Well...hopefully someone with more experience than I will chime in, but I believe blocking/bracing is important in this situation. You are basically building a raised deck.
Here is a loft I built w/2x12's 12" o.c., rim joist, heavy joist hangers & topped with 5/8" ply, screwed & glued. I still used blocking (you can see it down the middle of the pic). Span is 19'6"...:
