Size effect in large prestressed concrete deep beams

This paper presents an experimental investigation on the size effect in large prestressed-concrete deep beams. Twelve specimens with a total main-steel-and-strand ratio p_c maintained close to 2.50% were tested to failure; two parameters were studied: viz. beam height h, ranging from 500 to 1750 mm, and shear-span-to-height ratio a/h, varying between 0.50 and 1.00. All of the beams had cylinder strengths of about 40 MPa; web reinforcement was omitted to study the effect of beam size on concrete shear strength. In comparison to reinforced-concrete deep beams, it is evident that prestressing improves the diagonal cracking and service-ability strengths and generally enhances ultimate shear capacity if bearing failure can be prevented. Test results reveal that the ultimate shear stress is size-dependent but the diagonal cracking stress is not. With increasing h, prestressed deep beams tend to be more brittle, even though the brittleness is somewhat offset by the location and degree of prestressing. The 12 test results are then compared with predictions based on the American ACI Code, the Canadian CSA Code, and the U.K. CIRIA. The CSA Code predictions provide uniform safety margin for large-and medium-sized beams, but conservatism in both the ACI and CIRIA predictions reduces with increasing h and a/h ratio. An explanation is also given for the uniform safety margin predicted by the CSA Code.