Performance‐based testing specifications for seismic isolators

Abstract

This paper presents a study that determines the conditions of testing for determining the bounding properties of seismic isolators for use in analysis and design in a test that is meant to be a replacement of tests Items 2(a) and 3 of ASCE/SEI‐7. Then separately, the study determines the conditions of testing for determining the adequacy of isolators in the maximum considered earthquake in a test is meant to be a replacement of test Item 4 of ASCE/SEI‐7. The test conditions are determined by performing analyses with models that account for the time‐variant properties of the isolators and by computing the resultant isolator displacements. In performing the work, a large number of isolation systems (total of 96 systems, single friction pendulum, triple friction pendulum and lead‐rubber), three different seismic hazards as measured by the spectral acceleration at the period of one second, and a large collection of bi‐directional ground motions with short‐ and long‐duration characteristics (71 pairs of each duration, 96 systems, three seismic hazards) were considered. Fundamental in this methodology is the use of analysis models in which the properties of the isolators (friction or characteristic strength) are dependent on the instantaneous temperature at the sliding interfaces or in the lead core.

Validity of models for frictional heating in sliding isolators

“…The temperatures in the long-duration ground motion are larger and they are sustained for longer times when the plate thickness is small (20 mm in this study) to warrant consideration of their implications on testing requirements for the effects of the maximum earthquake using the procedures in Kim and Constantinou. 17,18 The results on temperature for the top sliding interface in Figure 14 appear unusual for the case in which the two plates have unequal thicknesses (Case 3 where L 1 = 20 mm, L 4 = 50 mm). It would have been expected that the temperature at the top sliding interface would have been larger by comparison to the reference case of indefinite plate thickness.…”

“…In standard ASCE/SEI-7, 19 this test is performed at the average gravity load on the isolator and for a lateral displacement having amplitude of 0.75D M and period of T M for at least 10 cycles of motion (where D M is the resultant maximum displacement and T M is the corresponding effective period computed for the maximum earthquake). Kim and Constantinou 17,18 established procedures for this test based on computed histories of temperature at the sliding interface of sliding isolators. Their study was based on the use of models for the heating of sliding isolators that assume indefinite plate thickness.…”

“…While the Kim and Constantinou 17,18 study presented results on median and various percentile temperature histories for use in the design of this test, this study only presents the 90 th percentile temperature histories as they comply with an appropriate confidence level that is most suitable for most applications but not for critical installations. 17,18,27 The rigid model structure with the two isolation systems (actually, three different single FP and four different triple FP systems due to different plate thicknesses) was analyzed for only the effects of the suites of 71 pairs of spectrally equivalent short-and long-duration ground motions scaled as discussed earlier and shown in Figure 8. Histories of temperature at the sliding interface of the single FP and at the two outer sliding interfaces of the triple FP isolators were computed.…”

“…The 90 th percentile histories of temperature were counted after shifting in time due to significant differences in the duration of the ground motions as described in Kim and Constantinou. 17,18 Figures 16 and 17 present the results for the single FP and the triple FP systems, respectively. Figure 17 includes two curves of 90 th percentile (empirical or counted) temperature histories: one for the bottom sliding interface (for which L 1 = 20 mm) and one for the top sliding interface (for which L 4 = 50 mm).…”

“…It is shown that the existing theories (a) predict well the displacement response of seismically isolated structures for all practical cases of plate depth regardless of the duration of ground motions, (b) slightly underpredict the peak temperature at the sliding interfaces and, more importantly, (c) underpredict the duration of exposure to the high temperature in long-duration ground motions. The latter is of some significance when using these theories to perform computations for developing testing protocol for isolators used in areas of long-duration ground motions, where more cycles of testing may be needed in the prototype testing of isolators for the effects of the maximum earthquake 17,18 as required by some standards. 19…”