What Are Heavy Hex Bolts?
Heavy hex bolts—often called structural bolts—are high-strength, externally threaded fasteners with a head that is larger and thicker than a standard hex bolt. The larger head provides a wider bearing surface for load distribution and wrench engagement in structural steel connections. Modern heavy hex structural bolts are supplied to ASTM F3125, which covers grades historically known as A325 and A490 (and their metric equivalents).
Benefits of Heavy Hex Bolts
- High strength: manufactured to structural bolt standards for demanding connections.
- Large bearing area: heavy-hex head improves clamping force distribution under the head.
- Reliable fit: defined head dimensions, thread lengths, and tolerances for repeatable assembly.
- Field-friendly: designed for installation methods used on structural steel (turn-of-nut, calibrated wrench, or with load-indicating devices when specified).
Heavy Hex Bolts Sizes, Grades & Finishes
- Typical diameters: 1/2" through 1-1/2" (imperial) and common metric sizes such as M12–M30.
- Grades (per ASTM F3125): grades commonly referenced as A325 (moderate strength) and A490 (higher strength). Metric equivalents are provided in the same standard.
- Materials: medium carbon or alloy steel; “Type 3” weathering-steel options may be specified on some projects.
- Finishes: plain/oil, mechanical or hot-dip galvanized for corrosion protection where permitted by the project specification. (Some coatings are restricted for certain grades—always verify.)
Common Applications for Heavy Hex Bolts
- Structural steel connections in buildings, bridges, towers, and industrial frames
- Base plates, splice plates, bracing connections, and equipment steelwork
- Retrofit and field erection where high clamp force and inspection are required
Installation Basics for Heavy Hex Bolts
- Use the installation method required by the governing specification (e.g., turn-of-nut or calibrated wrench procedures; load-indicating washers when specified).
- Pair bolts with matching heavy hex nuts (ASTM A563 per project grade) and hardened washers (ASTM F436) as detailed on the drawings.
- Observe hole type rules: additional washers are often required at slotted or oversized holes.
- Do not reuse structural bolts unless explicitly allowed by the project specification.
- For galvanized fasteners, use compatible galvanized nuts and washers to ensure proper thread fit and corrosion performance.
Related Hardware for Heavy Hex Bolts
- Heavy hex nuts (matching grade and finish)
- Hardened flat washers and load-indicating washers (when specified)
- Direct-tension indicators (DTIs) and bolt tension calibration tools as required by procedure