Check out appendix four of the fourth edition AISC Manual, Structural Design for Fire Condition.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
The ASME Boiler and Pressure Vessel Code includes temperature limitations and stress reductions for plate commonly used for boiler and vessel construction (A36=SA36 is included, I don't believe A572 Gr. 50 is). I mention this because I assume the reference above is targeted to short-term conditions, not sustained conditions. Anyway, you can have cases where long-term exposure results in metallurgical/chemical changes or where creep is the controlling factor rather than yield. A36 and A516-70 include ranges up to 650-1,000 degrees F, some of the stainless steels can be used up to 1,500 degrees F or so. In most cases, when you near the upper temperature range for the material, allowable stress will be very low, so if you want the maximum temperature without any reduction in design strength, that's a different question.
