Australian Bolt Hole Sizes — Standard, Oversized, and Slotted Holes per AS 4100 Clause 9.6.2

Complete reference for bolt hole dimensions in Australian structural steel connections per AS 4100:2020 Clause 9.6.2. Covers standard holes (STD), oversized holes (OVS), short slotted holes (SSL), and long slotted holes (LSL) for metric bolts M12 through M36. Includes hole tolerances, usage restrictions by connection type, and worked examples for hole selection.

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Hole Categories — AS 4100 Clause 9.6.2.1

AS 4100:2020 defines four distinct categories of bolt holes, each with specific dimensional standards and permitted applications:

1. Standard Holes (STD)

Standard holes are the default for all bearing-type connections. The hole diameter is the bolt diameter plus a clearance allowance:

• For bolts M24 and smaller: d_h = d_f + 2 mm
• For bolts M27 and larger: d_h = d_f + 3 mm

The 2 mm clearance provides adequate erection tolerance while maintaining sufficient bearing contact between the bolt shank and the hole wall. The 3 mm clearance for larger bolts recognises that erection alignment becomes more difficult with larger bolt diameters, and the proportional effect on bearing capacity is smaller.

2. Oversized Holes (OVS)

Oversized holes provide increased erection tolerance beyond standard holes:

Oversized holes are ONLY permitted in slip-critical connections (TF category) with fully tensioned bolts per AS 4100 Clause 9.6.2.3. The bolt pretension provides the clamping force that resists slip until the bolt bears against the far side of the oversized hole at the ultimate limit state.

3. Short Slotted Holes (SSL)

Short slotted holes have the same width as a standard hole and a length equal to the standard hole diameter plus an additional 2-7 mm:

• SSL width: same as standard hole diameter (d_h)
• SSL length: standard hole diameter + 2 mm (M20: 22 + 2 = 24 mm)
• Can extend up to standard hole diameter + 7 mm for larger bolts

Short slotted holes are permitted in ONE ply of a bearing-type connection (the ply that requires the adjustment), per Clause 9.6.2.3. The slot should be oriented perpendicular to the direction of the applied load to avoid reducing the bearing contact length. If the slot is parallel to the load, the bearing capacity is not reduced but the hole elongation under load will be larger.

4. Long Slotted Holes (LSL)

Long slotted holes have a width equal to the standard hole diameter and a length up to 2.5 times the standard hole diameter:

• LSL width: same as standard hole diameter
• LSL length: up to 2.5 x d_h (M20: up to 2.5 x 22 = 55 mm, rounded to 50 mm per standard sizes)

Long slotted holes are ONLY permitted in slip-critical connections, and only in one ply. They are typically used where significant erection adjustment is required, such as in base plate connections where column plumb adjustment is needed, or in long-span truss connections where thermal expansion must be accommodated.

Complete Bolt Hole Dimension Table — AS 4100 Table 9.6.2

Hole Tolerance

The tolerance on hole diameter is +0.5 mm / -0.0 mm for all hole types. This means holes may be up to 0.5 mm larger than the nominal size but must never be smaller. The tolerance is achieved by:

• Drilling: standard tolerance control with sharp drill bits
• Punching: requires regular die maintenance; punching is limited to material thickness <= hole diameter (t <= d_h) per AS 4100 Clause 9.6.2.4
• Flame cutting: permitted only for holes >= 24 mm diameter; the cut surface must be ground smooth

For slotted holes, the width tolerance is +0.5/-0.0 mm and the length tolerance is +/- 1.5 mm for short slots and +/- 3.0 mm for long slots.

Hole Type Restrictions by Connection Category — Clause 9.6.2.3

The distinction is fundamental: oversized and long slotted holes may NEVER be used in bearing-type connections because the bolt must slip through the excess clearance before bearing develops. In a bearing-type connection without pretension, this slip could cause excessive connection deformation at service load levels. In a slip-critical connection, the bolt pretension prevents slip at service loads, and the larger hole is only engaged in bearing at the ultimate limit state.

Net Area Deduction — Clause 9.1.10

When calculating the net cross-sectional area of a tension member at a bolt hole, the hole dimension used for the deduction is:

d_net = d_h + 2 mm (for all hole types)

where d_h is the actual hole diameter (or the slot width, not the slot length, for slotted holes).

The additional 2 mm accounts for fabrication damage around the hole (tearing, gouging from the drill or punch). This is a conservative allowance that ensures the net section capacity calculation provides a lower-bound estimate of the true fracture capacity.

For an M20 bolt with a standard hole (d_h = 22 mm):

d_net = 22 + 2 = 24 mm deduction per hole on the net section

For an M20 bolt with an oversized hole (d_h = 24 mm):

d_net = 24 + 2 = 26 mm deduction per hole

The larger deduction for oversized holes means the net section must be checked more carefully for slip-critical connections where oversized holes are used. The increased deduction can reduce the tension capacity of the connected member and may require a wider plate or additional bolt rows to compensate.

Staggered Hole Patterns and Net Section — Clause 9.1.10.3

When bolts are arranged in a staggered pattern, the critical net section path may follow a zig-zag (diagonal) line rather than a straight transverse line. The effective hole deduction for a chain of staggered holes is:

d_eff = sum(d_h + 2) - sum(s^2 / (4g))

where s is the staggered pitch (longitudinal spacing along the chain) and g is the gauge (transverse spacing between bolt rows). The s^2/(4g) term represents the beneficial effect of the diagonal path, which increases the net section length for the same number of holes.

The designer must check all possible failure paths:

1. Straight transverse path through the first row of bolts
2. Zig-zag path through bolt 1 (row 1) to bolt 2 (row 2) to bolt 3 (row 3) etc.
3. Combinations of straight and zig-zag segments

The critical path is the one yielding the minimum net width. For a compact bolt group with tight spacing, the zig-zag path typically gives the lowest net section.

Worked Example: Hole Selection for Slip-Critical Splice

Problem: A tension splice in a 20 mm thick Grade 350 plate uses M24 Grade 8.8/TB bolts in a slip-critical connection. The splice is in a bridge girder bottom flange where fatigue is a design consideration. The erection contractor has requested oversized holes to accommodate fabrication tolerances. Determine the hole dimensions, check compliance with AS 4100, and calculate the net section deduction.

Given:

• Bolt diameter: d_f = 24 mm
• Bolt grade: 8.8/TB (pretensioned)
• Connection category: TF (slip-critical)
• Plate: Grade 350, t = 20 mm, b = 300 mm wide
• Erection tolerance required: +/- 3 mm in both directions
• Applied tension in splice: N* = 1,200 kN (factored)

Solution:

Step 1: Select hole type

Oversized holes requested for erection tolerance. Per Clause 9.6.2.3, OVS holes are permitted in slip-critical (TF) connections. Per Table 9.6.2, M24 OVS hole diameter = 30 mm.

The 6 mm clearance (30 - 24 = 6 mm) provides 3 mm of adjustment in any direction, satisfying the erection tolerance requirement.

Step 2: Verify hole spacing compliance

Minimum edge distance for OVS hole: 1.5 x d_h = 1.5 x 30 = 45 mm. Minimum pitch: 2.5 x d_f = 2.5 x 24 = 60 mm (pitch is based on bolt diameter, not hole diameter).

Proposed layout: two rows of 4 bolts each, pitch = 80 mm, gauge = 100 mm between rows, edge distance = 50 mm.

Check: 50 mm > 45 mm -- OK. 80 mm > 60 mm -- OK.

Step 3: Net section calculation

Plate width b = 300 mm. Two bolt holes on the critical transverse path.

Gross area: A_g = 300 x 20 = 6,000 mm^2

Net width on straight path: b_net = 300 - 2 x (30 + 2) = 300 - 64 = 236 mm

Net area: A_n = 236 x 20 = 4,720 mm^2

Step 4: Alternative -- would standard holes work?

If standard holes were used instead (d_h = 26 mm, d_net = 28 mm):

Net width = 300 - 2 x 28 = 244 mm

Net area = 244 x 20 = 4,880 mm^2

Increase in net section capacity = (4,880 - 4,720) / 4,720 = 3.4%

The 3.4% reduction in net section capacity from oversized holes is small and acceptable. The slip-critical bolt design will govern before the net section fracture, so the reduced net area does not control the design.

Step 5: Document on the fabrication drawing

Specify on the splice detail: "M24 Grade 8.8/TB bolts in 30 mm diameter oversized holes (OVS). Slip-critical connection, faying surfaces blast-cleaned to Class 2-1/2 (mu = 0.50). Bolts pretensioned to 147 kN minimum per AS 4100 Clause 9.3.8."

Result: M24 OVS holes (30 mm diameter) comply with AS 4100 for this slip-critical connection. The 6 mm clearance provides adequate erection tolerance. Net section reduction of 3.4% is acceptable and does not govern the design.

Frequently Asked Questions

What is the standard hole size for M20 bolts in Australian structural steel?

The standard hole diameter for M20 bolts is 22 mm (bolt diameter + 2 mm), per AS 4100 Table 9.6.2. For M24 bolts, the standard hole is 26 mm (also +2 mm). For M27 and larger, the clearance increases to 3 mm: M27 standard hole = 30 mm, M30 = 33 mm, M36 = 39 mm. The tolerance is +0.5 mm / -0.0 mm, so the actual hole diameter may be up to 22.5 mm for M20.

Can I use oversized holes in a standard bearing-type bolted connection?

No. Oversized holes are only permitted in slip-critical (TF) connections where the bolts are fully pretensioned per AS 4100 Clause 9.6.2.3. In a bearing-type connection, the bolts are snug-tight and do not have sufficient clamping force to prevent slip. With an oversized hole, the bolt would have to slip through the excess clearance before engaging in bearing, causing connection deformation that may exceed serviceability limits. Oversized holes in bearing-type connections are a non-compliance with AS 4100 and a fabrication defect that requires rectification.

How is the net area calculated when slotted holes are used per AS 4100?

For slotted holes, the net area deduction uses the slot width (perpendicular to the load direction), not the slot length, plus the standard 2 mm fabrication damage allowance. If the slot is oriented perpendicular to the applied tension force, the slot width is approximately the standard hole diameter, and the deduction is the same as for standard holes. If the slot is oriented parallel to the applied force, the slot length is the relevant dimension for net section, and the deduction is d_slot_length + 2 mm, which can be significantly larger. For this reason, slotted holes in tension members should always be oriented with the slot perpendicular to the tension force.

Does punching affect the permissible hole diameter per AS 4100?

Yes. Per AS 4100 Clause 9.6.2.4, holes may be punched only when the material thickness does not exceed the hole diameter (t <= d_h) and when the hole diameter is not greater than 24 mm. For material thicker than the hole diameter, the hole must be drilled or sub-punched and reamed. Punching causes cold-working and micro-cracking around the hole periphery, which reduces the ductility of the net section. The t <= d_h restriction limits the severity of this cold-working effect. For Grade 350 and Grade 400 plates, drilling is preferred over punching regardless of thickness.

What hole type should be used for base plate connections per AS 4100?

Base plate holding-down bolt holes are typically oversized (OVS) or short slotted (SSL) to accommodate column plumb adjustment during erection. The hole type must be specified on the base plate fabrication drawing. Standard holes (STD) are rarely used for base plates because the 2 mm clearance is insufficient for the +/- 5 mm column plumb tolerance typical in Australian construction. The holding-down bolts are cast into the concrete and cannot be adjusted, so the base plate holes must provide the necessary positional tolerance. The oversize allowance does not affect the base plate bending capacity or the bolt tension capacity, but the washer size must be increased to ensure full bearing under the nut.

Educational reference only. All hole dimensions must be verified against the current edition of AS 4100:2020 and the project specification. This information does not constitute professional engineering advice.