Welcome to the HOT TOPIC in Earthquake Engineering  No 4 !
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).
          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). 
 
Regular building
Base-isolated building
      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
Facility
Earthquake
Maximum accelerations structure / ground
Type of isolation bearings
Santa Ana River Bridge
Whittier Narrows 
Oct.1, 1987
0.18g / 0.05g
Lead-rubber
Sierra Point Overpass
Loma Prieta 
Oct.17, 1989
0.41g / 0.09g
Lead-rubber
Mark II Detector, Stanford University
Loma Prieta 
Oct.17, 1989
Records suppressed
Lead-rubber
Liquid Argon Calorimeter, Stanford University
Loma Prieta 
Oct.17, 1989
Records suppressed
Lead-rubber
Eel River Bridge 
(not instrumented)
Petrolia 
Apr.25, 1992
Massive damage at strong shaking
Lead-rubber
Sylmar Substation 230kV Circuit Breakers
Northridge 
Jan.17,1994
Records suppressed
Elastomeric
Main Yard Vehicle Access Bridge
Northridge 
Jan.17,1994
Records suppressed
Lead-rubber
LA County Fire Command Facility
Northridge 
Jan.17,1994
0.35g / 0.19g
High-damping rubber
USC Teaching Hospital
Northridge 
Jan.17,1994
0.19g / 0.17g
0.21g / 0.37g
Rubber/Lead-rubber
Rockwell International Headquaters
Northridge 
Jan.17,1994
0.15g / 0.08g
Lead-rubber
3-story Residence Building
Northridge 
Jan.17,1994
0.63g / 0.44g
Spring & Viscodamper
       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 
         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.
Your questions and/or remarks on this page may be emaild to:   valentin.shustov@csun.edu.
You may also visit Dr.Shustov's  Home Page   or  CME research Web Page   or  " HOT TOPICS ".
Our address is: CME , 18111 Nordhoff Street, Northridge, California 91330-8347
  This page was last updated on 23 February 2000