About the reliability of punching verifications in reinforced concrete flat slabs

Reinforced concrete slabs are a widely diffused structural solution either in Italy, or abroad; this for a series of advantages connected to their structural conception and their performances. However, this series of advantages is obtained as a result of proper design, especially oriented to appropriately sizing the thickness of the plate itself. Moreover, for flat structures with concentrated loads, as the case of flat slabs on punctual supports, phenomenon of punching can't be neglected, as it inevitably affects the structure and so it must be taken into account even in the early stages of the project. In this paper an attempt to evaluate the reliability of punching verifications has been made, referring to some in force regulations; this has been possible making a comparison between the mean resisting value of punching, obtained applying law prescription and the real collapse load for some columns belonging to a building that collapsed during the early stages of its construction. For the specific case study analyzed, there has been the opportunity to perform an on-site investigation, collecting a great amount of information regarding the mechanical properties of the used materials, the real positioning of the rebars in the structural elements, the real amount of concrete cover and so on. Since in a punching failure mechanism a crucial element is the resistance of the concrete, the precise definition of its properties attains great importance, especially when existing buildings are involved. To analyze the exact conditions of collapse and the factors that may have originated it, an important procedure is the acquisition of information, as much as possible, about the mechanical properties of the onsite materials. After this first evaluation of the accuracy of the actual regulations, another interesting comparison has been carried out, studying the influence of two main factors in the definition of the punching shear resistance; more specifically, it has been observed which is the contribution of the variability of the compressive strength of the concrete on site and the effective depth of the plate. The first related to a series of circumstances that may affect the value of the resistance, reducing it from that established during the design phase, the second closely related to inaccuracy in the laying of the longitudinal reinforcement. Therefore two sensitivity analysis have been performed, either referring to Eurocode 2, or following the prescriptions given in Model Code 2010, varying once the mean value of the compressive strength of the concrete, once the effective depth d