You can use a plastic stress distribution for this, philosophically the same as for a typical composite beam. The first "gotcha" that I see is that, unlike a typical composite beam, much of your concrete is below the PNA so won't count for anything.

I don't know what type of coating is on the existing beams but I doubt that paint is a good thing. Does DG6 address the required coating, or lack thereof, for composite columns? It seems like the same standard should be applied here.

I think that encased beams are fully composite without shear studs, but check the Manual to make sure. I think it specifically says this. Even so, I'd feel better about it if I added some shear studs. I'd also add corner bars and stirrups at some kind of a wide spacing just for overall integrity.

It's been a while since your original post, so I'd be interested to know how this turns(ed) out.

DBD

Thanks for the reply DBDavis...the design is out and construction should begin in a month or so.  FYI, I ended up requiring paint removal and included shear studs even though AISC says beam alone will act composite as long as enough cover...also have longitudinal reinforcing as well as stirrups to help w/ durability.