Shear strength design formula for RC beams with stirrups

The shear strength increase due to stirrup inclusion in reinforced concrete beams is analyzed. A number of 126 experimental values of this increase is obtained from the literature as difference between the shear strength of a beam with stirrups and that of the same beam without stirrups, which is called reference beam. On the basis of these values and by means of a statistical regression, a formula for the shear strength increase due to stirrup inclusion is worked out. The proposed formula shows that the stirrup effectiveness in increasing the beam shear strength strongly depends on the concrete strength, and that the dependence on the stirrup nominal shear strength is non-linear. A corresponding design formula is also proposed. Then, by adding this formula to the published expression for the shear strength of beams without stirrups, a general design formula for the prediction of the ultimate shear strength of beams with stirrups failing under flexure and shear is proposed. A comparison of the results obtained by means of this general formula and Eurocode and ACI Code formulas with 474 experimental values of beams with stirrups found in the literature is performed. The comparison shows that the proposed design formula is more consistent and reliable than Eurocode and ACI Code ones.