Punching failure mechanism at edge columns of post-tensioned lift slabs

A test program designed to evaluate the punching shear strength of unbonded post-tensioned lift flat slabs at edge columns is described. Three isolated specimens, which represented a part of the edge panel in the vicinity of the column at full scale, were loaded to failure. The boundary conditions were controlled in order to accurately simulate the behavior of the prototype multipanel structure. The experimental investigation of one of the tested specimens was simulated using the non-linear FE analysis. A non-associated Mohr–Coulomb plasticity based formulation was applied for concrete in compression and a smeared crack approach for the tensile regime. Two different tension softening relations for concrete were used, brittle cracking behavior and linear tension softening. Comparison with test results showed that brittle cracking was more appropriate model of concrete post-cracking behavior for punching problem in this particular case. On the basis of numerical and experimental results, the attempt was made to explain the failure mechanism of the slab-edge column connections. Both test and measured results showed that punching failure was primarily a failure by splitting of concrete, with crushing of concrete as a secondary phenomena. It was concluded that the key to the explanation of the mechanics of punching failure is a proper modeling of the triaxial conditions that govern the strength of the area of the slab in the immediate vicinity of the edge column.