Answer:
In RFEM, the easiest way is to use the "Plane Surfaces" → "Annulus" function. This allows you to model the reinforcement rings very quickly and easily. Then, you can extrude the inner line into a surface; the chimney is already generated. Now, the existing annular surface is copied upwards and the corresponding stiffeners are modeled. You can use the "Rotate" command to distribute the stiffeners over the chimney perimeter.
It is then necessary to integrate the lines of the reinforcement ring into the quadrangle surface of the chimney. Otherwise, the elements are not connected to each other.
