Canadian Bolt Design Guide — CSA S16 Cl.21 Bearing-Type Connections

Quick Reference: Bolt shear Vr = 0.60 _ phi_b _ Ab _ Fu (AX, threads excluded). Bearing Br = 3.0 _ phi*br * t _ d_hole _ Fu. Tension Tr = 0.75 _ phi_b _ Ab _ Fu. Interaction: (Vf/Vr)^2 + (Tf/Tr)^2 <= 1.0. phi_b = 0.80 per CSA S16:24 Cl. 21.2.3.

Canadian structural steel design follows CSA S16:24, Design of Steel Structures. Clause 21 governs bolted connections — the most common method of joining structural steel in Canada. This guide covers bearing-type connections with A325M and A490M high-strength bolts, the backbone of Canadian steel construction from Vancouver high-rises to Alberta industrial plants to Toronto office towers. Slip-critical connections (Cl. 21.6), while important for specific applications, are treated separately.

The Canadian approach to bolted connections differs from AISC 360 in two important respects. First, phi_b = 0.80 (vs 0.75 in AISC), reflecting Canadian confidence in domestic bolt manufacturing quality. Second, CSA S16 uses metric bolts exclusively — the M suffix (A325M, A490M) denotes ISO metric threads, distinct from the imperial UNC threads used in US practice. Canadian structural engineers must be comfortable in both systems because many connection components (angles, plates, columns) are sourced from both domestic and US suppliers.

Bolt Material Properties and Selection

Canadian structural bolts are specified to ASTM standards with the M suffix for metric threads. The choice between A325M and A490M depends primarily on the required shear capacity per bolt and the number of bolts that can be accommodated within the available geometry.

A325M bolts are specified for approximately 90% of Canadian structural connections. They provide adequate strength for typical beam shear connections, bracing gusset plates, and column splices. A490M bolts, with 25% higher tensile strength, are reserved for connections where the number of A325M bolts would be impractical — deep moment-resisting frame connections, heavy column splices in tall buildings, and connections in transfer structures carrying discontinuous columns.

Nominal bolt body areas Ab per CSA S16:24 Table 2:

The body area Ab is the gross cross-sectional area of the unthreaded shank. It is used for shear resistance (AX condition) and forms the basis of the tensile stress area (effectively 0.75 * Ab for tension calculations). The metric bolt is identified by the nominal shank diameter — M20 has a 20 mm nominal shank, not the thread outer diameter.

Shear Resistance per Cl. 21.2.3

The shear resistance of a single bolt depends critically on whether the threaded portion intersects the shear plane. This distinction — AX vs AA — is the single most important detailing decision in bolted connection design, yet it is often overlooked in preliminary sizing.

AX Condition — Threads Excluded from Shear Plane

When the bolt grip is specified so the unthreaded shank lies within the shear plane:

Vr = 0.60 _ phi_b _ Ab * Fu

For M20 A325M (AX): Vr = 0.60 _ 0.80 _ 314 * 830 / 1000 = 125.1 kN per shear plane.

Achieving the AX condition requires coordination between the structural engineer's bolt schedule and the fabricator's bolt procurement. A M20 bolt with a 55 mm grip length will typically have 30-35 mm of unthreaded shank — sufficient for a double-shear connection with 27 mm total grip (two 9.5 mm angle legs + 8 mm beam web).

AA Condition — Threads Intercepted in Shear Plane

When the threaded portion lies in the shear plane, a 0.70 reduction factor accounts for the reduced root area:

Vr = 0.60 _ phi_b _ 0.70 _ Ab _ Fu

For M20 A325M (AA): Vr = 0.60 _ 0.80 _ 0.70 _ 314 _ 830 / 1000 = 87.6 kN per shear plane.

The 0.70 factor represents the ratio of the tensile stress area (root area at threads) to the gross body area Ab. It is approximate — the actual reduction depends on the thread pitch and class of fit. For M20 coarse thread (pitch 2.5 mm), the root diameter is approximately 17.3 mm, giving an area ratio of (17.3/20)^2 = 0.748. CSA S16 rounds to 0.70 for conservatism.

A325M Bolt Shear Capacities — Complete Table

For double shear connections — the most common case where a bolt passes through three plies — multiply by 2. For example, 4-M20 A325M bolts in double shear (AX) provide 4 _ 2 _ 125.1 = 1,001 kN shear capacity. This is sufficient for most beam shear connections in building construction.

A490M Bolt Shear Capacities

A490M bolts are 25% stronger in shear than A325M, but the higher strength comes with reduced ductility. A490M bolts are not permitted in connections expected to undergo significant inelastic deformation — moment-resisting frames in high-seismic zones should use A325M bolts for their superior ductility.

Tension Resistance per Cl. 21.2.3

When bolts carry axial tension — typical in hanger connections, moment-resisting end plates, and column base plates — the tensile stress area at the threads governs:

Tr = 0.75 _ phi_b _ Ab * Fu

The 0.75 factor converts gross body area Ab to effective tensile stress area. For M20 A325M: Tr = 0.75 _ 0.80 _ 314 * 830 / 1000 = 156.4 kN.

Tension resistance is independent of AX/AA condition because threads are always present in the cross-section resisting tension. However, prying action in T-stub and end plate connections can amplify the bolt tension force by 20-40% — the prying check per Cl. 21.8 must be satisfied separately.

Bearing Resistance per Cl. 21.2.4

Bearing governs only when the plate is thin relative to the bolt diameter. For standard holes with deformation at the bolt hole considered at service load:

Br = 3.0 _ phi_br _ t _ d_hole _ Fu

where phi_br = 0.80, t is the connected plate thickness (mm), d_hole is the nominal hole diameter (mm), and Fu is the plate ultimate tensile strength (MPa).

For a 12 mm Grade 350W plate (Fu = 450 MPa) with M20 bolt (d_hole = 22 mm):

Br = 3.0 _ 0.80 _ 12 _ 22 _ 450 / 1000 = 285.1 kN per bolt.

Compare to bolt shear: Vr = 125.1 kN AX per shear plane. Bearing (285 kN) greatly exceeds shear (125 kN) — shear governs. Bearing only becomes critical for plates thinner than approximately 6 mm. For a 6 mm 350W plate: Br = 3.0 _ 0.80 _ 6 _ 22 _ 450 / 1000 = 142.6 kN — still above the bolt shear capacity. This is why CSA S16 bolt design tables commonly list shear as the governing value.

End Distance Effect

The bearing formula above assumes a minimum end distance of 1.5 * d_hole in the direction of load. For shorter end distances, the effective bearing resistance is reduced proportionally:

Br*reduced = Br * (Le / (1.5 _ d_hole)) for Le < 1.5 * d_hole

where Le is the actual end distance from the bolt centre to the plate edge. For M20 with d_hole = 22 mm and Le = 30 mm (less than the minimum 33 mm): Br_reduced = 285.1 * (30/33) = 259.2 kN. Canadian detailing practice maintains a minimum end distance of 40 mm for M20 bolts — well above the code minimum of 33 mm — to avoid this reduction.

Combined Shear and Tension per Cl. 21.2.5

When a bolt group resists both shear and tension simultaneously — typical in moment connections and bracing connections with inclined force components — the interaction is elliptical:

(Vf/Vr)^2 + (Tf/Tr)^2 <= 1.0

This equation is identical in form to AISC 360 and AS 4100. The squared terms create a severe interaction: a modest shear demand significantly reduces available tension capacity.

Interaction Example: An M20 A325M (AX) bolt resists Vf = 60 kN shear and Tf = 80 kN tension.

Vr = 125.1 kN, Tr = 156.4 kN

(Vf/Vr)^2 + (Tf/Tr)^2 = (60/125.1)^2 + (80/156.4)^2 = 0.230 + 0.262 = 0.492 <= 1.0 — OK.

If shear increases to Vf = 100 kN: (Vf/Vr)^2 = 0.639. Available (Tf/Tr)^2 <= 1.0 - 0.639 = 0.361. Tf_max = 0.601 * 156.4 = 94.0 kN — a 40% reduction from the pure-tension capacity of 156.4 kN.

Bolt Spacing and Edge Distance per Cl. 21.3

Canadian detailing standards establish minimum geometric requirements that ensure bolts can be installed and tensioned, and that the connected plates have adequate net section for load transfer.

For M20 bolts (d = 20 mm, d_hole = 22 mm) with 12 mm plate:

• Minimum bolt spacing: 2.67 * 20 = 53.4 mm — use 60 mm minimum, 75 mm preferred
• Minimum edge distance: 1.5 * 22 = 33 mm — use 40 mm typical
• Maximum spacing: min(14*12, 200) = 168 mm — governed by stitch requirements for the connected plies

Worked Example — Beam Shear Connection with 4-M20 A325M Bolts

Problem: Design a simple beam shear connection for a W410x46 beam (Grade 350W) framing into a W310x118 column flange. Factored shear Vf = 320 kN, transferred through double angles bolted to the beam web and column flange. Verify the bolt group for 4-M20 A325M bolts in double shear (AX condition, threads excluded).

Given:

• Beam web thickness: tw = 7.0 mm
• Angle legs: 2-L102x102x9.5, Grade 350W
• Bolts: 4-M20 A325M, AX condition, double shear
• d_hole = 22 mm (standard holes per CSA S16 Table 22)

Step 1 — Bolt Shear Check

Vr per bolt per shear plane = 125.1 kN (M20 A325M AX) Vr per bolt (double shear) = 2 _ 125.1 = 250.2 kN Group capacity: Vr_group = 4 _ 250.2 = 1,001 kN

Utilisation: 320 / 1,001 = 0.320 — ample reserve.

Step 2 — Beam Web Bearing Check

Bearing per bolt on 7.0 mm beam web (Grade 350W, Fu = 450 MPa):

Br = 3.0 _ 0.80 _ 7.0 _ 22 _ 450 / 1000 = 166.3 kN per bolt

Group bearing on beam web: 4 * 166.3 = 665 kN > 320 kN. Utilisation = 320 / 665 = 0.481 — OK.

Step 3 — Angle Leg Bearing Check

Bearing per bolt on 9.5 mm angle leg:

Br = 3.0 _ 0.80 _ 9.5 _ 22 _ 450 / 1000 = 225.7 kN per bolt per leg

Bolts in double shear bear on two angle legs (each 9.5 mm). Combined bearing = 2 _ 4 _ 225.7 = 1,806 kN >> 320 kN. Bearing on the angles is not critical.

Step 4 — Block Shear Check (Beam Web)

For the beam web with 4 bolts in a single vertical line at 75 mm pitch, the block shear failure path involves shear rupture along the bolt line and tension rupture at the end bolt.

The block shear check per CSA S16 Cl. 13.11 yields Tr + Vr = 452 kN > 320 kN — OK, utilisation approximately 0.71.

Step 5 — Summary

All checks pass. The connection is governed by block shear in the beam web, a common outcome for thin-webbed W-shapes. Adding a fifth bolt or increasing the end distance to 50 mm would improve the block shear capacity. For a W410x46 with tw = 7.0 mm, specifying 5-M20 bolts at 75 mm pitch with 50 mm end distance is good practice.

Canadian vs International Practice

Canadian engineers working on cross-border projects should note:

• CSA S16 phi_b = 0.80 vs AISC 360 phi = 0.75: CSA provides slightly higher design resistances for identical bolt grades
• Metric vs Imperial: Canadian design uses MPa/kN/mm exclusively. A 3/4-inch A325 bolt has nominal diameter 19.05 mm and is not interchangeable with M20 (20.0 mm)
• Steel grades: Canadian 350W steel (Fy = 350 MPa, Fu = 450 MPa) differs from US A572 Gr.50 (Fy = 345 MPa, Fu = 448 MPa) — the 1.4% difference in yield strength is negligible for bearing calculations
• Hole sizes: CSA S16 Table 22 specifies d_hole = d + 2 mm for M16-M24, matching AISC Table J3.3 for standard holes

Frequently Asked Questions

What is the most common mistake in Canadian bolted connection design?

The most common error is assuming AX (threads excluded) capacity when the bolt grip length places threads in the shear plane — the AA condition. A M20 A325M bolt designed for 125.1 kN AX but actually in AA delivers only 87.6 kN — a 30% shortfall that may not be caught until the steel is fabricated. The fix: always specify the bolt length in the connection schedule, and verify that the specified grip places the unthreaded shank across all shear planes. For connections where the grip cannot be controlled (thin connected plies, field conditions with variable grip), conservatively assume AA.

How do Canadian winter conditions affect bolted connections?

Extreme cold — common in Prairie provinces and northern territories where temperatures drop below -40°C — affects bolt behaviour in two ways. First, the notch toughness of bolt material must be adequate for the minimum service temperature. CSA S16 Cl. 24 requires Charpy V-notch testing for bolts in tension applications below -30°C. A325M bolts typically meet 27 J at -20°C; for lower temperatures, supplementary CVN requirements should be specified. Second, differential thermal contraction between steel members and bolts (both steel, negligible) does not create significant thermal stress — the bolt and connected parts expand and contract together.

Should I pretension A325M bolts in bearing-type connections?

CSA S16 does not require pretension for bearing-type connections — snug-tight installation is sufficient. Snug-tight means the plies are brought into firm contact, typically achieved by the full effort of a worker using a spud wrench or a few impacts of an impact wrench. However, many Canadian project specifications require pretension for all A325M bolts regardless of connection type, as a quality assurance measure. Pretension adds approximately 15-20% to erection cost. The engineer should specify pretension only when required by the connection behaviour (slip-critical, fatigue, or connections subject to vibration) and not as a blanket requirement.

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Related Pages

• CSA S16 Bolt Design Guide — Shear, Bearing, Tension — Detailed capacity tables
• Canadian Steel Bolting Guide — A325M vs A490M — Grade selection
• Bolt Bearing and Tearout per CSA S16 — End distance effects
• Canadian Bolt Torque Chart — M16-M36 — Installation torque
• Bolt Hole Sizes — CSA S16 vs AISC 360 — Hole size comparison
• Canadian Steel Grades — G40.21 350W, 300W — Plate material properties

This page provides educational reference for Canadian bolted connection design. All capacities per CSA S16:24. For construction documents, the design must be verified and sealed by a Professional Engineer (P.Eng.) licensed in the province of practice. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent P.Eng. verification.

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