Concrete Slab-column Connections Research Articles

Punching and lateral cyclic behavior of concrete slab-column connections reinforced with steel sheets

During an earthquake, slab-column structures with high gravity-shear ratios often experience increased local shear stress and deformation in the connection area, leading to reduced unbalanced moment capacity and drift capacity of the connections. This study aims to create a new and more efficient method of enhancing the unbalanced moment capacity and deformation capacity of reinforced concrete slab-column structures through using steel sheets as punching reinforcement. Tests were conducted to evaluate the effects of the thickness, extension length, and layout of this new type of punching reinforcement on the unbalanced moment capacity and deformation capacity of the connections when subjected to vertical and horizontal cyclic loads. The results indicated that as the steel sheet thickness increased, the unbalanced moment capacity of SC-2 and SC-3 rose by 24 % and 48 % respectively, while the drift capacity improved by 20 % and 40 % compared to SC-1. Furthermore, the failure mode shifted from stress-induced punching to drift-induced punching. Compared to SC-1, a cruciform layout with an additional diagonal arrangement of sheets increased the unbalanced moment capacity and drift capacity by approximately 17 % and 20 %, respectively. The extension length of the sheets reduced from 600 to 420 had negligible effect on the unbalanced moment capacity and deformation capacity. Utilizing experimental data, this study compared the relationship between the drift capacity and gravity shear ratio of connections reinforced with different shear reinforcements. The underlying causes for the observed lower drift capacity of the specimens are analyzed, and design recommendations for optimizing the steel sheets are subsequently proffered. After thoroughly analyzing the experimental results, a simplified calculation methodology for assessing the shear strength of steel sheets was developed, taking inspiration from the shear strength calculation approach outlined in ACI 318–19. The efficacy of this simplified approach was subsequently validated by comparing the experimental results with those predicted by current design codes. Furthermore, by comparing the experimental results with the predicted values, the relationship between the punching failure mode of connections and their punching strength was discussed.

Effect of recycled concrete aggregates on the punching strength of slab-column connections without shear reinforcement

This paper presents the results of tests on six reinforced concrete slab-column connections. The slabs were octagonal, centrally supported on square columns, and concentrically loaded at their borders. The tests investigated the effects of replacing natural coarse aggregates with recycled concrete aggregates. The experimental results were also compared to resistance estimates from ACI 318 (2019) and Eurocode 2 (2004) and results from a database with tests on slabs without recycled concrete aggregates. The results show that using RCA reduces the concrete's modulus of elasticity and tensile strength but has little effect on its compressive strength. Furthermore, using RCA has little impact on the flexural response and punching resistance of the slabs. The findings suggest that RCA has potential for use in concrete applications, but further tests are required to draw definitive conclusions.

Comparative Analysis of Shear Strength Prediction Models for Reinforced Concrete Slab–Column Connections

This research focuses on a comprehensive comparative analysis of shear strength prediction in slab–column connections, integrating machine learning, design codes, and finite element analysis (FEA). The existing empirical models lack the influencing parameters that decrease their prediction accuracy. In this paper, current design codes of American Concrete Institute 318-19 (ACI 318-19) and Eurocode 2 (EC2), as well as innovative approaches like the compressive force path method and machine learning models are employed to predict the punching shear strength using a comprehensive database of 610 samples. The database consists of seven key parameters including slab depth (ds), column dimension (cs), shear span ratio (av/d), yield strength of longitudinal steel (fy), longitudinal reinforcement ratio (ρl), ultimate load-carrying capacity (Vu), and concrete compressive strength (fc). Compared with the design codes and other machine learning models, the particle swarm optimization-based feedforward neural network (PSOFNN) performed the best predictions. PSOFNN predicted the punching shear of flat slab with maximum accuracy with R2 value of 99.37% and least MSE and MAE values of 0.0275% and 1.214%, respectively. The findings of the study are validated through FEA of slabs to confirm experimental results and machine learning predictions that showed excellent agreement with PSOFNN predictions. The research also provides insight into the application of metaheuristic models along with ANN, revealing that not all metaheuristic models can outperform ANN as usually perceived. The study also highlights superior predictive capabilities of EC2 over ACI 318-19 for punching shear values.

Thick-shell finite element analysis of reinforced concrete flat plates with non-uniform connection stresses

This paper presents the application of a thick-shell nonlinear finite element analysis procedure to estimate the punching shear resisting performance of reinforced concrete slab-column connections under variable connection shear stress conditions. In the analyses performed, varied connection shear stress conditions stem from slabs being supported by columns with different cross-section aspect ratios, being subjected to different distributions of gravity loading, being constructed with different planar reinforcement ratios in orthogonal directions, as well as the application of unbalanced bending moments. Forty-eight isolated slab-column connection specimens presented in the literature were modelled and analyzed using a thick-shell finite analysis procedure. All numerical results were developed using a predefined set of material models and analysis parameters, a consistent meshing procedure, and are shown to provide meaningful agreement with experimental data without the need for case-specific model calibration or the adoption of specialised failure criteria. The results show that thick-shell modelling methods can be suitable for estimating the punching shear response of slabs subjected to non-uniform shear stress development in connection regions, and can provide similar levels of precision to that obtained for the analysis of idealised slab-column connections typically used for design procedure and model development.

Seismic Damage Assessment of Reinforced Concrete Slab-Column Connections—Review of Test Data, Code Provisions and Analytical Models

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.

Finite Element Model for Concrete Slab-Column Connections with Shear Reinforcement

Finite Element Model for Concrete Slab-Column Connections with Shear Reinforcement

Punching shear performance of star-shaped steel plates reinforced RC slab-column connections

The progressive collapse of flat plate structures due to brittle punching shear failure of reinforced concrete (RC) slab-column connections is a major concern in structural engineering. This paper proposed an innovative star-shaped steel plates (SSPs) system to achieve an effective yet practical enhancement of the punching and post-punching performance of RC slab-column connections. SSPs were fabricated from thin steel plates, welded together, and placed in the centre of the joint. Engineering cementitious composite (ECC) was also adopted to replace the joint core concrete and work with the SSPs further to improve the punching shear behaviour of the joint. To evaluate the effectiveness of the proposed system, five large-scale RC slab-column connections with different SSPs configurations and concrete materials were tested under monotonic load. The results showed that the initial stiffness of the joint was minimally affected by the presence of SSPs, but the system significantly improved the punching shear strength, post-punching load-bearing capacity, and deformation capacity of the joint. Specimens with ECC and SSPs exhibited an even higher punching shear strength and more ductile post-punching behaviour. The study also found that the optimum enhancement of the punching shear behaviour of the slab-column joint can be realised when the SSPs yield just before the joint experiences punching shear damage. Finally, formulas were developed to reasonably predict the punching shear strength and post-punching load-carrying capacity of the SSPs reinforced slab-column connections.

Experimental study on the effectiveness of strengthening reinforced concrete slab-column connections using CFRP sheets

This paper presents an experimental study on the effect of externally bonded carbon fiber-reinforced polymer (CFRP) sheets on the punching shear behavior of interior slab-column connections. Two square slabs with a concentric column were fabricated, measuring 1200 ´ 1200 ´ 80 mm for the slabs and 200 ´ 200 mm for the column. One specimen served as an unstrengthened control specimen, while the other specimen was strengthened using CFRP sheets. The specimens were simply supported along their edges and subjected to punching tests, with the vertical load applied through the central column. The experimental results revealed that the application of CFRP sheets significantly improved the performance of the slab-column connections. The strengthened slab exhibited a 16.3% increase in punching capacity compared to the control specimen. Additionally, a calculation was conducted to determine the punching shear strength of the test specimens, taking into account the interaction between shear and flexural capacities in a two-way flat slab. This calculation provided a deeper understanding of the impact of CFRP sheet strengthening on the enhancement of the strength of the slab-column connections.

Flat Slabs in Eccentric Punching Shear: Experimental Database and Code Analysis

Eccentric punching shear can occur in concrete slab–column connections when the connection is subjected to shear and unbalanced moments. Unbalanced moments occur in all floor slabs at the edge and corner columns. As such, this problem is of practical relevance. However, most punching experiments in the literature deal with concentric punching shear at internal columns. This paper presents a developed database of 128 experiments of flat slabs under eccentric punching shear, including a summary of the testing procedure of each reference and a description of the slab specimens. Additionally, a linear finite element analysis of all the specimens is included to determine the relevant sectional shear forces and moments. Finally, the ultimate shear stresses from the database experiments are compared to the shear capacities determined with ACI 318-19, Eurocode 2 and the Model Code 2010. The comparison shows that the Model Code 2010 is the most precise in the predictions with an average tested to predicted ratio of 0.82 and a coefficient of variation of 29.63%. It can be concluded that improvements to the current design methods for eccentric punching shear are necessary.

Strengthening of slab-column connections using ultra high-performance fiber concrete

This experimental work intends to assess the efficiency of employing ultra-high-performance fiber concrete (UHPFC) as a strengthening system for improving the punching shear capacity of slab-column connections (SCCs). Seven reinforced concrete SCCs were constructed and tested under axial load. One of them was retained as a control slab (un-strengthened), and the other six were strengthened with different strengthening schemes. The results revealed that the strengthening scheme is effective in improving the punching shear capacity of slab-column connections significantly. Strengthened slabs achieved increases in punching shear capacity, pre cracking stiffness, post-cracking stiffness, and toughness of up to 172.68 %, 235.70 %, 300 %, and 233.59 %, respectively. It turns out that it is important to use UHPFC with longitudinal reinforcement (R-UHPFC) to enhance the strength and toughness. Finally, the punching shear capacities of the tested specimens were compared to those predicted by the existing theoretical approach.

Punching shear behavior of reinforced concrete flat slab column connection containing recycled eggshell powder

Every day, millions of bio-waste eggshells are discarded in the environment by poultry farms, homeowners, bakeries, and the food industry around the world. The possibility of recycling eggshells as a cement replacement in concrete may solve the disposal issue of eggshells. Also, cement production causes the release of carbon dioxide, which contributes to global warming. Replacing cement with eggshell powder (ESP) reduces the level of cement production and indirectly decreases carbon dioxide emissions into the atmosphere. This paper aims to investigate the structural effect of the ESP content as a cement replacement on the punching shear behavior of reinforced concrete (RC) flat slab column connections. The possibility of ESP reuse in RC flat slab structures tested for punching shear was investigated. Four well instrumented RC flat slab column connections (S1, S2, S3, and S4) were cast using four different concrete mixtures containing 0, 5, 10, and 15% ESP, respectively. The slab panels (620x620x50) mm with a constant square column size of (100x100) mm were simply supported and tested under concentrated column load. The test results of different concrete mixes made with ESP content are reported. The test results of RC flat slab behaviors, including failure mode, load–deflection response, load-corner rise, load–strain curves, load-edge rotation response, punching perimeter, angle of punching, shear load, ductility, and stiffness of tested slab panels with and without ESP are discussed. The experimental punching shear load and punching perimeter were compared with those obtained by code model equations in ACI, BS, and Eurocode. It can be concluded that the experimental shear load and stiffness for the slab containing 10% ESP content increased by 5.85% and 24% than that of the control slab without ESP. Eggshell powder could be applied to a certain extent to replace cement in the RC flat slabs.

Finite Element Analysis of Punching Shear of Reinforced Concrete Slab–Column Connections with Shear Reinforcement

Shear reinforcement is one of the common measures used to strengthen slab–column connections. It can make the joint’s load capacity increase. However, when the connection is subjected to an unbalanced bending moment, the reduction effect of the shear reinforcement on the bending moment is worth investigating. Nonlinear finite element analysis was performed on reinforced concrete slabs with shear reinforcement for various moment-to-shear (M/V) ratios. The effects of the number and diameter of shear reinforcement and different loading methods on the slab were investigated, as along with a correction to the ACI-318 and China Concrete Structure Design code GB50010 formula for the moment transfer coefficient. The 3D finite element modeling was performed using appropriate element types and a constitutive model for concrete. The concrete damage plasticity model parameters were calibrated using the experimental results of a specimen. The amount of shear reinforcement had a certain contribution to the strength of the slab, but the diameter of the shear reinforcement had little effect. The plate’s bearing capacity decreased as the M/V increased. The corrected moment transfer coefficient formula demonstrates that M/V is positively correlated with the coefficient and that the calculated values specified in ACI-318 and GB50010 are overly conservative.

Experimental and numerical investigation of punching shear capacity of corroded reinforced concrete slab-column connections

Rebars in reinforced concrete (RC) slab-column structures may become corroded under adverse conditions, rendering slab-column connections (SCCs) more susceptible to punching shear (PS) failure. In this study, four RC SCC specimens were designed and fabricated, and their rebars were subjected to accelerated corrosion using an electrochemical approach. Next, pseudo-static tests were performed to determine the PS capacity of the RC SCC specimens with different corrosion ratios. The test results showed that the corrosion of the rebars in an RC SCC significantly reduced the angle of its PS plane and consequently lowered its PS capacity. On this basis, a high-fidelity finite-element model was generated using LS-DYNA, and its accuracy was validated through a comparison of the simulation and test results. Based on this model, numerical simulations were performed to examine the effects of four parameters, namely, the size of the corroded area, the reinforcement ratio, the slab thickness, and the boundary conditions, on the PS capacity of corroded RC SCCs. The numerical simulation results show that increasing the size of the corroded area reduces the PS capacity of an RC SCC and increasing slab thickness weakens the effect of rebar corrosion on the PS capacity of an RC SCC, while increasing the reinforcement ratio has a limited effect on the improvement in the PS capacity of a corroded RC SCC. Additionally, increasing the horizontal boundary stiffness significantly enhances the PS capacity of a corroded RC SCC.

Assessment of punching shear strength of FRP-RC slab-column connections using machine learning algorithms

Recently, the use of fiber-reinforced polymer (FRP) bars replacing steel reinforcement has been widely applied to overcome the corrosion issue, particularly concrete slab-column connections using FRP bars as flexural reinforcement (FRP-RC slabs). However, experimental studies showed that the use of FRP differentiates the punching shear behavior from steel-RC slabs. Various methods have been proposed to predict the punching shear strength of FRP-RC slabs, but existing design equations need improvement because their accuracy is low with wide scatteredness. Thus, this study aims at pioneering the application of machine learning (ML) algorithms for the prediction of the punching shear strength of FRP-C slabs without shear reinforcement. For this purpose, an experimental database with 104 specimens was compiled, with the input variables of the shear span-to-effective depth ratio, column perimeter-to-effective depth ratio, effective slab depth, concrete compressive strength, FRP reinforcement ratio, and ultimate tensile strength and elastic modulus of FRP. Three ML algorithms, including support vector regression (SVR), random forest (RF), and extreme gradient boosting (XGBoost), were evaluated for the application. To develop the ML-based models, a grid search method with a 5-fold cross-validation approach was used in the training process to determine the optimal hyperparameters. The performance of the ML-based models was estimated using various statistical estimators and compared with the current design codes and existing models. The comparisons showed that all three ML-based models could accurately predict the punching shear strength of the FRP-RC slabs without any significant and evident bias with the input variables. The XGBoost-based model displayed the best prediction with the coefficient of determination (R2) of 0.962, the root mean square error (RMSE) of 0.061 MN, mean absolute error (MAE) of 0.035 MN, and mean absolute percent error (MAPE) of 8.931% for testing dataset. The correlation coefficient, feature score, and sensitive analysis for the input variables indicated that the effective slab depth has the most substantial influence on the prediction performance. The prediction by the XGBoost-based model was more accurate and robust than that by the SVR- and RF-based models, current design codes, and existing models. These analysis results proved that the XGboost-based model can be used in the design and evaluation of FRP-RC slabs reliably and precisely.

Comparative evaluation of concrete constitutive models in non-linear finite element simulations of slabs with different flexural reinforcement ratios

Two types of failures can be observed in reinforced concrete slabs supported on columns: flexural and punching. Flexural failure occurs due to the formation of a yield-line mechanism, while punching failure occurs when a punching shear crack with a truncated cone shape develops around the connection. However, flexure-driven punching shear failures due to the yielding of the slab’s flexural reinforcement can occur in the immediate vicinity of the column. These failures appear in a similar manner to “pure” punching shear failures with an additional ductility resulting from the flexural reinforcement yielding. Since these flexure-driven punching failures occur in reinforced concrete slabs with low reinforcement ratio, the paper investigates the punching shear response of reinforced concrete slab-column connections with various flexural reinforcement ratios to distinguish the different failure modes that can be observed: punching and flexural-punching. First, the study presents the calibration procedure needed in three-dimensional non-linear finite element analyses (FEA) to simulate the punching shear failure of reinforced concrete two-way flat slabs without transverse reinforcement under static concentric loading. The calibration approaches are developed with reference to two advanced constitutive concrete models available in two different FEA software. Two series of experimental tests are selected from literature: plain concrete specimens under uniaxial and biaxial loadings and slab-column connections under concentric punching. It is shown that the numerical solutions using both concrete models can capture the cracking and failure mode accurately. It is emphasized that the numerical models provide a valuable tool to characterise failure processes and behavioural mechanisms for the simulation of slabs with varied reinforcement ratios. Numerical simulations showed that the failure mode transition within low and high reinforced concrete slabs can be highlighted by flexural-punching and punching shear mechanisms. In addition, the new code predictions of ACI 318-19 for punching shear of slabs with low reinforcement ratios are presented and discussed in comparison to EC2-2004 predictions and the experimental results. Finally, a numerical assessment of slab-column connections with reinforcement ratios varying between 0.2 and 2.2% is conducted to thoroughly investigate the effect of the flexural reinforcement ratio on the punching behaviour.

Punching shear strength of interior concrete slab-column connections reinforced with FRP flexural and shear reinforcement

To predict the punching shear strength of interior reinforced concrete (RC) slab-column connections using fiber reinforced polymer (FRP) bars as flexural and shear reinforcement, an analytical study was performed. Based on the punching shear failure mechanism of FRP-RC slab-column connections, a design method was proposed addressing the features of FRP reinforcement. For an accurate strength prediction, the depth of compression zone was evaluated by a modified approach, formulas for the effective strain and strength contribution of FRP shear reinforcement were proposed, and the effects of the size, aspect ratio, and load eccentricity were included. For verification, predictions by the proposed method were compared with existing test results for FRP-RC slab-column connection specimens with or without FRP shear reinforcement and subjected to concentric or eccentric load, and the accuracy and reliability of the proposed method were thoroughly examined by comparing with the current design codes and existing models. The comparisons showed that the proposed method best matched with the various test results consistently for all design parameters. To give an insight into the design of FRP-RC slab-column connections, the effects of various design parameters on the punching shear strength were also analyzed in detail.

Punching shear strength of corroded reinforced concrete slab-column connections

The use of de-icing salts on reinforced concrete (RC) car-park flat slab buildings is known to corrode the reinforcement, which may adversely affect the punching shear strength of the slab-column connections. The consequence of the punching shear failure of slab-column connections can be catastrophic, as the failure of one connection can easily spread to others and eventually leads to the progressive collapse of the flat-slab structure. To study the effects of corrosion of the flexural reinforcement on the punching shear behavior of slab-column connections, twelve full-scale RC slab-column connections were constructed and tested in this study. The experimental program was divided into two stages: 1) accelerated corrosion test; and 2) punching shear test. It was found that the rebar corrosion can change the failure mode of the connection from flexural or shear-flexural failure to punching shear and reduce the punching shear strength significantly. Moreover, the existing design formulas given by ACI 318–19, CSA 23.3–14, Eurocode 2, and BS 8110–97 were found to be unreliable for the prediction of the punching shear strength of corroded slab-column connections. The critical shear crack theory (CSCT) was modified in this work to predict the punching shear strength of the corroded slab-column connection well.

Numerical and analytical investigation on the moment and deformation capacity of interior slab-column connections without shear reinforcement

In this study, nonlinear finite element analyses of reinforced concrete interior slab-column connections were performed to investigate their punching shear behavior under gravity and lateral loads without shear reinforcement. A numerical model based on the concrete damaged plasticity (CDP) model in ABAQUS was developed with suitable constitutive models for concrete and reinforcements using eight-node brick elements with reduced integration and three-dimensional truss elements, respectively. The model was calibrated in comparison with the test results of an interior slab-column connection without shear reinforcement. Moreover, the validity of the calibrated model was verified using other test results. Then, a parametric study was conducted to examine the influence of different design variables on the unbalanced moment and deformation capacity of slab-column connections. Finally, based on the existing experimental data and finite element analysis results obtained in this work, formulas for calculating the ultimate unbalanced moment and drift ratio were proposed using the nonlinear fitting method. The proposed formulas were compared with the existing methods provided by other national codes and researchers, verifying the rationality of the proposed formulas.

Comparative analysis of unbalanced moment of reinforced concrete slab-column connections

The existing comparative analysis of the unbalanced moment mainly compares the differences between the calculated value and the experimental value of the unbalanced moment in different national codes and different calculation methods. There is less analysis on the change trend of the important parameters in the unbalanced moment calculation methods in national codes. A comparison of the provisions for unbalanced moment of reinforced concrete slab-column connections was studied based on Chinese code (GB 50010-2010), American code (ACI318-14) and European code (EN1992-04). The effects of concrete strength, longitudinal reinforcement ratio, the use of shear reinforcement and gravity-shear ratio on the unbalanced moment were studied through the calculation example and experimental results of 186 specimens. The results indicate that the calculation values of Chinese and American codes are conservative and dispersive, because the longitudinal reinforcement ratio and gravity-shear ratio on distribution coefficient of unbalanced moment were not considered, while the calculation results of European code are the closest to the test values.

Strengthening of bi-axially loaded RC slab-column connection using steel plates and stiffeners

This paper presented a strengthening technique for enhancement reinforced concrete (RC) slab-column connection behavior using steel plates and steel stiffeners. A bi-axial load was applied with an eccentricity of 150 mm in both x and z directions. Four specimens with dimensions of 1,600 × 1,600 × 100 mm were tested in this research. The steel plate dimensions were chosen to be 600 × 600 and 800 × 800 mm with 6 mm thickness. The steel stiffeners are used to support and enhance the steel plate that it is extended from column to the slab-column connection with different dimension. The results showed an improvement in the stiffness of slab and increasing in the ultimate capacity. Also, when the steel plates dimensions increased, the ultimate capacity and stiffness increased.