UK Bolt Hole Sizes -- EN 1993-1-8 Table 3.1 Clearances and Slot Types with UK NA

The selection of bolt hole type and diameter directly affects the fabrication cost, erection tolerance, and structural performance of bolted connections. EN 1993-1-8 Table 3.1 defines four hole types -- standard round, oversized round, short slot, and long slot -- each with prescribed clearances above the nominal bolt diameter. The UK National Annex adopts Table 3.1 without modification but supplements it with additional constraints for slotted hole usage in UK building and bridge structures. This reference covers the complete specification of bolt hole dimensions, the fabrication tolerances per BS EN 1090-2, the design implications of each hole type, and practical selection guidance for UK construction.

Regulatory Framework

The governing standard for hole dimensions is BS EN 1993-1-8:2005, Table 3.1. The fabrication and inspection requirements are specified in BS EN 1090-2:2018, Execution Class EXC2 and EXC3 for building structures. The Steel Construction Institute publication SCI P358 provides standard UK detailing for bolt holes in simple connections.

EN 1993-1-8 Table 3.1 specifies the hole clearance as a function of bolt diameter and hole type:

For standard round holes up to and including M24, the clearance is 2 mm (d0 = d + 2). For M27 and larger, the clearance increases to 3 mm (d0 = d + 3). This step change reflects the practical difficulty of maintaining tight clearances on larger diameter holes during the drilling or punching process.

Hole Type Selection and Design Implications

Standard Round Holes

Standard round holes are the default for all UK building connections. They provide sufficient clearance for normal fabrication and erection tolerances while maintaining reliable bolt-to-plate contact for bearing and shear transfer. Over 95% of UK steelwork bolted connections use standard round holes.

For preloaded slip-resistant connections (Categories D and E), standard round holes with ks = 1.0 provide the full slip resistance without reduction.

Oversized Round Holes

Oversized holes provide 4 mm clearance above the nominal bolt diameter (d0 = d + 4). They are intended for connections requiring extra erection adjustment, such as base plate holding-down bolt connections and column splices where positional tolerance accumulation across multiple storeys requires a larger hole.

EN 1993-1-8 places restrictions on oversized holes:

• They are permitted only in slip-resistant connections (Categories C, D, or E).
• They require hardened washer plates under both the bolt head and the nut.
• For Category C connections (slip-resistant at ULS), the ks factor reduces to 0.85, reducing the slip resistance by 15%.

In UK practice, oversized holes are relatively rare in building frames but common in holding-down bolt assemblies, where the base plate is grouted after column alignment.

Short Slots

Short slots are rectangular holes with the slot dimension parallel to the direction of the applied force limited to the standard round hole diameter. The slot length perpendicular to the force is three times the slot width. Short slots are used to permit adjustment in one direction while maintaining bearing in the direction of load transfer.

The UK NA requires that for short slots with the slot perpendicular to the load direction, the bearing check uses a reduced k1 factor (0.75 k1_round) to account for the reduced bearing contact area. When the slot is parallel to the load, the bearing check uses the standard formula but with the effective hole diameter for edge distance calculation taken as the slot width, not the slot length.

Long Slots

Long slots provide up to 2.5 times the bolt diameter in the slot length direction, offering maximum erection adjustment. They are restricted by EN 1993-1-8 to Category D and E connections (slip-resistant at SLS) and carry multiple design penalties:

• ks = 0.85, reducing slip resistance.
• The bearing check uses the reduced k1 for the perpendicular direction.
• The net section check must account for the full slot length as the effective hole dimension on the critical failure path.
• Long slots are not permitted in Category C connections (slip-resistant at ULS).

In UK steelwork, long slots are most commonly used for bracing connections where the brace member requires axial adjustment during erection, and for holding-down bolt holes in base plates where rotation adjustment is required.

BS EN 1090-2 Fabrication Tolerances

The execution standard BS EN 1090-2 specifies the permitted deviations from nominal hole positions and dimensions. For holes produced by drilling or punching:

These tolerances consume part of the clearance between the bolt shank and the hole. With a 2 mm positional tolerance and the standard 2 mm diametral clearance, it is theoretically possible for a bolt at one extreme of the tolerance band to contact the hole at the opposite extreme. In practice, good workmanship reduces the effective tolerance, and the standard spacing provisions (minimum edge distance 1.2d0) ensure that even with accumulated tolerances, edge distances remain adequate.

For punched holes, BS EN 1090-2 imposes additional constraints:

• The plate thickness must not exceed the hole diameter.
• Punched holes must be reamed to size if the connection is preloaded.
• Punched holes in material thicker than 25 mm require special approval.
• The punch side of the hole exhibits a burr that must be removed to achieve full bearing contact.

UK National Annex Provisions

The UK NA to BS EN 1993-1-8 adopts Table 3.1 without modification but includes supplementary guidance:

1. Slotted hole orientation: The UK NA recommends that where slotted holes are used to provide erection tolerance, the slot should be oriented perpendicular to the direction of applied force wherever possible, so that the bolt bears against the round hole face (not the slotted face) in the load direction.

2. Hardened washer requirements: For oversized and slotted holes, the UK NA mandates hardened washers (through-hardened to 300-370 HV) under both the bolt head and nut. Standard washers to BS 4320 Form A or G are insufficient.

3. Bridge structures: The UK NA to BS EN 1993-2 restricts hole types for bridge connections. Long slots are not permitted in primary bridge elements. Oversized holes require specific approval from the Technical Approval Authority.

4. Stainless steel bolting: For stainless steel structural connections to BS EN 1993-1-4, the UK NA confirms that Table 3.1 hole sizes apply equally to carbon and stainless steel bolts, but notes that stainless steel bolts in slotted holes may be prone to galling during tightening and recommends anti-seize compound application.

5. Countersunk bolts: The UK NA guidance on countersunk holes references BS EN ISO 10642, specifying a 0.5 mm clearance between the countersunk head and the countersunk hole for standard applications, reduced to 0.3 mm for slip-resistant connections.

Interaction with Spacing Requirements

Hole diameter d0 is a critical input to every bolt spacing check in EN 1993-1-8 Table 3.3. The minimum edge distance (1.2d0), minimum pitch (2.2d0), and minimum gauge (2.4d0) all scale with d0. Using an oversized or slotted hole therefore increases the required plate dimensions, as the spacing requirements are referenced to the hole diameter rather than the bolt diameter.

For an M20 bolt:

• Standard round (d0 = 22): e1_min = 26.4 mm, p1_min = 48.4 mm
• Oversize round (d0 = 24): e1_min = 28.8 mm, p1_min = 52.8 mm
• Short slot width (22 mm): e1_min = 26.4 mm (measured from slot end, not centre)

This increase in required edge distances may necessitate wider plates or longer end distances when oversized or slotted holes are specified, with consequent increases in connection plate sizes and weights.

Worked Example -- Hole Selection for a Column Splice

Consider a 356x406x235 UC column splice at mid-height of a multi-storey building. The splice must accommodate the accumulation of column fabrication and erection tolerances through the lower storeys.

Fabrication tolerances through three storeys: The BS EN 1090-2 permitted deviation for column length between adjacent floors is plus/minus 2 mm per storey. Over three storeys, the cumulative tolerance is approximately 6 mm.

Selection: If the lower column is positioned at the extreme of its tolerance range, the upper column splice requires 6 mm of adjustment in both principal axes. Standard round holes with 2 mm clearance will not provide this adjustment. Two options exist:

1. Oversized holes (d0 = 30 mm for M24): Provide 4 mm clearance, meaning 2 mm of adjustment per hole. With four bolts, the total adjustment is limited by the first bolt to contact its hole. This may be insufficient for 6 mm cumulative tolerance.

2. Short slots in the splice plates: Two short slots in each direction (one on each axis) oriented to permit adjustment. Slot length = 30 mm for M24, providing up to 15 mm of adjustment. This comfortably exceeds the 6 mm requirement.

The selected arrangement: flange cover plates with short slots in the x-direction on one column face, short slots in the y-direction on the perpendicular column face. Web cover plates with standard round holes. Bolts are M24 Class 8.8 preloaded (Category E). After column alignment, all bolts are fully tightened and the ks = 0.85 reduction is applied to the short-slotted connections.

Design Resources

• UK Steel Grades Reference -- EN 10025-2 grade selection
• UK Bolt Capacity Tables -- Class 8.8 and 10.9 bolt resistance
• 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 standard hole diameter for an M20 bolt in UK steelwork?

The standard round hole diameter for an M20 bolt is d0 = 22 mm per EN 1993-1-8 Table 3.1. This provides 2 mm of diametral clearance (1 mm radial), which accommodates normal fabrication tolerances (plus/minus 2 mm positional per BS EN 1090-2) while ensuring reliable bolt-to-plate bearing contact under load. For M24, d0 = 26 mm (2 mm clearance). For M27 and larger, the clearance increases to 3 mm: M30 d0 = 33 mm.

When are oversized holes permitted in UK structural connections?

Oversized holes (d0 = d + 4) are permitted only in slip-resistant connections (Categories C, D, or E) per EN 1993-1-8. They require hardened washers under both head and nut and attract a ks reduction of 0.85. In UK building practice, oversized holes are most common in column base plate connections for holding-down bolts and in column splice connections where cumulative verticality tolerances require additional erection adjustment. They are not permitted in standard shear-bearing connections.

What UK NA restrictions apply to slotted holes?

The UK NA to BS EN 1993-1-8 recommends orienting slots perpendicular to the applied load wherever possible, so the bolt bears on the round hole face. For bearing calculations with slotted holes perpendicular to load, the UK NA requires a reduced k1 = 0.75 x k1_round. Long slots are not permitted in Category C connections. For bridge structures, the UK NA to BS EN 1993-2 further restricts long slots in primary elements and requires Technical Approval Authority consent for oversized holes.

Can punched holes be used in UK steelwork connections?

Punched holes are permitted subject to BS EN 1090-2 constraints: the plate thickness must not exceed the hole diameter, and punched holes in material thicker than 25 mm require specific approval. For preloaded connections, punched holes must be reamed after punching to remove the work-hardened zone. In practice, most UK fabricators now use CNC drilling rather than punching for structural bolting -- drilling produces cleaner holes with better dimensional accuracy and no edge hardening. The BCSA National Structural Steelwork Specification recommends drilling for all holes in material thicker than 15 mm.

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.