Australian Braced Frame — AS 4100 Concentric Design Guide
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Reference for concentrically braced frame (CBF) design per AS 4100:2020 and AS 1170.4:2007 for Australian seismic and lateral load applications. Covers brace types, ductility classes, capacity design requirements.
Brace Configuration Types
| Type | Description | Ductility | Typical Use |
|---|---|---|---|
| X-braced | Diagonal braces cross | Moderate | Low-rise industrial |
| V-braced (chevron) | Braces meet at beam midspan | Low | Moderate height |
| Inverted V | V-braced inverted | Low | Architecture-driven |
| K-braced | Braces meet at column | Not permitted (seismic) | Gravity only |
| Single diagonal | One brace per bay per storey | Moderate | Regular frames |
| EBF | Eccentric brace (link beam) | High | Seismic frames |
Ductility Classes — AS 1170.4
| Class | Ductility μ | Structural FoS | Connection Design |
|---|---|---|---|
| D1 (Limited) | 1.25 | Elastic | Elastic forces |
| D2 (Moderate) | 2.0 | 1.5 times elastic | Capacity design |
| D3 (Moderate-High) | 3.0 | 1.5-2.0 times | Capacity design + detailing |
| D4 (High) | 4.0 | 2.0-2.5 times | Special detailing + testing |
Capacity Design — Connection Requirements
Overstrength factor: φo = Ry × ω × Sh
Where:
- Ry = ratio of expected to nominal yield (1.1-1.5)
- ω = strain hardening factor (1.1-1.2)
- Sh = system overstrength (1.1-1.3)
Connection design force: N*_conn = φo × N*_brace
Typical φo range: 1.5 (D2) to 2.5 (D4)
Worked Example
Problem: 3-storey braced frame, V-braced. Brace force N*=800 kN (earthquake). D3 ductility. Design brace and connection.
Solution:
- Brace: 200UC52 (φNc=1,450 kN). N*/φNc = 800/1450 = 0.55 OK
- Brace slenderness: Le/r = 0.85×5,000/52.4 = 81 < 120 OK
- Connection design force: N*_conn = 2.0 × 800 = 1,600 kN (D3 φo≈2.0)
- Gusset plate: 16 mm Grade 300, 6-M24 8.8/S bolts
- Gusset capacity per Whitmore + block shear ≥ 1,600 kN (governing connection)
Design Resources
- [[Australian Steel Grades|/reference/australian-steel-grades/]] | [[Australian Steel Properties|/reference/australian-steel-properties/]] | [[Australian Beam Sizes|/reference/au-beam-sizes/]] | [[Australian Bolt Capacity|/reference/australian-bolt-capacity/]] | [[AS 4100 Beam Design|/reference/as4100-beam-design-example/]] | [[All Australian References|/reference/]]
FAQ
What is capacity design for braced frames? Connections are designed for a multiple of the brace force to ensure ductile brace yielding precedes brittle connection failure. Essential for seismic design.
Is K-bracing permitted in seismic frames? No. K-bracing is not permitted for seismic applications per AS 1170.4 because brace buckling causes unbalanced column forces that can precipitate collapse.
What is the maximum brace slenderness per AS 4100? Le/r ≤ 120 for seismic frames, ≤ 200 for non-seismic. Tension-only braces are not permitted for seismic resistance.
Educational Use Only — This reference is for educational and preliminary design purposes only. All structural designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) in accordance with AS 4100:2020 and all applicable Australian Standards. Results are not for construction.