António Manuel Pinho Ramos

This work refers to experimental and 3D nonlinear FEA on punching. Numerical results were compared with experimental ones in order to benchmark the FE model and afterwards a parametric study was conducted, changing the reinforcement ratio, slab thickness, concrete strength and column dimensions, running a total of 360 models, where their effect on punching capacity is shown. EC2 and MC2010 provisions agreed approximately with experimental and FEA results. Based in the FEA results it is proposed an equation to predict the punching capacity with the introduction of fracture mechanics parameter, which was compared with several experimental results, giving good approximation.

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The use of prestress in flat slabs is a common solution, mainly because it allows larger spans and thinner slabs. Nevertheless, smaller thicknesses near the slab-column connections, along with the superposition of high shear and flexural stresses, arise the question of the slab capacity to resist punching. The punching failure results from the superposition of shear and flexural stresses near the column, and is associated to the formation of a pyramidal plug of concrete which punches through the slab. It is a local and brittle failure. The use of prestress can increase the punching capacity of flat slabs-column connections.This work presents the experimental analysis of flat slab specimens with tendons under punching. Nine slabs were tested using unbonded prestress with high strength steel tendons. The influences on the punching capacity of the vertical component of the prestress forces resulting from inclined tendons near the column and their distance to the column are analysed. The in-plane compression force due to prestress was not applied to the slabs, in order to evaluate only the deviation force influence. This work aims to improve the understanding of the behaviour of prestressed flat slabs under punching load in order to properly evaluate the punching resistance of this kind of structures. The experimental punching loads are compared with the provisions of EC2, ACI 318-11 and MC2010. © 2014 Elsevier Ltd.

Abstract This paper analyses the performance of the experimental setups to assess the punching strength of slab-column connections in continuous flat slabs under vertical and horizontal loading. In the last years, several experimental campaigns have been performed to investigate the punching strength of slab-column connections, but most of the experimental tests concerned isolated slab-column connections. Among the few setups aimed at reproducing the eccentric punching failure in continuous flat slabs, the setup developed at the NOVA School of Science and Technology in Lisbon is considered in this paper. The performance of the Lisbon setup is assessed through nonlinear finite element analyses, calibrated on experimental data, by comparison with numerical results of a theoretical continuous setup. Then, the performance of the isolated setups, used in many researches and at the base of some international codes, is also evaluated through the same finite element model. Numerical analyses highlight that the setup developed in Lisbon could provide reliable ultimate rotations of continuous flat slab connections, but it underestimates the punching strength. Despite isolated setups lead to similar results when compared with the Lisbon setup, the latter seems to provide a better representation of a continuous slab-column connection. The numerical analyses presented in this paper have been performed assuming monotonic lateral loading.

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Punching-shear capacity of slab-column connections in existing R/C structures may be inadequate to bear design loads, so strengthening works are required. The lack of punching resistance may be due to detailing, design or building errors; in other cases, such lack is due to a change of use, which requires an increase of resistance. Different techniques have been developed for strengthening R/C slabs against punching: enlargement of the support, gluing external fibre reinforced polymers or casting a bonded concrete overlay (BCO) on the slab's top surface, insertion of post-installed steel bolts, application of fibre reinforced polymers cords as shear reinforcement. In the paper, the authors apply the Critical Shear Crack Theory (CSCT) to all of these techniques and evaluate their efficacy with reference to a case study.

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This paper describes the experimental campaign to study the behaviour of reinforced-concrete flat slab structures with steel stirrups as punching shear reinforcement, under combined vertical and horizontal cyclic loading. The vertical load was first applied and kept constant during the test, while, regarding the cyclic horizontal loading, imposed cyclic drifts were increased until failure. Four slab specimens with shear reinforcement were tested and the results compared to a control slab specimen without shear reinforcement. The studied variables were different shear reinforcement ratios and the number of stirrup layers. The slabs were 4·15 × 1·85 m2 and 0·15 m thick, connected to two steel half-columns. The test setup used was developed by the research team and aimed to simulate the boundary conditions of a flat slab, representing the slab between middle spans in one direction and between zero bending moment points in the other direction. Results show that the use of steel stirrups as shear reinforcement is very effective, increasing shear, drift and energy dissipation capacities. The obtained results were also compared to the provisions given by European and American codes.

The current research aims to study the behavior of thin reinforced concrete (RC) slabs under concentrated loads as well as to investigate the application of Critical Shear Crack Theory (CSCT) to such slabs. For this purpose, four square 100-mm-thick slabs were cast and subjected to concentrated punching monotonic loading. The experimental parameters were the flexural reinforcement ratio, 0.38% and 1.00%, and the presence or absence of shear headed stud reinforcement. It is shown that the failure criteria of CSCT describe reasonably well the observed failure modes and the ultimate loads of the specimens. However, attention is brought to some peculiarities in the analytical derivation of the load-rotation curve for thin lightly reinforced flat slabs, in which large deformations are experienced. Results showed that in such slabs, the behavior can be highly influenced by the post-yield stress-strain curve of the flexural steel reinforcement. As a result, the constitutive law of steel reinforcement should be explicitly taken into account in such cases. The versatility of CSCT to adapt to these conditions is demonstrated.

Abstract An experimental work on reinforced concrete flat slab specimens to test the efficiency of postinstalled bolts, as punching shear reinforcement in resisting vertical and cyclic horizontal loads, was conducted and is presented in this paper. The test protocol consisted in increasing horizontal drifts combined with constant vertical load until failure. Two different detailing solutions for the shear reinforcement were considered, one using a radial distribution around the column and another using a cross distribution, being the results compared with a previously tested reference specimen. The dimensions of the specimens were 4.25 x 1.85 x 0.15 m3. The test setup used for these tests was developed by the research team and simulates the boundary conditions with already recognized good results. Postinstalled steel bolts were proven to be an efficient solution for strengthening of existing structures, improving the structural behavior, and the punching resistance.

Abstract An experimental work on reinforced concrete flat slab specimens to test the efficiency of postinstalled bolts, as punching shear reinforcement in resisting vertical and cyclic horizontal loads, was conducted and is presented in this paper. The test protocol consisted in increasing horizontal drifts combined with constant vertical load until failure. Two different detailing solutions for the shear reinforcement were considered, one using a radial distribution around the column and another using a cross distribution, being the results compared with a previously tested reference specimen. The dimensions of the specimens were 4.25 x 1.85 x 0.15 m3. The test setup used for these tests was developed by the research team and simulates the boundary conditions with already recognized good results. Postinstalled steel bolts were proven to be an efficient solution for strengthening of existing structures, improving the structural behavior, and the punching resistance.