Effectiveness of Variable Stiffness Systems in Base-isolated Bridges Subjected to Near-fault Earthquakes: An Experimental and Analytical Study

This article presents a novel semiactive independently variable stiffness (SAIVS) device, proposed for seismic response control of sliding base-isolated bridges. As a first step, force–displacement characteristics of the new SAIVS device are analytically and experimentally studied. It is demonstrated that the SAIVS device is capable of varying the stiffness, continuously and smoothly between minimum and maximum stiffnesses. This device is then incorporated into the sliding isolation system. In bridges, sliding isolation systems reduce pier drifts, but with increased bearing displacements. Such increased bearing displacements can be problematic under near-fault, large-velocity pulse-type earthquakes. To reduce bearing displacements, passive dampers are often incorporated into the isolation system. However, passive systems may result in increased pier drifts and isolation level forces. Semiactive variable stiffness systems, which can vary the period of the sliding isolated bridge in real-time, may reduce the bearing displacements and isolation level forces further than the passive systems; and hence, deserve investigation. In this study, the performance of a 1: 20-scaled sliding base-isolated bridge model equipped with the new SAIVS device is analytically and experimentally studied under several near-fault earthquakes. A new control algorithm for the control of the SAIVS device is developed and implemented in shake table tests. It is shown that the semiactive SAIVS device reduces bearing displacements further than the passive cases, while maintaining isolation level forces at the same level as in the minimum stiffness case.