Thank you Peter,

That's work but after sharing the whole structure with hidden bracings, you have to share the connexion between the structure and the bracings.

I did a modal analysis with the correct structure (verified in static) and the modal frequences are 0Hz with all the structure mass in the 3 directions.

How do you read that? Why the main modes are for 0 Hz?

Here a picture of the ratio between effective mass and total mass.

Thank you for your help.