Shear Resistance Of Flat Slabs Research Articles

Studying the Effects of Opening Size and Location on Punching Shear Resistance of Flat Slabs Using ANSYS V.19

To validate the accuracy of ANSYS V.19, an experimental model based on a previous study was used to determine the typical values of crack pattern, ultimate load, and deflection. The model was a flat slab with an internal column with dimensions of 1000 × 1000 × 100 mm³. Four groups used a slab with an edge column in parametric research. The first group studied the opening size relative to the column face. The second group of specimens studied opening sizes close to column corners. The third group consisted of four specimens: two had openings in front of the column face and two in front of the column corner to study how opening distance affected flat slab behaviour. The fourth group examined the optimum opening arrangement in slabs with two openings around the column. The findings showed that flat slab openings at the column corner had higher ultimate load capacity than those at the column face; however, increasing opening dimensions beyond the column dimension decreases punching shear capacity. If the column needs two openings, place them on opposite faces or near the corners; this minimizes ultimate load reduction.

Two-Way Slab Punching Shear Resistance: Experimental Insights into Basalt-FRP Bar as Flexural Reinforcement

This study seeks to experimentally evaluate the punching shear performance of two-way concrete slabs reinforced with conventional steel and basalt fiber-reinforced polymer (basalt-FRP) bars subjected to punching loading condition. Basalt-FRP bars offer high tensile strength and corrosion resistance but are understudied in two-way concrete slabs concerning punching shear. This study aims to fill this gap, with key implications for future structural design considerations. To achieve the objectives of the study, six large-scale square slabs were fabricated and subjected to a concentric load until failure. The parameters of the experiment included are the type of reinforcement used (either basalt-FRP or steel), the percentage of basalt-FRP used (ranging from 0.88% to 1.77%), the size of the basalt-FRP bars used (either 16 or 12 mm), and the concrete’s compressive strength (25, 30, and 35 MPa). The findings from the tests showed that incorporating basalt-FRP bars with one-quarter equivalent axial stiffness (ρ(Ef/Es)) to that of steel significantly enhanced the punching shear resistance of flat slabs, achieving approximately 65% of the capacity observed in steel-reinforced control sample. Moreover, increasing the amount of basalt-FRP bar reinforcement to half of the equivalent axial stiffness of steel had a substantial effect in improving shear strength, reaching approximately 89% of the capacity observed in the steel-reinforced specimen and concurrently reducing deflection during the failure. Additionally, the reinforcement type and concrete compressive strength played a crucial role in determining the ultimate load, failure modes, and crack propagation patterns. The study reveals discrepancies between experimental results and existing models for punching shear in FRP-reinforced slabs. Certain prevalent models prove to be conservative in their estimates, while others offer more accurate predictions, indicating the need for comprehensive model refinement. The investigation found that one model, encompassing the majority of variables affecting punching shear, exhibited the highest level of precision, with a slight adjustment recommended to enhance its accuracy further. This study suggests a sustainable, more durable way to reinforce concrete in bridges and high-rise buildings, potentially improving construction efficiency, enhanced service life, and potential updates to building codes.

UHPFRC hidden capitals as an alternative method for increasing punching shear resistance of LWAC flat slabs

Due to outstanding strength properties of contemporary ultra-high performance fiber reinforced concretes (UHPFRC), they can be successfully used to improve the punching shear resistance of flat slabs made from lightweight aggregate concrete (LWAC). However, the relatively high cost of UHPFRC means that the application is usually limited to the support zone or the subsurface layer of the slab.The paper presents the results of experimental investigations on application of the UHPFRC hidden capitals to increase punching shear capacity of LWAC slabs. The study was a continuation of the previous experimental program and included a total of three 200 mm thick slabs with the same longitudinal reinforcement ratio, equal to 1.23%. One was the control specimen made entirely from LWAC, while in the others 130 mm thick capitals from UHPFRC with 2.5% and 5.0% fiber content were cast. The application of hidden capitals allowed to increase the stiffness and reduce deflections of the slabs by about 16 ÷ 20%. The residual strength of UHPFRC was found to be the key feature. In case of the slab with capital from UHPFRC with 2.5% fibre content, shear crack formed at the edge of the column and crossed the capital. However, the contribution of UHPFRC strength allowed to increase the punching shear capacity by 29% with respect to the control specimen. The doubling of fiber content to 5.0% changed the failure mechanism, because the shear cracks formed at the edge of the capital, what was similar to observed in slabs failed outside punching shear reinforced area. This translated into a significant increase in the load capacity by 82% in comparison to the control slab.The performed analysis demonstrated the usefulness of the Critical Shear Crack Theory (CSCT) and the Variable Engagement Model (VEM) for the assessment of the punching shear resistance of LWAC slabs with hidden capitals from UHPFRC. A good agreement of load-rotation response and ratios of theoretical to experimental load equal to 0.87 and 1.16 were obtained. The calculations allowed for correct prediction of the failure mode – punching within or outside the capital.

Uncertainty Modeling and Analysis of Punching Shear Resistance of Flat Slabs

Uncertainty Modeling and Analysis of Punching Shear Resistance of Flat Slabs

Novel informational bat-ANN model for predicting punching shear of RC flat slabs without shear reinforcement

While design codes provide guidelines to prevent brittle punching shear failures in flat reinforced concrete (RC) slabs, they are associated with high inaccuracy. This study scrutinizes existing design provisions, highlighting its features and limitations. Sensitivity analysis is then used to identify the influential mechanical and geometric parameters. Subsequently, an artificial neural network coupled with a metaheuristic Bat algorithm (Bat-ANN) is used to develop a hybrid model for estimating punching shear strength. Several statistical metrics revealed that the Bat-ANN model achieved superior predictive accuracy. The novel hybrid model was deployed to assess the influence of key parameters affecting punching shear strength, including the slab effective depth, concrete strength, reinforcement ratio, reinforcement yield strength, and width of the square loaded area. The analysis identified the importance of the flexural reinforcement, which is not typically considered in estimating punching shear strength. Subsequently, using the supervised machine learning method through the EUREQA software, a new regression expression was proposed to estimate the punching shear resistance of flat slabs. This hybrid computational intelligence model could be integrated in future automated design platforms of RC structures.

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

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

Theoretical prediction of punching shear capacity of flat slabs without shear reinforcement strengthened by concrete topping

The main reasons for strengthening flat slabs are the change of the use of a building, increase in the value of loads, degradation of the concrete cover layer, or insufficient reinforcement. This paper is focused on the assessment of punching shear capacity of the strengthened flat slabs without shear reinforcement. One of the possibilities how to enhance punching shear capacity is the addition of reinforced concrete topping. The main goal of this paper is to compare the possibilities for calculation of the increase in the punching shear capacity by investigation of the influence of different thicknesses of concrete toppings and different reinforcement ratio. A reference specimen is represented by a fragment of a flat slab with the thickness of h = 200 mm supported by circular column with the diameter of 250 mm. Three different thicknesses (50 mm, 100 mm, 150 mm) of concrete toppings were considered together with three different reinforcement ratios for each thickness of concrete overlay. Theoretical predictions of the punching shear resistance of flat slabs were evaluated by design guidelines according to the relevant standards: Eurocode 2 (EN 1992-1-1), Model Code 2010 and draft of the second generation of Eurocode 2 (prEN 1992-1-1). The differences in the influence of reinforcement ratio are significant. In Model Code 2010 the reinforcement ratio in concrete topping was considered in equation of moment of resistance. This is unlike in both of the mentioned Eurocodes, where the reinforcement ratio was assumed as a geometric average value of the original reinforcement ratio in the slab before strengthening and of the reinforcement ratio of concrete topping. All the predicted theoretical calculations are based on the perfect connection and bond between the original and new layer of concrete. These predictions should be verified by experimental investigation, which is going to be prepared shortly. By the additional increase in the thickness of concrete topping or in the amount of added reinforcement the attention should be payed to the limitation of the punching shear resistance by the value of the maximum punching shear resistance in the compression concrete strut.

Investigation of the effects of opening size and location on punching shear resistance of flat slabs using Abaqus

The paper presents a numerical study on the effects of opening size and location on punching shear resistance of flat slabs without drop panels and shear reinforcement using ABAQUS. The study proposes an ABAQUS model that is enable to predict the punching shear resistance of flat slabs with openings. The model is validated well with the experimental data in literature. Using the validated numerical model, the effects of opening size and location on the punching shear resistance of flat slabs are then investigated, and the numerical results are compared with those predicted by ACI 318-19 and TCVN 5574:2018. The comparison between experimental and numerical results shows that the ABAQUS model is reliable. The punching shear resistances calculated by ACI 318-19 and TCVN 5574:2018 with different opening sizes and locations are agreed well to each other, since the design principles between two codes now are similar.

Punching Shear Resistance of Reinforced Concrete Flat Slabs Strengthened by CFRP and GFRP: A Review of Literature

Flat reinforced concrete (RC) slabs or plates are still widely used in buildings and are most popular in single or multiple story floor construction systems. This is due to the ease and speed of implementation as well as the continuous smoothness that is provided in relation to the locations of members. Flat slab systems have an inadequate shear strength in both directions. Thus, they are subjected to a shear failure at their intersections with columns, which results in the collapse of a larger part of the structure. Shear failure occurs due to many reasons including changing the functions of the facility, the technical errors in the design and implementation procedures, an increase in the load, deterioration of materials, and poor quality. The carbon fiber reinforced polymer (CFRP) sheets/strips and glass fiber reinforced concrete polymer (GFRP) are used as a composite section formulated when there is a structural deficiency. Strengthening by using CFRP and GFRP provide an improvement in the punching shear resistance in both directions as well as flexural strength, ductility, and hardness. They are more suitable for a practical use as a substitute for other costly and difficult approaches such as increase the cross-sectional area of columns and so on. This paper reviews the up to date studies of enhancing the shear resistance of flat slabs by CFRP/GFRP and discusses the used materials for strengthening flat slabs and the used methods, which are used to implement these materials. Also, a summary for the cited studies are stated and the possible future works are suggested.

Punching Shear Resistance of Flat Slabs Strengthened with Near Surface–Mounted CFRP Bars

Abstract This paper presents the effectiveness of strengthening slab–column connections against punching shear failure with near surface–mounted (NSM) carbon fiber–reinforced polymer (CFRP) bars. T...

Effect of load level of corner columns on punching shear resistance of flat slabs

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 ρl = 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.

Hidden Capital as an Alternative Method for Increasing Punching Shear Resistance of LWAC Flat Slabs

AbstractIn the paper an alternative method for increasing punching shear resistance of the flat slabs from lightweight aggregate concrete by means of hidden steel fibre reinforced capital was presented. Previous experimental studies demonstrated that the addition of steel fibres to concrete allows for increase in the punching shear resistance of flat slab. Steel fibres modify the tensile strength of concrete, which translates into increased ductility of the material. The results of the experimental investigations were presented, the aim of which was to assess the effectiveness of the proposed solution. For economic and technological reasons, a hidden capital of a height equal to half of the slabs depth was made so that the top reinforcement could be installed later. It was found that presented solution allowed to increase the load carrying capacity by about 36% with respect to the control element, made entirely of lightweight aggregate concrete.

Contribution of concrete and shear reinforcement to the punching shear resistance of flat slabs

The punching strength of flat slabs and footings without and with high amounts of shear reinforcement on the level of maximum punching shear capacity has been investigated extensively by various researchers over the last decades. The existing investigations suggest that the punching strength of RC slabs with shear reinforcement can be described by means of a contribution of concrete and a contribution of the shear reinforcement (steel contribution). However, a systematic evaluation of concrete and steel contribution depending on the amount of shear reinforcement has not yet been adequately possible since only few test series with low and medium amounts of shear reinforcement and a failure within the shear-reinforced zone have been available.This lack of experimental data has been partly filled with eleven systematic punching tests on flat slabs and eleven further tests on footings conducted at RWTH Aachen University, Germany. In this paper, the development of the concrete and steel contributions in shear-reinforced flat slabs is investigated in detail and a method for calculating and analyzing both contributions is presented. The development of concrete and steel contribution depending on the amount of shear reinforcement is discussed and compared to the design provisions according to Eurocode 2, Model Code 2010 and the draft (of April 2018) of the next generation of Eurocode 2.

3D finite element investigation of the compressive membrane action effect in reinforced concrete flat slabs

Design codes for punching shear resistance of flat slabs are based on test results obtained from isolated slab-column connections. However, by testing isolated slabs, the compressive membrane action of a continuous slab-column system is ignored. This can result in lower punching shear strength compared to the actual strength of the real slab system. Testing continuous slab system is very uneconomical and in most cases not possible. In this paper, finite element analyses (FEA) are performed in order to investigate the effect of the compressive membrane action in flat concrete slabs by comparing results from isolated specimens and continuous floor systems. The adopted FE formulation and the material parameters were previously calibrated on an isolated test specimen (SB1). At first, the calibrated FEA are used to investigate the boundary conditions of slab-column connection (SB1). Then, these boundary conditions are modified, and the slab is also considered to have larger in-plane dimensions in order to examine its continuity. Finally, numerical analyses of existing punching shear tests that examine the compressive membrane action effect are conducted to show the accuracy of the FEA model. All the numerical analyses indicate that the punching shear capacity of a continuous slab is higher compared to the capacity of a conventional isolated slab. The predictive capability of the FEA models can allow for future investigations on the effect of the compressive membrane action to supplement the limited testing background on this area, and to provide recommendations for future design provisions.

Effect of load history on punching shear resistance of flat slabs

The unloading of reinforced concrete slabs results in residual slab rotations and reloading to the same load results in irreversible rotation increases. Unloading and reloading (UR) cycles applied to non-strengthened and strengthened flat slabs may thus affect the punching resistance, which is rotation-dependent. A quintilinear moment–curvature relationship, which takes concrete softening and tension stiffening into account, combined with UR cycles, modeled as bilinear envelopes, is developed to predict residual slab rotations and irreversible rotation increases. A parametric study shows that the effect of UR cycles on the punching resistance of concrete is normally small, however, it may be significant if the slab is strengthened after unloading, particularly for thin and low-reinforced slabs, which exhibited plastic slab rotations before unloading. Prestressing of the strengthening system may reduce the residual slab rotations and thus limit or compensate the loss of punching resistance.

The Maximum Punching Shear Resistance of Flat Slabs

Results of the latest experiments have revealed that the maximum punching resistance defined from crushing of concrete struts at the perimeter of a column in EC2 is an insufficient criterion for limitation of maximum shear forces at the vicinity of columns. Therefore further limitation has been introduced in STN EN 1992-1-1/NA. The paper will deal with the new requirements concerning of maximum punching shear resistance which is based on the kmax factor and punching shear resistance without shear reinforcement vRd,c.

Punching Shear Resistance of Interior GFRP Reinforced Slab-Column Connections

This paper deals with the punching shear resistance of flat slabs reinforced with glass fiber-reinforced polymer (GFRP) reinforcing bars. A total of six large-scale interior GFRP and steel reinforced slab-column connections with the varying longitudinal tensile reinforcement ratio were tested. A new fracture-mechanics-based empirical formula for estimation of the punching shear resistance of interior GFRP reinforced slab-column connections is also presented in the paper. The formula takes into account effects, span to the effective slab-depth ratio, depth of the compression zone, dowel action, and the size effect on the punching shear resistance of flat slabs reinforced with GFRP bars. The accuracy of the proposed formula was verified against test results obtained by the authors and also by other researchers. In comparison with already known formulas, the proposed formula is in good agreement with the test results and provides predictions of the punching shear resistance with the smallest scatter. A physically reasonable relation between experimental punching resistances and reinforcement ratios of both the GFRP slabs and the steel reinforced concrete (SRC) slabs was obtained when equivalent GFRP reinforcement areas were used for SRC slabs. Predicted punching resistances of SRC slabs, calculated based on the equivalent GFRP reinforcement area, were found to be approximately equal to the resistances of GFRP slabs when the behavior (load-displacement diagram) of SRC slabs resembled the one of GFRP reinforced slabs.

Effect of fibers on the punching shear strength of slab-column connections

Twelve sets of small scale fiber reinforced concrete (FRC) slab-column connections were tested to investigate the effect of fiber reinforcement on the punching shear resistance of flat slabs. The parameters studied included type, content and aspect ratio of fibers, and span-depth ratio of the slab. The results indicate that the use of hooked steel fibers improves substantially the ductility of shear failure of slab-column connections, modifies their failure mode, favorably, from pure punching to flexural, and leads to a significant increase in their ultimate shear capacity. The increase in ultimate shear resistance varied almost linearly with the steel fiber content. While polypropylene fibers increase the ductility of shear failure, they are not as effective as steel fibers in increasing the punching shear resistance. Based on evaluation of the current test results and other experimental data reported in the technical literature, a design equation is proposed to predict the ultimate punching shear strength of slab-column connections containing deformed steel fiber reinforcement.