Shear Strength of Lightly Reinforced T-Beams in Negative Bending

Nine restrained T-beams were tested in a study of the negative moment region shear strength of lightly reinforced concrete T-beams. Variables included the longitudinal reinforcement ratio p, (0.47 percent and 0. 70 percent) and the nominal stirrup strength (0 to 84 psi [0 to 0.60 MPa]). The test results are compared with the shear provisions of "Building Code Requirements for Reinforced Concrete (ACI 318-83)." Both the concrete and stirrup contributions to shear strength are lower in negative than in positive moment regions. In both moment regions, the cracking shear is lower and the stirrup contribution is higher than predicted by A C/ 318. Current provisions are conservative for positive moment regions of beams with p, to ;;. 0.5 percent. The provisions are also conservative for negative moment regions in which the factored shear V, is less than or equal to the design shear strength of the concrete r/>V,. However, ACI 318 appears to be unconservative for negative moment regions of beams with p, .,; 0. 7 percent and V, > q, V,. The provisions may also be unconservative for joists with p • .,; I percent and ¢VJ2 .,; V, .,; q,V,.