Punching strength of actual two-way slabs

Many building codes rely on empirical formulations with respect to the punching shear design provisions. Such empirical equations are typically calibrated on the basis of laboratory tests performed on simplified geometries (usually isolated specimens) with usually statically-determined loading conditions. On the contrary, actual structures, particularly two-way slabs, are continuous and highly redundant systems. The isolated test specimens are thus unfortunately not always suitable to reproduce the actual behaviour of flat slabs and empirical models calibrated on their basis may not be applicable for their design. In this paper, the application of the Critical Shear Crack Theory (theoretical support of Model Code 2010 provisions) is shown for some unusual test specimens aiming at investigating actual inner bays of continuous flat slabs. This analysis shows that moment redistribution between the column region and mid-span develop after concrete cracking and that accounting for this behaviour is required to obtain good predictions of the actual flat slab response. It is also shown that the Critical Shear Crack Theory leads to consistent results in terms of failure mode, punching shear strength and deformation capacity as it allows accounting for this behaviour. On the contrary, empirical design formulas fail to predict the influence of continuous flat slabs on the punching strength.