A sawn hardwood timber beam, designed to AS 1720.1, applies the same family of k-factor modifications seen in AS 1720's glulam provisions — but for a solid sawn section rather than glulam, and it's a useful comparison point for how the same code family's stability and duration factors apply slightly differently depending on the specific timber product.
Every check above runs automatically inside the Structyze Timber 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: F17 stress grade hardwood, 100 mm × 300 mm, span 3.6 m, continuous lateral restraint along the compression edge via connected flooring
- Characteristic bending strength f'_b: 42 MPa
- Design bending moment M*: 10.8 kN·m
- k_1 (load duration, short-term floor live load combination): 0.94, k_4 (seasoning): 1.0
Working Through It
Because the beam has continuous lateral restraint along its compression edge (via the connected flooring providing effective bracing), the stability factor k_12 is not reduced below 1.0 by lateral-torsional buckling concerns — a materially different starting point from an unrestrained beam of similar proportions.
Design bending capacity φM_b = φ × k_1 × k_4 × k_12 × f'_b × section modulus = 0.9 × 0.94 × 1.0 × 1.0 × 42 × (100 × 300² / 6) × 10⁻&sup6; = 15.9 kN·m.
Utilization: 10.8 / 15.9 = 0.68.
Shear and deflection checks both clear with wider margins, at utilizations of 0.41 and 0.52 respectively, leaving bending as the governing check.
What Governs, and Why
Bending governs comfortably at utilization 0.68, and the comparison with an unrestrained glulam beam example elsewhere in this series is instructive: here, continuous lateral restraint from the connected flooring keeps k_12 at 1.0, meaning this beam's full material bending strength is actually usable — a much more favorable outcome than an equivalent beam left without that restraint would see.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Timber 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 Timber 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
Does restraint from connected flooring always count as adequate lateral restraint?
It generally does when the flooring is positively fixed to the beam's compression edge in a way that really prevents lateral movement and twist, but a design shouldn't assume this credit automatically — the specific fixing detail needs to actually deliver the restraint the calculation assumes, similar to the continuity detailing discussed for RCC slab support conditions elsewhere in this series.
Why does a hardwood stress grade like F17 have a different k_1 than a softwood species might?
k_1 (load duration factor) in AS 1720 is primarily a function of load duration class, not species — species and grade mainly affect the characteristic strength value (f'_b) itself, while the duration factor applies consistently across stress grades for the same load duration category.
Final Thoughts
The same lateral restraint principle that governed the earlier glulam beam example cuts the other way here — with restraint actually present and effective, this beam gets to use its full bending strength rather than a buckling-reduced fraction of it. That's why restraint condition, more than species or grade, often decides which of two similar-looking timber beams ends up with the more comfortable margin.