Slab-column Connections Research Articles

Aggregate Size on Punching Shear Behavior of Interior Slab-Column Connections

Aggregate Size on Punching Shear Behavior of Interior Slab-Column Connections

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.

Effect of column size on punching behavior of flat slabs with square columns: Numerical investigation

This study investigates the punching behavior of slabs supported by square columns of various sizes at different effective slab depths. Within the research context, a finite element model of the slab-column connections is developed using ABAQUS and calibrated based on 15 test results from the literature. Subsequently, 84 slabs with square columns are simulated, and the effect of column size is assessed in terms of load-rotation behavior, shear stress distribution, cracking patterns, punching capacity, and nominal ultimate shear stress. The findings indicate that all slabs undergo brittle punching failure due to the damage near the column corner, which occurs before the ultimate shear stress is fully reached around the column perimeter. The shear stress of the slab along the column perimeter concentrates at the column corner, with this concentration being more pronounced in slabs supported by large square columns. Moreover, as the column size increases, the punching capacity consistently rises while the nominal ultimate shear stress initially decreases, with the decline of the nominal ultimate shear stress being more significant in thinner slabs. The punching capacity predicted by the finite element analysis is compared to the values derived from ACI 318-19, EC2, MC2010, and CSCT models. The findings suggest that these formulas do not adequately capture the influence of column size on punching capacity, prompting the proposal of a new formula that refines the consideration of column size effects.

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.

Punching Behaviour of Flat Slab Using Recycled Aggregate

Recently, researchers have shown an interest in researching recycling aggregates from old concrete and using them in new construction as a means of protecting the environment and natural resources. This thesis includes an experimental investigation into the punching shear behaviour of continuously supported slabs of concrete with natural and recycled coarse aggregate. In the Laboratory at the MS Ramaiah Institute of Technology the tests on punching shear behaviour of inner slab-column connections in flat plates. ''Natural and recycled'' aggregates were used in six two-way slab models. The slabs were made to only fail while being used as a punching shear. Three specimens of recycled aggregate concrete (RAC) slabs and three specimens of natural aggregate concrete (NAC) slabs were used in the experimental study. Simply supported on four edges, all of the slabs were tested using a central patch load. In order to attain a 25 MPa concrete strength after 28 days, the IS mix design technique was used. For each aggregate size of 12.5mm (Downsize), five concrete mix proportions were used with (0%, 25%, 50%, 75%, and 100%) replacement of natural coarse aggregate with recycled coarse aggregate. Then, casting with a size of 1000 × 1000 × 100 mm was tried with 0% and the highest of the other four (75%). The test findings were then compared with the experimental data and the Codal Provisions for the Accuracy of Ultimate load.

Advanced soft computing techniques for predicting punching shear strength

Accurate prediction of punching shear strength is crucial for ensuring the structural integrity and safety of reinforced concrete (RC) flat slab-column connections. However, estimating punching shear strength in complex configurations, such as RC flat slabs, remains a challenging task due to intricate interactions between various parameters. To address this challenge, this study introduces an innovative approach that combines machine learning techniques with hyperparameter optimization strategies to enhance the precision and efficiency of predicting punching shear strength. Specifically, a comprehensive investigation is conducted to assess the performance of XGBoost optimized by different hyperparameter optimization algorithms such as the forensic-based investigation (FBI), Bayesian optimization (BO), grid search (GS), random search (RS), and default setting (DS). The experimental results of 40 runs on three different datasets confirmed the FBI-XGBoost model as the most appropriate for predicting punching shear strength in terms of efficiency and computation time by achieving the highest RMSE, MAPE, MAE, and R2 values. In addition, the statistical results exposed the infeasibility of using RS for the XGBoost model. While the GS method was able to configure the XGBoost model well, doing so required extensive computational time, while the BO-FBI model achieved acceptable prediction accuracy in a reasonable training time. The FBI-XGBoost model was further compared with empirical methods, including ACI-318-08, BS8110-97, and CEB-FIP-90, based on the predictive-to-actual ratio. FBI-XGBoost was identified as the best model by attaining the ratio value closest to 1 and the lowest standard deviation (1.011 ± 0.108). Overall, the statistical experimental results and hypothesis test values firmly support the FBI-XGBoost model as the most reliable free-formulae method for predicting punching shear strength in RC flat slab-column systems.

Predicting punching shear in RC interior flat slabs with steel and FRP reinforcements using Box-Cox and Yeo-Johnson transformations

This study presents a new approach for predicting the punching shear strength of reinforced concrete flat plates reinforced with steel and fiber-reinforced polymer (FRP) bars, using four machine learning regression models. A dataset of 505 interior flat plates from previous literature was used to develop the models. Input variables were transformed using Box-Cox and Yeo-Johnson transformations to improve predictive power by reducing white noise. The SHapley Additive exPlanations (SHAP) framework was utilized as the Explainable Artificial Intelligence (XAI) method to decipher model execution and feature the main info factors for anticipating the punching shear capacity. Additionally, design codes and equations from the literature are compared to the efficiency and accuracy of the power transformation featured XGBoost model. The findings demonstrate that the suggested prediction model performed very well and is appropriate for estimating the punching shear capacity of slab-column connections reinforced with steel and FRP bars. Moreover, the power transferred XGBoost model performed better than other numerical equations because it had the highest mean R2 (0.93) and lowest MAPE (0.20) for testing, which suggests that machine learning models may be able to offer an elective strategy to currently used mechanics-based models for configuration practice.

The effect of hybrid fiber and shear stud on the punching performance of flat-slab systems

In this paper, for the first time, the binary/ternary hybrid fiber and/or shear stud reinforcement as a measure was used to improve the punching performance of flat-slab systems by innovative self-compacting concrete (SCC). Because in these slab systems, sudden and brittle punching failure can be seen due to application and design errors, early removal of formwork, changes in the purpose of use of the building, earthquakes, etc. Besides, numerous studies investigated the punching performance of the single fiber and/or shear stud reinforced flat-slab systems, but research into the measures of the hybrid fiber or the combined use of hybrid fiber and shear stud reinforcement, which were the variable parameters of this study, was quite lacking. For this purpose, the half-scale slab-column connection elements were produced from SCC containing different punching measures (binary/ternary hybrid and/or shear stud) and tested to investigate the punching performance of flat-slab systems. In conclusion, it was found that hybrid fiber reinforcement was the best punching measure to improve the punching performance of slab-column connection elements with/out shear stud. Besides, 3D graphs were drawn so that designers and researchers could estimate the punching strength and energy absorption capacity for flat-slab systems with/out shear stud based on the parameters of micro fiber type and total volume fraction. On the other hand, empirical formulas were developed to predict the punching strength of binary/ternary hybrid fiber reinforced flat-slab systems with/out shear stud by compressive strength, fiber reinforcement index, the slab useful height, and the punching perimeter parameters.

NLFEA of one-way slabs in transition between shear and punching: Recommendations for modeling

Several studies investigated the precision of non-linear finite element analyses (NLFEA) to predict the shear capacity of beams or the punching capacity of slab-column connections. However, in the literature, there is little discussion regarding the results of NLFEA to predict the ultimate capacity and failure mechanism of slabs susceptible to different failure mechanisms. In this study, a set of tests from literature that developed different shear failure mechanisms was evaluated by NLFEA. A total of thirteen slabs were modeled, where slab’s width and shear span varied. A coupled damaged-plasticity model was employed to simulate the concrete behavior. The proposed model was calibrated to simulate specimens that failed as wide beams in one-way shear and specimens that failed by punching shear. Besides, the effect of different modeling choices was investigated: (i) the assumed stress–strain behavior in compression; (ii) tensile stress–strain behavior; (iii) the inclusion or not of damage parameters and (iv) the viscosity parameter. The results indicated that, on average, the proposed modeling strategy represented the failure mechanism and ultimate loads well. Also, the same calibrated model was found capable of representing one-way shear failure and punching shear failure or mixed modes. The sensitivity study demonstrated that the tensile stress–strain behavior and viscosity parameter influences the results of the numerical models more significantly than the assumed stress–strain behavior in compression or including/not including the damage parameters. This paper concludes that modeling strategies during the calibration process shall be checked carefully before performing any parametric analyses to identify the accuracy of the numerical models to represent the different failure mechanisms. Moreover, the validation step of the modeling strategy shall identify possible limits of the numerical model to be considered in the parametric analyses.

Experimental investigation of punching shear behaviour of ultra-high performance self-compacting concrete slabs

The use of heavy reinforced and thick transfer-slab in multipurpose tall buildings is crucial to dealing with many issues such as high bending and punching shear stresses created from relocation of columns in floors above transfer-slab. Ultra-High Performance Self-Compacting Concrete (UHPSCC) is a proper option for coping with the problem of dense reinforcement. In predicting the punching shear capacity of concrete slabs, most existing code provisions limited the concrete compressive strength up to 70 MPa. Furthermore, no code provisions deal with estimating the punching shear capability of UHPSCC slabs. For these reasons, the present paper aims to investigate experimentally the punching shear behaviour of the interior slab-column joint made of a developed UHPSCC blend had a compressive strength of more than 120 MPa. The study takes into account the effect of design parameters such as type of concrete, slab thickness, and reinforcement ratio on the punching shear capacity. The study explores if the existing code shear provisions could be utilized to estimate the punching shear strength of UHPSCC and UHPC slab-column connections. In terms of estimating the punching shear capacity of such slab-column joints using available code provisions, regardless of their concrete compressive strength limits (using the available code formulae to estimate punching shear strength twice, once with the code restrictions of concrete compressive strength and once without), the available code shear models were significantly conservative, indicating the need to develop a new punching shear model for such slab-column connections. The European Code (EC2, 2004) has the best appropriate formula for predicting the nominal punching load capacity of UHSC slabs among the present well-known code punching provisions. The critical punching section of the UHPSCC slab was detected at 1.5 times the slab's effective depth measured from the column face. For a slab thickness of 200 mm, increasing the tensile steel ratio from 1.3% to 2.0% increased the punching strength of UHPSCC by 77.6%. At a reinforcement ratio of 1.7%, raising the slab thickness from 160 to 240 mm boosted the punching strength by 118.6%. In view of the study's outcomes, in addition to its ability to address the issue of dense reinforcement, the use of UHPSCC provides great potential for improving the punching shear capacity of heavy-loaded slab-column connections.

Reduced area approach for predicting punching shear resistance of biaxial hollow slabs with openings

Due to the inclusion of hollow plastic spheres in bubbled slab systems, the shear resistance is greatly reduced. Also, the existence of the opening in the vicinity of columns takes away part of the concrete responsible for resisting shear forces and unbalanced moments, which in turn further reduces the punching shear capacity of the slab-column connection. In this study, experimental and theoretical investigations had been carried out on bubbled slabs with 2000 × 2000 × 230 mm dimensions. The investigated parameters were the size, position, distance of opening relative to the column stub, and the location of bubbles relative to the critical shear perimeter. A new calculation approach for determining punching shear resistance for bubbled slabs was developed. The proposed modification for calculating the effective critical perimeter showed an appropriate range of agreement of the punching shear strength, which is calculated by the methods adopted by (ACI 318, Eurocode 2, BS 8110, and AS 3600) codes, in comparison to the experimental data. The average ratios of the test results to the calculated punching shear capacity determined according to the proposed approach are between 1.007 and 1.417 with a standard deviation of 0.039–0.186 and a coefficient of variation of 0.038–0.131.

PUNCHING SHEAR BEHAVIOR AND STRENGTH OF FLAT SLABS

Prestressed and reinforced concrete flat plates are susceptible to failure due to punching shear. Modeling the punching shear strength of prestressed concrete slabs poses conceptual challenges, as it involves various factors, including the bonding of tendons, stress in tendons during punching failure (which can be indeterminate), tendon banding, and the presence of draped tendons that yield a beneficial vertical component of the prestressing force. Additionally, prestressed concrete slabs typically require supplementary bonded reinforcement. The objective of this research is to comprehensively investigate the physical behavior of each test specimen, considering parameters such as deflections, crack patterns, crack widths, mode of failure, and to compare the punching shear resistance of interior slab-column connections with various code provisions. The study involved testing a total of five interior slab-column connections, each measuring 1600×1600 mm in size, 80 mm in thickness, with square columns of 180×180 mm and a height of 300 mm. Keywords: Slab, Punching, Shear, Design, Codes, prestressing.

Punching tests on edge slab-column connections with refined measurements

Due to the significant shear forces developing around columns and the potential brittle failure associated, the punching capacity of flat slabs is the main concern at ultimate limit state. An effective way to understand the behaviour of slab-column connections has typically been by testing isolated specimens in the laboratory. While there exist many slab-column connection types in actual slabs, most research efforts have focused on the punching resistance at internal columns. Specifically, edge connections appear largely underrepresented in the literature when the number of existing tests is compared to their importance in typical building floor configurations. Furthermore, the favourable effect of slab continuity is generally neglected in this type of tests. This paper presents the results of an experimental campaign on two edge slab-column connections: one had no shear reinforcement, while the other had a large amount of shear reinforcement to evaluate maximum punching. A novel test setup was conceived to apply representative boundary conditions of actual continuous slabs, introducing moment continuity perpendicularly to the slab free edge. Refined measurement techniques were extensively used, monitoring the visible slab faces using Digital Image Correlation, and instrumenting the flexural and shear reinforcement with distributed fibre-optic sensors. The failure of the slab-column connection without shear reinforcement was triggered in the region of shear stress concentrations and propagated around the column thereafter. Well-anchored vertical legs of flexural reinforcement at the free face induced a smeared-like shear crack pattern. In the shear-reinforced slab-column connection, an initial crushing of the concrete at the column front corners was followed by the formation of a local yield line along the slab axis. In addition to the enhancement of the resistance and rotation capacity, the shear studs also served for increasing the moment redistribution capacity of the system. The experimental results are compared to different codes of practice, showing that improvements are needed to better assess the resistance of edge columns.

Effect of the Failure Surface Inclination of Punching with Studs as Shear Reinforcement

This paper presents experimental and theoretical results for the pullout resistance of anchor studs embedded in concrete blocks with inclined planes that simulate punching. This type of reinforcement has been widely studied to combat punching between the slab–column connection, but little information is available about its effect on the region of the inclined shear plane. To simulate this situation, six concrete blocks were made with a single embedded stud; the inclination planes were varied (18°, 30°, and 45°). The effective heights were 50 and 75 mm, and the section widths were 300 and 500 mm. The theoretical analysis was carried out by comparing the estimated load obtained from analytical models found in the literature with the failure load of the tested specimens. The results showed that the inclination plane did not significantly influence the pullout loads of the studs for the studied effective heights, because the configurations generated by the failure surface of the concrete cone trunks tended to stabilize in tension. However, the series with the 30° angle had slightly higher failure loads. The variation in concrete compressive strength did not influence the failure loads, revealing that the concrete strength grade <40 MPa can be an economical alternative in the design of concrete blocks or slabs reinforced with inclined studs.

Insulation effect on post-earthquake fire punching shear behavior of slab–column connections

Insulation effect on post-earthquake fire punching shear behavior of slab–column connections

A machine-learning-based model for seismic performance assessment of interior slab-column connections

Slab-column connections are the most critical components in flat plate systems under seismic actions due to their brittle failure mode. Finite element analysis (FEA) can be adopted as an extension of experimental testing to potentially lead to the development of modern design codes by providing significant information on the punching shear failure process of connections. However, the available concrete constitutive models in FEA packages fail in simulating the hysteretic response of the connections under cyclic loads. Moreover, it is shown that the available design code recommendations and empirical models are not accurate tools for assessing the seismic performance of these connections (e.g., unbalanced moment and/or drift capacity). In an effort to overcome these limitations, a dataset of previously tested experiments is compiled to predict the ultimate load and drift of the connections using the multi-variate non-linear regression model. Additionally, there is a need to develop a new constitutive material model for seismic performance assessment of the connections to capture the complete hysteretic response of the connection using numerical FEA. Consequently, a deep neural network is trained over the accumulated dataset and integrated into the OpenSees source code to determine the seismic performance of the connections. For data augmentation, a generative neural network model of autoencoders known as Generative Adversarial Networks (GANs) is constructed to generate synthetic data. The proposed material model is found to have significantly improved precision in capturing the hysteretic response of flat plates. The developed models showed higher accuracy than the design code estimations and empirical models.

Punching Shear Strength of Reinforced Concrete Flat Plates with Openings

Test results of six half-scale reinforced concrete flat plates connections with an opening in the vicinity of the column are reported. The test specimens represent a portion of a slab bounded by the lines of contraflexure around the column. The tests were designed to study the effect of openings on the punching shear behavior of the slab-column connections. The test parameters were the location and the size of the openings. One specimen had no opening and the remaining five had various arrangements of openings around the column. All specimens were cast with normal density concrete of approximately 30 MPa compressive strength. The openings in the specimens were square, with the sides parallel to the sides of the column. Three sizes of openings were used: the same size as the column (150 x 150 mm), 67 percent of the column size (100 x 100mm), and 150 percent of the column size (225×225mm). Due to the presence of the openings, the specimens showed a decrease in punching shear capacity ranged between 11.43% and 29.25% with respect to the control solid slab. Also, the stiffness decreased between 0.31% and 83.00%, depending on the size and location of these openings with respect to the column.

Openings effects on the structural behavior of interior voided slab-column connections: An experimental study

This paper studied the openings' influences on the structural responses of interior voided slab-column connections. Seven specimens were constructed with square slabs measuring 1100 × 1100 × 100 mm and contained voids distributed throughout their entire area. After construction, circular openings were introduced in six specimens; the left one was the control. Two specimens had one opening with a diameter of 100 mm and 150 mm. The others had two openings with a 100 diameter placed in different configurations about the column: on the same side, perpendicular, on the slab diagonal, and parallel. The specimens were tested under concentrated loads till the failure. The test results showed that all connections failed due to punching modes, and the strength declines increased with the opening size to reach an essential limit of 35% when the opening size was 150 mm compared with the control connection. Furthermore, the openings decreased the ability of specimens to absorb energy by about 48.1%− 74% and the specimens' stiffness to 33.5%. Moreover, the ductility of specimens with openings dropped nearly to 34.4%− 51.4%. The best openings arrangement was found when placing two openings on the slab diagonal, where the strength decline was only 4.7%; this reduction was lesser than introducing one opening in the front of the column.

Influence of openings on the punching shear behaviour of concrete slabs supported on special-shaped columns

Punching shear of special-shaped columns supporting reinforced concrete two-way slabs has been addressed in design codes since the late 1970s. While these provisions have existed for over 40 years their basis is unclear as previous studies related to the punching shear behaviour of irregular column geometries, such as L, T, and cruciform shapes, are limited. In this paper, the punching shear behaviour of L, T and cruciform slab-column connections, with or without slab openings near the connection, under concentric vertical loading is studied using a three-dimensional nonlinear finite element model (FEM) implemented in Abaqus/Explicit. The nonlinear FEM, based on the concrete damaged plasticity model, is used to study the influence of both the opening size and location on the punching shear behaviour of special-shaped (irregular) slab-column connections of various sizes. Based on the FEA results, the ideal location for openings around special-shaped slab-column connections (SS-SCCs) is between the column flanges. This location minimizes their negative influence on concentric punching capacity and is also beneficial architecturally. Finally, the accuracy of the ACI 318–19 punching shear provisions is evaluated. Based on the FEA results modifications to the existing provisions are necessary, particularly to account for the location of openings around special-shaped slab-column connections.

AN Experimental Study on Strengthening of R.C Flat Slab Edge Column Connections Subjected to Punching Shear Due to Eccentric Loads Using FRP

During the last few years, the need to flat slab systems has rapidly increased.The slab column connections behavior plays decisive role because of the suddenexpected brittle failure as the slab may be punched by column, so improving thebehavior of these connections is essential. Flat slab column connections can benefitfrom being strengthened by fiber reinforced polymers (FRP) as a structural applicationin punching shear [1]. The effect of using carbon fiber (CFRP) and glass fiber (GFRP)was investigated experimentally, also steel reinforcement bars were utilized asstrengthening element for comparison. Results of tested fifteen half-scale flat slab-edgecolumn connections reinforced concrete specimens under eccentric loads wereobtained. The investigated parameters were the eccentricity value, the type ofstrengthening material and the number of rows of strengthening elements. Five groupswere created from the fifteen tested specimens, every one contained three specimens.The first group consisted of three non-strengthening specimens, with variableeccentricity values (0, e & 2e), these specimens were considered as control ones. Eachof the other four groups consisted of three specimens, which were strengthened byCFRP, GFRP and steel bars. All the specimens of the second and fourth groups werestrengthened by one row of strengthening elements. All the specimens of the third and