Evaluation of Critical Input Directions for Establishing the Maximum Response of Structures under Bidirectional Seismic Action: Case Study of Eight-Story Structures

Abstract

This study quantitatively evaluated the level of conservatism or accuracy of four typical directions used to assess the maximum structural response over all nonredundant seismic input angles: fault-normal and fault-parallel (FN and FP), the direct maximum direction (MD), the maximum peak ground velocity (PGV) direction, and the principal axes directions. Two eight-story models of symmetric and asymmetric buildings were developed, and two ensembles of ground motion records were selected considering three varying ground motion intensities. Bidirectional nonlinear response-history analyses (RHAs) were performed by rotating each pair of ground motion horizontal components to 26 orientations, from 0° to 170° in increments of 10° as well as four critical and their orthogonal directions. The effect of rotating the ground motions on several engineering demand parameters (EDPs) was investigated and the seismic demands in four critical directions were compared and evaluated deterministically and probabilistically. The four critical directions were re-evaluated by considering the largest response between two orthogonal pairs. The results show that the response in the maximum PGV direction is very close or equal to the maximum value over all orientations, especially for high levels of ground motion intensities, and thus is recommended for ground motion selection in performance-based seismic design and assessment. In addition, the influence of rotation angle is effectively reduced when using the larger seismic response value of the two orthogonal pairs while maintaining the same number of RHAs.