A: The mechanical properties are the ultimate expression of the chemical and processing differences between these grades. They are rigorously defined in ASTM A572 and form the basis for all structural design calculations. The following table provides a comprehensive comparison for material up to 4 inches (100 mm) in thickness, which covers most H-beam applications.
| Mechanical Property | ASTM A572 Grade 50 | ASTM A572 Grade 60 | Engineering Implications |
|---|---|---|---|
| Yield Strength (Fy), min | 50 ksi (345 MPa) | 60 ksi (415 MPa) | This is the defining property. Yield strength is the stress at which the material begins to deform plastically (permanently). The 60 ksi grade allows designers to use approximately 20% higher allowable stress in bending, shear, and axial load calculations. This directly translates to smaller required cross-sectional areas for the same load, or higher load capacity for the same size member. |
| Tensile Strength (Fu), min | 65 ksi (450 MPa) | 75 ksi (520 MPa) | Tensile strength is the maximum stress the material can withstand while being stretched before necking and fracture. The Fu/Fy ratio is important: ~1.30 for Gr50, ~1.25 for Gr60. This ratio indicates the "reserve" strength beyond yield before failure. A higher ratio (as in Gr50) can be beneficial for seismic design where ductility and energy absorption are critical, as it allows for more plastic deformation. |
| Elongation in 2 inches (50 mm), min | 21% | 18% | Measured in a standard tensile test, elongation is a key indicator of ductility-the material's ability to deform plastically before fracture. The 3% reduction for Grade 60 is a direct consequence of its higher strength. It signifies marginally less capacity for inelastic deformation, which engineers must account for in detailing connections and considering failure modes. It does not imply brittleness; 18% is still a ductile material. |
| Bending Properties | Excellent. Bend test requirements ensure the material can be bent cold to a specific radius without cracking. | Very Good. Similar bend tests apply, but the slightly lower ductility may necessitate slightly larger bend radii for very sharp bends in fabrication shops, though this is rarely an issue for standard H-beam connections. | |
| Modulus of Elasticity (E) | ~29,000 ksi (200 GPa) | ~29,000 ksi (200 GPa) | Unchanged. This fundamental property, defining stiffness (stress/strain in the elastic region), is identical for all structural steels. A Gr60 beam of the same dimensions as a Gr50 beam will deflect the same amount under the same load, until the yield point is reached. The benefit of Gr60 is that it reaches its higher yield point at a smaller deflection. |
| Notch Toughness | Not specified in base A572. Can be supplied to supplemental requirements (e.g., S5, Charpy V-Notch) if required for low-temperature service. | Same as Grade 50. | For standard building applications in normal climates, impact testing is not mandated. For bridges, exposed structures, or cold regions, purchasers must explicitly specify supplemental toughness requirements (e.g., CVN at 40°F or 0°F), which the mill will meet through controlled chemistry and rolling. |
This mechanical profile clearly delineates the choice: Grade 50 for optimal balance and ductility, Grade 60 for maximum strength efficiency. The designer's handbook (e.g., AISC Steel Construction Manual) provides different design tables for each grade, reflecting these distinct Fy and Fu values.