Punching Resistance Of Flat Slabs Research Articles

Punching shear resistance of flat slabs with different types of stirrup anchorages such as shear reinforcement

Concrete codes demand that the stirrup type shear reinforcements must be adequately anchored in the longitudinal reinforcements. However, this requirement hinders the executive process and makes the use of some reinforcements unfeasible. The goal of the present study was to assess the influence of different types of anchoring of shear reinforcements with closed stirrups on the punching resistance of flat slabs that do not meet the code's standards. An experimental program was conducted assessing nine flat square slabs, a reference slab, and four different types of detailings concerning anchoring. Only one of those details met the normative prescriptions. As a result, it was found that the variation in the anchorage of the stirrups had not influenced the behavior of the slabs. All types of connections had similar punching shear resistance increases.

Design considerations on the influence of slab continuity on punching resistance of flat slabs

abstract: This paper assesses the influence of slab continuity on the punching resistance of a realistically proportioned flat slab floor plate without shear reinforcement. The edge column punching resistance of a symmetric flat slab extending bays in each direction was assessed by means of NLFEA with TNO DIANA, MC2010 levels II, III, IV, Eurocode 2 and NBR 6118. Both Eurocode 2 and NBR 6118 are seen to give similar predictions for punching resistance, while MC2010, which is based on the Critical Shear Crack Theory and depends on how rotations are calculated and FE modelling assumptions, varies significantly with its levels of approximation with Level IV agreeing reasonably well with predictions from NLFEA. Direction for the critical rotations is shown to vary and can also be influenced by the reinforcement over the span. For EC2, NBR 6118 and MC2010 LoA II and III punching shear design are independent of span, unlike the results obtained with MC2010 LoA IV.

Punching resistance of slab‐column connections with openings

AbstractThe paper deals with the punching resistance of flat slabs with openings adjacent to a column. The results of an experimental campaign carried out at a laboratory of the Slovak University of Technology in Bratislava (STU) are presented in the paper. A total of eight slabs with a thickness of 250 mm and without transverse reinforcement were tested under axis‐symmetric conditions. The six slabs had two symmetrically placed openings with different distances measured from the column's periphery and two reference slabs did that not have any openings. The results obtained were compared with punching resistances assessed using the design equations introduced in Eurocode 2, Model Code 2010, ACI 318‐14, and the CSCT model expressed in a closed form. Significant differences in the values of the Vtest/VR,c ratio were observed for the different positions of the openings for each design model. In order to increase the accuracy of the models, some changes in estimating the reduced lengths of the control perimeter were proposed.

Tests on the punching resistance of flat slabs with unbalanced moments

This paper presents results of tests on eight reinforced concrete slab-column connections without and with shear reinforcement under monotonic concentric and eccentric loading. The slabs were square, with sides of 2.5 m, 180 mm thick, and were centrally supported on square column stubs with sides of 300 mm. The slabs were cast with concrete with a mean compressive strength of 49 MPa, with constant flexural reinforcement ratio (1.5%) and with different amounts of double-headed studs as shear reinforcement. All the slabs failed by punching shear and their flexural and shear responses, as well as their ultimate punching strengths, are described and discussed by comparing the experimental observations with theoretical results obtained following the recommendations presented by ACI 318, Eurocode 2 and fib Model Code 2010.

Reliability of models aimed at evaluating the punching resistance of flat slabs without transverse reinforcement

Reinforced concrete flat slabs are structural members frequently used in building construction. Despite their many advantages, such structural systems suffer from several drawbacks. From a safety point of view, the concentration of shear forces at the vicinity of local supports, e.g., columns, edges and corners of walls, can lead to possible structural failure from punching. Punching is a dangerous phenomenon due to its brittle mode of failure and its ability to spread over a whole structure, which can be followed by a progressive collapse. Several models for the assessment of punching capacity have been developed and calibrated using experimental results from laboratory tests. Some models are fully empirical, e.g., EC2 (2004), while other models reflect the physical nature of the phenomenon; however, their safety level was calibrated using experimental data, as the model presented in Model Code 2010 or the CSCT-based model, which is expressed in a closed form. This paper deals with a statistical evaluation of the safety level of the previously mentioned models for punching resistance without transverse reinforcement. A database which includes the results of more than 600 experimental tests of flat slab specimens has been used. The relations between the safety of the model and the quality of the concrete used, the amount of the bending reinforcement, and the effective depth were found using advanced statistical methods.

Punching of reinforced concrete flat slabs with holes and shear reinforcement

Abstract Punching shear is a possible type of failure that occurs in reinforced concrete flat slabs, which can develop with an ultimate load below flexural capacity. Several researchers have studied the punching resistance of flat slabs over recent years. Although they have made great advances, there are codes that show different approaches to a singular design. Some codes show that there exist contradictions, even in the simplest situations, such as concentric loads. Most codes prescribe empirical expressions based in a theoretical model to analyze punching strength, but for flat slabs with holes around the column and shear reinforcement there are divergences between codes, justifying research in this area. This paper presents an experimental analysis of nine square reinforced concrete flat slabs under concentric loading (width: 1800 mm; thickness: 130 mm). The main variables used in the tests were: a) two square openings (150 mm) adjacent to the smallest side of the column and b) the use of shear reinforcement containing 3 layers, with 6 or 8 elements in each layer and radially distributed around the column. The research concludes that openings adjacent to the column affect punching shear strength, while the correct use of the shear reinforcement can minimize and even compensate this loss.

The Maximum Punching Capacity of Flat Slabs

Two ways how to determine maximum punching resistance of flat slabs with shear reinforcement are currently used. The first way is verification of the concrete strut capacity at the column periphery defined as VRd,max. The second limit is defined as kmax multiple of the punching shear resistance without shear reinforcement VRd,c. The values of kmax are proposed usually in between 1.4 and 2.0. Results of the experimental tests are presented in the paper that were focused on above mentioned limits, whether failure of the struts can precede any other form of punching failure that is limited by kmax*VRd,c. Two experimental slab samples reinforced with high amount of shear reinforcement that increased punching capacity above capacity of the concrete struts were tested together with two slab samples cast without shear reinforcement. Comparison has shown that punching resistance of flat slab with shear reinforcement has been 1.7 times higher than resistance without shear reinforcement. While some standards allow for use kmax value of 1.9 in this case. This indicates that limits based only on the kmax factors may overestimate actual maximum punching shear resistance.

Assessment of the effectiveness of steel fibre reinforcement for the punching resistance of flat slabs by experimental research and design approach

The present paper deals with the experimental assessment of the effectiveness of steel fibre reinforcement in terms of punching resistance of centrically loaded flat slabs, and to the development of an analytical model capable of predicting the punching behaviour of this type of structures. For this purpose, eight slabs of 2550 × 2550 × 150 mm3 dimensions were tested up to failure, by investigating the influence of the content of steel fibres (0, 60, 75 and 90 kg/m3) and concrete strength class (50 and 70 MPa). Two reference slabs without fibre reinforcement, one for each concrete strength class, and one slab for each fibre content and each strength class compose the experimental program. All slabs were flexurally reinforced with a grid of ribbed steel bars in a percentage to assure punching failure mode for the reference slabs. Hooked ends steel fibres provided the unique shear reinforcement. The results have revealed that steel fibres are very effective in converting brittle punching failure into ductile flexural failure, by increasing both the ultimate load and deflection, as long as adequate fibre reinforcement is assured. An analytical model was developed based on the most recent concepts proposed by the fib Mode Code 2010 for predicting the punching resistance of flat slabs and for the characterization of the behaviour of fibre reinforced concrete. The most refined version of this model was capable of predicting the punching resistance of the tested slabs with excellent accuracy and coefficient of variation of about 5%.

Model to Simulate the Contribution of Fiber Reinforcement for the Punching Resistance of RC Slabs

AbstractIn this paper analytical formulations are developed for predicting the punching resistance of flat slabs of steel fiber–reinforced concrete (SFRC) flexurally reinforced with steel bars. By performing statistical analysis with a database that collects experimental results on the characterization of the postcracking behavior of SFRC, equations are determined for evaluating the residual flexural tensile strength parameters (fRi) from fundamental data that characterize steel fibers. The fRi strength parameters proposed by CEB-FIP 2010 were used for the definition of the stress-crack width law (σ-w) that simulates the fiber-reinforcement mechanisms in cement-based materials. The second part of the paper describes an analytical formulation based on the concepts proposed by Muttoni and Ruiz, where the σ-w law is conveniently integrated for simulation of the contribution of steel fibers for the punching resistance of SFRC slabs. By using a database composed of 154 punching tests with SFRC slabs, the good ...

Contribuição ao estudo de lajes lisas unidirecionais de concreto armado com pilares retangulares ao puncionamento

São apresentadas análises das recomendações para estimar a resistência ao puncionamento de lajes lisas de acordo com a versão atual da norma brasileira, a NBR 6118:2003, que não considera o efeito da retangularidade de pilares internos, e sua predecessora, a NBR 6118:1980. Em seções transversais retangulares, o índice de polarização das tensões de cisalhamento é crescente. Isto altera a forma de ruptura da laje, já que a punção se inicia em torno das extremidades da seção do pilar, não se estendendo para os lados maiores, como prescreve a norma, comportamento este que reduz a resistência última das lajes frente às estimativas normativas. São apresentados os resultados de uma análise numérica por elementos finitos (M.E.F.) e uma proposta para melhorar a ductilidade das lajes lisas através das dimensões mínimas dos pilares. Observou-se que a NBR 6118:1980 mostrou-se conservadora e que as estimativas da NBR 6118: 2003 podem ser melhoradas.