Sometimes when A572 Grade 50 is a requirement on plates we use A36 when the tensile and/or yield meets Grade 50 criteria (50 ksi min. yield and 65 ksi min. tensile, compared to 36 ksi min. yield and 58-80 ksi min. tensile for A36). Many times the tensile properties of A36 meet the tensile properties of Grade 50, but the yield properties do not. If the yield properties are close, the engineer can evaluate whether the lesser yield will be sufficient. We have been able to get this approved many times, because we have found that sometimes the design requirements for yield fall somewhere between A36 and Grade 50. Being that it is a higher yield than A36, Grade 50 is specified. Sometimes Grade 42 can be used, but it is not as readily available as Grade 50 is.

Tim,
Years ago, Grade 50 was an extra cost. With an engineers approval, we used to substitute on wide flange beams too to keep from paying the extra charge for specifying Grade 50. Now, A36 and Grade 50 have been or are in the process of being phased out in favor of A992.