Fillet Weld Measurement: Leg Length, Actual and Design Throat
Introduction
Fillet welds are the most common joint type in steel fabrication. Their load-carrying capacity depends directly on weld dimensions — leg length and throat.
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Geometric Definitions
Leg Length (z)
- Distance from the weld root to the toe, measured along each plate face
- Symmetrical fillet: z₁ = z₂; Asymmetrical: z₁ ≠ z₂
Design Throat (a)
- Height of the largest isosceles triangle that can be inscribed in the weld cross-section
- For 90° joints: a = z × 0.707
- Value used in structural resistance calculations
- Specified as a in welding symbols (EN ISO 2553)
Actual Throat
- Minimum distance from the actual weld root to the surface
- Actual throat ≥ Design throat (minimum requirement)
| Dimension | Symbol | Relationship (90° joint) |
|---|---|---|
| Leg | z | z = a / 0.707 |
| Design throat | a | a = z × 0.707 |
| Actual throat | — | ≥ a |
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Measuring Instruments
| Instrument | What it measures | Accuracy |
|---|---|---|
| Leg gauge (V-WAC type) | Leg length z | ±0.5 mm |
| Throat gauge | Design throat a | ±0.5 mm |
| Multi-purpose gauge (Cambridge type) | z, a, excess weld metal, undercut depth | ±0.5 mm |
| Vernier calliper | z and a (direct) | ±0.1 mm |
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Acceptance Criteria — EN ISO 5817
Undersize
| Level | Admissible leg deficit (Δz) |
|---|---|
| D | ≤ 0.5 + 0.1·z or ≤ 2 mm |
| C | ≤ 0.5 + 0.1·z or ≤ 1.5 mm |
| B | ≤ 0.5 + 0.1·z or ≤ 1 mm |
Asymmetry (Δz)
| Level | Maximum asymmetry |
|---|---|
| D | 2 + 0.2·a or ≤ 4 mm |
| C | 1.5 + 0.15·a or ≤ 3 mm |
| B | 1 + 0.1·a or ≤ 2 mm |
Excess Weld Metal
| Level | Maximum (h) |
|---|---|
| D | 1 + 0.25·b or ≤ 5 mm |
| C | 1 + 0.15·b or ≤ 4 mm |
| B | 1 + 0.1·b or ≤ 3 mm |
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Practical Example
Specified fillet weld: a = 5 mm on a 90° joint
| Calculation | Value |
|---|---|
| Nominal leg (z) | 5 / 0.707 = 7.1 mm |
| Minimum acceptable leg (level C) | 7.1 − 1.5 = 5.6 mm |
| Maximum asymmetry (level C) | 1.5 + 0.15×5 = 2.25 mm |
If measurement gives z₁ = 7.0 mm and z₂ = 6.5 mm → Δz = 0.5 mm → Conforming at level B
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