EN 1993-1-8 Connection Design
Haunch & Stiffener Design — EN 1993-1-8 §6.2.6
Design haunch stiffeners for portal frame eaves connections. EN 1993-1-8 §6.2.6 worked example: IPE500/HEA340, 420 kNm, M24 10.9. Includes tension & compression stiffeners, weld sizing, column web panel shear check.
When Are Haunches and Stiffeners Required?
Haunch stiffeners are mandatory when portal frame eaves connection moment exceeds the column web panel shear capacity (VEd > Vpl,Rd per EN 1993-1-8 §6.2.6.1). A haunch extends the rafter bottom flange over the column; rib stiffeners reinforce the column web itself. Required at most 24m+ span portal frame connections.
§6.2.6(1) — Column web panel shear verification per §6.2.6.1 must be performed first.
If VEd > Vpl,Rd for the unstiffened web, stiffeners are mandatory on both flanges.
§6.2.7(1) — Haunches up to 0.6× rafter depth may be treated as part of the rafter for internal force calculation. Beyond 0.6d, formal Vierendeel analysis is required.
§6.2.7(1) — Haunches up to 0.6× rafter depth may be treated as part of the rafter for internal force calculation. Beyond 0.6d, formal Vierendeel analysis is required.
Figure 1 — Elevation of haunched eaves connection: IPE500 rafter haunched over HEA340 column. Tension stiffener (orange, top) and compression stiffener (green, bottom) reinforce the column web per EN 1993-1-8 §6.2.6. MEd = 420 kNm, VEd = 180 kN, M24 10.9 endplate.
Worked Example — 24 m Portal Frame, IPE500 / HEA340
24 m portal frame; IPE500 rafter (S355); HEA340 column (S355); MEd = 420 kNm; VEd = 180 kN; M24 10.9 endplate, 8 bolts. Panel utilisation 1.85 without stiffeners → PL-120×20 S355 tension + compression stiffeners.
Design moment: MEd = 420 kNm; VEd = 180 kN; IPE500 / HEA340
(a) Unstiffened column web panel utilisation
EN 1993-1-8 §6.2.6.1
VEd,panel = MEd / (dc − 2tf,c)
VEd,panel = 420,000,000 / 291 = 1,443 kN/m. Vpl,Rd = Av·fy/(√3·γM0) = 778 kN. Utilisation = 1,443/778 = 1.85 → stiffeners mandatory.
(b) Why tension + compression stiffeners are needed
EN 1993-1-8 §6.2.6.3
Stiffener carries diagonal panel shear
The 1.85× demand means the column web alone fails. Tension stiffener at upper flange carries diagonal tension. Compression stiffener at lower flange carries diagonal compression and prevents web buckling. Both act as compression members per EN 1993-1-1 §6.2.4, L = 0.75·dc = 247.5 mm.
(c) Stiffener size and weld sizing per §4.5.3
EN 1993-1-8 §4.5.3
a = 8 mm fillet; fvw,d = 277 N/mm²
PL-120×20 S355 per side. Buckling: λ = Le/i = 247.5/5.77 = 42.9 ≤ 50 ✓. Weld: a = 8 mm fillet both sides, Lw,eff = 275 mm. FRd = 8×277 = 2,216 N/mm > demand 1,828 N/mm. Weld utilisation 82.5%.
(d) Final utilisation — stiffened panel
EN 1993-1-8 §6.2.6
Vpl,Rd,stiffened = 1,762 kN
With PL-120×20 S355 on both sides: Vpl,Rd,stiffened = 778×(1 + 4,800/3,795) = 1,762 kN. Panel utilisation = 1,443/1,762 = 0.82. Combined (panel 0.82, stiffener 0.79, weld 0.825): Final utilisation = 0.91.
Figure 2 — Plan view of HEA340 column web panel. Diagonal tension (red) and compression (green) shear field per §6.2.6.1. Stiffened zones shaded orange/green. Panel: 291 mm deep × 230 mm wide.
Design Summary
| Check | Value | Limit | Result |
|---|---|---|---|
| Unstiffened panel utilisation | 1.85 | 1.0 | FAIL → stiffeners required |
| Stiffened panel utilisation | 0.82 | 1.0 | PASS |
| Stiffener buckling (λ) | 42.9 | 50 | PASS (Class 2) |
| Weld utilisation (a = 8 mm) | 82.5% | 100% | PASS |
| Final combined utilisation | 0.91 | 1.0 | PASS |
Common Failure Modes
Column web panel yielding without stiffeners
When VEd > Vpl,Rd, the column web yields under panel shear. Without tension + compression stiffeners the connection fails. This is the governing condition that triggers the entire haunch/stiffener design process.
Stiffener buckling — insufficient thickness
PL-100×15 stiffener failed the slenderness check (λ = 57.1 > 50 for Class 2). Upgrading to PL-120×20 brings λ = 42.9 ≤ 50 ✓. Always verify slenderness before specifying.
Weld rupture — undersized fillet
a = 6 mm fillet gave F_Rd = 1,662 N/mm < demand 1,828 N/mm → fails. Minimum a = 8 mm is required for this connection geometry. Always check weld resistance before specifying.
Vierendeel failure beyond haunch limit
EN 1993-1-8 §6.2.7(1): haunch depth > 0.6×rafter depth requires formal Vierendeel analysis. Keep haunch depth ≤ 0.6d or run the full check.
Detailing Gotchas
- Haunch depth limit: ≤ 0.6×rafter depth without Vierendeel analysis (EN 1993-1-8 §6.2.7)
- Stiffener weld quality: full-depth fillet weld required on both sides, minimum a = 8 mm for S355 ≥ 15 mm plate
- Stiffener material: use same grade as column (S355 here). Match Charpy requirements if T < −20°C.
- Both stiffeners required simultaneously — tension AND compression. One stiffener alone does not restore panel capacity.
Auto-Size Stiffeners with FrameAI
FrameAI auto-detects when a stiffener is required and sizes it — including weld throat a and effective length Lw,eff — per EN 1993-1-8 §4.5.3.
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EN 1993-1-8 Overview