Bolt pretension & turn-of-the-nut
The RCSC minimum pretension values for A325 and A490 structural bolts, the turn-of-the-nut rotation table, and what snug-tight actually means — the numbers a field crew and an inspector argue about, in one place.
Pretension and turn-of-the-nut lookup
A325 min pretension
28 kips
A490 min pretension
35 kips
Turn from snug-tight
1/2 turn (180°)
tolerance ±30°
RCSC values for bolts with both faces roughly perpendicular to the bolt axis. Sloped surfaces without beveled washers require more rotation. Installation method and inspection belong to the contract documents and the engineer of record.
Minimum bolt pretension
70% of the bolt's specified minimum tensile strength, in kips. These apply to pretensioned and slip-critical joints — a snug-tight joint has no pretension requirement at all.
| Diameter (in) | 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" | 64 | 80 |
| 1-1/4" | 81 | 102 |
| 1-3/8" | 97 | 121 |
| 1-1/2" | 118 | 148 |
Turn-of-the-nut rotation
From snug-tight, with match-marks on the nut and bolt, for bolts whose outer faces are roughly perpendicular to the bolt axis. Rotation is set by bolt length measured in diameters.
| Bolt length | Rotation from snug | Tolerance |
|---|---|---|
| Up to 4 diameters | 1/3 turn (120°) | ±30° |
| Over 4, up to 8 diameters | 1/2 turn (180°) | ±30° |
| Over 8, up to 12 diameters | 2/3 turn (240°) | ±45° |
Beyond 12 diameters, and for sloped surfaces without beveled washers, the RCSC Specification governs — the table does not extend.
Common questions
- What is the turn-of-the-nut method?
- A pretensioning method: bring the joint to snug-tight, match-mark the nut and bolt, then rotate the nut a specified fraction of a turn based on bolt length — 1/3 turn up to 4 diameters, 1/2 turn to 8, 2/3 turn to 12. The controlled rotation stretches the bolt to at least the required minimum pretension.
- What does snug-tight mean?
- The condition reached with a few impacts of an impact wrench or the full effort of a worker on an ordinary spud wrench — the plies are in firm contact. It is a starting condition, not a measured torque, and many shear connections need nothing more than snug-tight.
- How much pretension does a 3/4 inch A325 bolt need?
- 28 kips minimum — 70% of the bolt's minimum tensile strength. The same 3/4 inch diameter in A490 requires 35 kips. Values step up with diameter; see the full table on this page.
- When is pretensioning required instead of snug-tight?
- When the contract documents say so: slip-critical joints, joints subject to significant load reversal, fatigue, or where the specification requires it. The engineer of record decides — the erector's job is to install to the condition shown, and the inspector's is to verify it.
- What other installation methods exist besides turn-of-the-nut?
- Calibrated wrench (torque established daily in a tension calibrator), twist-off tension-control (TC) bolts whose splined end shears at the right tension, and direct tension indicators (DTI washers whose bumps compress). All four are recognized; torque tables alone are not an installation method.
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