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Due to the low lateral stiffness of slabs supported on columns alone, reinforced concrete flat plates are typically combined with other structural elements, such as shear walls. In these structures, the slab–column connections are designed to carry gravity loads only, and the shear walls are required to resist both gravity and lateral forces. Therefore, the slab–wall connections are essential for the performance of both the gravity and lateral force resisting systems. However, most punching shear research and design provisions are focused on slab–column connections, even though punching failures around slab–wall connections have occurred experimentally. Empirical testing of slab–wall connections is difficult due to the specimen size. This paper investigates the punching shear behavior of interior slab–wall connections subjected to concentric vertical loading, and combined concentric vertical loading and uniaxial unbalanced moment using a plasticity‐based nonlinear finite element model (FEM) in ABAQUS. The analysis of isolated slab–wall connections demonstrates that punching failures can occur before one‐way shear failures. Since the overall connection capacity is much higher than the expected loads in most structures, if these punching failures happen they would be localized to the region around the wall end and are not expected to lead to structural collapse. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The level of maximum punching strength is of major importance to design highly stressed slab‐column connections and to choose the type of punching shear reinforcement. Since a couple of years, there is consensus that the maximum punching strength of shear‐reinforced concrete slabs can be defined as a multiple of the reference punching strength without shear reinforcement. The upper bound of this load‐increasing effect depends mainly on the anchorage and arrangement of shear reinforcement elements. Despite numerous research studies and database evaluations, there are ongoing discussions regarding the final effectiveness of stirrups and double headed studs installed in flat slabs and column bases. In this paper, a strictly filtered punching database is used to highlight the significant impact of underlying reference strength comparing the maximum load increase according to current Eurocode 2 and the draft for the next generation of Eurocode 2 (prEC2). Based on this unifying database, the origin of current discrepancies is discussed and a combination of constant lower bound coefficients with a refined formula as specified in prEC2 is proposed. Additionally, the effect of adapting current European Technical Approvals for double‐headed studs and punching shear optimized lattice girders to prEC2 is shortly analyzed. A further database evaluation also allows for setting the basis for optimized detailing provisions regarding the maximum permissible diameter of shear reinforcement. [ABSTRACT FROM AUTHOR]

This paper is concerned with the finite element modeling of strengthened interior slab‐column connections in the existing continuous flat slab structural system by employing steel angle plates at the slab‐column junctions and fastened by shear bolts. This technique utilizes the advantage of combined action between the angle plates and bolts in resisting shear particularly at the slab‐column edges. Three‐dimensional analysis was carried out on such slabs employing concrete damaged plasticity model in ABAQUS to investigate their performance in terms of punching shear resistance, load–displacement response, and crack pattern at failure. The numerical models were first validated against the experimental results reported by other researchers. The proposed strengthening technique results in a substantial increase of load‐carrying capacity ranging from about 59 to 79% compared to the unstrengthened slab, and has also provided better stiffness, ductility and energy absorption in the slab system that change the mode of failure from a pure punching shear to flexural failure. The finite element results are compared with the calculated values according to provisions in the current design codes for evaluation purposes. A simple but reliable approach which is based on yield line theory to estimate the flexural capacity of the strengthened slabs is also outlined in this paper. [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) slabs are characterized by reduced construction time, versatility, and easier space partitioning. Their structural behavior is not straightforward and, specifically, punching shear strength is a current research topic. In this study an experimental database of 113 RC slabs without shear reinforcement under punching loads was compiled using data available in the literature. A sensitivity analysis of the parameters involved in the punching shear strength assessment was conducted, which highlighted the importance of the flexural reinforcement that are not typically considered for punching shear strength. After a discussion of the current international standards, a new proposed model for punching shear strength and rotation of RC slabs without shear reinforcement was discussed. It was based on a simplified load-rotation curve and new failure criteria that takes into account the flexural reinforcement effects. This experimental database was used to validate the approaches of the current international standards as well as the new proposed model. The latter proved to be a potentially useful design tool. [ABSTRACT FROM AUTHOR]

The problem related to the effect of the corner column load on the punching shear resistance of the slab was presented. Existing experimental studies on internal columns demonstrated that the column pressure could lead to an increase in the punching shear resistance. Because of different confinement conditions of corner column-slab connection joints, it is unclear if such an effect exists for corner columns. New experimental investigations were initiated to clarify this issue. They covered a total of three corner column-slab connection specimens - slabs with a thickness of 140 mm and a longitudinal reinforcement ratio -l = 1.09% connected with columns of a cross-section of 200×200 mm. The only variable parameter was the column load equal to 500, 1000 and 1500 kN. A reduction of the slab load-carrying capacity of about 9% due to a three-fold increase in the column load was noted. Therefore, the effect of the column load turned out to be opposite to that observed for most previous tests on internal column-slab connections, which could have a result of a limited capacity of the slab reinforcement due to additional tensile forces from the lateral expansion of joint concrete. Comparison in the light of test results demonstrated, that EN 1992-1-1 procedure allowed for safe, yet conservative estimation of the punching shear resistance. An average ratio of experimental to theoretical load of 1.82 was obtained. [ABSTRACT FROM AUTHOR]

The prefabricated reinforced concrete (RC) hollow slab bridges, with the advantages of high quality, lower cost and shorter construction period, have been widely used for small-to-medium-span highway bridges in China. Because of environmental deterioration and traffic volume increases, the performance of the bridge system deteriorates gradually. Accurate bridge system evaluation can provide a reliable basis for maintenance and management. A bridge system is composed of multiple interrelated components, which makes the system reliability evaluation become a computationally intractable work. In this paper, an effective method was proposed to evaluate the system reliability of the prefabricated RC hollow slab bridge considering hinge joint damage based on the modified analytic hierarchy process (AHP). Considering the subjectivity of the traditional AHP method in constructing the judgment matrix, this paper proposed an objective construction method of the judgment matrix to modify the traditional AHP. The modified hinge plate method (MHPM) proposed by the previous research was utilized to analyze the effect of hinge joint damage on system reliability. In order to verify the applicability of the proposed system reliability evaluation method, a simply supported RC hollow slab bridge was selected as the case study and the system reliability indexes were compared with the traditional series and parallel methods. The results indicated that the traditional methods were either too conservative or too radical to objectively evaluate the actual system reliability level of the structure. In contrast, the proposed method in this paper was more suitable for evaluating the system reliability of such bridges, and more accurate in providing maintenance decision makers with a relatively reasonable bridge condition information. [ABSTRACT FROM AUTHOR]

Highlights • The reliability of the models for the assessment of the punching capacity. • The modification of the failure criterion for the model based on the CSCT. • Relations between ratio V test / V Rc and parameters influencing the punching capacity. Abstract 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. [ABSTRACT FROM AUTHOR]

Highlights • Punching strengthening is required due to design/construction errors, load increase, etc. • Strengthening techniques are compared using the Critical Shear Crack Theory. • A case study is dealt with to give indications for strengthening pre-dimensioning. • Predicted punching strength of strengthened slabs agrees with experimental data. • A design tool is provided to choose which technique better fits the specific case. Abstract Over the years, the flat slab system has become a popular form of construction in many countries, particularly for multi-storey buildings such as offices or carparks. Nowadays, a considerable number of flat slab buildings requires strengthening against punching. Reasons are several, like design or construction errors, poor quality materials, not complying with new codes provisions, or increase of vertical load. This paper discusses different techniques for strengthening of R/C slabs against punching: insertion of post-installed shear reinforcement, application of external fibre reinforced polymers or a bonded reinforced concrete overlay (BRCO) on the slab top surface, enlargement of the support, application of a post-tensioning system. The authors propose a unified approach, based on the Critical Shear Crack Theory (CSCT), to evaluate the effectiveness of each strengthening technique. The attempt is to give a reading key capable of adapting to different strengthening techniques and providing a comprehensive view of the problem. The study shows that the CSCT could be easily adapted to all strengthening techniques and punching strength predictions are in good agreement with literature experimental results. This work also proposes to give designers the tool to choose which strengthening technique suits better to the specific case. [ABSTRACT FROM AUTHOR]

The provisions for design of shear reinforcement for flat slabs in the ACI 318 and Eurocode 2 building codes assume that punching failure and flexural failure can be treated as two mutually independent failure modes. However, if the flexural reinforcement yields, then wide flexural cracks will worsen the anchoring conditions for the shear reinforcement and a flexurally initiated punching failure may occur. However, shear reinforcement in the form of inclined stirrups is found to be effective even when wide flexural cracks develop and will, therefore, provide increased capacity and ductility when compared to a design with conventional shear reinforcement. Test results and design recommendations for ductile behavior as well as for maximum punching capacity are presented. [ABSTRACT FROM AUTHOR]

Limited research work exists on assessment of punching shear of reinforced concrete (RC) flat slabs made with blended cement incorporating ground-granulated blast-furnace slag (GGBS). This research is aimed at analyzing the punching shear strength of RC flat slabs cast from blended cement having GGBS in different proportions as partial replacement of cement. Four flat slabs supported on the ends were tested under column load such that one flat slab was cast from normal concrete with no GGBS and the remaining three flat slabs were cast with 30, 40, and 50% replacement of cement by GGBS. Experimental punching shear, midspan deflections, strain in the steel bars, and cracking pattern of the slabs were determined. The results of punching shear of flat slabs from the tests were compared with the nominal punching shear capacities proposed by BS 8110, BS EN1992-1-1/EC2, and ACI 318. The provisions of these building codes for the punching shear were observed as safe and conservative for the RC flat slab made from blended cement incorporating GGBS. [ABSTRACT FROM AUTHOR]

Over the last 20 years, the Institute of Structural Concrete at RWTH Aachen University has conducted numerous punching tests on interior slab-column connections with different types of punching shear reinforcement. Within the tests, different types and arrangements of stirrups, lattice girders, and double-headed studs were used as punching shear reinforcement. This paper summarizes the results of 39 punching tests and compares the punching shear resistances obtained from the tests with the punching shear capacity of flat slabs without punching shear reinforcement according to ACI 318-14 and Eurocode 2 along with the German Annex. [ABSTRACT FROM AUTHOR]

The paper deals with analysis of calculation of punching shear strength of reinforced concrete flat slabs with or without steel fibers in punching shear zone. The analysis shows, that the type, shape and strength of steel fibers has an influence on punching shear strength. The empirical method, evaluating these factors and calculating the punching shear strength according to Lithuanian and Euro Codes, is presented and discussed. [ABSTRACT FROM AUTHOR]

The fib Model Code for Concrete Structures 2010 introduces a new design concept for punching shear based on critical shear crack theory. This paper presents and provides the background to the design provisions for punching shear according to fib Model Code 2010, Eurocode 2 and the corresponding German National Annex to Eurocode 2. The different punching shear design provisions are critically reviewed by means of parameter studies and a comparison of the calculated resistances and test results. The safety levels of the code provisions are verified and the influence of the different punching parameters on the calculated resistances is examined in detail. [ABSTRACT FROM AUTHOR]

This paper presents the results of an extensive numerical parametric study to investigate the effects of different design parameters on the punching shear resistance of a hybrid steel tubular column/reinforced concrete flat slab system using shearhead arms. The parameters investigated include those related to the steel tubular column (diameter, shape), the shearhead arms (cross-section dimensions, length, end angle and position in the slab) and the slab (thickness and amount of reinforcement). The results of the parametric study indicate that for calculating the slab punching shear resistance, the shearhead system can be considered to be an enlarged column. The size of the enlarged column is equal to the original column dimensions plus the effective lengths of the shearhead arms. To calculate the effective length of the shearhead arm, it is assumed that the applied load is distributed underneath the inclined surfaces of the shearhead arm. Provided the shearhead arm has sufficient bending moment and shear resistances under the assumed load distribution, the entire shearhead arm is effective. The enlarged column size can be used to calculate the punching shear resistance as for reinforced concrete flat slab using either Eurocode EN 1992-1-1 or British Standard BS 8110. A comparison between the calculation results and the numerical simulation results suggests that the design calculation results are conservative and the BS 8110 calculations give slightly more accurate results. [ABSTRACT FROM AUTHOR]

Sometimes, due to design mistakes or a change in the construction function results in heavier or additional loads, a flat slab may be failed by punching shear which is considered as a brittle failure. A lot of research work has been carried with the objectives of strengthening existing flat slabs against punching shear. Nevertheless, very few experimental tests have been conducted for the investigation of the behavior of repaired flat slabs damaged due to punching shear. An experimental investigation with the objective of retrofitting flat slabs damaged due to punching shear using prestressed vertical bolts is presented in this paper. The parameters examined in this study are vertical prestressed bolts with different ratios within the slab thickness, slab thickness and central column size. Through the experimental tests the load carrying capacity, deformation characteristics and the cracking behavior have been investigated. A comparison between the behavior of retrofitted slabs and their references showed that the proposed system of repair is effective and could be used in practice. A comparison between the experimental results and calculated punching failure load based on the formulas adopted by different codes, showed a reasonable agreement. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The results of an experimental program on two-way flat slabs with central loading are presented in this paper. All the slabs were designed according to ACI 318-08 code provisions. One slab served as control without any modification while three slabs were strengthened in different ways with an innovative technique of using FRP fans after casting to increase shear capacity and provide sufficient anchorage. For strengthening in each case, 8, 16 and 24 strengtheners were used. The results of the experiments showed that the presented strengthening method is capable of enhancing both maximum loading capacity and deformation capacity as well as avoiding brittle failures that may occur under vertical loading. The dominant failure mode for flat slabs strengthened with FRP fans was deboning of FRP fans due to the small depth of slabs and simultaneously with separation of end anchorage from the concrete surface. It was shown that this method of strengthening not only can increase the shear capacity of the slab up to a high value around twice of that of controlled slab but also may change the slab failure mode from shear to shear–flexural and also flexural. The test results were also compared with the codes predictions to show the efficiency of the presented strengthening. The comparison showed that the ACI 318 code predictions are more conservative than those of JSCE code for results of this study. [ABSTRACT FROM AUTHOR]

Highlights: [•] 15 Test results are presented investigating the effects of prestressing on the punching shear strength. [•] Contrary to available tests, the different effects of prestressing are investigated separately. [•] The results are investigated on the basis of the physical model of the Critical Shear Crack Theory. [•] The suitability of this theory for punching design of prestressed slabs is demonstrated. [•] The evaluation of the physical parameters of this theory is adapted to the case of prestressed slabs. [Copyright &y& Elsevier]

Abstract: The use of randomly distributed steel fibres in the concrete mix improves its mechanical properties. In the particular case of a flat slab-column connection, this solution can provide slabs with an increased load capacity and deformation capacity, allowing a potential reduction of reinforcement. This work presents the experimental study of the behaviour of SFRC flat slabs up to failure under a concentrated loading, accompanied by the study of the mechanical properties of the SFRC, which consisted in three-point loading notched beams, compression and splitting tests. In this study, the hooked end steel fibre dosages varied between 0% and 1.25% by volume. Test results showed that the inclusion of steel fibres influences both slab stiffness and its load capacity. Increments of load capacity up to 64% were obtained in slabs with SFRC compared with the reference slab without fibres. The experimental results were compared with the predictions provided by several existing models. [Copyright &y& Elsevier]

Abstract: Normally in practice the situation of a slab-column structure is rarely analyzed after the occurrence of a local breakdown. According to the regulations quoted in CSA A23.3 an adequately calculated bottom reinforcement crossing above the column may be applied preventing a further development of the catastrophe. Instructions concerning such a structure are also contained in ACI 318 [1]. The paper presents the results of investigations carried out on model of slab-column connections in the scale 1:1. The aim of these investigations was to find out how far any additional bending moment resulting from the eccentric effect of the load affects the value of the destructive load imposed on the connection after its punching. Three variants of the position of the column in relation to the centre of the slab have been considered: the axial position, the position on a unidirectional eccentricity, and on bidirectional eccentricity. The tests were performed in two phases, viz. “Phase 1” preceding the punching and “Phase II” after punching to the complete destruction of the connection. Basing on these tests the results were compared with the standard, and it turned out that the cross-section of the reinforcement had been underestimated. Thus, it ought to be suggests that in the calculations the value of the ratio of the destructive force imposed on the column to the destructive load should be increased. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: This paper presents experimental data and results on the effect of externally bonded carbon fiber-reinforced polymer (CFRP) strips on the punching shear of interior slab-column connections. A total of six square slabs with a concentric column were constructed with overall dimensions of 1220mm by 1220mm and 100mm thick slab and 150×150mm column. The test variables were the configuration and amount of CFRP strips externally bonded to the tension face of the slab. The specimens were simply supported along their edges and tested in punching with vertical load applied through the central column. The test results clearly showed that CFRP strengthening leads to significant improvements in the structural behaviour of slab-column connections. The increase in punching capacity of strengthened slabs was up to 29%, while the increase in stiffness was up to 80% compared to the unstrengthened slab. Punching capacities of the test specimens were evaluated using a recently developed model that accounts for both configuration and amount of CFRP strips. The predicted punching capacities using the analytical model were compared with the experimental results, and good agreement was found. [Copyright &y& Elsevier]

Abstract: This paper is concerned with the behaviour of ductile shearheads for connecting reinforced concrete flat slabs to internal tubular steel columns. A novel detail is presented in which a gap is left around the column to enable the shearhead to yield in shear prior to the slab failing in punching shear. The proposed connection could serve as a primary lateral resisting system in low-rise buildings in regions of low to moderate seismicity or as a secondary system in areas of significant seismicity. Test results and numerical analyses are presented for a fully embedded ACI-type shearhead as well as the proposed partially embedded connection. The latter is shown to offer enhanced ductility compared with traditional forms. Experimental and numerical results are used to demonstrate the favourable inelastic performance of the proposed detail. [Copyright &y& Elsevier]

Abstract: This work aims to study a new flat slab strengthening technique based on post-tensioning with anchorages by bonding using an epoxy adhesive. The main advantages of this technique over the traditional prestress strengthening systems that use mechanical anchorages are that it does not need external permanent anchorages, meaning that the forces are introduced into the concrete gradually instead of being localized, thereby preserving aesthetics and useable space. The seven tested slab models show that this technique meets its objective as it is able to reduce reinforcement strains at service loads by up to 80% if the strengthening technique is applied in two directions and slab deformations by up to 70%, consequently making crack widths smaller. It can also increase punching load capacity by as much as 51% when compared to non-strengthened slabs. The results are compared with the EC2 (2004) , ACI 318-08 (2008)  and MC2010 (2010)  provisions. The main conclusions are that this strengthening technique is effective regarding ultimate and serviceability states and that it represents an advance in RC slab strengthening techniques. [Copyright &y& Elsevier]

The article focuses on the approach to predict both punching strength and rotation capacity of slabs without transverse reinforcement non-axis-symmetrical behavior. It says that the strength predictions of the tests is provided by Eurocode 2 and BS 8110 code formulas using the average reinforcement ratio between the x and y directions. It adds that the accurate predictions of strength and deformation capacity is provided through the development a rational analytical approach.

Abstract: The punching shear strength of two way slabs without shear reinforcement and without unbalanced moment transfer is estimated using both neural networks and new simplified punching shear equations. An artificial neural network (ANN) was used to predict the punching shear strength of internal slab–column connections. Neural network analysis is conducted using 244 test data available in the literature and experiments conducted by the authors to evaluate the influence of concrete strength, reinforcement ratio and slab effective depth on punching shear strength. A wide range of slab thicknesses (up to 500 mm) and reinforcement ratios were used. In general, the results obtained from the neural network are very close to the experimental data available. The test results were used to develop two new simplified practical punching shear equations. The equations also showed a very good match with available experimental data. Four equations for the punching shear strength prescribed in well-known specifications were evaluated based on the available experimental results. This paper includes a discussion of the parameters of punching shear strength in the American, Canadian, British and European specifications. [Copyright &y& Elsevier]

Strengthening two-way slabs by using fiber-reinforced polymer (FRP) is experimentally and analytically evaluated. Results show that the punching capacity of two-way slabs can increase to up to 40% greater than that of a reference specimen. A three-dimensional FEM program called 3D CAMUI, which was developed at Hokkaido University, was used to simulate the experimental slabs. Very good agreement is obtained in load-carrying capacity and modes of failure. An analytical model based on the numerical simulation, which discloses the mechanism of punching shear strength enhancement by FRP strengthening, is proposed to predict the punching shear strength of two-way slabs externally strengthened with FRP sheets. [ABSTRACT FROM AUTHOR]

Abstract: Experimental tests conducted on 27 square cementitious slabs of 490×490mm simply supported on four edges and subjected to patch load are presented. The slabs had a clear span of 400×400mm and provided with a 445×445mm closed frame of 8mm diameter steel bar to hold the reinforcement in place and to act as a line support. The test variables were the wire mesh volume fraction: four expanded and two square types; slab thickness: 40, 45, 50 and 60mm; and the patch load pattern: square and rectangular. The test results showed that as the volume fraction increased the punching strength of the slabs was also increased. Adding a wire mesh to ordinary reinforcement increases significantly the punching resistance at column stub. Moreover, as the loaded area size increases both ductility and stiffness increases and the bridging effect due to the difference in the reinforcement ratio in orthogonal directions was clearly noticed. More research was needed to identify the volume fraction ratio at which the mode of failure alter from flexure to punching. [ABSTRACT FROM AUTHOR]

The traditional approach of codes of practice for estimating the punching strength of shear-reinforced flat slabs is based on the assumption that concrete carries a fraction of the applied load at ultimate while the rest of the load is carried by the shear reinforcement. Concrete contribution is usually estimated as a fraction of the punching strength of members without shear reinforcement. The ratio between the concrete contribution for members with and without shear reinforcement is usually assumed constant, independent of the amount of shear reinforcement, flexural reinforcement ratio, and bond conditions of the shear reinforcement. The limitations of such an approach are discussed in this paper and a new theoretical model, based on the critical shear crack theory, is presented to investigate the strength and ductility of shear-reinforced slabs. The proposed approach is based on a physical model and overcomes most limitations of current codes of practice. Its application to various punching shear reinforcement systems is also detailed in the paper and its results are compared to available test data. [ABSTRACT FROM AUTHOR]

In determining the punching shear capacity of flat slabs, current codes of practice usually do not consider the effect of boundary restraint against rotation. The contribution of flexural steel reinforcement is ignored by some of the code provisions. Present paper is comprised of the test results of 15 model slabs in an effort to ascertain the influence of boundary restraint, the influence of flexural reinforcement and effect of thickness of the slabs on their structural behavior and punching load-carrying capacity. Cracking pattern and load-deflection behavior of the slabs tested have also been monitored closely. Significant effect of edge restraint on the punching failure load, resulting in an enhancing of the ultimate punching strength, and effect of flexural reinforcement on the punching failure load have been noticed. The code-specified strength of the specimens was calculated in accordance with the American, British, Canadian, European, and Australian codes. It has been understood that punching shear is not effectively estimated in some codes. Thus, inclusion of the findings of the study in the design codes is expected to result in a rational design of structural systems where punching phenomenon plays a vital role. [ABSTRACT FROM AUTHOR]

• Proposed a model to predict punching shear failure. • Predicted the punching shear failure by monitoring the column drift ratio. • The ASCE 41 stiffness can be used in conjunction with the proposed modeling method. • The lateral strength is estimated by flexural strength over slab column strip. • The proposed model provided good estimation for tests in the literature. A nonlinear modeling approach was developed to capture the response of slab-column (SC) frame members under combined gravity and lateral load demands. The occurrence of punching shear or flexure-punching shear failures at slab-column connections is detected by monitoring the column drift and providing a limiting drift value based on the gravity shear demand at the connection. Following detection of failure, plastic springs at the member ends are adjusted to provide a reduced stiffness and moment carrying capacity. Nonlinear modeling parameters developed for use in ACI 369.1 and ASCE 41 for SC connections without shear reinforcement were then reviewed by applying the developed nonlinear model to several SC connection tests reported in the literature. The model was implemented in the OpenSees computational platform. The backbone responses predicted by the proposed SC connection model are consistent with the experimental measurements. In addition, the proposed nonlinear modeling parameters are shown to provide a good estimate of the nonlinear response of SC connections under gravity and lateral loads that induce punching shear or flexure-punching shear failures. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the experimental investigations and a specific failure criterion for evaluating the ultimate punching load of concrete slabs strengthened by composite materials. First, the flexural mode failure is calculated by the yield lines method to ensure that it was the punching phenomenon that caused the break. The analytical model for punching failure of reinforced concrete slabs developed by Menetrey [Menetrey P. Synthesis of punching failure in reinforced concrete. Cem Concr Comp 2002;24:497–507] is used. For RC slabs reinforced by composite materials, the punching load is calculated by the Rochdi enhancement model [Rochdi EH, Bigaud D, Ferrier E, Hamelin P. Ultimate behaviour of CFRP strengthened RC flat slabs under a centrally applied load. Comp Struct 2006;72:69–78]. Finally, the experimental values are compared to the analytical model values to validate the method of calculation. [Copyright &y& Elsevier]

The response of high-performance fiber-reinforced cementitious composite (HPFRCC) slab panels to punching shear loading was investigated. The slabs were square, 180 mm (7 in.) thick, simply supported at their periphery, and concentrically loaded. Conventional distributed steel reinforcement, if provided, consisted of one or two layers of bottom reinforcement (one- or two-way reinforcement), or four layers of reinforcing bars (two on top and two on bottom, one in each principal direction). The four layers simulated a typical bridge deck designed according to the AASHTO LRFD specifications. Moreover, the effect on punching shear behavior of three different fibers (polyvinyl alcohol [PVA]; ultra-high molecular weight polyethylene identified as SPE; and twisted steel identified as Torex) was evaluated. Test results showed that the punching shear resistance, the energy absorption capacity, and the resistance to spalling of HPFRCC slabs having only two bottom layers of reinforcing bars (one in each direction) were significantly better than for the control specimen with four layers of reinforcing bars and regular concrete. Analysis of the results suggests that the punching shear resistance of HPFRCC slabs with geometric properties similar to those of the test specimens can be safely taken as twice that recommended for design by the ACI code. [ABSTRACT FROM AUTHOR]

The treatments of punching at interior supports of post-tensioned slabs given in ACI 318-02, Eurocode 2, and the FIP recommendations are presented. Sixteen new tests are reported and their results, together with those from 24 tests in the literature, are compared with the predictions from the three codes. Conclusions are drawn on the general adequacy of the design methods and on possible improvements. [ABSTRACT FROM AUTHOR]

The results of an experimental investigation undertaken to evaluate a new technique for strengthening interior slab–column connection in combined flexural and shear modes are presented. The technique consists of using a combination of shear bolts inserted into holes and prestressed against the concrete surface for improving the punching shear capacity, and external [fiber-reinforced polymer (FRP)] reinforcement bonded to the tension face of the slabs in two perpendicular directions for increasing the flexural strength of the slabs. Square slab specimens of 670×670 mm dimensions were tested and the main test variables included the ratio of steel reinforcement (1.0 and 1.5%), span–depth ratio or thickness (55 and 75 mm) of the slabs, area, and configuration of steel bolts, and area of FRP reinforcement. It was found that the use of shear bolts alone improves the punching shear strength and increases the ductility of failure by changing the failure mode from punching to flexural. However, the use of a combination of shear bolts and a moderate amount of FRP reinforcement increased the flexural strength and resulted in a substantial improvement of the punching shear capacity of the slabs. The corresponding increases attained levels between 34 and 77%. A design approach is presented for evaluating the ultimate capacity of the slab–column connections when strengthened using the proposed strengthening technique. Strength results predicted by the proposed approach were in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

This paper presents the direct decompression method for calculating the punching strengths of post-tensioned slabs. It is a method already used for determining the shear strengths of beams. In many respects it is similar to the Fédération Internationale de Ia Précontrainte (FIP) treatment of punching in post-tensioned slabs, but it is simpler to use. The predictions of two variants of the direct decompression approach, and of the FIP method, are compared with the results of tests of slabs with various arrangements and profiles of tendons. It is shown that the direct method reduces the scatter of ratios of experimental and calculated strengths although all three approaches provide reasonable results. [ABSTRACT FROM AUTHOR]

• Nonlinear finite analysis (NLFEA) of punching failure at biaxially loaded edge columns of flat slabs. • Development of Joint Shell Punching Model for edge columns. • Development of edge column shear enhancement factor for closed form version of Critical Shear Crack Theory. • Validation of proposed shear field methodology for calculating shear resisting perimeter for use with Critical Shear Crack Theory. The paper investigates the influence of biaxial loading on punching resistance at square and elongated edge columns of flat slabs which is virtually neglected in the literature. In the absence of experimental data, the influence of biaxial loading is determined using nonlinear finite element analysis (NLFEA) with 3D solid elements. The resulting baseline punching resistances are compared with the predictions of various implementations of the Critical Shear Crack Theory (CSCT) including a Joint Shell Punching Model (JSPME) in which punching failure is simulated using nonlinear joint elements inserted between the nodes of nonlinear shell elements located around the punching control perimeter. The failure criterion of the JSPME, which is most suited for structural assessment, implicitly accounts for the effect of biaxial loading unlike the original (classic) and closed form versions of the CSCT. The classic CSCT indirectly accounts for loading eccentricity by reducing the punching control perimeter by a multiple k e which is determined in this paper using shear field analysis. Conversely, the closed form CSCT, which is adopted in the draft for the next generation of Eurocode 2 (EC2), enhances the design shear force by a multiple β. The paper uses experimental data to determine an expression for β for edge column connections subject to inwards eccentricity normal to the slab edge. Subsequently, shear field analysis on representative flat slab to edge column connections is used to extend this expression for β to edge column connections subject to biaxial eccentricity. NLFEA simulations with 3D solid elements are used to validate the predictions of the JSPME, the shear field methodology used to determine k e in the classic CSCT and the proposed expression for β in the closed form CSCT. Reasonable agreement is achieved between all these analysis methods. The main advantage of the JSPME over NLFEA with 3D solid elements is its increased computational efficiency which makes it more suitable for the assessment of large structures. [ABSTRACT FROM AUTHOR]

• The activation of punching reinforcement in flat slabs and column bases is analyzed. • The mechanical background of activation for next Eurocode 2 (prEC2) is explained. • An extended activation formula considering bond and anchorage is derived for prEC2. • Different types of shear reinforcement are considered on a uniform basis. • Focus is on transformation from a mechanical model to a simplified design formula. • The proposed activation formula is validated by systematic punching test results. Various types of punching shear reinforcement have been developed during the past decades to increase the punching capacity and ductility of slab-column connections. Due to diverse geometries and thus varying anchorage conditions, the effectiveness of these elements contributing to total punching resistance can differ significantly. Generally, the degree of activation of shear reinforcement can be described by contributions of anchorage and of bond. Yet, most of current punching design provisions follow a simplified, empirical approach. Despite its mechanical background, the punching design according to the next generation of Eurocode 2 still does not consider the effect of better anchorage of, for example, double headed studs compared to simple stirrups. In this paper, an extended activation formula for flat slabs as well as column bases is proposed as refinement for the next generation of Eurocode 2. This unifying approach considers different anchorage categories and thus enables more progressive design results. After discussing the background of possible activation scenarios, the effects of bond and anchorage in cracked concrete and the kinematic deformation behavior of slabs subjected to punching are identified as governing constraints. The major focus is on the transformation process from mechanical considerations to a simplified design formula to ensure practical applicability. [ABSTRACT FROM AUTHOR]

• Load resisting mechanisms of eccentrically loaded slab-column joints with in-plane restraints were discussed. • Effects of eccentricity, slab thickness and steel ratio on the pre- and post-punching performances were studied. • Effects of in-plane restraint and unbalanced moment were quantified based on the FE analysis. • Interactions between the unbalanced moment and shear force were explored and compared with code predictions. In flat plate structures, interior slab-column joints are subjected to in-plane restraints and bending-shear actions, which can significantly impact their mechanical performance to resist progressive collapse. However, research in this area is still lacking. This paper presents experimental and numerical studies of pre- and post-punching performances of eccentrically loaded slab-column joints with in-plane restraints. Quasi-static tests were conducted on seven joint specimens, including one concentrically loaded (CL) joint and six eccentrically loaded (EL) ones. The effects of critical structural parameters (i.e., eccentricity, slab thickness and reinforcement ratio) on the pre- and post-punching performances were investigated by analysing the load–displacement relationships, failure modes and material strains. The experimental results indicated that larger eccentricity could lead to more reduction of the peak punching shear capacity by as much as 62%, whereas it has little effect on the post-punching capacity. In addition, the strain results of the through-column rebars showed that, with the well-restrained boundary conditions, the slab flexural reinforcement (FR) was found to resist the applied load to a large extent at small deformations and still able to contribute greatly during the post-punching stage. Numerical analysis was conducted to gain an in-depth understanding toward the effects of varied levels of in-plane restraints on both slab-column joints and actual flat plate structures. It was found that the lateral stiffnesses in the CL joint model with boundary beams and the substructure model were similar, which was around 400 N/mm. Finally, the interactions between the unbalanced moment and shear force were explored and compared with code predictions (ACI318-19 and EC2-04). Without considering the enhancement effects from in-plane restraints, the codes produced over-conservative predictions. Based on the FE analyses, the correlation between the dimensionless ratio e / d (eccentricity/effective thickness of slab) and punching shear strength was simplified by a combined straight line and a power curve. [ABSTRACT FROM AUTHOR]

• The behaviour of UHPC/NC hybrid slab is investigated. • A validated FE model is used to study a wide range of parameters. • A design model is developed to predict the punching capacity of hybrid slabs. Flat slabs are commonly used in the construction of multi-storey buildings due to their ease of placement and installation, relatively short construction time, minimal structural floor depth and economic credentials. However, flat slabs are vulnerable to punching shear, a failure mode that typically occurs without any warning at slab-column connections. While various methods exist to improve punching shear capacity, the use of ultra-high performance concrete (UHPC) offers a highly buildable alternative to use of shear reinforcement in normal strength concrete (NC). However, UHPC is an expensive material and recent research has focussed on its partial use in column zones only, with NC elsewhere i.e. hybrid NC/UHPC. In this paper, a non-linear finite element (FE) model using ABAQUS was developed to simulate the punching shear performance of hybrid NC/UHPC flat slabs without shear reinforcement. The FE model was validated against experimental results from the literature. The validated model was then used to investigate the influence of slab thickness, column size, UHPC area and UHPC compressive strength. The results show that punching shear capacity of the slabs increases with the area of the UHPC zone. Additionally, the ratio of concrete compressive strength of UHPC to that of NC has a significant effect on the behaviour of the hybrid slabs which should be accounted for in future design guides. In order to understand the performance of existing design guides and codes in this context, a comparison between FE results and code predictions is also presented. The results suggest that the code models including ACI 318-14, Eurocode2 and Model Code 2010 cannot accurately estimate the punching shear capacity of reinforced UHPC slabs and hybrid NC/UHPC slabs. A modification to the existing ACI equation is proposed which yields accurate predictions for reinforced UHPC and hybrid slabs. [ABSTRACT FROM AUTHOR]

• Tests to investigate the performance of a new type of punching shear reinforcement. • Punching shear reinforcement whose anchoring is independent of the flexural rebars. • Experimental tests on slab-column connections with high shear reinforcement ratios. • Use of complimentary reinforcement to avoid premature failure due to delamination. • ACI, EC2 and MC10 presented conservative strength predictions for the tested slabs. This paper presents the results of tests on slabs with punching shear reinforcement formed by prefabricated truss assemblies disconnected from the flexural reinforcement, developed to optimize the construction process. General guidelines for design, detailing, and assembly of this reinforcement are presented, based on the recommendations presented by ACI 318, Eurocode 2, and fib Model Code 2010. One slab without shear reinforcement and one slab with stud rails were tested as a reference for tests on two slabs with the prefabricated truss modules as punching shear reinforcement. The load-rotation response and the failure loads are presented and discussed in comparison with theoretical estimates obtained following the design codes. The profile of stress and strain distribution on the flexural and shear reinforcement are also presented and discussed. The slab with inclined prefabricated truss bars registered similar response and resistance compared to the one with stud rails. These tests confirm the effectiveness of this kind of punching shear reinforcement, as one of the slabs had almost twice the strength and more than three times the deformation capacity of the reference slab without shear reinforcement. [ABSTRACT FROM AUTHOR]

• Reliability assessment of punching shear capacity for? flat slabs without shear reinf. • Stochastic modelling of the random variables in the punching shear resistance function. • Safety level assessment of the resistance model and comparison to EN 1990 (β = 3.8). Punching shear is one of the main failure mechanisms in flat slabs. To prevent such failures, modern structural codes, as EN 1992-1-1, offer design provisions against punching shear. However, these provisions were calibrated based on experience, which can affect the precision of the resulting safety level. The use of modern reliability analysis techniques is required for a more precise evaluation of the safety level of these structural systems. This paper presents a comprehensive review of the state-of-the-art of reliability techniques where the safety level of design provisions for punching shear resistance without shear reinforcement is investigated. This study addresses three reliability analysis techniques: Mean-Value First Order Second Moment Method (MVFOSM), First-Order Second Moment Method (FOSM) and a Monte-Carlo simulation with Importance Sampling (MC-IS). Reliability indices β are used as a practical measurement of the safety level representing probabilities of failure. The parameters influencing the punching shear capacity without shear reinforcement were varied that the area of practical interest is covered. The estimated reliability indices β are evaluated and compared to a target safety level given by EN 1990 for a 50-year reference period where β is equal to 3.8. The results seem to confirm that the punching shear provisions for slabs without shear reinforcement according to EN 1992-1-1 achieve a required safety level, which is in accordance to the target level given by EN 1990. In addition, the study shows that the use of the techniques FOSM and MC-IS seems appropriate for determining the probability of failure of the design equation for the punching shear capacity without shear reinforcement. Furthermore, the study suggests that the method MVFOSM may not be suitable to assess the absolute safety level of such design equations. Finally, the study highlights the potential of using the aforementioned reliability analysis techniques and stresses the importance of modelling the uncertainties for a precise structural safety assessment of flat slabs without shear reinforcement. [ABSTRACT FROM AUTHOR]

The article discusses the paper "Punching of Post-Tensioned Slabs--Tests and Codes," by Ricardo J. C. Silva, Paul E. Regan, and Guilherme S. S. A. Melo. The reviewers of the paper found that in the calculation of Vp, defined as the sum of the vertical components of prestress forces assumed to be transferred directly to the support by inclined compression without producing shear in the slab, using the EC2, it was considered that all the tendons crossed the control perimeter, defined at a distance of 2d from the support. They also found that the final version of the EC2 however, has some contradictory information regarding this issue. The article also presents the comments of study authors regarding the review of their paper.