Main Tension Steel in High Strength Concrete Deep and Short Beams

Abstract

Twenty-two reinforced concrete deep beams with cylinder compressive strengths f 1 C generally exceeding 55 MPa (8000 psi) were tested under two-point symmetric top loading. Based on the main steel ratio ρ w the beams were organized into four groups with ρ W = 2.00, 2.58, 4.08, and 5.80 percent. Web reinforcement comprising 10 mm (0.4 m.) diameter plain mild steel stirrups at 300 mm (11.7 in.) centers was provided for all specimens, giving a vertical web steel ratio ρ ν of 0.48 percent. The beams were tested for different shear span-to-overall-height ratios a/h, ranging from 0.25 to 2.50 (equivalent to a/d from 0.28 to 3.14). The comparisons among the series serve to highlight the influence of ρ w and a/h ratio on the shear behavior of high strength concrete deep and short beams. It is shown that the a/h ratio (or equivalent a/d) dominates the failure modes while the beneficial effect of ρ w is more significant at the low end of a/d, say for a/d 1.50, the influence of the main steel ratio declines, except for the particularly high value of 5.80 percent, where the relative increase in shear strength due to main steel remains constantly high, regardless of a/d. The test results are then compared with predictions based on the current ACI Code, the Canadian Code, and the UK CIRIA Guide-2. It is shown that the ACT predictions are generally conservative, with the smallest standard deviation, though with a/d 1.50, a few cases are overestimated. The predictions from the Canadian Code are comparitively good, but the UK CIRIA Guide-2 estimations are generally unconservative, with the greatest scatter. The study shows that the CIRIA Guide-2 predictions may be unconservative for specimens with f' C ≥ 55 MPa (8000 psi) and with ρ W ≥2.58 percent.