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 diametre 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 diametre 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

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 diametre 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.