Fillet Weld Design Basis — CSA S16:24 Clause 13.13

The factored resistance of a fillet weld is based on the effective throat area:

Effective throat: t_e = 0.707 x a (for equal-leg 90-degree fillet)

Where a = leg size (mm). The 0.707 factor is sin(45 degrees) — the shortest distance from the root to the face of the weld.

Factored weld resistance per unit length (directional method, Clause 13.13.2.1):

Vr_weld = phi_w x 0.67 x t_e x Xu

Where:

• phi_w = 0.67 (welds — higher uncertainty than base metal)
• 0.67 = shear factor (weld metal treated as shear-critical)
• Xu = tensile strength of weld metal (MPa)
• For E49XX electrodes: Xu = 490 MPa
• For E48XX electrodes: Xu = 480 MPa

This gives the capacity in N/mm of weld length when loaded perpendicular to the weld axis.

For loading at an angle theta to the weld axis (Clause 13.13.2.2):

Vr_theta = Vr_weld / (sin^2(theta) + 0.75 x cos^2(theta))^0.5

This directional strength enhancement recognizes that fillet welds are stronger when loaded perpendicular (theta = 90 degrees) than in longitudinal shear (theta = 0 degrees). The enhancement factor is approximately 1 / sqrt(0.75) = 1.155 at theta = 0, meaning transverse loading provides about 15.5% more capacity than longitudinal.

E49XX vs E48XX Electrodes

Canadian practice (CISC) specifies E49XX electrodes (AWS A5.20 E71T-1 equivalent) as the standard for structural steel. The 49 indicates 490 MPa minimum tensile strength. E48XX (480 MPa) is an alternative, primarily for thinner materials or where lower heat input is desired.

The 2% reduction from E48XX is rarely worth the specification effort — standardize on E49XX throughout the project.

Base Metal Check — Clause 13.13.2.1

The connected base metal must also be checked. For fillet welds to the edge of a plate:

Vr_base = phi x 0.67 x t x Fu (shear rupture of base metal along weld line)

Where:

• phi = 0.90 (base metal in shear)
• t = minimum connected plate thickness (mm)
• Fu = plate tensile strength (MPa)

The base metal check often governs for thin plates with large welds. For a 6 mm fillet weld to a 10 mm CSA G40.21 350W plate:

• Weld capacity: 0.93 kN/mm
• Base metal: 0.90 x 0.67 x 10 x 450 / 1000 = 2.71 kN/mm

Base metal is not critical for this combination. However, for a 6 mm fillet to a 6 mm plate: base metal = 1.63 kN/mm — still above weld capacity but warranting verification.

Worked Example 1: Beam Web-to-Shear Tab Weld

Problem: A W410x60 beam is connected to a shear tab using two vertical fillet welds along the beam web. Factored shear Vf = 250 kN. Beam web thickness = 7.7 mm. Shear tab plate thickness = 10 mm. Use E49XX electrodes. Determine required fillet weld size and length.

Step 1 — Minimum fillet weld size (CSA W59 Table 5.1): For 10 mm plate thickness, minimum single-pass fillet weld = 5 mm. For 7.7 mm beam web, minimum = 5 mm. Use 6 mm fillet weld both sides.

Step 2 — Weld capacity (6 mm fillet, E49XX): t_e = 0.707 x 6 = 4.24 mm Vr_weld per mm = 0.67 x 0.67 x 4.24 x 490 / 1000 = 0.93 kN/mm (per side)

For two welds (both sides of web): Vr_total per mm = 0.93 x 2 = 1.86 kN/mm

Step 3 — Required weld length: L_weld = Vf / Vr_per_mm = 250 / 1.86 = 134.4 mm

Add 2x leg size for start/stop crater allowance (CSA W59 Clause 5.4.2): effective length = 134.4 + 2 x 6 = 146.4 mm. Use 150 mm minimum.

Step 4 — Base metal check (beam web shear rupture): Web Fu = 450 MPa, t = 7.7 mm. Weld length = 150 mm. Vr_base = 0.90 x 0.67 x 7.7 x 450 x 150 / 1000 = 313 kN >> 250 kN. OK.

Step 5 — Eccentricity check: The shear tab weld is loaded eccentrically. For a single-plate shear tab, the eccentricity e is typically 75-100 mm (distance from bolt line to weld face). The eccentric moment M_e = Vf x e = 250 x 0.080 = 20 kNm.

For a 150 mm long weld group, the polar moment of inertia J_w = 2 x (L^3 / 12) = 2 x 150^3 / 12 = 562,500 mm^3. The maximum force per mm from eccentricity = M_e x (L/2) / J_w = 20 x 10^6 x 75 / 562,500 = 2,667 N/mm = 2.67 kN/mm.

Resultant force per mm = sqrt((250/150)^2 + 2.67^2) = sqrt(1.67^2 + 2.67^2) = 3.15 kN/mm >> 1.86 kN/mm. NOT OK.

Extend weld length to 250 mm: J_w = 2 x 250^3 / 12 = 2,604,167 mm^3. Force from moment = 20 x 10^6 x 125 / 2,604,167 = 960 N/mm = 0.96 kN/mm. Direct shear = 250/250 = 1.00 kN/mm. Resultant = sqrt(1.00^2 + 0.96^2) = 1.39 kN/mm < 1.86 kN/mm. OK.

Final design: 6 mm fillet weld, 250 mm length both sides of beam web. E49XX electrode, all-position (AWS E71T-1 equivalent).

Worked Example 2: Column Stiffener Welds

Problem: A W310x158 column requires two full-depth transverse stiffeners between the flanges to resist a beam flange force of 850 kN from a moment connection. Stiffener thickness = 16 mm in 350W plate. Web thickness = 15.7 mm. Determine required fillet weld to the column web.

Step 1 — Force distribution: The stiffener-to-web weld transfers the beam flange force from the stiffener into the column web. The weld is loaded perpendicular to its axis (transverse loading, theta = 90 degrees).

Step 2 — Weld size selection: Minimum fillet weld for 16 mm plate: 6 mm (CSA W59 Table 5.1). Maximum single-pass: 8 mm for shop welding. Use 8 mm fillet weld both sides of stiffener.

Step 3 — Weld capacity (8 mm fillet, transverse loading, E49XX): t_e = 0.707 x 8 = 5.66 mm Vr_weld per mm (longitudinal) = 0.67 x 0.67 x 5.66 x 490 / 1000 = 1.24 kN/mm Directional enhancement factor (theta = 90 degrees): 1 / sqrt(sin^2(90) + 0.75 x cos^2(90)) = 1 / sqrt(1 + 0) = 1.0

Actually, the enhancement applies when loaded AT an angle. For pure transverse (perpendicular to weld axis): the formula gives Vr_trans = 0.67 x phi_w x t_e x Xu — same as longitudinal per CSA formulation. The enhancement in the Canadian code is embedded differently than AISC.

Using CSA S16:24 Clause 13.13.2.2 directly: the factored resistance of a fillet weld loaded at angle theta is taken as the minimum of the two base checks plus the weld itself. For stiffener welding to a web (load perpendicular to weld), the capacity is:

Vr_trans = 0.67 x phi_w x t_e x Xu per mm

For 8 mm fillet: Vr_trans = 0.67 x 0.67 x 5.66 x 490 / 1000 = 1.24 kN/mm

Step 4 — Required weld length: Stiffener depth = d - 2 x t_flange = 310 - 2 x 25 = 260 mm. Use full available depth. Available weld length per stiffener = 260 mm x 2 sides = 520 mm.

Capacity = 1.24 x 520 = 645 kN for one stiffener. With 2 stiffeners: 1,290 kN total.

Weld demand: 850 kN / 1,290 kN = 0.66 utilization. OK.

Step 5 — Base metal check (column web shear): The stiffener force is transferred into the column web. Check web local yielding per Clause 13.10. The web of W310x158 (t_w = 15.7 mm, Fy = 350 MPa):

Web panel zone shear: Vr_web = 0.90 x 0.55 x 350 x 15.7 x 310 / 1000 = 845 kN — but panel zone forces are more complex with the moment connection. For the stiffener itself, the weld length check governs.

Final design: 8 mm fillet weld both sides, full stiffener depth (260 mm). Two stiffeners each side of column web. E49XX electrode.

Minimum and Maximum Fillet Weld Sizes — CSA W59

Maximum weld size at lapped joints: a_max = t - 2 mm for thickness >= 6 mm (to avoid melting the top corner).

Frequently Asked Questions

When should I use the directional strength enhancement? CSA S16:24 Clause 13.13.2.2 provides the directional formulation. In practice, for beam web-to-shear tab welds (longitudinal shear), use the standard formula without enhancement. For stiffener welds, end plate welds, and other transverse loading cases, use the full capacity. The difference is modest (~15%) but can reduce weld sizes in fatigue-sensitive connections.

How do I specify welds on Canadian fabrication drawings? Use CSA W59 standard welding symbols. Specify: leg size (e.g., 6), both sides (arrow side + other side), length (if intermittent), and electrode classification (E49XX). Include a general note: "ALL WELDS TO CSA W59-24. ALL FILLET WELDS E49XX UNLESS NOTED OTHERWISE. MINIMUM FILLET SIZE PER W59 TABLE 5.1."

What is the effective length reduction for end craters? CSA W59 requires deducting 2 x leg size from each end of a fillet weld for start/stop craters. For a 6 mm fillet, deduct 12 mm total from the specified length. Intermittent welds also deduct craters at each segment. This is critical for short welds — a 50 mm intermittent fillet at 6 mm leg has an effective length of only 38 mm (24% reduction).

This page is for educational reference. Weld design per CSA S16:24 Clause 13.13 and CSA W59-24. Verify weld capacities per CISC Handbook Tables 3-30 through 3-34. All structural designs must be independently verified by a licensed Professional Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.