Progressive Collapse Resistance of Steel Frame Structures with Different Connection Stiffness Considering the Corrosion Effect

Steel corrosion reduces structural cross-sections, mechanical properties, and durability, threatening structural safety. Beam–column joint stiffness also significantly influences the overall performance of steel frames. This study investigated the progressive collapse resistance of corroded steel frame joints considering different connection stiffnesses and corrosion levels. A planar substructure consisting of one column and two half-span beams was adopted, and the central-column-loss scenario was investigated using displacement-controlled nonlinear static analysis to obtain the resistance–displacement response. The results showed that welded connections exhibited the highest strength and initial stiffness but experienced pronounced resistance degradation after fracture initiation. Fully bolted connections exhibited greater deformation accommodation capability but the lowest strength and initial stiffness. Welded–bolted connections provided intermediate strength and stiffness and achieved a relatively balanced response in terms of load-carrying capacity, energy absorption, and deformation accommodation capability. Under the investigated corrosion conditions, all three connection configurations exhibited progressive capacity degradation with increasing corrosion. The welded–bolted connection maintained relatively stable mechanical performance, whereas the fully bolted connection showed the greatest sensitivity to corrosion-induced deterioration among the three configurations. Among the three connection configurations examined, approximately 10% corrosion emerged as an indicative transition level for progressive collapse resistance within the investigated range. These findings provide a practical basis for connection selection and corrosion-management strategies in the progressive-collapse-resistant design and maintenance of steel frames.