The calculation of the flexural strength of concrete T-beams has been extensively discussed in recent issues of the PCI JOURNAL. The debate centers on when T-beam behavior is assumed to begin. The AASHTO LRFD Bridge Design Specifications (LRFD) maintain that it begins when c (distance from extreme compression fiber to neutral axis) exceeds the thickness of the flange. The AASHTO Standard Specifications for Highway Bridges (STD), and other references, contend that it begins when a (depth of equivalent rectangular stress block) exceeds the flange thickness. This paper examines the fundamentals of T-beam behavior at nominal flexural strength, and compares the results of LRFD and STD with more rigorous analyses, including the PCI Bridge Design Manual (PCI BDM) method and a strain compatibility approach using nonlinear concrete compressive stress distributions. For pretensioned T-beams of uniform strength, a method consisting of a mixture of LRFD and STD is investigated. For T-beams with different concrete strengths in the flange and web, the PCI BDM method is compared with the nonlinear strain compatibility analysis. High strength concretes (HSC) up to 5,000 psi (103 MPa) are considered. The selection of appropriate p factors and maximum reinforcement limits is also discussed. Comparisons with previous tests of T-beams are presented, and revisions to the relevant sections of LRFD are proposed.
