Structural steel connections use specialized bolt standards designed for high-strength, controlled-preload joints. The primary US standards are ASTM A325 (equivalent to ISO 8.8S / metric 8.8) and ASTM A490 (equivalent to ISO 10.9S / metric 10.9).
These are distinct from regular hex bolts — structural bolts have shorter thread lengths, larger heads, and are supplied with matched heavy hex nuts and hardened washers as an assembly.
| Bolt Diameter | A325 (kips) | A490 (kips) |
|---|---|---|
| 1/2" | 12 | 15 |
| 5/8" | 19 | 24 |
| 3/4" | 28 | 35 |
| 7/8" | 39 | 49 |
| 1" | 51 | 64 |
| 1-1/8" | 56 | 80 |
| 1-1/4" | 71 | 102 |
Per AISC Table J3.1 / RCSC Table 8.1. 1 kip = 1,000 lbs.
Turn-of-nut method: Snug bolts first, then apply a specified additional rotation (typically 1/3 to 1 full turn depending on bolt length). Most reliable for achieving target pretension. No torque wrench needed — uses geometry instead of friction.
Calibrated wrench method: Uses torque to achieve pretension. Requires daily calibration against a Skidmore-type bolt tension indicator. Less reliable than turn-of-nut because torque-tension relationship varies with lubrication.
Direct tension indicators (DTI): Washer-like devices with raised bumps that flatten under load. When the gap between the DTI and the hardened washer closes to the specified dimension, the bolt has reached minimum pretension. Provides visual confirmation.
Twist-off (TC) bolts: Have a splined end that shears off at a calibrated torque. Simple and fast — when the tip snaps off, the bolt is tightened. Most common in new construction.