Hello Gabriela
I am actually working together with christina in this project.
We followed your advice and used the 1D-Beam element. It seems to work fine.
The reason we initially tried to use the "volumetric" Beam element was that for our system it seemed to be more comfortable for the mesh building. It would have been nice to use the 3D beam because the coupling to other elements would have been a little easier and in the theory manual it was described that a real "beam kinematic" is used (not like with the isobeambrick where all strain components are used according to the manual).
However it is a bit of speacial case with us:
We work on a parametric study we do the "meshing" on our own and generate every node and element by hand with the help of another FE-Software where you can easily code the meshing routine and then kind of translate it to atena. In our special case this is very convenient for us as we just need to build the model once and we use the results of both programs to compare.
But now its all working - thank you for your help