Steel column design to AS 4100: slenderness, αc and section capacity
Why a steel column's capacity drops as it gets longer: effective length, the modified slenderness λn, the member slenderness reduction factor αc, and why UC sections suit columns.
A short steel column fails by squashing at φNs = φ kf An fy. A long one buckles sideways at a much lower load. AS 4100 Section 6 connects the two with a single reduction factor αc that depends on the modified slenderness λn and the section constant αb.
λn = (Le/r)·√kf·√(fy/250), where Le = ke·L. The effective length factor ke depends on end restraint: 0.7 for fixed–pinned, 1.0 for pinned–pinned and 2.2 for a sway cantilever. A ke of 1.0 is the usual braced-frame assumption.
The member capacity is then φNc = αc·φNs ≤ φNs, checked about each axis with its own effective length. Universal columns have similar radii of gyration about both axes, so they lose less capacity to minor-axis buckling than a UB of the same mass. That is why they are the default choice for columns.
- Bracing at mid-height halves the minor-axis effective length and can more than double capacity in the slender range.
- Combined actions (axial plus moment) need the Section 8 interaction checks. The toolkit's column tool reports the axial capacities only.
- Check the current AS 4100 edition and the section tables from your supplier.