On the numerical modelling of punching shear in flat reinforced concrete slabs by means of the finite element method

The slab-column connection is a critical point in the design of a structure, mainly buildings, since a high concentration of shear stresses can lead to punching, which is a localised failure mode that can take place in a brittle manner and with no previous warning. Currently, structural standards provide recommendations and general expressions to help design these structural connections, where the Critical Shear Crack Theory (CSCT), proposed by Muttoni, stands out. This approach considers the shear strength as dependent on the crack width developing in the shear-critical region and uses a control perimeter ($b_0$) that delimits the cracking region. Many efforts have been devoted to understanding the failure mechanisms involved in punching and to propose tools, such as CSCT, for a safe and efficient design; nevertheless, there is still no consensus on the mechanics governing this phenomenon. The present contribution uses the finite element method and takes advantage of material models based on continuum damage mechanics to reproduce punching failure in reinforced concrete slabs. This approach is not new and has been employed in the past, but with limitations and some issues still not completely solved. The aim of this work is to analyse different possible modelling techniques in order to obtain a numerical model that reproduces this phenomenon with accuracy. A major advantage of using a finite element model in this case is that the main fracture mechanisms involved in the failure process, which are varied and complex, can be identified. Bidimensional and tridimensional models are discussed, and the possibility of taking into account the slip between concrete and the reinforcement bars, which turns out to be a key mechanism in the evolution of punching failure, is analysed.