Multi-Ply Beam Lateral Torsional Buckling Effective Width Toggle

Feature Requests
1

When a multi-ply wood beam is not continuously braced, the calculator assumes full composite action between the member plies. This is unconservative and makes it hard to estimate if the beam will fail if it’s not fully braced.

Would it be possible to allow the user to manually specify the effective width to use for the CL calculation? Could it be done as a function of ply width (e.g. 1.2 x ply width)? It would give us some more control instead of assuming full composite action.

I did some research, and there’s a paper by Waltz et al. in 2025 that investigates this exact issue with LVLs. They give recommended values for composite action in multi-ply lateral torsional buckling.

2

Hi Andrew,

You’re correct. When a multi-ply wood beam is not continuously braced, the calculator currently treats the built-up section as fully composite for the C_L / LTB check, meaning it uses the total width (n × b_ply) in R_B and I_yy. That’s unconservative for typical nailed or bolted plies, and NDS doesn’t actually call for that.

We agree a user-specified effective width for the C_L calculation is the right fix, including formulas like 1.2 * b_ply. That width would apply only to the LTB check, flexure, shear, and deflection would still use the full built-up section.

The Waltz, Amini, and Douglas paper (J. Struct. Eng. 2025) looks at exactly this for nailed multi-ply LVL. They found solid-section rigidity overpredicts LTB capacity and independent plies underpredict it, and recommend around 6% torsional and 25% plank-flexural composite efficiency for the nailed LVL configurations they tested. We wouldn’t bake those factors in as a default (NDS hasn’t adopted them, and they don’t map cleanly to a single width), but an override would let you apply 1.2x ply width, single-ply, or another value based on that paper or manufacturer guidance.

Until that input exists, the conservative approach is to take C_L on a single ply (b = b_ply), which lines up with WoodWorks’ recommendation too. There isn’t a clean way to get full composite flexure plus reduced LTB width in one sheet today.

I’ve logged this as a feature request, the more detail you can share, the better we can assess where it sits on the roadmap. A couple of things that would help: what does your workflow look like right now when you hit this (are you manually adjusting b_ply outside the sheet, or just noting it as a limitation)? And roughly how often does this come up in your projects?

Worth mentioning too, we do have a team that takes on custom and prioritised development work, and depending on your setup it could potentially be part of your licensing. Happy to put you in touch if that’s something worth exploring.

3

Thanks for the reply. The Waltz paper is quite useful and I hope other researchers look into this topic and expand the testing data on this issue.

After giving it more thought, I think the best way to implement this is to allow the user to input a custom effective width for the C_L calculation with a check to ensure it doesn’t exceed the total beam width.

Currently, about 5-10% of my wood beams are affected by this. When this happens I either:

a. Ignore it when the loads are small.

b. Use another software package to design the beam.

c. Look at how one ply performs and extrapolate the performance of a multi-ply beam.

I hope this helps!

4

Thank you so much for the idea and suggestions :smiley: this is great