Diagonal cracking and ultimate shear strength of slender high strength concrete deep beams

To provide more information on the diagonal cracking in slender concrete deep beams and their design implications, loading tests were carried out on fifteen (15) slender concrete deep beams having height/thickness ratio in the range of twenty (20) to fifty (50). Due to the slenderness of the specimens, high strength concrete was used together with different arrangements and quantities of web reinforcement. Observations and measurements were made on the nature of cracking in general and on the diagonal cracks in particular and comparisons were made with those of stocky deep beams as reported in the literature. The effectiveness of web reinforcement in controlling diagonal cracks and enhancing the ultimate stength is discussed. The failure modes were described and both the diagonal cracking load and the failure load were recorded for comparison with those predicted by the CIRIA guide and by the ACI building code. Using the test results and the observations made during the testing, a modification of the CIRIA shear equation is proposed so that it can be used to predict safely the diagonal cracking loads and the shear strengths of slender concrete deep beams. With this modification, the safety factor (V-measured/V-predicted) approches the value of 2.0 which is what one would expect against a shear failure, particularly that high strength concrete tends to be a fairly brittle material. The CIRIA equation in its original form gives a safety factor just above 1.0, thus unsufficient against such failure. Indeed, the test results show that it may even be unsafe for the design of slender high strength concrete deep beams against shear.