Implications of strong earthquake ground motion duration on the response and testing of seismic isolation systems

This paper presents an investigation of the effects of the duration of earthquake ground motion on the peak displacement and total dissipated cumulative energy in seismic isolation systems designed by the procedures of ASCE/SEI 7-16. The seismic isolation systems considered in this study are elastomeric with lead and natural rubber bearings and sliding in the form of single and triple friction pendulum bearings. Long- and short-duration ground motions were selected so that the 5%-damped spectral acceleration values of each motion were closely matched over periods of 0.05 to 6 s. The results of the analysis demonstrate that the duration of ground motions have insignificant effects on the isolation system peak displacement—particularly in the lower bound conditions and sliding isolation bearings—but have important effects on the total cumulative energy for all conditions. This has implications in the testing of isolators, which are presented in an evaluation of the isolator test procedures of the ASCE/SEI 7-16 standard. The evaluation reveals that, if the cumulative energy dissipated by the isolation system is used as the controlling criterion, the specified tests are generally appropriate for locations characterized by short-duration ground motions, but are unconservative for locations where long-duration motions are anticipated. Additional results and discussion demonstrate that the cumulative energy may not be an appropriate criterion for designing isolator tests as it may lead to unrealistic demands on the tested isolators.