UK Brace Connection Design — EN 1993-1-8 CHS and RHS Bracing Reference
Design of brace connections for UK structural steel frames per EN 1993-1-8 with UK National Annex. Covers CHS (circular hollow section) and RHS (rectangular hollow section) brace connections to gusset plates, slotted connections, weld design, block shear checks, and a worked example for a UK bracing member in S355.
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Brace Connection Types in UK Steel Frames
| Type | Description | Typical UK Application |
|---|---|---|
| Gusset plate connection | Brace bolted or welded to a steel plate gusset, which is welded to the beam/column | Concentric braced frames (CBF), chevron bracing |
| Slotted gusset | Brace slotted to receive a gusset plate; welded on both sides | CHS braces — the most common UK detail for tubular bracing |
| Direct welded | Brace welded directly to another section (CHS-to-CHS, CHS-to-UC) | Heavy bracing, lattice girders |
| Cleat connection | Angles or channels connecting brace to frame | Light bracing, purlins, secondary members |
Slotted Gusset Connection for CHS Bracing
For CHS brace connections, the most common UK detail is the slotted gusset:
┌──────────────────┐ (beam/girder)
│ │
│ Gusset │ (welded gusset plate)
│ ────────── │
│ │ │
│ CHS │ │ (brace slotted over gusset)
│ │ │
│◄────► │ │
│ Weld │ │
Design Checks
| Check | Component | Method |
|---|---|---|
| Tension resistance | Brace (net section) | Nt,Rd = 0.9 × Anet × fu / γM2 |
| Shear resistance | Brace (gross) | Vpl,Rd per EN 1993-1-1 |
| Weld resistance | Brace-to-gusset | Fw,Rd per EN 1993-1-8 Clause 4.5 |
| Block shear | Gusset plate | Veff,Rd per Clause 3.10.2 |
| Gusset tension | Whitmore method | As per gusset plate section |
Weld Design for Slotted Connections
For a CHS brace slotted over a gusset plate, the weld is typically:
- Fillet weld on both sides of the gusset, along the slot length
- Return welds at the ends of the slot (typically 2 × weld size)
- The weld must develop the brace capacity (full-strength or design force)
Fw,Rd (fillet) = a × fu / (√3 × βw × γM2) per unit length
Worked Example — CHS 168×8 Brace Connection
Brace:
- CHS 168.3 × 8, S355
- Area: A = 4,030 mm²
- NEd = 550 kN (tension)
- Brace yield: Npl,Rd = 4,030 × 355 / 1.0 = 1,431 kN
- Brace rupture: Nu,Rd = 0.9 × (4,030 − 2×22×8) × 470/1.25 = 0.9 × 3,678 × 376 = 1,245 kN
Gusset Plate
Gusset: 250 × 16 mm, S355 (enhanced from 12 mm due to net section concerns) Slotted into CHS 168×8, slot width = 16.5 mm (gusset + 0.5 mm tolerance)
Weld Design
Fillet weld on both sides of gusset, along the CHS:
- Weld leg: 8 mm, throat a = 5.6 mm
- Fw,Rd = 5.6 × 470 / (1.732 × 0.85 × 1.25) = 1.43 kN/mm per weld
Welded length required per side: 550 / (2 × 1.43) = 192 mm
Slot length = welded length + 2 × return weld allowance = 200 mm minimum.
Weld Orientation Factor
For fillet welds in slotted connections, the weld is loaded parallel to its axis (longitudinal loading). The directional method per EN 1993-1-8 Clause 4.5.3 gives:
- Longitudinal shear: τ_parallel = Fw,Ed / (a × L_weld)
- Check: τ_parallel / (fu / (√3 × βw × γM2)) ≤ 1.0
Block Shear Check (Gusset Plate)
Check block shear at the gusset bolted connection to the beam/column:
- As per the gusset plate section above
Conclusion
The CHS 168×8 brace connection with a 16 mm gusset plate, 8 mm fillet welds, and a slot length of 200 mm is adequate for NEd = 550 kN. The brace itself has significant reserve capacity (1,431 kN yield). The connection design force is limited by the gusset plate or bolts rather than the brace.
Design Resources
- UK Steel Grades Reference — EN 10025-2 grade selection for UK projects
- UK Steel Mechanical Properties — fy, fu, and elongation tables
- UK Universal Beam and Column Sizes — UB/UC section dimensions and properties
- UK Bolt Capacity Tables — Class 8.8 and 10.9 bolt resistance
- UK Beam Design Guide — EN 1993-1-1 flexure, shear, and LTB
- UK Connection Design Guide — EN 1993-1-8 bolted and welded joints
- All UK Steel Design References — complete library
Frequently Asked Questions
What is the most common UK detail for CHS brace connections?
The most common UK detail for CHS brace connections is the slotted gusset connection. The gusset plate is welded to the beam-column joint, and the CHS brace is slotted longitudinally to receive the gusset plate. Fillet welds on both sides of the gusset within the slot transfer the brace force. This detail is preferred because it is simple to fabricate, provides good load transfer, and allows for standard NDT access. The slot is typically 16.5 mm wide for a 16 mm gusset plate.
How is the weld designed for a slotted CHS connection?
The fillet weld in a slotted CHS connection is designed using the directional method per EN 1993-1-8 Clause 4.5.3. The weld is loaded longitudinally (parallel to the weld axis). The design resistance per unit length is: Fw,Rd = a × fu/(√3 × βw × γM2). For a 8 mm fillet (throat a = 5.6 mm) in S355: Fw,Rd = 1.43 kN/mm per weld. The required weld length = NEd / (2 × Fw,Rd). Return welds at the slot end (typically 2a each side) are required for end shear transfer.
What is the minimum gusset plate thickness for a brace connection?
The minimum gusset plate thickness is determined by: (1) net section rupture at the bolt group, (2) bearing resistance under the bolts, (3) buckling for compression braces, and (4) slot width requirements for CHS connections. For M20 bolts, 12 mm is the practical minimum. For CHS connections, the gusset plate thickness should not exceed approximately 0.5 × CHS wall thickness to avoid excessive slot width. For the CHS 168×8 example above, t_gusset = 16 mm ≤ 0.5 × 8 = 4 mm? No — t_gusset should be similar to or slightly more than the CHS wall thickness. The wh:ole section needs careful consideration. For a CHS 168×8 with t_wall = 8 mm, the gusset should be 8-12 mm.
Does the UK NA modify brace connection design provisions?
The UK NA to BS EN 1993-1-8 does not specifically modify brace connection design rules. The standard EN 1993-1-8 provisions for fillet welds (Clause 4.5), block shear (Clause 3.10.2), and CHS joint design (Clause 7) apply without modification. The UK NA confirms γM2 = 1.25 for welds and bolts. UK practice follows SCI P358 for brace connection design and detailing.
Related Pages
- EN 1993 Steel Design Overview
- European Steel Properties
- EN 1993 Beam Design Guide
- EN 1993 Column Buckling
- EN 1990 Load Combinations
- UK Steel Chemical Composition
- UK Steel Charpy Values
Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK National Annex and BS EN 10025-2:2019. 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.