Australian Composite Column — AS 4100 Concrete-Filled Steel Design
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Reference for concrete-filled steel tube (CFST) column design per AS 4100:2020 and AS/NZS 2327:2017. Covers composite section capacity, axial compression, confinement effects, and construction sequence effects.
Composite Column Types
| Type | Description | Typical Application |
|---|---|---|
| Concrete-filled CHS | Steel tube filled with concrete | Columns, piles |
| Concrete-filled SHS | Square tube filled with concrete | Columns (architectural) |
| Partially encased | Steel section with concrete between flanges | High-rise columns |
| Fully encased | Steel section fully encased in concrete | Fire-rated columns |
Axial Compression Capacity — AS 4100 Clause 6 + AS/NZS 2327
Ns = As × fy + 0.85 × Ac × fc′
Where:
- As = steel area, Ac = concrete area
- fy = steel yield strength, fc′ = concrete compressive strength
- 0.85 = concrete confinement factor (may be higher for CFST)
Nc = αc × Ns (member buckling capacity per AS 4100 Clause 6.3)
Where αc = slenderness reduction factor based on modified slenderness λn.
CFST Capacity Example: CHS 219.1×8.2
| Property | Value |
|---|---|
| Steel area As | 5,400 mm² |
| Steel yield fy | 450 MPa (C450L0) |
| Concrete area Ac | 32,200 mm² (CHS 219.1×8.2) |
| Concrete strength fc′ | 32 MPa (normal weight) |
| Steel contribution Ns | 2,430 kN |
| Concrete contribution | 0.85 × 32,200 × 32 = 876 kN |
| Nominal capacity Ns | 2,430 + 876 = 3,306 kN |
| φNs design | 0.90 × 3,306 = 2,975 kN |
Buckling capacity (L=4m, pin-ended): λn depends on slenderness, typically αc ≈ 0.85-0.95.
Construction Requirements
| Aspect | Requirement |
|---|---|
| Concrete placement | Self-consolidating concrete (SCC) preferred |
| Minimum tube thickness | 2.5 mm (structural) |
| Concrete strength | 20-50 MPa typical |
| Shear connection | Natural bond (typically ignored for design) |
| Fire rating | Significantly improved vs bare steel |
| Steel contribution ratio | δ = As×fy/Ns ≥ 0.2 per AS/NZS 2327 |
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 the advantage of concrete-filled steel columns? Concrete filling increases axial capacity 30-60%, provides inherent fire protection for 60-120 min, and eliminates internal formwork.
How is composite action accounted for in design? Per AS/NZS 2327, natural bond between steel and concrete is typically ignored for capacity. Capacity is sum of steel plus concrete contributions with confinement factor.
What steel grade is used for CFST columns? C450L0 (fy=430-450 MPa) to AS/NZS 1163 is standard. CHS sizes 114.3 mm to 457 mm OD are most common.
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.