Experimental evaluation of supplemental viscous damping for a sliding isolation system under pulse-like base excitations

Near-fault earthquakes that usually contain a long-period pulse component may cause excessive responses to a base-isolated structure, especially with regard to its peak isolator displacement. Viscous-type dampers, whose damping force is proportional to the excitation velocity, can be used to mitigate the excessive responses of the isolated structure. In this study, the effect of viscous-type supplemental damping on a sliding isolation system subjected to pulse-like ground excitations is evaluated experimentally by a shaking table test. The test program involves a relatively rigid structure isolated by sliding-type isolators and a fluid viscous damper installed within the isolation layer. Seismic excitations with and without a long-period pulse component were imposed on the isolated system, and the experimental responses of the system with and without installation of the fluid damper were compared. The test results demonstrate that, for a rigid superstructure, the viscous damper is able to effectively suppress the peak isolator displacement induced by the long-period pulse component without increasing the acceleration level of the superstructure; however, it also slightly increases the structural acceleration in the seismic excitation without the pulse component. The results also show that a pulse-like ground motion is able to induce resonance-like behavior for the isolator displacement of a sliding system when the pulse period is close to the isolation period. However, this resonance-like behavior can be effectively mitigated by adding viscous damping to the isolation system. Finally, by using linear and nonlinear viscous models to simulate the experimental responses, the influence of the damper nonlinearity on the test results was also investigated.