EN 1993 Weld Capacity — Fillet Weld per Eurocode 3 Clause 4
Complete guide to fillet weld design per EN 1993-1-8:2005 Clause 4. Simplified (conservative) and directional (more accurate) methods for weld design. Throat thickness, design resistance, stress components, and worked examples for European structural connections.
Quick access: Weld Symbols → | Weld Inspection → | Welding Procedure →
Weld Geometry — Clause 4.3
The effective throat thickness a of a fillet weld is the height of the largest triangle inscribed within the weld (measured perpendicular to the weld throat). For a full-penetration butt weld, a = t (plate thickness).
| Leg Length s (mm) | Throat a (mm) for 45° fillet | Effective area per mm length |
|---|---|---|
| 4 | 2.8 | 2.8 mm²/mm |
| 5 | 3.5 | 3.5 mm²/mm |
| 6 | 4.2 | 4.2 mm²/mm |
| 8 | 5.7 | 5.7 mm²/mm |
| 10 | 7.1 | 7.1 mm²/mm |
| 12 | 8.5 | 8.5 mm²/mm |
a = s / √2 = s × 0.707 for a standard 45° fillet.
Directional Method — Clause 4.5.3.2
The weld throat is subject to three stress components:
- σ⊥ — normal stress perpendicular to the weld throat plane
- τ⊥ — shear stress perpendicular to the weld axis (in the throat plane)
- τ∥ — shear stress parallel to the weld axis
Design Criteria
σ_eq = √(σ⊥² + 3(τ⊥² + τ∥²)) ≤ f_u / (β_w × γ_M2)
And additionally: σ⊥ ≤ 0.9 × f_u / γ_M2
Where:
- f_u = nominal ultimate tensile strength of the weaker part joined
- β_w = correlation factor per Table 4.1
- γ_M2 = 1.25
Correlation Factor β_w — Table 4.1
| Steel Grade | β_w |
|---|---|
| S235 | 0.80 |
| S275 | 0.85 |
| S355 | 0.90 |
| S420 | 1.00 |
| S460 | 1.00 |
Simplified Method — Clause 4.5.3.3
The simplified method assumes the weld capacity is independent of the load direction:
F_w,Rd = f_vw,d × a
Where the design shear strength of the weld: f_vw,d = f_u / (√3 × β_w × γ_M2)
Weld Capacities per mm Run (Simplified Method)
| Weld Throat a | S235 (β_w = 0.80) | S275 (β_w = 0.85) | S355 (β_w = 0.90) |
|---|---|---|---|
| 3 mm | 0.80 kN/mm | 0.72 kN/mm | 0.82 kN/mm |
| 4 mm | 1.06 kN/mm | 0.96 kN/mm | 1.09 kN/mm |
| 5 mm | 1.33 kN/mm | 1.20 kN/mm | 1.37 kN/mm |
| 6 mm | 1.60 kN/mm | 1.43 kN/mm | 1.64 kN/mm |
| 8 mm | 2.13 kN/mm | 1.91 kN/mm | 2.19 kN/mm |
| 10 mm | 2.66 kN/mm | 2.39 kN/mm | 2.73 kN/mm |
Worked Example — 6 mm Fillet Weld, S355 Steel
Connection: 100 kN load transferred through 2 side fillet welds, 100 mm long each, 6 mm leg (4.2 mm throat), S355.
| Parameter | Value |
|---|---|
| Weld size | 6 mm leg (a = 4.2 mm) |
| Steel | S355 (f_u = 470 MPa) |
| β_w | 0.90 |
| Total weld length | 2 × 100 = 200 mm |
Directional Method
| Stress component | Value (kN/mm²) |
|---|---|
| τ∥ (parallel, assumed equal distribution) | 100000 / (200 × 4.2) = 119 MPa |
| σ⊥, τ⊥ | 0 (no transverse load) |
| σ_eq = √(0 + 3(0 + 119²)) | 206 MPa |
| f_u / (β_w × γ_M2) | 470 / (0.90 × 1.25) = 418 MPa |
| Check: 206 ≤ 418 | ✓ |
Simplified Method
| Per mm capacity | 1.64 kN/mm (from table) |
|---|---|
| Total capacity | 200 × 1.64 = 328 kN |
| Applied load | 100 kN |
| Utilization | 100/328 = 0.30 ✓ |
Minimum and Maximum Weld Sizes
Per EN 1993-1-8 Clause 4.5:
- Minimum throat thickness: a_min related to the thicker part joined
- Maximum weld size: generally a_max = 0.7 × t_min
For common thicknesses:
| Thicker Part t (mm) | Minimum a (mm) | Minimum Leg s (mm) |
|---|---|---|
| ≤ 4 | 2.5 | 3.5 |
| 4 < t ≤ 6 | 3.0 | 4.2 |
| 6 < t ≤ 12 | 4.0 | 5.7 |
| 12 < t ≤ 20 | 5.0 | 7.1 |
| 20 < t ≤ 30 | 6.0 | 8.5 |
| > 30 | 8.0 | 11.3 |
Frequently Asked Questions
What is the difference between the simplified and directional method for weld design?
The simplified method (Clause 4.5.3.3) assumes the weld capacity is independent of load direction and is always conservative. The directional method (Clause 4.5.3.2) resolves stresses into σ⊥, τ⊥, and τ∥ components and gives higher capacity for welds loaded parallel to the axis (up to 30% more). The directional method is recommended for heavily loaded connections.
What is the minimum fillet weld size per EN 1993-1-8?
Per EN 1993-1-8 Clause 4.5, the minimum fillet weld throat thickness a is 2.5 mm for plates up to 4 mm thick, increasing to 8 mm for plates over 30 mm. The minimum leg length s = a × √2. For standard structural connections in S355 (8-20 mm plates), minimum a = 4-5 mm (leg s = 5.7-7.1 mm).
Related Pages
- Weld Symbols — Standard weld symbols per EN 22553
- Weld Inspection — EN 1090-2 inspection methods
- Welding Procedure — EN 1090-2 WPS requirements
- All European References
Educational reference only. Design per EN 1993-1-8:2005 Clause 4.5. β_w per Table 4.1. γ_M2 = 1.25. Verify electrode classification and weld procedure specification. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.
Design Resources
Calculator tools
Design guides