Fragility and strength dependency of prefabricated concrete frames under sequential earthquake‐blast loading

Prefabricated concrete frames (PCFSs) subjected to blast impacts following initial earthquake damage exhibit significantly different damage evolution and failure probabilities compared to single‐hazard actions. This study establishes a refined finite element model accounting for cumulative damage to quantitatively assess the influence of concrete strength on structural blast fragility under earthquake‐blast sequences. A two‐stage numerical analysis workflow is constructed employing the RHT concrete constitutive model and a prefabricated joint interface degradation simulation method. A total of 756 sequence samples were generated using three concrete strength grades ( C 40, C 50, C 60), three earthquake intensity levels (PGA = 0.1g, 0.3g, 0.5g), six earthquake records, and 14 TNT equivalents. Blast intensity and limit states are characterized using the scaled distance Z and end rotation thresholds, and fragility curves were fitted using a log‐normal model and different damage levels. Results indicate that, for identical earthquake intensities, increasing concrete strength grades significantly reduces the exceedance probability across all damage states, with fragility curves shifting collectively toward smaller Z values. At equivalent strength grades, the overall structural damage probability elevates with PGA raising from 0.1g to 0.5g. Taking the C 40 structure at Z = 0.232 m/kg 1/3 as an example, the exceedance probabilities for Minor Damage, Moderate Damage, and Severe Damage increase by approximately 18.4%, 56.9%, and 38.7%, respectively, when PGA rises from 0.1g to 0.5g. These results demonstrate that post‐earthquake initial conditions significantly amplify subsequent blast response, with the moderate‐damage state being the most sensitive to seismic pre‐damage. The proposed two‐stage analytical framework and fragility results may serve as a reference for sequential disaster analysis and parameter sensitivity studies of similar structures.