Performance of base-isolated moment resisting steel frames subjected to pounding effects under near-field earthquake ground motions

Despite conservative requirements of existing building code regarding clear seismic gap distance of base-isolated (BI) structures to surrounding moat walls, seismic performance of these structures is still ambiguous under severe earthquake ground motions that may push the base slab of the isolation system to collide to the surrounding moat walls. Moreover, the temptation of reducing seismic gaps in congested urban areas exacerbates the risk of pounding. Excessive horizontal displacement response of these long-period structures subjected to a rare near-field ground motion may lead to pounding to adjacent structures and subsequently, severe and uncontrolled damage or even total collapse of the superstructure.Pounding of BI structures to moat walls is usually considered as an unwanted response that may inflict critical damage to a high importance structure designed for high performance levels. Pounding effects to the moat walls depend on several parameters including superstructure and isolated periods, damping, seismic gap as well as characteristics of the earthquake ground motion. This study aims to evaluate seismic response of base-isolated moment resisting steel frames subjected to pounding effects under near-field earthquake ground motions with different clear gap distances to surrounding moat walls. A seven story and a three story buildings isolated with elastomeric bearings have been modeled. Base slab displacement, global displacement ductility demands, yield strength reduction factors, and maximum inter-story drifts have been computed under recorded severe near-field earthquake ground motions. Results showed that for most of the seismic gaps lower than those of prescribed by codes, seismic demands remain in acceptable ranges corresponding to low performance levels, i.e., life safety or collapse prevention of fixed-base structures. This implies that performance of BI buildings with codified seismic gaps or insufficient seismic gaps is not much different than that of fixed-base buildings when they are pushed to their displacement limits under maximum considered earthquakes.