EN 1993 Brace Connection Design — Gusset, Splice, Slotted Tube per Eurocode 3-1-8
Complete guide to brace connection design per EN 1993-1-8:2005. Gusset plate connections for CHS and RHS braces, slotted tube connections, splice connections for built-up braces, block shear resistance, and weld design for brace-to-gusset connections. Worked example with CHS 139.7x8 brace in S355.
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Brace Connection Types
| Connection Type | Typical Brace Section | Application | EN 1993-1-8 Clause |
|---|---|---|---|
| Gusset plate | CHS, RHS, UC | Concentrically braced frames (CBF) | 2.5, 2.7 |
| Slotted tube | CHS | Simple bracing, tension-only | 4.5, 3.10 |
| Splice connection | 2 × channel, 2 × UC | Built-up braces, mega-bracing | 3.12 |
| Direct weld to column | RHS, UC | Eccentric braced frames (EBF) | 7.4, 7.5 |
| Bolted end plate | UC, CHS | Moment-resisting braces | 6.2 |
Gusset Plate Connection Design
Brace Force Components
The brace axial force N_Ed is resolved into horizontal and vertical components at the gusset-to-beam and gusset-to-column interfaces:
H_Ed = N_Ed \times cos(\theta)
V_Ed = N_Ed \times sin(\theta)
Where \theta is the brace angle from horizontal.
Design Checks per EN 1993-1-8
| Check | Formula | Clause |
|---|---|---|
| Gusset tension yielding | N_t,Rd = L_eff \times t_g \times f_y / \gamma_M0 | 6.2.3 |
| Gusset tension rupture | N_u,Rd = A_net \times f_u / \gamma_M2 | 6.2.4 |
| Block shear | V_eff,Rd = f_u\times A_nt/\gamma_M2 + f_y\times A_nv/(\sqrt3\times\gamma_M0) | 3.10.2 |
| Gusset buckling | N_b,Rd = \chi \times A \times f_y / \gamma_M1 | 6.3.1 |
| Weld at brace-gusset interface | F_w,Rd \ge N_Ed | 4.5 |
| Bolt bearing (if bolted) | F_b,Rd \ge N_Ed/n | 3.6 |
Slotted CHS Connection Design
For CHS braces connected via a slotted gusset plate through the tube:
Net Section Check
N_u,Rd = A_net \times f_u / \gamma_M2
Where the net area of the slotted CHS:
A_net = A_gross - t \times L_slot
Weld Design for Slot Weld
The slot weld (fillet weld on both sides of the plate inside the CHS):
- Weld length: L_weld = 2 \times L_slot (both sides)
- Required weld throat: a \ge N_Ed / (L_weld \times f_vw,d)
- Minimum a = 0.55 \times t_g (for full strength)
Shear Lag Factor
Per EN 1993-1-8 Clause 3.10.3, a shear lag factor may apply for slotted connections:
| L_slot / b_p | Shear Lag Factor U |
|---|---|
| < 1.0 | 0.60 |
| 1.0-1.5 | 0.75 |
| 1.5-2.0 | 0.85 |
| > 2.0 | 1.00 |
Worked Example — CHS 139.7\times8 Brace Connection
Connection details:
- Brace: CHS 139.7\times8, S355 (f_y = 355 MPa, f_u = 470 MPa)
- Brace capacity: N_pl,Rd = 1044 kN
- Gusset: 300 \times 300 \times 12 mm, S355
- Brace angle: \theta = 45\degree
- Bolts: 6 \times M20 8.8 (2 rows \times 3 bolts)
- Welds: 8 mm fillet (CHS to gusset), 6 mm fillet (gusset to beam/column)
Force Resolution
| Component | Value |
|---|---|
| Horizontal component | H_Ed = 1044 \times cos(45\degree) = 738 kN |
| Vertical component | V_Ed = 1044 \times sin(45\degree) = 738 kN |
Bolt Group Check (Gusset to Column)
| Check | Value |
|---|---|
| Bolt group (6\times M20 8.8) | Shear capacity per bolt: F_v,Rd = 94.1 kN |
| Total shear capacity | 6 \times 94.1 = 565 kN < 738 kN ✗ |
| Increase to 8 bolts (4 rows \times 2) | 8 \times 94.1 = 753 kN \ge 738 kN ✓ |
Block Shear Check (Brace Side)
| Parameter | Value |
|---|---|
| Shear planes | 2 \times (200 - 1.5 \times 22) \times 12 = 4008 mm² |
| Tension plane | (80 - 0.5 \times 22) \times 12 = 828 mm² |
| V_eff,Rd | 470\times828/1.25 + 205\times4008/1.00 = 311 + 822 = 1133 kN |
| Check | 1133 \ge 738 ✓ |
Gusset Buckling Check
| Parameter | Value |
|---|---|
| Unsupported free edge | 300 mm |
| t_g | 12 mm |
| \bar{\lambda} | 300 \times \sqrt12 / (12 \times \pi) \times \sqrt(355/210000) = 0.36 |
| \chi (curve c, \alpha = 0.49) | 0.92 |
| N_b,Rd | 0.92 \times 300 \times 12 \times 355 / 1.00 = 1085 kN |
| Check | 1085 \ge 1044 ✓ |
Brace Connection Detailing Requirements
Per EN 1993-1-8 and EN 1998-1 (seismic) for ductile braced frames:
| Detailing Rule | Requirement | Purpose |
|---|---|---|
| Gusset free edge | 2 \times t_g clearance from beam/column | Rotation capacity during buckling |
| Weld return | 2 \times t_g minimum return at gusset edge | Prevent weld end tear-out |
| Bolt edge distance | e1 \ge 2 \times d_0, e2 \ge 1.5 \times d_0 | Bearing and tearout resistance |
| Gusset aspect ratio | L_gusset / t_g \le 50 | Slenderness limit for buckling |
| Net section (slotted CHS) | A_net/A_gross \ge 0.75 for tension | Shear lag and net section efficiency |
| Plate thickness | t_g \ge 10 mm for brace force > 500 kN | Practical minimum for welding |
Frequently Asked Questions
What are the critical design checks for a gusset plate brace connection per EN 1993?
The critical checks are: (1) Whitmore section tension yielding, (2) block shear at the bolted connection, (3) gusset plate buckling for compression braces, (4) weld capacity at the brace-to-gusset and gusset-to-frame interfaces, and (5) bolt bearing/tearout. For braces in seismic frames, the connection must be designed for 1.1 \times R_y \times N_pl,Rd (overstrength) per EN 1998-1.
How is the effective length of a slotted CHS brace connection determined?
The net section of a slotted CHS is the gross area minus the slot width \times tube wall thickness. A shear lag factor per Clause 3.10.3 accounts for non-uniform stress distribution. For slots longer than 2\times the plate width, U = 1.0 (full efficiency). For shorter slots, the efficiency reduces significantly. The slot length should be at least 1.5 \times the gusset plate width for adequate load transfer.
When should brace connections be designed for overstrength?
Per EN 1998-1 Clause 6.5.3, brace connections in concentrically braced frames (CBF) designed for seismic action must be capacity-designed: the connection must resist 1.1 \times \gamma_ov \times N_pl,Rd of the brace, where \gamma_ov = 1.25 (default). This ensures ductile yielding occurs in the brace, not in the connection. For non-seismic design, connections may be designed for the design action effects (N_Ed).
Related Pages
- Gusset Plate Design — Whitmore method and buckling
- Shear Tab Connection — Fin plate per EN 1993-1-8
- End Plate Connection — Moment connection design
- Braced Frame Design — CBF per EN 1998-1
- All European References
Educational reference only. Design per EN 1993-1-8:2005. Brace connection detailing per EN 1998-1 for seismic design. Verify gusset geometry and bolt layout with actual member sizes. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.
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