Steel · Portal frames & crane runways
Portal rafter calculator
Steel UB rafter for a portal shed: gravity and wind-uplift combinations, flange restraint and deflection.
Standard: AS 4100 · AS/NZS 1170.0/.1/.2 combinations
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 |
|---|---|---|
| Portal span | 18 | m |
| Roof pitch | 10 | ° |
| Bay spacing | 6 | m |
| Rafter section | 360UB50.7 | |
| Roof dead load (sheeting + purlins) | 0.15 | kPa |
| Roof live load (AS 1170.1) | 0.25 | kPa |
| Net wind uplift (AS 1170.2) | 0.8 | kPa |
| Purlin spacing (top flange restraint) | 1.5 | m |
| Fly-brace spacing (bottom flange) | 3 | m |
| Analysis | Portal frame (fixed knees, approx.) | |
| Deflection limit span/ | 180 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| G line load (incl. self-wt) | 1.39 | kN/m |
| Q line load | 1.50 | kN/m |
| 1.2G + 1.5Q | 3.92 | kN/m |
| 0.9G − Wu | -3.55 | kN/m |
| M* gravity | 103 | kNm |
| φMb, top flange restrained | 238 | kNm |
| M* uplift | 93.4 | kNm |
| φMb, bottom flange (fly braces) | 194 | kNm |
| Deflection (G + Q) | 57 | mm |
- ✓ Gravity M* ≤ φMb
- ✓ Uplift M* ≤ φMb
- ✓ Deflection ≤ span/180
Method, formulas and assumptions
- Line load = pressure × bay spacing (+ rafter self-weight)
- Combinations: 1.2G + 1.5Q and 0.9G + Wu (AS/NZS 1170.0)
- Portal approximation: rafter M ≈ 0.65 × wL²/8 for a fixed-knee frame; choose ‘simply supported’ for an upper bound
- Gravity: compression on top flange, Le = purlin spacing; uplift: bottom flange, Le = fly-brace spacing; αm = 1.13
- A full portal design needs frame analysis (sway, knee/apex connections, columns).
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.