UK Weld Capacity — EN 1993-1-8 Fillet Weld Design

This reference covers weld capacity for UK steel design per EN 1993-1-8:2005 Clause 4 and UK NA. Welded connections are common in UK steel fabrication for beam-to-column connections, splices, bracket attachments, and portal frame haunches.

Design requirements, worked examples, and practical design guidance are provided for common design office applications.

Code Reference: EN 1993-1-8:2005 Clause 4 and UK NA

Fillet Weld Design Methods

EN 1993-1-8 provides two methods for fillet weld design:

1. Simplified method (directional): [ F*{w,Rd} = f*{vw,d} \times a \quad \text{(per unit length)} ]

Where:

• ( f*{vw,d} = \frac{f_u / \sqrt{3}}{\beta_w \gamma*{M2}} )
• a = weld throat thickness
• fu = ultimate tensile strength of weaker part
• βw = correlation factor (depends on steel grade)
• γM2 = 1.25 (UK NA)

2. Resolved stress method (exact): [ \sqrt{\sigma*\perp^2 + 3(\tau*\perp^2 + \tau*\parallel^2)} \leq \frac{f_u}{\beta_w \gamma*{M2}} ] and ( \sigma*\perp \leq 0.9 f_u / \gamma*{M2} )

Correlation Factor βw

Weld Design Strengths (Simplified Method)

Note: fvw,d = fu / (√3 × βw × γM2). For S355 into S275: use fu = 430 MPa (weaker part).

Minimum and Maximum Weld Sizes

Minimum weld length = 6a or 40 mm (whichever greater).

Effective Weld Length

The effective length of a fillet weld: [ L*{eff} = L*{actual} - 2a ]

For a 200 mm long fillet weld with 6 mm throat: Leff = 200 - 12 = 188 mm.

For end-loaded welds (long joints), the effective length is reduced if L > 150a:

• Reduction factor βLw = 1.2 - 0.2 × (Lw / 150a) but not < 0.6 or > 1.0

Worked Example — Fillet Welded Bracket

Given:

• Bracket: 10 mm plate in S275 welded to column flange
• Design shear: 150 kN (in-plane vertical)
• Fillet weld: 6 mm leg length (a = 4.2 mm), S355 member
• Weld length: 2 × 150 mm vertical (return welds at top)

Step 1 — Determine design strength: Weaker part = S275 plate (fu = 430 MPa, βw = 0.85) fvw,d = 430 / (√3 × 0.85 × 1.25) = 233.7 N/mm²

Step 2 — Weld capacity per unit length: Fw,Rd = 233.7 × 4.2 = 981.5 N/mm

Step 3 — Total weld capacity: Total effective length = 2 × (150 - 2 × 4.2) = 283.2 mm Total capacity = 283.2 × 981.5 × 10⁻³ = 278 kN

Step 4 — Check: UT = 150 / 278 = 0.54 — Satisfactory

Step 5 — Check weld return (top): The 12 mm return at top (typical detail) adds ~2% capacity, conservatively ignored.

Worked Example — Full Penetration Butt Weld

Given:

• 20 mm plate splice in S355
• Tension force: 1500 kN
• Plate width: 300 mm

Full penetration butt weld:

• Design strength = fy / γM0 = 355 / 1.0 = 355 N/mm² (tension yield)
• OR fu / γM2 = 510 / 1.25 = 408 N/mm² (tension fracture on net section)
• Conservative: use fy/γM0 = 355 N/mm²

Capacity = 300 × 20 × 355 / 1.0 × 10⁻³ = 2130 kN > 1500 kN — Satisfactory

Full penetration butt welds (subject to NDT verification) are designed as equal to the parent metal strength. No throat thickness calculation is required.

Partial Penetration Butt Welds

Weld Electrode Selection for UK Practice

For manual metal arc (MMA): E42 (S275), E46 (S355) per EN ISO 2560.

Design Resources

• UK Connection Design — General connection guidance
• UK Steel Properties — Material properties
• UK HSS Connections — Hollow section joints
• UK Steel Beam Sizes — Section data
• UK Bolt Capacity — Alternative connection methods
• All UK References

Frequently Asked Questions

How is fillet weld capacity calculated per EN 1993-1-8?

Two methods: simplified (Fw,Rd = fvw,d × a per unit length, fvw,d = fu/√3 / (βw × γM2)) and directional (stresses resolved on throat area). The simplified method is conservative and preferred for UK office use. UK NA specifies βw = 0.85 for S275, 0.90 for S355, γM2 = 1.25. For a 6 mm fillet weld on S355: fvw,d = 510/(√3 × 0.9 × 1.25) = 262 N/mm², capacity = 262 × 4.2 = 1100 N/mm.

What is the minimum fillet weld size in UK practice?

Minimum fillet weld throat thickness a = 3 mm per BS EN 1090-2. For thicker plates (> 20 mm), a minimum of 4-5 mm is typically specified. Minimum weld length = 40 mm or 6a (whichever greater). Maximum weld leg length should not exceed 0.7 × plate thickness (to avoid excessive heat input). The actual weld size is determined by the design force — UK fabricators prefer 6-8 mm leg length (4.2-5.6 mm throat) as the economic range for workshop welding.

When should the directional method be used instead of the simplified method?

The simplified method (Clause 4.5.3.2) is adequate for most UK design office applications and is more conservative. The directional (resolved stress) method (Clause 4.5.3.3) should be used when: (a) the weld is subject to complex multi-axial stress, (b) the weld orientation relative to the load direction significantly affects capacity (a transversely loaded fillet weld has ~25% higher capacity than a longitudinally loaded weld), or (c) the connection is highly stressed and the simplified method is too conservative.

What is the difference between full and partial penetration butt welds?

Full penetration butt welds (Clause 4.7) develop the full strength of the parent metal and do not require capacity checks beyond NDT verification. Partial penetration butt welds (Clause 4.2) are designed as fillet welds using the effective throat (depth of penetration). Full penetration is required for: tension members, moment connections, and fatigue-loaded details. Partial penetration is acceptable for: compression splices, shear connections, and non-structural attachments.

How are long fillet welds designed per EN 1993-1-8?

For fillet welds longer than 150a (where a = throat thickness), the stress distribution becomes non-uniform due to the flexibility of the connected parts. The effective length is reduced by factor βLw = 1.2 - 0.2(Lw/150a) but βLw ≥ 0.6. For example, a 900 mm long fillet weld with a = 6 mm: Lw = 900 > 150 × 6 = 900, so βLw = 0.6. This means only 60% of the weld length is considered effective. This is particularly relevant for long stiffener welds in plate girders.

Reference only. Verify all values against the current edition of EN 1993-1-8:2005 Clause 4 and UK NA. This information does not constitute professional engineering advice.