António Manuel Pinho Ramos

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The paper is focused on the calibration of non-linear 3D finite element (FE) analyses to simulate punching failure of R/C flat slabs. The calibration procedure is developed with reference to the code ABAQUS, which is one of the most used computer codes in nonlinear modelling of R/C structures. Generally, the calibration of a nonlinear FE model is grounded on one test only, so its reliability could be limited. Here a hybrid method for the calibration of FE models of R/C flat slabs failing in punching is proposed and discussed. The method consists in calibrating input data by comparison of finite element model (FEM) results with both experimental data and predictions provided by analytical models. The procedure allows for a consistent calibration to be performed, valid for a wide range of longitudinal reinforcement ratios, from 0.5% to 2.00%, and concrete grades, from C20/25 to C50/60. A case study is investigated using the proposed method. Results show that calibrated values of the fracture energy lie between those provided by Model Code 1990 and Model Code 2010. From the new calibration procedure, a relationship between fracture energy and concrete compressive strength is also derived and blind analyses are performed to check its reliability against experimental results.

Previous researches on punching of post-tensioned slabs have shown a number of phenomena significantly influencing their strength and behaviour. However, no general agreement is yet found on a physical theory (either in codes of practice or in design models) suitably describing the influence of prestressing and how should it be accounted on the punching shear behaviour. In this paper, the authors present the results of tests on 15 slabs (3000. ??. 3000. ??. 250. mm) tested to failure under different loading conditions. The aim of the tests was to investigate in a separate manner the different actions induced by prestressing on the punching shear strength (in-plane forces, bending moments and bonded tendons). These results are finally investigated on the basis of the physical model of the Critical Shear Crack Theory. The fundamentals of this theory are presented and adapted to post-tensioned slabs, providing a rational explanation of the observed phenomena and measured strengths. ?? 2014 Elsevier Ltd.

This paper studies the influence of different longitudinal top reinforcement detailing on the behaviour and punching capacity of flat slabs. Experimental and numerical studies were carried out. The experimental campaign consisted in testing three reinforced concrete slabs 2.20 m wide with a thickness equal to 0.15 m. The numerical study was conducted using the non-linear finite element software ATENA 3D. Using numerical models, which were calibrated using the experimental tests, a parametric study was carried out to include other design solutions beyond the tested ones. The parameters that were changed in this parametric study were the steel reinforcement ratio, the concrete strength and the detailing of the top steel reinforcement, namely using a solution with uniform distribution and other with a higher concentration of reinforcement near the column. Finally, the results were compared with predictions obtained using some of the existing codes (Eurocode 2 and Model Code 2010). From this study it can be concluded that concentrating the top flexural reinforcement, keeping the same total amount of top longitudinal reinforcement, results in a stiffer response, an increment in the punching resistance, and a decrease in the maximum vertical displacement.

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One possibility for strengthening existing flat slabs consists on gluing fibre reinforced polymers (FRPs) at the concrete surface. When applied on top of slab-column connections, this technique allows increasing the flexural stiffness and strength of the slab as well as its punching strength. Nevertheless, the higher punching strength is associated to a reduction on the deformation capacity of the slab-column connection, which can be detrimental for the overall behaviour of the structure (leading to a more brittle behaviour of the system). Design approaches for this strengthening technique are usually based on empirical formulas calibrated on the basis of the tests performed on isolated test specimens. However, some significant topics as the reduction on the deformation capacity or the influence of the whole slab (accounting for the reinforcement at mid-span) on the efficiency of the strengthening are neglected. In this paper, a critical review of this technique for strengthening against punching shear is investigated on the basis of the physical model proposed by the Critical Shear Crack Theory (CSCT). This approach allows taking into account the amount, layout and mechanical behaviour of the bonded FRP's in a consistent manner to estimate the punching strength and deformation capacity of strengthened slabs. The approach is first used to predict the punching strength of available test data, showing a good agreement. Then, it is applied in order to investigate strengthened continuous slabs, considering moment redistribution after concrete cracking and reinforcement yielding. This latter study provides valuable information regarding the differences between the behaviour of isolated test specimens and real strengthened flat slabs. The results show that empirical formulas calibrated on isolated specimens may overestimate the actual performance of FRP's strengthening. Finally, taking advantage of the physical model of the CSCT, the effect of the construction sequence on the punching shear strength is also evaluated, revealing the role of this issue which is also neglected in most empirical approaches.

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This work presents an experimental study concerning the post-punching behaviour of flat slabs strengthened with a new technique based on post-tensioning with anchorages by bonding using an epoxy adhesive. This strengthening technique proved efficient with respect to ultimate and serviceability states. Five slab specimens were tested in the post-punching range and it was found that the post-punching resistance was on average 78{%} of the punching resistance. This paper reports the development of strand forces and slab displacements from the beginning of the tests, including the bond stresses developed at several stages of the loading process. It was observed that top reinforcement bars were capable of transmitting post-punching loads to the prestressing strands. Taking this into account and based on the load bath envisaged from the column to the slab, expressions for the vertical load capacities corresponding to the parts of the load path are presented and compared with the experimental results, showing their ability to predict both ultimate loads and modes of failure. Compared with other strengthening techniques, the one proposed here not only upgrades ultimate and serviceability behaviour but also adds post-punching resistance, which is a great advantage in the event of progressive collapse, since it may avoid the collapse of an entire structure, thus reducing the risk of material and human losses.

The progressive collapse of structures generally results in the loss of human lives. Experience shows that flat slab structures are very sensitive to progressive collapse, which may cause serious accidents if special detailing is not provided. The post-punching behaviour of a column-supported slab is decisive in the progressive collapse development. This work presents the experimental research carried out to study the post-punching behaviour of prestressed concrete flat slabs. Six reduced-scale prestressed concrete flat slab models were tested. The tests were carried out in two different phases. In the first phase the models were loaded up to failure by punching. Afterwards, the models were loaded again to study the post-punching behaviour changes caused by the presence of unbonded, prestressed tendons and the influences of their distance to the column.

The objective of this study was to analyse the behaviour of prestress strand anchorages by bonding with an epoxy adhesive for structural strengthening use. Pull-out and push-in tests were carried out on 15·2 mm diameter prestress steel strands sealed in 18 mm diameter holes with several embedment lengths, complemented by long-term tests. Experimental results are presented and compared with theoretical results regarding maximum pull-out and transmittable loads and also draw-in results. Theoretical results are obtained by solving the governing equation of the bond phenomenon adopting a non-linear local bond/slip law derived from pull-out tests with short embedment length. The study shows that it is reasonable to assume an average constant bond stress for anchorage design with the studied epoxy adhesive in the range of the studied values of anchor embedment length and diameter. The average values for bond stress to be used for determining the maximum pull-out and transmittable loads were found to be 12·0 and 5·2 MPa, respectively. Experimental draw-in values show a great variability, and so determining transmission length based on draw-in values may lead to a false perception that the transmission length is very variable.

The aim of this work is to study the behaviour of reinforced concrete flat slab structures under combined vertical and horizontal cyclic loading. A total of five specimens were cast and tested: a control specimen was punched without eccentricity, one specimen was tested under constant vertical loading and monotonically increased eccentricity until failure and the remaining three were tested under constant vertical load, at different shear ratios, and cyclic horizontal loading with increasing horizontal drift ratios. All slabs were similar, measuring 4.25×1.85×0.15m3. The reinforced concrete slab specimens were connected to two steel half columns by 0.25×0.25m2 rigid steel plates, prestressed against the slab using steel bolts, to ensure monolithic behaviour. The cyclic tests were performed using an innovative test setup that allows bending moment redistribution, line of inflection mobility, assures equal vertical displacements at the North-South borders and symmetrical shear forces. Results show that cyclic horizontal actions are very harmful to the slab–column connection, resulting in low horizontal drifts and energy dissipation.

Flat slab structures are a very common structural solution nowadays, due to their architectural and economic advantages. However, flat slab-column connections may be vulnerable to punching failure, especially in an event of an earthquake, with potentially high human and economic losses. This type of structural solution is adequately covered by design codes and recommendations in North America, due to a large amount of experimental research carried out. In Europe the situation is different, missing specific guidance to flat slab design under earthquake action in most European codes. The ACI 318-14 prescriptive approach to the gravity shear ratio-drift ratio relationship shows good agreement with experimental results. Following a similar approach and, based in a databank containing cyclic horizontally loaded tests of slab-column connections found in literature, proposals are made applicable to EC2 and MC2010.

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