CSA S16 Bolt Design Guide — A325M/A490M Shear, Bearing & Tension

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 Cl. 21.2.3.

CSA S16:24 Clause 21 governs the design of bolted connections. This guide covers bearing-type connections using A325M and A490M high-strength bolts in standard holes. Slip-critical connections follow Clause 21.6 (serviceability) and are beyond the scope of this reference guide.

Bolt Material Properties

A325M bolts are the standard high-strength bolt for Canadian construction — the metric equivalent of ASTM A325. The M suffix indicates metric threads per ISO 261. CSA S16:24 Table 2 provides the nominal gross body area Ab for each bolt diameter.

Nominal bolt body areas Ab per CSA S16 Table 2:

Shear Resistance (Cl. 21.2.3)

Threads Excluded from Shear Plane (AX Condition)

When the bolt is detailed so that no threads lie in the shear plane:

Vr = 0.60 * phi_b * Ab * Fu

where phi_b = 0.80 for bearing-type connections.

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

Threads Intercepted in Shear Plane (AA Condition)

When threads lie in the shear plane, the effective area is reduced to approximately 0.70 * Ab:

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.

A325M Bolt Shear Capacities — M16 to M36

A490M Bolt Shear Capacities — M16 to M36

Note: For double shear connections (bolt passes through three plates), multiply these values by the number of shear planes. A M20 A325M bolt in double shear (AX) has Vr = 2 * 125.1 = 250.2 kN — this is the most common case for standard bolted connections.

Tension Resistance (Cl. 21.2.3)

When bolts carry axial tension (e.g., moment connections, hanger connections):

Tr = 0.75 * phi_b * Ab * Fu

The 0.75 factor accounts for the reduced tensile stress area at the threaded portion — the root area of the threads governs tension capacity. For M20 A325M:

Tr = 0.75 _ 0.80 _ 314 * 830 / 1000 = 156.4 kN

A325M and A490M Tension Capacities

Tension resistance is independent of the thread condition (AX/AA) because threads are always present in the cross-section subject to tension.

Bearing Resistance (Cl. 21.2.4)

Bearing resistance of the connected plate at the bolt hole, for standard holes with the bolt in a position such that deformation at the bolt hole at service load is a design consideration:

Br = 3.0 * phi_br * t * d_hole * Fu

where:

• phi_br = 0.80 (bearing resistance factor)
• t = thickness of the connected plate (mm)
• d_hole = nominal hole diameter (mm) — typically d + 2 mm for M16-M24, d + 3 mm for M27-M36
• Fu = specified minimum tensile strength of the plate material (MPa)

For a 12 mm thick 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

The bolt shear capacity (125.1 kN AX) is well below the bearing capacity — shear governs the connection. Bearing only governs for thin plates or low-Fu materials. For Grade 300W (Fu = 440 MPa):

Br = 3.0 _ 0.80 _ 12 _ 22 _ 440 / 1000 = 278.8 kN — still far above the shear capacity.

When Does Bearing Govern?

Bearing governs when Br < Vr. For M20 A325M (AX) with Vr = 125.1 kN, solving for t:

tmin for governing = Vr / (3.0 * phibr * d*hole * Fu) = 125,100 / (3.0 _ 0.80 _ 22 _ 450) = 5.3 mm

For plates thinner than ~5.3 mm, bearing governs over bolt shear. For typical structural connections (t >= 8 mm), bolt shear is the controlling limit state. This is why bolt design tables typically list shear as the design value.

Combined Shear and Tension (Cl. 21.2.5)

When a bolt group resists both shear and tension simultaneously:

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

This elliptical interaction curve means that the presence of shear reduces the available tension capacity (and vice versa). The interaction is severe — a 50% shear utilisation reduces tension capacity by 13%, and a 70% shear utilisation reduces tension capacity by 29%.

Interaction Worked Example

An M20 A325M bolt (AX) 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 — OK.

If shear increases to Vf = 100 kN:

(Vf/Vr)^2 = (100/125.1)^2 = 0.639

Available tension: (Tf/Tr)^2 <= 1.0 - 0.639 = 0.361, so Tf/Tr <= sqrt(0.361) = 0.601

Maximum allowable Tf = 0.601 * 156.4 = 94.0 kN — reduced from 156.4 kN.

Bolt Spacing and Edge Distance (Cl. 21.3)

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)
• Minimum edge distance: 1.5 * 22 = 33 mm (use 35-40 mm typical)
• Minimum end distance: 1.5 * 22 = 33 mm (use 40-45 mm for load direction)

These minimums are regulatory maximums for capacity — eroding below them reduces the design resistance. The bearing resistance formula Br above assumes a minimum end distance of 1.5 _ d_hole is satisfied. For end distances less than 1.5 _ d_hole, a reduced equation applies (see Cl. 21.2.4).

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

Problem: Design a simple beam shear connection (double angle, bolted to beam web and column flange). Factored shear Vf = 320 kN. Beam web 8 mm Grade 350W. Verify the bolt group for 4-M20 A325M bolts in double shear (AX condition).

Step 1 — Bolt Shear Check

4 bolts, double shear (each bolt has 2 shear planes), AX condition:

Vr per bolt per plane = 125.1 kN (M20 A325M AX)

Vr group = 4 _ 2 _ 125.1 = 1,001 kN

Check: Vf = 320 kN << Vr = 1,001 kN. Utilisation = 320/1,001 = 0.320 — OK by a large margin.

At this utilisation, fewer or smaller bolts could be considered. For 2-M20 A325M in double shear: Vr = 2 _ 2 _ 125.1 = 500.4 kN, utilisation 320/500.4 = 0.639. Still OK.

Step 2 — Bolt Bearing Check (Beam Web)

Beam web t = 8 mm, Grade 350W (Fu = 450 MPa), d_hole = 22 mm.

Br per bolt: Br = 3.0 _ 0.80 _ 8 _ 22 _ 450 / 1000 = 190.1 kN

Total bearing resistance: 4 * 190.1 = 760.4 kN. Vf = 320 kN. Utilisation = 320/760.4 = 0.421.

Bearing is not critical — shear controls, as expected for a typical web thickness.

Step 3 — Bolt Bearing Check (Angles)

Assuming 2-L102x102x9.5 angles, each leg t = 9.5 mm, Grade 350W.

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

Bolts in double shear pass through beam web (8 mm) and two angle legs (9.5 mm each). The thinnest connected plate — the beam web at 8 mm — governs bearing.

Step 4 — Summary

All checks pass with significant reserve. The connection could be reduced to 3-M20 or 2-M22 bolts.

CSA S16 vs AISC 360 — Bolt Design Comparison

The critical difference is phi_b = 0.80 (CSA) vs phi = 0.75 (AISC) — CSA allows 6.7% more bolt shear resistance. Conversely, CSA's 0.70 factor for AA condition is more conservative than the AISC Table J3.2 nominal stress approach for threads included. For practical design, the codes produce similar results: a M20 A325M bolt (AA) has Vr = 87.6 kN per CSA vs approximately 84.5 kN per AISC for a 3/4" A325 bolt — a 3.7% difference.

Frequently Asked Questions

How do I choose between bearing-type and slip-critical bolted connections?

Bearing-type connections (Cl. 21.2) rely on bolt shear and plate bearing for strength — the bolts may slip into bearing at service loads, which is acceptable for most connections. Slip-critical connections (Cl. 21.6) require the bolts to be pretensioned so that friction between faying surfaces resists the service load without slip. Use slip-critical when: (1) the structure is subject to fatigue or load reversal, (2) oversized or slotted holes are used with loads perpendicular to the slot, (3) slip would impair structural performance (e.g., moment connections in high-seismic frames), or (4) the project specification requires it. Slip-critical connections require Class A (0.30 coefficient) or Class B (0.50 coefficient) surface preparation per CSA S16.

What bolt hole size should I specify for M20 A325M bolts?

CSA S16:24 Table 22 specifies standard hole sizes: d_hole = d + 2 mm for M16-M24, d_hole = d + 3 mm for M27-M36. For M20: d_hole = 22 mm. Oversized holes (d_hole = d + 5 mm for M16-M24, d_hole up to d + 8 mm for M27+) are permitted but require slip-critical connections and hardened washers. Short-slotted and long-slotted holes have specific geometry limits in Table 22. Deformation of the hole is a design consideration — standard holes are assumed to deform at ULS, while oversized holes require verification.

Do I need to check bolt bearing for both connected plies in a double-shear connection?

Yes — bearing must be checked for every connected ply individually. In a beam-column double-angle connection with 4-M20 bolts, the bolt passes through: angle leg 1 (9.5 mm), beam web (8 mm), and angle leg 2 (9.5 mm). Check bearing for all three plies. Bearing Br is proportional to plate thickness t, so the thinnest ply (beam web at 8 mm) typically governs. If different materials are used (e.g., 300W angles with 350W beam), check bearing for each material's Fu separately.

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Related Pages

• CSA S16 Weld Design Guide — Fillet Welds — Weld design per CSA S16
• CSA S16 Beam Design — Flexure, LTB & Shear — Beam design reference
• Canadian Steel Bolting Guide — A325M vs A490M selection
• Bolt Capacity Tables — Multi-Code Reference — CSA, AISC, EN 1993
• Bolt Hole Sizes — CSA S16 vs AISC 360 — Hole size comparison
• Bolt Shear vs Bearing — Design Guide — When each limit state governs
• Block Shear Design per CSA S16 — Tearout and block shear checks

This page is for educational reference. All bolt capacities per CSA S16:24 Table 2 and Clause 21. Verify bolt specifications and plate materials with the fabricator. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent P.Eng. verification.

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