Dynamic punching shear of impacting RC flat slabs with drop panels

Reinforced concrete flat slabs (FS) are sensitive to punching shear failure. Such failure may progress to the slab loss of supports and its downwards motion that is followed by its impact with the slab underneath. This slabs impact may initiate a progressive collapse of the entire building. The design guidelines provided by modern standards to avoid punching shear failure and to prevent post-punching loss of slab support in an attempt to prevent progressive collapse are based on static loads considerations and tests and do not include considerations and guidelines that are needed to guarantee resistance to impact loading. In our studies on the impact response of impacting slabs we learnt that increasing the slab thickness and the amount of reinforcement do not improve the slabs resistance to impact and cannot prevent the severe resulting damage. Drop panels are a common means to enhance the static resistance of slabs against punching shear as they increase the slab thickness around the column, where highest shear stresses are developed. The present paper aims at the investigation of the impact response of flat slabs with drop panels and examine their added contribution to reduce the post-impact damage and achieve a stable state of the impacted slab, thus preventing a following progressive collapse scenario. In addition to the local slab depth increase, the drop panel also limits the contact zone of the impacting slabs. Both aspects positively contribute to reducing the impact induced damage. The drop panel provides a means to locally increase the slab resistance without considerably increasing its mass, thus controlling the impact load intensity while increasing the slab's resistance. The paper investigates the impacts of different possible combinations of two adjacent slabs with/without drop panels and finds that while flat slabs without drop panels fail upon impact, slabs with drop panels may survive the impact and avoid the following progressive collapse scenario. Opposed to static design where a drop panel contributes to the shear resistance, it was found that in the case of impact loading a large size drop panel increases the mass of the impacting slabs and may adversely affect the impact results, whereas a small size drop panel may not provide the required thickened zone. Examination of different drop panel geometries indicates that optimal drop panel dimensions exist which provide a stable final state which is associated with the minimum damage and a minimum permanent deflection. At larger or smaller drop panels the damage is larger and failure may occur. These are important findings that may help to improve the safe design of RC flat slabs.