Connections & Fasteners
High-Strength Structural Bolts for Steel Buildings
Definition: Structural bolts are high-strength steel bolt, nut and washer assemblies used to connect the main members of a pre-engineered building, such as rafter-to-column end plates, splices, bracing and secondary framing, so that the building can be assembled quickly on site without field welding.
Key facts
- Main frame connections normally use high-strength bolt assemblies; secondary members often use lower-strength bolts.
- Bolt grades follow the code family of the project, such as ASTM, AS/NZS or EN product standards.
- Joints are classified as snug-tight, pretensioned or slip-critical by the designer.
- Bolts, nuts and washers should be supplied as matched assemblies from the same lot where the standard requires.
- Tightening method and inspection are specified in the project documents.
Role in the structure
Bolts transfer forces between connected parts. In bearing-type joints they carry shear by bearing on the hole walls, and in end-plate moment connections they carry tension from frame bending. Pretensioned and slip-critical joints rely on a clamping force that resists slip by friction between the faying surfaces, which matters where load reversal, vibration or fatigue is present.
Which bolts are used in pre-engineered buildings?
Main frame connections, splices and crane brackets typically use high-strength heavy hex structural bolts, such as the ASTM grades used under the RCSC specification, property class 8.8 or 10.9 assemblies under EN and AS/NZS practice, or the equivalent required by the project code.
Purlins, girts and light secondary connections are commonly fixed with lower-strength bolts with or without washers. Anchor bolts that tie the frame to the foundation are a separate product family and are specified with the base plates.
Snug-tight, pretensioned and slip-critical joints
A snug-tight joint is one where the plies are in firm contact after a few impacts of an impact wrench or the full effort of a worker with a spud wrench. Many shear connections in metal buildings are designed this way.
Pretensioned joints are tightened to a specified minimum tension and are used where bolts carry tension or where the code requires them. Slip-critical joints are also pretensioned, and in addition the faying surfaces must meet a specified surface condition, so coatings at those joints must be controlled.
Tightening methods
Recognized methods for pretensioning include turn-of-nut, calibrated wrench, twist-off tension control bolts and direct tension indicating washers. Each method has its own procedure for pre-installation verification and inspection set out in the applicable specification.
Whatever the method, bolts should first be brought to snug-tight in a pattern that pulls the plies together from the stiffest part of the joint outward, then pretensioned in a similar systematic pattern.
Materials & coatings
- High-strength structural bolt assemblies to ASTM, AS/NZS or EN product standards
- Heavy hex nuts and hardened washers matched to the bolt grade
- Hot-dip galvanized or mechanically galvanized finish where corrosion protection is required
- Tension control bolts or direct tension indicators where those methods are specified
Design considerations
- Classify each joint as snug-tight, pretensioned or slip-critical on the drawings.
- Select bolt grade and diameter for shear, tension and combined effects.
- Check edge distances, spacing and hole types for the connection design.
- Specify faying surface preparation for slip-critical joints.
- Match bolt coatings to the member coating and the corrosion environment.
Fabrication
- Holes are drilled or punched to the size and tolerance permitted by the code.
- Faying surfaces of slip-critical joints are masked or prepared to the specified class.
- Bolts are packed by size, grade and lot with documentation for traceability.
Erection
- Store bolts dry and protected so lubrication and threads are not damaged.
- Install all bolts snug-tight before pretensioning any bolt in the joint.
- Pretension using the specified method and record the results.
- Do not reuse galvanized or pretensioned high-strength bolts unless the specification allows it.
Inspection & quality
- Check grade markings on bolts and nuts against the drawings.
- Carry out pre-installation verification of the tightening method where required.
- Inspect plies for firm contact and correct washer placement.
- Verify pretension by the method specified, such as indicator gaps or spline shear.
What to include in your RFQ
- Design code family and required bolt product standard.
- Bolt finish: plain, galvanized or other coating.
- Joint classifications and tightening method preferred by the erector.
- Allowance for spare bolts to cover site losses.
3D model & IFC of this component
Rotate the model, isolate the component and download its IFC4 file for your BIM coordination model.
IFC entity: IfcMechanicalFastener (BOLT). Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
Can galvanized bolts be used in main frame connections?
Yes, galvanized high-strength bolts are widely used, but they must be supplied with matched galvanized nuts and lubrication, and some higher grades are not permitted to be galvanized. Check the applicable standard.
Do all bolts in a metal building need to be pretensioned?
No. Many shear connections are snug-tight. Pretensioning is required where the designer specifies it, such as tension connections, slip-critical joints or where the code requires it.
What is turn-of-nut tightening?
It is a method where the nut is rotated a specified amount past snug-tight to achieve the required pretension. The rotation depends on the bolt length and diameter.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
