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   The current investigation is to determine how well four buildings employing the passive structural control called seismic ( base) isolation has survived the 1994 Northridge earthquake. To score the buildings performances with the help of field records, a procedure for comparison of  real base isolated structures with their hypothetical bench-marks had to be chosen. To compare  promise vs performance, a mathematical model had to be reduced to a SDOF system excited horizontally in accordance with a recorded on a basement real time history. No sophistication is warranted because the expectations were expressed in terms of the maximum acceleration of a SDOF system. Besides, the recording instruments in the buildings were mostly random, scanty and assumed that the superstructure would be rocking like a rigid body.
  FINDINGS: None of the base isolated buildings performed up to expectations. In fact, they performed much worse, namely: 
  1. Los Angeles Fire Command and Control Facility - 3.6 times,
  2. USC Hospital- 4.7 times,
  3. Los Angeles townhouse- 5.2 times,
  4. Rockwell International building (Seal Beach) - 17.0 times.
    There are at least two of  those buildings (index numbers 3 and 4) where the seismic isolation techniques were misused.  They performed, correspondingly, 1.3 and 1.8 times worse than their hypothetical non-isolated counterparts. 
     1. The results of this investigation cast serious doubts on the ability of  the current analytical approaches in base isolation research and engineering to predict accurately actual performances 
of the buildings targeted for earthquake protection. 
     2. Will  the Northridge earthquake experience with the buildings mounted on base isolators 
do us any good? - It depends on our attitude to the data presented in the chart above.
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This page was last updated on 07/29/99