Australian Column Design — AS 4100 Compression and Buckling Guide
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Comprehensive guide for column axial compression design per AS 4100:2020 Clause 6. Covers section capacity (φNs), member buckling capacity (φNc), buckling curves, effective length, and slenderness limits.
Compression Design Procedure
Section capacity: φNs = φ × fy × Ag (Clause 6.2)
Member capacity: φNc = φ × αc × Ns (Clause 6.3)
αc from AS 4100 Table 6.2.4 based on modified slenderness λn: λn = (Le/r) × √(fy/250)
Buckling curve selection based on section type (Table 6.2.3):
| Section Type | Axis | αb | Curve |
|---|---|---|---|
| UB (flange ≥ 0.5×depth) | Major | 0.5 | SHC |
| UB (flange < 0.5×depth) | Major | −1.0 | AISC |
| UC | Both | 0.0 | AISC |
| CHS (cold-formed) | — | −0.5 | AISC |
| SHS/RHS (cold-formed) | Both | −0.5 | AISC |
| Welded I-section | Major | 0.5 | SHC |
| Welded I-section | Minor | −0.5 | AISC |
αb: 0.0 = AISC curve (intermediate), −1.0 = AISC lower curve, +0.5 = SHC curve (higher capacity)
αc Values from λn
| λn | αb = 1.0 | αb = 0.5 | αb = 0.0 | αb = −0.5 | αb = −1.0 |
|---|---|---|---|---|---|
| 10 | 0.997 | 0.996 | 0.995 | 0.994 | 0.993 |
| 25 | 0.986 | 0.979 | 0.970 | 0.960 | 0.947 |
| 50 | 0.945 | 0.915 | 0.877 | 0.831 | 0.778 |
| 75 | 0.880 | 0.814 | 0.732 | 0.643 | 0.556 |
| 100 | 0.799 | 0.692 | 0.573 | 0.462 | 0.370 |
| 125 | 0.711 | 0.569 | 0.437 | 0.332 | 0.253 |
| 150 | 0.626 | 0.460 | 0.332 | 0.240 | 0.177 |
Column Capacity Tables — UC Sections
| Section | Ag (mm²) | φNs (kN) | Le=3m φNc | Le=4m φNc | Le=5m φNc | Le=6m φNc |
|---|---|---|---|---|---|---|
| 150UC37 | 4,740 | 1,248 | 1,180 | 1,120 | 1,045 | 960 |
| 200UC52 | 6,670 | 1,756 | 1,660 | 1,575 | 1,470 | 1,350 |
| 250UC89 | 11,400 | 3,002 | 2,840 | 2,690 | 2,510 | 2,300 |
| 310UC137 | 17,500 | 4,607 | 4,360 | 4,130 | 3,850 | 3,530 |
| 310UC158 | 20,100 | 5,292 | 5,010 | 4,740 | 4,420 | 4,050 |
Values: Grade 300, αb=0.0, pin-ended (k=1.0).
Worked Example
Problem: 310UC137 column, L=5.0 m, braced frame, pinned base, top GB=1.5. N*=2,800 kN. Grade 300.
Solution:
- Effective length: GA=10 (pinned), GB=1.5 → k=0.92 from alignment chart
- Le = 0.92×5,000 = 4,600 mm
- Slenderness: λn = (4,600/87.4) × √(300/250) = 52.6 × 1.095 = 57.6
- αb=0.0 (UC section). From table: αc ≈ 0.82 (interpolated)
- φNc = 0.90 × 0.82 × 300 × 17,500 = 3,874 kN > 2,800 OK
- Check: N*/φNc = 0.72 < 1.0 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 buckling curve is used for UC sections? αb=0.0 (AISC curve) per AS 4100 Table 6.2.3 for UC sections. This is the intermediate curve.
How does effective length affect column capacity? Capacity decreases rapidly with increasing Le. A 310UC137 at Le=3m has φNc=4,360 kN vs Le=6m φNc=3,530 kN (19% reduction).
What is the maximum slenderness for columns? AS 4100 Clause 6.1 limits Le/r ≤ 200 for primary members and ≤ 300 for secondary members.
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.