Post-punching behavior of reinforced concrete slabs

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.

Post punching behaviour of reinforced concrete slab-column connections

Yaser Mirzaei

Reinforced concrete flat slabs are extensively used in buildings and parking garages. At the ultimate limit state, their design is usually governed by punching shear. In the case where no punching shear reinforcement is provided, failure develops in a brittle manner. Furthermore, punching shear failure of one column may propagate to adjacent columns, eventually leading to the total collapse of the structure. Punching shear failure occurs with almost no warning signs, because deflections are small and cracks at top side of the slab are usually not visible. Over the past decades, several collapses due to punching shear failures have occurred, resulting in human casualties and large damages. Some solutions can be adopted to enhance the post punching behavior of flat slabs and to avoid progressive collapse. This paper presents some results of an extensive experimental campaign performed at the Ecole Polytechnique Fédérale de Lausanne. The post punching behavior of 24 tested slabs, with various flexural reinforcement layouts are introduced and compared. The performance and robustness of the various solutions is investigated to obtain physical explanations of the load-carrying mechanisms.

7th International fib Symposium in Civil Engineering2008

Poinçonnement non symétrique des dalles en béton armé

Luca Tassinari

Punching shear of reinforced concrete flat slabs supported on columns is a failure mode in which the slab fractures around the column. This type of failure is sudden and can be catastrophic in nature. Punching shear generally governs the ultimate limit state design of flat slabs and slab bridges. The Critical Shear Crack Theory (CSCT) developed by Prof. Muttoni can be used to calculate the punching shear strength in axis-symmetrical cases. The main objective of this work was to investigate non-symmetrical cases of punching shear and to extend the CSCT to such cases. The failure criterion proposed by Prof. Muttoni allows estimating the punching shear strength in terms of the slab rotation in the radial direction. A more general model is needed to predict the punching shear strength of slabs with non-symmetrical conditions. A numerical model was developed, which is based on the finite difference method, to predict the flexural behaviour of flat slabs. It takes into account the non-linear behaviour of concrete and reinforcement steel and can be applied to non-symmetrical cases and complex structures such as flat slabs with different span length in each direction and slab bridges. Moreover, this model satisfactorily predicts the behaviour of slabs with orthogonal reinforcement and general load cases. A series of punching shear tests on non-symmetrical slabs was carried out to validate an analytical approach which considers the redistribution of shear stresses along the perimeter. This refined approach provides accurate predictions of strength and slab rotations in both orthogonal directions at failure. Moreover, this approach can be a useful tool for the structural assessment of existing structures. An analytical model is also presented which extends the CSCT to punching shear cases of internal columns with moment transfer, edge columns, and corner columns, which are commonly found in practice. The comparison between the analytical and experimental results shows that the approach developed combined with the failure criterion used in the CSCT provides accurate predictions of strength and deformation capacity for internal columns with moment transfer, edge columns, and corner columns. This work was also used to validate the formulation of the first complete draft of the Model Code 2010. In this work, the punching shear behaviour of flat slabs with bent-up bars was also investigated. Bent-up bars were commonly used in the past and are still used nowadays for particular design situations. A reliable model is needed to design structures and to assess existing structures with bent-up bars. To address this, five punching shear tests were performed on symmetrical slabs with bent-up bars. The strain measurements along the reinforcement bars provided a better understanding of the anchorage mechanisms. A model was developed, which is consistent with the CSCT and provides satisfactory predictions of the distribution of strains along the bent-up bar. The proposed model can be used to calculate the contribution of the bent-up bars, which needs to be added to the concrete contribution from the CSCT to obtain the punching shear strength.

EPFL2011

Seismic behaviour of slab-column connections without transverse reinforcement

Ioannis Sokratis Drakatos

Reinforced concrete (RC) flat slabs supported by slender columns are often used as gravity load resisting system for buildings in regions of moderate and high seismicity. Such buildings are typically braced by RC walls, which carry the largest portion of the horizontal loads generated during earthquakes. Therefore, the slab-column system does not contribute significantly to the lateral stiffness and strength of the structure, but each slab-column connection must be able to accommodate the seismically induced drifts of the building while maintaining its capacity to transfer vertical loads from the slab to the columns. Otherwise, brittle punching failure of the slab occurs and the deformation capacity of the entire building is limited by the deformation capacity of the slab-column connection if the building is not designed to resist progressive collapse. The first part of this work presents an experimental investigation on 13 full-scale internal slab-column connections without transverse reinforcement. The objective of the test campaign is to assess the influence of the loading history (monotonic vs. reversed cyclic) for different gravity loads and reinforcement ratios. The study shows that cyclic loading leads, in particular for slabs subjected to low gravity loads, to significant moment strength and deformation capacity reduction when compared to results obtained from monotonic loading tests. The effect of cyclic loading is more pronounced for slabs with low reinforcement content. In the second part, a mechanical model is presented for predicting the moment-rotation relationship of interior slab-column connections without transverse reinforcement when subjected to seismically induced drifts. The model accounts explicitly for the three load transfer mechanisms between slab and column contributing to the unbalanced moment resistance, i.e., eccentric shear, flexure and torsion. The moment resistance and deformation capacity are deduced from the intersection of the moment-rotation curve with a failure criterion that is based on the Critical Shear Crack Theory and distinguishes between monotonic and cyclic loading conditions. The model predicts well the moment strength and the deformation capacity of slabs tested within this research and reported in the literature. The third part of this thesis proposes an extension of the mechanical model for the moment-rotation relationship presented earlier to account for the hysteretic behaviour and cumulative damage effects on slab-column connections subjected to cyclic loading. A hysteretic moment-curvature relationship is proposed for the radial direction, based on local deformation measurements from the cyclic tests. Cyclic damage is considered by adopting a damage index proposed by a previous study. The extended model predicts more accurately the response of cyclic tests than the simplified approach based on the monotonic model. Finally, based on the theoretical investigation of the two previous parts, two methods are proposed for the numerical analysis of flat slab buildings to simulate the column deformation and the slab deformation until midspan. First, an Effective Beam Width method is presented and compared to test results of flat slab buildings with over-hangs. Then, a simplified method is proposed for the analysis of slab-column connections not part of the lateral force-resisting system. This method allows estimating the contribution of column and slab deformation to the interstorey drift.