Section properties explained
A practical guide to A, I, Z, S, r, J, Iw, kf and compactness, and which property controls which design check.
| Property | What it tells you | Used for |
|---|---|---|
| Ag (mm²) | Cross-section area | Mass (A × 7850 kg/m³), tension and squash capacity |
| Ix, Iy (mm⁴) | Second moment of area: bending stiffness about each axis | Deflection, frame stiffness, buckling loads |
| Z (mm³) | Elastic section modulus, I / ymax | First-yield moment, serviceability stress checks |
| S (mm³) | Plastic section modulus | Full plastic moment of compact sections |
| Ze (mm³) | Effective section modulus to AS 4100 Cl. 5.2 | Section moment capacity φMs = φfyZe |
| r (mm) | Radius of gyration, √(I/A) | Column slenderness Le/r |
| J (mm⁴) | Torsion constant (St Venant) | Twisting stiffness, lateral-torsional buckling |
| Iw (mm⁶) | Warping constant | Lateral-torsional buckling of I-sections and channels |
| kf | Form factor: effective fraction of area in compression | Local buckling in compression members |
| Compact / non-compact / slender | Whether plate elements can reach full plasticity before local buckling | Determines Ze |
Which property matters?
- Beam strength: Ze (and S for compact sections), reduced by lateral-torsional buckling through Iy, J and Iw when the compression flange is unrestrained.
- Beam deflection: Ix.
- Column strength: A, kf and the smaller radius of gyration (usually ry).
- Torsion: closed sections (SHS, RHS, CHS) have J values hundreds of times larger than open sections of similar mass.
See notation & method for exactly how each value on this site is calculated.