Australian Bolt Hole Sizes — Standard, Oversized & Slotted Holes per AS 4100
Complete reference for bolt hole dimensions in Australian steel design per AS 4100:2020 Clause 9.6. Metric bolt hole sizes for Grade 8.8 and 10.9 bolts from M12 to M36: standard holes, oversized holes, short slotted holes, and long slotted holes. All dimensions in millimetres.
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AS 4100 Bolt Hole Categories
AS 4100:2020 Clause 9.6.2 defines four categories of bolt holes for structural steel connections:
Standard holes (STD) — Bolt hole diameter equal to the bolt diameter plus 2 mm for bolts up to M24, or plus 3 mm for M27 and larger. Standard holes are the default for bearing-type connections and provide the most economical fabrication.
Oversized holes (OVS) — Larger holes providing additional erection tolerance. Permitted only in slip-critical connections. The oversized dimension is typically 3-8 mm larger than the bolt diameter depending on bolt size.
Short slotted holes (SSL) — Elongated holes with length limited to the standard hole diameter plus 2 mm. Permitted in either bearing-type or slip-critical connections. Slot width equals standard hole diameter.
Long slotted holes (LSL) — Elongated holes up to 2.5 times the standard hole diameter in length. Only permitted in slip-critical connections. The slot should be oriented perpendicular to the direction of applied load where possible.
Hole Type Usage Per Connection Type
| Connection Type | STD | OVS | SSL | LSL |
|---|---|---|---|---|
| Bearing-type (snug-tight) | Yes | No | Yes (one ply) | No |
| Slip-critical (pretensioned) | Yes | Yes | Yes | Yes (fillers only) |
| Tension-bearing (TB) | Yes | No | Yes (one ply) | No |
Per AS 4100 Clause 9.6.2.3, oversized and long slotted holes are only permitted in connections where the bolts are fully tensioned to the requirements of Clause 9.3.8.
Complete Bolt Hole Dimension Table
All dimensions per AS 4100:2020 Table 9.6.2. Tolerance on hole diameter: +0.5 mm / -0.0 mm. Slotted hole width tolerance: +0.5 mm / -0.0 mm. Slot length tolerance: +/-1.5 mm for short slots, +/-3.0 mm for long slots.
| Bolt Dia (mm) | STD (mm) | OVS (mm) | SSL Width x Length (mm) | LSL Width x Length (mm) |
|---|---|---|---|---|
| M12 | 14 | 16 | 14 x 16 | 14 x 30 |
| M16 | 18 | 20 | 18 x 22 | 18 x 40 |
| M20 | 22 | 24 | 22 x 26 | 22 x 50 |
| M22 | 24 | 28 | 24 x 28 | 24 x 55 |
| M24 | 26 | 30 | 26 x 32 | 26 x 60 |
| M27 | 30 | 35 | 30 x 37 | 30 x 67 |
| M30 | 33 | 38 | 33 x 40 | 33 x 75 |
| M36 | 39 | 45 | 39 x 46 | 39 x 90 |
Hole Nomenclature
- STD (Standard hole): dh = d + 2 mm (d <= M24), dh = d + 3 mm (d >= M27)
- OVS (Oversized hole): dh = d + 4 mm to d + 9 mm depending on bolt diameter
- SSL (Short slotted hole): Width = dh (std), Length = dh + 2 mm to dh + 7 mm
- LSL (Long slotted hole): Width = dh (std), Length <= 2.5 x dh
Edge Distance Requirements
AS 4100 Clause 9.6.3 specifies minimum and maximum edge distances from the centre of the hole to the edge of the member.
Minimum Edge Distance (ae)
The minimum edge distance depends on the edge type and the bolt diameter:
| Edge Type | Minimum Edge Distance |
|---|---|
| Sheared or flame-cut edge | 1.5 x dh (or 1.5 x bolt diameter for STD holes) |
| Rolled or sawn edge | 1.25 x bolt diameter |
| For M20 standard hole (22 mm) | ae >= 33 mm (sheared edge), ae >= 25 mm (rolled edge) |
Alternatively, the minimum edge distance can be calculated from the bearing capacity requirement: the design bearing capacity (phi Vb) must be at least equal to the design shear capacity of the bolt.
Maximum Edge Distance
The maximum edge distance is the lesser of:
- 12 x t (where t is the thickness of the connected part)
- 150 mm
This limit prevents prying action at the edge of the member and ensures the connection remains tight under load.
Bolt Spacing Requirements
AS 4100 Clause 9.6.4 specifies minimum and maximum centre-to-centre spacing of bolts.
Minimum Pitch (p) and Gauge (g)
| Direction | Minimum Spacing |
|---|---|
| Between bolt centres (pitch) | 2.5 x d (where d = nominal bolt diameter) |
| Between bolt rows (gauge) | 2.5 x d |
| For M20 bolts | 50 mm minimum pitch and gauge |
Maximum Pitch
| Member Type | Maximum Pitch |
|---|---|
| Tension members | 300 mm |
| Compression members (painted) | 200 mm |
| Compression members (weathering steel) | 150 mm |
| Stitch bolts in built-up compression members | 600 mm |
Net Area Deduction for Bolt Holes
Per AS 4100 Clause 9.1.1, the net area of a member at a bolt hole connection is calculated by deducting the projected area of the hole from the gross cross-sectional area.
The hole dimension used for net area calculation is:
- Standard holes: Actual hole diameter + 2 mm allowance for fabrication damage
- Oversized holes: Actual hole diameter + 2 mm allowance
- Slotted holes: Slot width (not length) + 2 mm allowance
For an M20 standard hole (22 mm actual diameter), the net area deduction width = 22 + 2 = 24 mm per hole.
Worked Example: Bolt Hole Sizing
Problem: A bolted connection uses M24 Grade 8.8 bolts in a slip-critical connection. The plate edges are flame-cut. Determine the standard hole size, the minimum edge distance, and the minimum bolt spacing.
Given:
- Bolt diameter: d = 24 mm
- Connection type: slip-critical (pretensioned)
- Edge type: flame-cut
- M24 is <= M24, so standard hole = d + 2 mm
Solution:
- Standard hole diameter: dh = d + 2 = 24 + 2 = 26 mm
- Minimum edge distance (flame-cut): ae >= 1.5 x dh = 1.5 x 26 = 39 mm
- Minimum bolt spacing: p >= 2.5 x d = 2.5 x 24 = 60 mm
Check: If oversized holes are needed for erection tolerance: OVS = 30 mm diameter (per AS 4100 Table 9.6.2). Permitted only in slip-critical connections with fully tensioned bolts.
Design Resources
- Australian Steel Design Guide — AS 4100 overview
- Australian Bolt Grades — Grade 8.8 and 10.9 properties
- Australian Bolt Capacity — Shear and tension tables
- Australian Bolt Pretension — Slip-critical requirements
- AS 4100 Base Plate Design — Base plate reference
- Bolted Connections Calculator
- Bolt Torque Calculator
Frequently Asked Questions
What is the standard hole diameter for M20 bolts per AS 4100? The standard hole diameter for M20 bolts is 22 mm (bolt diameter + 2 mm), per AS 4100 Table 9.6.2. For oversized holes, the diameter is 24 mm. For short slotted holes, the dimension is 22 x 26 mm. For long slotted holes, the dimension is 22 x 50 mm. Tolerance on hole diameter is +0.5 mm / -0.0 mm.
Can oversized holes be used in bearing-type connections per AS 4100? No, oversized holes are only permitted in slip-critical connections (TF category) per AS 4100 Clause 9.6.2.3. In bearing-type connections (snug-tight or TB), only standard holes and short slotted holes (in one ply) are permitted. Using oversized holes in bearing-type connections would allow excessive slip before bearing develops, potentially causing connection deformation under service loads.
What is the minimum edge distance for M24 bolts in AS 4100? For M24 bolts with standard holes (26 mm diameter), the minimum edge distance from the centre of the hole to a flame-cut or sheared edge is 1.5 x dh = 39 mm. For rolled or sawn edges, the minimum is 1.25 x d = 30 mm. These limits per AS 4100 Clause 9.6.3 ensure that the plate has adequate bearing and tearout capacity at the edge.
How is hole oversize accounted for in AS 4100 net section calculations? Per AS 4100 Clause 9.1.1, the hole dimension used for net area calculation is the actual hole diameter plus 2 mm, regardless of whether the hole is standard, oversized, or slotted. For M20 standard holes (22 mm actual), the deduction is 22 + 2 = 24 mm. For oversized holes (24 mm actual), the deduction is 24 + 2 = 26 mm. For slotted holes, the slot width (not length) plus 2 mm is used.
What is the maximum bolt spacing per AS 4100? Maximum spacing between bolt centres per AS 4100 Clause 9.6.4: 300 mm for tension members and 200 mm for compression members with painted surfaces. For weathering steel, maximum edge distance reduces to 150 mm for compression members. Stitch bolts in built-up compression members have a maximum spacing of 600 mm. These limits prevent member separation under load and ensure adequate corrosion protection.
Educational reference only. All design values must be verified against the current edition of AS 4100:2020 and the project specification. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.