Structural bolting requires precise adherence to tensile strength, shear ratings, and proper tensioning protocols. Here is how A325 and A490 compare.
Understanding Heavy Hex Structural Specifications
When fastening structural steel members in commercial buildings, highway bridges, or industrial processing plants, standard Grade 5 or Grade 8 hex bolts are insufficient. Building codes require heavy hex structural bolts conforming to ASTM standards (now unified under ASTM F3125).
The two most common high-strength structural bolt grades are ASTM A325 (Grade A325) and ASTM A490 (Grade A490).
Key Mechanical Property Comparison
| Property | ASTM A325 (F3125 Grade A325) | ASTM A490 (F3125 Grade A490) |
|---|---|---|
| Tensile Strength | 120 ksi (up to 1”) / 105 ksi (> 1”) | 150 ksi to 173 ksi (all diameters) |
| Yield Strength | 92 ksi (min) | 130 ksi (min) |
| Material Chemistry | Medium Carbon or Alloy Steel | Alloy Steel (Quenched & Tempered) |
| Galvanizing Options | Hot-Dip Galvanized (ASTM F2329) or Mechanical | Plain or specialized zinc coatings (HDG prohibited due to hydrogen embrittlement) |
When to Specify A325 vs A490
1. ASTM A325: The Versatile Workhorse
A325 bolts are the standard specification for the vast majority of commercial building frames, equipment skid mounting, and industrial structures.
- Corrosion Protection: Because A325 bolts have a maximum tensile strength below 150 ksi, they can be safely Hot-Dip Galvanized without significant risk of hydrogen embrittlement. This makes them ideal for exposed outdoor infrastructure in the wet Pacific Northwest climate.
- Tensioning: Installed using turn-of-nut, calibrated wrench, or twist-off tension control (TC) bolt methods.
2. ASTM A490: Maximum Shear & Tensile Capacity
A490 bolts provide 25% higher tensile strength than A325, enabling structural engineers to reduce bolt count or smaller connection plate footprints.
- Critical Coating Caution: Standard hot-dip galvanizing is strictly prohibited on A490 bolts because the acid pickling process introduces hydrogen that causes sudden catastrophic brittle failure under sustained tensile load. If corrosion resistance is required, mechanically deposited zinc or specialized protective coatings must be specified.
Complete Lot Traceability & Mill Test Reports (MTRs)
Every structural bolt installed on commercial and defense infrastructure requires full chemical and physical test reports.
Northwest Fasteners maintains complete lot traceability from domestic mills for heavy hex structural bolts, matched 2H heavy hex nuts, and F436 hardened flat washers. Contact our Lakewood sales desk for immediate stock checks and certified submittal packages.
