The shear strength of slabs with mechanically anchored shear reinforcement

Abstract

The use of reinforced concrete flat plates (slabs) in floor construction is often limited by the punching shear strength of the slab-column connection. The purpose of this study is to investigate the use of shear reinforcement, consisting of thin vertical bars with square anchor plates welded at both ends, as a means of improving shear strength and ductility of slabs. Eight specimens were tested, including one without shear reinforcement. Slab dimensions were 1900 x 1900 mm and 150 mm deep (effective depth d = 113 mm) and the column dimensions were 250 x 250 mm. Only axial forces were applied to the centrally located column. Test variables consisted of the number of shear reinforcement elements, the area of each element and the distribution of the elements within the slab. Two types of shear failure are possible; failure by yielding of the shear reinforcement adjacent to the column (under-reinforced for shear) or failure by exhaustion of the shear strength of the slab concrete on a section to the outside of the shear reinforcement, with little or no yielding of the shear reinforcement (over-reinforced for shear). The nominal concrete shear stress which allows the latter failure condition to occur was calculated using a design perimeter extending 'd/2' beyond the outer row of shear reinforcement and was found to be inversely proportional to the length of the design perimeter. The former failure condition depends on the yield force developed by the shear reinforcement within a distance 'd' of the column. Of the seven shear reinforced slabs, one failed after the shear reinforcement yielded; the remainder failed by punching along a section outside the shear reinforcement. A procedure for the design of a slab under-reinforced for shear is presented. The concrete shear strength is calculated using the current Canadian CSA Code equation for the nominal slab punching strength. The contribution by the shear reinforcement to the shear strength of a slab is determined by the elements located within 'd' of the column face, using the truss analogy to account for the effects of spacing and inclination of the failure surface. The ultimate shear strength of a thin slab can be increased by at least 33 percent over the current Code provisions for slabs containing mechanically anchored shear reinforcement so long as the shear reinforcement is suitably detailed. While slab ductility was improved by including shear reinforcement, full yield lines were not observed for any test.