Heavy Hex Bolts
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
Heavy Hex Bolts FAQs
See the accordion below for grade selection, coating guidance, length/threads, and installation questions.
Disclaimer: This page provides general guidance only. Always follow the product datasheet, project specification, and applicable standards.
Heavy hex bolts have a larger, thicker head and are made to structural standards for high-strength joints. Standard hex bolts are used for general fastening and have smaller heads and different tolerances.
In common usage, yes—heavy hex structural bolts are the typical fastener used in structural steel connections when specified by the project.
They’re strength grades historically used for structural bolts. Today they’re covered under ASTM F3125. A490 is higher strength than A325; choose the grade shown on your drawings.
Use heavy hex nuts to ASTM A563 (grade per project—often DH for high-strength assemblies) and hardened washers to ASTM F436. Follow the drawing notes for quantity and placement.
Many A325-type bolts are available galvanized. Coating options and restrictions vary by grade—verify what’s allowed for your project before ordering.
Hardened washers are typically required under the turned element and may be required under both head and nut for slotted/oversize holes or specific details. Check the plans/specs.
Common methods include turn-of-nut and calibrated wrench procedures; some jobs use load-indicating washers or other devices. Use the method specified by the governing code and project.
Inch bolts are usually UNC (coarse). Metric bolts use the standard metric pitches for the diameter (e.g., M20-2.5). Order the thread shown on the drawings.
Bolt length is measured from under the head to the tip. Structural bolts have defined thread lengths per standard so the unthreaded shank can be placed in shear planes when required.
Reuse is generally not allowed unless explicitly stated by the project specification or code. Replace bolts that have been tensioned.
When the structure uses weathering steel and the design calls for matching corrosion behavior and appearance. Confirm availability and coating rules for the grade.
Structural bolting is typically inspected for pretension using the method specified (e.g., rotation marks for turn-of-nut, feeler-gage on DTIs, or calibrated wrench verification). Follow the project’s QA/QC plan.