What ASTM A709 Covers
ASTM A709/A709M is the standard specification for structural steel for bridges. It is written as a performance-based document: the same specification number covers carbon steel, high-strength low-alloy steel and quenched-and-tempered alloy steel, and the grade suffix determines which property package applies. Strength, toughness and corrosion resistance are therefore all read from the grade designation and from the product thickness, not from the specification number alone.
The grades most often ordered as plate are Grade 36, Grade 50, Grade 50W, Grade 50S, Grade HPS 50W, Grade HPS 70W, Grade 100 and Grade 100W. AASHTO M270 mirrors the specification for the overlapping grades, which is why transportation authorities accept an A709 plate on M270 projects.
Grade-by-Grade Strength Requirements
Minimum yield strength and tensile strength are the first properties a designer checks, because they set the section size and the connection design. The values below apply to plate in the thickness ranges shown.
| Grade | Minimum yield (MPa) | Tensile strength (MPa) | Typical application |
|---|---|---|---|
| Grade 36 | 250 | 400-475 | Bearings, stiffeners, secondary members |
| Grade 50 | 345 | 450 min | General bridge plate work |
| Grade 50S | 345 | 450 min | Fracture-critical members with enhanced toughness |
| Grade 50W | 345 | 485 min | Unpainted exposed weathering plate |
| Grade HPS 50W | 345 | 485 min | High performance weathering plate with tight toughness control |
| Grade HPS 70W | 485 | 585-760 | Long-span girders where weight must be reduced |
| Grade 100 | 690 | 828 min | Quenched and tempered plate up to 64 mm thick |
| Grade 100W | 690 | 828 min | Quenched and tempered weathering plate up to 64 mm thick |
Quenched and tempered grades show a further thickness effect: Grade 100 and Grade 100W plate thicker than 64 mm and up to 100 mm drops to a 620 MPa minimum yield and 793 MPa minimum tensile strength, because the hardening response falls off as section size increases.
Physical and Elastic Properties
Physical constants are effectively grade-independent across the weathering plate family, which simplifies deflection and thermal calculations.
| Property | Metric value | Imperial value |
|---|---|---|
| Young's modulus | 205,000 MPa | 29,700 ksi |
| Density | 7,850 kg/m³ | 490 lb/ft³ |
| Poisson's ratio | 0.29 | 0.29 |
| Coefficient of thermal expansion | 12 × 10-6 per °C | 6.7 × 10-6 per °F |
| Thermal conductivity | 50 W/m·K | 34.5 BTU·in/h·ft²·°F |
| Melting range | 1425-1540 °C | 2600-2800 °F |
Toughness and the Effect of Thickness
Bridge steel is governed as much by toughness as by strength. Charpy V-notch testing to ASTM E23, with specimen preparation and testing to ASTM A370, is required for the bridge grades, and the acceptance energy depends on the exposure zone and on whether the member is fracture critical. Heavier plate generally shows lower toughness at a given temperature because the cooling rate during rolling is slower, so a designer specifying 100 mm plate may need a grade with tighter impact control than for 25 mm plate.
Bridge grades require longitudinal Charpy V-notch testing at the specified design temperature.
Fracture-critical members carry higher energy requirements than non-fracture-critical members.
Higher-strength quenched and tempered grades need careful welding procedures to preserve heat-affected zone toughness.
Atmospheric Corrosion Behaviour of the W Grades
The W suffix identifies a weathering chemistry with copper, chromium and nickel additions that satisfy the atmospheric corrosion resistance index of the specification. Exposed to alternating wet and dry conditions, these grades form a dense, adherent oxide layer that limits further attack. The benefit depends on exposure: rural and moderately industrial sites favour patina formation, while chlorides, permanent moisture, crevices and ponding prevent it, and in those situations a protective coating or a non-weathering grade is the correct choice.
How Plate Thickness Changes the Property Rules
| Thickness band | Practical consequence |
|---|---|
| 6-40 mm | Highest toughness and most predictable weldability; typical for girder webs and flanges |
| 40-64 mm | Strength unchanged for Grade 50W; impact testing still specified by zone |
| 64-100 mm | Quenched and tempered grades step down to the lower strength level; toughness control becomes critical |
| Over 100 mm | Outside the usual plate provisions of the bridge specification; a special order is required |
Frequently Asked Questions
Q: What is ASTM A709 steel?
It is the standard specification for structural steel for bridges, covering carbon, high-strength low-alloy and quenched and tempered grades supplied as plate, shapes and bars. The grade suffix defines the strength, toughness and corrosion package.
Q: Which A709 grades are supplied as weathering steel?
Grade 50W, Grade HPS 50W, Grade 100W and Grade HPS 100W carry the weathering chemistry. Grade 36, Grade 50 and Grade 50S are not weathering grades unless the order states otherwise.
Q: What is the yield strength of A709 Grade 50 plate?
345 MPa, equal to 50 ksi, with a minimum tensile strength of 450 MPa for plate in the standard thickness range.
Q: What is the yield strength of A709 Grade 100 plate?
690 MPa (100 ksi) up to 64 mm thick, with a minimum tensile strength of 828 MPa. From 64 mm to 100 mm the minimum yield falls to 620 MPa and the minimum tensile strength to 793 MPa.
Q: Is density different for weathering plate?
No. Weathering grades have the same density as other structural steel, about 7,850 kg/m³, so weight calculations are unchanged when a W grade replaces a plain grade of the same strength class.
Q: How does plate thickness affect mechanical properties?
Strength requirements are stated per thickness band, and toughness generally falls as thickness rises because of slower cooling after rolling. Thick plate therefore needs closer attention to impact testing and to welding procedure qualification.
