UK Gusset Plate Design — EN 1993-1-8 Bracing Connection Reference

Design of gusset plates for bracing connections per EN 1993-1-8 with UK National Annex. Covers the Whitmore effective width method for tension yield, block shear (tearing) per Clause 3.10.2, gusset plate buckling in compression, weld design for brace-to-gusset and gusset-to-beam connections, and a worked example for a UK S355 brace connection.

Quick access: UK Steel Grades | UK Steel Properties | UK Beam Sizes | UK Bolt Capacity | All UK References

Gusset Plate Design per EN 1993-1-8

Gusset plates transfer forces between bracing members and the main structural frame. The design must address:

Failure Mode Method EN Reference
Tension yielding (gross section) Whitmore effective width Clause 3.10
Tension rupture (net section) Net area × fu / γM2 Clause 3.10.1
Block shear (tearing) Veff,Rd Clause 3.10.2
Buckling (compression brace) Plate buckling model EN 1993-1-1 Cl. 6.3
Bolt bearing/tear-out Fb,Rd per bolt Clause 3.6
Weld (gusset to beam/column) Fw,Rd Clause 4.5

Whitmore Effective Width Method

The Whitmore effective width is the width at the centre of the gusset plate through which the brace force is distributed at 30° on each side from the first row of bolts:

W_eff = (n_bolts − 1) × p + 2 × L_30 × tan(30°)

Where:

The tension resistance of the gusset plate is then:

Nt,Rd = min(A × fy / γM0, 0.9 × Anet × fu / γM2)

Block Shear (Clause 3.10.2)

Block shear checks the tear-out of a block of plate material around the bolt group:

Veff,Rd = min(fu × Ant / γM2, fy × Anv / (√3 × γM0))

Where:

Gusset Plate Buckling (Compression)

For brace connections subject to compression, the gusset plate must be checked for buckling. The SCI model treats the gusset as a rectangular plate with effective length based on the free edge:

Lcr = β × L_free (where β depends on edge fixity)

The buckling resistance is:

Nb,Rd = χ × A × fy / γM1 (using buckling curve c for plates)

Worked Example — UK Gusset Plate

Connection details:

Step 1 — Gross Section Yield (Whitmore Method)

Bolt spacing: p1 = 70 mm, gauge = 60 mm Distance from first bolt row to critical section: L_30 = 50 mm (to the gusset edge)

W_eff = (2 − 1) × 70 + 2 × 50 × tan(30°) = 70 + 57.7 = 127.7 mm

A_eff = 127.7 × 12 = 1,533 mm²

Nt,Rd = 1,533 × 355 / 1.0 = 544 kN — marginally exceeds 500 kN. Increase plate or reduce bolt spacing.

Step 2 — Net Section Rupture

Anet = A_eff − 2 × d0 × t = 1,533 − 2 × 22 × 12 = 1,533 − 528 = 1,005 mm²

Nu,Rd = 0.9 × 1,005 × 470 / 1.25 = 0.9 × 377,880 = 340 kN — governs!

The net section rupture resistance is only 340 kN, below the 500 kN design force. Options:

  1. Increase plate thickness to 16 mm: Anet = 127.7 × 16 − 528 = 2,043 − 528 = 1,515 mm² → Nu,Rd = 513 kN — OK
  2. Use staggered bolt pattern to reduce net section loss
  3. Use welded brace-to-gusset connection (no net section loss)

Step 3 — Weld Design (Gusset to Supporting Beam)

Fillet weld: 10 mm leg both sides, throat a = 7.1 mm Weld length: 300 mm

Fw,Rd = 7.1 × 470 / (1.732 × 0.85 × 1.25) = 1,813 N/mm = 1.81 kN/mm

Total weld resistance: 2 × 300 × 1.81 = 1,086 kN — OK (factor of 2 on 500 kN).

Design Conclusion

The 12 mm gusset plate fails in net section rupture. Increase to 16 mm plate or use welded brace connection to avoid net section issues. The bolts and weld are adequate.

Design Resources

Frequently Asked Questions

What is the Whitmore effective width for gusset plate design?

The Whitmore effective width distributes the brace force at 30° each side from the first bolt row: Weff = (n − 1) × p + 2 × L30 × tan30°, where n = bolts in first row, p = bolt pitch, L30 = distance to critical section. The resulting effective area (Weff × t) is used for tension yielding check. This method was developed by Whitmore in 1952 and is a standard approach in European and UK design practice for gusset plates.

How is gusset plate buckling checked for compression braces?

Gusset plate buckling is assessed using the plate buckling model per EN 1993-1-1 Clause 6.3. The effective length Lcr is based on the free edge length of the gusset plate, with a factor β depending on edge rotational restraint. For a gusset with both edges bolted/welded to beams and columns: β ≈ 0.7. For a gusset with one free edge: β ≈ 1.2. The buckling curve c (α = 0.49) is typically used for welded gusset plates. The SCI recommends Lcr ≤ 2 × t × √(E/fy) to prevent local buckling.

Does the UK NA modify gusset plate design rules?

The UK NA to BS EN 1993-1-8 does not specifically modify gusset plate design. The design follows the standard EN 1993-1-8 provisions for bolted (Clause 3.6) and welded (Clause 4.5) connections, combined with EN 1993-1-1 for the plate buckling check. The UK NA confirms γM0 = 1.00, γM1 = 1.00, and γM2 = 1.25. UK practice follows SCI P358 guidance for gusset plate detailing.

What is the minimum gusset plate thickness in UK practice?

The minimum gusset plate thickness in UK practice is typically 10 mm for light bracing and 12-15 mm for primary brace connections. The thickness must satisfy: (1) net section rupture (0.9 × Anet × fu/γM2 ≥ NEd), (2) bearing resistance (Fb,Rd per bolt), (3) buckling resistance for compression braces, and (4) welding requirements (minimum 6 mm fillet weld). For M20 bolts, a 12 mm plate is the minimum to develop the bolt bearing capacity fully.

Related Pages

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.