UK Bolt Pretension -- EN 1993-1-8 Clauses 3.8 and 3.9 Slip-Resistant Connection Design
Preloaded bolts provide a clamping force between connected plies that generates friction resistance to slip, enabling the design of connections where relative movement between connected parts cannot be tolerated. EN 1993-1-8 Clause 3.8 defines the design preload force Fp,Cd, and Clause 3.9 provides the slip resistance formula. The UK National Annex confirms the partial factors and provides supplementary guidance on tightening methods and slip factor determination for UK practice. This reference covers the complete preloaded bolt design methodology, including the slip resistance calculation for Categories D and E, tightening methods per BS EN 1090-2, and worked examples for moment and bracing connections.
Bolt Categories per EN 1993-1-8 Clause 3.4
EN 1993-1-8 classifies bolted connections into five categories based on their mode of load transfer and the design checks required:
| Category | Load Transfer Mechanism | Critical Checks | Slip Check |
|---|---|---|---|
| A | Bearing type, non-preloaded | Shear Fv,Rd, bearing Fb,Rd | None |
| B | Slip-resistant at SLS, bearing at ULS | Preload, slip at SLS, shear/bearing at ULS | At SLS |
| C | Slip-resistant at ULS | Preload, slip at ULS, net section | At ULS |
| D | Slip-resistant at ULS, non-preloaded bolts permitted | Preload, slip at ULS, net section | At ULS |
| E | Slip-resistant at SLS, non-preloaded equivalent permitted | Preload, slip at SLS | At SLS |
In UK practice:
- Category A is the default for simple shear connections (fin plates, partial-depth end plates).
- Category D is specified for connections subject to load reversal, vibration, or fatigue where slip at the ULS would compromise structural integrity.
- Category E is specified for connections where slip must be controlled at SLS for serviceability (e.g., avoiding visible gap opening at bolted splice plates under wind loading).
Design Preload Force Fp,Cd per Clause 3.8
The design preload force is the minimum clamping force that must be developed in the bolt at the time of tightening. Per EN 1993-1-8 Clause 3.8:
Fp,Cd = 0.7 x fub x As / gamma_M7
Where:
- 0.7 is the ratio of preload to ultimate tensile strength, reflecting the target 70% of fub as the installed preload
- fub is the bolt ultimate tensile strength
- As is the tensile stress area of the bolt
- gamma_M7 = 1.10 (UK NA value, confirmed in the UK NA to BS EN 1993-1-8)
The 70% factor accounts for:
- Relaxation of bolt tension over time due to embedment and creep in the connected plies.
- A safety margin below the bolt yield point to prevent yielding during tightening.
- Variability in the torque-tension relationship for torque-controlled tightening.
Fp,Cd Values for UK Structural Bolts
| Bolt | As (mm^2) | Class 8.8: Fp,Cd (kN) | Class 10.9: Fp,Cd (kN) |
|---|---|---|---|
| M12 | 84.3 | 42.9 | 53.6 |
| M16 | 157 | 79.9 | 99.9 |
| M20 | 245 | 124.7 | 155.9 |
| M22 | 303 | 154.2 | 192.7 |
| M24 | 353 | 179.7 | 224.6 |
| M27 | 459 | 233.6 | 292.0 |
| M30 | 561 | 285.5 | 356.9 |
| M36 | 817 | 415.9 | 519.9 |
Slip Resistance Fs,Rd per Clause 3.9
The design slip resistance of a preloaded Class 8.8 or 10.9 bolt is:
Fs,Rd = ks x n x mu x Fp,Cd / gamma_M3
Where:
- ks = hole type factor: 1.0 for standard round holes, 0.85 for oversized or short slotted holes, 0.70 for long slotted holes
- n = number of friction interfaces (1 for lap joint, 2 for cover plate splice)
- mu = slip factor dependent on surface preparation (EN 1993-1-8 Table 3.6)
- Fp,Cd = design preload force per Clause 3.8
- gamma_M3 = 1.25 for ULS (Category C and D), 1.10 for SLS (Category E) per UK NA
Slip Factors mu per EN 1993-1-8 Table 3.6
| Surface Treatment Class | Preparation | mu | UK Application |
|---|---|---|---|
| Class A | Grit blasted, no paint, light rust removed | 0.50 | Faying surfaces of moment connections, bridge splices |
| Class B | Grit blasted, alkali-zinc silicate paint (50-80 micron) | 0.40 | Standard UK treatment for preloaded building connections |
| Class C | Cleaned by wire brushing or flame cleaning, loose rust removed | 0.30 | Minimum standard, maintenance of existing structures |
| Class D | Untreated surfaces, as-rolled | 0.20 | Not recommended for slip-resistant connections |
For UK building frame connections, Class A and Class B are the standard specifications. Class A (mu = 0.50) provides the highest slip resistance but requires strict site control to prevent surface contamination after blasting. Class B (mu = 0.40) with zinc silicate paint is more practical for building construction because the paint protects the surface until connection assembly.
Fs,Rd Values for Standard UK Configurations
For M20 Class 8.8 bolts, single interface (n = 1), standard holes (ks = 1.0), Class B surface (mu = 0.40):
At ULS (gamma_M3 = 1.25): Fs,Rd = 1.0 x 1 x 0.40 x 124.7 / 1.25 = 39.9 kN
At SLS (gamma_M3 = 1.10): Fs,Rd = 1.0 x 1 x 0.40 x 124.7 / 1.10 = 45.3 kN
For the same configuration with Class A surface (mu = 0.50): Fs,Rd (ULS) = 49.9 kN, Fs,Rd (SLS) = 56.7 kN.
For a cover plate splice in a bridge girder (n = 2, Class A surface): Fs,Rd (ULS) per bolt = 1.0 x 2 x 0.50 x 124.7 / 1.25 = 99.8 kN.
Tightening Methods per BS EN 1090-2
The UK NA to BS EN 1993-1-8 references BS EN 1090-2 for the approved tightening methods. Four methods are available, each with specific requirements for UK execution:
Torque Control Method (Method T)
The bolt is tightened with a calibrated torque wrench to a target torque that corresponds to the required preload. The torque coefficient k (or nut factor) must be determined by pre-installation verification testing (PIVT) using a representative sample of the bolt-nut-washer assemblies to be used on site.
Target torque: T = k x Fp,Cd x d
For an M20 Class 8.8 bolt with an experimentally determined k = 0.20: T = 0.20 x 124.7 x 20 = 499 N.m
This is the standard UK site method for building connections. The calibrated wrench must be checked daily against a torque analyser, and the bolt assemblies must be from the same production batch as those used in the PIVT.
Combined Method (Method CM)
A part-turn is applied after the snug-tight condition is achieved. The snug-tight condition is defined as the point where the connected plies are in firm contact. A further rotation of the nut through a prescribed angle then develops the required preload.
For bolts up to 4d grip length:
- Snug-tight, then 1/3 turn for M12-M20
- Snug-tight, then 1/2 turn for M22-M36
Method CM is less common in UK building construction but is used for large-diameter bolts (M30 and above) and bridge connections where torque control becomes less reliable due to thread friction variability.
Direct Tension Indicator Method (Method HRC)
Special washers with raised protrusions (DTI washers) are placed under the bolt head. As the bolt is tightened, the protrusions crush, and the gap between the DTI washer and the bolt head is measured with a feeler gauge. When the gap reduces to the specified feeler gauge thickness, the required preload has been achieved. DTI washers provide a direct visual indication of preload and are increasingly common in UK bridge construction and wind turbine tower connections.
Nut Rotation from Snug-Tight
This method, defined in AISC/RCSC specifications but less common in UK practice, relies on the linear relationship between nut rotation and bolt elongation in the elastic range. It is rarely used in UK steelwork due to the difficulty of establishing a reliable snug-tight reference point on site.
Worked Example -- Slip-Resistant Brace Connection
Consider a bracing connection in a UK multi-storey braced frame. The brace is a CHS 168.3 x 8 in S355, connected to a gusset plate via a slotted connection. The gusset plate is bolted to the beam-column joint with 4 M20 Class 8.8 preloaded bolts (Category E, slip-resistant at SLS).
Given:
- Brace force at SLS: N_SLS = 180 kN (wind)
- Bolt configuration: 4 M20, standard round holes, single shear interface (n = 1)
- Surface: Class B (grit blast + zinc silicate paint, mu = 0.40)
- Category E: gamma_M3,SLS = 1.10
SLS slip check: Fs,Rd per bolt = 1.0 x 1 x 0.40 x 124.7 / 1.10 = 45.3 kN Total slip resistance: 4 x 45.3 = 181.2 kN > 180 kN. OK.
ULS bearing check (slip overcome, connection bears in shear): Fv,Rd per bolt = 94.1 kN (threads in shear plane) Total shear resistance: 4 x 94.1 = 376.4 kN >> N_ULS = 1.5 x 180 = 270 kN. OK.
The connection meets both SLS slip and ULS bearing requirements with adequate margin.
UK National Annex Provisions for Preloaded Bolts
The UK NA to BS EN 1993-1-8 makes the following key confirmations and additions for preloaded bolt design:
gamma_M7 = 1.10 is confirmed for the design preload force Fp,Cd.
gamma_M3 = 1.25 (ULS) and gamma_M3 = 1.10 (SLS) are confirmed for slip resistance.
The slip factors mu in Table 3.6 are adopted without modification. However, the UK NA recommends that for UK projects, the slip factor be verified by slip testing to BS EN 1090-2 Annex G for the specific surface treatment and bolt assembly combination, unless the designer can demonstrate that the Table 3.6 conservative values are acceptable.
Bolt tightening to be in accordance with BS EN 1090-2, with the method specified on the construction drawings.
For connections subject to impact or fatigue, the UK NA requires that all bolts be fully preloaded and that the slip resistance be demonstrated at ULS (Category C or D).
Design Resources
- UK Steel Grades Reference -- EN 10025-2 grade selection
- UK Bolt Capacity Tables -- Fv,Rd and Ft,Rd tables
- UK Bolt Spacing Requirements -- Edge distance and pitch
- UK Bolt Grades Guide -- Class selection
- 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 design preload for an M20 Class 8.8 bolt per UK NA provisions?
The design preload force Fp,Cd = 0.7 x 800 x 245 / 1.10 = 124.7 kN, where gamma_M7 = 1.10 per the UK National Annex to BS EN 1993-1-8. This represents 70% of the bolt's ultimate tensile capacity, providing a 30% margin against bolt fracture during tightening and accounting for relaxation over the service life of the connection.
How is the slip factor mu determined for a UK connection?
The slip factor mu is selected from EN 1993-1-8 Table 3.6 based on the surface treatment class. For Class A (grit blasted, no paint): mu = 0.50. For Class B (grit blasted, zinc silicate paint): mu = 0.40. The UK NA recommends verifying the slip factor by slip testing to BS EN 1090-2 Annex G, particularly for Class A surfaces where site contamination can reduce the effective mu below the tabulated value. The test involves loading a preloaded bolt assembly in shear and measuring the load at which macroscopic slip (0.15 mm displacement) occurs.
What is the difference between Category E and Category D preloaded connections?
Category E connections are designed to be slip-resistant at the serviceability limit state (SLS) only. At the ultimate limit state (ULS), the connection may slip and bear in shear against the bolt holes. The SLS slip check uses gamma_M3,SLS = 1.10. Category D connections must remain slip-resistant at ULS, using gamma_M3,ULS = 1.25, which provides a lower slip resistance per bolt. Category D is specified for connections where slip at ULS would cause unacceptable structural behaviour -- typically connections subject to load reversal, fatigue, or those critical to overall frame stability.
Are preloaded bolts always required for UK moment connections?
Not all moment connections require preloaded bolts. Category A (non-preloaded) bolted end plate connections are acceptable for many UK building applications provided the connection is designed to be nominally pinned under gravity loads (simple construction) or the moment transfer does not rely on friction. Preloaded bolts (Category E minimum) are required for moment-resisting frames where: (1) the frame stability relies on a limited number of stiff joints, (2) the connection is subject to frequent load reversal, (3) fatigue governs design, or (4) the connection is classified as a full-strength or rigid joint in a sway frame. The UK NA provides specific guidance on the interaction between frame classification and bolt preloading requirements.
Educational reference only. All design values are per BS EN 1993-1-8:2005 + UK National Annex and BS EN 1090-2:2018. 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.