Governing Failure Mode of Reinforced Concrete Flat Slabs Under Gravity Load Using the Codes Guidelines and Yield-Line Theory

Nowadays, reinforced concrete (RC) flat slabs frequently known as a gravity-load resisting system are being used in almost every building structure due to a number of distinct advantages. On the minus side, the greatest drawback of the RC flat slabs is punching shear failure still hotly debated by researchers. Nevertheless, there is not a source of information to accurately predict the governing failure mechanism of the slabs concerning the severe shortage of a certain hypothetical method. This study was targeted at investigating the governing failure mode of the slabs monotonically subjected to vertical load by reviewing the recommendations of the current codes, i.e., ACI 318-19, EC 2, MC2010, also by using the yield line theory (YLT), as was foremost amongst the literature. For this purpose, the results numerically assessed by the punching shear equations of the codes were compared to those of experimental tests categorized in a database with 88 RC flat slabs. To predict the failure mode of the slabs, the punching shear strength obtained by the codes was compared to the flexural capacity calculated by the YLT. Finally, two case studies on RC flat slabs were proposed to elucidate the changes of shear load-carrying capacity and flexural strength while the flexural reinforcement ratio differed, as were to predict the governing failure mechanism. The slabs’ geometry was identical; one slab was served as the control specimen while the other had four openings located adjacent and parallel to the column. Shear reinforcement was, moreover, employed as a key parameter for improving the punching shear strength of the slabs at the rate of 50%. It was proved that the presented failure-mode prediction methodology is applicable to the design purposes of the slabs by comparing the predicted failure modes with the experimental counterparts. According to the case studies, the governing failure mode of 50%-shear-reinforced slab was punching shear as the flexural reinforcement ratio was more than 0.7% while the failure mode was flexural for lower flexural reinforcement ratios.