Post-fire progressive collapse resistance of beam-column substructures with RBS connections

Steel frame structures repaired after fire exposure exhibit markedly different collapse behavior, compared to their performance under ambient conditions, when subjected to extreme loads. This study investigates the progressive collapse resistance of steel frame structures with reduced beam section (RBS) connections in post-fire conditions, using ten beam-column substructures: one tested at room temperature and nine exposed to various fire conditions. Results show that fire exposure shifts the failure from the RBS to the beam-column connection, significantly impairing the RBS's ability to relocate the plastic hinge, especially at higher fire temperatures. Fire temperature significantly affects collapse resistance, especially above 600 °C, whereas fire duration has a comparatively smaller influence on deformation capacity, particularly at 800 °C. Elevated temperatures weaken tensile catenary action (TCA), with substructures exposed to 800 °C for 90 min failing to transition to the TCA-dominated stage. Numerical simulations show that for substructures exposed to 400 °C and 600 °C, collapse resistance increases with greater flange reduction length, while the relationship between collapse resistance and starting reduction distance follows a rise-and-fall pattern. At 800 °C, collapse resistance remains relatively consistent across different starting reduction distances, but increasing the reduction length initially enhances and then reduces resistance. Increasing the reduction depth to 30 mm significantly reduces both the flexural and tensile capacities of the RBS region, shifting the failure mode from the beam-column connection to the RBS region.