Seismic rocking fragility analysis of unanchored nonstructural components under combined horizontal and vertical near-fault ground motions

Earthquake reconnaissance reports often emphasize the loss of building contents, drawing significant attention to the seismic performance of unanchored nonstructural components (NSCs). However, the effect of vertical near-fault earthquake component on the seismic fragility of these unanchored NSCs has been rarely reported. This research evaluates the seismic fragility of unanchored NSCs under bidirectional near-fault seismic effects. The study investigates the rocking responses of slender NSCs, modeled as rectangular rigid blocks, under multi-directional excitations (both horizontal and vertical). Floor acceleration responses of three different archetype structures, subjected to near-fault ground motions, are used as inputs for NSCs. OpenSees models are developed for simplified NSCs under multi-directional inputs, and the peak rocking angle is taken as the engineering demand parameter. This study offers normalized fragility models to estimate the likelihood of NSCs' overturning, using different limit states of rocking responses for the development of fragility models. The results reveal that the vertical excitations increase the probability of exceeding the slightly rocking limit state, but have limited effects on fragility for moderate rocking and overturning limit states. In contrast, horizontal components remain the dominant factor in affecting severe rocking response and the overturning of unanchored NSCs.