I have a CLT beam model under combined effects of out-of-plane loading, in-plane loading, and axial loading. Why is the bending normal stress from "in-plane bending" being considered as if it resulted from "axial" loading. It is the stress resulting out-of-plane bending (only!) being considered as "bending normal stress". It finally resulted in incorrect combined-stress uls verifications. Am I missing something here?
Hi MedTek,
Welcome to our community 
To consider the bending effect correctly for your design, you have to analyze the results with a result beam.
You will find more information inside this KB-Article:
https://www.dlubal.com/en/support-and-learning/support/knowledge-base/001685
Best regards
Paul Sivolgin
Hi Paul,
Thank you for your response. I understand what you mean. I, however, don't think I made my point very clear. Let me rephrase it ...
I modelled the CLT beam as a multilayer surface and I wanted to do a layer-wise ULS design check (via the 'Timber Design add-on') at surface-grid points (and/or mesh-node points). Here the stress-interaction formulations are called up on where the bending normal stress from "in-plane bending" and the normal stress from "axial loading" are treated as if they both resulted from "axial loading". Only the stress resulting from out-of-plane bending is considered as "bending normal stress" in the design checks. In the end, a uniaxial (out-of-plane) bending normal stress is interacting with an "axial normal stress" resulting from both in-plane bending (!) and axial force instead of an interaction between biaxial bending normal stresses and a normal stress from an axial load. I hope I explained it a little better. Thank you once again Paul!
Hi MedTek,
When dealing with in-plane stress, it can be unclear whether the stress is caused by bending or normal force. Without knowing the neutral axis (which, in this case, is unknown), it is not possible to differentiate between bending stress and normal stress.
What Paul is trying to convey is that the neutral axis should be defined based on the resulting beam. However, for timber, we face the challenge that tensile strength is lower than bending stress.
Unfortunately, the only solution is to check the stresses per resulting beam, analyze the stress ratio, and work with the effective tension/compression strength in this area. I understand this isn't the most comfortable solution, but at the moment, I don't see any other option.
This issue isn't limited to CLT; it's a general challenge for all timber materials modeled using surface elements.
Hi Gerhard,
Thank you anyway! I appreciate your effort/time.
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