UK Bolt Torque Chart — EN 1993-1-8 Preload and Tightening for 8.8 and 10.9 Bolts
Torque and preload values for UK structural bolting per EN 1993-1-8 Clause 3.8 with UK National Annex. Class 8.8 and Class 10.9 bolts from M12 to M36, including preload force Fp,Cd, tightening torque values, and installation procedures for non-tension (Categories A-C) and preloaded (Categories D-E) connections in UK steelwork to BS EN 1090-2.
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Preload Force Fp,Cd per EN 1993-1-8
For preloaded bolts (Categories D — slip-resistant at ULS and E — slip-resistant at SLS), the design preload force is:
Fp,Cd = 0.7 × fub × As / γM7
Where:
- fub = bolt ultimate tensile strength
- As = tensile stress area
- γM7 = 1.10 (UK NA value) — Note: the UK NA modifies γM7 from the recommended 1.10 (confirmed as consistent)
Class 8.8 Bolt Preload and Torque (fub = 800 MPa)
| Bolt | As (mm²) | Fp,Cd — Preload (kN) | Tightening Torque (N·m) | Torque (lbf·ft) |
|---|---|---|---|---|
| M12 | 84.3 | 42.9 | 86 | 63 |
| M16 | 157 | 79.9 | 213 | 157 |
| M20 | 245 | 124.7 | 416 | 307 |
| M22 | 303 | 154.2 | 565 | 417 |
| M24 | 353 | 179.7 | 719 | 530 |
| M27 | 459 | 233.6 | 1,051 | 775 |
| M30 | 561 | 285.5 | 1,427 | 1,053 |
| M36 | 817 | 415.9 | 2,495 | 1,840 |
Torque calculated as T = 0.20 × Fp,Cd × d (coefficient of friction μ = 0.20, UK standard assumption).
Class 10.9 Bolt Preload and Torque (fub = 1,000 MPa)
| Bolt | As (mm²) | Fp,Cd — Preload (kN) | Tightening Torque (N·m) | Torque (lbf·ft) |
|---|---|---|---|---|
| M12 | 84.3 | 53.6 | 107 | 79 |
| M16 | 157 | 99.9 | 266 | 196 |
| M20 | 245 | 155.9 | 520 | 383 |
| M22 | 303 | 192.7 | 706 | 521 |
| M24 | 353 | 224.6 | 898 | 662 |
| M27 | 459 | 292.0 | 1,314 | 969 |
| M30 | 561 | 356.9 | 1,785 | 1,316 |
| M36 | 817 | 519.9 | 3,119 | 2,300 |
Installation Methods per BS EN 1090-2
| Method | Abbreviation | Description | Coefficient | UK Practice |
|---|---|---|---|---|
| Torque control | T | Calibrated torque wrench | μ ≈ 0.14-0.24 | Standard method for UK site bolting |
| Combined method | CM | Part-turn after snug-tight | μ calculated | Used for large diameter (≥ M24) |
| HRC (tension) | HRC | Direct tension indicator | μ-independent | Increasingly common — uses DTI washers |
| Nut rotation | — | Specified nut rotation from snug | μ estimated | Rare in UK — verification bolts only |
UK NA Modifications
The UK NA to BS EN 1993-1-8 confirms:
- γM7 = 1.10 for preloaded bolts (Category D and E connections)
- Slip factor μ = 0.30-0.50 depending on surface treatment per Table 3.6
- UK practice uses M20 class 8.8 as the default for preloaded connections in building frames
- M24 class 10.9 is standard for bridge connections and heavily loaded moment connections
Design Resources
- UK Steel Grades Reference — EN 10025-2 grade selection for UK projects
- UK Steel Mechanical Properties — fy, fu, and elongation tables
- UK Universal Beam and Column Sizes — UB/UC section dimensions and properties
- UK Bolt Capacity Tables — Class 8.8 and 10.9 bolt resistance
- UK Beam Design Guide — EN 1993-1-1 flexure, shear, and LTB
- UK Connection Design Guide — EN 1993-1-8 bolted and welded joints
- All UK Steel Design References — complete library
Frequently Asked Questions
What is the tightening torque for an M20 Class 8.8 bolt in UK practice?
The tightening torque for M20 Class 8.8 with an assumed friction coefficient of 0.20 is approximately 416 N·m (307 lbf·ft). This corresponds to a preload force Fp,Cd = 124.7 kN. The actual torque depends on the friction coefficient (μ = 0.14-0.24), which should be verified by torque-tension testing per BS EN 1090-2 for the specific bolt-nut-washer combination.
What γM7 value does the UK NA specify for preloaded bolts?
The UK NA to BS EN 1993-1-8 specifies γM7 = 1.10 for preloaded bolts (Categories D and E). This is the same as the recommended value. The γM7 factor is applied to the design preload force: Fp,Cd = 0.7 × fub × As / γM7. Note that γM7 applies only to preloading; bolt shear and tension resistance use γM2 = 1.25.
When are preloaded bolts required in UK steelwork?
Preloaded bolts are required per BS EN 1090-2 for: (1) slip-resistant connections at ULS (Category D), (2) connections subject to frequent load reversal or vibration (e.g., crane runway beams), (3) bridge connections where slippage cannot be tolerated, (4) connections in wind turbines and dynamically loaded structures. For simple shear connections in UK building frames, non-preloaded bolts (Categories A-C) are standard.
What is the standard bolt specification for UK moment connections?
The standard UK specification for moment-resisting end plate connections is M20 Class 8.8 preloaded bolts for typical building frames, and M24 Class 10.9 for heavy moment connections or bridge applications. British Steel recommends M20 8.8 as the default for steel moment connections in multi-storey buildings. M36 is used only for very large connections such as those in power generation, heavy industrial, or major infrastructure.
Related Pages
- EN 1993 Steel Design Overview
- European Steel Properties
- EN 1993 Beam Design Guide
- EN 1993 Column Buckling
- EN 1990 Load Combinations
- UK Steel Chemical Composition
- UK Steel Charpy Values
Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK National Annex and BS EN 10025-2:2019. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.