Post-Punching Behavior of Flat Slabs

Reinforced concrete flat slabs are a common structural system for cast-in-place concrete slabs. Failures in punching shear near the column regions are typically governing at ultimate. In case no punching shear or integrity reinforcement is placed, failures in punching develop normally in a brittle manner with almost no warning signs. Furthermore, the residual strength after punching is, in general, significantly lower than the punching load. Thus, punching of a single column of a flat slab overloads adjacent columns and can potentially lead to their failure on punching, thus triggering the progressive collapse of the structure. Over the past decades, several collapses have been reported due to punching shear failures, resulting in human casualties and extensive damage. Other than placing conventional punching shear reinforcement, the deformation capacity and residual strength after punching can also be enhanced by placing integrity reinforcement to avoid progressive collapses of flat slabs. This paper presents the main results of an extensive experimental campaign performed at the Ecole Polytechnique Fédérale de Lausanne (EPFL) on the role of integrity reinforcement by means of 20 slabs with dimensions of 1500 x 1500 x 125 mm (≈5 ft x 5 ft x 5 in.) and various integrity reinforcement layouts. The performance and robustness of the various solutions is investigated to obtain physical explanations and a consistent design model for the load-carrying mechanisms and strength after punching failures.

Post-punching behavior of reinforced concrete slabs

Yaser Mirzaei

Reinforced concrete flat slabs are extensively used in buildings and parking garages. Their design is governed by deflection at the serviceability limit state and punching shear at the ultimate limit state. When no punching shear reinforcement is provided, failure develops in a brittle manner. Punching shear failure occurs with almost no warning signs, because deflections are small and cracks at the top side of the slab are usually not visible. Over the past decades, several structural collapses occurred due to punching shear failure resulting in human casualties and large damages. These collapses revealed some shortcomings in codes of practice and the necessity of reconsidering punching provisions. The investigations of these collapses showed that the collapse initiated from a local punching failure and propagated throughout the structure, in a progressive collapse. The term progressive collapse refers to the spreading of an initial local failure triggered by the loss of one or more load carrying members and leading to partial or total collapse of the structure in a manner analogous to the chain reaction. As a local punching failure can trigger progressive collapse, the study of the post-punching behavior can help adopting constructive solutions to avoid progressive collapse. The post-punching behavior of flat slabs supported by columns has not yet been thoroughly investigated. Therefore, an extensive experimental campaign was performed in the framework of this dissertation to investigate the post-punching behavior of 24 slabs with various reinforcement layouts. The effects of bending reinforcement, integrity reinforcement, bent-up bars, steel type, and anchorage conditions on the post-punching behavior of slab-column connections were investigated. The performance and robustness of the various solutions was investigated to obtain physical explanations of the load-carrying mechanisms. Test results showed that the post-punching strength provided by the top reinforcement is small because the concrete cover is thin and spalling of the concrete cover occurs leaving the reinforcement ineffective. In addition, it was observed that integrity reinforcing bars passing through the column significantly improve the post-punching behavior in terms of strength and deformation capacity. The integrity bars behave as a tensile membrane inclined to the plane of the slab and are able to sustain damaged portions from the column. Thus, one possibility to enhance the robustness of the structure against progressive collapse is to provide well-anchored bottom reinforcing bars passing through the column. A mechanical model capable of predicting the post-punching behavior of slab-column connections without shear reinforcement was developed. The model predicts the contribution of the tensile reinforcement and of the integrity reinforcement to the post-punching strength. The model accounts for possible failure modes including the fracture of the bars and the destruction of the concrete over the integrity bars. The progressive destruction of the concrete within and outside the punching cone is treated by considering the pullout behavior of reinforcement embedded in the concrete. Finally, a parametric study was performed to evaluate the influence of various parameters and their relative importance in order to develop practical proposals for the estimation of the post-punching strength. It showed that the post-punching strength is not only a function of the cross sectional area and yield strength of the integrity reinforcement as it appears in provisions and codes of practices but also of the diameter of the bars, the effective depth, the ductility, and the type of reinforcement.

EPFL2010

Experimental investigation of reinforced concrete slabs with punching shear reinforcement

Stefan Lips, Aurelio Muttoni

Over the past century, the use of flat slabs in buildings and especially in parking garages has been growing as it is an economic and efficient solution. Flat slabs are easy to build and have, through their smaller depth, an economical and architectural advantage compared to slabs on girders. Because of their limited depth, flat slabs are especially sensitive to deflections and to punching shear, which are their main design criteria. Furthermore, flat slabs without punching shear reinforcement have a rather brittle failure mode with little deformation capacity. This behavior potentially limits the redistribution of the internal forces in flat slabs in the case of punching of an isolated column and can thus lead to progressive collapse of the entire structure. To increase both the strength and the deformation capacity of flat slabs, punching shear reinforcement can be provided in the vicinity of the columns. Although the influence of punching shear reinforcement on the strength of flat slabs has been intensively investigated, there are still fundamental uncertainties such as the contribution of the shear reinforcement and the strength of the concrete struts close to the column. The paper presents the results of an extensive experimental campaign performed at the Ecole Polytechnique Fédérale de Lausanne (EPFL). Sixteen full-scale slab specimens (3.0 x 3.0 m in plane) with varying parameters such as the column size, the slab thickness, the shear reinforcement ratio, and the shear reinforcing system have been investigated. The performance of these specimens is analyzed and compared to modern design codes and to the critical shear crack theory.

Proceeding of the 8th fib-PhD Symposium2010

Post-punching behavior of reinforced concrete slabs

Yaser Mirzaei

EPFL2010