Amplification of seismic demands in inter-storey-isolated buildings subjected to near fault pulse type ground motions

Abstract

Inter Storey Isolation (ISI) shows promise for seismic vibration control in relatively tall buildings. With proper design, an ISI system isolates the upper storey block (USB) and emulates the effect of a nonconventional Tuned Mass Damper (TMD) on the Lower Storey Block (LSB) to reduce the seismic responses considerably. The vulnerability of conventional Base Isolation (BI) under pulse type motions was reported in the past. This study demonstrates the adverse effect of pulse type motions on the performance of ISI systems. An inclusive set of building configurations and bearing typology for the ISI system are employed and are subjected to two suites of unidirectional pulse and non-pulse type ground motions pertaining to varying hazard levels. Extensive nonlinear dynamic analysis of the frame-ISI systems are carried out. Representative statistics of the pertinent responses from each suite are contrasted to demarcate the effect of pulse type motions. The pulse type motions are shown to degenerate the efficiency of an ISI system to significantly amplify the seismic demands (and associated uncertainties) by allowing transmission of low frequency components (reminiscent of pulse(s)) to the response time histories. Response-specific amplification factors for the demands are assessed. The FEMA suggested guideline is shown to severely underestimates the bearing displacements, which is potentially dangerous. This deficiency is addressed by suggesting a modification factor in the FEMA formulae. The influence of the pulse period, mass ratio, damping in the bearing and hazard levels on the amplification of demands are discussed. However, this investigation considers a limited number of framing systems and isolation bearings subjected to a limited number of ground motions, which may be expanded in future study.