ASTM A709 covers structural steel for bridges, with grades categorized by minimum yield strength (e.g., 50 = 50 ksi) and specialized properties (weathering resistance, fracture-critical use). Below is a comprehensive breakdown:
A709 Grades Summary
| Grade | Min Yield Strength | Min Tensile Strength | Key Properties | Applications |
|---|---|---|---|---|
| 36 | 36 ksi (250 MPa) | 58-80 ksi (400-550 MPa) | General-purpose carbon steel | Non-critical bridge components |
| 50 | 50 ksi (345 MPa) | 65 ksi (450 MPa) | Standard strength | Girders, beams |
| 50W | 50 ksi (345 MPa) | 70 ksi (485 MPa) | Atmospheric corrosion resistance | Exposed bridge structures |
| HPS 50W | 50 ksi (345 MPa) | 70 ksi (485 MPa) | Higher toughness, weldability vs. 50W | Fracture-critical members |
| 70W | 70 ksi (485 MPa) | 90 ksi (620 MPa) | Weathering + high strength | Long-span bridges, heavy loads |
| HPS 70W | 70 ksi (485 MPa) | 90 ksi (620 MPa) | Optimized toughness, low P/S for weldability | Seismic zones, fatigue-prone areas |
| 100 | 100 ksi (690 MPa) | 110 ksi (760 MPa) | Ultra-high strength | Specialty components (rare) |
| 100W | 100 ksi (690 MPa) | 110 ksi (760 MPa) | Weathering + ultra-high strength | Extreme-load scenarios |
| HPS 100W | 100 ksi (690 MPa) | 110 ksi (760 MPa) | Enhanced toughness vs. 100W | Critical infrastructure |
Critical Classifications
Weathering Steels (W Grades):
Form protective patina (rust layer) to resist atmospheric corrosion.
Grades: 50W, 70W, 100W, HPS 50W, HPS 70W, HPS 100W.
Reduce maintenance costs by 30-50% vs. painted carbon steel.
Fracture-Critical (F Suffix):
Tighter impact toughness requirements (e.g., 50W-F).
Testing: Charpy V-notch ≥ 35 ft-lb (47 J) at lowest service temperature.
Mandatory for components where failure would collapse the bridge.
High-Performance Steel (HPS):
Superior weldability (low P/S ratio), toughness, and weathering resistance.
Key grades: HPS 50W, HPS 70W, HPS 100W.
Replace traditional ASTM A588 in modern bridges.
Chemical Composition Highlights
Weathering Grades:
Cr (0.40-0.80%), Cu (0.20-0.50%), Ni (0.10-0.65%) for corrosion resistance.
HPS Grades:
Lower C (≤0.10%) and P/S ratios (≤0.025) for enhanced weldability.
Impact Toughness Requirements
| Grade | Test Temp. | Charpy V-Notch (Min) |
|---|---|---|
| 50W / 50W-F | -34°C (-30°F) | 15 ft-lb (20 J) / 35 ft-lb (47 J) |
| 70W / 70W-F | -34°C (-30°F) | 15 ft-lb (20 J) / 35 ft-lb (47 J) |
| HPS 70W | -51°C (-60°F) | 35 ft-lb (47 J) |
When to Use Which Grade?
Standard Applications:
Grade 50/50W: Most highway bridges, piers.
Long-Span/Heavy Loads:
70W/HPS 70W: Trusses, cantilevered sections (e.g., San Francisco-Oakland Bay Bridge).
Corrosive Environments:
50W/70W: Coastal bridges, industrial areas.
Earthquake/High Fatigue Zones:
HPS Grades (e.g., HPS 70W for Golden Gate Bridge seismic retrofits).
Compliance Notes
Annex Requirements: Grades with suffix "-F" (fracture-critical) mandate supplementary testing per Annex A5.
Certification: Mill test reports must confirm chemistry, tensile strength, and impact values.
Global Equivalents
| A709 Grade | EN Standard | JIS Standard |
|---|---|---|
| 50W | S355J2W+N | SMA 50AW |
| 70W | S460J2W+N | SMA 70W |
For bridge design, prioritize weathering grades (W) for lifespan and HPS for fracture-critical systems. Always verify project-specific codes (AASHTO LRFD, AWS D1.5) for material selection.
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