Punching Shear Strength Research Articles

The calibration of NLFE model for the assessment of the maximum punching capacity of flat slabs

The requirements on structural engineers to design more subtle flat slabs are still increasing and therefore they must often design a large amount of reinforcement against punching. The maximum amount of transverse reinforcement is limited by the maximum punching capacity of slab, so the precise determination of the limit is essential. In the second generation of Eurocode 2, the model for the assessment of the maximum possible increase of the punching shear resistance of slab is less empirical and it considers the performance of the punching shear reinforcement. The main objective of the paper is to develop nonlinear finite element model, which would determine as closely as possible the maximum punching capacity of flat slab, strongly reinforced with punching shear reinforcement. So that, an effect of some transverse reinforcement’s parameters on the maximum punching shear strength could be investigated quite easily. For this purpose, the software GID was used. The calibrated NLFE model showed quite good agreement with experiment. The difference is less than 8 %, even for slabs reinforced with double-headed studs it did not exceed 1%.

FRP Strengthening of RC Structures: Sustainable, Environmental and Structural Evaluations

Strengthening and rehabilitation have been widely implemented for many years to extend the service life of reinforced concrete structures. The paper begins with a comprehensive review of the fiber-reinforced polymers (FRP) utilization on strengthening particularly over the traditional materials formerly used in practice with respect to materials, manufacturing, operation, construction, and maintenance phases, as well as the engineering and environmental performance of such materials. Carbon and Glass FRP, the most frequently used strengthening materials, are particularly designated in the study and are employed to conduct an environmental performance evaluation using the previously published data in the literature. The paper then investigates the punching shear strength of flat slab-column connections strengthened with externally bonded FRP by means of a nominated database comprising 57 number of data points harvested from the recent literature. The database is used in the evaluation of the test data with TS 500 code equations and the recent modification of Chen and Li. The study enabled the key factors affecting the punching shear strength of such connections to be emphasized and highlighted the fact that the TS 500 code equations fall conservative in predicting the punching shear strength of slab-column connections strengthed with FRP. The study is novel as it provides a comprehensive review of the FRP as a strengthening material with regards to environmental sustainability and also provides an insight into the structural implications of this material by evaluating the current TS 500 code provisions and recent modifications.

Effect of using Engineered Cementitious Composites (ECC) on failure behavior of flat slab-column connections

Load capacity and ductility are two key characteristics of the flat slab-column connection in high seismic areas. This study specifically examined the effect of strengthening layer thickness of Engineered Cementitious Composites (ECC) on punching shear strength and post punching behavior of flat slabs. The effect of the unbalanced moment, ECC thickness of the slab and the strengthening ECC layer on flat slab punching and post punching behavior were also investigated. Thirteen reinforced concrete flat slab specimens were made and examined. It was observed that the replacement of slab concrete with ECC increased the punching capacity and improved post-punching behavior without a large and sudden drop in load. In addition, the use of ECC as a strengthening layer for the flat slabs significantly increased the punching and post-punching shear strength capacities. The change in failure mechanism was observed from a sudden and abrupt failure to a ductile failure with high-energy absorption when more ECC layer thicknesses were used.

Performance of reinforced concrete flat slab strengthened with CFRP for punching shear

Existing reinforced concrete (RC) slabs need punching shear reinforcement due to design defects, structural damage, increased loading, corroded reinforcement bars, and change in the use or function of the buildings. Strengthening methods such as using carbon fibre reinforced polymer (CFRP) sheets as external reinforcements in the tension area have proven to be useful in increasing the punching shear capacity and ductility of RC members. Therefore, this study aims to investigate the effect of using CFRP strengthening method in two-way RC slabs. Three strengthened RC solid flat slab samples in addition to unstrengthened slab samples were tested and subjected to static load to simulate the punching shear effect at the middle of the slab. Based on the test results, the samples strengthened with CFRP showed better performance in terms of delaying the appearance of the first crack, improving the resistance of shear crack propagation, and enhancing the load-displacement and ductility capacities compared to the control sample. In addition, the failure mode of the strengthened samples with CFRP was improved and the capacity for punching shear was increased. Furthermore, the analytical model to predict the ultimate punching shear strength has good agreement with the requirements of the practical codes, thus it can be used for the design and analysis of CFRP strengthened slab.

Influence of the Distribution and Level of Post-Tensioning Force on the Punching Shear of Flat Slabs

Punching shear is the most common failure mechanism of slabs that are supported directly on columns. The slab–column connection is always vulnerable to critical punching shear; thus, it is necessary to investigate the effect of various parameters on the punching shear behavior of the flat slabs. This work presents an experimental study to evaluate the effect of the level of prestressing force and layout of the strands on the punching shear behavior of the slab–column connection in terms of the maximum load, deflection, stiffness, ductility, and the absorbed energy. Five square post-tension flat slabs (2000 mm × 2000 mm) with 150 mm thickness were tested. Increasing the prestressing force increased the maximum load and the ductility with a delay in damage in the case of uniformly distributed strands. Additionally, the banded layout of the post-tensioning strands significantly increased the punching shear capacity of the post-tensioned flat slabs at all levels of prestressing. The influence of the layout of the strands on the flat slab ductility is clearly visible in cases of high prestressing force as the ductility decreases in cases with distributed strands when compared to the same flat slabs with banded strands. Finally, the predicted values of the ultimate load of the punching shear using different codes, including the Egyptian Code of Practice (ECP-203), the American Building code (ACI-318), the CEB-FIP Model code and the Euro code, are compared to the experimental values of the ultimate punching shear strength of the post-tensioned and non-post-tensioned flat slabs.

Nonlinear finite element analysis of non-symmetrical punching shear of rectangular flat slabs supported on square columns

Different tests and calculation methods for determining the punching shear of flat slabs are based on symmetrical punching shear. However, in practical engineering, most flat slabs are not subjected to symmetrical punching shear due to the differences of two orthogonal directions in load, slabs reinforcement, column section size and slabs plane dimensions. The calibrated nonlinear finite element model (FEM) effectively expands the test database, performs variable parameter analysis, and verifies the calculation method. In this paper, the FEM of isolated slab-column connections is developed using ABAQUS and calibrated based on 14 test results from the literature. The simulation of punching behavior of 49 slab-column connections is carried out, the corresponding effects of the shear span ratio and the slab aspect ratio are evaluated. The results indicate that the shear span ratio and the slab aspect ratio significantly affect the crack development, reinforcement stress, stiffness, and punching shear strength. Furthermore, the slab aspect ratio influences the symmetry of the response. With the increase in the slab aspect ratio, the failure mode changes from symmetrical punching shear failure to non-symmetrical punching shear failure and flexural punching failure. The finite element analysis (FEA) prediction results of the ultimate load are compared with the calculation results of four equations of ACI, EC2, CSCT-simplified, and CSCT (ψX − ψY). The results confirm that the prediction using equations of CSCT (ψX − ψY) is reasonable, ACI method gives conservative results, whereas the safety factor of EC2 and CSCT-simplified is less than 1.0.

Implementation of boosting algorithms for prediction of punching shear strength of RC column footings

For more accurate and reliable prediction of punching shear strength of reinforced concrete (RC) column footings, machine learning (ML) was utilized. To develop ML-based strength prediction models, an experimental database was built by collecting test results (total 218 specimens without shear reinforcement and subjected to concentric loads) from the literature, and three boosting algorithms including the adaptive boosting (AdaBoost), gradient boosting (GBR), and extreme gradient boosting (XGBoost) were implemented. Input variables were carefully selected based on existing study results, and optimal hyperparameters of the ML-based models were determined based on the grid search with k-fold cross-validation. The ML-based models with the optimal hyperparameters showed highly accurate predictions without any significant and evident bias for all input variables, and outperformed current design codes and existing models. The relative feature importance of input variables showed that the effective footing depth is the most important variable for the prediction performance, and the boxplot analysis, ANOVA test, and t-test results revealed that spring or uniform load supports can be used instead of sand supports to avoid the complexity of the test setup. Finally, based on the study results, a simple design equation was proposed to widen the scope of application and to consider the effects of the shear span-to-depth ratio, steel yield strength, and test setup.

Prediction of punching shear strength of slab-column connections: A comprehensive evaluation of machine learning and deep learning based approaches

Despite the complex punching shear behavior of reinforced concrete slabs have been comprehensively addressed in the literature, it is further essential to develop a universal design model comprising high accuracy and the simplicity for design practicability, adaptable to diverse conditions encountered in practice. Artificial intelligence applications, artificial neural networks (ANN), and more recently, various machine learning (ML) and deep learning (DL) techniques veer off in a new direction in structural engineering context with improved accuracy and efficiency. The paper begins with the assessment of the capabilities of various artificial intelligence applications in predicting the punching shear strength of slab-column connections without shear reinforcement through the extensive database using 650 punching shear experiments from the literature. Critical parameters influencing the punching shear strength as well as the precision of the current code provisions in predicting this feature were then thoroughly examined in the paper. The results shown in this paper validated the competency of artificial intelligence applications in predicting the punching shear strength of such connections with increased accuracy and improved simplicity in practical terms. The proposed models utilizing the artificial intelligence applications encourage the ultimate rehabilitation policies to be proposed and improved code provisions to be developed for contemporary structures.

Punching Shear Capacity of Recycled Aggregate Concrete Slabs

This article investigates the punching shear behavior of recycled aggregate concrete (RAC) two-way slabs. Ten 1500 mm × 1500 mm × 100 mm slabs were tested monotonically. Eight slabs were cast with RAC, whereas two control slabs were cast with natural aggregate concrete (NAC). The RAC incorporated coarse recycled concrete aggregate (RCA) at replacement levels of 25%, 50%, 75% and 100%. Two flexural reinforcement ratios (0.8% and 1.5%) were examined. The results show that the normalized punching shear strength of 100% RAC slabs decreased by 6.5% and 9% compared to NAC slabs for ρ = 1.5% and ρ = 0.8%, respectively. Doubling the amount of flexural reinforcement can increase the punching shear capacity of 100% RAC slabs by up to 45%. A punching shear database of 44 RAC slabs from literature and the 8 RAC slabs presented in this study revealed that the punching shear strength of RAC slabs predicted by ACI 318 was conservative, except for slabs with low reinforcement ratios (<0.6%). The punching shear strength predicted by Eurocode 2 gave more conservative results for all levels of RCA replacement and all flexural reinforcement ratios. A yield-line analysis also showed that the failure mode of the RAC slabs was controlled by punching shear.

Prediction of punching shear capacity of concrete slabs reinforced with additional middle-thickness rebar mesh

This paper aims to evaluate the influence of using additional middle-thickness rebar mesh (mid-mesh) on improving the punching shear behavior of interior slab-column connection. Sixteen two-way slabs were cast and tested including different flexural, compression, and mid-mesh reinforcement ratios at different levels of concrete strength. The performance of code-based expressions in predicting the punching shear capacity of reinforced concrete flat plates was also ascertained. The results indicated that using additional mid-mesh showed an increase in the punching shear capacity of slabs reached up to 17.7%. The results also indicated that the ACI 318-19 and CSA A23.1-19 codes underestimate the punching shear strength of tested slabs. Eurocode showed the closest predictions of the ultimate shear load compared to the other design codes investigated in this study. The authors also proposed an equation that can help the designers/engineers to accurately predict the punching shear capacity of intermediate slab–column connection with and/or without additional mid-mesh reinforcement.

Experimental and Theoretical Investigation of the Punching Shear Strength of GFRP-Reinforced Two-Way Slabs

This study aims at investigating the punching shear behavior of square RC interior slab–column connections reinforced with glass fiber reinforced polymer (GFRP) rebars. A total of eight specimens are tested under vertical shear until failure. Five of the specimens are reinforced with GFRP rebars and three specimens with conventional steel reinforcement for comparison. The investigated test parameters are flexural reinforcement type (GFRP or steel), column stub dimensions (100 mm × 100 mm or 150 mm × 150 mm), slab dimensions (900 mm × 900 mm or 1200 mm × 1200 mm), and slab thickness (100 mm or 120 mm). The slenderness effect is also taken into account by testing specimens with varying span lengths and thicknesses. The 3D nonlinear finite element method (FEM) is also implemented to validate the experimental findings. Additionally, the accuracy of existing design equations is assessed against the experimental results. The results reveal that GFRP-reinforced specimens yield higher deformation capacity, yet lower punching shear strength, as compared to steel-reinforced counterparts, owing to the lower elastic modulus of GFRP rebars. Although neglected in most studies, the net span-to-depth ratio (L net /d) is a substantial parameter and should be considered in design.

Punching shear behavior of geopolymer concrete two-way flat slabs incorporating a combination of nano silica and steel fibers

This research was conducted by performing center point load tests on the two way flat slab samples to investigate the combined effects of steel fiber (SF) and Nano-silica (NS) on the punching shear and deflection capacities of ten slag-based Geopolymer Concrete (GC). This study consists of two parts; in the first part, the effects of only SFs were investigated, while in the second part the combined effects of NS and SFs on the properties of GC were examined. During the study, two types of hook-end SFs (0.5% and 1%) and only 2% NS were used. The tested slabs showed that the energy absorption capacity/toughness of GC slabs was positively affected by the inclusion of NS and SFs. Also, it was favorably influenced by SFs content and aspect ratio. Furthermore, these slabs were categorized. While, NS and SFs enhanced the punching shear strength of the slabs, the combined influence of NS and SFs exhibited better the punching shear capacity compared to individual utilization. It is clear from the experimental studies that the slab with the combination of NS and SF2 outperformed all of them and showed 131.08% higher strength bearing capacity than the slab without SFs and NS.

Experimental Evaluation of the Punching Shear Strength of Interior Slab-column Connections with Different Shear Reinforcement Details

This research aims to evaluate the effect of using different shear reinforcement details on the punching shear behavior of interior slab column connections. A comprehensive experimental program is conducted on sixteen specimens having the same concrete dimensions of 1100×1100×160 mm where the slab depth is chosen to be less than that stipulated by different design codes. The parameters under examination were the type of shear reinforcement arranged in a cross shape perpendicular to the column edges (single leg, multi-leg, and closed stirrups), the spacing between stirrups (25 and 50 mm), and the extended length covered by the stirrups (300 and 425 mm). Experimental results showed that slabs reinforced with multi-leg or closed stirrups, even for slabs with a thickness of 160 mm, had an increase in the shear capacity by up to 40% depending on the stirrup amount. A noticeable enhancement in ductility was also observed. Slabs reinforced with vertical single-leg stirrups did not exhibit any improvements. A finite element analysis was conducted to further assess the punching shear behavior of the tested specimens. A comparison between the test results and values obtained using design codes such as ACI 318-19 and ECP 203-2018 showed that the two design codes give a rather underestimated prediction of the punching shear capacity. Doi: 10.28991/CEJ-2022-08-09-015 Full Text: PDF

Development of machine learning models for reliable prediction of the punching shear strength of FRP-reinforced concrete slabs without shear reinforcements

This study aims to examine the punching shear strength of FRP-reinforced concrete slabs, which is a complex behavior affected by several mechanisms and many variables. In this study, assessment of selected strength models available in the literature using a simplified reliability analysis method showed the need for more accurate and consistent strength models. Thus, two machine learning (ML) models were developed and proposed. Both models accurately predict the strength compared to the available models, which provides an alternative method to the available ones. In addition, the effect of the main variables on the strength using the proposed models was compared to that using the available models, which provided an insight on the impact and interrelationship of effective variables on such a complex problem. Such insight can be helpful in future development of design codes.

Punching Shear Strength of Voided Slab: Literature Review and Evaluation of Design Codes

Despite of the economic advantages provided by the voided slabs, their design against punching shear failure, as suggested by the current building codes, are based on the equations derived from tests on flat slabs. Previous tests on voided slabs showed that the punching shear capacity of such slabs are different from that of flat slabs due to the differences in cross-sections. This paper reviews the previous works conducted to investigate the punching shear behavior of voided slabs. In addition, it evaluates the punching shear design provisions of the current building codes using the test results of voided slabs from literature. It was found that the punching shear design equations of the current building codes are inconsistent when they applied to estimate the punching shear capacities of voided slabs

Pre- and post-punching failure performances of flat slab-column joints with drop panels and shear studs

To investigate the pre- and post-punching failure performances of flat slab-column joints with drop panels and shear studs, static loading tests were conducted on three joint specimens subjected to in-plane restraints. The pre- and post-punching failure modes and load capacities of the joints were analysed. The tests identified successive punching failures at two critical regions (column perimeter and slab-drop panel interface) in all three joints. Compared with the specimen with a drop panel only, installing shear studs within the drop panels contributed to improving the punching shear strength. Although the post-punching capacity was not significantly improved, utilisation of shear studs effectively increased the ductility of the joints, rendering the punching failure around the column area more ductile. Punching shear strength validation against the international design codes reveals that they were conservative for the specimen with a drop panel only but overestimated for specimens with drop panels and shear studs.

On Punching Shear Strength of Steel Fiber-Reinforced Concrete Slabs-on-Ground

On Punching Shear Strength of Steel Fiber-Reinforced Concrete Slabs-on-Ground

Punching Shear Strength of Post-Tensioned Transfer Slab-Column Connections

Punching Shear Strength of Post-Tensioned Transfer Slab-Column Connections

Residual punching shear capacity of corroded reinforced concrete slabs

Steel reinforcement corrosion is becoming a critical issue due to the amount of old buildings still in operation with a high probability of substantial corrosion in their reinforcements. The strength and serviceability of corroded reinforced concrete (RC) slab is remarkably impacted by corrosion. Therefore, it is vital to understand more about the punching shear mechanism of slabs subjected to corrosion effects in order to make a good prediction regarding residual capacity. In this paper, critical shear crack theory (CSCT) was further developed to predict the residual punching shear strength of corroded RC slabs, considering the deterioration of material properties induced by corrosion. The effectiveness of modified CSCT was verified by comparison with the available experimental results. In addition, a validated finite element model that considers material deterioration and bond degradation was proposed to verify the accuracy of the proposed model. Finally, a parametric study of 240 cases was conducted to investigate the effect of some important factors on the punching shear behaviour of corroded RC slabs based on modified CSCT. The outcomes demonstrated that the residual punching shear capacity predicted by the proposed model is more accurate than existing design codes. The predicted failure modes also showed fairly good agreement with the experimental results.

Resistance of Punching Shear Strength of RC Flat Slabs According to Laboratory and Finite Element Analysis Study Comparison of International Codes

Resistance of Punching Shear Strength of RC Flat Slabs According to Laboratory and Finite Element Analysis Study Comparison of International Codes