UK Connection Design — EN 1993-1-8 UK NA Guide
This reference covers connection design for UK steel design per EN 1993-1-8:2005 and UK NA. Connection design is one of the most detailed aspects of UK structural steelwork, with specific rules for bolt groups, welds, stiffeners, and rotational capacity.
Design requirements, worked examples, and practical design guidance are provided for common design office applications.
Code Reference: EN 1993-1-8:2005 and UK NA
Connection Types in UK Practice
| Connection Type | Typical Application | Moment Capacity | Rotation Capacity | UK Standard Detail |
|---|---|---|---|---|
| Flexible end plate | Beam-to-beam, beam-to-column | ~10-30% of beam | Full (ductile) | SCI P358 |
| Fin plate | Beam-to-column | — | Full (ductile) | SCI P358 |
| Partial-depth end plate | Beam-to-column | ~30-60% of beam | Moderate | SCI P358 |
| Full-depth end plate | Moment connection | ≥ 50% of beam | Limited | SCI P398 |
| Splice (cover plate) | Column/beam continuity | Full (by design) | Limited | SCI P398 |
| Bolted bracing | Bracing to gusset | — | — | SCI P358 |
| Welded bracing | Bracing to gusset | — | — | SCI P358 |
Partial Factors per UK NA
- γM0 = 1.00 — Cross-section resistance (matches recommended)
- γM1 = 1.00 — Member buckling resistance
- γM2 = 1.25 — Bolt, weld, and plate in tension resistance
- γM3 = 1.25 — Slip resistance at ULS
- γM3,ser = 1.10 — Slip resistance at SLS
Flexible End Plate Connection Design
Components to check:
- Bolts in shear and tension (EN 1993-1-8 Clause 3.4)
- End plate in bending (Clause 6.2.4 — T-stub model)
- Beam web in shear (Clause 6.2.6)
- Welds between beam web and end plate (Clause 4.5)
- Column web in bearing and buckling (Clause 6.2.6)
Typical design capacities — end plate connections (Grade 8.8 bolts, S275 plate, S355 beam):
| Beam Section | Bolt Configuration | Shear Capacity (kN) | Tension Capacity (kN) | Plate Size (mm) |
|---|---|---|---|---|
| 305×165 UB 40 | 2 rows M20 (4 bolts) | 300 | 200 | 200×200×10 |
| 406×178 UB 60 | 3 rows M20 (6 bolts) | 450 | 350 | 250×200×10 |
| 457×191 UB 89 | 3 rows M20 (6 bolts) | 550 | 400 | 280×220×12 |
| 533×210 UB 92 | 4 rows M20 (8 bolts) | 600 | 500 | 300×220×12 |
| 610×229 UB 125 | 4 rows M24 (8 bolts) | 750 | 650 | 320×240×15 |
Capacities are approximate and depend on exact geometry, edge distances, and stiffener presence. Based on UK standard details per SCI P358.
Fin Plate Connection Design
Components to check:
- Bolts in shear and bearing (EN 1993-1-8 Clause 3.6)
- Fin plate in shear, bending, and block tearing
- Beam web in bearing and block tearing
- Welds connecting fin plate to column
- Column web in bearing and local buckling
Typical fin plate capacities:
| Beam Section | Bolt Config | Plate (mm) | Welds | Shear Capacity (kN) |
|---|---|---|---|---|
| 305×165 UB 40 | 3 M20 | 180×100×10 | 8mm FW | 300 |
| 406×178 UB 60 | 4 M20 | 220×120×10 | 8mm FW | 400 |
| 457×191 UB 89 | 5 M20 | 260×120×12 | 10mm FW | 500 |
| 533×210 UB 92 | 5 M20 | 280×120×12 | 10mm FW | 550 |
Worked Example — Flexible End Plate Connection
Given:
- Beam: 406×178 UB 60 in S355
- Reaction at support: VEd = 300 kN
- Bolt: 6 × M20 Grade 8.8 (3 rows of 2)
- End plate: 250 × 200 × 10 mm, S275
- Column: 203×203 UC 46 in S355
- γM2 = 1.25, γM0 = 1.00
Step 1 — Bolt group shear check: Bolts in vertical shear with eccentricity eg (distance from bolt group centre to weld line, approximately 50 mm): For simplicity, assume equal distribution (torsional effect checked separately): Fv,Ed per bolt = 300/6 = 50 kN
Bolt shear capacity (M20 8.8, thread in shear plane): Fv,Rd = 0.6 × 800 × 245 / 1.25 × 10⁻³ = 94.1 kN per bolt UT = 50/94.1 = 0.53 — Satisfactory
Step 2 — End plate in bending (T-stub per row): Row 1 (top row) is most heavily loaded in tension due to prying forces. End plate thickness = 10 mm, S275 (fy = 275 N/mm²)
T-stub effective length per bolt row (end plate in bending): leff = min(2πm, 4m + 1.25e) where m is the distance from bolt centreline to weld toe
Mode 1 (complete flange yielding): FT,1,Rd = 4 Mpl,1,Rd / m = 4 × (leff × 10²/6 × 275/1.0) / (40) × 10⁻³ ≈ 80 kN per bolt (simplified)
Combined with the shear check above, the connection capacity is governed by the weld group.
Step 3 — Weld between beam web and end plate: Web thickness tw = 7.9 mm, use 6 mm fillet weld (throat a = 4.2 mm), S355
Fillet weld capacity per unit length (simplified method): fvw,d = fu/√3 / (βw γM2) = 510/(√3 × 0.9 × 1.25) = 262 N/mm² Fw,Rd = fvw,d × a = 262 × 4.2 = 1100 N/mm
Weld length available = 2 × 406.4 mm = 813 mm Total weld capacity: 1100 × 813 × 10⁻³ = 894 kN > 300 kN — Satisfactory
Step 4 — Column web bearing: Column web thickness twc = 9.0 mm (203×203 UC 46) Check web bearing: Fb,Rd = (k1 × αb × fu × d × t) / γM2 Severe tension from the top bolt rows may require stiffening.
Moment-Resisting Connections (SCI P398)
For moment connections (full-depth end plate), the connection must transfer:
- Bending moment (MEd) through tension in top bolt rows and compression in bottom flange
- Shear (VEd) through the bolt group
- Combined effects: MEd = Ft,Ed × lever arm
Typical moment capacities — full depth end plates (M20 Grade 8.8, S355 beam, S275 plate):
| Beam Section | Bolts per Flange | Plate Thickness (mm) | Mc,Rd (kNm) | Stiffeners? |
|---|---|---|---|---|
| 406×178 UB 60 | 4 M20 | 15 | 450 | No |
| 457×191 UB 89 | 6 M20 | 20 | 650 | No |
| 533×210 UB 92 | 6 M20 | 20 | 800 | Yes |
| 610×229 UB 125 | 8 M20 | 25 | 1200 | Yes |
| 610×305 UB 149 | 8 M24 | 25 | 1500 | Yes |
Design Resources
- UK Bolt Capacity — Bolt shear and tension data
- UK Weld Capacity — Fillet weld design
- UK Bolt Grades — Bolt materials
- UK Beam Design — Beam section data
- UK Column Design — Column interaction checks
- UK Steel Beam Sizes — Section dimensions
- All UK References
Frequently Asked Questions
What partial factors apply per UK NA to EN 1993-1-8?
UK NA specifies: γM2 = 1.25 (bolt resistance), γM3 = 1.25 (slip resistance at ULS), γM3,ser = 1.10 (slip resistance at SLS). These match the recommended Eurocode values. γM0 = 1.00 applies to cross-section resistance (end plate, fin plate, structural tee). The UK NA does not modify any of these partial factors from the recommended values.
How does UK NA modify weld design per EN 1993-1-8?
UK NA does not modify the directional method (simplified) or the resolved stress method (exact) for fillet weld design. γM2 = 1.25 applies. Fillet weld throat thickness a ≥ 3 mm minimum per BS EN 1090-2. The correlation factor βw depends on steel grade: βw = 0.80 (S235), 0.85 (S275), 0.90 (S355), 1.00 (S460). UK practice typically uses the simplified method for most connections.
When are stiffeners required on column webs?
Column web stiffeners are required when: (a) the column flange is too thin to resist the tension from beam flange forces without excessive deformation, (b) the column web is too slender to resist compression from the opposite beam flange (web buckling), or (c) for moment connections with high bolt tension forces where the column web yielding or crippling resistance is exceeded. The check is per EN 1993-1-8 Clause 6.2.6.2 (transverse compression) and Clause 6.2.6.3 (transverse tension).
What is the difference between simple and continuous connections in UK practice?
Simple connections (flexible end plate, fin plate) are designed to transfer shear only, with negligible moment resistance. They allow end rotation of the beam (pinned assumption). Continuous connections (full-depth end plate, haunched) are designed to transfer both moment and shear, providing continuity at the joint. The SCI "Green Books" (P358 for simple, P398 for moment) are the standard UK references. Simple connections account for approximately 80% of all beam-to-column connections in UK multi-storey buildings.
How is block tearing checked in UK connection design?
Block tearing (block shear) per EN 1993-1-8 Clause 3.6.2 checks a block of material tearing out from the end of a member. The design resistance is: Veff,2,Rd = (0.5 fu Ant) / γM2 + (fy Anv / √3) / γM0 for concentric loading Veff,1,Rd = (fu Ant) / γM2 + (fy Anv / √3) / γM0 for eccentric loading Block tearing often governs for fin plate connections with thin beam webs and short end distances. UK practice checks both tension and shear rupture paths on the net section.
Reference only. Verify all values against the current edition of EN 1993-1-8:2005 and UK NA. This information does not constitute professional engineering advice.