UK Bolt Hole Sizes — EN 1993-1-8 & BS NA Table

This reference covers bolt hole sizes for UK steel design per EN 1993-1-8:2005 Table 3.1 and UK NA. Hole size selection is critical to connection performance — oversized holes provide erection tolerance but reduce bearing resistance and limit the use of preloaded assemblies.

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

Code Reference: EN 1993-1-8:2005 Table 3.1 and UK NA

Bolt Hole Types (EN 1993-1-8 Table 3.1)

Standard Hole Diameters

Minimum Spacing and Edge Distances (EN 1993-1-8 Table 3.1)

All spacing values apply to standard round holes. For slotted holes, additional checks for washer plate requirements and bearing resistance must be made.

Bolt Spacing Worked Example — M20 End Plate

Given: End plate connection for 406×178 UB 60, M20 bolts in 22 mm standard holes.

Minimum edge distances: e1,min = e2,min = 1.2 × 22 = 26.4 mm → use 30 mm typically Minimum spacing (load direction): p1,min = 2.2 × 22 = 48.4 mm → use 50 mm typically Minimum spacing (transverse): p2,min = 2.4 × 22 = 52.8 mm → use 60 mm typically

Typical end plate layout (3 rows of 2 M20):

• End distance (top): 30 mm
• Row spacing: 50 mm (giving 3 rows over 100 mm total)
• Gauge (transverse spacing): 60 mm
• Total plate height: 30 + 50 + 50 + 30 = 160 mm (minimum)
• Practical plate height: 200 mm

Net Section Verification

When bolt holes reduce the cross-section of a tension member:

[ A_{net} = A - \Sigma (d_0 \times t) ]

For staggered holes: ( A_{net} = A - \Sigma (d_0 \times t) + \Sigma (s^2 / (4g) \times t) )

Where s = staggered pitch, g = gauge (transverse spacing), t = thickness.

Hole Clearance Guidelines for Erection

Effect of Hole Size on Bearing Resistance

Bearing resistance per EN 1993-1-8 Clause 3.6:

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

Where αb and k1 depend on end distances, spacings, and the ratio d0/d:

OSHA (Oversized) holes may reduce bearing capacity by up to 25% compared to standard holes.

Design Resources

• UK Bolt Grades — Bolt property classes
• UK Bolt Capacity — Shear and tension tables
• UK Bolt Pretension — Preloaded bolt installation
• UK Connection Design — Full connection guidance
• UK Steel Beam Sizes — Section dimensions
• All UK References

Frequently Asked Questions

What are the standard bolt hole sizes per EN 1993-1-8?

EN 1993-1-8 Table 3.1 specifies standard holes as bolt diameter + 1 mm for M12-M24, or + 2 mm for M27-M36. Nominal holes are + 1 mm oversize. UK NA does not modify hole diameters. For M20 bolts: standard hole = 22 mm diameter. For M24 bolts: standard hole = 26 mm. For M30 bolts: standard hole = 33 mm diameter.

What are the UK NA edge distance requirements?

UK National Annex to EN 1993-1-8 specifies minimum edge distance e1 ≥ 1.2 d0 (loaded end) and e2 ≥ 1.2 d0 (unloaded edge). These match EN 1993-1-8 Table 3.1 recommended values. The UK NA also confirms the recommended maximum edge distance of 4t + 40 mm (where t is the thinnest connected part). For bolted tension members, exceeding the maximum edge distance can reduce the effectiveness of the edge zone in resisting prying forces.

When should oversized holes be used in UK steelwork?

Oversized holes are used when: (a) significant erection tolerances are required between steelwork and foundations (base plates), (b) enabling large-scale prefabricated modules to be joined with some adjustability, or (c) accommodating unavoidable dimensional variations in long-span trusses. However, oversized holes cannot be used in slip-critical connections (Category B or C per EN 1993-1-8) unless the designer specifically approves and the connection is designed for the reduced bearing resistance.

What washer requirements apply to slotted holes?

For slotted holes, BS EN 1090-2 requires: (a) flat washers with a minimum thickness of 0.3 × bolt diameter (Grade 8.8) or 0.4 × bolt diameter (Grade 10.9), (b) washer external diameter must fully cover the slot width, and (c) for long slotted holes, a backing plate or continuous washer strip must be used. For short slotted holes, a single oversized round washer is typically sufficient. For long slotted holes, a rectangular washer or plate is required.

How do bolt holes affect the net section of tension members?

Per EN 1993-1-1 Clause 6.2.3, the net area Anet must be used for the tension resistance check of members with holes: Nt,Rd = Anet × fu / γM2 (for fracture) or Nt,Rd = A × fy / γM0 (for yield). For staggered holes, the deduction uses the s²/(4g) correction. For a single bolt row in a 10 mm plate with M20 bolts: Anet = A - 22 × 10 = A - 220 mm². This can reduce tension capacity by 15-25% for typical sections.

Reference only. Verify all values against the current edition of EN 1993-1-8:2005 Table 3.1 and UK NA. This information does not constitute professional engineering advice.