CSA Bolt Hole Sizes — Standard, Oversized & Slotted Holes per CSA S16-19

Complete reference for bolt hole dimensions in Canadian steel design per CSA S16-19 Clause 22. Metric bolt hole sizes for A325M and A490M bolts from M16 to M36: standard holes, oversized holes, short slotted holes, and long slotted holes. All dimensions in millimetres.

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CSA S16 Bolt Hole Categories

CSA S16-19 Clause 22.3 defines four categories of bolt holes for structural joints:

1. Standard holes (STD) — Bolt hole diameter equal to bolt diameter plus 2 mm for bolts up to M24, or plus 3 mm for bolts M27 and larger. Standard holes are used in bearing-type connections and provide the most economical fabrication.

2. Oversized holes (OVS) — Larger holes that provide additional tolerance for erection. Used in slip-critical connections only. The oversized dimension is typically 3-8 mm larger than standard holes depending on bolt diameter.

3. Short slotted holes (SSL) — Elongated holes oriented in any direction. Used in slip-critical or bearing-type connections to accommodate minor misalignment. Slot length = hole diameter + 2 mm.

4. Long slotted holes (LSL) — Elongated holes up to 2.5 times the hole diameter in length. Used for expansion provisions or significant tolerance adjustment. Only permitted in slip-critical connections with the slot oriented perpendicular to the direction of load.

Hole Type Usage per Connection Type

Per CSA S16-19 Clause 22.3.3, oversized and slotted holes are permitted in slip-critical connections only, and the long slot direction should be oriented perpendicular to the applied load where possible to minimise slip deformation.

Complete Bolt Hole Dimension Table

All dimensions per CSA S16-19 Table 3 (Hole Dimensions). Standard tolerance on hole diameter is +0.5 mm / -0.0 mm. Slotted hole width tolerance is +0.5 mm / -0.0 mm. Slot length tolerance is +/-1.5 mm for short slots and +/-3.0 mm for long slots.

Bolt Designation Dimensional Reference

Canadian structural bolts (A325M and A490M) use metric thread dimensions per ISO metric thread profile:

For CSA S16-19 bolt capacity calculations, the area used is the nominal body area Ab = pi * d^2 / 4 where d is the nominal bolt diameter. The root area (through threads) is not directly used in CSA S16 capacity equations — instead the capacity tables in Clause 13.12 already account for whether threads are in the shear plane.

Edge Distance Requirements

Per CSA S16-19 Clause 22.4, minimum edge distances from the centre of the hole to the edge of the member:

Edge distance is measured from the centre of the bolt hole to the nearest edge of the connected part. Minimum edge distances ensure that bearing and tear-out resistances per CSA S16-19 Clause 13.11.2 can develop fully. If edge distance is less than specified, the bearing resistance Br must be reduced proportionally.

Maximum Edge Distance

CSA S16-19 Clause 22.4.3 also specifies a maximum edge distance for exposed steelwork:

• Maximum edge distance = 300 mm for painted steel
• Maximum edge distance = 200 mm for weathering steel

These limits ensure that exposed edges do not separate under load and that weathering steel edges remain tight enough to prevent moisture ingress.

Bolt Spacing Requirements

Per CSA S16-19 Clause 22.4:

Minimum spacing between bolt centres:

• Standard holes: 2.7 * d (where d = bolt diameter)
• Oversized holes: 3.0 * d
• Slotted holes: 3.0 * d (measured parallel to slot direction)

For M20 bolts in standard holes: minimum spacing = 2.7 x 20 = 54 mm. In practice, 60 mm (3 x bolt diameter) is the most common working spacing for bolted connections.

Maximum spacing:

• For compression members in contact: 200 mm
• For tension members: 300 mm
• For stitch bolts in built-up compression members: 600 mm

Hole Deductions for Net Area

Per CSA S16-19 Clause 12.3.2, when calculating the net area of a tension member, the hole diameter for deduction is taken as:

Hole deduction diameter = bolt hole diameter + 2 mm

This accounts for damage to the edge of the hole during punching or drilling. For example, in an M20 standard hole (22 mm diameter), the net section deduction uses a 24 mm hole:

An = Ag - sum(d_hole + 2) * t

For sections with staggered holes (zig-zag pattern), the net section can be determined per CSA S16-19 Clause 12.3.3 using the standard pitch line method:

An = Ag - sum(d*hole * t) + sum(s^2 / (4g) _ t)

Where s is the staggered pitch (longitudinal spacing) and g is the gauge (transverse spacing).

Hole Fabrication Methods

CSA S16-19 Clause 22.5 specifies hole fabrication requirements:

For material thicker than bolt diameter plus 3 mm, holes must be drilled or sub-punched and reamed. For thickness up to 12 mm, punching is acceptable. In slip-critical connections, sub-punching and reaming or drilling is preferred to avoid the hardened edge zone that reduces slip coefficient.

Design Example: Bolt Hole Selection for a Shear Connection

Problem: Select bolt holes for a shear connection using M22 A325M bolts in CSA G40.21 350W steel (12 mm gusset plate). Connection is a beam web shear tab subject to reversal (fatigue loading).

Step 1 — Determine hole type: Fatigue loading requires slip-critical connection per CSA S16-19 Clause 22.8. For slip-critical connections, standard, oversized, or slotted holes are permitted.

Given that this is a beam connection requiring some erection tolerance, use short slotted holes (SSL) in the beam web and standard holes (STD) in the shear tab:

• Beam web (SSL): 24 x 28 mm slots (width x length)
• Shear tab (STD): 24 mm diameter holes

Step 2 — Check edge distances: For M22 bolts with sheared edges, minimum edge distance = 35 mm. If the shear tab is 180 mm deep with M22 holes at standard spacing (60 mm centres), the edge distance = (180 - 5 x 60) / 2 = 30 mm from the centre.

With minimum edge distance of 35 mm required, redesign with 220 mm deep shear tab: edge distance = (220 - 5 x 60) / 2 = 35 mm. Acceptable.

Step 3 — Check spacing: Minimum spacing = 2.7 x 22 = 59.4 mm. Using 60 mm spacing is acceptable for standard holes. For short slotted holes, minimum spacing = 3.0 x 22 = 66 mm when slots are parallel to load direction. Increase spacing to 70 mm to accommodate slots.

Step 4 — Net section check: Gusset plate (12 mm thick, 350W): Net area at first bolt line with SSL holes: Hole deduction = 24 mm slot width + 2 mm = 26 mm An = (220 x 12) - (26 x 12) = 2,640 - 312 = 2,328 mm^2

Tr = phi*u * An _ Fu = 0.85 x 2,328 x 450 / 1000 = 890 kN (net section fracture)

Compare with Ag _ Fy yield capacity: Ty = phi _ Ag * Fy = 0.90 x 2,640 x 350 / 1000 = 831 kN

Net section fracture governs. Connection adequate for the design tension force.

Frequently Asked Questions

What is the standard hole diameter for M20 bolts per CSA S16? Standard hole diameter for M20 bolts is 22 mm (bolt diameter + 2 mm). For oversized holes, 24 mm. For short slotted holes, 22 x 26 mm. For long slotted holes, 22 x 50 mm. These dimensions per CSA S16-19 Table 3. The tolerance on hole diameter is +0.5 mm / -0.0 mm.

Can oversized holes be used in bearing-type connections? No, oversized holes are only permitted in slip-critical connections per CSA S16-19 Clause 22.3.3. In bearing-type connections, the standard hole or short slotted hole in one ply is permitted. Oversized holes in bearing connections would allow excessive bolt slip that could cause connection deformation under service loads. If oversized holes must be used in a bearing application, the designer must verify there is adequate rotation and deformation capacity in the connection.

What is the minimum edge distance for M24 bolts in CSA S16? For M24 bolts, minimum edge distance from the centre of hole to the sheared edge is 40 mm. For rolled or gas-cut edges, minimum edge distance is 30 mm. These values per CSA S16-19 Clause 22.4 ensure full bearing resistance can develop. Reducing edge distance below these values requires proportional reduction of the bearing resistance Br per Clause 13.11.2.

How is hole oversize accounted for in net section calculations? Per CSA S16-19 Clause 12.3.2, the hole dimension for net section calculations is the actual hole diameter plus 2 mm, regardless of whether the hole is standard, oversized, or slotted. For M20 oversized holes (24 mm actual diameter), the deduction is 24 + 2 = 26 mm. This additional 2 mm accounts for edge damage from the hole fabrication process. For slotted holes, the slot width (not length) is used for area deduction, and the deduction = slot width + 2 mm.

What is the maximum bolt spacing per CSA S16? Maximum spacing between bolt centres per CSA S16-19 Clause 22.4: 300 mm for tension members and 200 mm for compression members with painted surfaces. For weathering steel, maximum edge distance is reduced to 200 mm. These limits prevent member separation under load and ensure adequate sealing against moisture for exposed steelwork. Stitch bolts in built-up compression members have a maximum spacing of 600 mm.

Related Pages

• CSA Bolt Capacity — A325M & A490M Tables
• CSA Bolt Pretension — S16 Required Clamping
• Canadian Steel Bolting — A325M Fasteners & CSA S16
• Canadian Steel Grades — G40.21 300W, 350W, 400W
• CSA S16 Code Overview
• Bolted Connection Calculator
• Bolt Torque Calculator
• AISC Bolt Hole Sizes Reference

This page is for educational reference. All hole dimension data per CSA S16-19 Table 3. Verify dimensions against the current code edition before design. For connections in seismic force-resisting systems, follow additional detailing requirements per CSA S16-19 Clause 27. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.