Title: Punching Shear Strength in Concrete Slabs

Author(s): Lionello BortolottiI

Publication: Structural Journal

Volume: 87

Issue: 2

Appears on pages(s): 208-219

Keywords: concrete slabs; failure mechanisms; loads (forces); plastic theory; punching shear; shear strength; tests; yield strength; Structural Research

DOI: 10.14359/2717

Date: 3/1/1990

Abstract:
A theoretical solution for the punching shear strength of concrete slabs is presented. The problem is treated as three-dimensional axisymmetrical, the material to be assumed rigid plastic. A Coulomb yield criterion for concrete as modified by the author in earlier work is introduced in the present paper. By applying the theory of plasticity, the form of the failure surface generatrix visualizes processes of strain softening by tension and compression in concrete. Moreover, by applying the virtual work equation, an upper bound solution for the failure load is obtained. A variational procedure is then employed to evaluate the minimum of this bound, through an analysis that also provides the shape of the failure surface of the slab. A comparison with the experimental results in literature shows that the theoretical equations are valid as long as the slabs are rigid enough to prevent displacements of the border of the slab, depending on the entity of the relevant parameters. If this does not happen and border displacements and rotations are allowed (e.g., in simply supported slabs), the theoretical values disagree with the experimental. This fact means that effective failure mechanisms are different from the one assumed in the present paper. The evidence of some linearities of theoretical to experimental load ratio in function of the relevant parameters of slabs leads to corrective mechanism factors for theoretical failure load. This avoids searching for the appropriate failure mechanism. Some of these expressions are suggested here in view of providing punching failure loads in agreement with the experimental results examined in this paper.