CSA W59 Minimum Fillet Weld Size — Requirements & Design Resistance
Complete reference for minimum fillet weld sizes per CSA W59-18 (Welded Steel Construction) Table 5.1, and fillet weld design resistance per CSA S16-19 Clause 13.13. Includes leg dimension requirements, effective throat thickness, directional strength method, and design examples.
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CSA W59 Minimum Fillet Weld Size
CSA W59 Table 5.1 specifies the minimum fillet weld leg dimension (w) based on the thickness of the thicker part being joined. These minimums ensure proper fusion and adequate cooling rate to prevent rapid quenching in thicker sections.
Minimum Fillet Weld Leg Dimensions
| Thickness of Thicker Connected Part (mm) | Minimum Weld Leg (mm) | Single Pass Maximum Leg (mm) |
|---|---|---|
| t <= 6 | 3 | 5 |
| 6 < t <= 13 | 5 | 8 |
| 13 < t <= 19 | 6 | 8 |
| 19 < t <= 38 | 8 | 8 |
| 38 < t <= 57 | 10 | 10 |
| 57 < t <= 150 | 12 | 12 |
| t > 150 | 16 | 16 |
Source: CSA W59-18 Table 5.1. All dimensions in millimetres.
The minimum weld leg dimension ensures that:
- The weld has adequate heat input to achieve fusion with the base metal
- The cooling rate is slow enough to prevent martensite formation in the heat-affected zone
- The weld is large enough to resist handling forces during fabrication and erection
Maximum Fillet Weld Size
Per CSA W59 Clause 5.14.2, the maximum fillet weld leg dimension along edges of connected parts:
- Along rolled edges: maximum leg = thickness of the thinner part minus 1 mm
- Along sheared or gas-cut edges: maximum leg = thickness of the thinner part minus 2 mm
For a 10 mm gusset plate with a rolled edge: maximum weld leg = 10 - 1 = 9 mm. For a 10 mm plate with a sheared edge: maximum weld leg = 10 - 2 = 8 mm.
Effective Throat Thickness
The design resistance of a fillet weld depends on the effective throat thickness, not the leg dimension.
Effective throat (Te) = 0.707 * w (for 90-degree fillet welds with equal legs)
Where w is the leg dimension. For unequal-leg fillet welds, the throat is measured as the minimum distance from the root to the face of the weld.
| Weld Leg w (mm) | Effective Throat Te (mm) | Design Area per mm Length (mm^2/mm) |
|---|---|---|
| 3 | 2.1 | 2.1 |
| 5 | 3.5 | 3.5 |
| 6 | 4.2 | 4.2 |
| 8 | 5.7 | 5.7 |
| 10 | 7.1 | 7.1 |
| 12 | 8.5 | 8.5 |
| 16 | 11.3 | 11.3 |
| 20 | 14.1 | 14.1 |
Fillet Weld Design Resistance per CSA S16-19
CSA S16-19 Clause 13.13.1 specifies the factored shear resistance of a fillet weld:
Vr = 0.67 _ phi_w _ Aw * Xu
Where:
- phi_w = 0.67 (resistance factor for welds)
- Aw = effective weld area = Te _ Lw (throat _ weld length)
- Xu = ultimate tensile strength of the electrode (e.g., 480 MPa for E48XX electrodes)
Weld Resistance per Unit Length
For E48XX electrodes (Xu = 480 MPa), the most common electrode for G40.21 350W steel:
| Weld Leg w (mm) | Vr per mm (kN/mm) | Vr per 100 mm (kN) |
|---|---|---|
| 3 | 0.452 | 45.2 |
| 5 | 0.754 | 75.4 |
| 6 | 0.905 | 90.5 |
| 8 | 1.206 | 120.6 |
| 10 | 1.508 | 150.8 |
| 12 | 1.810 | 181.0 |
| 16 | 2.413 | 241.3 |
Calculation for 6 mm fillet weld with E48XX: Vr per mm = 0.67 _ 0.67 _ (0.707 _ 6) _ 1.0 _ 480 / 1000 = 0.67 _ 0.67 _ 4.24 _ 480 / 1000 = 0.905 kN/mm = 90.5 kN per 100 mm
For E55XX Electrodes (Xu = 550 MPa)
| Weld Leg w (mm) | Vr per mm (kN/mm) | Vr per 100 mm (kN) |
|---|---|---|
| 3 | 0.518 | 51.8 |
| 5 | 0.864 | 86.4 |
| 6 | 1.036 | 103.6 |
| 8 | 1.382 | 138.2 |
| 10 | 1.727 | 172.7 |
| 12 | 2.073 | 207.3 |
| 16 | 2.764 | 276.4 |
For E62XX Electrodes (Xu = 620 MPa)
| Weld Leg w (mm) | Vr per mm (kN/mm) | Vr per 100 mm (kN) |
|---|---|---|
| 3 | 0.584 | 58.4 |
| 5 | 0.974 | 97.4 |
| 6 | 1.169 | 116.9 |
| 8 | 1.559 | 155.9 |
| 10 | 1.948 | 194.8 |
| 12 | 2.338 | 233.8 |
| 16 | 3.118 | 311.8 |
Directional Strength Method
Per CSA S16-19 Clause 13.13.1, the directional strength enhancement for fillet welds loaded at an angle to their longitudinal axis can be considered. This method increases the weld resistance when the load is not parallel to the weld axis.
Vr*theta = Vr * (1.0 + 0.5 _ sin^1.5(theta))
Where theta = angle between the weld axis and the line of action of the applied force.
| Load Angle theta (degrees) | Enhancement Factor | Effective Vr (6 mm E48XX, kN/mm) |
|---|---|---|
| 0 (parallel to weld) | 1.00 | 0.905 |
| 15 | 1.02 | 0.924 |
| 30 | 1.09 | 0.987 |
| 45 | 1.21 | 1.098 |
| 60 | 1.35 | 1.221 |
| 75 | 1.46 | 1.320 |
| 90 (transverse to weld) | 1.50 | 1.357 |
The directional strength method is useful for connections where the weld orientation can be optimised. A fillet weld loaded transversely (perpendicular to the weld axis) has 50% greater resistance than the same weld loaded longitudinally. This is consistent with AISC 360's approach in the AISC Steel Construction Manual.
Electrode Strength Matching Requirements
Per CSA W59 Table 5.2, the electrode must be matched to the base metal grade:
| Base Metal Grade | Min Electrode Grade | Xu (MPa) |
|---|---|---|
| CSA G40.21 260W, 300W, 350W | E48XX | 480 |
| CSA G40.21 350A, 350WT, 350AT | E48XX | 480 |
| CSA G40.21 400W | E55XX | 550 |
| CSA G40.21 480W | E55XX | 550 |
| CSA G40.21 700Q | E62XX | 620 |
Undermatching
Undermatching (using a lower-strength electrode than specified for the base metal) is permitted per CSA W59 Clause 5.6 provided the weld can be designed with adequate size to compensate for the lower electrode strength. Common undermatching scenarios:
- E48XX electrodes welded to 400W base metal (Fy = 400 MPa) — the weld strength is 480 MPa while the base metal is 400 MPa. Since the weld resistance is based on electrode strength, not base metal strength, undermatching is conservative for the weld but may require larger welds than minimum.
Undermatching is NOT permitted for:
- Welds in seismic force-resisting systems (CSA S16-19 Clause 27.5)
- Welds in tension applications for fracture-critical members
- Welds on quenched and tempered steels (700Q) without qualified welding procedure
Design Example — Fillet Welded Connection
Problem: Design a fillet welded connection for a gusset plate (12 mm thick, G40.21 350W) to a column flange (W310x97, flange thickness 15 mm). The factored load Vf = 400 kN applied parallel to the weld. Use CSA W59 minimum requirements and E48XX electrodes.
Step 1 — Determine minimum weld size: Thicker part: column flange = 15 mm. From CSA W59 Table 5.1, for 13 < t <= 19 mm, minimum weld leg = 6 mm.
Step 2 — Determine required weld length: For 6 mm fillet weld with E48XX: Vr = 0.905 kN/mm (longitudinal loading, no directional enhancement).
Required total weld length = 400 / 0.905 = 442 mm
Use two 225 mm welds (one on each side of the gusset plate): total = 450 mm > 442 mm.
Step 3 — Check maximum weld size: Gusset plate edge is sheared (12 mm). Maximum weld leg = 12 - 2 = 10 mm. The 6 mm leg is within limits.
Column flange edge is rolled (15 mm). Maximum weld leg = 15 - 1 = 14 mm. The 6 mm leg is within limits.
Step 4 — Check weld return (end return): Per CSA W59 Clause 5.15, fillet welds terminating at the end of a member must have a return of at least 2 * w (12 mm for 6 mm weld). The return length should be included in the effective weld length.
Step 5 — Check base metal strength: The weld is on G40.21 350W (Fu = 450 MPa). The base metal shear resistance at the weld:
Base metal shear per unit length = 0.80 _ phi _ t _ 0.67 _ Fu = 0.80 _ 0.90 _ 12 _ 0.67 _ 450 / 1000 = 2.60 kN/mm
The base metal shear (2.60 kN/mm) exceeds the weld shear (0.905 kN/mm), so the weld metal governs. This is typical — the weld throat almost always governs over base metal shear for properly sized fillet welds.
Selection: 6 mm fillet weld, E48XX electrode, two 225 mm longitudinal welds on gusset plate sides. Total weld capacity = 2 _ 225 _ 0.905 = 407 kN > 400 kN.
Fillet Weld Fatigue Considerations
Per CSA W59 Clause 6.5, fillet welds in fatigue applications (crane runways, bridges, vibrating equipment) have reduced design stresses:
| Fatigue Category | Allowable Stress Range (MPa) at 2,000,000 cycles | Typical Detail |
|---|---|---|
| E' | 40 | Transverse fillet welds in shear |
| F | 50 | Longitudinal fillet weld ends |
| F2 | 55 | Weld attachments on flanges |
For fatigue-loaded connections, the minimum fillet weld size may need to be increased beyond CSA W59 Table 5.1 requirements to reduce the stress range at the weld toe. Per CSA S16-19 Clause 26, fatigue design follows a stress-range-based approach consistent with the AASHTO fatigue categories.
Frequently Asked Questions
What is the minimum fillet weld size for 12 mm steel per CSA W59? For 12 mm thicker part (6 < t <= 13 mm bracket), the minimum fillet weld leg is 5 mm per CSA W59 Table 5.1. In practice, 6 mm is the most commonly specified minimum for structural connections because it provides better resistance to handling stresses and accommodates typical tolerances. The maximum single-pass fillet weld leg size is 8 mm.
What is the difference between weld leg and effective throat in CSA fillet weld design? The weld leg (w) is the distance from the root of the weld to the toe, measured along the base metal surface. The effective throat (Te) is the minimum distance from the root to the weld face, measured perpendicular to the weld axis. For a 90-degree fillet weld with equal legs, Te = 0.707 _ w. CSA S16-19 Clause 13.13 uses the effective throat area (Aw = Te _ Lw) to calculate weld resistance, not the leg dimension.
What electrode grade is used for welding G40.21 350W steel? E48XX electrodes (Xu = 480 MPa) are used for welding 350W steel per CSA W59 Table 5.2. Common electrodes: E48018 (SMAW), E48S-6 (GMAW with solid wire), and E480T-1 (FCAW with rutile slag). For 400W steel, E55XX electrodes are required. For 700Q, E62XX electrodes are required. Undermatching (using E48XX on 400W) is permitted with adequate weld size compensation.
How is fillet weld resistance calculated per CSA S16? Vr = 0.67 _ phi_w _ Aw _ Xu, where phi_w = 0.67 (weld resistance factor), Aw = effective throat _ weld length, and Xu = electrode tensile strength. For a 6 mm fillet weld with E48XX (Xu = 480 MPa): Vr = 0.67 _ 0.67 _ (0.707 _ 6 _ 1.0) * 480 / 1000 = 0.905 kN per mm of weld length. The directional strength method permits up to 50% enhancement for transversely loaded welds per Clause 13.13.1.
What is the resistance factor for fillet welds in CSA S16? The resistance factor for fillet welds is phi_w = 0.67 per CSA S16-19 Clause 13.13. This is lower than the AISC 360 phi = 0.75 for welds. The Canadian factor accounts for greater variability in weld metal strength and the consequences of weld failure (typically sudden and without ductility). Combining the 0.67 resistance factor with the 0.67 factor in the equation gives an effective factor of approximately 0.45 on the nominal weld strength.
Related Pages
- CSA W59 Weld Electrodes — E48XX Selection Guide
- CSA Bolt Capacity — A325M & A490M Tables
- Canadian Steel Grades — G40.21 300W, 350W, 400W
- CSA S16 Code Overview
- Weld Joint Types and Symbols Reference
- Welded Connection Calculator
- Weld Symbol Chart
- Fillet Weld Size Chart
This page is for educational reference. All weld data per CSA W59-18 and CSA S16-19. Verify welding procedure specifications against the current code edition. For fracture-critical and seismic connections, follow additional toughness and quality control requirements per CSA W59. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.