Peak Sliding Demands on Unanchored Equipment and Contents in Base-Isolated Buildings under Pulse Excitation

Unanchored equipment and contents (EC) in buildings are of primary concern during an earthquake because not only can they contribute to substantial nonstructural losses but also pose a serious safety risk to building occupants. Base isolation is widely considered to be an effective technique for reducing seismic demands on nonstructural components, but recent research has noted situations, under broadband ground motions, where isolating a building can in fact amplify the response of unanchored EC that are prone to sliding. This study investigates the sliding response of unanchored EC in base-isolated buildings subjected to analytical pulse excitation representing pulselike ground motions. Describing the excitation by analytical pulses enables the application of dimensional analysis on the problem to provide insight into how different parameters of the nonlinear problem affect the response. The EC are idealized as freestanding rigid bodies, and the contact surface between the EC and floor is described by a Stribeck friction model taking into account the transition from static to kinetic friction. The isolation system is treated as either viscoelastic or bilinear. The study shows that the peak sliding response of rigid objects exhibits complete similarity in the ratio of kinetic-to-static friction coefficient. Therefore, a simple Coulomb model with a single friction coefficient value is adequate for estimating the peak sliding displacement. Moreover, it is observed that as the isolation damping increases, the sliding response exhibits complete similarity in the ratio of isolation-to-pulse period. The study concludes that certain combinations of the isolation design parameters can result in amplification in the peak sliding response of contents, compared to the fixed-base building.