Testing seismic isolators for the effect of maximum earthquake based on measure of cumulative energy

This paper presents results on what the conditions of testing for assessing the adequacy of seismic isolators in the maximum considered earthquake should be by utilizing an approach in which the cumulative dissipated energy history is used as the performance parameter. This test is what is described as Item 4 in section 17.8.2.2 of standard ASCE/SEI 7. The approach used follows the paradigm of another study in which the history of temperature at the sliding interfaces or in the bulk of the lead core of isolators was used. In conducting this work, many isolation systems (a total of 96 systems, single friction pendulum, triple friction pendulum, and lead-rubber) were used, with isolator models capable of simulating the effects of heating on the strength of the isolators. The study utilized a large set of bi-directional ground motions with short- and long-duration characteristics (71 pairs of short- and long-duration ground motions, 96 isolation systems, three seismic intensities, for a total of 40,896 ground motion pairs). The adequacy tests were designed to reproduce various percentile cumulative energy histories obtained in the response history analysis. The requirements of these tests are compared to the testing requirements obtained by the use of temperature history as the performance criterion, and the test currently prescribed as test item 4 of section 17.8.2.2 of standard ASCE/SEI-7. The study shows that use of the cumulative dissipated energy as the performance criterion for designing tests to represent the effects of maximum earthquake results in onerous testing requirements. This is because the cumulative energy increases monotonically without accounting for fluctuations in the seismic ground motion intensity, unlike the history of temperature at the sliding interfaces or the lead core of isolators. Accordingly, tests designed using the temperature as the performance criterion are recommended for use in specifications for testing.