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Welcome
to the HOT
TOPIC in Earthquake Engineering No
4 !
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BASE
ISOLATION: PROMISE, DESIGN & PERFORMANCE
The goal of
this investigation is to scrutinize the concept of Seismic
Isolation System and determine
how well the structures employing the new technology survived an earthquake
exposure.
Seismic
Isolation System is a collection of structural elements that
should substantially decouple a structure from the horizontal components
of ground shaking thus protecting the building's integrity. Isolation
System consists of Isolation
Units with or without Isolation
Components.
Isolation
Units are the basic elements of an Isolation
System which are intended to provide the decoupling effect.
Isolation Components are the connections between Isolation
Units and their parts having no decoupling effect of their own.
Initial implementation
of the new technology coincided with the romantic era of earthquake engineering
and public relations associated with the dreams of "beating the quake"
(see below). |
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From the very beginning, the theory of seismic (or base) isolation engineering
rested on two pillars: heavy
damping and frequency
separation. Unfortunately, nobody paid any attention that heavy
damping is a sort of a strong connection between a substructure and superstructure,
and that the idea of decoupling them with the help of such connections
is no good. As a result, the only problem left seemed to be development
of proper design provisions for seismic-isolated structures (Shustov,
1994). |
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Regular building
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Base-isolated building
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The current provisions, incorporated as an appendix into UBC, are underlain
by the indisputable assumption that
base isolation technique should separate a building from shaking ground
just by definition. Therefore, the very idea of seismic isolation looks
extremely appealing, especially, taking into account that researchers and
engineers predict a sizable reduction of seismic forces, usually no less
than 5-10-fold .
However, if anybody decides
to verify a degree of expected mitigation per UBC by comparing the seismic
forces applied to a regular building and the identical one but put on base
isolators, there may be a surprise similar to the figurative picture presented
to the left. |
| Though in theory, there should
not be practically any deviation between the currently existing theory
of base isolation and the practice of its implementation, in practice,
however, there is a huge one (see below). |
PERFORMANCE
OF SEISMIC ISOLATED STRUCTURES IN CALIFORNIA
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Facility
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Earthquake
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Maximum
accelerations structure
/ ground
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Type
of isolation bearings
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Santa Ana River Bridge
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Whittier Narrows
Oct.1, 1987
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0.18g / 0.05g
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Lead-rubber
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Sierra Point Overpass
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Loma Prieta
Oct.17, 1989
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0.41g / 0.09g
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Lead-rubber
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Mark II Detector, Stanford University
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Loma Prieta
Oct.17, 1989
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Lead-rubber
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Liquid Argon Calorimeter, Stanford University
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Loma Prieta
Oct.17, 1989
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Lead-rubber
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Eel River Bridge
(not instrumented)
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Petrolia
Apr.25, 1992
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Massive damage at strong shaking
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Lead-rubber
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Sylmar Substation 230kV Circuit Breakers
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Northridge
Jan.17,1994
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Elastomeric
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Main Yard Vehicle Access Bridge
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Northridge
Jan.17,1994
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Lead-rubber
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LA County
Fire Command Facility
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Northridge
Jan.17,1994
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0.35g / 0.19g
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High-damping rubber
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USC
Teaching Hospital
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Northridge
Jan.17,1994
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0.19g / 0.17g
0.21g / 0.37g
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Rubber/Lead-rubber
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Rockwell
International Headquaters
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Northridge
Jan.17,1994
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0.15g / 0.08g
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Lead-rubber
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3-story
Residence Building
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Northridge
Jan.17,1994
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0.63g / 0.44g
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Spring & Viscodamper
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The field experiments supported by numerous instrumental records of earthquake
performances of the structures mounted on seismic bearings, which happened
to be a way below predictions and expectations, inspire continuous efforts
to improve both the concept and practical design of seismic isolation systems.
One of the recent laboratory tests is shown in the right picture where
two identical building models are shaken vigorously and identically.
The only difference is: the model on right rests on the earthquake
protective foundation using an innovative design approach. For
a movie demonstration of the testing, click on the picture    |
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Preliminary results of the analytical and experimental research on a new
generation of seismic isolation technology, which is under way at the Cal
State University Northridge now, are encouraging. Thus, when the shake
table was vibrating in resonance with the fundamental frequencies of the
tested superstructures and the amplitudes of acceleration were around 0.75g,
the recorded maximum acceleration at the roof level of the unprotected
model reached 2g while the protected model sustained negligible
deformations ( Shustov,
2000). |
Shustov,
V., 1994, “Adaptive Systems: A New Application”, Proc. 1st World
Conf. on Struct. Control, 3:
FP-32-FP-41, Pasadena, CA.
Shustov,
V., 1999, “Briefing on the 1994 Northridge Earthquake”,
http://www.seaint.org/seaocconvention/convention1999/Proceedings/BRIEFING_ON_THE_1994.pdf
Shustov, V., 2000, “Earthquake-Protective Pneumatic Foundation”,
Proc. Smart Systems for Bridges,
Structures, and Highways, 3988-45, Newport
Beach, CA. |
This
page was last updated on 23 February 2000
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