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Structural bolts are designed for high-strength connections where size, fit, and specification matter. Whether you're sourcing bolts for structural steel, bridges, buildings, industrial equipment, or infrastructure, ordering the correct diameter and length is critical to a properly designed bolted connection.
But structural bolt sizing can be confusing. Nominal diameter, bolt length, thread length, head dimensions, grip length, and compatible nuts and washers all need to be considered. This guide covers the basics and what buyers should know before requesting a quote.
Browse structural bolts →Key Dimensions to Understand
Nominal diameter
The outside diameter of the bolt threads. Common structural bolt diameters range from 1/2" to 1-1/2", though available sizes depend on grade and specification.
Nominal length
Measured from the bearing surface beneath the head to the end of the bolt. The head itself is not included in the nominal length.
Grip length
The total thickness of materials being clamped together. Determines the appropriate bolt length before accounting for washers, nuts, and required thread projection.
Thread length
Structural bolts are commonly partially threaded. The unthreaded shank portion can be important to the design and performance of the connection.
Head dimensions
Structural bolts use a heavy hex head — larger than a standard hex bolt head of the same diameter. The larger head provides a greater bearing surface.
Hole size
A 3/4" bolt doesn't go into a 3/4" hole. Permitted hole dimensions depend on the connection type (standard, oversized, slotted) and applicable structural specifications.
Common Structural Bolt Diameters
Available diameters depend on the applicable grade and specification. Always verify project requirements rather than selecting a diameter based solely on availability.
Understanding Grip Length
Grip length is the total thickness of the materials being clamped. Choosing the right bolt length starts with understanding the grip, then accounting for all additional components in the assembly.
Example grip calculation
How to Read a Structural Bolt Description
A typical structural bolt request looks like this:
The Structural Bolt Assembly: More Than Just a Bolt
A structural bolted connection is a system. The bolt, nut, and washer all need to meet applicable specification requirements for the connection to perform as designed.
Structural bolt
ASTM F3125 Grade A325 or A490. Heavy hex head. Partially threaded shank.
Heavy hex nut
Grade and finish must match bolt specification. Larger than standard hex nuts of the same thread diameter.
Structural washer
Must meet hardness and dimensional requirements (ASTM F436). Not interchangeable with standard flat washers.
A325 vs. A490 Sizing
A325 and A490 structural bolts share many of the same dimensional conventions but have different mechanical properties and material requirements. Two structural bolts with identical nominal dimensions should never be assumed to be interchangeable.
| Feature | A325 | A490 |
|---|---|---|
| Specification | ASTM F3125 Grade A325 | ASTM F3125 Grade A490 |
| Min. tensile strength | 120 ksi (common diameters) | 150 ksi |
| Head style | Heavy hex | Heavy hex |
| Hot-dip galvanizing | Available | Not permitted |
| Interchangeable by size? | No — never substitute based on dimensions alone | |
What to Provide When Requesting a Quote
The more complete your specifications, the easier it is to source the correct hardware. Whenever possible, provide:
If you're working from a construction drawing, bill of materials, or existing part specification, providing that documentation helps eliminate ambiguity and speeds up the quoting process.
Common Structural Bolt Sizing Mistakes
Frequently Asked Questions
Get the Right Structural Bolt the First Time
Structural bolt sizing involves more than choosing a diameter and length. Grade, thread configuration, grip, nuts, washers, coatings, and certification requirements all contribute to a properly specified assembly.
Have a structural bolt specification or bill of materials ready to quote? Send Blue Ribbon Fastener the size, grade, finish, quantity, and certification requirements and our team can help source the correct components.