Australian Brace Connection — AS 4100 Design Guide
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Reference for brace connection design per AS 4100:2020 for concentrically braced frames. Covers gusset plate, block shear, Whitmore section, and gusset-to-beam/column interface weld design.
Connection Types
| Brace Type | Connection | Typical Capacity |
|---|---|---|
| UB/UC brace | Bolted gusset (both ends) | 500-2000 kN |
| CHS brace | Welded gusset + slot | 300-1000 kN |
| SHS brace | Welded gusset + gusset plate | 400-1500 kN |
| Double angles | Bolted gusset (back-to-back) | 200-800 kN |
Design Checks — AS 4100
1. Brace net section:
- φNt = 0.90 × 0.85 × fu × An (fracture at bolt holes)
- φNt = 0.90 × fy × Ag (gross section yield)
2. Gusset plate checks:
- Tension: Whitmore section (as per gusset plate design)
- Block shear: φVb = φ × (0.5×fu×Ant + 0.6×fy×Agv)
- Buckling: Unbraced gusset edge L/r ≤ 50
3. Bolts:
- Shear: φVfn per bolt
- Bearing: φVb per Clause 9.3.2.2
- Combined (if tension present)
4. Gusset-to-beam/column weld:
- Fillet weld along gusset perimeter
- Subjected to N* and eccentric moment M* = N* × e
Worked Example
Problem: 200UC52 brace (N*=600 kN tension). Bolted gusset: 12 mm plate, Grade 300. 6-M20 8.8/S bolts. Check connection.
Solution:
- Brace net section (2 bolt holes): An = Ag - 2×dh×tf = 6,670 - 2×22×12.7 = 6,111 mm²
- Fracture: φNt = 0.90×0.85×440×6,111 = 2,056 kN > 600 OK
- Yield: φNt = 0.90×300×6,670 = 1,801 kN > 600 OK
- Block shear: Ant=630 mm², Agv=2,880 mm². φVb = 0.90(0.5×440×630 + 0.6×300×2,880) = 125+466 = 591 kN < 600 NG
- Increase plate: 16 mm plate or add bolt row to increase Agv.
- With 16 mm plate: φVb = 788 kN > 600 OK
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 governs brace connection capacity? Block shear is often critical for brace gusset connections. Bolt bearing and Whitmore section tension also common limit states.
How is gusset-to-beam weld designed? Fillet weld along gusset perimeter designed for N* plus the eccentric moment from the work point offset.
Minimum gusset plate thickness for brace connections? 12 mm minimum for buildings, 16 mm for seismic frames. Governed by block shear and bolt bearing.
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.