Comparative Study between Horizontal–Vertical Mixed and Vertical Progressive Collapse Responses of Reinforced Concrete Planar Frames

Inadequate lateral restraint of side columns in RC frames can lead to lateral deformation of the side columns during progressive collapse, causing RC frames to transition from vertical progressive collapse to horizontal and vertical mixed progressive collapse. In this study, two quasi-static tests were performed on 1:3 scaled-down specimens of RC planar frames under the scenario of middle column removal. The collapse responses of the RC planar frames under mixed and vertical collapse conditions were compared by changing the restraint conditions of the side columns from free lateral restraint to fixed lateral restraint. Based on the energy conservation principle, the dynamic progressive collapse resistances of the RC planar frames under mixed and vertical collapse conditions were evaluated. A finite-element method was applied to supplement the intermediate lateral restraints of the side column between the free and fixed restraints, and the functional relationships between the peak resistance amplification factors of the RC planar frames and the number of bays of the adjacent frames were fitted. The results indicated that insufficient lateral restraint of side columns may hinder the development of progressive collapse resistance of RC planar frames. Under the mixed conditions of horizontal and vertical mixed collapse, the maximum collapse resistance of the RC planar frame is less than 0.28 times that of the vertical collapse condition, and the maximum collapse resistance occurs earlier than the vertical collapse. The lateral load exerted by the frame beams on the side columns during the catenary action stage can not only cause a large lateral deformation of the side columns, but also transfer the final failure position of the RC planar frame from the middle beam–column joint to the side column. The maximum dynamic collapse resistance of the RC planar frame under the mixed collapse condition decreased by 51.83% compared with the vertical dynamic collapse resistance. Increasing the number of bays in the unilaterally restrained frame had no effect on the collapse resistance of the RC planar frame. However, as the number of bays in the bilateral restrained frame increased, the lateral restraints of the side column changed from free to fixed restraints, and the collapse direction of the RC planar frame shifted from mixed to vertical.