EN 1993 Braced Frame — Concentrically Braced Frame Design per Eurocode 3 & 8
Complete guide to concentrically braced frame (CBF) design per EN 1993-1-1 and EN 1998-1. Cross-bracing (X-bracing), chevron (V and inverted-V) bracing, and K-bracing configurations. Ductility classes DCM (medium) and DCH (high), behaviour factors q for CBF, capacity design requirements, brace slenderness limits, and gusset plate connection design. Worked 3-storey CBF example with HEA 200 braces in S355 steel.
Quick access: Moment Frame | Framing Systems | Seismic Design
Braced Frame Configurations
| Configuration | Brace Arrangement | EN 1998-1 | Advantages | Disadvantages |
|---|---|---|---|---|
| Cross-bracing (X) | Diagonal pairs | DCM, DCH | High stiffness, ductile | Blocks openings |
| Chevron (V) | Braces meet at beam | DCM only | Open bay below | Beam carries unbalanced force |
| Inverted-V | Braces below beam | DCM only | Open bay above | Beam unbalanced load |
| K-bracing | Braces at column mid-height | NOT permitted | — | Column buckling risk |
| Tension-only (X) | Slender rods | Not recommended | Simple, economical | Low stiffness |
K-bracing is not permitted in seismic design (EN 1998-1 Cl. 6.7.1) because brace buckling induces large moments at column mid-height, risking column failure.
Behaviour Factors q for CBF (EN 1998-1 Table 6.2)
| Ductility Class | q Factor | Brace Section | Slenderness |
|---|---|---|---|
| DCM (medium) | 4.0 | Class 1, 2, or 3 | bar_lambda <= 2.0 |
| DCH (high) | 4.0 | Class 1 only | 1.3 <= bar_lambda <= 2.0 |
| Low ductility | 1.5 | Any class | Any |
For DCH, braces must have bar_lambda >= 1.3 to ensure ductile behaviour (yield in tension before buckling).
Capacity Design (EN 1998-1 Cl. 6.7.4)
Connections and adjacent members must resist the overstrength brace force:
N_ov,Rd = 1.1 x gamma_ov x N_pl,Rd,brace
Where gamma_ov = 1.25 (material overstrength), 1.1 = strain-hardening factor.
Worked Example — 3-Storey CBF with HEA 200 Braces
3 storeys at 4.0 m, 3 bays at 6.0 m. X-bracing both directions. S355, HEA 200. DCM, q = 4.0.
| Storey | N_Ed (kN) | HEA 200 N_pl,Rd (kN) | bar_lambda | Governing |
|---|---|---|---|---|
| Roof | 180 | 2128 | 1.8 | Tension |
| 3rd | 420 | 2128 | 1.8 | Tension |
| 2nd | 650 | 2128 | 1.8 | Tension |
Overstrength connection design: N_ov,Rd = 1.1 x 1.25 x 2128 = 2926 kN
| Storey | Drift (mm) | Ratio | Limit (H/200) |
|---|---|---|---|
| Roof | 18 | 1/222 | OK |
| 3rd | 22 | 1/182 | FAIL |
| 2nd | 24 | 1/167 | FAIL |
Drift failure means increased brace size or additional braced bay required.
Brace Section Selection Guide
| Brace Force (kN) | Recommended Section | Typical bar_lambda |
|---|---|---|
| < 300 | CHS 88.9x5 | 1.5-2.0 |
| 300-600 | CHS 139.7x8 | 1.3-1.8 |
| 600-1000 | HEA 240 / CHS 168.3x10 | 1.3-1.6 |
| 1000-1500 | HEB 260 / CHS 219.1x12 | 1.0-1.4 |
| > 1500 | HEB 300+ / built-up | 0.8-1.3 |
Frequently Asked Questions
Key differences between DCM and DCH for CBF?
DCM: braces Class 1-3, bar_lambda <= 2.0, pinned beam-to-column joints acceptable. DCH: braces must be Class 1, bar_lambda 1.3-2.0, beam-to-column joints must be rigid or specifically detailed. Both use q = 4.0.
Why is K-bracing prohibited in seismic design?
K-bracing (per EN 1998-1 Cl. 6.7.1) induces unbalanced horizontal force at column mid-height when one brace buckles, causing large bending moments and potential column plastic hinging. Only X, V, and inverted-V bracing permitted.
Related Pages
Educational reference only. CBF design per EN 1993-1-1:2005 and EN 1998-1:2004. Verify National Annex. Results are PRELIMINARY - NOT FOR CONSTRUCTION without independent verification.
Design Resources
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Reference pages