SIMULATION ANALYSIS ON PANCAKE COLLAPSE RESPONSE OF A STEEL FRAME UNDER SEISMIC ACTION

A numerical model based on the explicit finite element package LS-DYNA was proposed to investigate the pancake collapse response of a 9-story steel frame under seismic action. The numerical model was validated by a shaking table collapse test for a spacial slab-column structure, and then the pancake collapse process of the steel frame was simulated and analyzed. It is found that the drift angle of the weak story expanding larger than 1/10 leads to pancake collapse, then the upper part of the structure moves downwards and collides with the lower part of the structure, and causes a large axial force in the sub-story column, which expands the P-Δ effect and leads to progressive collapse of the steel frame. The dynamic increase factor of the sub-story column may be larger than the proposed value in the code. It is indicated that the pancake collapse can be prevented by enhancing the lateral stiffness of the weak story, and the collapse would not happen by a reasonably design if the drift angles of the story converge though larger than the code limit.