A simplified approach for including the incidence angle effect in seismic risk assessment

A simplified procedure is developed to consider the azimuthal orientation of buildings when estimating seismic risk. Two square‐plan reinforced concrete building models are considered as a testbed, one with similar and one with dissimilar properties along the two principal horizontal axes. The fragility of both structures is analysed using a set of ground motion records rotated to multiple incidence angles to develop orientation‐dependent fragility functions. It has been observed that, re‐orienting all records so that these structures have the same azimuth vis‐à‐vis the corresponding epicentre leads to significant differences compared to assuming random orientations. Additional results stemming from single‐degree‐of‐freedom oscillators further confirm such findings, showing a dependence to the proximity to the faults and the level of dissimilarity in the principal horizontal axes of the structure. The end results point to a non‐negligible bias in assessment studies when a structure's orientation with respect to governing rupture scenarios is not taken into account. It is shown that the median of fragility curves calculated for un‐rotated incidence angles can be bias‐corrected through shifted by an amount that depends on the azimuthal orientation and level of axes‐dissimilarity of structures.