Bidirectional seismic fragility analysis of double-deck viaduct frame piers based on stripe-cloud method

To evaluate the influence of bidirectional seismic action on the seismic fragility of double-deck viaduct fram pier (DVFP), this study employs a vector-valued intensity measure (IM) [PGA, Sa (T₁, 5 %)], in tandem with the stripe-cloud method. The study compares the median values of Sa and the damage probability difference index (DI) to analyze the fragility of DVFP under both unidirectional and bidirectional seismic actions. Additionally, this research investigates the sensitivity of bidirectional seismic fragility of DVFP to the incidence angles. The results indicate a notable decrease in the median Sa values for DVFP are observed under bidirectional seismic action, with decreases of 28.6 %, 24.3 %, 20.9 %, and 18.8 % corresponding to four damage states, as compared to those under unidirectional seismic action, and the corresponding parameter DIs are measured at 0.122, 0.182, 0.245, and 0.291, respectively. Notably, the most severe damage to DVFP occurs when the bidirectional seismic incidence angle is within the range of 120–135°. At this angle, the median Sa values at four damage states are 85.8 %, 86.1 %, 87.6 %, and 88.6 %, respectively, compared to the angle of 0°. Therefore, neglecting the influence of bidirectional seismic incidence angles may lead to an underestimation of the bidirectional seismic fragility of DVFP.