Heavy Hex Structural Bolts (ASTM A325 / A490 & EN 14399)
In steel construction, a bolt is not merely a fastener; it is a calibrated tensioning device essential for the integrity of slip-critical and bearing-type connections. Standard hex bolts often fail to provide the necessary bearing area or pretension retention required in high-load structural nodes.
Our Structural Bolts are engineered with a larger “Heavy Hex” head geometry to distribute clamping load effectively across the washer face, preventing localized yielding of the connected material. Manufactured strictly to ASTM F3125 (covering Grades A325 and A490) and EN 14399 (HV/HR systems), these fasteners are heat-treated to balance high tensile strength with the ductility needed to survive slight plastic deformation during seismic events or settling. From bridge girders to high-rise framing, we deliver traceable lots that ensure predictable torque-tension relationships.
- Larger bearing surface (Heavy Hex).
- Controlled ductility for seismic loads.
- Traceable Heat Lots (MTR Type 3.1).
- Short thread lengths exclude shear planes.
- Compatible with DTI tensioning methods.
- Corrosion-resistant finishes (HDG/Geomet).
Technical Specifications
Product Name
Heavy Hex Structural Bolt / Pre-load Bolt Assemblies
Standards(USA)
ASTM F3125 (Grades A325, A490, F1852, F2280), ANSI/ASME B18.2.6 (Dimensions)
Standards (EU)
EN 14399-4 (System HV), EN 14399-3 (System HR), ISO 7411
Material Grades
Medium Carbon Boron Steel (Type 1), Weathering Steel (Type 3), Alloy Steel (42CrMo4)
Strength Levels
120 ksi (A325), 150 ksi (A490), 10.9 HV/HR
Diameter Range
1/2″ – 1-1/2″ (Imperial); M12 – M36 (Metric)
Surface Finish
Hot Dip Galvanized (ASTM F2329), Zinc Flake (Geomet/Dacromet), Plain Oiled
Certifications
ISO 9001:2015, CE Marking (EN 15048), Mill Test Reports (MTR) included
1. The “Shear Plane” Protection
Pain Point: In standard bolts, the threaded portion is often long. If the threads align with the “shear plane” (the interface between two steel plates), the connection’s shear capacity is reduced by ~25% due to the smaller root diameter area.
The Engineering Solution: Our structural bolts feature shorter thread lengths specific to structural standards. This ensures the full shank diameter occupies the shear plane, maximizing the load-bearing capacity of the joint (Designation: X-connection vs N-connection).
2. Prevention of Hydrogen Embrittlement (in Grade A490)
Pain Point: High-strength bolts (tensile >1000 MPa, like A490) are susceptible to hydrogen embrittlement if acid-pickled or galvanized incorrectly, leading to delayed catastrophic failure.
The Engineering Solution: We strictly prohibit Hot Dip Galvanizing for standard A490 bolts (per ASTM recommendations). Instead, we offer Zinc/Aluminum Flake coatings (Geomet/Dacromet) which provide superior salt-spray resistance without the acid pickling process, eliminating the risk of internal hydrogen fissures.
3. Consistent Tensioning (K-Factor Control)
Pain Point: A structural bolt’s job is to create clamping force (preload). Inconsistent lubrication leads to scattered torque values—you might hit the torque target, but miss the tension target, resulting in a loose joint.
The Engineering Solution: Our assemblies are supplied with factory-lubricated nuts (molybdenum disulfide or wax). This stabilizes the Torque Coefficient (K-factor) to a predictable range (e.g., K=0.11-0.15), ensuring that your calibrated wrench settings actually achieve the required pretension.
Dimensions for ASTM A325 / A490 Heavy Hex Structural Bolts. Note the head size is larger than standard hex bolts.
| Nominal Size (d) | Body Diameter (E) | Width Across Flats (F) | Head Height (H) | Thread Length (T) |
| 1/2″ | 0.500″ | 7/8″ | 5/16″ | 1.00″ |
| 5/8″ | 0.625″ | 1-1/16″ | 25/64″ | 1.25″ |
| 3/4″ | 0.750″ | 1-1/4″ | 15/32″ | 1.38″ |
| 7/8″ | 0.875″ | 1-7/16″ | 35/64″ | 1.50″ |
| 1″ | 1.000″ | 1-5/8″ | 39/64″ | 1.75″ |
| 1-1/8″ | 1.125″ | 1-13/16″ | 11/16″ | 2.00″ |
| 1-1/4″ | 1.250″ | 2″ | 25/32″ | 2.00″ |
1. Installation Methods For slip-critical joints, simple torqueing is often insufficient. We recommend and support the following approved installation methods (RCSC):
Turn-of-Nut: Rotating the nut a specified degree past “snug tight.”
Calibrated Wrench: Using a daily-calibrated impact wrench or hydraulic torque wrench.
DTI Washers: Using Direct Tension Indicators (squirter washers) to visually verify tension.
2. Rotational Capacity Test (RoCap) Before installation, verify the assembly’s integrity. The nut must be able to rotate freely on the bolt threads. If the nut binds due to galvanized buildup or damaged threads, the torque reading will be false, and the bolt will not have sufficient tension.
3. Washer Placement Always use a hardened ASTM F436 washer under the turned element (usually the nut). Standard mild steel washers will crush/yield under the high clamping force of an A325 bolt, causing immediate preload loss (relaxation).
Related Products
FAQ
What is the difference between A325 and A490 structural bolts?
The primary difference is strength and application. A325 (Group 120) is a medium-strength carbon steel bolt (120 ksi tensile) used in standard structures. A490 (Group 150) is a high-strength alloy steel bolt (150 ksi tensile) used for heavy loads. Crucially, A490 bolts should generally not be hot-dip galvanized due to hydrogen embrittlement risks.
Why do structural bolts have a "Heavy Hex" head?
Structural bolts use a “Heavy Hex” design (wider across flats than standard bolts) to increase the bearing surface area. This distributes the massive clamping force over a wider area of the steel connection, preventing the bolt head from crushing or embedding into the structural member.
Can I reuse structural bolts after they have been fully tightened?
Generally, no. ASTM A490 bolts and galvanized A325 bolts should strictly never be reused after being fully tensioned, as the threads may have yielded. Plain “black” A325 bolts may be reused once if approved by the Engineer of Record (EOR), but it is industry best practice to use new fasteners to ensure safety.
What is a "slip-critical" connection?
A slip-critical connection relies on the friction between the clamped steel plates to transfer loads, rather than the bolt bearing against the side of the hole. This requires the structural bolt to be pretensioned to a specific minimum load (e.g., 70% of tensile strength) to generate the necessary clamping force.
Do you sell "wholesale structural bolts" with Mill Test Reports?
Yes, as a wholesale structural bolts manufacturer, we provide full traceability. Every shipment includes Type 3.1 Mill Test Reports (MTRs) detailing the chemical composition, heat number, and mechanical test results (tensile, proof load, Charpy V-notch) required for structural inspections.