Impact of two columns missing on dynamic response of RC flat slab structures

Sudden removal of columns caused by unexpected extreme loading may increase the bending moment and shear force at surrounding column-slab connections significantly, which may trigger punching shear failure at these connections and lead to progressive collapse of entire reinforced concrete (RC) flat slab structures. To quantify the dynamic load redistribution of flat slab structures subjected to different extents of initial local damage (one-column or two-column removal), two multi panel RC flat slab substructures were tested subjected to simulated sudden column removal scenarios. These two specimens have identical dimensions and reinforcement details. One of the substructures suffered a loss of an interior column scenario while another one was subjected to a two columns (one interior column and one edge column) missing scenario. The dynamic response, deformation shape, failure mode, and local strain gauge results are presented. It is found that although both specimens had exceeded their yield load capacity, no collapse occurred as considerable compressive membrane action developed in the RC slab to help redistribute the loads. With the drop panels, no punching shear failure was observed in the slab-column connections after removal of the column. To further study the progressive collapse robustness of flat slab structures, finite element (FE) models were validated and parametric studies were carried out. Numerical analysis indicated that the axial force initial carried by the lost columns may amplify up by 1.25 times before redistribution into surrounding columns. Moreover, the dynamic ultimate load capacity of Specimens S1 and S2 are as large as 1.92 and 1.18 times of their design service load, respectively.