There are several grades of steel that were used in the construction of the World Trade Center. The most commonly used grade of steel was A36, which is a mild steel with a yield strength of 36,000 pounds per square inch (PSI). Other grades of steel used included A572, which has a higher yield strength of 57,000 PSI, and A992, which is a high-strength steel with a yield strength of 90,000 PSI.