Punching strength of continuous flat slabs

Reinforced concrete slabs with uniform thickness are common in residential and commercial buildings but can also be found in other types of structures. Such slabs are susceptible to punching shear failures, where a supporting column penetrates through the slab and leads to an immediate local failure that may trigger a progressive collapse of the building. Provisions for punching shear in most codes of practice are still mainly empirical, calibrated on the basis of experiments on test specimens that traditionally model only an isolated part of the slab within the points of contraflexure around the column. However, the punching behavior of actual continuous slabs may be influenced by effects that cannot occur in isolated specimens, such as moment redistribution between hogging and sagging moments, which changes the location of the points of contraflexure, and compressive membrane action. These effects can lead to higher punching strengths of actual continuous slabs compared to isolated specimens. The first part of the thesis introduces an axisymmetric model to analyze the influence of these effects on the flexural deformations of continuous flat slabs. Combined with the failure criterion of the Critical Shear Crack Theory, the model can be used to predict the punching capacities of such slabs. Good agreement was found between the model predictions and the results of some unconventional punching tests from the literature. A simplified method, sufficiently straightforward to be used in design or assessment and given in a format compatible with the punching provisions of the Model Code 2010, is also proposed for calculating the load-rotation curves of continuous slabs. The second part of the thesis contains the results of a test campaign comprising 13 isolated symmetric punching specimens. The study focuses on the influence of the size of the supported area and the slenderness of the slab. Other investigated parameters are the flexural reinforcement ratio and the presence of shear reinforcement. A novel experimental approach is used for tracking the formation and development of internal cracks. Measurement points were installed inside small holes drilled on the slab soffit on two sides of the column in the regions were punching cracks were expected to appear. Displacements of these points at various stages of loading were followed with a high-precision coordinate measuring arm. In most cases, the punching failure cracks were seen to develop independently of the flexural cracks, either appearing at the moment of failure or, in some cases, already at earlier stages of loading. Although the slabs were nominally axis-symmetric, different crack development patterns could be observed on the two monitored sides of the columns. On the basis of the experimental evidence, a new punching model is proposed for slabs without shear reinforcement. Punching failures are assumed to occur due to reaching a critical triaxial stress state below the flexural cracks in the compression strut and a consequent formation and propagation of a failure crack. The proposed model uses the theory of plasticity with a general triaxial yield criterion together with an effectiveness factor based on fracture mechanics that is a function of the depth of the compression zone and the size of the column. The influence of membrane forces in continuous slabs on their punching strength is taken into account by adjusting the depth of the compression zone

Experimental investigation of soil - structure interaction for transition slabs of integral bridges

This paper presents the results of an experimental test series on the soil-structure-pavement interaction in the vicinity of the transition slab at the end of an integral bridge. The main function of transition slabs is to ease the transition between the bridge deck and the embankment in case of differential settlements. Additionally, in the case of integral bridges, they can solve the problem of moderate imposed longitudinal deformations at the bridge ends. In this case, the displacements imposed to the transition slab can lead to vertical and longitudinal surface displacements and to cracking of the pavement. Based on the observed behaviour, some recommendations are proposed for the geometry and surface conditions to optimise the behaviour of the transition slabs

Punching resistance and flexural behaviour of continuous flat slabs

The punching resistance of actual reinforced concrete flat slabs is potentially influenced by the compressive membrane action, which may significantly increase their resistance compared to isolated laboratory specimens. However, design codes have been developed using the results of laboratory experiments and therefore usually neglect this phenomenon. In this paper, a numerical model is presented to describe the behaviour of continuous and confined flat slabs under distributed loads. The flexural strength and stiffness are shown to increase due to the redistribution of moments and in-plane forces in the slab arising from the confinement of the slab dilation. The failure criterion of the Critical Shear Crack Theory is applied to predict the punching strength. The modelling results are compared to experimental data of some unconventional punching tests from the literature and satisfactory correlation is found

Influence of moment redistribution and compressive membrane action on punching strength of flat slabs

The punching shear strength of interior column connections of flat slabs has traditionally been investigated with isolated test specimens subjected exclusively to hogging bending moments. However, the behavior of such specimens is different from that of actual flat slabs, as the potentially beneficial phenomena of moment redistribution between hogging and sagging moments and compressive membrane action cannot take place in the conventional experiments. In the present paper, an axisymmetric numerical model is introduced that allows analyzing the role and significance of these effects on the flexural deformations of continuous flat slabs. Combined with the failure criterion of the Critical Shear Crack Theory, this model can be used to predict the punching capacities of such slabs. Comparisons are made to the results of some unconventional punching tests from the literature showing sound agreement between the modeling results and the experimental observations. The results suggest that the punching capacity of continuous slabs with low amounts of flexural reinforcement in the interior column regions may be underestimated in the current codes of practice

Measurements of internal cracking in punching test slabs without shear reinforcement

Advancing the knowledge about the punching behaviour of flat slabs has been held back by the fact that only indirect measurements of the internal failure mechanism have typically been available. In this paper, a novel experimental approach for tracking the formation and development of cracks inside punching test specimens is presented. In six full-scale specimens without shear reinforcement, 48–64 measurement points were installed inside small holes drilled on the soffits of the slabs. During the punching tests, the displacements of these points were followed at various levels of load with a high-precision coordinate measuring arm. Development of inclined flexural cracks (called the critical shear cracks) from the tension chord to the neutral axis was observed in every specimen. In some cases, the punching failures occurred along these cracks. However, in other cases, new lower-angled failure cracks developed, either at the moment of failure or already at earlier stages of loading. These cracks had very different patterns of development, even between two monitored sides of the same specimen, although the specimens were nominally axis-symmetric

Performance of punching shear reinforcement under gravity loading: Influence of type and detailing

The performance of 11 different shear reinforcement systems against punching of inner slab-column connections under gravity loading was compared on the basis of experiments on 12 full-scale specimens, 8 of them newly reported. The slab geometry and flexural reinforcement ratio (1.5%) were kept constant. The shear reinforcement systems included different layouts of double-headed studs, individual links, bent-up bars and bonded post-installed reinforcement. All the systems were found to increase both the strength and the deformation capacity of the members but exhibited varying performances. The factors influencing the maximum punching strength of different systems, such as the layout and the anchorage conditions of the transverse reinforcement units, are described and analyzed. The mechanical model of the Critical Shear Crack Theory is used to explain the observed differences and provide design guidance. Comparisons to the codes of practice (ACI 318, Eurocode 2 and Model Code 2010) are also presented

Punching shear capacity of continuous slabs

Provisions for punching shear design of reinforced concrete slabs are usually calibrated on the basis of results from tests on isolated specimens that simulate the slab zone within the points of contraflexure around a column. However, the punching behavior of interior slab-column connections in actual continuous slabs without transverse reinforcement may be influenced by the effects of moment redistribution and compressive membrane action, which can lead to higher punching strengths and lower deformation capacities compared to those in isolated specimens. This paper discusses these behavioral differences on the basis of experiments performed on symmetric edge-restrained slabs and investigates available test data by means of a numerical model. A simplified calculation method (based on the Critical Shear Crack Theory) that accounts for these effects is also proposed. The calculation model shows consistent agreement with the results of the numerical evaluation and is sufficiently simple to be used in design and assessment

On the efficiency of flat slabs strengthening against punching using externally bonded fibre reinforced polymers

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 approache