Fragility analysis of rectangular and circular reinforced concrete columns under bidirectional multiple excitations

This paper presents a numerical investigation on seismic performance of reinforced concrete (RC) cantilever columns under unidirectional and bidirectional excitations. The influence of cross-sectional geometry and multiple excitations have been examined in this study. An advanced nonlinear finite element model is employed to model different failure modes of RC columns under seismic excitation. The model simulates degradation of materials under cyclic loading, including inelastic buckling and low-cycle fatigue degradation of longitudinal reinforcement. A series of monotonic pushover and incremental dynamic analyses (IDA) are conducted on hypothetical rectangular and circular columns. Proposing a unique algorithm, an existing inclusive damage index is implemented to quantify the different sources of damage including flexural, shear and reinforcement slippage damage under bidirectional excitation. Ground motion records are carefully selected using conditional mean spectrum (CMS) to generate as-recorded real mainshock and aftershock (MSAS) sequences. Results show that multiple bidirectional excitations significantly increase the damage that accumulates in RC columns. Moreover, inelastic buckling and low-cycle fatigue degradation of longitudinal bars have an evident contribution to the failure of RC columns. It is also found that the rectangular column is more prone to collapse under bidirectional loading in comparison to circular section. The analyses results show that the impact of bidirectional excitation on the seismic performance of the studied cantilever columns is considerable. This implies that seismic performance assessment of RC structures using unidirectional excitation can be biased.