A roof purlin reuses the identical DSM flexural engine as a general CFS beam — the same formula set, just applied to lighter, narrower, longer-spanning geometry. And that shift in proportions alone is enough to flip which check actually governs the design.
Every check above runs automatically inside the Structyze CFS Purlin / Girt module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
The Example We'll Use
- Section 65 × 150 × 1.5 mm (box/tube)
- Span 6.0 m
- Factored moment 2.0 kN·m, shear 1.35 kN
- Service UDL 0.45 kN/m
- Material ASTM A1003 Grade 50, Fy = 345 MPa
Working Through It
Gross section properties give Sg ≈ 24,923 mm³ and a flat flange width of 62 mm. The elastic local buckling stress Fcrl ≈ 423.4 MPa works out higher than the yield-based My ≈ 8.60 kN·m's own reference stress this time, but λl = √(8.60/10.55) = 0.903 still sits past the DSM transition of 0.776, so the reduced capacity Mnl ≈ 7.81 kN·m applies — φMn ≈ 7.03 kN·m against a 2.0 kN·m demand (utilisation 0.284).
Shear carries enormous margin at utilisation 0.017 — nowhere close to governing for a section this lightly loaded in shear.
Deflection tells the real story: δ = 5(0.45)(6000)⁴/[384(200,000)(1.869×10⁶)] ≈ 20.3 mm against the L/240 limit of 25.0 mm — utilisation 0.813, close to its limit.
What Governs, and Why
Deflection governs decisively at utilisation 0.813, close to its limit, while both strength checks carry wide margin (flexure 0.284, shear 0.017). A long, light purlin is almost always a deflection-governed design — pushing the span or the service load up even modestly would fail this check well before flexure or shear became a concern.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze CFS Purlin / Girt module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
For CFS Purlin / Girt and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Why does the same DSM engine give such a different governing check here versus a heavier CFS joist?
Which check governs is a property of the actual proportions chosen, not a fixed rule of the member type — a comfortable joist geometry (shorter span, stiffer section) can leave deflection as the tightest but still loose check, while a actually light, long-span purlin section pushes the identical L/240 limit far closer to its edge simply because span enters the deflection formula to the fourth power.
Does the roof sheeting help this purlin at all?
In reality, yes — a purlin's compression flange is very often laterally and torsionally restrained by the roof or wall sheeting screwed to it, which can meaningfully increase both effective flexural capacity and resistance to distortional buckling. This module doesn't credit that restraint, a disclosed conservative simplification, so the real purlin likely has more margin than shown.
Final Thoughts
A light, long-span purlin is the clearest illustration in this whole CFS series that member proportions, not member type, decide whether strength or deflection ends up governing.