Effect of Web Reinforcement on High-Strength Concrete Deep Beams

Abstract

Results of an experimental investigation on the behavior and ultimate shear strength of 18 high strength concrete deep beams are summarized. The concrete cylinder compressive strength f' c ranges from 55 to 86 MPa (approx. 8000 to 12500 psi). The test specimens are divided into three series based on the shear-span-to-overall-height ratio a/h. Each series consists of six beams with different arrangements of horizontal and vertical web reinforcements, i.e., the main variables are the horizontal and the vertical web steel ratios. Observations are made on mid-span deflections, crack widths, failure modes and ultimate strengths. The test results show that for deep flexural members with a/h exceeding 1.00 (or shear-span-to-effective-depth ratio a/d ≥ 1.13), the vertical web reinforcement is more effective than the horizontal web reinforcement. It is also shown that orthogonal web reinforcement comprising both vertical and horizontal reinforcements is the most efficient in increasing the beam stiffness, restricting the diagonal crack width development and enhancing the ultimate shear strength. The test results are then compared with the ultimate strength predictions obtained using the current ACI Code, the Canadian Code, and the UK CIRIA Guide. The deep-beam provisions in the ACI Code overestimate the contribution of the horizontal web steel to shear strength. Based on the test results, a revision to ACI Eq. (11-31) for web steel contribution is suggested. The Canadian Code shows the most consistent and yet conservative predictions of the test beams with different web reinforcements, while the UK CIRIA Guide is unconservative for beams with horizontal web reinforcement.