Estimating seismic response under bi-directional shaking per uni-directional analysis: Identification of preferred angle of incidence

Implications of incidence angle of ground motions on inelastic demand of bridge piers under bi-directional seismic excitation have been studied against the corresponding uni-directional counterparts. Recognizing that the bi-directional analysis is complex and computationally intensive, to improve the use of less-rigorous unidirectional analysis, present study identifies the most and the least preferred orientations for a given pair of horizontal ground motion. We define the most and least preferred orientations, where the difference between bidirectional and uni-directional response is respectively minimized and maximized. These orientations can be uniquely identified for both peak and cumulative demand in terms of appropriate ground motion parameters (energetic length and characteristic intensity respectively for peak and cumulative demand) independent of structural properties. It has been shown that the improved estimate of bi-directional response may be obtained from simplified uni-directional analysis by utilizing these preferred incidence angles in conjunction with appropriate combination rules (30% for peak and 40% for cumulative demand).