Have you already had a look at out Girder_wBars.cc3 example (installed in the Examples\ATENA Engineering\Tutorial subdirectory of the ATENA program directory)?

I guess the simplest torsion setup would be 2 elastic loading plates, one at the top and another one at the bottom, each loaded in one corner, e.g., the top one on the left by a displacement or force in -Z direction, the bottom one on the right in +Z.

I am trying to model a concrete beam under torsion using ATENA 3D
I used the first load case as fixed supports to one end of the beam.
Could you please help how to apply torsion to the other end:
1- as prescribed force
2- as prescribed deformation

best regards