Blast-induced damage and residual performance assessment of steel box arch ribs under out-of-plane close-in explosions

Steel box arch ribs, as critical load-bearing components of arch bridges, may be subjected to explosion threats such as drone attacks during service. To evaluate the blast resistance and post-blast residual bearing capacity of steel box arch ribs under out-of-plane close-in explosions, in-plane and out-of-plane blast tests were conducted, followed by residual bearing capacity tests. A finite element model was established and validated based on the experimental results. The damage evolution mechanism, response characteristics, and effects of key parameters under out-of-plane explosions were further investigated. Compared with in-plane explosions, out-of-plane explosions induced more severe local damage and out-of-plane deformation. The failure modes were characterized by local buckling, denting, tearing, localized collapse, and global deformation, and were classified into slight damage, moderate damage, severe damage, and localized collapse. The arch plate was identified as the primary energy-dissipating component. Parametric analyses showed that increasing the cross-sectional size, arch plate thickness, and thickness and number of internal stiffeners effectively reduced the maximum and residual displacements. Among these parameters, the cross-sectional size had a more significant influence on the internal force response. Out-of-plane explosions resulted in greater deformation responses and energy inputs than in-plane explosions, indicating that blast direction is a critical factor affecting the blast resistance of steel box arch ribs. Compared with steel beams, steel box arch ribs exhibited larger residual displacements and smaller internal force responses, demonstrating different blast-response mechanisms. Post-blast residual bearing capacity tests further revealed that structural failure was mainly governed by local buckling. Increasing the arch plate thickness effectively reduced the damage indices of initial stiffness and ultimate bearing capacity, thereby improving the post-blast residual performance. A blast damage assessment method for steel box arch ribs was established based on the damage indices of initial stiffness and ultimate bearing capacity, providing a basis for blast-resistant design and post-disaster performance evaluation of steel arch bridges.