Retrofitting of flat plates against punching shear

Abstract

Punching shear failure is usually the governing failure mode of flat plate structures. Punching failure is brittle in nature which induces more vulnerability to this type of structure. The objective of the present study is to develop a three dimensional finite element model for determination of the punching capacity of flat plates before and after retrofitting and to verify the model developed with reference to experimental study carried out by various researchers. Another objective of the study is to investigate the punching and overall behavior of flat plate under cyclic lateral load. Twelve finite element models have been developed and verified with the experimental setup of Lips et al (2012). On the basis of the numerical simulation and experimentally available data, it can be concluded that the developed numerical model along with the material property can be used for analyzing flat plate where experimental setup is unavailable and expensive. Integrity reinforcement can be used to substantially increase the punching capacity as observed from the numerical data. Numerical simulation shows that provision of integrity reinforcement increase 19 percent of punching capacity and 101 percent of ductility. Developed numerical model can be used to quantify the influence of reinforcement ratio on the behavior of flat plate. Variation of slab thickness along with column reinforcement has noteworthy impact on punching capacity which is also presented in this study. Increase in slab thickness increases the punching strength and ductility. Based on the comparison between experimental and numerical results it can be concluded that base plate of stud type reinforcement contribute a significant role in carrying punching load. The developed finite element model with stud type transverse reinforcement without base plate can attain only 60 to 70 percent of the strength of the mechanical model with stud along with base plate. Inclusion of column reinforcement increases the punching capacity as well as ductility. Cages of continuous stirrup provide higher punching strength than conventional stirrup and inclined stirrup. Inclined stirrup also provides a significant punching strength and hence can be used for the retrofitting purpose.In the experimental investigation of the present study, eight half scale frame specimens with varying plate thicknesses and concrete strengths were tested under lateral cyclic loading to observe the punching capacity of flat plates. The samples were subjected to incremental cyclic loading provided by hydraulic jacks under constant axial or gravity load and their load-deformation behavior was measured by dial gauges and video extensometer. Finite element models have been developed and verified with the experimental result. Good correlations exist between the proposed and experimental results. Some useful design charts have been prepared by using the finite element model developed in the present study, for retrofitting design of flat plate deficient in punching shear.