Nonlinear finite element analysis of reinforced concrete plates in punching shear

Abstract

Punching shear is one of the most critical phenomena for flat plate building systems due to the brittle nature of this failure mode. The region of a slab in the vicinity of a column could fail in shear by developing a failure surface in the form of a truncated cone or pyramid. This type of failure, called a punching shear failure, is usually the source of collapse of flat plate and flat slab buildings. It is one of the topics of intensive research in recent years by various researchers in the field of concrete structure. Numerous tests have been carried out to evaluate the punching shear strength of slabs. Several theories have been put forward to predict the strength observed in these tests. An attempt has been made in this research to study the punching shear of reinforced concrete flat plates. Before carrying out the systematic computational investigation, existing literature on the relevant field based on experimental investigation, analytical methods, numerical models and various Codes of Practice is thoroughly reviewed. A 3-dimensional reinforced concrete flat plate model was generated. The formulation has been checked against experimental results of tests on slabs subjected to punching shear. From a comparison of the numerical and the experimental results, it is concluded that the adopted model of the Finite Element Analysis package (ANSYS)is suitable for analyzing reinforced concrete plates. The good agreement obtained between the numerical and the experimental results establish the validity and accuracy of the computational model. A systematic study is carried out to determine the punching shear capacity due to variations of different structural parameters like span-depth ratio, reinforcement ratio, support condition, shear reinforcement etc. The parametric study indicated that the analysis is able to predict the influence of several of the most commonly considered parameters on the punching shear behaviour of slabs. From the graphical presentation of the various parameters a designer can visualize and choose the most suitable one to enhance the punching shear strength. The results of the analysis confirm the general understanding of the behaviour of slabs in punching shear . .It was seen that the addition of shear reinforcement has a significant effect on the punching shear strength of the slab/plates. The models with shear reinforcement showed a more ductile behavior than those without shear reinforcement. The results indicate that the inclined stirrups are technically superior. These are also very easy to install, and would thus seem to be a promising form of shear reinforcement for flat plates. This study shows that both span-depth ratio effects and the type of support condition have significant influence on the punching shear strength in concrete slabs. Size effect and reinforcement ratio effects are not considered in ACI 318- 02 Code (also BNBC 1993) provision; as such there is scope for review of these codes taking all the parameters into consideration. Hence, a modification to the ACI 318-02 Code equation is proposed and the performance of the proposed modification is verified. Finally, based on the findings of the study, conclusion and recommendations are put forward.