Assessing punching shear failure in reinforced concrete flat slabs subjected to localised impact loading

Reinforced concrete flat slab structures are used widely in construction projects due to their economic and functional advantages. Punching shear failure in such structures can have catastrophic effects in the case of, for example, multi-storey framed structures and the designer aims to ensure that ductile flexural deformation occurs before the brittle shear failure. Shear mechanisms generally govern the behaviour of reinforced concrete structures subjected to localised impact loads. Existing experimental results investigating punching shear in flat slabs subjected to impact loading shows that when increasing the loading rate, the punching shear strength also increases whereas the deformation capacity reduces. This behaviour is due to a combination of inertial effects and material strain-rate effects which leads to a stiffer behaviour of the slab for higher loading rates. This can also lead to a change of mode of failure from flexural to pure punching shear with increasing loading rates. Current empirical formulae for punching shear are unable to predict this behaviour since the slab deformations are not considered for calculating the punching shear strength. This paper presents an analytical model based on the Critical Shear Crack Theory which can be applied to flat slabs subjected to impact loading. This model is particularly useful for cases such as progressive collapse analysis and flat slab-column connections subjected to an impulsive axial load in the column. The novelty of the approach is that it considers (a) the dynamic punching shear capacity and (b) the dynamic shear demand, both in terms of the slab deformation (slab rotation). The model considers inertial effects and material strain-rate effects although it is shown that the former has a more significant effect. Moreover, the model allows a further physical understanding of the phenomena and it can be applied to different cases (slabs with and without transverse reinforcement) showing a good correlation with experimental data. (C) 2014 Published by Elsevier Ltd.

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

Poinçonnement symétrique des dalles en béton armé

Stefano Guandalini

Punching of reinforced concrete and prestressed slabs is usually a critical failure mode for the design and verification of structures such as flat slabs or bridge slabs. Although codes of practice propose several rules for common cases (usually with an empirical basis), they do not provide a general tool for studying the punching strength because they are not based on a physical model. Furthermore, a better accuracy in the determination of the punching strength is needed when investigating the ultimate load of existing structures. Recently, test results from a series of 10 concrete slabs without punching reinforcement (performed within this thesis) as well as different tests performed by other researchers allowed to check and validate the application limits of a failure criterion proposed by Professor Muttoni for slabs without punching reinforcement. This failure criterion defines the punching strength mainly as a function of the radial rotation of the slab in the vicinity of the column. Even if a punching failure is predominantly a shear failure, the vertical displacements and the plate rotations before failure are governed mainly by the flexural characteristics of the slab. A computational model for the flexural behavior of concrete slabs has been developed, considering the different material non linearities and allowing also to include the effect of prestressing. Finally, both the failure criterion and the computational model are merged into a physical model which is able to determine the punching strength of symmetrical slabs, with any flexural reinforcement layout (prestressed or not). The comparison between theoretical and experimental results shows good agreement, better than provided by current codes of practice. With this physical model, it is also possible to determine the punching strength for particular cases, not covered by building codes. For instance, in the case of an inner column at a flat slab, it is possible to compute the enhanced punching strength due to the restraint effect exerted by the rest of the slab. The model can also be used to determine the failure load of a foundation plate, considering the interaction between the soil pressure and the slab displacement. Furthermore, it is possible to include temperature effects on the punching strength evaluating the loss of resistance due to fire exposure of the slab. The proposed model is very flexible and can easily be adapted to the different cases which an engineer is confronted to. It revealed itself as a very helpful tool for determining the failure load of an existing structure as well as for designing the reinforcement layout for new projects. Within this thesis, only axisymmetrical cases have been studied. To analyse border or edge columns as well as other non symmetrical cases, the model should be adapted.

EPFL2006

Deformation capacity evaluation for flat slab seismic design

Lorenzo Martinelli, Aurelio Muttoni, António Manuel Pinho Ramos, Andri Setiawan

In flat-slab frames, which are typically designed as secondary seismic structures, the shear failure of the slab around the column (punching failure) is typically the governing failure mode which limits the deformation capacity and can potentially lead to a progressive collapse of the structure. Existing rules to predict the capacity of flat slab frames to resist imposed lateral displacements without losing the capability to bear gravity loads have been derived empirically from the results of cyclic tests on thin members. These rules account explicitly only for the ratio between acting gravity loads and resistance against concentric punching shear (so-called Gravity Shear Ratio). Recent rational models to estimate the deformation capacity of flat slabs show that other parameters can play a major role and predict a significant size effect (reduced deformation for thick slabs). In this paper, a closed-form expression to predict the deformation capacity of internal slab-column connections as a function of the main parameters is derived from the same model that has been used to develop the punching shear formulae for the second generation of Eurocode 2 for concrete structures. This expression is compared to an existing database of isolated internal slab-column connections showing fine accuracy and allowing to resolve the shortcomings of existing rules. In addition, the results of a testing programme on a full-scale flat-slab frame with two stories and 12 columns are described. The differences between measured interstorey drifts and local slab rotations influencing their capacity to resist shear forces are presented and discussed. With respect to the observed deformation capacities, similar values are obtained as in the isolated specimens and the predictions are confirmed for the internal columns, but significant differences are observed between internal, edge and corner slab-column connections. The effects of punching shear reinforcement and of integrity reinforcement (required according to Eurocode 2 to prevent progressive collapse after punching) are also discussed.