Steel · Cold-formed & secondary steel
Purlin / girt check calculator
Gravity and uplift moments for single- or double-span purlins and girts, compared with the section's design capacity and deflection.
Standard: AS/NZS 4600 · AS/NZS 1170.0 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 |
|---|---|---|
| Span (frame spacing) | 6 | m |
| Purlin spacing | 1.2 | m |
| Dead load (sheeting + self) | 0.12 | kPa |
| Roof live load | 0.25 | kPa |
| Net wind uplift (ULS) | 0.9 | kPa |
| Continuity | Double span (continuous) | |
| Section modulus Ze (from supplier) | 18.6 | ×10³ mm³ |
| Second moment Ix | 1.2 | ×10⁶ mm⁴ |
| fy | 450 | MPa |
| Uplift capacity reduction (LTB, bridging) | 0.7 | |
| Deflection limit span / | 150 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Gravity w* | 0.62 | kN/m |
| Uplift w* (Wu − 0.9G) | 0.95 | kN/m |
| M* gravity | 2.80 | kNm |
| M* uplift | 4.28 | kNm |
| φMb gravity | 7.53 | kNm |
| φMb uplift (×0.7) | 5.27 | kNm |
| Required Ze | 10.6 | ×10³ mm³ |
| Service deflection | 10.3 | mm |
- ✓ Gravity M* ≤ φMb
- ✓ Uplift M* ≤ reduced φMb
- ✓ Deflection ≤ span/150
Method, formulas and assumptions
- Double span: hogging over the centre support wL²/8, deflection wL⁴/(185EI)
- Uplift capacity is usually governed by lateral-torsional buckling of the unrestrained flange: use the supplier's tabulated uplift capacity where available
- Enter Ze and Ix from the purlin supplier's section tables
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.