INFLUENCE OF ANGLE OF INCIDENCE ON THE SEISMIC DEMANDS FOR INELASTIC STRUCTURES SUBJECTED TO BI-DIRECTIONAL GROUND MOTIONS

Abstract

This study investigated the influence that angle of incidence of applied bi-directional ground motions had on several engineering demand parameters (EDPs) for inelastic structures. The EDPs of interest in this study were peak drift, peak ductility, and peak slab rotation demands. The structural models had various degrees of inelasticity, plan irregularities, 5% damping ratios, and fundamental periods that ranged from 0.2 seconds to 2.0 seconds. This work utilized suites of ground motions recorded on stiff soils and on rock. The critical angle (the angle of incidence at which an EDP achieves a maximum) for a given EDP and bi-directional ground motion was found to occur at virtually any angle of incidence. For a given bi-directional ground motion and given fundamental period, the critical angle was found to vary unpredictably with increasing degree of inelasticity. The results also indicated that, on average, applying bi-directional ground motions only along the principal axes of an inelastic building underestimated the peak deformation demands when compared to those obtained at other ground motion angles of incidence. For a given degree of inelasticity, the average ratio of peak deformation responses based on all angles of incidence to the peak deformation response when the ground motion components were applied along the principal building orientations increased with fundamental period of vibration. Specifically, the results from this study indicated that for small and moderate degrees of inelasticity, average values for maximum inelastic deformation demands relative to the principal orientation ranged from 1.1 to 1.8 (for fundamental periods ranging from 0.2 seconds to 2.0 seconds). In addition, the standard deviations of such ratios are typically on the order of 0.15 to 0.8 which can be approximated by the standard deviation of the spectral ground motion component ratios. In this context, spectral component ratios refer to the ratio of spectral horizontal component accelerations at the fundamental period of the structure. A statistical analysis of the results also demonstrated that these ratios of peak deformation demands are weakly dependent on moment magnitude and distance for ground motions and structural systems with characteristics consistent with those used in this study.