A steel structure is assembled on site, and most of the connections are bolted. The bolts themselves are a small share of the cost and a large share of the risk: the wrong grade, hole size or tightening method usually shows up during erection, not in the shop.
This guide covers the decisions that have to be settled at enquiry stage: bolt grade and system, connection type, hole size and tolerance, coating, and how the bolts are tightened and inspected.
The bolt systems most often specified
The bolt standard matters because it sets the grade, the dimensions, the marking and the tightening requirements. A metric bolt and an imperial bolt with a similar strength are not interchangeable.
| System | Typical grades | Where it is used | What to watch |
|---|---|---|---|
| GB/T 1228 to 1231 | 8.8, 10.9 | Domestic projects and many export projects where no other system is named | The default system for Chinese fabrication; confirm the marking and the test certificate |
| ASTM F3125 | A325, A490 | United States and projects following AISC | Replaced ASTM A325 and A490; diameters and thread pitch are imperial |
| EN 14399 / EN 15048 | 8.8, 10.9 | European projects | EN 14399 covers preloaded bolts; EN 15048 covers non-preloaded bolts |
| AS/NZS 1252 | 8.8, 10.9 | Australia and New Zealand | Metric dimensions with specific marking and batch testing requirements |
Bearing and slip-critical connections
The connection type decides how the load crosses the joint, and that changes the bolt grade, the hole size, the surface preparation and the tightening method.
| Connection type | How the load crosses | Hole size | Tightening | Inspection focus |
|---|---|---|---|---|
| Bearing (shear) | Bolt shear and bearing on the plate | Standard or oversize holes within the code limits | Snug-tight, or a specified torque where required | Bolt grade, hole size and fit-up |
| Slip-critical | Friction between the faying surfaces | Standard holes, often with tighter clearance | Preloaded to a specified tension by torque or turn-of-nut | Calibration, tightening records and surface preparation |
Hole size, tolerance and site fit-up
- Hole diameter is set by the bolt standard and the connection type; it is not a workshop preference.
- Standard, oversize and slotted holes have different limits, and oversize holes are not allowed in every connection.
- Drilling gives a cleaner hole than punching; punching is limited by thickness and by the project standard.
- Reaming is used to correct misalignment, but it changes the hole and has to be approved.
- Connection geometry and hole position tolerance decide how much site fit-up the frame can absorb.
- Anchor bolts and base plates are a separate interface: the setting plan has to match the foundation, not the other way round.
Coating, tightening and what to send with the enquiry
- The bolt system, grade and diameter, and the standard they are supplied to.
- The connection type: bearing or slip-critical, and the faying surface preparation.
- The hole size and tolerance, and whether oversize or slotted holes are permitted.
- The bolt coating: zinc plating, hot-dip galvanizing, zinc flake or another system, and how it affects thread tolerance and lubrication.
- Whether bolts, nuts and washers are supplied with the structure or by the site contractor.
- The tightening method, the torque or tension values, and who calibrates the equipment.
- The inspection records: batch certificates, calibration records and tightening inspection.
This guide describes general practice for steel structure projects. It is not a design, and it does not replace the specification or the engineer responsible for your project.