[RSA 2023] Type 1 Instabilities with User Defined Timber Material

[DonBAE]

Attached model ran with no instabilities originally, inclusive of releases, Timber material was changed and now the model indicates several rotational instabilities. Review of the nodal rotations does not show anything excessive.

In addition when entering user defined Timber sections the property dialog window does not appear to correctly show all of the input parameters:

Robot Native Glulam Member:

User Defined Glulam, tab 5A is selected however the parameters do not coincide with tab 5A for the Robot default material:

[DonBAE]

It appears several material properties are not being computed with the user defined material which is causing the instabilities, switching to one of the default glulam materials in Robot leads to no reported instabilities in the model.
Robot Default Material:

User Defined Timber Material:

[Krzysztof_Wasik]

Hi @DonBAE 

Structure is horizontaly not stable in X direction due to too many releases defined (also for steel sections).

When top node(s) are loaded in X direction displacement is almost infinite.

Start material modification when GL name and GL softwood tab 5A category are selected

Then you can modify material name and define required properties according to table 5.A

[DonBAE]

@Krzysztof_Wasik "due to too many releases defined" it is a wood structures these are necessary releases I have changed the panel calculation model to stiffening diaphragm in X,Y which stabilizes the model globally as the edge nodes are supported in both X,Y directions. 

The instabilities appear to be caused by the lack of a shear modulus for the user defined timber material which remains uncalculated even after making a new glulam material as you noted above.

"Start material modification when GL name and GL softwood tab 5A category are selected"

this is a joke right, shouldn't the input parameters change to match the selected table. Why even let us select a different table if the parameters don't update, seems like this is faulty GUI coding.

[DonBAE]

User Defined Glulam (Computed G = 0.0 ksi):

Stiffened Diaphragm, lateral load produces very little deformations. 

Robot Default Glulam:
Shear modulus computed as 120 ksi all type 2 instabilities are gone

[Krzysztof_Wasik]

Hi @DonBAE 

Ad 1

You have not defined panels rigid in XY direction

When diaphragm is defined model reports instability type 3 regardless section type defined (steel, timber, concrete) and displacements for horizontal load is reduced to reasonable value. As far as I know G is not used (for bars) unless Consider shear forces is activated in Advanced Bar properties dialog.(tension /compression bars definition)

ad2.

This approach will let you to define required material parameters (regardless possible issue with GUI). Due to specific approach to American timber databases I would start new timber definition from selecting and Applying required set of parameters from More timbers  (available in Name drop down menu).

[DonBAE]

@Krzysztof_Wasik  see message 5 I have revised my model to include stiffening in x,y for the panel calculation, the original model attached to my first post did not have this. 

As you can see in message 5 the only change between the two screenshots is the material selection between a built in Glulam definition and  a new user defined material which used the built in glulam definition as a basis.

[DonBAE]

As you noted with the Robot Default Glulam there is still a Type 3 - UX instability.

This instability is reported at a node within the rigid panel plane how is this instability, UX - Translation in X, possible if the x,y plane is rigid and supported?

[Krzysztof_Wasik]

HI @DonBAE 

I have checked again with your updated model. You are right. User defined timber material does not save G for American database. It is of course incorrect software behavior.

G is used directly for torsion rigidity calculation (sorry for my mistake in previous email). Timber members with user defined material have no torsional rigidity. This is the reason for unexpected instabilities.

I am registering that as the issue for correction

[DonBAE]

Any chance this correction can be fast tracked, this leaves Robot unusable for several open projects?

[Krzysztof_Wasik]

Hi @DonBAE 

I have registered issue with high priority.

[DonBAE]

@Krzysztof_Wasik understood, Thank you for taking the time to look into this!

[DonBAE]

@Krzysztof_Wasik are you able to share any news about a hotfix to correct this issue, we have a few project deadlines approaching and it would be extremely helpful to be able to use Robot for the analysis? 

[Krzysztof_Wasik]

Hi @DonBAE 

I have checked ticket work log, and I see that Development is working on it. For now I do not have more information

[okapawal]

Hi @DonBAE 

The problem with saving G value for timber material has been corrected in the 2023 Hot-fix 1.

Which is just released.  Please find more details on

Robot Structural Analysis 2023 Hotfix 1

[DonBAE]

@okapawal excellent news thank you.

how is G being calculated as there is no entry for poissons ratio for timber material?

EDIT:

Also installing the hotfix via the Autodesk Desktop app the app indicates the install fails.

the about dialog shows the correct information, seems a disconnect between the installer and the desktop app, but now this update notification will persist in the Desktop App.

Also your teams need to communicate better as there is no About under the help menu, it is located in another dialog.

[okapawal]

G value is assumed 120 ksi for all US timber materials.

Did the installation finish with success?

[DonBAE]

@okapawal "G value is assumed 120 ksi for all US timber materials." 

Umm that is entirely incorrect, G will vary based on the various species Youngs Modulus.

"Did the installation finish with success?"

yes

---

[okapawal]

Timber is not an isotropic material, I wouldn't apply the standard formula E=2G(1+v).

[DonBAE]

Agreed, however applying a single value to all species is grossly wrong. Please refer to this document which provides stiffness ratios in the three member planes: https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch04.pdf