A theory of failure for reinforced concrete beams without web reinforcement subjected to bending and shear.

This thesis presents an investigation into the failure of reinforced concrete beams without web reinforcement under the action of bending and shear. A total of 39 singly-reinforced concrete beams of various cross-sections were tested with two concentrated loads. The test specimens and the instrumentation were designed to yield detailed information on shrinkage strains, and the load-induced strains in concrete prior to and after the formation of diagonal tension cracks. The behaviour of the beams up to the failure load was carefully observed and studied. The mode of formation and propagation of cracks, and the failure surfaces were given special attention. The influence of bond between concrete and the tensile reinforcement and the contribution of the "dowel force" to shear resistance was considered. Quantitative analyses show that shear failure is not a bond failure and that the "dowel force" is a variable parameter which can assume any value including zero. From the analysis of the computed strains and the observed phenomena, a simple and rational expression is presented for the failure in shear of reinforced concrete beams subjected to concentrated loads. It is thought that the equation represents more accurately the behaviour of reinforced concrete beams under bending and shear than other available equations. The proposed equation was applied to other test data from different authors and the results are compared with the data obtained from this project. Errors inherent in the determination of diagonal cracking loads and the unreliable nature of visual observations are discussed. It is thought that this investigation and analysis will lead to a better understanding of the failure of reinforced concrete beam in shear, and therefore to a more rational basis for the design of web reinforcement. The nominal shear stress at the formation of diagonal cracks and at shear failure is compared with the permissible shear stress in beams without web reinforcement, recommended by the British and other Codes of Practice, It was observed that these permissible stresses which are currently in use are too low particularly for beams with shear-span to effective depth ratio of less than 3. It is therefore suggested that the nominal shear stress based on the external diagonal cracking reaction be considered as the permissible stress for beams without web reinforcement. This will ensure economy and safety even under the more severe loading conditions obtained in practice.