What fixities would you consider for both column and beam, in a scenario where beam is modeled as a stub cantilever, but with the back span?

In a 4 story building, I have a beam in the third level, which I have modeled as a stub cantilever with a back span, since I wanted  two different beam sizes .I have modeled them as lateral to modify the fixities. I have defined the fixities as shown for the beams. For the Column, both top and bottom are fixed.  RAM steel beam gives me a design for both the back span and the stub cantilever. (Back span is treated as simply supported I suppose). The steel post processor in RAM frame shows that the back span member is failing with a S.R. = 3.5. Could anyone please suggest what should the fixities for the beam and the column be?