The American Concrete Institute (ACI) has released a paper called Constitutive Modelling and Overstrength Factors for Reinforcing Steel.
The paper presents probability density function parameters and a mathematical model to simulate the stress-strain behaviour of reinforcing steel bars under uniaxial monotonic loading at different rates.
ACI says model parameters were obtained from statistical analyses of tension test data and mill reports for historical and contemporary reinforcing steel used in US building construction. The reinforcing steel types include ASTM A15 intermediate grade; ASTM A615 Grade 60, 75, 80, and 100; ASTM A706 Grade 60 and 80; and ASTM A1035 Grade 100.
A critical assessment of current engineering practices related to material stress multipliers is presented based on results from the statistical analyses.
“Stress multipliers for each steel type at yield and two other strain demands are presented for the fifth percentile, mean, and 95th percentile, to facilitate calculating the lower-bound, expected, and upper-bound reinforcement stress at failure,” ACI adds.
According to ACI, these values are compared with stress multipliers specified in seismic and blast-resistant design guidelines.
“It was found that stress multipliers are dependent on reinforcing bar grade, and that current stress multipliers underestimate the expected reinforcing steel stress at failure of most commonly used reinforcement grades if the effects of bar slip are not significant,” ACI concludes.
This paper was written by Thomas J. Mander and Adolfo B. Matamoros.
