Design Check on Composite Steel Girder

[Stewie_]

Hi, Dear All,
Recently I am working on a composite steel girder project. I am using MIDAS Civil. I had a question regarding the design module and I asked the tech support. They answered as follows.
------------------------------
The Steel Composite Girder design module can only be used if you select the "All Frame" option while generating the model. In other scenarios, such as when modeling the girder as a beam and the deck as a plate, the girder should be designed using the Steel Design module, and the deck should be designed as a plate using the RC Design module.
------------------------------
I have no problem with the 1st part of their answer. However, the 2nd part, they said for a composite steel girder, we can actually design the steel portion and concrete portion separately. I am a bit confused. I think we can do that, but there must be something missing here.
Have you guys ever heard/tried this method to design a composite steel girder? Please advise and let me know your opinions on this.
Thanks,
Stewie