Australian Column Effective Length Factor K — AS 4100 Clause 6.3
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Reference for column effective length factor (k) per AS 4100:2020 Clause 6.3. Covers alignment charts, end restraint conditions, braced vs sway frames, and worked examples.
Effective Length Factor k — AS 4100 Table 6.3.1
| End Condition | Braced Frame (k) | Sway Frame (k) |
|---|---|---|
| Fixed-fixed | 0.65 | 1.0 |
| Fixed-pinned | 0.85 | 1.2 |
| Pinned-pinned | 1.0 | ∞ (2.0 min) |
| Fixed-free | 2.0 | 2.0 |
| Fixed-partial fixity | Use alignment chart | Use alignment chart |
Alignment Chart Method — Clause 6.3.3
Braced frames: k = function of GA and GB from alignment chart
GA (or GB) = Σ(Ic/Lc) / Σ(Ib/Lb)
Where:
- Ic, Lc = column second moment of area and length
- Ib, Lb = beam second moment of area and length
- Σ = sum for all members meeting at the joint
Practical values:
- GA = 10 for pinned base
- GA = 1.0 for fixed base
- GA = 0 for perfectly fixed (theoretical)
Sway vs Non-Sway Frames
| Condition | Non-Sway (Braced) | Sway (Unbraced) |
|---|---|---|
| Lateral stability | Bracing walls, cores | Moment frames |
| P-Δ effects | Second-order < 5% | Second-order > 5% |
| k factor range | 0.65 - 1.0 | 1.0 - ∞ |
| Design approach | Effective length method | Effective length + P-Δ |
| AS 4100 reference | Clause 6.3.2 | Clause 6.3.4 |
Worked Example
Problem: Column in braced frame, pinned base, top connection gives GB = 1.2. Determine k.
Solution:
- Pinned base → GA = 10 (practical)
- GB = 1.2 (from beam/column stiffness ratio)
- From AS 4100 alignment chart for braced frames: k ≈ 0.87
- Effective length L̅e = k × L = 0.87 × 4,000 = 3,480 mm
- Use Le to determine λn and αc for buckling capacity.
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 k factor for a pinned-pinned column? k = 1.0 for braced frames, per AS 4100 Table 6.3.1. This is the fundamental case with no end restraint.
How is the alignment chart used? Calculate GA and GB from column/beam stiffness ratios. Read k from the chart intersection. GA = Σ(Ic/Lc)/Σ(Ib/Lb).
What k factor for cantilever columns? k = 2.0 (fixed-free) per Table 6.3.1. For fixed cantilever columns in sway frames, use k = 1.0 if the free end is laterally restrained.
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.