Gilbert

SECTION 1 INTRODUCTION

1.1 Lead Rubber Bearings

Lead Rubber Bearing or LRB is a type of base isolation employing

a

heavy

damping.

It

was

invented

by

Bill

Robinson, a new Zealander.

Heavy

damping

mechanism

incorporated

in

vibration

control technologies and, particularly, in base isolation devices, is often consider a value source of suppressing vibrations thus enhancing a building's seismic performance. However, isolated

for

the

rather

structures,

pliant

with

a

system

such

relatively

as

low

a

base

bearing

stiffness but with a high damping, the so called “damping force” may turn out the main pushing force at a strong earthquake. It was a uni-axial test in which the bearing was also under a full structure load. Many buildings and bridges, both in New Zealand and elsewhere, are protected with lead dampers and lead and rubber bearings. Te Papa Tongarewa, the museum of New Zealand, and the New Zealand Parliament Buildings have been fitted with the bearings. 1

Both are in Wellington, which sits on active earthquake fault.

Laminated Elastomeric Bearings with one or more lead cylinder / plug in the center are named as lead core rubber bearings where these lead plugs facilitates very effective damping during extreme movements of these bearings. Lead core rubber bearings and how they can help reduces damages of a major earthquake is explained here.

Recent devastation of many earthquakes and tidal wave activities around the world has awakened many designers in construction industry and subject that is much looked up is to design structures such a way that even after seismic activates much less damages can be incurred, resulting less human

causalities.

designers

are

resistant

design

strength,

stiffness

which

are

great

Few

of

points

conventional of

approaches

buildings; and

enough

providing

inelastic to

raised

2

structural

-

earthquake

building

deformation

withstand

earthquake generated force.

by

a

given

with

capacity level

of

Above can be generally accomplished through selection of an appropriate structural configuration and with careful detailing of structural members, such as beams and columns, and

connections

recommended earthquake

between

them

additional

to

etc...

resistance,

not

above only

Few

designers

have

basic

approach

for

to

strengthen

building

structure, but to look at options to reduce earthquakegenerated advanced

forces

acting

techniques

of

upon

it.

Among

earthquake

most

resistant

important design

and

construction are base isolation system.

According alternate

to

layers

the of

Algasism rubber

LRB

and

isolators

vulcanized

consist

of

reinforcement

steel plates of limited thickness and a central lead core. They

allow

selection

the of

isolation the

of

horizontal

the

structure

stiffness

by

and

a

proper

they

can

dissipate energy up to 30% damping due, to the high damping capacity of the lead core. The damping value of 30% is normally the maximum damping to model the device as linear according

to

the

international

3

design

standards.

The

Algasism

LRB

isolators

fulfill

the

following

due

permanent

requirements: -Transmit

the

vertical

loads

to

accidental

and

effects;

-Capacity to support horizontal loads due to service load conditions -Capacity

with to

very

isolate

the

low

displacements.

structure

by

shifting

the

fundamental vibration period to an optimal and safe level. -Capacity

to

dissipate

energy

to

reduce

the

horizontal

displacement of the isolated structure with respect to the ground.

LRB isolators assure the following advantages: -

No

damage

elastic

to

the

structure

response

for

high

that

intensity

remains

due

earthquakes

to with

the no

interruption of the structural function. This is a primary goal for strategic structures (hospitals, control rooms, etc…) - Very well known and used in many application both for building -

Very

visual

and simple

maintenance

inspection

mainly

for

all

bridges limited the

to

a

design

periodic life

- Capacity to reduce the seismic energy from the ground to 4

the

structure

with

consequent

simplification

of

the

structure design

1.2 Base Isolated Structures

A base isolated structure is one, which is supported on a series of isolation supports, which are placed between building and its foundation. Many structural designers have developed different types of base isolation systems around the world and one of them, which are practical and cost effective, are usage of lead-rubber bearings.

A lead rubber bearing is nothing else but a bigger laminated

bearing

manufactured

from

layers

of

rubber,

sandwiched together with layers of steel, except for that in middle of bearing there will be a solid lead "plug." Top and bottom of the bearing is fitted with steel plates, which are used to attach bearing to building through its foundation. These lead rubber bearings are designed in such a way that bearing is very stiff and strong in vertical direction, but flexible in horizontal direction.

5

SECTION 2 MATERIALS AND METHODS

2.1 Construction of Base Isolation

Lead rubber bearings were developed as base isolators in the 1970s. They consist of three basic components – a lead plug, and rubber and steel, which are generally placed in layers.

2.1.1 Rubber

The rubber provides flexibility through its ability to move but return to its original position. At the end of an

6

earthquake, if a building hasn’t returned to its original position, the rubber bearings will slowly bring it back. This might take months, but it will return to its original position.

2.1.2 Lead

Lead

was

chosen

because

of

its

plastic property –

while it may deform with the movement of the earthquake, it will revert to its original shape, and it is capable of deforming

many

times

without

losing

strength.

During

an

earthquake, the kinetic (movement) energy of the earthquake is absorbed into heat energy as the lead is deformed.

2.1.3 Steel

Using

layers

of

steel

with

the

rubber

means

the

bearing can move in a horizontal direction but is stiff in a vertical direction.

7

2.2 The Design and Characteristics of (MAURER-LRB)s

MLRBs

are

consisting

of

a

regular

elastomeric

laminated rubber bearing. The rubber compound can be made of

natural

usually

rubber(NR)

European

or

chloroprene

Standards

were

rubber

considered.

(CR), On

while

request

other standards like SETRA,ASSHTO, etc. can be applied. The shape can be either round, square or rectangular (Fig. 8 and 9).

The MLRBs are generally constructed with low-damping (unfilled) elastomers with shear-moduli of 0,6-1,35 N/mm² and lead cores with diameters ranging 15% and 33% of the bonded

bearing

diameter

for

round

bearings.

The

surface

relation is kept the same for rectangular bearings. The elastomer provides the isolation and recentring, while the lead core offers the necessary energy dissipation or damping component.

The maximum shear strain value for MLRBs is generally between 125% and 200%.

8

The inner steel shims do not only grant for good load capacity, but also for a proper confinement of the lead core.

2.3

Basic

Principle

of

Seismic

Isolation

by

Energy

Mitigation realized with (MLRB)s

According to the MAURER Lead Rubber Bearings (MLRB) are based on the design principles of the EN1337(Structural Bearings)

and

the

prEN15129(Anti

device is a regular rubber

Seismic

Devices).

The

bearing for service condition

and it is a seismic isolator for the seismic condition. The bearing plan shape can be round, square or rectangular, whereas the design rules according to the EN standards were applied.

The production of the (MLRB)s is within the DIN ISO 9001 quality management system. (MLRB)s can be applied for buildings

and

for

bridges.

The

seismic

isolation

of

a

structure is based on the concept of ENERGY MITIGATION and costly strengthening measures are avoided as the resulting forces are mitigated within the structure.

9

Two methods are simultaneously applied: 1. Seismic isolation by (MLRB)s:

The superstructure gets de-coupled from the ground. The so called seismic-isolation limits automatically the energy to a minimum to enter the superstructure during an earthquake. Due to this fact the natural period of the structure

is

increased,

therefore

reduces

the

spectral

acceleration during a seismic attack. Depending on the type of

the

this

employed

case

multidirectional

(MLRB)s

-

they

grant

seismic for

isolators

the

–

vertical

in

load

transmission but also for the active re-centring of the superstructure during and after an earthquake. Recentring means that the bridge deck displaced due to the seismic energy input is automatically shifted back by the seismic isolators into its original position.

2.

Energy dissipation by (MLRB)s:

By

means

of

passive

energy

dissipation

(=

energy

transformation into heat) the seismic rest energy entering into the superstructure will be effectively dissipated by

10

additional relieving

damping

within

the

lead

core

of

the

MLRB

the entire structure from additional strain.

2.3.1 The fundamental Functions of MAURER-(LRB)s

The four fundamental functions of MAURER (LRB)s are: 1. Transmission of vertical loads. 2. Allowance of displacements on the

horizontal plane

providing the horizontal flexibility. 3. Dissipation of substantial quantities of energy. 4. Assurance of self-re-centring.

The

first

function

means

that

the

MLRB

acts

as

a

conventional rubber bearing, i.e. transfers vertical loads in the intended location from the superstructure to the substructure. between

foundation

transmitted which

The

is

forces

second and

function

produces

superstructure

or

the

amount

of

essentially

the

same.

The

and

uncoupling

thus

reduces

mechanical

energy,

uncoupling

allows

horizontal flexibility of the structure. The flexibility is provided by the rubber of the MLRB.

11

The dissipation of energy limits relative displacement of

the

isolated

structural

mass

and

provides

better

structural control with bigger safety for the structure. The energy dissipation is realized by the rubber and by the inner lead core of the MLRB.

The

purpose

of

the

self-re-centring

capability

requirement – return of the structure to former neutral mid position - is not so much to limit residual displacements at

the

end

of

a

seismic

attack,

but

rather,

prevent

cumulative displacements during the seismic event.

Self-re-centring

assumes

particular

importance

in

structures located in close proximity to a fault, where earthquakes

characterized

by

highly

asymmetric

accelerograms are expected (Near Field or Fling effect). The re-centring effect is based on the natural elasticity of the applied rubber.

It

should

be

noted

that

energy

dissipation

and

selfcentring capability (sometimes referred to as restoring force)

are

two

antithetic

functions

and

their

relative

importance depends primarily on the case under examination. 12

Basically the structural designer is providing certain conceptual

requirements

of

the

isolation

unit,

like

for

load capacity, damping, stiffness, etc. and the MLRB unit will then be adapted to these requirement by MAURER.

2.4 Research at EERC

Research on the development of natural rubber bearings for isolating buildings from earthquakes began in 1976 at the

Earthquake

PEER,

the

Engineering

Pacific

Research

Engineering

Center

Research

(EERC)

Center)

of

(now the

University of California at Berkeley. The initial research program was a joint effort by EERC and the Malaysian Rubber Producers Research Association (MRPRA), U.K. The program was funded by MRPRA through a number of grants over several years, with later funding provided by the National Science Foundation

and

the

Electric

Power

Research

Institute.

Professor James M. Kelly directed the research at EERC, which

included

considerable

theoretical

and

experimental

contributions by graduate students.

Although not an entirely new idea at the time—a few methods

using

rollers

or

sliders 13

had

been

proposed—the

concept

of

impractical

base by

isolation most

was

of

the

considered

to

structural

be

very

engineering

profession. The research project began with a set of handmade bearings of extremely low-modulus rubber used with a simple three-story, single-bay, 20-ton model. Shaking table tests

showed

that

isolation

bearings

could

bring

about

reductions in acceleration by factors of as much as ten when compared to those of conventional design and that, as predicted, the model would respond as a rigid body with all deformation concentrated in the isolation system. It was also clear that a certain degree of damping was needed in the system and that the scale of the model was too small to allow more practical rubber compounds to be used.

In

1978,

a

more

convincing

demonstration

of

the

isolation concept was achieved with a more realistic fivestory,

three-bay

model

weighing

40

tons

and

by

using

damping-enhanced bearings made by commercial techniques. A strong interest throughout the EERC research program was in the influence of isolation on the response of equipment and contents in a structure, which tend to sustain more damage when conventional methods of seismic-resistant design are used and which, in many buildings, are much more costly 14

than the structure itself. An extensive series of tests on the

five-story

frame

demonstrated

that

isolation

with

rubber bearings could provide very substantial reductions in

the

accelerations

exceeding

the

However,

reductions

the

elements

experienced

same

(such

experienced

tests

as

by

showed

steel

internal by

that

equipment,

the

structure.

when

additional

energy-absorbing

devices,

frictional systems, or lead plugs in the bearings) were added

to

the

reductions

isolation

in

system

acceleration

to

to

increase

the

damping,

equipment

were

the not

achieved because the added elements also induced responses in

the

higher

equipment.

It

modes became

of

the

clear

structure,

that

the

affecting

optimum

method

the of

increasing damping was to provide it in the rubber compound itself.

This

developed

by

method MRPRA

was and

applied used

in

later the

in

first

the

compound

base-isolated

building in the United States, described below.

Rubber bearings are relatively easy to manufacture, have no moving parts, are unaffected by time, and are very resistant to environmental degradation.

15

Test of bearing used in the Indonesian demonstration building. Photo: I. D. Aiken

The

bearings

are

made

by

vulcanization

bonding

of

sheets of rubber to thin steel reinforcing plates. Because the bearings are very stiff in the vertical direction and very flexible in the horizontal direction, under seismic loading the bearing layer isolates the building from the horizontal vertical

components components

relatively

of

the

are

unchanged.

ground

transmitted

Although

movement to

vertical

the

while

the

structure

accelerations

do

not affect most buildings, the bearings also isolate the building from unwanted high-frequency vertical vibrations produced by underground railways and local traffic. Rubber bearings

are

suitable

for

stiff

buildings

up

to

seven

stories in height. For this type of building, uplift on the bearings will not occur and wind load will be unimportant.

16

2.5 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 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

multilayer natural rubber bearings reinforced with steel plates. The superstructure of the building has a structural steel frame stiffened by braced frames in some bays.

Foothill Communities Law and Justice Center. Photo: I. D. Aiken.

17

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

accounted

selected

for

for

possible

the

torsion,

isolation

system,

incorporated

a

which maximum

horizontal displacement demand of 380 mm (15 in.) in the isolators at the corners of the building. Tests of fullscale 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 18

system assumes minimum values of stiffness and damping and a linear 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. Photo: I.D. Aiken

19

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 costs

of are

protection first

are

costs.

considered.

Life-cycle

Furthermore,

costs

are

these

even

more

favorable. Also noteworthy is that the conventional code design requires only a minimal level of protection that the structures not collapse; whereas isolation design provides a higher level of protection.

The Hospital

University in

concentrically

of

Southern

eastern

Los

braced

steel

California

Angeles frame

is

an

supported

Teaching

eight-story 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 20

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 torsion 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.

2.6 Nuclear Applications

Isolation used in conventional nuclear plants greatly simplifies

the

expensive

and

time-consuming

design

and

qualification of the equipment, piping, and supports for seismic loading. In addition, when seismic design criteria are increased due to the discovery of nearby faults, for example, the plant need not be redesigned; upgrading the isolation system is sufficient.

In an experimental program at EERC isolation bearings were designed, produced, and tested for two types of liquid 21

metal reactor designs. The first, called PRISM, uses highshape

factor

isolation

bearings

designed

to

provide

horizontal isolation only. In the other design, SAFR, the reactor

is

supported

on

low-shape

bearings

that

provide

both horizontal and vertical isolation. The results of this test series extended the range of the isolator types with well-understood characteristics.

22

SECTION 3 RESULTS

3.1

Function of Base Isolation

To get a basic idea of how base isolation works with lead rubber bearings, let us examine how will an earthquake act up on both base isolated building where super structure is attached to foundations through a sets of lead rubber bearings and a standard conventional fixed base building where super structure is attached direct to its foundation.

As a result of an earthquake, ground beneath building begins to move. Each building responds with movement: which tends

toward

opposite

side

of

movement

of

ground.

In

reality buildings undergoes displacement towards opposite side of ground movement. This building's displacement in direction

opposite

ground

motion

is

actually

due

to

inertia. The inertial forces acting on a building are the most important of all those generated during an earthquake.

The inertial forces by which a building undergoes at time of a major earthquake are proportional to building's 23

acceleration

during

ground

motion

so

buildings

don't

actually shift in not only in one direction but because of complex nature of earthquake ground motion; building tends to vibrate back and forth in varying directions.

In addition to displacing toward one side, the unisolated

building

changing

its

basically

(without

shape-from

deforming

earthquake

damage

to

the

a

lead

rubber

rectangle

building.

buildings

is

bearing)

to

The

a

will

be

parallelogram,

primary

deformation

cause by

of

which

building undergoes as a result of inertial forces acting upon it.

By contrast, even though building with base isolation too

is

being

displaced,

under

different

directional

movement of ground due to earthquake, building with lead rubber

bearing

base-isolation

retains

its

original,

rectangular shape and it will be the lead-rubber bearings supporting the building that will be deformed. The baseisolated building itself escapes deformation and damage-which implies that inertial forces acting on base-isolated building have been reduced by the usage of proper lead rubber bearings. 24

Experiments

and

observations

of

base-isolated

buildings in earthquakes have been shown to reduce building accelerations to as little as one fourth of acceleration of comparable

to

fixed-base

buildings,

which

each

building

undergoes as a percentage of gravity as inertial forces increase,

and

decrease,

proportionally

as

acceleration

increases or decreases. Acceleration is decreased because lead rubber base isolation system lengthens a building's period of vibration, (the time it takes for the building to rock back and forth and then back again). And in general, structures with longer periods of vibration tend to reduce acceleration,

while

those

with

shorter

periods

tend

to

increase or amplify acceleration.

3.2 Advantages of Lead Center core in Bearings.

Finally, during and after a seismic activities on a building

with

lead

rubber

bearing

base

isolation,

what

happens to lead rubber bearings?. Main body of the bearing, rubber being highly elastic, does not suffer any damages, whereas lead plug in middle of bearing will experience the same deformation as rubber but generates heat and reduces in

size,

or

dissipates,

energy 25

of

motion--i.e.,

kinetic

energy--by converting that energy into heat, also thus by reducing the energy entering building. This helps to slow and eventually stop the building's vibrations sooner than would otherwise be the case: helping to dampen building's vibrations.

(Damping is the fundamental property of all vibrating bodies, which tends to absorb the body's energy of motion, and

thus

reduce

the

amplitude

of

vibrations

until

the

body's motion).

Thus, dissipates

lead the

inserted energy

of

as

center

earthquake

core

of

bearing

while

the

rubber,

reinforced with steel plates, provides stability, supports structure

and

isolates

vibrations.

LRB

bearings

also

provide excellent base isolation provided there is enough space for bearing and thermal movements are not too extreme.

Lead rubber bearings can, not only be used with new building but also can be incorporated into foundations of existing buildings.

26

3.3 U.S. Application Response

University of Southern California University Hospital. Photo: P. W. Clark.

The University of Southern California (USC) Teaching hospital was 36 km (23 miles) from the epicenter of the Mw 6.8

1994

Northridge

acceleration

outside

earthquake.

the

building

The

was

peak

0.49

g,

ground and

the

accelerations inside the building were around 0.10 to 0.13 g.

In

isolated

this

earthquake

from

ground

significant

damage

center.

records

The

to

the

structure

was

motions

strong

enough

other

buildings

obtained

from

the

in USC

effectively to

the

cause medical

hospital

are

particularly encouraging in that they represent the most severe test of an isolated building to date.

27

3.4 Base Isolation in Japan

After

a

development

in

slow

start,

Japan

base

increased

isolation rapidly.

research

The

first

and large

base-isolated building was completed in 1986. Although such buildings

in

Japan

require

special

approval

from

the

Ministry of Construction, as of June 30, 1998, 550 baseisolated buildings had been approved.

Base several

isolation reasons.

development

in

has The

advanced

expenditure

engineering

amount

designated

large

construction

rapidly

is

specifically companies

high for

in

Japan

for

research

with

a

base

and

significant

isolation;

aggressively

for

market

the the

technology; the approval process for constructing a baseisolated

building

is

a

straightforward

and

standardized

process; and the high seismicity of Japan encourages the Japanese to favor the long-term benefits of life safety and building

life-cycle

costs

when

making

seismic

design

decisions.

The system most commonly used in the past has been natural rubber bearings with mechanical dampers or lead28

rubber

bearings.

Recently,

however,

there

has

been

an

increasing use of high-damping natural rubber isolators. There are now several large buildings that use these highdamping bearings: an outstanding example is the computer center for the Tohoku Electric Power Company in Sendai, Miyako Province.

Tohoku Electric Power Company, Japan. Photo: P. W. Clark

Currently the largest base-isolated building in the world is the West Japan Postal Computer Center, located in Sanda,

Kobe

(500,000

ft

Prefecture. square)

This

six-story,

structure

is

47,000

supported

m

square on

120

elastomeric isolators with a number of additional steel and lead dampers. The building, which has an isolated period of 3.9 sec, is located approximately 30 km (19 miles) from the epicenter of the 1995 HyogokenNanbu (Kobe) earthquake, and

29

experienced

severe

acceleration

under

ground the

motion.

isolators

was

The

peak

ground

400

cm/sec

square

(0.41 g) but was reduced by the isolation system to 127 cm/sec square (0.13 g) at the sixth floor. The estimate of the displacement of the isolators is around 12 cm (4.8 in.). A fixed-base building adjacent to the computer center experienced some damage, but there was no damage to the isolated building.

The use of isolation in Japan continues to increase, especially in the aftermath of the Kobe earthquake. As a result of superior performance of the West Japan Postal Computer Center, there has been a rapid increase in the number of permits for base-isolated buildings, including many apartments and condominiums.

30

SECTION 4 CONCLUSION

After

of

several

experimental

analysis

about

lead

rubber bearing it was proven to be an ideal bases isolation system in terms of seismic event in building construction. Seismic occurrence is more dramatically than other natural hazards

because

it

is

unpredictable

so

that

engineers

design this kind of isolator to reduce the damages of a certain buildings and protect the human kind.

As a result of an earthquake, movement beneath the ground

of

a

building

begins.

Building

respond

with

the

movement: which tend towards opposite side of movement of ground. In reality buildings also undergoes displacement towards opposite side of ground movement. Displacement of building

is

due

to

inertia.

Inertial

force

by

which

a

building undergoes of a time of as major earthquake are proportional

to

building's

acceleration

during

ground

motion so buildings don't actually shift in not only in one direction

but

because

of

complex

nature

of

earthquake

ground motion; building tends to vibrate back and forth in varying directions. 31

In addition to displacing towards one side, the unisolated

building(without

changing

its

basically earthquake

shape-form

deforming damage

the

to

lead a

rubber

rectangle

building.

buildings

bearing)

to

The

is

a

will

be

parallelogram,

primary

deformation

cause by

of

which

building undergoes as a result of inertial force acting upon it.

Even though building with base isolation too is being displace, under different directional movement of ground due to seismic activity. Building with rubber bearing baseisolation retains its original rectangular shape and it be the lead-rubber bearings supporting the building that will be deformed.

Experiments

and

observations

of

base-isolated

buildings in earthquakes have been shown to reduce building accelerations to as little as one fourth of acceleration of comparable

to

fixed-base

buildings,

which

each

building

undergoes as a percentage of gravity as inertial forces increase,

and

decrease,

proportionally

as

acceleration

increases or decreases. Acceleration is decreased because lead rubber base isolation system lengthens a building's 32

period of vibration, (the time it takes for the building to rock back and forth and then back again). And in general, structures with longer periods of vibration tend to reduce acceleration,

while

those

with

shorter

periods

tend

to

increase or amplify acceleration.

The advantage of lead center core in bearings is it help to slow and eventually stop the building's vibration sooner than would otherwise be the case: helping to dampen building's vibration. Damping is the fundamental property of all vibrating bodies, which tends to absorb the body's energy

of

motion,

and

thus

reduce

the

amplitude

of

vibrations until the body's motion.

In U.S. Application response due to seismic activity the

University

of

Southern

California(USC)

Teaching

Hospital was 36 km(23 miles) from the epicenter of the Mw 6.8

1994

North-ridge

acceleration

outside

the

earthquake. building

The

was

peak

0.49

g

ground and

the

acceleration inside over 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. 33

Base several

isolation reasons.

development

in

has The

advanced

expenditure

engineering

amount

designated

large

construction

rapidly

is

specifically companies

high for

in

Japan

for

research

with

a

base

and

significant

isolation;

aggressively

for

market

the the

technology; the approval process for constructing a baseisolated

building

is

a

straightforward

and

standardized

process; and the high seismicity of Japan encourages the Japanese to favor the long-term benefits of life safety and building

life-cycle

costs

when

making

seismic

design

decisions.

The system most commonly used in the past has been natural rubber bearings with mechanical dampers or leadrubber

bearings.

Recently,

however,

there

has

been

an

increasing use of high-damping natural rubber isolators. There are now several large buildings that use these highdamping bearings: an outstanding example is the computer center for the Tohoku Electric Power Company in Sendai, Miyako Province.

34

Currently the largest base-isolated building in the world is the West Japan Postal Computer Center, located in Sanda, Kobe Prefecture.The building, which has an isolated period

of

3.9

sec,

is

located

approximately

30

km

(19

miles) from the epicenter of the 1995 Hyogoken Nanbu (Kobe) earthquake, and experienced severe ground motion. The peak ground

acceleration

under

the

isolators

was

400

cm/sec

square (0.41 g) but was reduced by the isolation system to 127 cm/sec square (0.13 g) at the sixth floor. The estimate of the displacement of the isolators is around 12 cm (4.8 in.). A fixed-base building adjacent to the computer center experienced some damage, but there was no damage to the isolated building.

The use of isolation in Japan continues to increase, especially in the aftermath of the Kobe earthquake. As a result of superior performance of the West Japan Postal Computer Center, there has been a rapid increase in the number of permits for base-isolated buildings, including many apartments and condominiums.

35

SECTION 5 REFERENCES

1.Earthquake Engineering, Access at http://en.wikipedia.org/wiki/Earthquake_engineering#Lead_ru bber_bearing>

Access on March 7, 2012.

2.Algagism LRB,Access at Access on March 8,2012.

3.Base Isolation: Origins and Development,” EERC News, Vol. 12, No. 1, January 1991. Access at Access on March 7,2012.

4.Lead Rubber Bearings ,Access at Access on March 7, 2012.

36

5.SCIENCE LEARNING sparking fresh thinking,Access at Access on March 8, 2012.

6.MAURER Seismic Isolation System with Lead Rubber Bearing(MLRB), Access at Access at March 7, 2012.

37