SEISMIC RESPONSE OF BASE ISOLATED STRUCTURES WITH INSUFFICIENT SEISMIC GAPS

Increasing number of seismically isolated structures mostly in seismic-prone regions of the world has raised a concern on seismic performance of these structures in severe earthquake ground motions. On one hand, base-isolated buildings should have a clear certain gap with adjacent structures to avoid pounding; otherwise developed seismic force and displacement demands during impact with neighboring buildings or surrounding moat walls may be far beyond the capacity of the superstructure. In this case, the advantage of using base-isolation technology over traditional seismic design may be totally lost. On the other hand, considering the high cost of lands and other urban limitations do not allow for large seismic gaps to completely eliminate the risk of pounding. Moreover, isolation hardwares should remain stable under vertical loads at their maximum horizontal displacement demand under maximum considered earthquake motion. This results in expensive isolation devices that may prevent the use of isolation technology for a wide range of residential constructions. Additionally, the structural performance level of base-isolated buildings has a sharp change from immediate occupancy for moderate seismic hazard levels to collapse prevention for the large ones. Consequently, structural elements of the superstructure usually do not experience damage-control performance levels. Aforementioned performance objectives may not be compliant with the performance goals considered for residential buildings with normal importance. This study aims to evaluate seismic performance of base-isolated buildings with insufficient seismic gaps that do not conform to minimum codified gap requirements. A wide range of isolated and fixed-base elastic superstructures has been subjected to analytical near field pulse-type motions to evaluate seismic demands in the superstructure during impact with retaining walls. Different values for stiffness of moat walls has been considered with a stereomechanical impact model. The results of the parametric study show that the developed seismic demands in the superstructure depends on the pulse duration, gap size and the ratio of the isolated period to the fixed-base period as well as the moat wall stiffness. As a conclusion, this study shows that by using available gap sizes and utilizing lower cost isolation devices with a smaller displacement capacity compared with the current code-based design requirements, a more economical performance-based seismic design for base-isolated structures is achievable adopting a more gradual transition from immediate occupancy performance level to the collapse prevention one.