Technical Foundation
What Is a Structural Bolt?
Structural bolts are high-strength fasteners governed by ASTM standards specific to structural steel connections — A325 (Type 1 medium carbon steel, or Type 3 weathering steel) and the higher-strength A490 — and are always specified, ordered and installed as a matched assembly of bolt, nut and washer rated to the same standard, since mixing components from different standards can compromise the connection's rated capacity even if the thread fits. Selection between A325 and A490 depends on the structural engineer's design requirements for the connection's required strength, not a general 'go stronger' default — A490 bolts have specific restrictions on use with galvanized coatings and certain steel types that A325 doesn't share.
Installation method (turn-of-nut, calibrated wrench, direct tension indicator, or twist-off tension control bolts) must achieve the specified pretension for a slip-critical or bearing-type connection per the applicable structural steel construction code (commonly RCSC specification in the US), and proper installation is verified and inspected as a structural life-safety item — an under-tensioned high-strength bolted connection can fail to perform as the structural design intended even though the bolt itself meets material spec.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Grade/Standard | ASTM A325 (Type 1/3) vs. A490 | Sets the fastener's strength class per structural engineering design requirements — not interchangeable by preference |
| Assembly Matching | Bolt, nut and washer rated to the same standard | Mixing components across standards can compromise the connection's rated capacity |
| Finish/Coating | Plain, hot-dip galvanized, weathering steel (Type 3) | A490 bolts have specific restrictions on galvanizing not shared by A325 — confirm compatibility before specifying |
| Installation Method | Turn-of-nut, calibrated wrench, direct tension indicator, twist-off (TC) bolts | Must achieve specified pretension per applicable structural bolting code (e.g., RCSC specification) |
| Connection Type | Slip-critical vs. bearing-type | Determines required installation tensioning method and surface preparation at the connection |
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Applications
Where Structural Bolts Get Used
Structural bolts are specified by structural engineering design requirements as life-safety fasteners in load-bearing steel connections.
Structural Steel Building Construction
A325 and A490 bolted connections in steel frame, beam and column construction.
Bridge Construction & Infrastructure
High-strength structural bolts in slip-critical connections for bridge steel structures.
Industrial Structures & Platforms
Structural bolted connections in equipment platforms, pipe racks and structural steel support.
Wind Energy & Tower Structures
High-strength bolted flange connections in wind turbine tower sections.
Pre-Engineered Metal Buildings
Structural bolts in bolted frame connections for pre-fabricated steel building systems.
Marine & Port Structures
Weathering steel and galvanized structural bolts in corrosion-exposed dock and marine structures.
FAQ
Structural Bolts Questions, Answered
What's the difference between A325 and A490 structural bolts?
A490 bolts have a higher minimum tensile strength than A325, allowing a structural connection to carry greater load with the same bolt size and count, but A490 also has specific restrictions — for example, on use with galvanized coatings and on certain steel types — that A325 doesn't share. The choice between them is a structural engineering design decision based on the connection's required capacity, not simply choosing the stronger option by default.
Can I mix a bolt, nut and washer from different manufacturers or standards?
This should be avoided unless the components are all rated to the same governing standard (e.g., all A325) and the assembly is verified compatible — structural bolt assemblies are tested and rated as a matched set, and mixing components across standards can compromise the connection's actual performance even if everything physically threads together.
What's the difference between slip-critical and bearing-type connections?
In a bearing-type connection, the bolt shank itself resists shear load by bearing against the connected plates. In a slip-critical connection, the bolted joint is designed to resist load through friction between clamped surfaces, requiring the bolt to be tensioned to a specified level and often requiring specific faying surface preparation — the two connection types have different installation and inspection requirements even using the same bolt grade.
How is proper bolt tension verified during installation?
Common methods include turn-of-nut (rotating the nut a specified amount past snug-tight), calibrated wrench (torque-controlled), direct tension indicator washers (which show a gap closing as tension is achieved), and twist-off tension control (TC) bolts (where a spline shears off at the correct tension). The applicable structural bolting code, commonly the RCSC specification in the US, governs which methods are acceptable and how installation is inspected.
Are A490 bolts allowed to be galvanized?
A490 bolts have specific restrictions on hot-dip galvanizing due to hydrogen embrittlement concerns at their higher strength level, and many specifications limit or prohibit galvanized A490 bolts, using mechanically galvanized or other coatings instead — always confirm current code and manufacturer guidance for the specific coating and grade combination rather than assuming A325 galvanizing practices apply.
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