Non-symmetrical punching of flat slabs and slab bridges without transverse reinforcement
Punching of flat slabs without transverse reinforcement has mostly been investigated in the past for slabs with equal reinforcement ratios in the two directions and loaded under axis-symmetrical conditions. However, in practice, slab bridges as well as many flat slabs have different span lengths and reinforcement ratios along the two principal directions. For such cases, where punching shear is typically the governing design criterion for the ultimate state, two major differences with respect to axis-symmetrical slabs are found. Firstly, the shear forces developed in a flat slab with different span lengths might lead to concentrations of shear stresses near the column region, which in turn can reduce the punching shear strength. Secondly, the larger width of the flexural cracks along one direction of the slab with respect to the other direction can influence the punching shear strength, since the capacity to transmit shear forces is reduced as crack width increases. In this paper the phenomenon of non-symmetrical punching shear is revised according to the Critical Shear Crack Theory (CSCT) and compared with other design approaches suggested in codes such as EC2, BS8110 and ACI-318. The results of an experimental series of 7 tests (3×3×0.25m) carried out at Ecole Polytechnique Fédérale de Lausanne (EPFL) on non-symmetrical conditions and with various concrete strengths and reinforcement ratios are presented. A comparison between the experimental results and the different theoretical models is finally introduced. Two approaches are presented with respect the CSCT, in order to estimate the required load-rotation response of the tests; firstly by means of approximate design formulas and secondly with a more refined non-linear finite element analysis using bending shell elements. Both approaches can provide accurate predictions of the ultimate strength and ductility when used in combination with the failure criterion of the Critical Shear Crack Theory.