designed for a Richter magnitude 8.3 earthquake,

the maximum credible earthquake for that site. The design selected for the isolation

system, which accounted for possible torsion, incorporated a maximum horizontal

displacement demand of 380 mm (15 in.) in the isolators at the corners of the

building. Tests of full-scale sample bearings verified this capacity.

The highly filled natural rubber from which the isolators are made, developed

as part of the EERC research program, has mechanical properties that make it ideal

for a base isolation system. The shear stiffness of this rubber is high for small strains

but decreases by a factor of about four or five as the strain increases, reaching a

minimum value at a shear strain of 50 percent. For strains greater than 100 percent,

the stiffness begins to increase again, providing a fail-safe action under a very high

load. The damping follows the same pattern but less dramatically, decreasing from an

initial value of 20 percent to a minimum of 10 percent and then increasing again. The

design of the system assumes minimum values of stiffness and damping and a linear



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response. The high initial stiffness is invoked only for wind load design and the large

strain response only for fail-safe action.

This high-damping rubber system was also adopted for the Fire Department

Command and Control Facility (FCCF) of Los Angeles County, completed in 1990.

(The same type of high-damping rubber bearing was also used for the Italian

telephone company, S.I.P., Ancona, Italy, the first modern base-isolated building in

Europe.) The FCCF building houses the computer systems for the emergency

services of the county and is therefore required to remain functional after an extreme

event.

Fire Department Command and Control Facility.

The decision to use base isolation for this project was reached by comparing

conventional and isolation schemes designed to provide the same degree of

protection. In most projects, the isolation design costs five percent more. Not only

was the isolation design estimate 6 percent less in this case but is less for any

building when equivalent levels of protection are considered. Furthermore, these

costs are first costs. Life-cycle costs are even more favorable. Also noteworthy is that

the conventional code design requires only a minimal level of protection, that the


(1990)


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structure do not collapse; whereas isolation design provides a higher level of

protection.

The University of Southern California Teaching Hospital in eastern Los

Angeles is an eight-story concentrically braced steel frame supported on 68 lead

rubber isolators and 81 elastomeric isolators. The building was instrumented by the

California Strong Motion Instrumentation Program soon after its completion in 1991.

The foundation system consists of spread footings and grade beams on rock. Because

of functional requirements, both the building plan and elevation are highly irregular

with numerous setbacks over the height. Two wings at either side of the building are

connected through what is referred to as the "necked-down" portion of the building,

and in the original fixed-base design the irregular configuration led to both coupling

between the lateral and torsional vibration modes and very large shear force demands

in the slender region between the two rings. (Even in the isolated design steel trusses

are required to carry the shears in the necked-down region.) These were two of the

main reasons that seismic isolation was eventually chosen for this structure.

University of Southern California University Hospital.


(1991)


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The University of Southern California (USC) Teaching hospital was 36 km

(23 miles) from the epicenter of the Mw 6.8 1994 Northridge earthquake. The peak

ground acceleration outside the building was 0.49 g, and the accelerations inside the

building were around 0.10 to 0.13 g. In this earthquake the structure was effectively

isolated from ground motions strong enough to cause significant damage to other

buildings in the medical center. The records obtained from the USC hospital are

particularly encouraging in that they represent the most severe test of an isolated

building to date.

To date 45 base-isolated buildings in the U.S. are planned, under

construction, or completedfor new construction and for retrofitting. The use of

base isolation applications up to this time in the U.S. has been for structures with

critical or expensive contents, but there is increasing interest in applying this

technology to public housing, schools, and hospitals in developing countries, where

the replacement cost due to earthquake damage can be a significant part of the gross

national product. The cooperation between EERC and MRPRA led to a new joint

effort supported by the United Nations Industrial Development Organization

(UNIDO) that developed low-cost isolation systems for such countries, and several

demonstration projects are in place in Indonesia, the People's Republic of China, and

Armenia.



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CONCLUSION

Today this kind of seismic technology is like a boon to the developing

countries like India where any calamity can affect the gross national income of the

country. This kind of setbacks can be spine breaking to the economical growth of

developing countries. Considering the national interest and value of each life the

calamity claims, paying just 2%more on the constructional cost should not be a

hurdle in assimilating the technology.



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BIBLIOGRAPHY

1. Encyclopedia of polymer science and engg. A wiley-interscience publication

Second Edition ( Vol. 17)

Page No. 666-675

2. Rubber Technology Sir. Maurice Morton

Third Edition

Page No. 11-19

3. Technical Development of Sir. Hiyoyuki Aoyama

Earthquake resistance structure (Civil Engg.)

Page No. 7,8,10,27

4. Reinforced concrete Sir. Ashok K. Jain

(Civil Engg.)

Fifth Edition

Page No. 471-473

Web site :-

www. seismictech.serch.com

www.seismicisolatetion.com

rubber used in seismic technology

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