HI Artur,
Sorry for my "not answer" of yesterday but I was out of office for work.
The following picture show what I mean for "going crazy":

Well, I used the midline points of the complex shell to create the underwall as a stiffener, but the shape of the upper complex slab it show an out of plane shape, I can't understand what really happened.
However, working with materials and loads (depnding by the construction method of the stair) I found a possibly solution. My question now is that:
How can I set the reinforcing direction parallel and orthogonal to the midline of the complex slab? I'd like to use curved bars for longitudinal rebar and radial ones for transverse rebar, as shown in the following picture:

How can I do this??????
Here comes the model.
Thanks a lot, cheers

PasProStudio
www.pasquiniprogetti.eu
Structural + Detailing engineers