Punching shear in waffle slabs in the presence of biaxial moment transfer

An extensive amount of works have been carried out to develop the current understanding in punching shear mechanism noted in reinforced concrete slabs. However, despite the increasing popularity of waffle slabs, the current understanding about punching behaviour is mainly focused on solid flat slabs, and only limited amount of works have been carried out on waffle slabs and in the presence of biaxial moment. Thus, there is a need to carry out a research in this area to aid the understanding about punching mechanism of waffle slabs in the presence of biaxial moment for the internal column and edge column connections. The experimental work carried out in this research included destructive testing of thirty-eight 1/10th scale model waffle slab specimens, which consists of fifteen internal column slabs and twenty-three edge column slabs. The main variables were, for the internal column slab, the principle angles of biaxial moment transfer, the column eccentricity, the column orientation and the size of solid sections, and for the edge column slab, the principle angles of biaxial moment transfer, the column eccentricity, the column location and the size of solid sections. From the experimental investigations, three distinct failure mechanisms were observed: the concentric punching at internal column mechanism; the eccentric punching at internal column mechanism; and the edge punching mechanism. In general, the observed punching shear failure mechanisms of waffle slabs were found identical to solid flat slabs; but the punching shear capacities reduced due to some losses in potential failure surface within the waffle section. The principle angle of biaxial moment transfer was found varying the shear surface area that was being mobilized, thus affecting the punching capacity of the slabs. An analytical study was carried out, using an upper-bound plastic model, to simulate the observed punching shear mechanisms, and hence, to predict the punching capacity of the slabs. A theoretical model was developed for each of the identified failure mechanism. In addition, three design models based on the current UK code, Eurocode 2, have been developed. In all cases, these models have achieved good agreements with the test results