Structural flat steel must meet specific yield strength minima to ensure load-bearing capacity. In European standards (EN 10025), S235 grade requires ≥235 MPa, while S355 demands ≥355 MPa. ASTM A36 (U.S.) specifies ≥250 MPa, and China's GB/T 700 mandates ≥235 MPa for Q235. These values represent the stress at which permanent deformation begins, ensuring the steel can support static and dynamic loads in buildings, bridges, and machinery without excessive deflection. Higher-strength grades (e.g., S460 with ≥460 MPa) are used in weight-critical applications, reducing material thickness while maintaining structural integrity. Yield strength is measured via tensile testing, with samples required to elongate by 0.2% under load before permanent deformation occurs.