Experimental tests of post-fire beam-column assemblies with WUF-B connections against progressive collapse

Fire-induced loss of column members is a unique scenario of localized failure that could further trigger the progressive collapse of frame structures. This paper presents an experimental study on the anti-collapse performance of beam-column assemblies with WUF-B connections under a column removal scenario due to fire conditions. A total of ten assemblies included nine specimens subjected to different fire conditions, and the remaining single specimen without fire treatment was designed and tested under a fire-induced middle column loss scenario. The effect of the fire conditions on the anti-collapse mechanism along the whole loading process was comprehensively analyzed. The test results demonstrated that all beam-column assemblies fractured at one beam end of the middle column whether the specimens had been subjected to the fire treatment or not. Owing to the pressure-bearing failure process with high ductility after the fracture of the lower flange, the WUF-B connections could effectively maintain the capacities of the specimens at high levels and made the specimens develop greater deformation capacity. During the overall loading sequence, the flexural mechanism was the principal mechanism against the progressive collapse of the structure, while the catenary mechanism generally played a leading role after the vertical displacement exceeded 50% of the maximum displacement. Both the flexural and catenary mechanism resistances declined to different extents. The anti-collapse performances of the post-fire assemblies deteriorated compared with those without fire treatment. Significantly, with the increasing fire temperatures, the mechanical flexural actions gradually degraded and the respective catenary actions did strengthen instead. However, the fire durations exerted a minor influence on the development process of the resistance mechanism. The plastic rotation angles of assemblies with WUF-B connections after fire conditions changed at the failure state within the range of 0.161 to 0.209 rad and the respective variation at the limit state was 0.202 to 0.278 rad. By contrast with the test plastic rotation angles at both failure and limit states, it was found that the modeling parameters provided in DoD guidelines are too conservative for evaluating the progressive collapse performance of beam-column assemblies with WUF-B connections.