Interactive Mechanical Model for Shear Strength of Deep Beams

The current American Concrete Institute code states that the nominal shear strength Vn of a reinforced concrete deep beam consists of the concrete contribution Vc and shear reinforcement contribution Vs, i.e., Vn=Vc+Vs. In this paper, however, a very different yet original approach is presented. The method is based on strut-and-tie approach, with the effect of transverse tensile stresses on concrete compressive strength of the diagonal strut properly accounted for. Two common failure modes, namely, diagonal splitting and concrete crushing, are examined in the paper. Premature failures such as shear tension failure (due to insufficient anchorage of main longitudinal reinforcement) and bearing failures are not considered. The resistance to diagonal splitting is mainly provided by the main and shear reinforcement. Additional resistance from concrete tensile strength is included in the analysis. The resistance to crushing of concrete is contributed by the concrete compressive strength. Ultimate shear strengths of deep beams are governed by both the transverse tensile stresses perpendicular to the diagonal strut, and the compressive stresses in the diagonal strut, resulting in an interaction between the two failure modes. Predictions by the proposed modal are compared with experimental results and other established calculation methods. Generally, the predictions are not only accurate and consistent in each case study, but also conservative.