Progressive collapse behaviour of composite substructure with large rectangular beam-web openings

This study explores the progressive collapse resistance of composite substructures with the fully-welded connection (FC) and fully-welded connection with rectangular web openings (FCRO). Quasi-static loading tests were conducted to analyze failure modes, internal forces, and resistance mechanisms. The effect of substructure's boundary constraints on its anti-collapse behaviour was investigated through validated numerical models. The results show that the damage mode of the FC specimen involves tensile beam flange fracture at the beam root, while the FCRO specimen experiences fracture of the rectangular web openings. The resistance development of the FCRO specimen includes bending, bending-catenary mixed, and catenary stages, whereas the FC specimen undergoes bending and bending-catenary mixed stages. In the bending-catenary mixed stage, the FCRO specimen exhibits a 69.2% higher maximum resistance and a 15.3% larger displacement compared to the FC specimen. Despite the reduction in flexural capacity, incorporating large rectangular openings in the beam web enables plastic hinge to shift outward at the beam root, promoting the effective evolution of the composite beam’s axial force. The study derived expressions for the boundary axial constraint coefficient and attenuation coefficient to the FCRO specimen. Traditional strengthening methods were found to have limited effectiveness in enhancing the collapse resistance of unreinforced composite substructures. Consequently, a novel and effective construction measure was proposed, resulting in a significant enhancement of 78% in maximum resistance and 85% in corresponding deformation of the substructure.