Impact of failure mode uncertainty on seismic fragility and collapse risk of buildings

Laboratory tests on nominally identical reinforced concrete (RC) components have demonstrated the existence of failure mode variability and its significant impact on the strength and deformation capacity of RC components. In comparison with record-to-record and modeling uncertainties, the impact of failure mode uncertainty on the seismic fragility of RC structural systems has received less attention. This study presents a methodology for propagating failure mode variability in the probabilistic seismic assessment of RC structural systems. In the proposed methodology, strength hierarchy calculations are used to identify the structural system's susceptibility to failure mode variability. Subsequently, a number of segregate models corresponding to the number of failure mode combinations are developed. Nonlinear response history analyses of the segregates are used to quantify each segregate's seismic fragility and risk. Finally, the total probability theorem is used to derive the combined seismic fragility of the structure. The proposed methodology is demonstrated using an older-type (pre-1970s) four-story RC frame building archetype with ground floor columns susceptible to failure mode switch between flexure- and flexure-shear mechanisms. The results show that the seismic fragility and collapse risk of the RC buildings with failure mode variability significantly changes when failure mode variability is propagated. In the example, accounting for component-level failure mode variability can shift the median collapse fragility by more than 20%. Furthermore, the collapse risk (i.e., probability of collapse in 50 years) of the archetype changed by at least 30%. Similar changes may be observed in other types of structures with significant failure mode uncertainty, not limited to RC structures.