EN 1993 Gusset Plate Design — Brace Connection per Eurocode 3
Complete guide to gusset plate design per EN 1993-1-8:2005. Whitmore effective width method, block shear resistance (Clause 3.10.2), gusset plate buckling (Clause 6.3.1), and weld design for brace-to-gusset connections. Worked example with CHS brace and S355 gusset plate.
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Whitmore Effective Width Method
The Whitmore section accounts for stress dispersion at 30° from the bolted or welded connection:
L_eff = L_weld + 2 × L_disp × tan(30°)
Where L_weld is the weld length at the brace end and L_disp is the distance from the connection face to the critical cross-section of the gusset.
The Whitmore width gives the effective gusset section for tension yielding check:
N_t,Rd = L_eff × t_g × f_y / γ_M0
Where t_g = gusset plate thickness.
Design Checks
1. Tension Yielding (Gross Section)
N_t,Rd = L_eff × t_g × f_y / γ_M0
2. Tension Rupture (Net Section at Bolt Holes)
N_u,Rd = A_net × f_u / γ_M2
For bolted brace connections, net area = gross area minus bolt hole area.
3. Block Shear — EN 1993-1-8 Clause 3.10.2
V_eff,Rd = f_u × A_nt / γ_M2 + (1/√3) × f_y × A_nv / γ_M0
Where A_nt = net area in tension, A_nv = net area in shear.
4. Gusset Plate Buckling
For gusset plates in compression, check per EN 1993-1-1 Clause 6.3.1:
N_b,Rd = χ × A × f_y / γ_M1
Use buckling curve c (α = 0.49) for gusset plates. The effective length may be taken as 0.65 × L (rigid frame) or 1.0 × L (simple support), where L is the free edge length.
5. Weld Design
Fillet welds between brace and gusset per Clause 4.5:
- Total weld capacity ≥ applied brace force
- Weld return at the gusset edge (25-50 mm each side) for ductility
Worked Example — CHS 139.7×8 Brace, Gusset Plate
| Parameter | Value |
|---|---|
| Brace | CHS 139.7×8, S355 (f_y = 355 MPa) |
| Brace capacity N_pl,Rd | 2940 × 355 / 1.00 = 1044 kN |
| Gusset plate | 300 × 250 × 12 mm, S355 |
| Welds | 8 mm fillet (a = 5.7 mm), full perimeter |
Tension Yielding Check
| Parameter | Value |
|---|---|
| Whitmore width L_eff | 250 + 2 × 100 × tan(30°) = 365 mm |
| N_t,Rd | 365 × 12 × 355 / 1.00 = 1555 kN |
| Check | 1555 ≥ 1044 ✓ |
Block Shear Check
| Parameter | Value |
|---|---|
| Shear planes | 2 vertical shear planes: A_nv = 2 × (250 - 1.5×22) × 12 = 5208 mm² |
| Tension plane | A_nt = (90 - 0.5×22) × 12 = 948 mm² |
| V_eff,Rd | 470×948/1.25 + 205×5208/1.00 = 356 + 1068 = 1424 kN |
| Check | 1424 ≥ 1044 ✓ |
Gusset Buckling Check
| Parameter | Value |
|---|---|
| Free edge length L | 350 mm |
| t_g | 12 mm |
| λ̄ = (L × √12) / (t × π) × √(f_y/E) | 350 × 3.46 / (12 × π) × √(355/210000) = 0.42 |
| χ (curve c, α = 0.49) | 0.88 |
| N_b,Rd | 0.88 × 300 × 12 × 355 / 1.00 = 1125 kN |
| Check | 1125 ≥ 1044 ✓ |
Weld Design
| Parameter | Value |
|---|---|
| Weld length | ~450 mm (around CHS perimeter) |
| Required throat | 1044000 / (450 × 241) = 9.6 mm |
| Provide | 8 mm fillet (a = 5.7 mm) — NOT adequate |
| Required | 14 mm fillet (a = 9.9 mm) |
Note: For the full brace capacity to be developed, large welds or longer weld length is required. In practice, the connection detail may use a slot-and-plate or doubler plate to reduce weld demand.
Minimum Gusset Plate Thickness
Per EN 1993-1-8 and common practice:
| Brace Force (kN) | Minimum t_g (mm) | Recommended t_g (mm) |
|---|---|---|
| ≤ 300 | 8 | 10 |
| 300-600 | 10 | 12 |
| 600-1000 | 12 | 15 |
| 1000-1500 | 15 | 20 |
| > 1500 | 20 | 25 |
Frequently Asked Questions
What is the Whitmore section and why is it used for gusset plate design?
The Whitmore section is the effective width of the gusset plate through which the brace force is distributed, assuming a 30° stress dispersion angle from the connection. It accounts for the non-uniform stress distribution across the gusset plate. The Whitmore width L_eff = L_connection + 2 × L_disp × tan(30°), where L_connection is the connection length and L_disp is the distance from the connection to the critical section.
When should gusset plate buckling be checked?
Gusset plate buckling must be checked when the brace is in compression. Per EN 1993-1-8, the gusset is treated as a compression element. The buckling check considers the free edge of the gusset as a column. For gussets with stiffened edges (folded gusset, stiffeners), the buckling capacity increases significantly. For braces with tension-only design (e.g., cross-bracing), buckling of the gusset in compression is not critical.
Related Pages
- Brace Connection — Brace connection per EN 1993-1-8
- Shear Tab Connection — Fin plate per EN 1993-1-8
- End Plate Connection — Moment connection
- Column Design — Compression per EN 1993-1-1
- All European References
Educational reference only. Design per EN 1993-1-8:2005. Whitmore method per AISC/SCI guidance. Verify gusset dimensions against actual brace angle and connection geometry. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.
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Design guides
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- Steel Connection Calculator Guide
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- EN 1993-1-8 Bolted Connection Worked Example
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