Effect of ground motion orientation on response of mass eccentric seismic isolated structures

The superstructure of a seismically isolated structure behaves in an elastic manner and all nonlinearity takes places on seismic isolation level. Accordingly, for such structures, maximum isolator displacement (MID) turns out to be the most important demand parameter during the design stage. For this reason, MID needs to be calculated accurately by considering the factors that can be essential on structural response during the dynamic analyses such as amplitude of scale factor, ground motion direction and superstructure irregularities. Thus, this study focuses on the parameters that are involved in the applied scaling procedure of ground motions, in order to determine the variation in amplitude of scale factors. Then, sensitivity of MID to these parameters was examined by means of nonlinear dynamic analyses. Moreover, effect of ground motion incidence angle on the amplitude of scale factors was studied and variation of isolator displacement according to the ground motion rotation was discussed. Finally, amplification on MID due to mass eccentricity on superstructure was investigated systematically by considering a wide range of eccentricity ratio, lead rubber bearings (LRB) with different characteristic strength to weight ratio and isolation period. Amplification in MIDs due to mass eccentricity in the superstructure was also compared with the suggested values of simplified analysis method for eccentric seismically isolated structures. In all analyses, nonlinear modeling of LRBs was performed by means of deteriorating hysteretic behavior in which the lateral strength of isolator reduces due to temperature rise in the lead core during cyclic motion.