Abstract:To investigate the post-fire residual capacity and failure mode of the concrete beam-slab specimens, the mechanical tests of three fire-damaged specimens and two reference specimens were conducted. The cracks, failure modes, deflections, and concrete and steel strains were observed during each test. Based on the conventional yield line method, the stiffness and deflection of the beam were considered to establish the theoretical method, and the residual ultimate load and failure modes of the specimens can be analyzed, and the predicted results were compared to the experimental results. Results showed that different mechanisms of the reference and fire-damaged specimens led to different failure modes, including the first-panel failure and the first beam failure. In addition, due to the post-fire spalling, the concrete cover of the fire-damaged panel fell off, and its effective thickness and bond strength degraded. Thus, apart from the flexural failure, the punching failure mode of the fire-damaged specimen also occurred. Compared to the reinforcement layout, the reinforcement ratio had a considerable effect on the ultimate loads of the beam-slab specimens, increasing the reinforcement ratio of the panel led to its higher carrying capacity and the brittle failure easily appeared. The conventional simply supported and fixed yield line method tended to underestimate or overestimate the ultimate loads of the specimens. For the present method, the effect of the edge beam’s stiffness and the vertical deflection was considered, and thus it could reasonably predict the ultimate loads, the failure mode and the reinforcement or repair method of the fire-damaged beam-slab specimens.