Structural steel plate – grades for steel structures in five standards

A plate grade fixes four things: strength (minimum yield and tensile strength, which fall slightly as thickness increases), toughness (Charpy impact energy at a test temperature – the sub-grade such as J2, L0 or C), weldability (carbon equivalent) and, for thick plates in welded joints, through-thickness properties (Z-grades) that resist lamellar tearing.

Most steel structures use a ≈ 350 MPa grade – S355, A572 Gr 50, Grade 350, SM490 or Q355 – for main members and a ≈ 250 MPa grade for secondary plates. Higher strengths save weight on heavy members; weathering grades avoid painting in suitable climates.