Weld Design Methods — EN 1993-1-8 Clause 4.5

Two methods for fillet weld design are permitted, both adopted by the UK NA:

Electrode Selection for UK Steelwork

Per BS EN ISO 2560 and UK practice:

UK NA rule: For S355 steel, E42 electrodes are standard for fillet welds up to 8 mm leg length. E50 electrodes are used when: (1) higher weld strength is needed to match the parent metal; (2) fatigue loading governs; (3) the weld is subject to impact at low temperatures (Charpy requirement). The UK NA adopts the EN 1993-1-8 recommended correlation factor beta_w = 0.85 for S235/S275/S355 and beta_w = 0.90 for S420/S460.

Weld Sizing Rules — UK Practice and SCI P358

Minimum and maximum fillet weld sizes for UK steelwork:

UK practice rules:

• For fin plate connections (SCI P358): weld leg = 0.5 x plate thickness (minimum), 0.6 x t (standard)
• For partial-depth end plates: weld leg = 6 mm standard for plates <= 12 mm
• For column base plates (pinned): 6 mm CFW all around
• For full-strength butt welds: full-penetration with run-on/run-off tabs

Directional Method — Worked Example

Calculate the design resistance of a 6 mm fillet weld between a 10 mm S275 fin plate and an S355 column flange, for a 200 mm weld length, using E42 electrodes.

Weld throat: a = 0.7 x 6 = 4.2 mm

Design shear strength of weld (directional method):

fw,d = fu / (sqrt(3) x beta_w x gamma_M2) fw,d = 410 / (1.732 x 0.85 x 1.25) = 223 MPa

(base metal governs: use fu of weaker connected part = 410 MPa for S275)

Longitudinal resistance:

Fw,L,Rd = fw,d x a = 223 x 4.2 = 936 N/mm per unit length

Total longitudinal resistance for 200 mm:

Fw,total,Rd = 936 x 200 = 187.2 kN

For shear on a fin plate connection with 2 weld lines (2 x 200 mm):

Total Vw,Rd = 2 x 187.2 = 374.4 kN

This exceeds the typical bolt group resistance for a standard fin plate, confirming that the weld is not governing in a well-proportioned UK simple connection.

Simplified Method Comparison

For comparison, the simplified method (Clause 4.5.3.3) gives:

Fw,Rd = fvw,d x a

Where fvw,d = fu / (sqrt(3) x beta_w x gamma_M2) = same 223 MPa

The simplified method gives identical results to the directional method for longitudinal fillet welds loaded in shear. The benefit of the directional method is when the weld carries combined transverse tension and longitudinal shear — it permits a more favourable interaction (1.22 x fvw,d for transverse loading) than the simplified scalar check.

Transverse Fillet Weld — Increased Resistance

Per Clause 4.5.3.2(6), the design resistance of a transverse fillet weld is:

Fw,T,Rd = 1.22 x fvw,d x a (for the directional method)

For the 6 mm leg weld: Fw,T,Rd = 1.22 x 223 x 4.2 = 1,142 N/mm (22 % increase over longitudinal loading).

This explains why end plate welds (primarily transverse) can be smaller than web welds (longitudinal) in typical UK beam-to-column connections.

Typical UK Connection Weld Sizes — SCI P358 Summary

Weld Execution Classes — BS EN 1090-2

UK structural steelwork is fabricated to BS EN 1090-2 execution classes:

Most UK building structures are EXC2. Specifying EXC3 or EXC4 significantly increases fabrication costs and is usually unnecessary for building frames.

Butt Weld Design — EN 1993-1-8 Clause 4.7

Butt welds transfer force directly through the full cross-section of the connected parts. Per Clause 4.7.1, the design resistance of a full-penetration butt weld is taken as equal to the design resistance of the weaker connected part, provided the weld metal has matching or overmatching mechanical properties (fy_weld >= fy_parent, fu_weld >= fu_parent).

Design resistance (full penetration):

sigma_w,Rd = fy / gamma_M0 (tension/compression perpendicular to weld axis) tau_w,Rd = fy / (sqrt(3) x gamma_M0) (shear)

For the UK, gamma_M0 = 1.00 (UK NA confirmed), so the butt weld effectively achieves 100% of the parent metal capacity.

Partial penetration butt welds:

Where the weld does not extend through the full thickness, Clause 4.7.2(2) requires the throat thickness to be used instead of the plate thickness. The design resistance is:

F_w,Rd = a x Leff x fu / (beta_w x gamma_M2)

Where a is the design throat thickness (penetration depth) and beta_w = 0.85 for S235/S275/S355 and 0.90 for S420/S460, as for fillet welds. The UK NA does not modify the beta_w correlation factors.

UK butt weld applications:

Cost note for UK practice: Full-penetration butt welds cost approximately 3-5 times more than fillet welds per linear metre due to weld preparation (bevel cutting or grinding), backing strip installation, multi-pass welding, inter-pass cleaning, and mandatory NDT. For building frame connections, fillet welds are the default; butt welds are reserved for moment-critical splices and connections where full continuity is essential.

Plug and Slot Weld Design — EN 1993-1-8 Clause 4.6

Plug welds (circular holes filled with weld metal) and slot welds (elongated holes filled with weld metal) are used primarily to transmit shear in lapped joints where access for fillet welding is limited, or to prevent buckling of lapped plates in compression. Per Clause 4.6.1:

Design shear resistance of a plug weld:

F_vw,Rd = f_vw,d x A_w

Where:

• f_vw,d = fu / (sqrt(3) x beta_w x gamma_M2) -- the same design shear strength as for fillet welds
• A_w = pi x d^2 / 4 -- the cross-sectional area of the plug (nominal hole diameter)
• beta_w = 0.85 for S235/S275/S355; 0.90 for S420/S460
• gamma_M2 = 1.25 (UK NA confirmed)

For a 20 mm diameter plug weld in S275 plate with E42 electrodes:

f_vw,d = 410 / (1.732 x 0.85 x 1.25) = 223 MPa (same as fillet weld shear strength) A_w = pi x 20^2 / 4 = 314.2 mm^2 F_vw,Rd = 223 x 314.2 / 1000 = 70.1 kN per plug

UK practical rules for plug welds (BS EN 1090-2 + UK practice):

UK applications: Plug welds are used in UK practice for: (1) attaching web stiffeners to beams where fillet weld access is obstructed by the flange; (2) connecting lapped plates in strengthening and remedial works where fillet welding at the plate edge is impractical; (3) preventing buckling of cover plates in compression members. Slot welds (elongated holes, typical 2d x d) provide higher shear capacity per weld and are preferred over plug welds for heavy connections where the plate thickness permits slot cutting.

Slot weld design capacity (slot 40 x 20 mm): A_w = 40 x 20 + pi x 10^2 = 800 + 314 = 1,114 mm^2 (rectangular area + two semi-circular ends) F_vw,Rd = 223 x 1,114 / 1000 = 248.4 kN

Slot welds are 3-4 times stronger than plug welds of the same nominal diameter because the slot length can be increased independently of the plate thickness. However, slot welds require plate slot cutting (laser or plasma), which adds fabrication cost.

Weld Sizes for Common UKB and UKC Sections

The following table provides standard fillet weld sizes for typical UK beam and column connections, based on SCI P358 (Simple Connections) and UK industry practice. All values assume S275 or S355 steel with E42 electrodes.

Beam Web Connections (Fin Plates, Partial-Depth End Plates)

Column Connections (End Plates, Column Splices, Base Plates)

Weld size rules: Web weld leg = 0.5 x web thickness (min 4 mm, max 6-8 mm). Flange weld leg = 0.5 x flange thickness (min 6 mm). For S355 with E50 electrodes, welds are designed per the higher electrode strength; sizes may reduce slightly but 6 mm remains the practical minimum.

For hollow section columns (SHS/RHS/CHS): Fillet welds to hollow sections follow the same sizing rules but the connected plate thickness governs (not the hollow section wall thickness). A 10 mm end plate on a 200 x 200 x 10 SHS column takes a 6 mm fillet weld (0.5 x 10 = 5 mm, rounded to 6 mm minimum).

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Frequently Asked Questions

What is the minimum fillet weld size for UK steel building frames?

Per BS EN 1993-1-8 Clause 4.5.2(2), the minimum throat thickness is 3 mm for t <= 15 mm. In practice, UK fabricators can reliably deposit a 4 mm fillet weld, and 6 mm is standard for structural connections. Welds smaller than 6 mm are avoided for primary members due to: (1) difficulty of achieving consistent 3-4 mm throat thickness in production; (2) sensitivity to fit-up gaps (EN 1090-2 limits gaps); (3) inspection requirements (visual inspection is less reliable for very small welds).

When should I use full-penetration butt welds instead of fillet welds in UK design?

Full-penetration (FP) butt welds are used when: (1) full continuity of the section is required (splices, moment connections, column splices); (2) the connection must develop the full strength of the parent metal; (3) fatigue resistance requires Category 71 or higher (fillet welds are Category 36-56); (4) the member is subject to direct tension across the weld (fillet welds in tension have additional reduction per Clause 4.5.3.2(5)). FP welds require weld preparation, backing, and NDT — they cost 3-5 times more than fillet welds per linear metre.

How does the UK NA modify EN 1993-1-8 weld design?

The UK NA does not modify the EN 1993-1-8 weld design rules significantly. The main UK-specific aspects are: (1) adoption of BS EN 1090-2 for execution, which mandates UK welding coordination per BS EN ISO 14731; (2) UK practice favours E42 electrodes for S355 fillet welds (some EU NAs require E50 matching electrodes); (3) UK inspectors typically require MPI on butt welds at 10 % sampling rate for EXC2, which is at the upper end of the EN 1090-2 range.

What is the interaction between fillet welds and parent metal in UK design?

When a fillet weld is loaded, the failure can occur in the weld metal or the parent metal heat-affected zone (HAZ). EN 1993-1-8 Clause 4.5.3.2(5) requires the plate to be checked for tensile rupture at the weld root. For fillet welds loaded in transverse tension, the plate capacity may govern over the weld. In UK practice, this is automatically satisfied when the weld is at least 0.5 x the connected plate thickness and the plate is checked for tension (Clause 6.2.3).

Related Pages

• UK Weld Capacity Calculator — EN 1993-1-8 weld resistances
• UK Weld Size Tables — Standard fillet weld sizes
• UK Weld Electrodes — E42, E50 electrode selection
• UK Welding Procedure Guide — WPS and welder qualification
• UK Weld Inspection — NDT and visual inspection
• UK Weld Symbols Guide — BS EN 22553 symbols
• UK Connection Design Guide — EN 1993-1-8 bolted and welded joints

Educational reference only. All design values are per BS EN 1993-1-8:2005 + UK NA, BS EN 1090-2:2018, BS EN ISO 2560, and SCI P358. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.