Composite beams are complex.
What should RISA-3D use as the "effective" area of the beam for axial stiffness? Should this area be different for tension vs. compression?
Then temperature loads add an extra level of complexity. Part of the area is due to concrete, part of it is for steel. What sort of thermal expansion coefficient should you use? Clearly, it would have to be weighted in some way based on the area of steel vs. the area of the concrete, but what method would you use?
What's worse is that because of the different expansion coefficients of the two materials, you will get differential expansion of the concrete and steel. This would induce a bending moment into the beam, wouldn't it? I cannot think of a reasonable way to model this short of building the composite beam out of plate elments and such per the Modeling Tips section I pointed out in my previous response.
In the end, RISAFloor is really designed to handle gravity type loading on typical commercial buildings. It's a great tool for that and can be expanded out into some other building sectors as well. But, the more you lean towards specialty loading (like thermal loads or bridge loading) the more you're getting away from that program's strong points and the more limitations you will encounter.