New Base Isolation Technique for Earthquake Resistant Constr
Transcript of 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/