Evaluation and Modifications of the AASHTO Procedures for Flexural Strength of Prestressed Concrete Flanged Sections

Different interpretations of the equivalent rectangular stress block approximation used by the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) and AASHTO Standard Specifications may lead to inconsistencies in the sectional response of the prestressed concrete flanged sections predicted by the two specifications. According to the LRFD specifications, the limits on the contribution of the compression flange overhangs causes the neutral axis to be located lower in the web of the section to satisfy the internal force equilibrium. This artificial overestimation of the neutral axis depth according to the LRFD Specifications as compared to the Standard Specifications, leads to inconsistencies in the provisions that depend on neutral axis depth, such as whether or not the section is over-reinforced, as well as differences in nominal moment capacity. In this paper, provisions of the AASHTO LRFD and Standard Specifications are compared to the results from the nonlinear strain compatibility analyses of prestressed concrete nonrectangular sections. The purposes of this research are to illustrate the discrepancies that exist in the sectional response predicted by these specifications and to identify design procedures that more accurately represent the actual behavior of such members. Modifications to the AASHTO LRFD procedure are proposed to correct for errors in determining the contribution of compression flange overhangs. Improvements in the accuracy of predicted sectional response and maximum reinforcement limits are demonstrated through a set of examples. Flexural strengths predicted by the specifications, the proposed modified LRFD procedure, and the strain compatibility analyses were also compared to measured flexural strengths of prestressed concrete I-beams found in the literature to validate the proposed modifications.