Australian Bolt Spacing — AS 4100 Clause 9.6 Edge Distance and Pitch Requirements

Complete reference for minimum and maximum bolt spacing, edge distance, and pitch requirements per AS 4100:2020 Clause 9.6. Covers standard holes, oversized holes, slotted holes, staggered hole patterns, and stitch bolt provisions for built-up members. All dimensions in millimetres unless noted otherwise.

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Governing Clause: AS 4100 Clause 9.6

AS 4100:2020 Clause 9.6 governs the positioning of bolts in structural steel connections. The clause addresses three distinct geometric requirements: minimum edge distance (Clause 9.6.3), minimum pitch and gauge (Clause 9.6.4), and maximum pitch (Clause 9.6.5). Each requirement serves a different structural purpose.

Edge distance requirements prevent bearing and tearout failure of the plate material at the bolt hole. Pitch requirements ensure that individual bolts can develop their full design capacity without overlapping zones of influence. Maximum pitch limits prevent plate separation, moisture ingress, and corrosion between bolts in a connection.

Minimum Edge Distance — Clause 9.6.3

The minimum distance from the centre of a bolt hole to the edge of a plate or the end of a member is specified as a function of the bolt hole diameter and the edge condition.

For a sheared or flame-cut edge the minimum edge distance is:

a_e >= 1.5 d_h

where d_h is the hole diameter (mm).

For a rolled, sawn, or machined edge the minimum edge distance is:

a_e >= 1.25 d_f

where d_f is the nominal bolt diameter (mm).

Edge Distance by Bolt Size and Edge Type

Rationale for Edge Distance Limits

The 1.5 d_h factor for sheared edges accounts for the reduced ductility of the plate material adjacent to a thermally cut or sheared edge. The heat-affected zone and cold-worked region extend approximately 0.5 d_h from the edge, and the bolt bearing stresses should not overlap with this zone.

For rolled or machined edges, the material properties at the edge are essentially the same as the bulk plate properties, allowing the tighter 1.25 d_f limit.

Maximum Edge Distance — Clause 9.6.3

The maximum edge distance from the centre of a bolt hole to the nearest plate edge is limited to the lesser of:

• 12 t_p, where t_p is the thickness of the connected ply (mm)
• 150 mm

Excessive edge distance can lead to local buckling of the ply between bolts or prying action that reduces the effective clamping force in pretensioned bolts.

Minimum Pitch and Gauge — Clause 9.6.4

The minimum centre-to-centre distance between bolts (pitch in the direction of force, gauge perpendicular to the direction of force) is:

p_min >= 2.5 d_f

where d_f is the nominal bolt diameter.

Minimum Pitch by Bolt Size

The 2.5 d_f minimum ensures that the bearing stress fields of adjacent bolts do not overlap significantly. If bolts are placed closer than this limit, the combined bearing stress reduces the effective bearing capacity of the connected plies. The minimum also provides adequate wrench clearance for installation with standard tools.

Maximum Pitch — Clause 9.6.5

The maximum centre-to-centre bolt pitch is governed by corrosion protection and plate separation criteria rather than strength:

Rationale for Maximum Pitch Limits

Tension members: The 300 mm limit ensures that connected plies remain in contact between bolts, preventing gap formation that would allow moisture ingress. At spacings greater than 300 mm, differential thermal expansion between plies can cause separation.

Compression members (painted): The tighter 200 mm limit is imposed because paint systems can crack at the bolt-to-plate interface, creating corrosion initiation sites. Closer bolt spacing maintains ply contact and seals these interfaces.

Weathering steel (unpainted): The 150 mm limit is the tightest because weathering steel relies on a tightly adherent oxide layer for corrosion protection. Plate separation disrupts this oxide layer and accelerates section loss.

Stitch Bolts in Built-Up Members — Clause 9.6.6

For built-up compression members where two or more components are bolted together to form a single cross-section, stitch bolts are required to prevent individual component buckling between connection points.

Maximum Stitch Bolt Spacing

The maximum longitudinal spacing of stitch bolts (s_stitch) is governed by the minimum radius of gyration of the individual component (r_min) and the slenderness ratio of that component:

s_stitch <= min(r_min / r_y of built-up section x 200, 600 mm)

For typical built-up I-sections with bolted flange-to-web connections:

Stitch Bolt Edge Distance Adjustment

For stitch bolts connecting cover plates or component elements, the minimum edge distance may be reduced to 1.25 d_h (in place of the usual 1.5 d_h for sheared edges) provided that the bolt shear capacity is verified and the reduced edge distance does not control the design.

Staggered Bolt Pattern Provisions — Clause 9.6.7

When bolts are arranged in a staggered pattern, the net section of the connected ply may be governed by a zig-zag failure path rather than a straight transverse path. AS 4100 Clause 9.6.7 specifies the effective hole deduction for staggered patterns:

For a chain of holes at spacing s (pitch along the member) and g (gauge across the member), the effective deduction for each hole in the chain is:

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

where the s^2/(4g) term accounts for the diagonal component of the failure path. This formula is applied to determine the critical net section for tension capacity calculations per Clause 7.2.

Worked Example: M20 Bolted Splice Connection

Problem: A tension splice in a 250UB37.3 Grade 300 beam uses M20 Grade 8.8 bolts in standard holes (d_h = 22 mm). The connected plies are 10 mm thick Grade 300 plate. The plate edges are flame-cut. The connection has two vertical rows of four bolts each. Determine the required edge distances and spacing.

Given:

• Bolt diameter: d_f = 20 mm
• Hole diameter: d_h = 22 mm
• Edge condition: flame-cut (sheared edge)
• Connection type: bearing-type, snug-tight

Solution:

1. Minimum edge distance (sheared edge): a_e_min = 1.5 x 22 = 33 mm. Use 35 mm to allow for fabrication tolerance.

2. Minimum end distance (in direction of load): a_e_min = 1.5 x 22 = 33 mm. Use 40 mm to provide additional tearout resistance at the loaded end of the splice.

3. Minimum pitch: p_min = 2.5 x 20 = 50 mm. Use 70 mm to provide adequate wrench clearance for M20 bolts (standard M20 nut across flats = 30 mm, wrench clearance approximately 42 mm).

4. Minimum gauge (between bolt rows): g_min = 2.5 x 20 = 50 mm. Use 70 mm centre-to-centre between the two vertical rows.

5. Maximum pitch check: For tension member, p_max = 300 mm. 70 mm < 300 mm -- OK.

6. Maximum edge distance check: a_e_max = min(12 x 10, 150) = 120 mm. 35 mm < 120 mm -- OK.

7. Check staggered layout: With bolts at 70 mm pitch and 70 mm gauge, s^2/(4g) = 70^2 / (4 x 70) = 17.5 mm. Effective hole deduction for net section = 22 + 2 - 17.5 = 6.5 mm per hole on the staggered path. This is less than the straight-path deduction of 24 mm per hole, confirming the staggered pattern is more favourable for net section capacity.

Result: Edge distance 35 mm, end distance 40 mm, pitch 70 mm, gauge 70 mm. All values comply with AS 4100 Clause 9.6.

Practical Detailing Notes

For bolted connections fabricated in Australian workshops, standard practice is to use 5 mm increments for edge distances and 10 mm increments for pitch and gauge to match typical fabrication templates and CNC drilling tables. Common standard spacings are:

• M16 bolts: 40 mm edge, 60 mm pitch
• M20 bolts: 35-40 mm edge, 70 mm pitch
• M24 bolts: 45-50 mm edge, 80 mm pitch
• M30 bolts: 55-60 mm edge, 100 mm pitch

These dimensions satisfy AS 4100 minimum requirements while maximising erection tolerance and simplifying fabrication.

Frequently Asked Questions

What is the minimum edge distance for M20 bolts per AS 4100?

For M20 bolts in standard holes (d_h = 22 mm) with sheared or flame-cut edges, the minimum edge distance is 1.5 x 22 = 33 mm per AS 4100 Clause 9.6.3. For rolled or machined edges, the minimum reduces to 1.25 x 20 = 25 mm. In Australian practice, 35 mm is commonly used for sheared edges to provide fabrication tolerance.

What is the difference between pitch and gauge in bolted connections?

Pitch is the centre-to-centre distance between bolts measured parallel to the direction of the applied force. Gauge is the centre-to-centre distance measured perpendicular to the direction of the applied force, typically across the width of the member. Both are subject to the same minimum of 2.5 d_f per AS 4100 Clause 9.6.4.

Can bolt spacing vary within a single connection per AS 4100?

Yes. AS 4100 does not require uniform bolt spacing within a connection. Variable spacing is permitted provided that every bolt individually satisfies the minimum edge distance and pitch requirements. However, non-uniform spacing complicates fabrication and is generally avoided unless structurally justified by variable force distribution within the bolt group.

What is the maximum bolt pitch in a tension splice?

The maximum bolt pitch in a tension member is 300 mm per AS 4100 Clause 9.6.5. This applies to all bolts connecting the tension member regardless of whether they are in a splice, end connection, or intermediate joint. The 300 mm limit prevents plate separation between widely spaced bolts.

How does staggered bolt layout affect net section calculation?

Per AS 4100 Clause 9.6.7, staggered bolt patterns reduce the effective hole deduction for net section calculations through the s^2/(4g) term. For a straight transverse path, each hole deducts d_h + 2 mm. For a staggered path, the deduction per hole is reduced by the diagonal component. The critical net section is the path with the minimum net width after all deductions are applied. The engineer must check all possible failure paths: straight transverse, zig-zag through one row of staggered holes, and zig-zag through multiple rows.

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.