Stress limits and minimum reinforcement requirements in shear design provisions

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AbstractThere are substantial differences in the maximum stress limits and minimum shear reinforcement requirements that are used in codes of practice. A database of 1287 shear test results was used to evaluate the requirements/limits that are used in the ACI318‐02 Building Code, the AASHTO‐LRFD Bridge Design Specifications, and Eurocode 2. Owing to concerns and uncertainties about the behaviour of members cast with high‐strength concrete, codes of practice place a limit on the maximum concrete compressive strength (f′c) that may be used in design relationships. This limit may be as low as 50 MPa. An examination of test data reveals that these maximum stress limitations may be unnecessary, except for members that are large, lightly reinforced in flexure, and which do not contain shear reinforcement.Codes also limit the maximum allowable shear stress in a member. In this case, the limitation is principally to guard against brittle compression failures. The shear stress limits employed in the three codes examined were found to differ by up to 2.5 times. This illustrates a lack of international agreement on fundamental issues related to shear behaviour. While only a limited amount of test data is available to evaluate the appropriateness of the selected limits, these data suggest that the ACI318‐02 limitation may be needlessly conservative.Minimum shear reinforcing requirements also differ substantially between these three codes of practice. The test data indicate that there is a small positive correlation of the shear strength ratio (Vtest/Vcode) with the amount of shear reinforcement and little to no correlation of this ratio with the spacing of the shear reinforcement.It is suggested that more accurate shear design provisions, minimum reinforcement requirements, and maximum shear limits will be achieved if researchers and code committee members work together to develop a qualified test database from which future research needs may be defined and from which improved provisions can be developed.