Small amounts of shear reinforcement are often assumed to increase the shear capacity of RC beams, compared to an identical beam without shear reinforcement. However, in a recent experimental campaign, the shear capacity of beams with a shear reinforcement ratio below the minimum requirements according to the design standards turned out to be similar to identical beams without shear reinforcement. This paper presents a detailed analysis of why the shear capacity may be similar for beams without-and beams with small amounts of shear reinforcement. This includes the influence of small amounts of shear reinforcement on the shear behaviour and shear-transferring mechanisms. The analysis shows that the crack development is more severe at the ultimate load for beams with a small amount of shear reinforcement compared to beams without shear reinforcement. This more severe crack development is shown to cause an overestimation of the shear contribution from aggregate interlock when applying a well-known constitutive model often used for beams without shear reinforcement. Therefore, a new expression for the aggregate interlock stresses is proposed. A comparison of the proposed expression with Mixed-Mode crack opening tests shows a good agreement with the test for both small and large crack openings. By applying the proposed expression on the measured crack kinematics it is shown that for a large shear contribution from aggregate interlock the shear contribution from the shear reinforcement is very limited and as the aggregate interlock stresses decrease the shear contribution from the shear reinforcement increases. This shift in the governing shear-transferring mechanism can help to improve the requirements for the minimum shear reinforcement often found in the design standards.Discussion on this paper must be submitted within two months of the print publication. The discussion will then be published in print, along with the authors' closure, if any, approximately nine months after the print publication.
<p>This paper deals with the influence of high axial tension on the shear strength of beams without shear reinforcement. An experimental program with shear-tension tests was carried out. The experimental results have been used to evaluate the applicability of the Eurocode 2 (EC2) design formula in cases with large normal forces. In addition, the experiments have been used to evaluate an extension of the plasticity based Crack Sliding Model (CSM) to cover cases with large normal forces. The test results show, that even in the present of very high axial tensile stresses and strains, the member is still able to carry significant shear stresses. The analysis reveals that the EC2 formula is over conservative in this regard.</p>
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