1.1 ,This specification covers alloy steel in bars, plates up to and including 4 in. [100 mm] in thickness and shapes of structural quality with improved yield strength at high temperature. Two grades, 36 [250] and 50 [345] are available for use in bolted or welded buildings or for general structural purposes. Class 2 requires a maximum yield to tensile ratio –, this ratio is not required for Class 1.