Finite element study of RC slab in punching shear

Abstract

Current design procedures and provisions for preventing punching shear failure of reinforced concrete slabs, given in various codes of practice, are largely based on studies of the behavior and strength of simply supported, small sized conventional specimens extending to the nominal line of contra-flexure. Present day codes of practice usually do not consider the effect of boundary restraint against rotation. The contribution of flexural steel reinforcement is ignored by some of the code provisions. Also, the effect of slab thickness and column size are not considered for calculating punching shear capacity of slab in some codes. A finite element study of punching shear behavior of reinforced concrete slab is presented in this thesis. The numerical simulation is based on previous experimental study of 15 reinforced concrete model slabs. Finite element analysis of reinforced concrete slabs subjected to load producing punching shear is evaluated and the validity of FE analysis has been verified through comparison with available experimental data from other researchers as well. It has been shown that the load vs. deflection diagrams and ultimate load capacities obtained from FE analysis closely agree with the experimental results. Comparison of crack pattern of the slab also shows good agreement between experiment and numerical prediction. It has been shown that using appropriate method of solution and material model for numerical simulation, significant benefit can be achieved employing finite element tools and advanced computing facilities in obtaining safe and optimum solutions without doing expensive and time-consuming laboratory tests. Following the establishment of the validity and reliability of the FE modeling scheme, a parametric study has been carried out to investigate the influence of the flexural reinforcement on ultimate load capacity of slabs. Code-specified strength of the specimen was calculated in accordance with the American, British, Canadian, European, German and Australian codes. It has been observed from the study that punching shear capacity may not be efficiently predicted in some of the codes. The study is then extended for reinforced concrete multi-panel flat plates subjected to punching shear. The study involves employment of a nonlinear material model in finite element method of analysis based on past experimental investigations which provides solution for realistic behavior of reinforced concrete slabs with punching shear for concrete strengths, flexural reinforcement ratio, slab thickness and column size. It has been observed that depending on the degree of variation in these parameters, the overall behavior of RC slab with punching shear changes significantly. A proposal for a reasonable estimate of punching shear capacity of flat plate has been made in this thesis based on the findings of parametric study. The proposal includes the effect of flexural reinforcement in addition to concrete strength in calculating the punching capacity. The size effect of slab and column on punching shear behavior of flat plate is also included in the proposal. The punching shear capacity calculated using the proposed method is compared with the results of nonlinear finite element analysis and values from different codes equations and have been found to be in good agreement. It is expected that the findings of this study would result in a more rational design of structural floor systems where concrete punching phenomenon plays an important role.