Assessing punching shear failure in reinforced concrete flat slabs subjected to localised impact loading

Reinforced concrete flat slab structures are used widely in construction projects due to their economic and functional advantages. Punching shear failure in such structures can have catastrophic effects in the case of, for example, multi-storey framed structures and the designer aims to ensure that ductile flexural deformation occurs before the brittle shear failure. Shear mechanisms generally govern the behaviour of reinforced concrete structures subjected to localised impact loads. Existing experimental results investigating punching shear in flat slabs subjected to impact loading shows that when increasing the loading rate, the punching shear strength also increases whereas the deformation capacity reduces. This behaviour is due to a combination of inertial effects and material strain-rate effects which leads to a stiffer behaviour of the slab for higher loading rates. This can also lead to a change of mode of failure from flexural to pure punching shear with increasing loading rates. Current empirical formulae for punching shear are unable to predict this behaviour since the slab deformations are not considered for calculating the punching shear strength. This paper presents an analytical model based on the Critical Shear Crack Theory which can be applied to flat slabs subjected to impact loading. This model is particularly useful for cases such as progressive collapse analysis and flat slab-column connections subjected to an impulsive axial load in the column. The novelty of the approach is that it considers (a) the dynamic punching shear capacity and (b) the dynamic shear demand, both in terms of the slab deformation (slab rotation). The model considers inertial effects and material strain-rate effects although it is shown that the former has a more significant effect. Moreover, the model allows a further physical understanding of the phenomena and it can be applied to different cases (slabs with and without transverse reinforcement) showing a good correlation with experimental data. (C) 2014 Published by Elsevier Ltd.