New Base Isolation Technique for Earthquake Resistant Constr

8/11/2019 New Base Isolation Technique for Earthquake Resistant Constr

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BASE ISOLATIONTECHNIQUE FOREARTHQUAKE RESISTANT

CONSTRUCTION

BY

ROHITH.K


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The concept of Base isolation

The base isolation attempts todecouple the structure from base

excitations It does not absorbs the energy but

deflects through its dynamics.


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ISOLATION SYSTEMS

In first approach the isolation systemintroduces a layer of low lateral

stiffness between the structure andthe foundation.

Isolation layer lengthens natural

period of the structure . Reduces the earthquake induce force.

Increase in deformation is

concentrated in isolation.


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Contd/.

. The second type of isolation system

uses sliding elements.

Shear force is limited by coefficient offriction.

Restoring mechanism is provided.


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Elastomeric-Based Systems

Natural rubber bearing

First used in 1969 in Switzerland .

Compress by about 25% .

Vertical stiffness only a few timeshorizontal stiffness.

It includes foam-glass blocks to actsas fuses.


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Low-Damping Natural and

Synthetic Rubber Bearings Bearings with internal steel

reinforcing plates .

The steel shims provide a high verticalstiffness .

The material is not subject to creep.

Supplementary damping system isneeded.


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Lead Plug Bearing (LRB)

Invented inNew Zealand

in 1975.Yeilding of

lead plug

generatesdampingeffect.


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High-Damping Natural

Rubber (HDNR) Systems To eliminate the need for

supplementary damping elements .

The development of a natural rubbercompound with enough inherentdamping.

The damping is increased by addingextra-fine carbon black, oils or resins,and other proprietary fillers.

More sensitive to temperature


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Isolation systems based

on sliding A system using pure sliding was first proposed in

1909 by Dr. Calantarients.

Chinese earthquake engineers observed the sliding

phenomena following the 1976 Tangshanearthquake.

Buildings that survived the earthquake had ahorizontal crack at the bottom of the walls thatallowed a slip.

A 4-story building for the Earthquake StrongMotion Observatory in Beijing in which the slidingsurface is a layer of specially selected sand.


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on

As the slider moves over the spherical surfacesupported mass rise and provides therestoring force for the system.

Isolation period is controlled by radius ofcurvature of concave surface.

The friction between the slider and surfacegenerates damping in the isolators.


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EERC combined systems

A combination of electrometric andsliding system.

The interior columns of the structurewere carried on sliding elements andexterior columns on rubber bearing.

The electrometric bearing providedrecentering capability.

Sliding element provided damping.


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Sleeved-pile isolation

system For buildings on very soft soil

The piles are made flexible

The union House in Auckland,NewZealand


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Applications of base

isolation Both types of isolation systems have

also been used for retrofit of existing

buildings. Isolation system minimizes the need

for new strengthening structural

member.Appearance of historical or

architectural buildings is preserved.

It is difficult and expansive to modify


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Retrofitted San Francisco

city Hall A 5-story building is 309 ft by 408 ft in plan

the structural system is a steel frame andconcrete slab with unreinforced brick

masonry. Substantial damage sustained from the

1989 Loma Prieta earthquake, centeredabout 60 miles away, necessitated repairsand strengthening.

In addition to base isolation thesuperstructure was strengthened by newshear wall.


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d


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Contd

The isolation system consisted of 530

isolators, each a laminated rubber bearingwith lead plugs located at the base ofeach column and at the base of the shear

walls. The 21-in.-high bearings variedfrom 31 to 36 in. in diameter.

The columns are supported by one or

more isolators. The building estimated to move 18 to 26in

Isolation period of 2.5sec.


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Response of base isolated

buildings to earthquake Response of buildings to 1994

northridge earthquake.

The northridge earthquake has beenthe most significant test of modernisolated buildings providing a valuable

data set of response of various typesof structure incorporating varioustypes of isolation systems.

Name Epicenter Isolation Lateral PGA Roof


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Name Epicenter

(km)

Isolation

system

Lateral

system

PGA

(g)

Roof

acceleration

system

(g)

Private

residencies

21

GERB

Steel braced

frame

0.44

0.63

LA fire

commandand control

center

38 HDR Steel braced

frame

0.22 0.32

Rockwellcomputer

center seal

beach

66

LRB

Concretemoment

frame

0.08

0.15


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conclusion

Base isolation provides an alternative to theconventional, fixed-base design of structures -andmay be cost-effective for some new buildings in

locations where very strong ground shaking islikely. It is an attractive alternative for buildingsthat must remain functional after a majorearthquake (e.g., hospitals, emergencycommunications centers, computer processing

centers, etc). Base isolation is an attractive retrofit approach for

buildings of historical or architectural merit whoseappearance must be preserved


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References

Anil k. chopra , dynamics of structures ,

printice-hall of India.

James M. Kelly , earthquake engineering: from engineering seismology to

performance based engineering Base isolation origins and development :

EERC news, volume 12 january 1991.

www.celebratingEQsafety.com
http://www.celebrating/http://www.celebrating/

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New Base Isolation Technique for Earthquake Resistant Constr