The load path of a small steel-framed house
Following loads from the roof sheet to the footing: wind classification (AS 4055), purlins and rafters, portal action, columns and tie-down, with the matching toolkit calculators at each step.
Every structure is a chain. A small steel-framed house or shed is a good way to see the whole chain: sheeting spans to purlins, purlins to rafters, rafters to columns, and columns to footings. Wind uplift reverses much of it, so the connections have to work both ways.
Step by step
- Wind: AS 4055 classifies houses (N1–N6, C1–C4) from region, terrain, shielding and topography. Larger or non-house buildings need the full AS/NZS 1170.2 route: VR × Md × Mz,cat × Ms × Mt.
- Purlins and girts: continuous cold-formed Z or C sections. Uplift usually governs because the unrestrained inner flange is in compression.
- Rafters and columns: portal action. Knee moments from gravity and from wind, then member checks to AS 4100.
- Tie-down: the column base sees uplift and shear, so size anchor bolts and the footing for uplift with a stability factor.
- Footings: dead load minus uplift with load factors (0.9G − Wu) for stability, and gravity cases for bearing.
The toolkit's small steel-framed house tool joins these steps for a simple gable building with SHS/RHS framing. It is meant for preliminary sizing and for understanding the load path, not for certification.