Canadian Weld Capacity — CSA S16-19 Clause 13.13 Fillet and Butt Weld Design

Complete reference for welded connection design per CSA S16-19 Clause 13.13 and CSA W59-18. Covers fillet weld shear capacity per mm for E48XX, E49XX, and E55XX electrodes, directional strength enhancement for transversely loaded welds, electrode selection for CSA G40.21 steels, and practical weld sizing.

Related pages: CSA Weld Electrode Guide | CSA W59 Welding Procedures | Canadian Weld Symbols | Welded Connection Calculator

CSA S16 Weld Design Formula — Clause 13.13.1

The factored shear resistance of a fillet weld per unit length is:

Vr = 0.67 x phi_w x Xu x A_throat

Where:

• phi_w = 0.67 (resistance factor for welds — lower than the 0.90 for base metal, reflecting the higher variability of weld metal properties)
• Xu = electrode classification tensile strength (MPa)
• A_throat = effective throat area per unit length = 0.707 x D for a 90-degree fillet with equal legs

For a fillet weld with leg size D (mm): Vr (kN/mm) = 0.67 x 0.67 x Xu x 0.707 x D / 1000 = 0.317 x Xu x D / 1000

The coefficient 0.317 incorporates phi_w = 0.67, the 0.707 throat conversion for a 45-degree fillet leg, and the 1/1000 for unit conversion to kN.

Fillet Weld Capacity Tables

The capacity per unit length depends on the electrode classification (Xu) and the weld leg size (D). Below are design capacities in kN/mm and kN per 100 mm length.

E48XX Electrodes (Xu = 480 MPa) — Standard for 350W Steel

E49XX Electrodes (Xu = 490 MPa) — GMAW Solid Wire

E55XX Electrodes (Xu = 550 MPa) — Matching for 480W Steel

Directional Strength Enhancement — Clause 13.13.2

Fillet welds loaded transversely to their axis (at 90 degrees) are stronger than those loaded longitudinally (along the axis). This is because the failure plane in a transverse fillet passes through a larger cross-section of weld metal. The CSA S16 enhancement factor mirrors the AISC formula:

Enhancement = 1.0 + 0.50 x sin^1.5(theta)

This 1.50 factor for transverse welds is significant. A bracket with a transverse end weld can carry 50% more at that segment than a side weld of the same size. For connections where the load direction is known and purely static, the enhancement should always be applied.

Electrode Selection — CSA W59-18 Matching Requirements

The electrode must be matched to the base metal to ensure the weld is not the weak point in the connection:

For 350W (the most common Canadian structural steel), E48XX (SMAW) or E49S-6 (GMAW) is the default. The slight undermatch for 350W (Xu = 480-490 MPa vs Fu = 450 MPa) is acceptable because the fillet weld shear capacity per Clause 13.13 is based on the electrode Xu, not the parent metal Fu, and the capacity check ensures adequate strength.

Minimum and Maximum Fillet Weld Sizes — CSA W59-18

Minimum fillet weld leg size per CSA W59 Table 5.2:

Maximum fillet weld size along plate edges:

• For t <= 6 mm: D_max = t (full edge thickness)
• For t > 6 mm: D_max = t - 2 mm (to avoid melting away the edge)

These limits ensure that the weld adequately fuses into the base metal without excessive heat input that could distort thin plates or burn through edges.

Parent Metal Capacity Check

The weld may be strong enough, but the plate must also be capable of transferring the design force into the weld. The parent metal shear capacity is:

Vr_parent = phi x 0.66 x Fy x t (per unit width)

For a 10 mm 350W plate: Vr_parent = 0.90 x 0.66 x 350 x 10 / 1000 = 2.08 kN/mm.

Compare with an 8 mm fillet E48XX: Vr = 1.22 kN/mm. The weld metal governs (1.22 < 2.08), which is the desired design outcome. However, for a 5 mm plate with an 8 mm fillet:

Vr_weld = 1.22 kN/mm Vr_parent = 0.90 x 0.66 x 350 x 5 / 1000 = 1.04 kN/mm

The parent metal governs — the plate will fail before the weld, regardless of how large the weld is. When parent metal governs, either increase the plate thickness or increase the weld length (more continuous weld, lower stress per mm).

Intermittent Fillet Welds

CSA W59 permits intermittent fillet welds when the required continuous weld capacity exceeds the demand:

• Minimum segment length: 40 mm or 4 x leg size (whichever is greater)
• Maximum clear spacing: 300 mm or 16 x t_min (the thinner plate), whichever is less
• End segments: Continuous for at least 2 x leg size at all ends or corners

Intermittent welds are common for stiffeners, gusset-to-beam connections, and lightly loaded bracing. They are not permitted for:

• Members subject to fatigue (cyclic loading, vibration)
• Connections exposed to corrosion where gaps could trap moisture
• Compression members where the intermittent nature could induce local buckling between weld segments

Worked Example — Welded Bracket to Column

Problem: A steel bracket plate (12 mm, 350W) is welded to a column flange (25 mm, 350W) with fillet welds on two sides and one end. The bracket supports a factored load Vf = 120 kN applied at the bracket tip, 200 mm from the column face. E48XX electrodes. Design the welds.

Step 1 — Weld geometry: Two side welds: each 200 mm long (vertical, parallel to load). One end weld: 150 mm (horizontal at bottom, transverse to load).

Step 2 — Weld group centroid and eccentricity: ybar = (2 x 200 x 100 + 150 x 200) / (2 x 200 + 150) = (40000 + 30000) / 550 = 127.3 mm from top. e(from centroid to load line) = 200 + (200 - 127.3) = 272.7 mm.

Moment: Mf = 120 x 0.2727 = 32.7 kNm.

Step 3 — Polar moment of inertia of weld group (line elements): I_p = sum of (d^3/12 + d x d_y^2) for each segment plus I_y contributions for the end weld.

Side weld 1 (y relative to centroid = 100 - 127.3 = -27.3 mm): I_1 = 200^3/12 + 200 x 27.3^2 = 666,667 + 149,058 = 815,725 mm^3

Side weld 2 (same): I_2 = 815,725 mm^3

End weld (y = 200 - 127.3 = 72.7 mm, plus I about its own axis): I_3 = 150^3/12 + 150 x 72.7^2 = 281,250 + 793,364 = 1,074,614 mm^3 (line units, vertical contribution) Plus I_y for end weld: 150 x (0^2...) — negligible for a horizontal weld with centroid at x = 0.

Total I_p ~ 815,725 + 815,725 + 1,074,614 = 2,706,064 mm^3.

Step 4 — Critical weld stress at top of side weld: r_top = sqrt((100)^2 + (200 - 127.3)^2) = sqrt(10000 + 5286) = sqrt(15286) = 123.6 mm.

Torsional stress component (perpendicular to r_top): f_t = Mf x r_top / I_p = 32.7 x 10^6 x 123.6 / 2,706,064 = 1,493 N/mm = 1.493 kN/mm.

Direct shear component: f_v = Vf / L_total = 120/550 = 0.218 kN/mm (vertical).

Resultant on the critical weld segment (top of side weld): Vector addition depends on the angle between f_t and f_v, approximately 54 degrees. Using approximate vector addition:

f_res ~ sqrt(1.493^2 + 0.218^2 + 2 x 1.493 x 0.218 x cos(54)) = sqrt(2.229 + 0.048 + 0.384) = sqrt(2.661) = 1.631 kN/mm.

Step 5 — Weld size selection: Try 8 mm fillet, E48XX: Vr = 1.220 kN/mm. Directional enhancement:

• Side welds (theta = 0): enhancement = 1.00. Vr_side = 1.220 kN/mm.
• End weld (theta = 90): enhancement = 1.50. Vr_end = 1.831 kN/mm.

The critical point is at the top of the side weld (theta = 0). Vr_available = 1.220 kN/mm < f_res = 1.631. 8 mm inadequate.

Try 10 mm fillet: Vr = 1.525 kN/mm. Vr_available = 1.525 (side) < 1.631. Still inadequate.

Try 12 mm fillet: Vr = 1.830 kN/mm > 1.631. OK for side welds. Check maximum fillet: 12 mm plate with t = 12 mm, D_max = 12 - 2 = 10 mm. 12 mm exceeds the permitted maximum along the plate edge!

Resolution: The 12 mm fillet is not permitted on the 12 mm plate edge. Options: (a) Increase side weld length to 250 mm each — reduces the stress per mm by spreading the load over a longer weld (b) Increase end weld contribution by making end weld primary load path (c) Increase plate thickness to 16 mm, permitting D_max = 14 mm, and use 12 mm fillet

Option (a) with revised dimensions: side welds = 250 mm, end = 150 mm. L_total = 650 mm. y_bar = (2 x 250 x 125 + 150 x 250) / 650 = (62500 + 37500) / 650 = 153.8 mm. e' = 200 + (250 - 153.8) = 296.2 mm. Mf = 120 x 0.2962 = 35.5 kNm. I_p recalculated: larger due to longer side welds. r_top = sqrt(100^2 + (250 - 153.8)^2) = sqrt(10000 + 9254) = 138.7 mm. f_t = 35.5 x 10^6 x 138.7 / I_p_revised. This significantly reduces the stress.

Rather than iterate, check simply: with 250 mm side welds, f_res expected below 1.3 kN/mm based on increased length and I_p. 10 mm fillet (1.525 kN/mm) likely adequate.

Final specification: 10 mm fillet weld, E48XX, all three sides. Use 12 mm thick bracket plate, 350W. Provide nominal erection bolts (2-M16) to hold bracket against column during welding.

Weld Inspection Categories — CSA W59-18

For standard building connections with fillet welds: visual inspection (CSA W59 Category "Standard") is adequate. For moment frame connections with CJP beam flange welds: UT (ultrasonic testing) is required per CSA W59 Category "Special".

Frequently Asked Questions

What is the shear capacity of an 8 mm fillet weld in 350W steel per 100 mm length? With E48XX electrodes (Xu = 480 MPa): Vr = 0.317 x 480 x 8 x 100 / 1000 = 122.0 kN per 100 mm. This is the most commonly quoted capacity in Canadian fabrication shops. The 8 mm fillet is the default specification for plate thicknesses 12-20 mm and covers approximately 70% of all structural welds.

How does the directional strength enhancement work in practice? For a typical bracket with fillet welds on three sides: the side welds (longitudinal) use the base capacity, and the end weld (transverse) uses 1.5x the base capacity. The total capacity is the sum of individual segments with their respective enhancement factors. For an asymmetric weld group, compute the resultant force on each segment accounting for both the load direction and the weld orientation at that segment.

When should I use E55XX electrodes instead of E48XX? E55XX is required for welding CSA G40.21 480W steel (Xu = 550 MPa matches the Fu = 570 MPa of 480W). It may also be specified for 400W steel when a matching (not undermatched) electrode is desired. In practice, E48XX is adequate for 400W (Fu_weld = 490 vs Fu_base = 510). The slight undermatch is acceptable for fillet welds where the shear capacity is governed by the weld metal, not the base metal.

What is the difference between SMAW (stick) and GMAW (MIG) weld capacity? For the same electrode tensile strength (Xu), the capacity formula is identical. The practical difference is that GMAW (solid wire, E49S-6, Xu = 490 MPa) provides a smoother weld profile with less spatter, while SMAW (covered electrode, E48XX, Xu = 480 MPa) is more tolerant of field conditions (wind, rust, position). The capacity difference between 480 and 490 MPa Xu is approximately 2% and is negligible for design purposes.

This page is for educational reference. Weld design per CSA S16-19 Clause 13.13 and CSA W59-18. Verify electrode selection and minimum weld sizes for the specific plate gauges. All structural designs must be independently verified by a licensed Professional Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.