I want to model the ceilings as force-flow-oriented ribbed slabs in a skeleton structure made of reinforced concrete (similar to the type shown in the picture).
What is the best way to model the ribs and the slab on top in RFEM6? With beam members and rib members on top?
Hello 
I would generally not recommend a combination of beam bars and additional rib bars. Rib bars are basically intended for coupling with a surface.
My general suggestion would therefore be:
Define the slab as a surface
Define the desired beams as lines
Model the beams as rib bars (assign bars to the lines)
Modeling beams is also clearly demonstrated in the following webinar 
:
https://www.dlubal.com/de/support-und-schulungen/schulungen/videos/002953
In this particular case, however, it might make sense to model the bars as beam bars and represent the offset via eccentricities 
. I would base the chosen modeling on the desired results.
Feel free to get in touch if you have any further questions. 