CSA S16 Weld Design Guide — Fillet Welds, Effective Throat & Electrodes

Quick Reference: Fillet weld resistance Vr = min(base metal, weld metal). Base metal: Vr = phi _ 0.67 _ Fy _ Am. Weld metal: Vr = phi_w _ 0.67 _ Xu _ Aw per unit length. phi = 0.90 for base metal, phi_w = 0.67 for weld metal. All per CSA S16:24 Clause 21.

CSA S16:24 Clause 21 governs welded connection design in conjunction with CSA W59-18 (Welded Steel Construction). This guide covers the design strength of fillet welds — the most common weld type in structural steel construction, accounting for roughly 80% of all structural welds. Fillet welds are used in lap joints, T-joints, and corner joints to resist shear forces parallel and transverse to the weld axis.

CSA S16 Fillet Weld Design Philosophy

CSA S16:24 uses a two-part verification for fillet welds:

1. Base metal check: The base metal adjacent to the weld must resist the shear flow through the connected plate.
2. Weld metal check: The deposited weld metal itself must carry the applied force through the effective throat area.

Both checks must be satisfied independently. The governing design strength is the lower of the two. For most connections, the weld metal check governs because phi_w = 0.67 is significantly lower than phi = 0.90 for base metal.

Why is phi_w = 0.67?

The phi_w = 0.67 factor is 12% more conservative than AISC 360's phi = 0.75 and 18% lower than AS 4100's phi = 0.80. The Canadian approach reflects:

1. The inherent variability of deposited weld metal compared to rolled base metal
2. The quality uncertainty of field welds — CSA W59 inspection is visual for most structural welds, unlike shop welds which may receive UT/RT
3. Cold-weather welding — Canadian field welds are often performed in sub-zero temperatures where cooling rates can produce harder, more brittle weld metal

For critical-demand welds, specifying 100% UT inspection and pre-qualified welding procedures can justify an increase in effective strength, but the phi_w = 0.67 remains the code-default resistance factor.

Base Metal Shear Resistance at Weld (Cl. 13.4, applied to connection)

For base metal shear adjacent to the weld:

Vr_bm = phi * 0.67 * Fy * Am

where:

• phi = 0.90 (base metal resistance factor)
• Fy = yield strength of the base metal (MPa)
• Am = effective shear area of the base metal along the weld fusion face (mm^2 per unit length)

For a fillet weld along a plate edge, Am per unit length = t (plate thickness per mm of weld length). The 0.67 factor is 0.60 / sqrt(3) _ sqrt(3), representing the shear yield stress of the base metal (approximately 0.577 _ Fy, rounded to 0.67 * Fy for design convenience).

For a 10 mm Grade 350W plate, base metal shear resistance per mm of weld length:

Vr*bm = 0.90 * 0.67 _ 350 * 10 / 1000 = 2.11 kN/mm

This is the maximum shear the base metal can transfer through the fusion face. For a 100 mm long weld: Vr_bm = 211 kN.

Weld Metal Resistance (Cl. 21.2.3)

For the deposited weld metal, the design shear resistance per unit length of fillet weld:

Vr_wm = phi_w * 0.67 * Xu * Aw

where:

• phi_w = 0.67 (weld metal resistance factor)
• Xu = ultimate tensile strength of the weld metal (MPa), per electrode classification
• Aw = effective throat area per unit length = 0.707 * D (mm² per mm length)

For a matching electrode, Xu is the minimum tensile strength of the electrode, typically the electrode classification number in ksi * 6.895. An E49XX electrode has Xu = 490 MPa.

For a 6 mm fillet weld (leg size D = 6 mm) with E4918 electrode (Xu = 490 MPa):

Aw = 0.707 * 6 = 4.24 mm^2 per mm

Vr*wm = 0.67 * 0.67 _ 490 * 4.24 / 1000 = 0.934 kN/mm

For a 100 mm long weld: Vr_wm = 93.4 kN.

Fillet Weld Capacity Table — E49XX Electrode (Xu = 490 MPa)

For E48XX electrodes (Xu = 480 MPa, slightly undermatching for 350W steel), multiply by 480/490 = 0.980. For E43XX electrodes (Xu = 430 MPa, matching 300W steel), multiply by 430/490 = 0.878.

Minimum Fillet Weld Sizes (CSA S16 Table 25)

CSA S16:24 Table 25 specifies minimum fillet weld sizes based on the thickness of the thicker connected part:

These minimums exist to ensure the weld has sufficient heat input to achieve adequate fusion and to prevent cracking from rapid cooling. Single-pass maximum is typically 8 mm (flat position, SMAW) or 10 mm (horizontal, FCAW). For larger sizes, multiple passes are required.

The maximum fillet weld size at an edge is t - 2 mm (where t is the thickness of the edge being welded) to prevent the weld from melting away the plate edge. For a 10 mm plate: maximum leg size = 8 mm.

Electrode Selection for Canadian Steel

CSA W59-18 Table 10 provides the electrode classification matching requirements:

For 350W, the standard electrode is E4918 (SMAW, low-hydrogen, all-position). This overmatches the base metal Fu by 8.9%, ensuring the weld is not the weakest link in the connection. For FCAW, E491T-9CH is common. For SAW (submerged arc), F48A2-EM12K is typical.

Worked Example — Beam Web-to-End Plate Fillet Weld

Problem: Design a fillet weld connecting a W310x39 beam web (tw = 6.1 mm, Grade 350W) to a 12 mm end plate (Grade 350W). Factored shear Vf = 160 kN. Determine the required fillet weld size and length.

Step 1 — Minimum Weld Size

Thicker connected part = 12 mm (end plate). Per Table 25: minimum fillet weld = 5 mm.

Try D = 6 mm fillet weld (one size up from minimum, common for beam webs).

Step 2 — Weld Metal Capacity

E4918 electrode, Xu = 490 MPa, D = 6 mm:

Vr*wm = phi_w * 0.67 _ Xu _ 0.707 _ D = 0.67 _ 0.67 _ 490 _ 0.707 _ 6 / 1000 = 0.934 kN/mm

Required weld length: L = Vf / Vr_wm = 160 / 0.934 = 171 mm

Since the beam web depth is approximately 310 - 2*9.7 = 290 mm, a 171 mm weld can easily be accommodated on one side. For a typical beam shear connection, welds are placed on both sides of the web.

With two 6 mm fillet welds (one each side of web), total required length per side = 171 / 2 = 86 mm per side.

Step 3 — Base Metal Check

Base metal shear at the weld fusion face (beam web, tw = 6.1 mm):

Vr*bm = phi * 0.67 _ Fy _ tw = 0.90 _ 0.67 _ 350 _ 6.1 / 1000 = 1.29 kN/mm

This exceeds Vr_wm = 0.934 kN/mm — weld metal governs. For the 171 mm weld length:

Vr_bm = 1.29 * 171 = 220.6 kN > Vf = 160 kN — OK.

Step 4 — Block Shear Check (End Plate)

Block shear of the end plate — the welded connection creates a potential tearout pattern at the plate edge. Per CSA S16 Cl. 13.11, the block shear resistance is:

Tr = phi _ (0.60 _ Fy _ Agv + Fu _ Ant) for fracture governing

Where Agv is the gross area in shear and Ant is the net area in tension. For this end plate with welds along the beam web, the shear area is L_weld * t_plate and the tension area is from the plate edge.

This check is typically satisfied for standard details. Refer to the CSA S16 Clause 13.11 for complete block shear verification.

Step 5 — Summary

The 6 mm fillet weld, 86 mm on each side of the web, is adequate. Specify: "6 mm fillet weld, both sides of beam web, minimum length 90 mm, E4918 electrode, all around return 2x weld size at ends."

Weld Length Limitations (Cl. 21.2.3)

CSA S16 limits the effective length of fillet welds:

• Minimum length: 4 * D (weld size) or 40 mm, whichever is greater. For D = 6 mm: minimum 40 mm.
• Maximum effective length: No explicit CSA S16 limit, but for very long welds (L > 100 _ D), stress distribution becomes non-uniform. The CISC Handbook recommends limiting effective length to 100 _ D = 600 mm for D = 6 mm. For longer connections, intermittent welds or multiple shorter weld segments are used.
• End returns: Welds should return around the corners for a distance of at least 2 * D (12 mm for D = 6 mm) to prevent stress concentrations at the weld terminations.

Transverse vs Longitudinal Fillet Welds

CSA S16:24 Cl. 21.2.3 does not distinguish between longitudinal (parallel to load) and transverse (perpendicular to load) fillet welds — the same 0.67 _ phi_w _ Xu _ Aw formula applies for both orientations. This differs from AISC 360 which allows a 50% increase for transverse fillet welds (1.0 + 0.50 _ sin^1.5(theta)).

In practice, transverse fillet welds ARE stronger (test data shows 30-50% higher strength), but CSA S16 conservatively uses the same formula for design simplicity. This is conservative by 30-50% for purely transverse welds. When a weld group contains both longitudinal and transverse segments, the CSA S16 approach still treats all segments at the longitudinal capacity — which is why CSA S16 fillet weld designs often appear more conservative than their AISC equivalents.

Frequently Asked Questions

How do I decide between a 5 mm, 6 mm, or 8 mm fillet weld?

The choice depends on three factors: (1) required capacity — use the table above to size for demand; (2) minimum size per Table 25 — the thicker connected part dictates the minimum; (3) constructability — 6 mm is the minimum practical size for SMAW welders in the field, and 5 mm is typically only used for light-gauge connections or shop welding with GMAW. For heavy sections (t > 20 mm), 8 mm is the practical minimum to ensure fusion. Weld size should not exceed the plate thickness for a single pass — multi-pass welds are specified as total leg size.

Does CSA S16 require weld inspection and what level?

CSA W59-18 governs weld inspection, not CSA S16 directly. For statically loaded structures, visual inspection (VT) of all completed welds is the minimum requirement. The project specification or engineer of record defines supplementary NDT: magnetic particle (MT) for surface defects, ultrasonic testing (UT) for internal defects in complete-joint-penetration (CJP) groove welds, or radiographic testing (RT) for critical welds. Fillet welds typically receive VT only unless specified otherwise. CJP welds in tension applications often require UT per CSA W59 Table 20.

Can I use intermittent fillet welds under CSA S16?

Yes, CSA S16 Cl. 21.2.3 permits intermittent fillet welds. The effective capacity is proportional to the ratio of welded length to total length. For a 6 mm fillet weld with E4918, intermittent at 75 mm weld / 150 mm pitch: effective capacity = 0.934 * (75/150) = 0.467 kN/mm. Intermittent welds are cost-effective for lightly loaded connections but have minimum segment length and maximum pitch requirements: minimum segment = the larger of 4D or 40 mm, maximum pitch = 12t for compression or 16*t for tension (where t is the thinner plate thickness). Intermittent welds are NOT permitted in corrosive environments, fatigue applications, or where water/condensation can enter gaps.

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Related Pages

• CSA S16 Bolt Design Guide — A325M/A490M — Bolted connection design
• CSA S16 Beam Design — Flexure, LTB & Shear — Beam design reference
• Fillet Weld Size Chart — Multi-Code Reference — CSA, AISC, EN 1993
• Weld Capacity Calculator — Multi-Code — Weld strength reference
• Minimum Weld Size Requirements — CSA vs AISC — Code comparison
• Weld Inspection Guide — CSA W59 — Inspection requirements
• Welding Electrode Selection Guide — E43XX to E55XX selection

This page is for educational reference. All weld capacities per CSA S16:24 Clause 21 and CSA W59-18. Verify electrode specifications with the fabricator. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent P.Eng. verification.

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