Answer:
In the case of using timber materials in RFEM, a message appears when creating the material, saying that the "isotropic linear elastic" material model is only correct for the calculation with members because the Poisson's ratio is very large. For surfaces, it is necessary to select a different material model.
If you have assigned the material with the orthotropic material model to the surface, it is not possible to use certain orthotropy types. This material cannot be used for ribbed plates.
However, if you have assigned the material to the surface with an isotropic linear elastic material model, you can access all types of orthotropy.
When entering the geometry for the ribbed plate, Error No. 30343 appears due to the incorrectly assigned timber material for the surface: "The computed stiffness matrix is not positive definite".
