Australian Weld Sizes — AS 1554.1 Fillet Weld Leg Length and Throat Thickness
Complete reference for fillet weld sizes in Australian steel design per AS 1554.1 (Structural Steel Welding) and AS 4100:2020 Clause 9.7.3.10. Minimum weld leg dimensions by thicker part thickness, SP (Structural Purpose) and GP (General Purpose) weld categories, effective throat thickness, and factored weld capacities per unit length.
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AS 1554.1 Weld Categories
AS 1554.1 defines two categories of fillet welds based on the level of quality assurance and inspection:
SP Category (Structural Purpose)
SP welds require full quality assurance including:
- Welder qualification per AS 1796 (Certificate of Competency)
- Welding procedure specification (WPS) per AS 1554.1
- Non-destructive testing (NDT) — magnetic particle or dye penetrant inspection
- Visual inspection of all welds
- Compliance with all AS 1554.1 requirements for weld profile, leg length, and throat thickness
SP welds are used in primary structural connections, moment-resisting frames, seismic load-resisting systems, and any connection where weld failure would compromise structural integrity.
GP Category (General Purpose)
GP welds have reduced quality assurance requirements:
- No mandatory NDT beyond visual inspection
- Less restrictive weld profile tolerances
- Reduced documentation requirements
GP welds are used in secondary connections, bracing connections in non-seismic frames, shear connections in low-rise buildings, and non-structural attachments.
Weld Category Comparison
| Requirement | SP Category | GP Category |
|---|---|---|
| Welder qualification | AS 1796 | AS 1796 |
| Welding procedure specification | Required | Required |
| Visual inspection | 100% | 100% |
| NDT (MPI or dye penetrant) | Required (sampling) | Not required |
| Weld profile tolerance | Tight | Standard |
| Design capacity factor phi | 0.80 | 0.80 |
| Typical use | Primary connections | Secondary connections |
Minimum Fillet Weld Leg Size — AS 1554.1
AS 1554.1 Clause 5.8 specifies minimum fillet weld leg sizes based on the thickness of the thicker part being joined. This ensures adequate fusion and prevents rapid cooling that could cause hydrogen cracking.
| Thickness of Thicker Part (t) mm | Minimum Weld Leg (w) mm |
|---|---|
| t <= 6 | 3 |
| 6 < t <= 10 | 4 |
| 10 < t <= 16 | 5 |
| 16 < t <= 25 | 6 |
| 25 < t <= 40 | 8 |
| 40 < t <= 60 | 10 |
| 60 < t <= 80 | 12 |
| t > 80 | 14 |
Notes on Minimum Weld Size
- Maximum single-pass fillet weld leg: 8 mm. Welds larger than 8 mm require multiple passes.
- Minimum weld leg for tack welds: 4 mm unless otherwise specified in the WPS.
- Minimum weld leg for intermittent fillet welds: 5 mm.
- Thin material exception: Where the thinner part is less than 3 mm, the minimum weld leg may be equal to the thinner part thickness.
- Preheat consideration: For thicker parts, the minimum weld size may need to increase if preheat is not applied, particularly for Grade 350 and higher-strength steels with higher CEV.
Fillet Weld Effective Throat Thickness
AS 4100 Clause 9.7.3.10 uses the effective throat thickness (tt) of the fillet weld for capacity calculations, not the leg length.
For a standard 90-degree fillet weld with equal legs: tt = 0.707 x w
Where tt = effective throat thickness (mm) and w = weld leg length (mm).
| Weld Leg (w) mm | Effective Throat (tt) mm | Notes |
|---|---|---|
| 3 | 2.1 | Minimum leg for t <= 6 |
| 4 | 2.8 | Minimum for 6 < t <= 10 |
| 5 | 3.5 | Minimum for 10 < t <= 16 |
| 6 | 4.2 | Minimum for 16 < t <= 25 |
| 8 | 5.7 | Minimum for 25 < t <= 40 |
| 10 | 7.1 | Minimum for 40 < t <= 60 |
| 12 | 8.5 | Minimum for 60 < t <= 80 |
| 14 | 9.9 | Minimum for t > 80 |
Weld Design Capacity — AS 4100 Clause 9.7.3.10
AS 4100:2020 Clause 9.7.3.10 defines the design capacity of a fillet weld per unit length:
phi vw = phi x 0.6 x fuw x tt x kr
Where:
- phi = 0.80 (capacity factor for welds, Table 3.4)
- fuw = nominal tensile strength of weld metal (MPa)
- W50X electrode: fuw = 490 MPa
- W502 electrode: fuw = 490 MPa
- W55X electrode: fuw = 540 MPa
- W62X electrode: fuw = 610 MPa
- tt = effective throat thickness (mm)
- kr = reduction factor for weld length (Clause 9.7.3.10.2)
- kr = 1.0 for weld length >= 50 x tt
- kr = 0.85 for weld length < 50 x tt
Factored Weld Capacity Per mm — W50X (fuw = 490 MPa)
| Weld Leg (mm) | Effective Throat (mm) | phi vw (kN/mm) |
|---|---|---|
| 3 | 2.1 | 0.49 |
| 4 | 2.8 | 0.66 |
| 5 | 3.5 | 0.82 |
| 6 | 4.2 | 0.99 |
| 8 | 5.7 | 1.34 |
| 10 | 7.1 | 1.67 |
| 12 | 8.5 | 2.00 |
| 14 | 9.9 | 2.33 |
Factored Weld Capacity Per mm — W55X (fuw = 540 MPa)
| Weld Leg (mm) | Effective Throat (mm) | phi vw (kN/mm) |
|---|---|---|
| 3 | 2.1 | 0.54 |
| 4 | 2.8 | 0.73 |
| 5 | 3.5 | 0.91 |
| 6 | 4.2 | 1.09 |
| 8 | 5.7 | 1.48 |
| 10 | 7.1 | 1.84 |
| 12 | 8.5 | 2.21 |
| 14 | 9.9 | 2.57 |
Factored Weld Capacity Per mm — W62X (fuw = 610 MPa)
| Weld Leg (mm) | Effective Throat (mm) | phi vw (kN/mm) |
|---|---|---|
| 3 | 2.1 | 0.61 |
| 4 | 2.8 | 0.82 |
| 5 | 3.5 | 1.02 |
| 6 | 4.2 | 1.23 |
| 8 | 5.7 | 1.67 |
| 10 | 7.1 | 2.08 |
| 12 | 8.5 | 2.49 |
| 14 | 9.9 | 2.90 |
Directional Strength Enhancement
AS 4100 Clause 9.7.3.10.3 permits a directional strength enhancement for fillet welds loaded transversely (perpendicular to the weld axis):
phi vw_theta = phi vw x (1.0 + 0.5 x sin^1.5 theta)
Where theta is the angle between the weld longitudinal axis and the line of action of the resultant force.
For a transverse fillet weld (theta = 90 degrees): Enhancement factor = 1.0 + 0.5 x sin^1.5(90) = 1.0 + 0.5 x 1.0 = 1.5
This means a transverse fillet weld can carry up to 50% more load than a longitudinal fillet weld.
Base Metal Shear Capacity Check
AS 4100 also requires checking the shear capacity of the base metal adjacent to the weld (Clause 9.7.3.4):
phi vbm = phi x 0.6 x fy x tw
For Grade 300PLUS (fy = 300 MPa): phi vbm = 0.90 x 0.6 x 300 x tw / 1000 = 0.162 kN per mm per mm thickness
The base metal check often governs for thin plates welded to thick sections — the weld may be stronger than the plate.
Worked Example: Fillet Weld Design
Problem: A beam-to-column connection requires a 6 mm SP category fillet weld using W50X electrodes for Grade 300PLUS steel. Determine the factored weld capacity per mm length.
Given:
- Weld leg: w = 6 mm
- Electrode: W50X (fuw = 490 MPa)
- Category: SP
- Steel grade: 300PLUS (Fy = 300 MPa)
Solution:
- Effective throat: tt = 0.707 x 6 = 4.2 mm
- Factored weld capacity: phi vw = 0.80 x 0.6 x 490 x 4.2 / 1000 = 0.99 kN/mm
- If loaded transversely: phi vw_90 = 0.99 x 1.5 = 1.48 kN/mm
- Base metal capacity (6 mm plate): phi vbm = 0.90 x 0.6 x 300 x 6 / 1000 = 0.97 kN/mm
For longitudinal loading, the weld capacity (0.99 kN/mm) slightly exceeds the base metal capacity (0.97 kN/mm), so base metal governs. The design capacity is 0.97 kN/mm.
Maximum Weld Size Limitations
AS 1554.1 also specifies maximum fillet weld leg sizes to prevent weld metal wastage and excessive heat input:
- Along edges of plates: w <= t - 1 mm (where t = plate thickness)
- Along edges of thin sections: w <= t (for sections up to 6 mm thick)
- Multiple-pass welds: No single pass should exceed 8 mm leg length
- Over-filling: The weld face should not exceed 1 mm reinforcement beyond the base metal surface for SP category
Weld Length Reduction Factor
AS 4100 Clause 9.7.3.10.2 specifies a reduction factor for short welds:
- If weld length L >= 50 x tt: kr = 1.0 (no reduction)
- If weld length L < 50 x tt: kr = 0.85 (reduction applies)
For a 6 mm fillet weld (tt = 4.2 mm), 50 x tt = 210 mm. Any weld shorter than 210 mm receives the 0.85 reduction, reflecting the lower load-sharing capacity of short welds.
Design Resources
- Australian Steel Design Guide — AS 4100 overview
- Australian Weld Electrodes — Electrode classification
- Australian Bolt Capacity — Bolt design reference
- AS 4100 Base Plate Design — Base plate reference
- Beam Capacity Calculator
- Column Capacity Calculator
- Section Properties
Frequently Asked Questions
What is the minimum fillet weld size for 12 mm thick steel per AS 1554.1? For 12 mm thicker part (10 < t <= 16 mm bracket), the minimum fillet weld leg is 5 mm per AS 1554.1 Clause 5.8. 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.
What is the difference between SP and GP weld categories in AS 1554.1? SP (Structural Purpose) welds require NDT beyond visual inspection, tighter weld profile tolerances, and more comprehensive documentation. GP (General Purpose) welds only require visual inspection. The design capacity factor (phi = 0.80) is the same for both, but SP welds provide greater confidence in weld quality.
How is fillet weld capacity calculated per AS 4100? phi vw = phi x 0.6 x fuw x tt x kr, where phi = 0.80, fuw = electrode tensile strength, tt = effective throat (0.707 x weld leg), and kr = length reduction factor. For a 6 mm fillet weld with W50X: phi vw = 0.80 x 0.6 x 490 x 4.2 / 1000 = 0.99 kN/mm.
What is the directional strength enhancement for fillet welds? AS 4100 Clause 9.7.3.10.3 permits an enhancement factor of (1.0 + 0.5 x sin^1.5 theta). For a weld loaded at 90 degrees (transverse), the factor is 1.5 — a 50% increase. For a weld loaded at 0 degrees (longitudinal), the factor is 1.0 — no enhancement.
What electrode should be used for welding Grade 300PLUS steel? Grade 300PLUS is welded using W50X or W502 electrodes (fuw = 490 MPa) per AS 1554.1 Table 5.2. Common consumables: E4818 (SMAW), ES6 (GMAW solid wire), or E480T-1 (FCAW). For Grade 350 steel, W55X electrodes are recommended.
Educational reference only. All design values must be verified against the current editions of AS 1554.1, AS 4100, and the project specification. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.