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Abstract

Determination of the shear strength of prestressed concrete beams has become complicated in recent years. Much of the complexity comes from determining the concrete contribution (V_c) to the overall shear resistance. Because of the empirical nature of the calculation of V_c by various codes and specifications and because of the insignificant cost impact of shear reinforcement, this complexity is not warranted. A simplified shear design method is proposed. A detailed example is presented to demonstrate a design that is based on the current AASHTO Load and Resistance Factor Design (LRFD) Specifications and the proposed method. An important issue in the shear design of thin-web, precast-prestressed concrete members is the maximum allowed shear reinforcement. AASHTO Standard Specifications require that V_s not exceed 0.66 $\sqrt{f_{c}^{'}} b' d$ ( $f_{c}^{'}$ in MPa) or 8 $\sqrt{f_{c}^{'}} b' d$ ( $f_{c}^{'}$ in psi). This limit has become a controlling design factor when wide-beam spacing and high-performance concrete are used. It is almost always a controlling parameter in the design of posttensioned I-beams. Recent experiments have shown that (a) the AASHTO Standard Specifications limit is too conservative, and (b) the limit of 0.25( $\sqrt{f_{c}^{'}} b_{v} d_{v}$ ) on V_c + V_s in the AASHTO LRFD Specifications can be reached if adequate anchorage of the prestressing steel at member ends is provided. A summary of the full-scale test results is presented.

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Simplified Method for Shear Design Based on AASHTO Load and Resistance Factor Design Specifications

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