Seismic overturning fragility analysis for freestanding building contents subjected to horizontal bidirectional floor motions

Abstract

The rocking response of freestanding building contents simplified as rigid bodies was usually investigated in two dimensions with ground motions as the input, while their three-dimensional (3-D) rocking response subjected to horizontal bidirectional floor motions needs further investigation in both deterministic and probabilistic views. In this paper, the rocking response of freestanding 3-D rectangular blocks subjected to seismic excitations with two horizontal components are first investigated by shake table tests and finite element (FE) numerical simulation. An incremental dynamic analysis-based overturning fragility assessment method for the blocks within a building subjected to bidirectional ground motions is then proposed, using uncoupled nonlinear FE models of the block-building systems. The overturning fragility of six blocks within a four-story reinforced concrete frame structure is assessed. The influence of excitation directions, floor levels, block slenderness ratios (2.5, 3, 3.5 and 4), block sizes (size parameters of 91 mm, 182 mm and 364 mm) and intensity measurements (IMs) on the overturning fragility of the block is characterized. The experimental overturning fragility curves of three blocks in real conditions under bidirectional seismic excitations are finally obtained from the shake table tests. Rocking-torsional rotations are observed for the block with sliding not allowed. The block rocks around a side or a vertex while overturns around a side. Except the largest block, the block under bidirectional excitations is usually more and sometimes equally vulnerable to overturn in comparison to that under unidirectional excitations. The block on a higher floor is found to be more vulnerable to overturn than that on a lower floor considering the peak ground acceleration-based IM. These observations highlight on the significance of 3-D analysis for the rocking blocks considering floor acceleration amplification. This paper also enlarges the experimental database of seismic rocking response of rectangular blocks under bidirectional horizontal motions.