MAIN TENSION STEEL IN HIGH-STRENGTH CONCRETE DEEP AND SHORT BEAMS

Twenty-two reinforced concrete deep beams with cylinder compressive st rengths f(c)', generally exceeding 55 MPa (8000 psi) were tested under two-point symmetric top loading. Based on the main steel ratio rho(w) , the beams were organized into four groups with rho(w) = 2.00, 2.58, 4.08, and 5.80 percent. Web reinforcement comprising 10 mm (0.4 in.) d iameter plain mild steel stirrups at 300 mm (11.7 in.) centers was pro vided for all specimens, giving a vertical web steel ratio rho(v) 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 3.141. The comparisons among the series serve to highlight the influence of rho(w) and a/d ratio on the shear behavior of high stren gth concrete deep and short beams. it is shown that the a/h ratio (or equivalent a/d) dominates the failure modes while the beneficial effec t of rho(w) is more significant at the low end of a/d, say for a/d les s than or equal to 1.50. With a/d > 1.50, the influence of the main st eel ratio declines, except for the particularly high value of 5.80 per cent, 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 ACI predictions a re generally conservative, with the smallest standard deviation, thoug h with a/d > 1.50, a few cases are overestimated. The predictions from the Canadian Code are comparatively good, but the UK CIRIA Guide-2 es timations are generally unconservative, with the greatest scatter The study shows that the CIRIA Guide-2 predictions may be unconservative f or specimens with f(c)' greater than or equal to 55 MPa (8000 psi) and with rho(w) greater than or equal to 2.58 percent.