Behavior of slender beams without stirrups: effects of load distribution and member depth

Though uniform loading is common in structures, the vast majority of all shear strength tests on slender reinforced concrete members without stirrups have been performed using concentrated loading. Furthermore, the uniform load tests that have been conducted typically involve members with smaller specimen depths (d) and larger reinforcement ratios (ρ) than are commonly used in practice. Previous studies usually agree that a noticeable increase in shear strength can be expected when a specimen is subjected to uniform loading as opposed to concentrated loading. Six shear tests were performed on four slender beams without stirrups at The University of Texas at Austin. Two of the specimens had approximately double the effective depth (d) as the other two. For a given depth, two concentrated load tests were carried out on either end of one specimen, and one uniform load test was carried out on the second specimen. Thus, four reinforced concrete beams were used to perform a total of four concentrated load tests and two uniform load tests, with the objective of determining the influence of load distribution as member depth (d) increases. To ensure that a direct comparison could be made between each load distribution, the ratio between maximum bending moment and maximum shear force was maintained for all tests. Additionally, to provide consistency with typical design practice, the reinforcement ratio (ρ) was selected to match that of a typical beam. The experimental results presented an influence of load distribution opposite to that of previous studies, with a range of increase in shear strength at first diagonal cracking of concentrated load tests of -16 to 50 percent, with an average increase of 18 percent, over uniform load tests. Additionally, the tests with smaller effective depths (d) saw a percent increase in shear strength of 31 to 68 percent, with an average increase of 50 percent, over tests with larger effective depths (d).