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In collaboration with practicing structural engineers and experts in anchoring to concrete, the authors undertook a research project to explore the requirements for anchoring columns to reinforced concrete foundations. In early discussions it was realized that, while there were design procedures that were in use in different design offices, the design procedures differed from office to office and few of the methods had been verified by laboratory testing. Based on this knowledge, a series of laboratory tests was designed and carried out to develop benchmark data on the following types of column-foundation problems:- Interior footings supporting columns in direct tension and anchored by multiple anchor bolts, either with or without additional footing transverse reinforcement to increase strength and deformation capacity.- Interior footings supporting column in direct bending and anchored by multiple anchor bolts, either with or without additional footing transverse reinforcement to increase strength and deformation capacity. The laboratory tests were supplemented by nonlinear finite element studies using the software ATENA, both to calibrate the model material parameters and to extrapolate results from the laboratory tests to geometries that were not tested in the laboratory. Together, the laboratory and numerical studies were used to derive a design method to calculate the strength of connections in either direct tension, direct moment transfer, or combinations of the two, with or without additional transverse reinforcement intended to increase strength and deformation capacity. The authors subsequently worked with ACI Committee 318 Structural Concrete Building Code of the American Concrete Institute to develop design provisions that were approved and adopted in ACI 318-25 Building Code Requirements for Structural Concrete and Commentary.

This study investigates the structural behavior of reinforced concrete (RC) flat slabs supported on edge columns and the influence of eccentricity direction on their punching shear strength. For this purpose, code provisions were compared based on the ultimate capacities of experimental tests reported in the literature. The comparison included the approaches provided by the current Eurocode 2, fib Model Code 2010 and the final draft of the second generation of Eurocode 2 (FprEC2). Then, a numerical investigation was conducted in which full‐scale slabs tested by Albuquerque et al. (2016) were simulated. Numerical analyses were carried out in the software Diana using a total strain‐based crack model to simulate the concrete nonlinear behavior. The input parameters for the simulations were obtained through a stochastic calibration approach to account for uncertainties related to the concrete properties. A reasonable agreement was found between the calibrated numerical models and the experimental results regarding load‐deflection curves and crack patterns. Furthermore, important differences were identified in the failure mode of edge column‐slab connections subjected to inward and outward eccentricities. Finally, additional simulations were carried out to obtain a full shear‐moment interaction diagram and investigate the shear distribution at the control perimeters around the column. The proposed method in FprEC2 exhibited a good estimation with a relatively low scatter of the punching resistance of flat slab‐edge column connections analyzed in this work. [ABSTRACT FROM AUTHOR]

The behaviour of lightweight concrete (LWC) edge column-slab connections reinforced with two types of flexural reinforcement glass fiber-reinforced polymer (GFRP) and steel bars with different ratios of shear reinforcement are investigated experimentally and analytically using ABAQUS software. The experimental protocol covered evaluation of eight slabs and edge columns measuring subjected to static loading. Slabs are evaluated as supported, free-standing at one edge, and the column connection is included. The slabs are divided into two groupings, with four slabs in each group. The first group was reinforced with steel flexural, while the second group was reinforced with GFRP reinforcement. Within each group, one slab lacked shear reinforcement, while the other was reinforced with a varying shear reinforcement ratio. A good agreement was found between the numerical FE model and experimental outcomes in most tested slabs. [ABSTRACT FROM AUTHOR]

The slab is more petite and space-efficient in flat plate buildings since it is supported directly by columns rather than beams or drop panels, allowing additional floors to be added. Despite these benefits, a flat plate slab is vulnerable to brittle punching shear, a catastrophic collapse caused by the abrupt propulsion of a slab piece out from underneath by a column. In this study, Finite Element Analysis (FEA) was provided and carried out in ABAQUS/CAE 2019 to model the effects of punching shear impact on a flat plate reinforced with horizontal steel bars that vary in position, diameter, and number. Concrete was represented in the model by 8-noded hexahedral 3D brick elements and steel reinforcements by 2-noded linear 2D beam elements. The model has been modified according to the results of the experiments. In order to determine how different quantities and sizes of horizontal steel bars placed at different locations in a flat plate affected the slab's performance, parametric analysis was conducted. According to the outcomes, the shear capacity increases between 0.37 and 9.85% as the diameter of bars increases, between 1.2 and 22.9% as the quantity of bars increases, and between 1.99 and 26.1% as the bars shift from the tension side to the compression side. [ABSTRACT FROM AUTHOR]

In this paper, the punching shear performance of 8 steel fiber reinforced recycled coarse aggregate concrete (SFRCAC) two-way slabs with a size of 1800 mm × 1800 mm × 150 mm was studied under local concentric load. The effects of RCA replacement ratio (rg) and SF volume fraction (Vf) on the punching shear performance of SFRCAC two-way slabs were investigated. Digital Image Correlation (DIC) measurement and Acoustic Emission (AE) technique were introduced to collect pictures and relevant data during the punching shear test. The test results show that the SFRCAC two-way slab mainly exhibits punching shear failure and flexure failure under local concentric load. The punching shear failure space area of SFRCAC two-way slab has no obvious change with increasing rg, however, show a gradual increase trend with increasing Vf. Both of the punching shear ultimate bearing capacity (Pu) and its deflection of SFRCAC two-way slab decrease with increasing rg and increase with increasing Vf, respectively. Finally, through the regression analysis of the results from this study and the data collected from related literature, the influence of rg and Vf on the Pu of two-way slabs were obtained, and the equations in GB 50010-2010, ACI 318-19, and Eurocode 2 Codes were amended, respectively. Furthermore, the amended equations were all applicable to predicted the ultimate bearing capacity of the ordinary concrete two-way slab, RCAC two-way slab, SFRC two-way slab, and SFRCAC two-way slab. [ABSTRACT FROM AUTHOR]

The present study aims to parameterize the numerical models of flat slabs based on five experimental prototypes developed previously. The analyzed slabs had dimensions of 1800 mm × 1800 mm and a thickness of 130 mm. Two slabs did not have openings, and three slabs had stud‐type shear reinforcement. Subsequently, five new slab models were selected with position variations and several openings, with a fixed diameter of 75 mm and without shear reinforcement. After calibration of the models, the following results were obtained: the numerical rupture load approached at least 90% of the experimental rupture load, and the rupture modes and cracking patterns were similar to those observed in the experimental models. When it is impossible to move the column openings vertically, it is more beneficial to position them at the corners of the column rather than close to the same face of the column. Thus, it was possible to numerically analyze the effects of the position and number of openings in flat slabs. [ABSTRACT FROM AUTHOR]

Punching shear failure of flat slabs is a critical safety issue since it may occur without any prior signs of damage. Welded shear reinforcement fabricated by longitudinal chords and diagonal webs is proposed as a novel type of shear reinforcement to significantly improve punching shear strength and ductility of flat slabs. However, there are few investigations on its force mechanism at present. To compensate for the objective flaws in the experiment, this research uses ABAQUS finite element software to distinguish the influence of welded shear reinforcements on punching shear behavior of flat slabs. A nonlinear finite element model was developed and validated with experimental results. The failure mode, concrete stress, flexural reinforcement strain, and welded shear reinforcement strain are all investigated in failure mechanism of flat slabs. The webs of weld shear reinforcement can limit the development of cracks and strengthen the aggregate interlocking at the cracks. The parametric analysis of the welded shear reinforcement under different geometrical parameters proves that punching shear effect of diagonal webs of welded shear reinforcement is much larger than that of chords. The definition of the web shear reinforcement ratio provides a theoretical basis for the design of such punching shear keys. [ABSTRACT FROM AUTHOR]

Flat plate structures consist of a slab supported directly by columns without beams or drop panels, resulting in a thinner slab with more efficient use of space. Despite these advantages, a flat plate slab is subjected to brittle punching shear. Sudden collapse may occur when a column pushes a piece of concrete from the slab above it. This paper displays Finite Element Analysis (FEA) using ABAQUS/ CAE 2019 to simulate the punching shear impact on a flat plate strengthened with horizontal steel bars of varying number and diameter, located at the compressive side of the slab. A numerical model was constructed with 8-noded hexahedral 3D brick elements for concrete and 2-noded linear 2D beam elements for steel reinforcement. The model was adapted based on experimental data. A parametric analysis was conducted to evaluate the impact of placing horizontal steel bars at the compression side of the flat plate and changing the quantity and size of these bars on the slab's performance. The results illustrate that the shear capacity increases from 17.07% to 28.13% as the bar diameter increases and from 19.17% to 54.82% as the number of bars increases. [ABSTRACT FROM AUTHOR]

The behavior of precast concrete tunnel lining (PCTL) segments reinforced with glass fiber-reinforced polymer (GFRP) bars under punching loads is one area in which no research work has been conducted. This paper reports on an investigation of the punching-shear behavior of GFRP-reinforced PCTL segments induced by soil conditions, such as rock expansion or the geotechnical conditions surrounding a tunnel. Six full-scale rhomboidal PCTL specimens measuring 1500 mm (59 in.) in width and 250 mm (9.8 in.) in thickness were constructed and tested up to failure. The investigated parameters were: 1) reinforcement type (GFRP or steel); 2) reinforcement ratio (0.46 or 0.86%); 3) stirrups as shear reinforcement; and 4) segment length (2100 or 3100 mm [82.7 or 122 in.]). The experimental results are reported in terms of cracking behavior, punching-shear capacity, deflection, strain in the reinforcement and concrete, and failure modes. The results reveal that the GFRP-reinforced PCTL segment was comparable with its steel counterpart with the same reinforcement ratio and satisfied serviceability limits. Increasing the reinforcement ratio and decreasing the segment length enhanced the punching-shear strength. The shear stirrups improved the structural performance and increased the punching and deformation capacities of the GFRP-reinforced PCTL segments. In addition, theoretical predictions of the punching-shear capacity using the current design provisions were compared to the experimental results obtained herein. The theoretical outcomes show the suitability of using current FRP design provisions for predicting the punching capacity of PCTL segments reinforced with GFRP bars. [ABSTRACT FROM AUTHOR]

Abstract In structural design practice and scientific research on flat slabs, estimating the punching shear resistance of slab-column connection with shear reinforcement involves checking the shear stress acting on a control perimeter outside the shear-reinforced zone. Depending on the arrangement chosen for the shear reinforcement, the design codes` recommendations can lead to different interpretations of the geometry of the external control perimeter, which are, in some cases, also complex to formulate into closed-form equations. Thus, it is common that, especially in hand-made estimates, the slab-column connections are drawn, and dimensions of the external control perimeters are individually measured and applied to the design equations. This paper presents equations developed for ABNT NBR 6118, Eurocode 2 and ACI 318 to account for the length of the control perimeter outside the shear-reinforced zone, allowing the automated calculation for different shear reinforcement arrangements. The different interpretations of the geometry of the external control perimeter of these design codes are discussed and clarified. The proposed equations were validated by comparing their results with those obtained from individual drawing analysis of 62 slab-column connections with varied geometries and shear reinforcement arrangements, and the maximum error was up to 0.1%. This paper significantly contributes to the automation and optimisation of the punching shear strength estimates of slab-column connections with shear reinforcement, clarifying obscure points and automating the definition of the geometry of the external control perimeter.

The behaviour of lightweight concrete (LWC) edge column-slab connections reinforced with two types of flexural reinforcement glass fiber-reinforced polymer (GFRP) and steel bars with different ratios of shear reinforcement are investigated experimentally and analytically using ABAQUS software. The experimental protocol covered evaluation of eight slabs and edge columns measuring subjected to static loading. Slabs are evaluated as supported, free-standing at one edge, and the column connection is included. The slabs are divided into two groupings, with four slabs in each group. The first group was reinforced with steel flexural, while the second group was reinforced with GFRP reinforcement. Within each group, one slab lacked shear reinforcement, while the other was reinforced with a varying shear reinforcement ratio. A good agreement was found between the numerical FE model and experimental outcomes in most tested slabs.

For the last five decades, researchers have been studying the punching shear of two-way reinforced slabs at column supports in structural testing laboratories. Limited tests are available on retrofitted waffle slab punching behavior. These are necessary since they establish a baseline against which punching shear design provisions can be created, calibrated, and evaluated. This work presents an experimental approach to investigate the punching shear of reinforced concrete waffle slabs strengthened with carbon fiber reinforced polymers (CFRP) sheets. Retrofitted waffle slabs were tested under concentric monotonic loading. A series of tests on construction materials were also conducted. The testing program was divided into two categories (IWS1 and IWS2), with a total of ten specimens: solid sections (275 × 275) and (515 × 515) mm, respectively. The IWS1 category consisted of eight slabs; one slab was without strengthening as a reference slab, and the remaining slabs were strengthened. While IWS2 consisted of only two slabs. However, the waffle slabs’ observed punching failure mechanism was like flat slabs. All waffle slabs were punctured in a sudden position. Research showed that the CFRP retrofitting process enhanced the ultimate punching shear performance by 47.1% and the initial cracking load by 67.02%, respectively. When the solid size regions were increased, the ultimate punching load increased by 41.5 %, and the initial cracking load increased by 40.8%, respectively. Significant enhancements were documented in the proposed retrofitting mechanism.

The literature review showed insufficient relevant research on the application of Glass-Fiber-Reinforced-Polymers (GFRP) gratings in the structural elements, while GFRP bars, laminate, sheets, and strips, have been extensively explored. This research aims to present a proposal for a new reinforcing system using GFRP gratings to improve the punching shear resistance of RC flat slabs. Results of seven specimens tested experimentally under vertical static loading are displayed, taking into account the influence of the gratings variables. Test results revealed an improvement in the ultimate load ranging between 9.03% and 27.67% for the specimens strengthened by the proposed GFRP grating system. A Nonlinear Finite Element Analysis (NLFEA) was carried out using the ANSYS program with correlational evaluation using load-deflection response and cracking pattern, which resulted in a good convergence of numerical simulations and experimental performance results ranging from 1.0% to 8.0%. Key parameters, namely the concrete compressive strength, steel reinforcement yield strength, main steel reinforcement ratio, secondary steel reinforcement ratio, column dimensions, slab thickness, concrete cover, and GFRP gratings characteristics, were investigated through a parametric study adopting NLFEA by the ANSYS program, where the output results were compared to the recent code provisions

The punching shear behavior of interior flat slab was examined in this study in the absence of shear reinforcement using numerical and analytical techniques. The effect of sensitive parameters was investigated and their influence was examined and reported including the opening existence and location, loading eccentricity, and slab height. The effect of the reinforcement ratio is neglected in most of the literature models and code provisions with the influence of loading eccentricity being addressed inappropriately. Therefore, this study adopted the nonlinear finite element analysis (NLFEA) method for the numerical part where the flat slab punching shear behavior was studied in detail. Results were compared in their behavior at both cracking and ultimate stages, absorbed energy, elastic and plastic stiffnesses, and general trend lines were provided. The effect of the reinforcement ratio was assessed indirectly with slabs of similar reinforcement area and different slab effective depths (65, 95, 125, and 155) mm where a 25 mm concrete cover was used. In addition, the NLFEA punching shear capacity results were compared with three international codes (American Concrete Institute (ACI318-119), EuroCode (EC2), and Model Code (MC2010)) to highlight their potential predictability and weakness points. It was found that the MC2010 has the most conservative prediction, followed by the EC2 and the ACI318-19 predictions. Therefore, two modification parameters were proposed for the ACI318-19 punching shear capacity calculation with R2 value of 0.98. The modified version of the ACI318-19 was validated using experimental data from the literature proving a high prediction capability.

Given the rise in sustainable use of recycled aggregate concrete (RAC) in new construction, weaker properties may compromise the integrity of structural components, including the slab-column connections. Low-strength concrete can be encountered, not only by using weak RAC in new construction, but also in the case of normal concrete in aged and deteriorated historic or conventional structures. To uphold the sustainable use of such materials in critical structural components, the need for restoring the strength via external strengthening using near-surface-mounted (NSM) fiber-reinforced polymer (FRP) bars is presented in this study. The paper experimentally investigates the behavior of NSM FRP strengthening against punching shear failure in slab-column connections produced from RAC and steel fibers with low structural strength. The three variables studied are steel fiber addition, recycled aggregate replacement, and FRP bar material type – Glass FRP (GFRP) versus Carbon FRP (CFRP). Five tests were conducted, utilizing three and two specimens strengthened with GFRP and CFRP bars, respectively. The three GFRP bar strengthened specimens differ in the type of concrete used, namely, ordinary concrete without steel fibers, ordinary steel fiber reinforced concrete (FRC), and RAC without steel fibers. The two CFRP bar strengthened specimens were of ordinary concrete and RAC, both with steel fibers, resulting in recycled aggregate FRC. RCA’s incorporation positively affected the results of the ultimate punching shear, flexural stiffness, and the maximum displacement due to the likely role of the recycled aggregate angularity in resisting the punching shear better. Due to higher flexural stiffness of the NSM CFRP, it entails more likelihood of debonding failure that limits its full utilization in increasing the punching shear capacity compared to that of NSM GFRP. However, both the NSM strengthening materials appear promising in low-strength (f’c ≤ 17 MPa) concrete slabs to reach acceptable capacities. Assessment based on different design code provisions and an analytical model revealed better accuracy of the Critical Shear Crack Theory (CSCT) and the fib Model Code in predicting punching shear capacity for NSM strengthened ordinary concrete column-slab connections, but with some slight deviations when applied to specimens made of RAC.

Although it has been shown that using recycled concrete aggregate in new structural concrete is economical and sustainable, the use of this material for such applications is still not widespread. One of the reasons is that manufacturers, designers, engineers, owners, and other market players are not familiar with specifying and utilizing this material—although standards are starting to incorporate provisions for recycled aggregate concrete, successful, practical example projects are needed. The current paper describes the results of a partnership between universities and a precast concrete manufacturer of hollow core slabs. Seven hollow core slabs with volumetric replacement ratios of natural aggregate with recycled aggregate from 0% to 60% were tested to failure in both bending and shear, and then undamaged portions of the slabs were subjected to punching shear until failure. The results showed only mild differences in strength, with different replacement percentages of recycled aggregate under the various loading scenarios. Numerical simulations performed in Abaqus demonstrated the feasibility of analyzing recycled aggregate concrete structural elements and provided important insights into their behavior. [ABSTRACT FROM AUTHOR]

This article explores the punching shear behavior of GFRP-RC interior slab–column connections. The parameters tested included the column–aspect ratio (1.0, 2.0, 3.0, 4.0, and 5.0), perimeter-to-depth ratio for square column stubs with side lengths of 0.3, 0.4, 0.5, 0.6, and 0.7 m, and span-to-depth ratios of 4, 6, 8, 10, and 12. A review of the literature revealed that no previous study has investigated the effect of these parameters or their interactions on this type of connection. Numerically, twenty-five slabs were created using finite element (FE) software (V3), each with square dimensions of 2.5 m and a constant thickness of 0.2 m. The central column extended 0.3 m from the top and bottom of the slab. All four sides of the slabs were supported, and the specimens underwent pure static shear load testing. The test results demonstrated that all slabs failed due to punching shear. Increasing any parameter value reduced the punching shear stresses. Additionally, the results indicated that Canadian (CSA-S806-12) and Japanese (JSCE-97) standards for FRP-RC materials generally provided the closest predictions of punching shear capacity compared to the American guideline, ACI 440.1R-22. However, all standards exhibited shortcomings and require enhancement and modifications, particularly to consider the impact of the span-to-depth ratio. Therefore, three equations were developed to predict the shear strength of the connections, yielding better results than those prescribed by the North American and Japanese standards. [ABSTRACT FROM AUTHOR]

In the present research, an experimental study was performed to investigate the punching shear strength of isolated concrete column footings having low shear span-depth ratios (a/d). The primary test parameters included the a/d, concrete compressive strength, and soil-bearing stiffness. Twelve column footings were tested for a/d ranging from 1.0 to 2.5. A support system using rubber-wood composite blocks was designed to simulate the equivalent soilbearing stiffness. The test results showed that as the a/d decreased, the punching shear strength of the footings significantly increased with a change in the angle of the punching failure surface. Contrarily, the soil-bearing stiffness did not significantly affect the punching shear strength, although it did affect the stiffness and deflection of the footings, as well as the contact pressure distribution beneath the footings. Furthermore, to evaluate the punching shear strength of footings with low a/d, an analytical model was proposed based on the web-shear cracking mechanism. [ABSTRACT FROM AUTHOR]

This article discusses the use of Glass Fiber Reinforced Polymer (GFRP) gratings to enhance the punching shear behavior in reinforced concrete flat slabs. The article presents various studies that have investigated the influence of GFRP reinforcement on punching shear resistance in concrete slabs. Experimental testing and numerical modeling were conducted to analyze the effects of different parameters on the punching shear behavior of the slabs. The results showed that the presence of GFRP gratings increased the failure load and toughness of the slabs, with varying effects depending on the specific parameters. The study also compared the results with different building codes and found variations in punching shear estimations. Overall, this research provides valuable insights into the use of GFRP gratings for strengthening concrete structures. [Extracted from the article]

Flat slab is a widely utilized structural system, especially in commercial, residential, and parking garage structures. It has major advantages, including its architectural flexibility and low construction cost. In contrast, it undergoes a major deficiency where bending and shear stresses are concentrated within a very small region, resulting in the well-known brittle punching shear failure. In addition, the slab's capacity is further reduced under the effect of elevated temperature or opening existence. In this study, the RC flat slab behavior was examined under the effect of different temperatures (23, 200, 400, and 600) °C, square opening side length (70 or 200) mm, and the opening location with respect to the column's side in a parallel direction (0 (directly) or 60 mm away). A total of twenty square-shaped specimens of 1000 mm side length and 100 mm total depth were tested, with each of five tested at one temperature value under the effect of concentrated loading applied at the top of a 150 mm square central column. Results revealed that the load–deflection curve behaves like the concrete stress–strain curved under elevated temperatures, where it flattens and moves downward and to the right under increasing temperature values. In addition, increasing the exposed temperature reduces the stiffness of the specimen under investigation significantly, even at a low-temperature value (200 °C). The most significant reduction is observed at the initial loading stage, and differences regarding the opening existence and its size are major. The differences in toughness were found to be significant upon creating an opening within the flat slab regardless of its size, with its being increased by increasing the opening size and location. Finally, the ACI 318 code punching shear predictions were also provided and compared with the experimentally obtained ones. [ABSTRACT FROM AUTHOR]

Several design standards have been developed in the last two decades to estimate the punching capacity of two-way reinforced concrete (RC) slabs reinforced with fiber-reinforced polymer (FRP) reinforcement. FRP-RC design standards include the recently published ACI 440.11-22, CSA/S806-12, and JSCE-2007. These models are either based on empirical data or semi-empirical methods and calibrated using different databases. Additionally, these standards do not have provisions for connections with shear reinforcement. Therefore, a reliable worldwide database for developing and assessing the applicability of such provisions with test results is vital. This study presents a worldwide and up-to-date database for punching shear of FRP-RC slabs. The database includes 197 tested connections, comprising interior and edge connections, with and without shear reinforcement, and a wide range of materials and cross-sectional properties. The database was used to evaluate the accuracy of the mentioned standards in predicting the punching shear capacity. For connections without shear reinforcement, it was determined that the three design standards yielded similar performance with different conservatism levels. ACI 440.11-22 yielded the most conservative results, with average Vexp/Vpred ratios of 2.04 compared to 1.28 and 1.3 for other models. For connection with shear reinforcement, specimens with Evf> 100 GPa resulted in Vexp/Vpred ratios less than 1.0 for ACI and CSA standards. [ABSTRACT FROM AUTHOR]

In concrete slabs, due to the absence of support beams, three-dimensional shear stresses are created around the slab-column connection area, leading to radial shear cracks and punching shear in the column-slab connection area. In this article, for evaluating the strengthening of the slabcolumn connection using CFRP sheets under the conditions of punching shear and seismic loading, the numerical method has carried parametric studies out. Therefore, considering 5 variables: loading conditions, number of fiber layers in the sheet, angle of fiber placement, sheet thickness, and dimensions of CFRP sheets, parametric studies have been carried out in the form of 33 separate models by Abaqus finite element software. Validation of the numerical model was evaluated based on the modeling of the laboratory sample. The results showed that the average error of numerical modeling was 5.2% compared to the laboratory sample, indicating the numerical model's acceptable accuracy. Also, the results showed that the two parameters of sheet thickness and dimensions are the main parameters in strengthening the RC slab-column connection, so considering the optimal values of about 98% reduces the damage to the concrete slab. The results of this research can be used as a comprehensive model for the design and application of CFRP sheets in strengthening the concrete column-slab connection. [ABSTRACT FROM AUTHOR]

In reinforced concrete flat slab buildings, the transference of unbalanced moments in the slab–column connections usually results from the asymmetry of spans, vertical loads, and horizontal forces from the wind. The punching strength of the slab–column connections can limit the load-carrying capacity of the structure in these cases, leading to structural collapse. The design code provisions are still based on empirical or semi-empirical equations; as the punching shear failure mechanisms are complex, and the ultimate strength is affected by several parameters. In this context, this paper presents the results of the computational investigation of the mechanical behaviour of flat slabs subjected to balanced and unbalanced moments using numerical Finite Element models. The numerical models were calibrated and accurately reproduced the behaviour and the punching resistance for concentric and eccentric loading. Furthermore, a parametric study was conducted to evaluate the mechanical behaviour of flat slabs under different load eccentricities, confirming that the increase in the unbalanced moment negatively impacts the load-carrying capacity of the slab–column connection. Furthermore, it was observed that all computational results obtained from models with unbalanced bending moments were higher than those estimated by the design codes. [ABSTRACT FROM AUTHOR]

This research investigates the use of carbon fiber reinforced polymer (CFRP) sheet and bar in concrete to increase punching shear capacity. The study focuses on the use of 1000×1000×100 mm³ reinforced flat plates and their flexure in both directions. The flat plates are subjected to a single-point load and a column stub. CFRP has a high modulus of elasticity and low coefficient of thermal expansion. Fifteen reinforced concrete flat plates were investigated, with three without FRP as control flat plates and twelve divided into two groups. The results showed that using CFRP composites to reinforce slabs at columns postponed the first slab flexural fractures, resulting in an increase in slab 2's ultimate punching shear capacity of 127.4 kN to 141.2 kN and a rise in sustained deflection from 33 mm to 46 mm. Strengthening slabs with CFRP delays the onset of concrete cracking and increases the initial slab stiffness by about 20% compared to an non-reinforced slab. Ultimate punching shear capacity improved using CFRP, making it on par with the slab without apertures. This research highlights the importance of using CFRP in concrete manufacturing systems to prevent failures and ensure long-term stability. [ABSTRACT FROM AUTHOR]

The bubbled slab, a type of reinforced concrete (RC) slab with plastic voids, is an innovative design that employs a biaxial distribution of voiding formers within the slab to reduce the slab's self-weight while preserving a load-carrying capacity that is approximately comparable to that of solid slabs. This paper presents a new approach for figuring out the effective critical shear perimeter of voided slabs using the reduced-volume concept of concrete. This approach aims to reduce the coefficient of variation of the current design standards, namely the ACI 318-19 and Eurocode 2, for assessing the slabs' resistance to punching shear. Our experimental program investigated the impact of voiding former patterns and the location of an opening near a column on the punching shear resistance of biaxial hollow slabs. The factors under consideration included the opening's size, location, and distance from the loaded area, as well as the voiding formers' placement concerning the critical shear boundaries. The results of experiments on 10 full-scale, 2000 × 2000 × 230 mm, reinforced concrete biaxial voided slabs with an opening are presented in this study. Two design expressions were used to estimate the biaxial hollow slabs' shear strength. These expressions take into account the reduced volume of concrete and the distribution of voiding formers up to the section 4 d from the periphery of the column. The proposed approach to determine the effective punching shear perimeter has the lowest coefficient of variation among the methods suggested by these standards. This indicates the validity of our proposed expressions. The coefficient of variation of the proposed expressions does not exceed 0.057. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate the performance of normal- and high-strength concretes including limestone powder (LP) through their mechanical properties. Moreover, sustainable flat plates made of these concretes were investigated through their punching strength. For this purpose, two different types of concrete (normal- and high-strength) with various limestone replacement ratios of 0%, 5%, 15%, and 20% by weight were designed. The fresh and hardened characteristics of the mixtures were investigated at various ages. By this means, the experimental behavior of reinforced concrete (RC) flat plate slabs made with limestone powder subjected to punching shear failure was studied. Slump value increased up to a 5% replacement of LP; after that, there was a tendency for the slump value to decrease as the replacement of limestone in normal-strength LP concrete increased. However, slump values for high-strength LP concrete increased as the LP replacement amount increased. There was a steady decrease in the compressive strength and splitting tensile strength values with the increase in LP content in normal concrete. However, in the high-strength LP concrete, with more than 10% of replacement LP, a decrease in the compressive strength values and splitting tensile strength values occurred. Compared to the control slab specimen without LP, in normal strength, the slab specimens with LP exhibit a larger ultimate shear load for slab specimens containing 5% and 10% of LP. The maximum increment for RC slabs containing 10% limestone powder was 3.8%. However, in high-strength concrete, the slab specimens with LP remained at the same ultimate shear load as control slabs, up to 10% of LP. high-strength concrete slabs with 5–20% LP showed an overall increase of (17.2%) in punching strength over the corresponding LP normal-strength concrete slabs. The corresponding increase for control slabs was 18.8%. It can be concluded that introducing LP improves the slab punching strength in a similar way that is found in non-sustainable slabs when using either normal- or high-strength concrete. [ABSTRACT FROM AUTHOR]

It is common in concrete flat plate construction to connect the exterior columns with marginal, or edge, beams. In addition to the benefit gained from marginal beams in stiffening the slab edges and supporting the concentrated loads on the slab edge, they aid in resisting punching shear at the exterior columns. However, in most cases the marginal beam dimensions are limited by architectural constraints rendering it very small relative to the adjoining columns. In such cases, especially ones when the column protrudes beyond the beam width, the effect of the beam in resisting punching shear is doubtful. Most design codes do not provide a direct method for checking punching shear taking into account the effect of the beam, which might encourage designers to neglect punching shear design due to beam effect. This paper investigates the effect of marginal beams on the punching shear capacity of slabs at edge and corner columns. To the authors’ knowledge, only the Australian Standards (AS3600) accounts for that case, however; the AS3600 provisions are based on studies conducted on beams with width equal to the adjoining column width, a case which is out of the scope of this paper. The results of eight experimentally tested slabs were used to verify the modeling approach used to generate the parametric study models. The test specimens chosen for verification of the numerical model represented two possible modes of failure, punching shear at the column slab interface and torsion shear failure of the marginal beam. Numerical models of 80 slabs were developed using ABAQUS software package. The study results were compared to three international code provisions, the ACI318, EC2 and ECP-203. The study concluded that the presence of a beam with width less than 80% of the column width does not eliminate the punching shear mode of failure, however; the beam increases the overall capacity of the connection by transferring part of the load directly to the column and reducing demand on two-way shear. Design guidelines are presented to check shear capacity of such connections.

Ultrahigh-performance concrete (UHPC) has become the most promising high-performance material, and has achieved excellent application in the field of impact and explosion protection engineering. Normal reinforced concrete (NC) beams subjected to impact load are prone to local punching shear failure. However, although the impact performance can be improved using UHPC beams, their application is limited by high costs. To achieve impact resistance at a lower cost, this work proposes a design scheme for locally replacing and wrapping NC beams with UHPC, with the aim of improving the impact resistance of NC beams. In this study, differing research conditions were employed (NC, UHPC, and NC–UHPC composite beams), and the impact resistance of specimens was compared and analyzed. Numerical models of NC and UHPC beams under impact loads were first established to verify the reliability of the modeling method, which is contact method, load application, initial velocity, boundary conditions, etc. The NC, UHPC, and NC–UHPC composite beam models were then established, and the impact performances of different NC–UHPC beam combinations were analyzed. The results revealed that compared with NC beams, the UHPC local replacement scheme effectively avoided local punching shear failure of beams. Under impact load, bending and shear cracks occurred on both sides, and the damage degree on both sides decreased with the increase of the local replacement length. For the UHPC wrapping, the failure mode of the beam changed from punching shear failure to bending failure. With an increase in UHPC thickness, the damage degree of the beam span increased, and it gradually reached that of the pure UHPC beam. Furthermore, both schemes effectively reduced peak displacement and residual displacement in the mid-span. Compared with the UHPC wrapping scheme, the peak displacement and residual displacement in the mid-span of the UHPC local replacement beam were reduced, and the mid-span bearing capacity was improved. Thus, we recommend that a local replacement length of more than twice the beam height should be selected to avoid the occurrence of local punching shear failure.

A thorough study of the literature indicated that there is not much relevant research on the application of GFRP gratings. Otherwise, extensive study has been done on FRP bars, laminates, sheets, strips, and rods. This study proposes a novel approach to improve the punching shear resistance of flat slab column connections by inserting GFRP gratings across the slab thickness. The results of seven specimens tested under vertical static loading are presented, considering the influence of the grating�s characteristics. All specimens were tested as simply supported slabs under one point of static loading. Experimental results, including crack patterns, slab deflection, concrete compressive strain, tensile steel strain, GFRP grating strain, and failure load, were recorded using extensive electric instrumentation. Test results revealed an enhancement in the failure load ranging between 9.03% and 27.67% for the specimens provided with the GFRP grating. In addition, a nonlinear finite element numerical model analysis was carried out using the ANSYS 15 program. Correlational studies based on the load-deflection response and crack patterns were utilized, resulting in a good agreement between numerical simulations and experimental results with differences ranging from 1.0% to 8.0%.

Reinforced concrete slab-column connections are vulnerable to punching shear failure when subjected to combined gravity and lateral loadings during earthquakes. Over the years, many experimental campaigns have been conducted and assessed the seismic performance of flat slabs. The experimental findings were used to derive empirical equations for the design provisions. The paper aims to provide a detailed description of the code development for seismic punching shear. Two current code provisions (ACI 318-19, EC 2 & EC 8-2004) for seismic punching shear are presented and discussed. Relevant and updated experimental data of interior slab-column connections without and with shear reinforcement are selected and assessed against the current design provisions with a focus on key response parameters such as the drift ratio and the gravity shear ratio. The ACI 318-19 limit drift line is considered to assess the lateral deformation demand for the connections with respect to the updated database. The gravity shear ratio is shown to have a considerable impact on the limit drift ratio of slab-column connections without shear reinforcement. The majority of the test specimens with no shear reinforcement experienced punching shear failure, followed by the specimens which showed a combined flexure–punching failure. Punching shear failures occurred for a range of gravity shear ratios of 0.2 to 0.9, while all combined flexure–punching shear failures occurred for gravity shear ratio values below 0.5. The shear-reinforced concrete slab-column connections can achieve much higher drift ratios. Most of the slabs were reinforced with shear studs, particularly in the range of gravity shear ratios of 0.4 to 0.6. Most test specimens failed by flexure for gravity shear ratio values of 0.1 to 0.7, followed by the specimens which failed in combined flexure–punching for gravity shear ratio values that ranged from 0.3 to 0.9. Finally, the available numerical and analytical models for seismic punching shear are presented with the objective of observing their potential strengths and limitations. [ABSTRACT FROM AUTHOR]

This paper presents a novel mathematical model to determine the minimum cost for the design of reinforced-concrete strap combined footings under biaxial bending, with each column using a genetic algorithm. The pressure is assumed to be linearly distributed along the contact area. This study comprises two steps: firstly, identifying the smallest ground contact area, and secondly, minimizing the cost. The methodology integrates moment, bending shear, and punching shear calculations according to the ACI standard. Some authors present a smaller area (but limited to one or two property lines) and the design considers that the thickness of the footings and beam are equal, and do not show the lower cost of a strap combined footing; generally, the beam has a greater thickness than the footings and therefore the footings would have an unnecessary thickness that would generate a higher cost. A numerical example is shown to find the lowest cost for the design of strap combined footings considering four different conditions such as square footings and other limitation at the ends of the footings. The minimum area does not guarantee that it is the lowest cost. The proposed model is versatile, applicable to T-shaped and rectangular combined footings, and is not restricted to specific property lines. The contributions include eliminating trial and error practices, accommodating various design conditions, and emphasizing equilibrium in the derived equations. The model is adaptable to different building codes, offering a comprehensive approach to achieving optimal design and cost considerations for strap combined footings. [ABSTRACT FROM AUTHOR]

Punching shear resistance of flat slabs with combined reinforcement utilising steel fibres and stirrups To reinforce areas subject to punching shear, punching shear reinforcement systems such as stirrups or double‐headed anchors are commonly used. An alternative is to utilise steel fibre reinforced concrete (SFRC), which can achieve a similar increase in load‐bearing capacity as stirrups. In the current German codes, a combined utilisation of SFRC and punching shear reinforcement is excluded due to a lack of experimental data to validate a design approach. On the other hand, both Model Code 2010 and Annex L of the next generation of Eurocode 2 already contain design principles that consider steel fibres and punching shear reinforcement additively. However, the suitability in terms of reliability/safety and economic efficiency has not yet been sufficiently validated due to missing experimental results. In order to close the existing gap, a total of eight large‐scale punching shear tests with combined reinforcement (stirrups, steel fibres) were carried out by the Chair of Concrete and Masonry Structures at the Technical University of Munich (TUM) following initial positive findings from preliminary tests. This paper describes and discusses in detail the setup, testing procedure and main results and findings of these experimental investigations. [ABSTRACT FROM AUTHOR]

In this paper, the feasibility of strengthening a flat column–slab connection within the carbon fiber reinforced polymer (CFRP) has been investigated through experimental study. The experimental program includes a set of nine reinforced concrete flat slab specimens. Three unaltered specimens served as control slabs, while an additional six samples were strengthened with various CFRP configurations to enhance their shear capacity. The strain distribution, ductility, punching shear resistance, stiffness, and crack formation were studied. The result of experimental studies showed that in the direct method of strengthening in which two layers of unidirectional CFRP sheets were employed in two opposite directions, the ultimate punching shear resistance improved by 64%, 44.7%, and 15.3%, with respect to the location of the column connection, as compared with the control specimens. In the case of using one layer of unidirectional CFRP strips, the punching shear resistance was enhanced by approximately 16% and 39%, considering the configuration of CFRP sheets and the amount of strengthened and adhesive layers used. Following the outcomes of this research, the application of CFRPs in improving the resistance capacity of flat slabs against the punching shear is considerable. The reported outcomes were compared with the latest provisions of ACI to show the efficiency of the presented strengthening. Finally, a parametric study was performed assuming different loading locations to assess the effect of the loading region on the response of RC slabs. Results indicate that approaching the loading location toward the RC slab supports results of an increase in the load-bearing capacity and a reduction in the ductility of the RC slab. [ABSTRACT FROM AUTHOR]

Reinforced concrete flat slab projects usually require openings close to the columns for electrical and hydraulic installations. Since such openings reduce the punching shear of slabs, a comprehensive study is needed. In this paper, numerical models of slabs tested in literature were elaborated upon, using the commercial software in finite elements DIANA. The physical nonlinearity of the materials was considered. After the calibration and validation of the numerical parameters related to concrete and reinforcement, 62 numerical models were created for performing a parametric analysis. The numerical analysis aimed to study the effect of openings close to the columns and verify the punching resistance reduction. In this parametric study, the main variables were the compressive strength of the concrete, the bending reinforcement ratio, the number of openings, and the distance from the openings to the column's face. Finally, the numerical results obtained were compared with the punching strength values provided by the main structural design codes, viz., NBR 6118 (2014), Eurocode 2 (2004), ACI 318 (2019), fib Model Code (2010), and the next generation of Eurocode 2, which are constantly questioned in academic papers regarding their safety. [ABSTRACT FROM AUTHOR]

Much of the current code provisions for designing slab-column connections against punching shear are based on empirically derived formulations based on tests of partial scale isolated slab-column specimens. Although many experiments have been conducted on shear reinforced concrete flat slabs supported on columns, due to cost or time constraints, there are still many parameters that have not been adequately studied in the laboratory. These tests can be supplemented by analytical results of properly calibrated nonlinear finite element analysis (NLFEA) to enhance the existing experimental database and formulate rational design recommendations for future codes. This paper presents a rational approach for calibrating an NLFEA model in ABAQUS using interior and edge slab-column connection specimens. The calibration includes a study to determine how to effectively model the shear reinforcement and shear reinforced area. This further includes a detailed analysis of the modeling of the shear reinforcing elements to ensure appropriate rotational capability of the shear reinforced region of the slab without significantly reducing the predicted capacity of the model. The calibrated models show good agreement with test data based on load-deflection, moment-curvature, and bolt strain behavior. [ABSTRACT FROM AUTHOR]

Punching shear is one of the possible failures that must be taken into consideration in designing concrete slabs and foundations. Several factors control the occurrence of this failure mechanism, such as slab thickness, concentrated loads, and the cross-section area of the supporting columns. The cross-section shape of columns may also affect the punching shear in flat slabs. Therefore, this research was specified to study the effect of the columns' cross-section shape on the punching shear in flat slabs by conducting a simulation analysis for two multi-story concrete buildings with a flat slab but with different structural stability systems, building layout, and column distribution, where five cross-section shapes (square, rectangular, circular, hexagonal, and octagonal) were tested. ETABS 2016 software was adopted for modeling and analysis to identify the optimum column shape leading to minimizing punching shear. Although the cross-section area and the reinforcement area of the columns were fixed while changing the models' column shapes, it was found, in both buildings, that hexagonal columns performed better than others under punching shear and provided the lowest punching shear ratio. In contrast, circular columns gave the lowest resistance. Octagonal, square, and rectangular columns, respectively, follow the hexagonal columns in resisting punching shear. [ABSTRACT FROM AUTHOR]

With the emergence of fiber-reinforced polymer (FRP) reinforcement as a substitute for conventional steel reinforcement, different design codes have been developed to account for the different mechanical properties of the FRP, specifically the elastic modulus. The ACI 440.11-22, CSA/S806-12, and JSCE-97 are well-known standards for FRP-reinforced concrete structures. In particular, these design standards show significant variations in estimating the punching shear resistance and accounting for the elastic modulus. This study provides a statistical and machine learning-based evaluation of the punching shear models in design standards. An Artificial neural network (ANN) framework is used to develop a generalized punching resistance model of flat slabs reinforced with steel and FRP bars utilizing a large experimental dataset with 539 tests. The study presents a parametric study to examine the effect of different factors on the punching shear strength. The parametric study provided a graphical presentation and comparison between the design models and the developed ANN model.

This study introduces a unified two-way (punching) shear design model for concrete reinforced with slabs steel- and FRP bars. The unified model modifies the ACI 318-19 punching shear model to account for the axial stiffness of the tension reinforcement by providing a new modification term (ρ Ereft./Ec)0.26, representing the reinforcing ratio multiplied by the elastic modular ratio of longitudinal bars to concrete. The unified punching shear model has been evaluated over two experimental datasets: FRP-RC slabs (145 specimens) and steel-RC slabs (390 specimens). A comparative assessment between the ACI 440.11-22, CSA S806, JSCE, and the proposed models to test their generalization abilities for slabs reinforced with both types of reinforcement. The assessment indicated a similar performance of the proposed model to the CSA S806 and the JSCE models in terms of statistical measures. The ACI 440.11–22 showed a high average strength ratio (Vexp./Vpred.) of 1.83. However, the ACI and the CSA models showed reduction in the conservatism at high reinforcement axial stiffness. In addition, reliability analysis using Monte Carlo simulation indicated that the unified punching shear model provides a satisfactory safety level with a reliability index between 3.5 and 4.0 for both steel and FRP-RC slabs.