Influence of Effective Depth on Shear Strength of Concrete Beams—Experimental Study

Abstract

Laboratory tests of reinforced concrete beams without shear reinforcement have shown that the shear strength (in terms of average shear stress) decreases as the size (depth) of the member increases. This paper discusses the results of experimental research performed to test the hypothesis that the effective depth influences the shear strength of reinforced concrete flexural members that do not contain web reinforcement in the range of overall depth between 12 to 36 in. (610 to 900 mm) where ACI 318-08 does not require skin reinforcement. The results of tests on eight simply supported reinforced concrete beams without shear and skin reinforcement are described, discussed, and correlated herein. The longitudinal reinforcement ratio was approximately 1.25%. The target concrete compressive strength was 10, 000 psi (70 MPa). The beam width varied between 8 and 24 in. (203 and 610 mm). All of the beams were simply supported and monotonically loaded in increments at midspan up to destruction. The shear span-depth ratio was maintained at 3.0. Test results show a reduction in shear strength with increasing effective depth; however, significant differences in behavior were observed between the 12 in. (305 mm) specimens and the larger specimens in terms of the amount of flexural cracking, crack progression, load-displacement, and load-strain measurements despite holding other traditionally considered influential parameters constant. These differences suggest that the reduction in shear strength was influenced not only by a size effect but also by differences in behavior and mode of shear transfer at failure (beam action versus arch action). For the beams tested in this study, flexural crack spacing did not scale with beam size. The change in ACI 318-08 restricting isolated beams without minimum shear reinforcement to heights not greater than 10 in. (250 mm) is supported by the findings of this study.