UK Bolt Capacity Tables -- EN 1993-1-8 Shear and Tension Resistance with UK National Annex
The design resistance of structural bolts forms the foundation of every UK steel connection design. EN 1993-1-8 Table 3.4 provides the general formula for bolt shear resistance Fv,Rd and bolt tension resistance Ft,Rd, referencing the tensile stress area As and the bolt ultimate strength fub. This reference presents complete capacity tables for M12 through M36 bolts in Class 4.6, 8.8, and 10.9, covering shear with threads in and outside the shear plane, tension, combined shear and tension interaction, and the UK NA partial factors that govern UK practice.
Regulatory Framework
The bolt shear resistance formula derives from EN 1993-1-8 Clause 3.6 and Table 3.4:
Fv,Rd = alpha_v x fub x As / gamma_M2
Where:
- alpha_v = 0.6 for Classes 4.6, 5.6, 8.8 when the shear plane passes through the threaded portion
- alpha_v = 0.5 for Classes 4.8, 5.8, 6.8, 10.9 when the shear plane passes through the threaded portion
- alpha_v = 0.6 for all classes when the shear plane passes through the unthreaded shank
- fub = bolt ultimate tensile strength (400, 500, 600, 800, or 1000 MPa depending on class)
- As = tensile stress area of the bolt thread
- gamma_M2 = 1.25 (UK NA confirmed value)
The tension resistance formula:
Ft,Rd = k2 x fub x As / gamma_M2
Where k2 = 0.9 for all bolt classes, and gamma_M2 = 1.25.
Bolt Tensile Stress Areas
The tensile stress area As represents the effective cross-sectional area of the bolt at the threaded section, accounting for the reduced diameter at the thread root. Values are per ISO 898-1 and BS EN ISO 898-1:2013:
| Bolt | As (mm^2) | d (mm) | Thread Pitch (mm) |
|---|---|---|---|
| M12 | 84.3 | 12 | 1.75 |
| M16 | 157 | 16 | 2.0 |
| M20 | 245 | 20 | 2.5 |
| M22 | 303 | 22 | 2.5 |
| M24 | 353 | 24 | 3.0 |
| M27 | 459 | 27 | 3.0 |
| M30 | 561 | 30 | 3.5 |
| M36 | 817 | 36 | 4.0 |
Class 8.8 Bolt Capacity Tables (fub = 800 MPa)
Class 8.8 bolts are the workhorse of UK structural steelwork, specified on the vast majority of building frame connections. The 8.8 designation indicates a minimum ultimate tensile strength of 800 MPa and a yield ratio of 0.8, giving fyb = 640 MPa nominal minimum.
Shear Resistance Fv,Rd (kN) -- Threads in Shear Plane (alpha_v = 0.6)
| Bolt | As (mm^2) | Fv,Rd (kN) |
|---|---|---|
| M12 | 84.3 | 32.4 |
| M16 | 157 | 60.3 |
| M20 | 245 | 94.1 |
| M22 | 303 | 116.4 |
| M24 | 353 | 135.6 |
| M27 | 459 | 176.3 |
| M30 | 561 | 215.4 |
| M36 | 817 | 313.7 |
Shear Resistance Fv,Rd (kN) -- Threads Outside Shear Plane (alpha_v = 0.6, use gross area A)
| Bolt | A (mm^2) | Fv,Rd (kN) |
|---|---|---|
| M12 | 113 | 43.4 |
| M16 | 201 | 77.2 |
| M20 | 314 | 120.6 |
| M24 | 452 | 173.6 |
| M30 | 707 | 271.5 |
When the shear plane passes through the unthreaded shank, the larger gross area A is used instead of As, yielding significantly higher shear resistance. This condition is achievable when the grip length is controlled and the bolt stick-out is specified to ensure threads are outside the shear plane.
Tension Resistance Ft,Rd (kN) (k2 = 0.9)
| Bolt | As (mm^2) | Ft,Rd (kN) |
|---|---|---|
| M12 | 84.3 | 48.6 |
| M16 | 157 | 90.4 |
| M20 | 245 | 141.1 |
| M22 | 303 | 174.5 |
| M24 | 353 | 203.3 |
| M27 | 459 | 264.4 |
| M30 | 561 | 323.1 |
| M36 | 817 | 470.6 |
Class 10.9 Bolt Capacity Tables (fub = 1000 MPa)
Class 10.9 bolts are specified for heavily loaded connections, bridge structures, and preloaded slip-resistant joints where higher preload forces permit higher slip resistance. The bolt shear resistance formula for Class 10.9 uses alpha_v = 0.5 when threads are in the shear plane, reflecting the reduced ductility of higher-strength bolts and the more conservative treatment of thread effects.
Shear Resistance Fv,Rd (kN) -- Threads in Shear Plane (alpha_v = 0.5)
| Bolt | As (mm^2) | Fv,Rd (kN) |
|---|---|---|
| M12 | 84.3 | 33.7 |
| M16 | 157 | 62.8 |
| M20 | 245 | 98.0 |
| M24 | 353 | 141.2 |
| M30 | 561 | 224.4 |
| M36 | 817 | 326.8 |
Note that for Class 10.9, the Fv,Rd with threads in the shear plane is only marginally higher than for Class 8.8 (e.g., M20: 98.0 vs 94.1 kN) despite the 25% increase in fub, because alpha_v reduces from 0.6 to 0.5.
Tension Resistance Ft,Rd (kN) (k2 = 0.9, fub = 1000 MPa)
| Bolt | As (mm^2) | Ft,Rd (kN) |
|---|---|---|
| M12 | 84.3 | 60.7 |
| M16 | 157 | 113.0 |
| M20 | 245 | 176.4 |
| M24 | 353 | 254.2 |
| M30 | 561 | 403.9 |
| M36 | 817 | 588.2 |
Class 4.6 Bolt Capacity Tables (fub = 400 MPa)
Class 4.6 bolts represent the lowest strength class commonly specified in UK steelwork, typically reserved for secondary members, purlin connections, and non-structural attachments where strength requirements are modest. The lower fub reduces all capacities proportionally.
Shear Resistance Fv,Rd (kN) -- Threads in Shear Plane
| Bolt | Fv,Rd (kN) | Fv,Rd (kN) -- Threads Outside |
|---|---|---|
| M12 | 16.2 | 21.7 |
| M16 | 30.1 | 38.6 |
| M20 | 47.0 | 60.3 |
| M24 | 67.8 | 86.8 |
| M30 | 107.7 | 135.7 |
Tension Resistance Ft,Rd (kN)
| Bolt | Ft,Rd (kN) |
|---|---|
| M12 | 24.3 |
| M16 | 45.2 |
| M20 | 70.6 |
| M24 | 101.7 |
| M30 | 161.6 |
Provision for Preloaded Bolts -- Categories D and E
For preloaded bolt connections designed as slip-resistant at the serviceability limit state (Category E) or ultimate limit state (Category D), the design slip resistance per bolt is:
Fs,Rd = ks x n x mu x Fp,Cd / gamma_M3
Where:
- ks = 1.0 for standard clearance holes, 0.85 for oversized or short slotted holes
- n = number of friction interfaces (typically 1 for a lap joint, 2 for a cover plate splice)
- mu = slip factor, taken as 0.30 to 0.50 depending on surface preparation per EN 1993-1-8 Table 3.6
- Fp,Cd = 0.7 x fub x As / gamma_M7 (design preload force)
- gamma_M3 = 1.25 (UK NA value for slip resistance at ULS)
- gamma_M3 = 1.10 (UK NA value for slip resistance at SLS, Category E)
For a connection with Class A surface (grit blasted, no paint, mu = 0.50), M20 Class 8.8 bolts, standard holes (ks = 1.0), single interface (n = 1):
Fp,Cd = 0.7 x 800 x 245 / 1.10 = 124.7 kN Fs,Rd (ULS) = 1.0 x 1 x 0.50 x 124.7 / 1.25 = 49.9 kN Fs,Rd (SLS) = 1.0 x 1 x 0.50 x 124.7 / 1.10 = 56.7 kN
Combined Shear and Tension
EN 1993-1-8 Table 3.4 provides the interaction equation for bolts subject to combined shear and tension:
(Fv,Ed / Fv,Rd) + (Ft,Ed / 1.4Ft,Rd) <= 1.0
The 1.4 factor on the tension denominator reflects the reduced tension capacity in the presence of shear, but the interaction is asymmetric -- shear is penalised more heavily than tension. For a bolt with Fv,Ed/Fv,Rd = 0.5, the permitted tension ratio is Ft,Ed/Ft,Rd = 0.7 (i.e., (1.0 - 0.5) x 1.4 = 0.7).
This interaction is relevant for:
- End plate moment connections where bolts in upper rows are primarily in tension with some shear
- Bracket connections subject to eccentric loading
- Bolted splices in combined axial and moment transfer
- Bracing connections where the brace force resolves into combined bolt shear and tension
UK National Annex Modifications
The UK NA to BS EN 1993-1-8 confirms the following bolt resistance parameters:
gamma_M2 = 1.25 for bolt shear and tension resistance, without modification from the recommended value.
gamma_M7 = 1.10 for preloaded bolt design preload force Fp,Cd.
gamma_M3 (ULS) = 1.25 and gamma_M3 (SLS) = 1.10 for slip-resistant connections.
No modification to alpha_v values. The thread-in-shear-plane alpha_v values of 0.6 (8.8) and 0.5 (10.9) are adopted as recommended.
The UK NA permits the use of LSCC (Limited Slip Capacity Connections), where the connection is designed as shear-bearing for ULS but slip is checked at SLS. This hybrid approach is common in UK multi-storey frames where thermal movements are accommodated by simple connections but lateral stability depends on a limited number of slip-resistant joints.
Design Resources
- UK Steel Grades Reference -- EN 10025-2 grade selection
- UK Steel Mechanical Properties -- fy, fu tables
- UK Bolt Spacing Requirements -- Table 3.3 edge distance and pitch
- UK Bolt Bearing and Tear-Out -- Fb,Rd calculation
- 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 shear capacity of an M20 Class 8.8 bolt per UK NA provisions?
The shear resistance per shear plane for M20 Class 8.8 bolts with threads in the shear plane is Fv,Rd = 0.6 x 800 x 245 / 1.25 = 94.1 kN. With threads outside the shear plane: Fv,Rd = 0.6 x 800 x 314 / 1.25 = 120.6 kN. Single-shear connections (lap joints) use one shear plane; double-shear connections (cover plate splices) use two shear planes, effectively doubling the bolt group shear resistance.
Why does the Class 10.9 shear capacity not increase proportionally with bolt strength?
For Class 10.9 bolts, alpha_v reduces from 0.6 to 0.5 when threads are in the shear plane, per EN 1993-1-8 Table 3.4. The 25% increase in fub (800 to 1000 MPa) is partially offset by the alpha_v reduction (0.6 to 0.5, a 17% reduction). The net effect for M20: Fv,Rd_8.8 = 94.1 kN vs Fv,Rd_10.9 = 98.0 kN -- only a 4% increase. The reduced alpha_v reflects the lower ductility of higher-strength bolts and the associated reduction in stress redistribution capacity at the bolt-pin interface.
What UK NA factor applies to bolt tension resistance checks?
The UK NA to BS EN 1993-1-8 confirms gamma_M2 = 1.25 for bolt tension resistance. The tension resistance is Ft,Rd = 0.9 x fub x As / 1.25. For M20 Class 8.8: Ft,Rd = 0.9 x 800 x 245 / 1.25 = 141.1 kN. The gamma_M2 factor is not modified by the UK NA from the recommended value.
Educational reference only. All design values are per BS EN 1993-1-8:2005 + UK National Annex and BS EN ISO 898-1:2013. 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.