------------- | ------ | ------------------------------------------------------------------- | ----------------- | | Leg length | s | Length of weld leg measured from toe to root | 6.0 mm | | Throat thickness | a | Height of inscribed triangle, measured perpendicular to throat face | 4.2 mm | | Effective throat | a | Actual throat used for strength calculation | 4.2 mm | | Actual length | L | Overall weld length as specified | 200 mm | | Effective length | Leff | L - 2a (deduct end craters) | 192 mm |

Relationship: a = s / √2 = s × 0.707 (for a 90° fillet weld with equal legs)

Standard Fillet Weld Sizes

Capacity fw,Rd = (fu/√3)/(βw × γM2) × a. S275: fu = 430, βw = 0.85. S355: fu = 510, βw = 0.90. γM2 = 1.25.

Minimum Weld Sizes per BS EN 1090-2

Maximum Weld Sizes

The maximum fillet weld leg length is limited to prevent excessive heat input:

• Along a plate edge: s_max = t (the plate edge thickness)
• In a corner: s_max = 0.7t
• Single-run fillet weld: s_max ≈ 8 mm (approximately 1 pass)
• Multi-run fillet: s_max unlimited with adequate inter-pass temperature control

Effective Length Requirements

Weld Size Selection Guide — Typical UK Details

VT = Visual Testing, MT = Magnetic Particle Testing, UT = Ultrasonic Testing. Inspection levels per BS EN 1090-2.

Weld Symbol Conventions (BS EN 22553)

UK fabrication drawings use BS EN 22553 (equivalent to ISO 2553) weld symbols:

Example weld call-up: 8 (200) - 300 → 8 mm leg fillet, 200 mm long at 300 mm centres.

Worked Example — Determining Fillet Weld Size for a Beam Connection

Given:

• Beam: 406×178 UB 60 (S355), end plate connection
• Design shear: VEd = 300 kN
• Weld length available: 2 × 400 mm (two sides of web)
• Electrode: G 46 4 M21 3Si1

Step 1 — Required weld capacity per unit length: FEd = 300 × 10³ / (2 × 400) = 375 N/mm

Step 2 — Design strength (S355): fvw,d = 510 / (√3 × 0.90 × 1.25) = 261.7 N/mm²

Step 3 — Required throat thickness: a_req = FEd / fvw,d = 375 / 261.7 = 1.43 mm

Step 4 — Apply minimum size: For t = 10.2 mm (UB 60 web): minimum a = 3.0 mm Also minimum for structural welds: a_min = 3.0 mm

Step 5 — Select weld: Use a = 3.0 mm throat → leg = 4.2 mm. Specify 5 mm leg fillet weld.

Check capacity: Fw,Rd = 261.7 × 3.0 = 785 N/mm Total capacity = 785 × 2 × 400 × 10⁻³ = 628 kN > 300 kN — Satisfactory

Design Resources

• UK Weld Capacity — Full weld design guide
• UK Weld Electrodes — Electrode selection
• UK Connection Design — Connection types
• UK Steel Properties — Material data
• All UK References

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Frequently Asked Questions

What are the standard fillet weld sizes in UK fabrication?

The most common fillet weld sizes in UK structural steelwork are 6 mm leg (4.2 mm throat) for standard beam-to-column connections, 8 mm leg (5.7 mm throat) for heavier connections, and 10-12 mm leg (7.1-8.5 mm throat) for moment-resisting connections and splices. The minimum structural weld is 4 mm leg (2.8 mm throat) for secondary steel. The relationship between leg length (s) and throat (a) is a = s × 0.707 for a 90° fillet.

What is the minimum weld size for UK structural connections?

BS EN 1090-2 specifies minimum throat thickness based on the thicker part: 3 mm throat (4-5 mm leg) for plates 5-10 mm thick, 4 mm throat (6 mm leg) for 10-15 mm plate, and 5 mm throat (7-8 mm leg) for 15-30 mm plate. The minimum weld length is 40 mm or 6a (whichever greater). These minimums ensure adequate fusion and prevent rapid cooling that could cause hydrogen cracking.

How are intermittent fillet welds specified?

Intermittent fillet welds (stitch welds) are specified as: leg length × effective length — pitch, e.g., 6 (100) - 200 meaning 6 mm leg fillet, 100 mm weld length at 200 mm centres (100 mm weld, 100 mm gap). The minimum effective length of each intermittent segment is 40 mm or 6a. Intermittent welds are used for: (a) non-structural attachments, (b) built-up sections where continuous welds are not required, and (c) reducing distortion and fabrication cost.

What is the maximum effective length of a fillet weld?

For fillet welds longer than 150a (a = throat thickness), the effective length is reduced by βLw = 1.2 - 0.2(Lw/150a) with βLw ≥ 0.6. For example, a 4 mm throat weld (a = 4): 150a = 600 mm. A weld of 1200 mm length gives βLw = 1.2 - 0.2(1200/600) = 0.8, meaning 80% effective. This reduction accounts for the non-uniform stress distribution along the weld due to shear lag in the connected parts.

How are weld sizes determined for full-strength butt welds?

Full penetration butt welds are designed to develop the full strength of the parent metal — no throat calculation is needed. The weld is specified by the plate edge preparation: single-V, double-V, single-U, or double-U groove per BS EN ISO 9692-1. The groove angle (typically 45-60° for single-V) and root gap (2-4 mm) determine the weld volume. NDT inspection (UT or RT) is required for full-strength butt welds in tension or fatigue. Partial penetration butt welds use the effective throat method as for fillet welds.

Weld Design Methods

Fillet Weld Design

Fillet welds are the most common weld type in structural steel construction. The design strength is calculated based on the weld throat dimension and effective length.

For AISC 360 LRFD:

• φRn = φ × 0.60 × FEXX × (0.707w) × L × (1.0 + 0.50 sin¹·⁵θ)
• Where φ = 0.75, FEXX = electrode classification strength, w = weld leg size

For EN 1993-1-8:

• Fw,Rd = fu / √3 × a / (βw γM2)
• Where a = weld throat thickness, βw = correlation factor (0.80-1.0 depending on steel grade)

Design Procedure for Fillet Welds

1. Determine the required weld size from the applied load
2. Select the appropriate electrode (E70XX for steels with Fu ≤ 480 MPa, E80XX for higher strength)
3. Calculate the weld capacity per unit length
4. Determine the required weld length
5. Check minimum and maximum weld size limitations
6. Verify weld termination details (return welds, end returns)

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Frequently Asked Questions

What is the recommended design procedure for this structural element?

The standard design procedure follows: (1) establish design criteria including applicable code, material grade, and loading; (2) determine loads and applicable load combinations; (3) analyse the structure for internal forces; (4) check member strength for all applicable limit states; (5) verify serviceability requirements; and (6) detail connections. Computer analysis is recommended for complex structures, but hand calculations should be used for verification of critical elements.

How do different design codes compare for this calculation?

AISC 360 (US), EN 1993 (Eurocode), AS 4100 (Australia), and CSA S16 (Canada) follow similar limit states design philosophy but differ in specific resistance factors, slenderness limits, and partial safety factors. Generally, EN 1993 uses partial factors on both load and resistance sides (γM0 = 1.0, γM1 = 1.0, γM2 = 1.25), while AISC 360 uses a single resistance factor (φ). Engineers should verify which code is adopted in their jurisdiction.

Reference only. Verify all values against the current edition of BS EN 1090-2 and EN 1993-1-8. This information does not constitute professional engineering advice.