Punching Shear Strength Prediction for Reinforced Concrete Flat Slabs without Shear Reinforcement

Failure of flat slabs usually occurs by punching shear mode. Current structural codes provide an experience-based design provision for punching shear strength which is often associated with high bias and variance. This paper investigates the effect of adding a horizontal reinforcement mesh at the top of the slab-column connection zone on punching the shear strength of flat slabs. A new equation considering the effect of adding this mesh was proposed to determine the punching shear strength. The proposed equation is based on the Critical Shear Crack Theory combined with the analysis of results extracted from previous experimental and theoretical studies. Moreover, the equation of load-rotation curves for different steel ratios together with the failure criterion curves were evaluated to get the design points. The investigated parameters were the slab thicknesses and dimensions, concrete strengths, size of the supporting column, and steel ratios. The model was validated using a new set of specimens and the results were also compared with the predictions of different international design codes (ACI318, BS8110, AS3600, and Eurocode 2). Statistical analysis provides that the proposed equation can predict the punching shear strength with a level of high accuracy (Mean Square Error =2.5%, Standard Deviation =0.104, Mean=1.0) and over a wide range of reinforcement ratios and compressive strengths of concrete. Most of the predictions were conservative with an underestimation rate of 12%. Doi: 10.28991/CEJ-2022-08-01-013 Full Text: PDF

Influence of the Shear Cap Size and Stiffness on the Distribution of Shear Forces in Flat Slabs

The scope of this paper is to investigate analytically and numerically the influence of shear cap size and stiffness on the distribution of shear forces in flat slabs in a slab–column-connections-reinforced concrete structure. The effect of support (shear cap) stiffness on the calculation of the length of the shear control perimeter according to the available methods is presented. Based on the analysis, the authors indicate in what range of support stiffness the corner concentrations become important in the calculation of the punching resistance. For shear caps with high flexibility (α1 ≤ 0.5), the concentration of internal forces in the corners does not occur. The authors compare the numerical results obtained from the calculation methods and indicate the correlations, which can be useful guidance for structural designers. In the case of large shear caps, the simplified MC2010 method gives a significantly lower value of the effective control perimeter length compared to more accurate methods. This paper is intended to provide scientists, civil engineers, and designers with guidelines on which factors influence punching shear load capacity of the slab–column connections with shear caps.

Cooling of the continental plate during flat-slab subduction

Most flat-slab subduction regions are marked by an absence of arc volcanism, which is consistent with closure of the hot mantle wedge as the subducting plate flattens below the continent. Farther inland, low surface heat flow is observed, which is generally attributed to cooling of the continent by the underlying flat slab. However, modern flat slabs have only been in place for <20 Ma, and it is unclear whether there has been sufficient time for cooling to occur. We use numerical models to assess temporal variations in continental thermal structure during flat-slab subduction. Our models show that the flat slab leads to continental cooling on timescales of tens of millions of years. Cool slab temperatures must diffuse through the continental lithosphere, resulting in a delay between slab emplacement and surface cooling. Therefore, the timescales primarily depend on the flat-slab depth with shallower slabs resulting in shorter timescales. The magnitude of cooling increases for a shallow or long-lived flat slab, old subducting plate, and fast convergence rates. For regions with flat slabs at 45–70 km depth (e.g., Mexico and Peru), shallow continental cooling initiates 5–10 Ma after slab emplacement, and low surface heat flow in these regions is largely explained by the presence of the flat slab. However, for the Pampean region in Chile, with an ~100-km-deep slab, our models predict that conductive cooling has not yet affected the surface heat flow. The low heat flow observed requires additional processes such as advective cooling from the infiltration of fluids released through dehydration of the flat slab.

Application of critical shear crack theory on punching of flat slabs

This article deals with the punching capacity of a flat slab fragment supported by an internal atypically elongated column. Based on the results of this analysis and the application of Critical Shear Crack Theory, the reliability of two design models was determined. The CSCT model is a mechanical model where the shear force transferred by concrete in shear crack can be determined by accounting for the roughness and opening of a critical shear crack. The crack width is proportional to the slab rotation, which was obtained from a nonlinear program Atena and from experimental test and shear capacity was obtained by integrating the shear strength along the control perimeter. The aim of this analysis was to compare the application of CSCT in non-linear analysis and experimental test to point out the significant difference between obtained results, which shows the importance of experimental tests realization. Non-linear analyses provided unsafe results. Contrary the currently used EC2 model provided safe results when reduction of the control perimeter was applied. The best results were obtained in a combination of the CSCT model with measured rotations of the slab specimen.