UK Bolt Spacing and Edge Distance — EN 1993-1-8 Table 3.3 with UK National Annex

The correct placement of bolt holes in structural steel connections is governed by minimum and maximum spacing rules that prevent premature failure modes and ensure adequate access for tightening. EN 1993-1-8 Table 3.3 defines these geometric constraints for the European design framework, and the UK National Annex to BS EN 1993-1-8:2005 adopts the recommended values without modification. This reference covers the complete spacing requirements, the rationale behind each limit, standard UK fabrication practice, and worked examples for common connection types.

Regulatory Framework

The governing standard for bolt spacing in UK steelwork is BS EN 1993-1-8:2005, Design of Joints. Table 3.3 of Clause 3.5 defines minimum and maximum values for end distance (e1), edge distance (e2), pitch (p1), and gauge (p2). Execution requirements are detailed in BS EN 1090-2:2018, Execution of Steel Structures and Aluminium Structures, which specifies the permitted deviations from nominal hole positions. The Steel Construction Institute publication SCI P358, Joints in Steel Construction: Simple Connections, provides standardised UK detailing practice that satisfies the code requirements while optimising for fabrication efficiency.

The UK NA to BS EN 1993-1-8 makes no modifications to Table 3.3. The minimum edge distance of 1.2d0, minimum pitch of 2.2d0, and minimum gauge of 2.4d0 are adopted verbatim. However, the UK NA includes supplementary guidance on the application of these values to slotted holes and oversized holes, noting that the reference point for edge distance measurement shifts from the hole centre to the physical extremity of the slot or oversize.

Definitions and Notation

EN 1993-1-8 Clause 3.5 establishes the following geometric parameters:

• End distance e1: Distance from the centre of a bolt hole to the adjacent end of the connected plate, measured in the direction of load transfer. This dimension governs tear-out resistance and is directly linked to the alpha_b coefficient in Clause 3.6.

• Edge distance e2: Distance from the centre of a bolt hole to the adjacent edge of the connected plate, measured perpendicular to the direction of load transfer. This dimension influences the k1 coefficient in bearing checks.

• Pitch p1: Centre-to-centre distance between bolts in a row measured parallel to the direction of load transfer. Pitch controls the interaction between adjacent bolts in bearing and the susceptibility to stagger cracking.

• Gauge p2: Centre-to-centre distance between adjacent rows of bolts measured perpendicular to the direction of load transfer. Gauge affects plate bending between bolt rows and provides clearance for tightening tools.

• Hole diameter d0: The nominal diameter of the bolt hole, which depends on bolt size and hole type per EN 1993-1-8 Table 3.1. For standard round holes in UK practice, d0 = d + 2 mm for bolts up to and including M24, and d0 = d + 3 mm for M27 and larger.

Minimum Spacing Requirements — Table 3.3

End Distance e1

The minimum end distance is e1_min = 1.2d0. This requirement prevents shear tear-out of the plate end ahead of the bolt in the direction of load. The value of 1.2d0 corresponds to an alpha_b value of 0.4 when applying the bearing formula in Clause 3.6. For an M20 bolt with a standard 22 mm round hole: e1_min = 1.2 x 22 = 26.4 mm. UK standard practice typically adopts significantly larger values -- commonly 40 mm for M20 bolts -- to provide robustness against fabrication tolerances and to achieve higher alpha_b values, which increase the bearing resistance.

Edge Distance e2

The minimum edge distance is e2_min = 1.2d0. This requirement prevents the bolt from breaking out through the side of the plate under bearing or shear loading. For an M20 bolt with a 22 mm hole: e2_min = 26.4 mm. UK standard practice for edge distance is typically 30-35 mm for M20 bolts in building structures.

Pitch p1

The minimum pitch in the direction of load transfer is p1_min = 2.2d0. This requirement prevents the bearing stress fields of adjacent bolts from overlapping excessively, which would reduce the net section resistance and increase the risk of stagger-crack propagation between holes. For an M20 bolt with 22 mm hole: p1_min = 2.2 x 22 = 48.4 mm. Standard UK pitch for M20 is 70 mm, comfortably exceeding the minimum.

Gauge p2

The minimum gauge perpendicular to the direction of load is p2_min = 2.4d0. For an M20 bolt: p2_min = 2.4 x 22 = 52.8 mm. Standard UK gauge for M20 is 55-60 mm.

Staggered Rows

For staggered bolt rows, the effective minimum pitch depends on whether the ratio of the staggered distance to the gauge exceeds or falls below certain thresholds. Where the staggered distance is less than 1.4 times the minimum gauge, the effective pitch is adjusted downward, potentially increasing the number of bolts required.

Maximum Spacing Requirements

EN 1993-1-8 Table 3.3 also specifies maximum spacing limits, which serve three principal functions: prevention of local buckling between widely spaced fasteners in compression elements, ensuring adequate clamping to prevent moisture ingress and corrosion between connected plates, and providing sufficient out-of-plane restraint to thin plates.

For compression elements exposed to the environment, the maximum pitch is 14t_min, where t_min is the thickness of the thinner connected plate. For a 10 mm plate, this corresponds to 140 mm maximum pitch. For tension elements in non-corrosive environments, the maximum pitch doubles to 28t_min = 280 mm for a 10 mm plate.

Additional provisions apply to weathering steel and structures in corrosive environments, where the reduced spacing of 14t_min applies universally. The maximum edge distance from any bolt centre to the nearest plate edge is limited to 4t + 40 mm, with an absolute maximum of 100 mm for exposed plates and 150 mm for enclosed plates.

Standard UK Spacing for M12 to M36 Bolts

The following table presents the code minimum values alongside standard UK practice for structural bolting. Standard values reflect UK fabrication norms documented in SCI P358 and the BCSA National Structural Steelwork Specification.

The standard UK values are approximately 1.5 times the code minimums for most bolt sizes. This margin accommodates the combined effects of marking-out tolerances, drilling positional deviations, and shop assembly requirements.

Worked Example — Fin Plate Connection

Consider a simply supported 406x178x54 UB in S355 connected to a 254x254x89 UC column via a 10 mm thick fin plate with M20 Class 8.8 bolts in single shear.

Given: Bolt diameter M20, standard round holes d0 = 22 mm, plate thickness t = 10 mm, plate width = 220 mm, tensile strength fu = 470 MPa (S355).

Vertical edge distance (end distance e1): The bolt nearest the top of the fin plate requires e1 >= 27 mm (code minimum). UK practice adopts 40 mm for M20. The bearing check for the top bolt with reduced end distance may govern. Confirm: 40 mm >> 27 mm, therefore adequate.

Horizontal edge distance e2: With the 220 mm plate width and holes centred on the plate, e2 = (220 - 10 - 2x22)/2 = 83 mm for slotted holes or 110 mm for a single column of bolts. This substantially exceeds the 27 mm minimum. The bearing check will use the k1 value corresponding to e2.

Pitch p1 (vertical): With 70 mm standard pitch: p1 = 70 mm > 2.2 x 22 = 48.4 mm. OK. Also check maximum: for a tension element with 10 mm plate in non-corrosive environment, max pitch = 28 x 10 = 280 mm. The 70 mm pitch is well within limits.

Gauge p2 (horizontal): For a single column of bolts, p2 is not applicable. Where two columns of bolts are required, the gauge must exceed 2.4d0 = 52.8 mm. Standard gauge of 60 mm satisfies this.

The bolt layout is compliant with all Table 3.3 minimum and maximum spacing requirements.

Worked Example — End Plate Moment Connection

Consider a 533x210x82 UB beam moment-connected to a UC column via a full-depth end plate. The connection uses 8 M20 Class 8.8 bolts in two columns of four rows.

End plate dimensions: 210 mm wide x 529 mm deep x 20 mm thick, S355.

Bolt layout: Two columns with p2 = 110 mm gauge. Rows at p1 = 100 mm pitch vertically. Top bolt end distance e1 = 50 mm from top of end plate. Bottom row located in the compression zone.

e1 check: e1 = 50 mm > 1.2 x 22 = 26.4 mm. OK. The generous end distance ensures adequate tear-out resistance for the top bolt row, which carries the highest tension.

e2 check: e2 = (210 - 110 - 22)/2 = 39 mm. > 26.4 mm. OK.

p1 check: p1 = 100 mm > 48.4 mm minimum. Less than 28 x 20 = 560 mm (tension element, non-corrosive). OK.

p2 check: p2 = 110 mm > 52.8 mm minimum. The gauge accommodates the M20 bolt tightening clearance (typically 60 mm minimum), suitable for calibrated wrench access.

All spacing requirements are satisfied. The T-stub model from Clause 6.2.4 can be applied with these geometric parameters to determine the connection moment resistance.

Fabrication Tolerances per BS EN 1090-2

BS EN 1090-2 Table B.1 specifies the permitted positional deviations for bolt holes, which directly influence the adopted spacing practice. For standard round holes up to diameter 26 mm, the positional tolerance is plus or minus 2 mm. For holes 27 mm and larger, the tolerance is plus or minus 3 mm. These tolerances consume a portion of the margin between the code minimum spacing and the UK standard values. The standard UK edge distance of 40 mm for M20 provides a margin of 13 mm above the absolute minimum, meaning a hole positioned 2 mm out of position still leaves 11 mm of margin -- adequate for normal fabrication practice.

For slotted holes, the positional tolerance applies to the centre of the slot. The edge distance is measured from the end of the slot, not the centre, which effectively increases the required edge distance. The UK NA emphasises this measurement convention.

Design Guidance for UK Practice

When selecting bolt spacings for a connection, the designer must balance several competing considerations. Tighter spacings reduce the connection footprint and plate sizes but may trigger interactive bearing effects that reduce per-bolt resistance. Wider spacings increase connection dimensions but simplify fabrication and erection. The SCI recommends that UK designers standardise on the values in the table above unless project-specific constraints dictate otherwise.

For bolted splices in bridge construction, the UK NA to BS EN 1993-2 specifies additional requirements for maximum spacing to ensure adequate clamping of the splice plates against corrosion. For bridge structures in the UK network, the maximum pitch is uniformly 14t_min regardless of whether the element is in tension or compression, reflecting the more aggressive exposure conditions.

For stainless steel connections, the bolt spacing requirements are governed by BS EN 1993-1-4, which references Table 3.3 but includes supplementary guidance on the prevention of galvanic corrosion through appropriate edge distances that maintain full bolt-to-plate contact.

Design Resources

• UK Steel Grades Reference -- EN 10025-2 grade selection for UK projects
• UK Steel Mechanical Properties -- fy, fu, and elongation tables
• UK Universal Beam and Column Sizes -- UB/UC section dimensions and properties
• UK Bolt Capacity Tables -- Class 8.8 and 10.9 bolt resistance
• UK Beam Design Guide -- EN 1993-1-1 flexure, shear, and LTB
• 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 minimum end distance for an M20 bolt per EN 1993-1-8?

The minimum end distance e1 = 1.2 x d0 = 1.2 x 22 = 26.4 mm for a standard round hole, per EN 1993-1-8 Table 3.3. The UK NA does not modify this value. In practice, UK steelwork contractors adopt 40 mm as the standard end distance for M20 bolts, which provides adequate bearing resistance and absorbs positional tolerances of plus or minus 2 mm per BS EN 1090-2. The larger e1 increases the alpha_b factor in bearing calculations, yielding higher Fb,Rd values.

How does edge distance differ from end distance in a bolt layout?

End distance e1 is measured parallel to the direction of the applied force, from the bolt hole centre to the end of the plate. Edge distance e2 is measured perpendicular to the force direction, from the bolt hole centre to the side of the plate. Both have the same minimum value of 1.2d0, but they influence different bearing coefficients: e1 affects alpha_b, while e2 affects k1.

What is the maximum pitch permitted between bolts in a compression flange?

The maximum pitch for bolts in a compression flange exposed to the environment is 14 x t_min, where t_min is the thickness of the thinner connected plate. For a flange splice with 15 mm plates, the maximum pitch is 14 x 15 = 210 mm. This limit prevents local buckling of the plate between widely spaced fasteners. For tension flanges in non-corrosive environments, the limit is 28 x t_min.

Does the UK NA modify the Table 3.3 spacing requirements?

The UK National Annex to BS EN 1993-1-8 adopts Table 3.3 without modification. The minimum edge, end, pitch, and gauge values are unchanged. The UK NA provides supplementary guidance on the measurement conventions for slotted and oversized holes, confirming that edge distance for slotted holes is measured from the physical end of the slot rather than the hole centre. This convention increases the required plate width for slotted hole connections.

What hole diameter should be used for an M24 bolt in a UK building connection?

For M24 bolts in standard round holes, EN 1993-1-8 Table 3.1 specifies d0 = 26 mm. This is the default for UK building connections. Oversized holes (d0 = 30 mm) are permitted but require hardened washer plates and are limited to slip-resistant connections. Short slots of 26 x 30 mm and long slots of 26 x 60 mm are available for connections requiring erection adjustment.

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.