Abstract: To investigate the progressive collapse resistance of steel frame structures after fire exposure, pushdown loading tests were conducted on beam-column substructures with bolt-welded hybrid connections and middle column failure, under both ambient and post-fire conditions. The study examined the resistance mechanisms and failure modes during the collapse process. The test results showed that under ambient conditions, the substructure failed at the first row of bolts, while the post-fire substructure failure occurred closer to the beam-column connection. Compared to ambient conditions, the catenary effect in the later stages of loading showed significant degradation in the post-fire substructure, with ultimate load and ultimate displacement decreasing by 13.7% and 16.9%, respectively. Additionally, a refined numerical model was developed using the finite element software ABAQUS, and its accuracy was validated against the experimental results. The study was then extended to explore the effects of three reinforcement methods-cover plate reinforcement, rib plate reinforcement, and web plate reinforcement-on the progressive collapse resistance of post-fire substructures. The finite element results indicated that all three reinforcement methods enhanced the collapse resistance and deformation capacity of the substructures with bolt-welded hybrid connections effectively.

Key words: fire, steel frame structure, progressive collapse, beam-column reinforcement