EN 1993-1-8 Bolt Hole Sizes — M12 to M36 Clearance, Oversized, Slotted Holes

Complete reference for bolt hole dimensions in European structural steel connections per EN 1993-1-8 Table 3.3. Includes standard clearance holes, oversized holes, short slotted holes, and long slotted holes for metric bolts M12 through M36. Edge and end distance requirements, spacing rules, and practical detailing guidance per Eurocode 3.

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EN 1993-1-8 Hole Types

EN 1993-1-8 Clause 3.6.2 and Table 3.3 define four types of bolt holes for structural steel connections. The hole type influences the slip resistance, the bearing resistance of the connected plates, and the ease of erection.

Holes larger than normal reduce the bearing resistance (alpha_b and k1 factors per Table 3.4) and may affect slip resistance in Category B and C connections. Standard clearance holes are used in the vast majority of structural connections.

Normal Clearance Hole Sizes — EN 1993-1-8 Table 3.3

Standard clearance holes provide adequate tolerance for erection while maintaining full bearing resistance. These are the default hole dimensions for structural steelwork.

The clearance of 2 mm for M16-M24 and 3 mm for M27-M36 reflects practical steel fabrication tolerances. For M12 bolts, a 1 mm clearance is standard. These clearances are identical to those used in UK practice (BS 5950 legacy) and similar to AISC (which uses 1.6 mm clearance for 5/8"-1" bolts, increasing to 3.2 mm for larger diameters).

Oversized Hole Sizes

Oversized holes are used where additional erection tolerance is needed. They reduce the bearing capacity and are not permitted in slip-resistant Category B or C connections without additional measures.

EN 1993-1-8 restricts the use of oversized holes: they are permitted only in Category A (bearing) connections and require hardened washers under both the bolt head and nut. For slip-resistant connections (Categories B and C), oversized holes are not permitted without specific verification.

Slotted Hole Sizes — Short and Long

Slotted holes provide tolerance in one direction. The orientation of the slot relative to the load direction is critical for design.

Short Slotted Holes

Long Slotted Holes

For long slotted holes with the slot axis perpendicular to the load direction, the bearing resistance factor alpha_b is reduced (see EN 1993-1-8 Table 3.4, Note 2). Hardened washers are always required over slotted holes, and for long slotted holes, a cover plate or additional hardened plate washer may be needed to distribute the bolt preload over the full slot length.

Edge and End Distance Requirements — EN 1993-1-8 Table 3.3

Adequate edge and end distances are critical to prevent tear-out failure of the connected plate. The minimum values per EN 1993-1-8 depend on the hole type and whether the loaded edge is subject to bearing or is unloaded.

For M20 bolts with d0 = 22 mm (normal clearance):

• Minimum end distance e1 = 1.2 x 22 = 26.4 mm (use 30 mm)
• Minimum edge distance e2 = 1.2 x 22 = 26.4 mm (recommend 1.5 x 22 = 33 mm)

In UK practice, e1 >= 2d and e2 >= 1.5d (based on bolt diameter, not hole diameter) is commonly adopted as a conservative rule that satisfies EN 1993-1-8.

Bolt Spacing Requirements

EN 1993-1-8 Clause 3.5 specifies the following minimum and maximum bolt spacings:

Where t is the minimum thickness of the connected plies. The minimum spacing ensures adequate bearing resistance and prevents overlapping stress cones between adjacent bolts. The maximum spacing prevents buckling of the plate between bolts and ensures the plate acts compositely.

Effect of Hole Size on Bearing Resistance

The hole diameter d0 directly influences the bearing resistance of the connected plate per EN 1993-1-8 Table 3.4:

Fb,Rd = k1 x alpha_b x fu x d x t / gamma_M2

Where:

• k1 = min(2.8 e2/d0 - 1.7, 2.5) for edge bolts; min(1.4 p2/d0 - 1.7, 2.5) for inner bolts
• alpha_b = min(e1/(3d0), p1/(3d0) - 1/4, fub/fu, 1.0)
• fu = tensile strength of the connected plate
• d = bolt diameter (not hole diameter)
• t = plate thickness
• gamma_M2 = 1.25

Larger holes (oversized, slotted) reduce alpha_b and k1, thereby reducing the bearing resistance. For this reason, standard clearance holes are used whenever possible, and oversized holes are used only where erection tolerances demand them.

Practical Detailing Rules for Bolt Holes

Based on common European fabrication and erection practice:

1. Always use standard clearance holes — oversized or slotted holes should only be specified when erection tolerance demands them and the reduced bearing capacity has been accounted for.
2. For M20 bolts at standard gauge (90 mm): Use pitch p1 = 70 mm (>= 2.2 x 22 = 48.4 mm, OK) and end distance e1 = 40 mm (>= 1.2 x 22 = 26.4 mm, OK).
3. For fin plate connections: Long slotted holes in the fin plate (with slot perpendicular to the beam axis) are standard to allow for beam length tolerance — the fin plate acts as an erection aid.
4. Hardened washers: Required over oversized and slotted holes, and on the outer face of the outer plies in preloaded connections.
5. Hole forming: Holes may be punched, drilled, or flame-cut. Punched holes are acceptable for t <= 25 mm and where the hole diameter d0 >= t. Reaming to full diameter after punching is required for thicker material.
6. Alignment: All holes in a connection stack must be aligned within fabrication tolerance. Drifting should not damage the bolt threads or enlarge the hole beyond the specified d0.

Frequently Asked Questions

What is the standard clearance for M20 bolt holes per EN 1993-1-8? For M20 bolts, EN 1993-1-8 Table 3.3 specifies a hole diameter d0 = 22 mm, giving a clearance of 2 mm. This is the standard clearance for structural steelwork across Europe and is identical to the UK legacy standard (BS 5950). The 2 mm clearance provides adequate tolerance for fabrication and erection while maintaining full bearing resistance.

What are the edge and end distance requirements for bolted connections? EN 1993-1-8 requires minimum end distance e1 >= 1.2 d0 for normal clearance holes (loaded edge), where d0 is the hole diameter. For an M20 bolt with d0 = 22 mm, this gives e1 >= 26.4 mm. For oversized holes, e1 >= 1.5 d0. For long slotted holes with the slot perpendicular to the edge, e1 >= 2.0 d0. The minimum edge distance e2 (unloaded edge) is also 1.2 d0, though 1.5 d0 is recommended for practical fabrication. The reduced end/edge distances for slotted and oversized holes reflect the larger hole in relation to the loaded plate material.

When should I use oversized or slotted holes instead of normal clearance holes? Oversized or slotted holes should be used only when erection tolerances demand them. Common applications include: fin plate connections (long slotted holes in the fin plate perpendicular to the beam axis for length tolerance), column base plates (oversized holes for anchor bolt positioning tolerance), and connections to existing structures where field measurements have uncertainty. Always re-calculate the bearing resistance with the appropriate alpha_b and k1 reductions for the larger hole diameter, and note that oversized holes are not permitted in slip-resistant connections (Categories B and C) without specific verification.

What is the minimum bolt pitch (centre-to-centre spacing) per EN 1993-1-8? The minimum bolt pitch p1 (spacing along the load direction) is 2.2 d0, where d0 is the hole diameter. For M20 bolts (d0 = 22 mm), p1 >= 48.4 mm. The minimum gauge p2 (spacing perpendicular to load) is 2.4 d0 = 52.8 mm. The standard gauge for M20 bolts in European sections is 90 mm for beams up to ~300 mm depth and 140 mm for larger beams — both well above the minimum. The maximum spacing is 14t or 200 mm, whichever is smaller, to prevent plate buckling between bolts and ensure the ply acts compositely.

How does hole size affect bolt bearing resistance in EN 1993-1-8? The hole diameter d0 directly reduces the bearing resistance through the alpha_b and k1 coefficients: alpha_b includes e1/(3d0) and p1/(3d0) terms, and k1 includes e2/d0. Larger holes reduce these coefficients, reducing Fb,Rd. This is why standard clearance holes are preferred — they provide the highest bearing resistance. For an M20 bolt in a 10 mm S355 plate with standard e1 = 40 mm and p1 = 70 mm: alpha_b = min(40/(3x22), 70/(3x22)-0.25, 1.0) = min(0.606, 0.811, 1.0) = 0.606. With oversized hole d0 = 24 mm: alpha_b = min(40/(3x24), ...) = min(0.556, ...) = 0.556 — approximately 8% reduction.

Related Pages

• EN 1993-1-8 Connection Design Guide — Bolt categories, welds, joint types
• EN 1993-1-8 Bolt Grades — 4.6, 5.6, 8.8, 10.9 properties
• EN 1993-1-8 Bolt Capacity Tables — Fv,Rd and Ft,Rd values
• EN 1993-1-8 Bolt Pretension — Slip-resistant Cat B/C
• EN 1993-1-1 Beam Design Guide — Section classification, Mc,Rd
• EN 1993 Column Buckling Guide — Nb,Rd, buckling curves
• Bolted Connections Calculator
• Bolt Torque Calculator
• Beam Capacity Calculator
• Column Capacity Calculator

Educational reference only. All hole dimensions and spacing rules are per EN 1993-1-8:2005. Verify all values against the current edition of the Eurocode and the applicable National Annex for your project. Hole sizes shown are for standard structural steelwork — special applications (bridges, crane girders, fatigue-loaded connections) may have additional requirements. Results are PRELIMINARY — NOT FOR CONSTRUCTION. All details must be independently verified by a licensed Professional Engineer or Chartered Structural Engineer.