EN 1993-1-8 Weld Sizes — Minimum Fillet Weld Throat Thickness and Leg Length
Complete reference for fillet weld sizing in European steel design per EN 1993-1-8:2005 Clause 4.5. Including minimum fillet weld throat thickness (a) per Table 4.1, the relationship between throat thickness a and leg length z, design weld resistance Fw,Rd, and practical weld sizing guidance for the European steelwork industry.
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Fillet Weld Geometry — Throat Thickness a vs Leg Length z
A fillet weld is defined by its throat thickness a, which is the shortest distance from the root of the weld to the face of the weld, measured in the plane of the weld cross-section. For an equal-leg fillet weld with a flat or slightly convex face:
a = z / sqrt(2) = 0.707 x z
Where:
- a = throat thickness (design dimension)
- z = leg length (fabrication dimension)
Conversely: z = a x sqrt(2) = 1.414 x a
The throat thickness a is used for design calculations because the weld failure plane is through the throat. The leg length z is used for fabrication and inspection because it is the measurable external dimension.
| Throat a (mm) | Approx Leg z (mm) | Standard Electrode |
|---|---|---|
| 3.0 | 4.2 (use 5) | 3.2 mm |
| 3.5 | 5.0 | 3.2 or 4.0 mm |
| 4.0 | 5.7 (use 6) | 4.0 mm |
| 5.0 | 7.1 | 4.0 or 5.0 mm |
| 6.0 | 8.5 | 5.0 mm |
| 7.0 | 10.0 | 5.0 or 6.0 mm |
| 8.0 | 11.3 | 6.0 mm |
| 10.0 | 14.1 | 6.0 or 8.0 mm |
| 12.0 | 17.0 | 8.0 mm |
In European practice, fillet welds are designated by their throat thickness: "a6" means a 6 mm throat fillet weld (z ~ 8.5 mm leg). In UK and US practice, fillet welds are designated by leg length: "6 mm fillet" means z = 6 mm (a ~ 4.2 mm). Always clarify which dimension is being referenced in the specification.
Minimum Fillet Weld Throat Thickness — EN 1993-1-8 Table 4.1
The minimum throat thickness a_min for fillet welds depends on the thickness of the thicker connected part, per EN 1993-1-8 Clause 4.5.2(2) and Table 4.1. This requirement prevents the weld from cooling too quickly (leading to hardening and cracking) and ensures adequate weld strength relative to the connected plate.
| Thicker Connected Part t (mm) | Min Throat a (mm) | Typical Application |
|---|---|---|
| t <= 5 | 3.0 | Thin gauge connections, cladding rails, purlin cleats |
| 5 < t <= 10 | 3.0 | Standard secondary connections, fin plates |
| 10 < t <= 15 | 4.0 | Beam-to-column shear connections |
| 15 < t <= 20 | 5.0 | Standard end plates, beam splices |
| 20 < t <= 25 | 6.0 | Heavy beam connections, column base plates |
| 25 < t <= 30 | 7.0 | Thick end plates, moment connections |
| 30 < t <= 40 | 8.0 | Very heavy connections, crane girders |
| 40 < t <= 50 | 10.0 | Bridge girder connections, offshore structures |
The minimum throat thickness may be reduced if it exceeds the thickness of the thinner connected part, but the weld must still be capable of transmitting the design force. Additionally, the effective throat shall not exceed 0.7 x t_min (where t_min is the thickness of the thinner connected part) for structural adequacy.
Effective Length and End Returns
EN 1993-1-8 Clause 4.5.1 specifies the effective length of a fillet weld:
L_eff = L_real - 2 x a (for welds with start/stop craters)
Where L_real is the actual deposited weld length. The 2 x a deduction removes the crater regions at the start and end of the weld run, where full throat thickness is not achieved. For automated or mechanised welding processes with run-on and run-off tabs, the deduction may be omitted.
Welds shorter than the greater of 30 mm or 6 x a shall not be considered as effective for load transfer, per Clause 4.5.1(2). For a6 welds, the minimum effective length is max(30, 6 x 6 = 36) = 36 mm.
Design Weld Resistance — Fw,Rd
Fillet Weld Resistance per Unit Length
Per EN 1993-1-8 Clause 4.5.3.3(3):
Fw,Rd = fvw,d x a
Where the design shear strength of the weld fvw,d is:
fvw,d = fu / (sqrt(3) x beta_w x gamma_M2)
And:
| Symbol | Description | Value |
|---|---|---|
| fu | Nominal ultimate tensile strength of the weaker connected part | From EN 10025 |
| beta_w | Correlation factor for fillet welds | From Table 4.1 |
| gamma_M2 | Partial factor for welded connections | 1.25 (UK NA) |
Correlation Factor beta_w — EN 1993-1-8 Table 4.1
beta_w accounts for the difference between the filler metal strength and the parent metal strength:
| Steel Grade (EN 10025) | beta_w |
|---|---|
| S235 | 0.80 |
| S275 | 0.85 |
| S355 | 0.90 |
| S420 | 0.88 |
| S460 | 0.85 |
A lower beta_w indicates that the weld metal is stronger relative to the parent plate — S235 has beta_w = 0.80 (most favourable for weld size), while S355 has beta_w = 0.90 (relatively larger welds required). S460 benefits from beta_w = 0.85 because the matching filler metal is proportionally stronger.
Weld Resistance per mm Throat — S355 Steel (fu = 470 MPa, beta_w = 0.90, gamma_M2 = 1.25)
fvw,d = 470 / (sqrt(3) x 0.90 x 1.25) = 470 / (1.732 x 0.90 x 1.25) = 470 / 1.949 = 241.2 MPa
| Throat a (mm) | Fw,Rd per mm length (kN/mm) | Fw,Rd for 100 mm weld (kN) |
|---|---|---|
| 3.0 | 0.724 | 72.4 |
| 4.0 | 0.965 | 96.5 |
| 5.0 | 1.206 | 120.6 |
| 6.0 | 1.447 | 144.7 |
| 7.0 | 1.688 | 168.8 |
| 8.0 | 1.929 | 192.9 |
| 10.0 | 2.412 | 241.2 |
For a typical fin plate connection with a 6 mm throat fillet weld (a6) over a 200 mm plate depth: Fw,Rd = 1.447 x 200 = 289.4 kN total weld resistance. Double-sided welds double the capacity.
Weld Resistance per mm Throat — S275 and S460 Comparison
| Steel Grade | fu (MPa) | beta_w | fvw,d (MPa) | Fw,Rd for a6 (kN/mm) |
|---|---|---|---|---|
| S275 | 410 | 0.85 | 222.7 | 1.336 |
| S355 | 470 | 0.90 | 241.2 | 1.447 |
| S460 | 540 | 0.85 | 331.8 | 1.991 |
For S275, the weld resistance is approximately 8% lower than S355, reflecting the lower fu. For S460, the resistance is approximately 38% higher than S355, reflecting the proportionally higher fu and the favourable beta_w = 0.85.
Maximum Fillet Weld Size
EN 1993-1-8 does not prescribe an absolute maximum fillet weld throat thickness, but practical limits apply:
- Through-thickness limitation: a <= 0.7 x t_min where t_min is the thickness of the thinner connected part. Welds larger than this risk over-matching the plate and inducing through-thickness stresses that may cause lamellar tearing.
- Heat input: Very large single-pass fillets (a > 8 mm) require multi-pass welding, which adds cost and may distort the plate. For plate t >= 20 mm, multi-pass welds are routine.
- Single-pass capability: A 5 mm or 6 mm electrode in a single pass can typically deposit a throat of 5-6 mm. For larger throats, specify multi-pass welding.
Fillet Weld vs Butt Weld — When to Choose Each
| Factor | Fillet Weld | Full-Penetration Butt Weld |
|---|---|---|
| Strength | Governed by throat a and fu of weaker part | Equal to parent plate strength |
| Preparation | No edge preparation required | Bevel, J-prep, or V-prep required |
| Cost | Lower fabrication cost | Higher fabrication cost (edge prep + weld volume) |
| Inspection | Visual + surface NDT (MPI/DPI) | Visual + volumetric NDT (UT/RT) typically required |
| Through-thickness stress | Avoid where lamellar tearing is a risk | Full through-thickness connection — Z-quality steel may be needed |
| Fatigue | Lower fatigue class (FAT 36 or 40) | Higher fatigue class (FAT 71 for ground flush) |
| Common use | Shear connections (fin plates, web cleats, stiffeners) | Moment connections, tension members, dynamic loading |
Practical Weld Sizing — Standard European Details
Based on common UK and European fabrication practice:
| Connection Type | Typical Plate t (mm) | Typical Weld | Notes |
|---|---|---|---|
| Fin plate to beam web | 8-12 | a4 or a5 | Single-sided fillet, standard UK detail |
| Web cleat to column | 10-15 | a5 or a6 | Double-sided fillet |
| Stiffener to column web | 10-20 | a5 double-sided | Continuous fillet |
| End plate to beam (moment) | 12-25 | a6 or a8 | Full-strength fillet or PJP butt + fillet |
| Column base plate to column | 20-40 | a8 or a10 | Multi-pass fillet, often partial-penetration butt as alternative |
| Bracing gusset to beam/column | 8-15 | a5 or a6 | Double-sided fillet |
For connections designed to transmit the full strength of the connected member, use either: a full-penetration butt weld, or a combination of partial-penetration butt weld with reinforcing fillet (PJP + fillet), sized to match the plate capacity.
Frequently Asked Questions
What is the minimum fillet weld size for a 20 mm thick plate per EN 1993-1-8? Per EN 1993-1-8 Table 4.1, for a thicker connected part t = 20 mm, the minimum fillet weld throat thickness is a = 5.0 mm (leg length z ~ 7.1 mm). This requirement prevents excessively rapid weld cooling, which can lead to martensite formation, hardening, and hydrogen-induced cracking (cold cracking). For S355 steel with a5, Fw,Rd = 1.206 kN per mm of weld length.
How do I convert between weld throat thickness a and leg length z? For an equal-leg flat-face fillet weld: a = z / sqrt(2) = 0.707z, and z = a x sqrt(2) = 1.414a. A common UK specification "6 mm fillet" means z = 6 mm, which gives a = 4.2 mm (effectively a4). A European specification "a6" means a = 6 mm, which gives z = 8.5 mm. The European convention (designating by throat) is used for design; the UK convention (designating by leg) is used for fabrication. Always clarify which dimension is intended in the weld symbol or specification.
What is the beta_w correlation factor in EN 1993-1-8? beta_w is the correlation factor for fillet welds from EN 1993-1-8 Table 4.1. It accounts for the difference between the filler metal strength and the parent plate strength: beta_w = 0.80 (S235), 0.85 (S275), 0.90 (S355), 0.88 (S420), 0.85 (S460). A lower beta_w reflects a higher ratio of weld metal to parent metal strength, meaning smaller welds are adequate. For S355 (beta_w = 0.90), the weld design strength fvw,d = fu/(sqrt(3) x 0.90 x 1.25).
What is the weld design resistance Fw,Rd per unit length for S355 steel? fvw,d = fu / (sqrt(3) x beta_w x gamma_M2). For S355: fu = 470 MPa, beta_w = 0.90, gamma_M2 = 1.25. fvw,d = 470 / (1.732 x 0.90 x 1.25) = 241.2 MPa. Fw,Rd = fvw,d x a. For a6: Fw,Rd = 241.2 x 6 = 1.447 kN/mm. For a 100 mm long weld: Fw,Rd = 144.7 kN. Always check the fu of the weaker connected part — if connected to S275 (fu = 410 MPa), use fu = 410 MPa.
When should I use a full-penetration butt weld instead of a fillet weld? Use full-penetration butt welds when: (1) the design requires the full strength of the connected plate (i.e., the weld must develop the plate capacity), (2) the connection is subject to tension or moment (end plate to beam flange, column splices), (3) fatigue is a design consideration (butt welds have higher FAT classes than fillet welds), (4) through-thickness loading demands a full fusion connection, or (5) the connection must be sealed (watertight, corrosion protection). Fillet welds are adequate for shear connections (fin plates, web cleats, stiffeners) and where the plate capacity exceeds the weld demand.
Related Pages
- EN 1993-1-8 Connection Design Guide — Bolt categories, welds, joint classification
- EN 1993-1-8 Weld Electrode Selection — EN ISO 2560/14341 matching to S235-S460
- EN 1993-1-8 Bolt Grades — 4.6 to 10.9 mechanical properties
- EN 1993-1-1 Beam Design Guide — Section classification, Mc,Rd
- European Steel Properties — EN 10025 S235-S460 Fy and Fu
- Welded Connections Calculator
- Beam Capacity Calculator
- Column Capacity Calculator
- Section Properties Lookup
Educational reference only. All weld sizing values and formulas are per EN 1993-1-8:2005. Verify all values against the current edition of the Eurocode and the applicable National Annex for your project jurisdiction. Weld sizes shown assume matched electrodes per EN ISO 2560/14341. For automated or mechanised welding, different procedures may be applicable. Results are PRELIMINARY — NOT FOR CONSTRUCTION. All designs must be independently verified by a licensed Professional Engineer or Chartered Structural Engineer.