Evaluation of punching shear design criteria to prevent progressive collapse of RC flat slabs

This study focuses on a RC building with flat slabs that were designed according to current standards. A scenario of a slab with failed connections that is detached from its supporting columns and is falling downward is considered, and the impact results with the slab underneath are assessed. The suitability of the standards design criteria to provide safe design against impact loading is evaluated. It was found that larger span slabs experience heavier damage. Falling from a floor height and from a quarter floor height are analyzed and the damage results of these impacts are examined. At the lower impact velocity, the concrete slab surrounding the column undergoes major damage and shear deformations; In the case of a relatively short span slab, the rebars undergo large plastic deformations and almost reach the ultimate strain, such that a slightly higher impact velocity would cause rebars fracture and total failure of the slab-column connection. In the case of high impact velocity, the concrete in the slab around the column is fully crashed and the longitudinal and the bent up rebars are ruptured. The yield of the rebars occurs within a few milliseconds. During this extremely short time the impacted slab hardly starts developing its downward displacement. The impacted slab responds like a rigid body with severe damage concentration at the slab-column connection region. Different parameters affecting the slabs dynamic response are examined, and new insight is gained on the complex impact response of flat RC slabs. This study finds that the current design standards that are based on static loading considerations do not provide resilience to flat slab connections that are subjected to impact loading and therefore cannot prevent a progressive collapse scenario.