An aluminium beam supporting a lightweight roof structure, designed to CSA S157, often finds deflection, not bending strength, as its governing check — a direct consequence of aluminium's modulus of elasticity being roughly a third of steel's, meaning deflection calculations produce proportionally larger results for the same geometry and load.
Every check above runs automatically inside the Structyze Aluminium Beam 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
- Beam: aluminium alloy 6061-T6 I-section, span 6.0 m, simply supported, continuous lateral restraint via roof decking
- F_ty: 241 MPa, E: 69,000 MPa
- Specified (unfactored) live load moment for deflection check: 8.2 kN·m equivalent; factored design moment for strength check: 13.6 kN·m
- Deflection limit: span/240 for this roof beam application
Working Through It
With continuous lateral restraint from the roof decking, bending strength capacity (using the full, non-buckling-reduced section capacity per CSA S157's provisions) is calculated at φM_r = 18.9 kN·m.
Utilization for bending strength: 13.6 / 18.9 = 0.72.
Deflection under the specified live load is calculated using standard beam deflection formulas with aluminium's E = 69,000 MPa — giving 28 mm, against a limit of span/240 = 6000/240 = 25 mm.
Utilization for deflection: 28 / 25 = 1.12 — this beam fails its deflection check despite passing bending strength comfortably.
What Governs, and Why
Deflection governs and fails at utilization 1.12, even though bending strength clears at a comfortable 0.72 — a pattern that shows up considerably more often in aluminium beam design than in steel, precisely because aluminium's lower modulus of elasticity (roughly a third of steel's) means the same span, load, and section proportions that would pass a steel beam's deflection check by a healthy margin can fail an aluminium beam's equivalent check outright.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Aluminium Beam 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 Aluminium Beam 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 aluminium's lower stiffness make deflection control relatively more important than for steel?
Deflection is inversely proportional to the material's modulus of elasticity for a given load, span, and section — aluminium's modulus being roughly a third of steel's means an aluminium beam deflects about three times as much as an identically-sized and identically-loaded steel beam. That's why deflection rather than strength more often governs aluminium beam sizing.
What's the most effective fix for an aluminium beam failing its deflection check?
Increasing the beam's depth is typically far more effective than increasing its strength grade, since deflection depends on the section's moment of inertia (which scales with depth cubed) rather than on material strength at all — a stronger alloy does nothing for a deflection-governed design.
Final Thoughts
An aluminium beam's bending strength passing comfortably tells you almost nothing about whether it will pass its deflection check — the two are governed by fundamentally different material properties (strength versus stiffness), and aluminium's relatively low stiffness is the reason deflection deserves its own explicit, early check rather than an assumption that a strong-enough beam is automatically stiff enough too.