What are structural fasteners?
Structural fasteners are bolts, nuts, washers, and rivets designed to clamp structural members together in buildings, bridges, towers, and heavy frames. They are engineered for high clamp load, consistent preload, and reliable performance under vibration, shear, and tension. On steel-to-steel work, common callouts are A325 heavy hex bolts, A563 heavy hex nuts, and F436 flat washers. Structural blind rivets are used where only one side is accessible or where speed and vibration resistance are priorities.
Common types of structural fasteners
- A325 heavy hex bolts for steel connections that require high clamp force and controlled installation.
- A563 heavy hex nuts matched to bolt grade and finish to develop the joint’s full strength.
- F436 flat washers hardened washers placed under the bolt head and nut to distribute load and protect the bearing surface.
- Structural blind rivets interlock-style or multi-grip designs for strong, consistent joints from one side of the work.
When to choose A325 bolts for steel connections
A325 heavy hex bolts are specified for most building and bridge steel joints that need high, repeatable pretension. Their head and nut geometry accept installation tools and the hardened F436 washer interface needed to reach design preload. Use matching A563 heavy hex nuts and hardened washers to maintain clamp load over time.
What does A563 nut grade mean?
A563 is the standard for carbon and alloy steel nuts used with structural bolts. Heavy hex geometry offers more bearing area and thread engagement than finished hex patterns, improving joint reliability on steel connection plates.
Why hardened F436 washers matter
F436 washers are heat-treated to resist brinelling and embedment. The hardness and thickness help distribute load evenly, protect the plate surface during tightening, and improve slip resistance in pretensioned connections.
Structural blind rivets for high-vibration joints
Interlock and structural blind rivets expand and lock on the blind side, creating high shear and tensile values from one-side access. They are useful for heavy sheet to structure, truck bodies, equipment enclosures, and any joint that sees vibration where a through-bolt is not practical.
Structural Fasteners FAQs
Quick answers to the questions buyers ask most about structural fasteners.
A325 vs A490 bolts, what is the difference?
Both are structural heavy hex bolts with different strength levels. A325 is medium carbon steel, widely specified for building and bridge work. A490 is higher strength alloy steel and requires additional considerations for installation and reuse. Follow project specs and match the nut and washer grades called out in the drawings.
Which nut goes with an A325 bolt?
Use an A563 heavy hex nut compatible with the bolt finish and strength class. Heavy hex geometry provides the thread engagement and bearing area needed for structural pretension.
Do structural bolts require hardened washers?
Yes, place an F436 hardened washer under the turned element, and as required under both head and nut. Hardened washers limit embedment, help reach the specified pretension, and maintain slip resistance.
What finish should I choose for outdoor structural steel?
Follow project documents. Common finishes include plain for shop-controlled environments and protective coatings where corrosion is a concern. The nut and washer finishes should be compatible with the bolt to maintain fit and performance.
When are structural blind rivets a better choice than bolts?
Use structural blind rivets when only one side is accessible, when rapid installation is needed, or when a locked-mandrel design provides sufficient shear and tensile strength for sheet and fabricated assemblies subject to vibration.
Sizing and selection tips
- Match bolt diameter to hole size and plate thickness to achieve required shear area and clamp load.
- Use heavy hex nut and F436 washer sets that match the bolt grade and finish for consistent installation.
- Confirm installation method and pretension requirements in the project specification before ordering.