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

After steel frame structures experience a fire and are subsequently subjected to extreme loads, the mechanisms of progressive collapse resistance and the weak points in the load transfer path of the remaining structure may differ from those observed under ambient conditions. This study investigates the progressive collapse performance of steel frame structures with reduced beam section (RBS) connections post-fire, using ten beam-column substructures—one at ambient temperature and nine exposed to varying fire conditions. Results indicate that fire temperature more significantly impacts collapse resistance and deformation capacity than fire duration. Post-fire, the failure mode shifts from the reduced section to the joint weld connection, compromising the RBS’s ability to relocate the plastic hinge. Numerical simulations show that reinforcing the beam-column weld delays failure but does not substantially improve collapse resistance. However, flexible reinforcement with V-shaped stiffening plates markedly enhances both collapse resistance and deformation capacity, with ultimate load improvement being approximately twice that of ultimate displacement. Determining the appropriate corrugation height is crucial; insufficient height impedes deformation, while excessive height becomes effective only after substantial damage to the RBS. This study underscores the significance of selecting a corrugation height of 0.15 times the beam depth, which optimally balances energy dissipation and plastic deformation capacity with the post-fire progressive collapse resistance of the substructure, offering critical guidance for the design of reinforcement in steel frame structures.