UK Bolt Capacity — EN 1993-1-8 Shear & Tension Guide

This reference covers bolt capacity for UK steel design per EN 1993-1-8:2005 Clause 3.4 and UK NA. The design of bolted connections in UK practice follows the resistance formulas given in Eurocode 3, with partial factors defined in the UK National Annex.

Design requirements, worked examples, and practical design guidance are provided for common design office applications.

Design Formulas per EN 1993-1-8 Clause 3.4

Bolt shear resistance per shear plane (Category A — bearing type):

[ F*{v,Rd} = \frac{\alpha_v f*{ub} A}{\gamma_{M2}} ]

Where:

• ( \alpha_v = 0.6 ) for Grade 8.8, 0.5 for Grade 10.9
• ( f_{ub} ) = ultimate tensile strength of bolt (800 MPa for 8.8, 1000 MPa for 10.9)
• ( A ) = gross cross-section area (shank) or tensile stress area ( A_s ) (threaded)
• ( \gamma_{M2} = 1.25 ) (UK NA — matching recommended value)

For bolts in double shear, the total shear resistance is 2 × Fv,Rd per bolt (assuming both shear planes are through similar threads or shanks).

Bolt tension resistance:

[ F*{t,Rd} = \frac{k_2 f*{ub} As}{\gamma{M2}} ]

Where:

• ( k_2 = 0.9 ) for standard hexagon bolts, 0.63 for countersunk head bolts
• ( A_s ) = tensile stress area of the bolt

Punching shear resistance (thin plate connected to thicker bolt):

[ B*{p,Rd} = \frac{0.6 \pi d_m t_p f_u}{\gamma*{M2}} ]

Where ( d_m ) is the mean of across-points and across-flats dimensions.

Bolt Shear and Tension Capacity Table — Grade 8.8

Values computed with γM2 = 1.25 per UK NA. Threads assumed in shear plane; for shank shear use gross area.

Bolt Shear and Tension Capacity Table — Grade 10.9

Combined Shear and Tension (EN 1993-1-8 Clause 3.6)

For bolts subject to combined shear and tension:

[ \frac{F*{v,Ed}}{F*{v,Rd}} + \frac{F*{t,Ed}}{1.4 F*{t,Rd}} \leq 1.0 ]

This interaction applies to Category A and B connections at ULS. The 1.4 factor accounts for the reduced tension resistance when shear is present.

Connection Categories per EN 1993-1-8 Clause 3.4

Worked Example — M20 Grade 8.8 in Double Shear

Given:

• Bolt: M20 Grade 8.8 (fub = 800 MPa, As = 245 mm²)
• Double shear connection (both threads in shear plane)
• γM2 = 1.25 (UK NA)

Shear capacity: Fv,Rd = 2 × (0.6 × 800 × 245 / 1.25) × 10⁻³ = 2 × 94.1 = 188.2 kN

Tension capacity: Ft,Rd = 0.9 × 800 × 245 / 1.25 × 10⁻³ = 141.1 kN

Combined check: If applied shear Fv,Ed = 120 kN and tension Ft,Ed = 50 kN: 120/188.2 + 50/(1.4 × 141.1) = 0.64 + 0.25 = 0.89 ≤ 1.0 — Satisfactory

Bearing Resistance at Bolt Holes (EN 1993-1-8 Clause 3.6)

[ F*{b,Rd} = \frac{k_1 \alpha_b f_u d t}{\gamma*{M2}} ]

Where:

• ( k_1 ) depends on edge distance; ( \alpha_b ) depends on bolt spacing and end distance
• For end bolts: ( e_1 \geq 1.2 d_0 ), minimum edge distance governs
• For inner bolts: ( p_1 \geq 2.2 d_0 ), spacing governs

Typical bearing resistance for a M20 bolt in 12mm S355 plate (fu = 470 MPa) with end distance 40mm: Fb,Rd = (2.5 × 0.69 × 470 × 20 × 12) / 1.25 = 155.7 kN

Design Resources

• UK Bolt Grades — Property classes and materials
• UK Bolt Pretension — Preloaded bolt installation
• UK Bolt Hole Sizes — Hole dimensions and spacing
• UK Connection Design — Full connection design guidance
• UK Steel Properties — S275/S355 material data
• UK Steel Beam Sizes — UB/UC section dimensions
• All UK References

Frequently Asked Questions

How is bolt shear capacity calculated per EN 1993-1-8?

EN 1993-1-8 Clause 3.4: Fv,Rd = αv fub A / γM2. For Grade 8.8: αv = 0.6. For Grade 10.9: αv = 0.5. UK NA specifies γM2 = 1.25 (matching the recommended value). The area A depends on whether the shear plane passes through the threaded portion (use tensile stress area As) or the shank (use gross area). In UK practice, shear planes are conservatively assumed to pass through threads unless specifically detailed otherwise.

What are the bolt tension capacity formulas per EN 1993?

Bolt tension capacity per EN 1993-1-8 Clause 3.4: Ft,Rd = k2 fub As / γM2. k2 = 0.9 for standard bolts, 0.63 for countersunk. UK NA uses γM2 = 1.25. Tension resistance is always based on the tensile stress area As, not the shank area, because threads extend close to the bolt head.

When do I use the combined shear and tension check?

The interaction formula Fv,Ed/Fv,Rd + Ft,Ed/(1.4 Ft,Rd) ≤ 1.0 is required when a bolt is simultaneously subject to applied shear force and applied tension. This is common in end-plate connections where beam shear is transferred through bolts in the tension zone. The interaction is required for Category A (bearing type) and Category B (slip-resistant at SLS) connections per EN 1993-1-8 Clause 3.6.

What is the difference between single shear and double shear capacity?

In single shear, the bolt resists force across one shear plane. In double shear, two shear planes share the load, giving approximately 2× the single shear capacity. For a typical beam-to-column connection with bolt groups, the end plate bolts are often in single shear, while a fin plate bolt group acts in single shear but with multiple bolt lines. The actual distribution depends on connection stiffness and eccentricity.

What edge distances and spacings are required per UK NA?

UK NA to EN 1993-1-8 specifies minimum edge distance e1 ≥ 1.2d0 (loaded end) and e2 ≥ 1.2d0 (unloaded edge). Minimum spacing p1 ≥ 2.2d0 in the direction of load, p2 ≥ 2.4d0 transverse. Maximum edge distance ≤ 4t + 40mm (t = thinnest connected ply). Maximum spacing for exposed members ≤ 14t (minimum) or 200mm, whichever is smaller.

Reference only. Verify all values against the current edition of EN 1993-1-8:2005 Clause 3.4 and UK NA. This information does not constitute professional engineering advice.