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.

UK Brace Connection Design -- EN 1993-1-8 CHS and RHS Bracing with UK National Annex

Q: 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.

Brace connections transfer axial forces from the bracing members into the main frame, forming the lateral stability system in braced steel frames. The connection must develop the full design force of the brace while accommodating the geometric constraints of intersecting members at the beam-column joint. EN 1993-1-8 provides specific design rules for brace-to-gusset and direct brace-to-frame connections, and the UK National Annex confirms these provisions for UK practice. This reference covers the complete design methodology for circular hollow section (CHS) and rectangular hollow section (RHS) brace connections, including slotted gusset plate details, weld design per Clause 4.5, block shear per Clause 3.10.2, and worked examples for tension and compression braces in UK frames.

Brace Connection Typology for UK Steel Frames

UK steel frames employ four principal brace connection types, selected primarily by brace section type and force magnitude:

Slotted Gusset Connection: The most common detail for CHS bracing in the UK. A flat plate is welded to the beam-column joint, and the CHS brace is longitudinally slotted to receive the plate. Fillet welds on both sides of the gusset, within the slot, transfer the brace force. This detail is preferred for its simplicity, construction tolerance, and the clear load path from the CHS wall through the welds into the gusset.

Gusset Plate with Bolted Lap: An RHS or angle brace is bolted to a gusset plate that is shop-welded to the beam-column joint. This detail is common for RHS bracing and allows the brace connection to be made on site by bolting, eliminating site welding. The gusset plate may require stiffening for compression braces.

Direct Welded Connection: Common in heavy bracing and lattice girders, this detail eliminates the gusset plate entirely by welding the brace directly to the main member. For CHS-to-CHS connections, EN 1993-1-8 Clause 7 provides the joint strength formulae. For CHS-to-UC or CHS-to-UB connections, the brace is profiled to fit the flange or web of the main member and welded with a full-penetration butt weld or reinforced fillet.

Cleat Connection: Light bracing (typically angle or channel sections) connected to the main frame through bolted cleats. These connections are designed as simple pin-ended connections and are common for secondary bracing in purlin and rail systems.

Slotted Gusset Connection for CHS Bracing -- Design Procedure

The slotted gusset connection is the standard UK detail for CHS brace members. The design procedure involves the following checks:

Step 1: Brace Member Resistance

The brace section must be verified for the design axial force (tension or compression):

For tension: Nt,Rd = min(A x fy / gamma_M0, 0.9 x Anet x fu / gamma_M2)

For compression: Nb,Rd = chi x A x fy / gamma_M1

The net section check for tension includes the deduction for the slot, which removes material from the CHS wall on both sides. The net area Anet = A - 2 x t_slot x t_wall, where t_slot is the slot width and t_wall is the CHS wall thickness.

Step 2: Weld Design -- Brace to Gusset

Fillet welds on both sides of the gusset plate, within the slot, transfer the brace force by longitudinal shear. The required weld length L_w is:

L_w = N_Ed / (2 x Fw,Rd)

Where Fw,Rd per unit length per fillet weld is:

Fw,Rd = a x fu / (sqrt(3) x beta_w x gamma_M2)

For S355 parent material (fu = 470 MPa) and a 6 mm fillet weld (throat a = 4.2 mm): Fw,Rd = 4.2 x 470 / (1.732 x 0.85 x 1.25) = 1.07 kN/mm per weld

For a design brace force N_Ed = 350 kN: L_w = 350 / (2 x 1.07) = 164 mm.

Return welds of length 2a at each end of the slot provide end anchorage and improve the weld group ductility.

Step 3: Gusset Plate Design

The gusset plate is checked for:

Tension yield across the gross section at the narrowest point
Tension rupture across the net section at the bolt group (for bolted gusset to frame connections)
Buckling for compression braces -- the gusset plate unsupported length must be limited
Block shear at the bolted connection to the beam-column interface

For compression braces, the Whitmore section method provides a rational estimate of the effective gusset width for buckling checks. The effective width extends at 30 degrees from the last row of connection fasteners to the gusset tip. The UK NA does not specifically mandate the Whitmore method but it is widely used in UK design practice for gusset plate buckling.

Step 4: Gusset-to-Frame Connection

The gusset plate connection to the beam-column joint must transfer the brace force into the main frame. Two configurations are common:

Shop-welded gusset: The gusset is welded to the beam and column flanges/webs, typically with fillet welds. For CHS braces in tension, the weld connecting the gusset to the frame must develop the full brace design force.

Site-bolted gusset: The gusset is bolted to a cleat or bracket that is shop-welded to the beam-column joint. This provides erection tolerance and avoids site welding but introduces additional bolt checks.

Worked Example -- CHS Brace in Tension

Given:

Brace: CHS 168.3 x 8.0, S355J2H (EN 10210-1 hot-finished)
A = 4030 mm^2, d = 168.3 mm, t = 8.0 mm
Design tension force: N_Ed = 450 kN
Gusset plate: 250 x 16 mm, S355J2
Bolted to beam-column cleat with 4 x M20 Class 8.8 bolts

Brace Tension Resistance: N_pl,Rd = 4030 x 355 / 1.0 = 1431 kN >> 450 kN. OK.

Net area accounting for slot (slot width = 16.5 mm, gusset 16 mm + 0.5 mm tolerance): Anet = 4030 - 2 x 16.5 x 8.0 = 4030 - 264 = 3766 mm^2 N_u,Rd = 0.9 x 3766 x 470 / 1.25 = 1273 kN >> 450 kN. OK.

Weld Design: Fillet welds both sides of gusset, 6 mm leg (throat a = 4.2 mm): Fw,Rd = 4.2 x 470 / (1.732 x 0.85 x 1.25) = 1.07 kN/mm per weld Required weld length: 450 / (2 x 1.07) = 211 mm. Use 220 mm slot length, providing approximately 5% reserve.

Gusset Plate Tension: Gross section: N_pl,Rd_gusset = 250 x 16 x 355 / 1.0 = 1420 kN >> 450 kN. OK. Net section at bolt holes (4 holes, d0 = 22 mm): Anet = 16 x (250 - 4 x 22) = 16 x 162 = 2592 mm^2 N_u,Rd_gusset = 0.9 x 2592 x 470 / 1.25 = 877 kN > 450 kN. OK.

Bolt Group Check (gusset to cleat): Shear per bolt (4 bolts): Fv,Ed = 450 / 4 = 112.5 kN Fv,Rd per M20 8.8 (single shear, threads in plane) = 94.1 kN 112.5 kN > 94.1 kN -- NOT OK.

The bolt shear governs. Options:

Increase to 6 M20 bolts: Fv,Ed = 450 / 6 = 75.0 kN < 94.1 kN. OK.
Use M24 bolts: Fv,Rd = 135.6 kN > 112.5 kN. OK.

Select: 6 M20 Class 8.8 bolts in double column of 3 rows.

Block Shear (gusset): Check block shear failure path around the 6-bolt group. For a symmetric bolt group with the tension face at 45 degrees from the load direction, Veff,Rd is typically adequate for well-proportioned gusset plates. Detailed calculation per Clause 3.10.2 is project-specific.

UK National Annex Provisions for Brace Connections

The UK NA to BS EN 1993-1-8 does not specifically modify the brace connection design rules. The standard provisions for fillet welds (Clause 4.5), block shear (Clause 3.10.2), and CHS joint strength (Clause 7) apply without UK-specific amendments. The UK NA confirms:

gamma_M2 = 1.25 for weld design and block shear rupture checks.
beta_w = 0.85 for S355 steel (correlation factor for fillet weld design).
For fillet welds in CHS connections, the directional method per Clause 4.5.3 is recommended rather than the simplified method, because the brace-to-gusset weld is loaded in pure longitudinal shear and the simplified method is conservative for this case.
The UK NA references SCI P358 for standardised brace connection details, including recommended gusset plate thicknesses, bolt configurations, and slot dimensions for common CHS sizes.

Compression Brace Considerations

For compression braces, the design must additionally consider:

Gusset plate buckling: The unsupported edge of the gusset plate, from the last row of bolts to the Whitmore section, must be checked for buckling. The effective length is typically taken as the free edge length, with buckling curve c (alpha = 0.49) per EN 1993-1-1.
Brace buckling interaction: The slotted connection reduces the effective section of the brace at the slot, potentially creating a weaker zone that may govern the compression resistance.
Second-order effects: For slender braces, the P-delta moment at the connection due to brace curvature under compression must be considered. The moment is typically small for braces with L/r < 150.

Design Resources

UK Steel Grades Reference -- EN 10025-2 grade selection
UK Bolt Capacity Tables -- Fv,Rd and Ft,Rd
UK Connection Design Guide -- EN 1993-1-8 overview
UK Column Buckling Reference -- Compression member design
All UK Steel Design References -- complete library

Frequently Asked Questions

What is the standard UK detail for a CHS brace connection?

The slotted gusset plate connection is the standard UK detail for CHS bracing. The gusset plate (typically 12-20 mm thick in S355) is shop-welded to the beam-column joint, and the CHS brace is longitudinally slotted to accept the plate. Fillet welds on both sides of the gusset within the slot transfer the brace force. The slot width is the gusset plate thickness plus 0.5 mm fabrication tolerance. This detail is standardised in SCI P358.

How is the weld length determined for a slotted CHS brace connection?

The required fillet weld length per side L_w = N_Ed / (2 x Fw,Rd), where Fw,Rd is the design fillet weld resistance per unit length per EN 1993-1-8 Clause 4.5.3. For a 6 mm fillet in S355: Fw,Rd = a x fu / (sqrt(3) x beta_w x gamma_M2) = 4.2 x 470 / (1.732 x 0.85 x 1.25) = 1.07 kN/mm. The slot length = L_w + 2 x return weld allowance (2a each end). For a 450 kN brace: L_w = 450/(2 x 1.07) = 211 mm, slot length = approximately 220 mm.

Does the UK NA modify the brace connection design rules in EN 1993-1-8?

The UK NA to BS EN 1993-1-8 does not modify the brace connection design rules. The standard provisions for welds (Clause 4.5), bolts (Clause 3.6), and block shear (Clause 3.10.2) apply without amendment. The UK NA confirms gamma_M2 = 1.25 and references SCI P358 for standardised UK bracing connection details. The UK NA also references BS EN 1090-2 for the execution requirements specific to brace connections, including permissible slot straightness tolerances and weld access requirements.

When is a gusset plate stiffener required for a compression brace?

A gusset plate stiffener is required when the gusset plate free edge length (from the last bolt to the Whitmore section tip) exceeds approximately 8 times the gusset plate thickness. For a 12 mm gusset, this equates to a free edge of 96 mm. The stiffener prevents local buckling of the gusset plate under compression. In UK practice, stiffeners are typically provided as triangular plates welded perpendicular to the gusset, or the gusset thickness is increased to eliminate the need for stiffening. SCI P358 provides specific geometry limits.

Educational reference only. All design values are per BS EN 1993-1-8:2005 + 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.