Influence of size on punching shear strength of concrete slabs

The punching shear behaviour of interior slab-column connections in flat plates is investigated. The response of six two-way slab specimens, which were designed such that they would fail in punching shear, are presented. The parameter introduced in the experimental program is member depth. The effects of this parameter on the punching shear capacity of slab elements are investigated. The results show a strong "size effect", with deeper members having a smaller shear stresses at failure than shallow ones.Test results obtained from this experimental program are compared with the punching shear predictions of the Canadian CSA A23.3-94 Standard, the American ACI 318-95 Code, the British BS 8110-85 Standard and the European CEB-FIP 1990 Model Code. Predictions were also made using computer program "Response 2000", assuming an equivalent beam analogy to represent the slab. It is concluded that the shear design of slabs, according to the current Canadian and American codes, can be unconservative under certain conditions, particularity for thick slabs. It is recommended that the punching shear expressions of the CSA Standard and the ACI Code be modified to take into account the "size effect" on the punching shear strength of slabs