How does the yield strength of angles vary with thickness in GB Q355 vs. ASTM A572 Grade 50?

Both GB Q355 and ASTM A572 Grade 50 exhibit a decrease in specified minimum yield strength as the material thickness increases, a phenomenon known as strength de-rating. For GB Q355B, the yield strength is 355 MPa for thicknesses (t) ≤ 16mm. It decreases to 345 MPa for 16mm < t ≤ 40mm, 335 MPa for 40mm < t ≤ 63mm, and 325 MPa for 63mm < t ≤ 80mm. Similarly, for ASTM A572 Grade 50, the yield strength is 50 ksi (345 MPa) for shapes up to 4 inches (101.6mm) in thickness. For plates over 4 inches, the strength requirement drops. This parallel behavior is crucial for substitution. A designer must ensure that for a given angle thickness, the specified minimum yield strength of the substitute material meets or exceeds the design requirement. A simple 345 MPa vs. 345 MPa comparison is only valid for a specific thickness range; outside of that, the equivalence may break down and require re-calculation.