Beam torsion effect of monolithic precast concrete beam-column substructures during progressive collapse

To investigate the effect of spandrel beam torsion on the progressive collapse resistance of the monolithic precast concrete (PC) spatial frames, three PC substructures (a cantilever beam substructure PC-c, a double-span planar substructure PC-p and a spatial substructure PC-s) were designed. They were tested under a middle column removal scenario, to simulate the progressive collapse of the PC frame structures. The test results indicated that the torque developed in spandrel beams of the spatial substructure PC-s leaded to more severe concrete cracking and crushing at its beam ends. The existence of torque in the PC spatial substructure weakened the mobilization of compressive arch action (CAA) and catenary action (CA). The CA capacity of the spatial PC substructure was reduced by 27.23% compared with the planar substructure PC-p, and the CA action was ended in advance. Additionally, a three-dimensional joint model considering the torsion effect was proposed herein, which accurately captured the progressive collapse behavior of spatial substructures. Based on the verified numerical model, the difference between the combined torsion-bending interaction in the spandrel beam of planar and spatial substructures was analyzed. The effects of reinforcement configuration and span-depth ratio of spandrel beams on the collapse resistance were studied. Finally, a conservative calculation method of the increased gravity loads GN was given when the torsion effect needs to be considered.