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

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 ( Vc) to the overall shear resistance. Because of the empirical nature of the calculation of Vc 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 Vs not exceed 0.66 [Formula: see text] ([Formula: see text] in MPa) or 8 [Formula: see text] ([Formula: see text] 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([Formula: see text]) on Vc + Vs 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.