EN 1993-1-8 Bolt Pretension — Slip-Resistant Connections Categories B and C
Complete reference for preloaded bolts in European steel connections per EN 1993-1-8 Clause 3.6.1 and 3.9. Design preload force Fp,Cd tables for Class 8.8 and 10.9 bolts M12-M36, slip resistance Fs,Rd per Category B (SLS) and Category C (ULS), ks surface preparation factors, and EN 14399 assembly requirements.
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Why Preloaded Bolts?
In standard bearing-type connections (Category A), bolts transfer shear through bearing of the bolt shank against the hole wall. Slip at the faying surfaces occurs before the bolt engages in bearing.
Preloaded bolts (Categories B and C) apply a clamping force that compresses the connected plies together, developing friction resistance that prevents slip. This is required when:
- Slip at service load would be unacceptable (aesthetic, alignment, fatigue)
- The connection is subject to load reversal or impact
- The joint must behave as rigid (slip-critical at ULS)
- Prying forces in tension connections must be controlled
- The connection is in a fatigue-sensitive location
EN 1993-1-8 Bolt Categories
EN 1993-1-8 Table 3.1 defines five bolt categories for shear and tension connections.
Shear Connection Categories
| Category | Type | Bolting | Criterion |
|---|---|---|---|
| A | Bearing type | Any class 4.6-10.9, snug-tight or preloaded | Fv,Rd (shear), Fb,Rd (bearing) |
| B | Slip-resistant at SLS | Preloaded 8.8 or 10.9 | Fs,Rd,ser (slip at SLS), Fb,Rd (bearing at ULS) |
| C | Slip-resistant at ULS | Preloaded 8.8 or 10.9 | Fs,Rd (slip at ULS), Fb,Rd (bearing at ULS) |
Tension Connection Categories
| Category | Type | Bolting | Criterion |
|---|---|---|---|
| D | Non-preloaded | Any class 4.6-10.9 | Ft,Rd (tension), Bp,Rd (punching) |
| E | Preloaded | Preloaded 8.8 or 10.9 | Ft,Rd (tension), Bp,Rd (punching) |
For Category B, slip must not occur at the serviceability limit state (SLS). For Category C, slip must not occur at the ultimate limit state (ULS) — this is the most demanding category.
Design Preload Force — Fp,Cd
EN 1993-1-8 Clause 3.6.1 defines the design preload force:
Fp,Cd = 0.7 x fub x As / gamma_M7
Where:
- 0.7 = factor accounting for relaxation and scatter in preload application
- fub = bolt ultimate tensile strength (800 MPa 8.8; 1000 MPa 10.9)
- As = tensile stress area of the bolt
- gamma_M7 = 1.10 (partial factor for preloading per EN 1993-1-8)
Preload Force Table — Class 8.8 (fub = 800 MPa, gamma_M7 = 1.10)
| Bolt Size | As (mm2) | Fp,Cd (kN) | 0.7 fub As (kN) |
|---|---|---|---|
| M12 | 84.3 | 42.9 | 47.2 |
| M16 | 157 | 79.9 | 87.9 |
| M20 | 245 | 124.7 | 137.2 |
| M22 | 303 | 154.2 | 169.7 |
| M24 | 353 | 179.6 | 197.7 |
| M27 | 459 | 233.6 | 257.0 |
| M30 | 561 | 285.5 | 314.2 |
| M36 | 817 | 415.8 | 457.5 |
Preload Force Table — Class 10.9 (fub = 1000 MPa, gamma_M7 = 1.10)
| Bolt Size | As (mm2) | Fp,Cd (kN) |
|---|---|---|
| M12 | 84.3 | 53.6 |
| M16 | 157 | 99.9 |
| M20 | 245 | 155.9 |
| M22 | 303 | 192.8 |
| M24 | 353 | 224.6 |
| M27 | 459 | 292.0 |
| M30 | 561 | 357.0 |
| M36 | 817 | 519.8 |
For the same bolt size, Class 10.9 provides 25% higher preload than Class 8.8, which directly translates to 25% higher slip resistance (Fs,Rd).
Slip Resistance — Fs,Rd
Category C — Slip Resistance at ULS
Fs,Rd = ks x n x mu x Fp,Cd / gamma_M3
Category B — Slip Resistance at SLS
Fs,Rd,ser = ks x n x mu x Fp,Cd / gamma_M3,ser
Where:
| Symbol | Parameter | Value / Source |
|---|---|---|
| ks | Hole type factor | 1.00 (normal clearance); 0.85 (oversized); 0.70 (long slotted) |
| n | Number of friction surfaces | 1 for single-shear; 2 for double-shear |
| mu | Slip factor (surface preparation dependent) | See table below |
| gamma_M3 | Partial factor for slip at ULS | 1.25 (UK NA) |
| gamma_M3,ser | Partial factor for slip at SLS | 1.10 (UK NA) |
Slip Factor (mu) by Surface Preparation
The slip factor mu is determined by the surface preparation of the faying surfaces. EN 1993-1-8 Table 3.5 provides indicative values:
| Surface Class | Surface Preparation | Slip Factor mu |
|---|---|---|
| A | Shot-blasted or grit-blasted, no pitting, lightly rusted | 0.50 |
| B | Shot-blasted or grit-blasted, spray-metallised with aluminium or zinc | 0.40 |
| C | Cleaned by wire-brushing or flame-cleaning, loose rust removed | 0.30 |
| D | As-rolled (mill scale intact) | 0.20 |
Practical defaults for European construction:
- Shot-blasted + inorganic zinc silicate paint (common shop-primed surfaces): mu = 0.40 (Surface Class B)
- Shot-blasted only, exposed for light rusting (standard for slip-critical): mu = 0.50 (Surface Class A)
- Galvanised (hot-dip) — a separate test is required per EN 1090-2 Annex G; typical mu after surface treatment: 0.35-0.40
Surface preparation must be verified by slip factor testing per EN 1090-2 Annex G for projects where the code-prescribed mu values are not directly applicable. For surface classes not listed in Table 3.5, the slip factor must be determined experimentally.
Slip Resistance Tables — Category C (ULS, gamma_M3 = 1.25)
Single-shear connection (n = 1), normal clearance holes (ks = 1.0), mu = 0.50 (Class A, shot-blasted):
| Bolt Size | Fs,Rd Class 8.8 (kN) | Fs,Rd Class 10.9 (kN) |
|---|---|---|
| M12 | 17.2 | 21.5 |
| M16 | 32.0 | 40.0 |
| M20 | 49.9 | 62.4 |
| M22 | 61.7 | 77.1 |
| M24 | 71.9 | 89.8 |
| M27 | 93.5 | 116.8 |
| M30 | 114.2 | 142.8 |
| M36 | 166.3 | 207.9 |
Double-shear connection (n = 2): double the values above.
For shot-blasted + painted (Surface Class B, mu = 0.40): multiply values by (0.40/0.50) = 0.80.
Slip Resistance vs Bearing Resistance — When Slip Governs
An important check: is the slip resistance (Category C) lower than the bearing bolt shear resistance (Category A)? If yes, slip resistance governs the design.
For M20 Class 8.8, single-shear:
- Category A (bearing): Fv,Rd = 47.0 kN (threads in shear plane)
- Category C (slip): Fs,Rd = 49.9 kN (Class A, mu = 0.50)
- Category C (slip, painted): Fs,Rd = 39.9 kN (Class B, mu = 0.40)
For a shot-blasted surface (Class A), the slip resistance (49.9 kN) slightly exceeds the bolt shear resistance (47.0 kN), so shear governs at ULS — but slip must still be checked at SLS for Category B. For painted surfaces (Class B), slip governs (39.9 kN < 47.0 kN), reducing the effective connection capacity. The surface preparation decision has a first-order effect on the connection design.
EN 14399 Preloaded Bolt Assembly Requirements
Preloaded bolts must comply with EN 14399, which specifies the complete assembly (bolt + nut + washer) as a matched, tested system. Key requirements:
- Matched assemblies: Nuts and washers must be from the same manufacturer. Mixing components voids the preload certification.
- k-class: The bolt assembly must be designated with a k-class (k-factor) that covers the torque-preload relationship — k-class K1 or K2 per EN 14399-2.
- Lubrication: Preloaded bolts must be lubricated per the manufacturer's specification. Dry or corroded bolts will not achieve the specified preload.
- Tightening methods:
- Torque method (HR system, EN 14399-3): Preload controlled by calibrated torque wrench
- Combined method (HV system, EN 14399-4): Torque + part-turn angle control
- Direct tension indicator (DTI): Load-indicating washers per EN 14399-9
- Tension control (TC) bolts: Spline shear-off at calibrated torque
- Inspection: Preloaded bolts must be inspected per EN 1090-2, typically by checking that 5% of bolts in a random sample achieve the calibrated tightening criterion.
Practical Guidance — When to Use Each Category
| Scenario | Category | Reason |
|---|---|---|
| Simple beam-to-column shear connection (fin plate, web cleat) | A (snug-tight 8.8) | Slip at connection is not detrimental; ductile behaviour preferred |
| End plate moment connection (building) | A or B (preloaded 8.8) | A if prying is controlled; B if slip at SLS is a concern |
| Column splice in multi-storey building | A (snug-tight 8.8) | Compression-dominant; slip accommodated by column continuity plates |
| Bracing connection with load reversal | C (preloaded 10.9) | Reversal means slip at ULS would compromise frame stiffness |
| Crane runway girder connection | C (preloaded 8.8) | Fatigue-sensitive; slip at ULS must be prevented |
| Bridge girder splice | C (preloaded 10.9, Class A surface) | Slip-critical at ULS; high preload from 10.9 preferred |
| Tension-only hanger connection | E (preloaded 8.8) | Preload controls prying; tension governs at ULS |
| Secondary member (handrail, purlin) | A (snug-tight 4.6 or 8.8) | Non-critical; bearing-only sufficient |
Frequently Asked Questions
What is the design preload force Fp,Cd for an M20 Class 8.8 bolt? Fp,Cd = 0.7 x fub x As / gamma_M7 = 0.7 x 800 x 245 / 1.10 = 124.7 kN. The partial factor gamma_M7 = 1.10 accounts for losses due to relaxation and scatter in the applied preload. The nominal (unfactored) preload 0.7 x 800 x 245 = 137.2 kN is the target tightening force at the time of installation.
What is the difference between Category B and Category C slip-resistant connections? Category B checks slip resistance at the serviceability limit state (SLS) only; at ULS, the connection reverts to bearing (Category A checks apply). Category C checks slip resistance at the ultimate limit state (ULS) — this is the most demanding category. Category C connections use gamma_M3 = 1.25 (at ULS), while Category B uses gamma_M3,ser = 1.10 (at SLS). Category C is required for connections subject to load reversal, impact, or where slip at ULS would compromise structural behaviour (e.g., bracing in seismic frames).
How does surface preparation affect slip factor mu in preloaded connections? The slip factor mu ranges from 0.20 (as-rolled mill scale) to 0.50 (shot-blasted, clean, lightly rusted). Shot-blasting to Surface Class A (mu = 0.50) provides 2.5 times the slip resistance of untreated as-rolled surfaces. Paint and galvanising reduce mu to 0.30-0.40. For critical slip-resistant connections (Category C), specifying Surface Class A (shot-blasted, no painting of faying surfaces) is standard. The mu value specified must be substantiated by testing per EN 1090-2 Annex G for the specific surface preparation used on the project.
Why is gamma_M7 used for preload instead of gamma_M2? gamma_M7 = 1.10 is a lower partial factor than gamma_M2 = 1.25. EN 1993-1-8 applies gamma_M7 specifically to preloading because the preload force is a controlled installation parameter, not a material resistance. The preload is verified during installation (torque checks, DTI washers, TC spline shear-off), providing a higher level of reliability than the material-based bolt resistances, which use gamma_M2 = 1.25. In effect, EN 1993-1-8 gives credit for the quality control inherent in preload application.
Can Class 10.9 bolts be used in Category E (tension) preloaded connections? Yes, Class 10.9 preloaded bolts are permitted in Category E tension connections per EN 1993-1-8 Table 3.1. The higher fub (1000 vs 800 MPa) provides higher tension resistance (Ft,Rd) and higher preload force (Fp,Cd), which helps control prying action. However, Class 10.9 bolts are more notch-sensitive than Class 8.8 and require careful hydrogen embrittlement control (plating, pickling, and environmental exposure). For standard building structures, Class 8.8 preloaded bolts in Category E are typically adequate.
Related Pages
- EN 1993-1-8 Connection Design Guide — Bolt categories, welds, joint classification
- EN 1993-1-8 Bolt Grades — 4.6, 5.6, 8.8, 10.9 mechanical properties
- EN 1993-1-8 Bolt Capacity Tables — Fv,Rd, Ft,Rd, bearing
- EN 1993-1-8 Bolt Hole Sizes — Clearance, oversized, slotted
- EN 1993-1-1 Beam Design Guide — Mc,Rd, Vc,Rd, LTB
- European Steel Properties — EN 10025 S235-S460
- Bolted Connections Calculator
- Bolt Torque Calculator
- Beam Capacity Calculator
- Column Capacity Calculator
Educational reference only. All preload values and slip resistance formulas are per EN 1993-1-8:2005. Verify surface preparation and slip factors against EN 1090-2 Annex G for the specific project conditions. Preloaded bolt assemblies must comply with EN 14399 as a matched system. Results are PRELIMINARY — NOT FOR CONSTRUCTION. All designs must be independently verified by a licensed Professional Engineer or Chartered Structural Engineer.