Influence of angle of incidence on seismic demands for inelastic single-storey structures subjected to bi-directional ground motions

This study examines the influence that the angle of incidence of the ground motion has on several engineering demand parameters (EDPs) for a single-storey structure subjected to bi-directional ground motions. The models in this work had various degrees of inelasticity, were subjected to a set of 39 ground motion pairs for which nonlinear time histories were conducted, and had fundamental periods that ranged from 0.2 to 2.0 s for both symmetrical and asymmetrical structures. It is demonstrated that applying bi-directional ground motions only along the principal axes of an inelastic building underestimates the inelastic peak deformation demands when compared to those obtained at other angles of incidence. Although an optimal building orientation that minimizes demands for all the EDPs considered for a given model cannot be determined explicitly, 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 motions are applied at principal building orientations shows stable trends. Generally, these ratios increase with the fundamental period of vibration. These average ratios typically vary between 1.1 and 1.6; however, ratios for individual ground motions can be as high as 5 for the EDPs examined. Maximum responses for individual ground motions were found to occur for virtually any angle of incidence and varied with the degree of inelasticity, which implies that inaccurate estimates of structural performance and damage may result if based on ground motions applied at principal orientations alone.