Defining Yield Strength for Nonprestressed Reinforcing Steel

Analytical strengths of reinforced concrete beams and columns based on reinforcing steel stress-strain curves with and without a sharp yield plateau and the nonlinear stress-strain behavior of concrete are compared with strengths calculated in accordance with the ACI Building Code to determine the feasibility of using the 0.2% offset method to define the yield strength of reinforcing steel. Comparisons include steel yield strengths of 60,000 and 80,000 psi (420 and 550 MPa) and concrete compressive strengths of 5000, 8000, and 12,000 psi (34, 55, and 83 MPa). For beams with reinforcement ratios below three-fourths of the balanced ratio, the stress-strain curves based on the 0.2% offset method produce analytical strengths greater than or equal to those corresponding to the provisions of the ACI Code. For columns with total reinforcement ratios of 1 or 2%, the lowest ratio of analytical to Code-calculated strength for reinforcement with yield strength defined based the 0.2% offset method is 97%. The ratio drops with increasing reinforcement ratio, with the lowest value, 93%, occurring at total reinforcement ratios of 6 to 8%. The lower ratios occur for combinations of moment and axial load where the strength reduction factor is 0.65 for tied columns and 0.75 for spirally reinforced columns, thus maintaining an adequate margin of safety. The results justify use of the 0.2% offset method to define the yield strength of reinforcing steel.