Critical Assessment of ACI 318 Eq. (18-3) for Prestressing Steel Stress at Ultimate Flexure

An attempt is made by the authors to explore the background and feasibility of Eq. (18-3) of the ACI Code 318-89. Advantages and disadvantages of the equation are discussed in detail, and results of laboratory testing conducted by the authors on 270-ksi low-relaxation strands for the Precast/Prestressed Concrete Institute also are included. A lower-bound curve based on 56 actual stress-strain curves is compared with other prediction methods, and an alternative approach for calculating prestressing steel stress at ultimate flexure is proposed. Recommendations are made to revise some of the existing Building Code and Commentary sections. Two design examples are presented to show the versatility of the iterative strain-compatibility method and to show the invalidity of Eq. (18-3) for use in some practical cases.