A cold-formed truss member's tension path skips the Direct Strength Method entirely, while its compression path needs the full global-plus-local buckling check — the same physical section, but a truly different level of calculation depending purely on which way the axial force points.
Every check above runs automatically inside the Structyze CFS Truss Member 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 100 × 100 × 2.0 mm (box/tube)
- Unbraced length 2500 mm
- Factored axial load 100 kN (compression case)
- Material ASTM A1003 Grade 50, Fy = 345 MPa
Working Through It
For compression, the identical governing-minimum method from the CFS stud/column chapter applies: global buckling Pne ≈ 203.3 kN, DSM local buckling Pnl ≈ 183.9 kN (local governs), φPn = 0.85(183.9) ≈ 156.3 kN against the 100 kN demand — utilisation 0.640.
For the identical section and force in tension instead, the calculation is dramatically simpler: φTn = 0.90(345)(784)/1000 ≈ 243.4 kN — no buckling reduction of any kind, since a thin plate in pure tension simply doesn't buckle. At the same 100 kN demand, utilisation drops to just 0.411, noticeably more margin than the compression case.
This tension/compression asymmetry is how DSM's local-buckling machinery exists specifically for compressive and flexural capacity — it has nothing to check in pure tension, where gross-section yielding alone governs, identical to how a hot-rolled tension member would be checked.
What Governs, and Why
The compression case governs at utilisation 0.640 versus tension's 0.411 for the identical section carrying the identical force — a direct, numeric illustration of how much capacity local buckling costs a thin-walled compression member relative to the same section in tension, where no such penalty applies at all.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze CFS Truss Member 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 Truss Member 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 a CFS truss module even offer a tension/compression toggle if the geometry is identical?
Because which failure mode governs, and by how much margin, genuinely depends on the direction of force — a diagonal that's in tension under one load case and compression under a reversed load case (say, wind suction versus wind pressure) needs both checks run explicitly, not just whichever one happens to be more convenient.
Does this module check the end connection separately?
No — net-section rupture and block shear at a bolted end connection aren't checked here, in either tension or compression; only gross-section capacity (and, for compression, the local-buckling reference) is covered. The actual end connection needs its own separate check via the CFS Connection module.
Final Thoughts
Watching the identical CFS section's capacity swing from 156.3 kN in compression to 243.4 kN in tension is a clean, concrete demonstration of exactly how much local buckling costs a thin-walled member — a cost that simply doesn't exist on the tension side.