I believe the 85 represents the ultimate tensile strength of the bar in ksi. Do you design concrete reinforcing using ASD? I didn't think anyone did anymore so why do you need the "allowable"?
ASTM A615-85, A615 is the designation for billet steel, 85 is the year of designation such as 1985. A615 is available in 40, 60, qnd 75 ksi, with 60 ksi being used most often (due to availability).
Ultimate strength of any section of a reinforced conrete beam may be computed assuming the following:
Strain in the concrete is directly proportional to the distance from the nuetral axis.
Except in anchorage zones, strain in reinforcing steel equals strain in adjoining concrete.
At ultimate strength, maximum strain at the extreme compression surface equals 0.003 in/in.
When reinfrocing steel is not stressed to its yield (fy), the steel stress is 29,000,000 psi times the steel strain, in/in. After the yield strength has been reached, the stress remains constant at fy, though the strain increases.
