Steel · Portal frames & crane runways
Small steel-framed house (preliminary) calculator
From plan, roof pitch, wind region and terrain: site wind speed and class, then purlin, rafter, stud and lintel sizes and the bracing (racking) demand, with an isometric frame.
Standard: AS 4055 wind classes · AS/NZS 1170.2 (2011) · AS/NZS 4600 / AS 4100 sizing
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| House width (rafter span) | 8 | m |
| House length | 15 | m |
| Wall height | 2.7 | m |
| Roof pitch | 22.5 | ° |
| Wind region | B (intermediate) | |
| Terrain category | TC3 suburban | |
| Shielding multiplier Ms | 0.85 | |
| Topographic multiplier Mt | 1 | |
| Roof dead load (sheet + battens) | 0.15 | kPa |
| Rafter / truss spacing | 1.2 | m |
| Batten / purlin spacing | 0.9 | m |
| Stud spacing | 0.6 | m |
| Widest opening (lintel span) | 2.4 | m |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Site wind speed Vh,u = VR·Mz,cat·Ms·Mt | 39.9 | m/s |
| Indicative wind class (AS 4055) | N2 | |
| Design wind pressure q | 0.96 | kPa |
| Net roof uplift (Cpe −0.9, Cpi +0.2) | 0.66 | kPa |
| Factored roof gravity 1.2G + 1.5Q | 0.55 | kPa |
| Battens / purlins @ 0.9 m | 50x50x2.0 (3.0 kg/m) | M* 0.09 kNm |
| Rafters @ 1.2 m | 75x75x3.0 (6.8 kg/m) | M* 1.33 kNm |
| Wall studs @ 0.6 m | 50x50x2.0 (3.0 kg/m) | M* 0.52 kNm |
| Stud axial N* | 1.3 | kN |
| Lintel over 2.4 m opening | 75x50x2.5 (4.7 kg/m) | M* 1.86 kNm |
| Racking force on end walls (wind on side) | 51.7 | kN |
| Racking force on side walls (wind on end) | 20.0 | kN |
| Frames / studs (approx.) | 14 / 82 |
- ✓ Within AS 4055 range (≤ N6 / C4)
- ✓ Rafter section found within table
- ✓ Lintel section found within table
Method, formulas and assumptions
- VR (R = 500) from AS/NZS 1170.2:2011 Table 3.1 formulae; Mz,cat from Table 4.1(B) at eaves-ridge mean height
- Wind class from Vh,u: N1 34, N2 40, N3 50, N4 61, N5 74, N6 86 m/s (non-cyclonic). Confirm with AS 4055 site classification
- Members sized as simply supported cold-formed SHS/RHS, fy = 450 MPa, φ = 0.9, elastic Z (no local buckling reduction), deflection span/150 (purlins, studs), /250 (rafters), /300 (lintels)
- Racking: 1.2 × q × projected elevation area (half wall height + roof). Provide bracing capacity ≥ demand in each direction
- Preliminary only. Light-gauge framing must be designed to AS/NZS 4600 / NASH standards or the manufacturer's span tables and certified
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.