Flexural Behavior and Design with High-Strength Bars and Bars without a Well-Defined Yield Point

Abstract

An ongoing project, NCHRP Project 12–77, was formed to evaluate and prepare appropriate revisions to the AASHTO load and resistance factor (LRFD) bridge design specifications and the AASHTO LRFD bridge construction specifications in reference to high-strength reinforcement as well as to other types of steel not having a well-defined yield plateau. Analytical and experimental studies are being conducted to identify and overcome deficiencies in the existing knowledge base. The focus of this study is on behavior and design of flexural members reinforced with high-strength reinforcement (ASTM A1035), as well as other types of steel without well-defined yield plateaus. Analytical formulations and experimental testing of full-scale beams are reported. For members with most commonly encountered amounts of longitudinal reinforcement ratio, flexural capacity can be established by using well-established strain compatibility analysis procedures in which the steel stress–strain behavior is idealized as being elastic–perfectly plastic, with the yield point taken as the stress at strain equal to 0.0035 or 0.005. For beams with reinforcement ratios larger than 3% and concrete strength exceeding 69 MPa (10 ksi), the use of stress corresponding to strain equal to 0.0035 is conservative and recommended. For high-strength reinforcement, the current strain limits of 0.002 and 0.005 should be changed to 0.004 and 0.008, respectively, for tension-controlled and compression-controlled members. Beams reinforced with high-strength ASTM A1035 bars exhibit adequate ductility and do not suggest any unexpected response characteristics.