I'd like as few posts as possible.
OK,
You have choices to make.
The long beam across the front of the mezzanine can rest on 2 columns, one column or it can free span the entire 30' distance wall to wall, leaving you with no obstructions.
Understand that with each column removal and span lengthening, the beam needs to become able to support more load and thus becomes deeper (Taller), thus restricting your total available height.
The first suggestion I will make, is to use a double LVL beam here.
So let's work from the use and load in the mezzanine. Since this is so short a space, it isn't considered habitable. So you can set the live load at whatever is reasonable. A living space would have a 40#/SF live load, but we can set it at 30# or even #25. Let's try 25# LL and 10# DL (Dead load is the weight of the construction.
Based on that, if you span the entire 30', you will have a 105/LF (Lineal foot) total load on the beam. Using a 1.9E LVL, you will need a double 1 3/4" x 14" beam. You can hang your floor joists so the bottom of the joists are level with the bottom of the beam, and where the beam rises above the mezzanine floor level, just use it as a toe kick,
2x6 joists at 24" on center are more than adequate. Use at least 5/8" plywood decking.
Since you are building a wall around the perimeter, no supporting ledger board, used as a beam, is required. Just close off the open ends of the joists with some 2x's to give them horizontal support. Think how you will prevent things from dropping down behind the perimeter walls. Maybe continue the flooring structure into the face of the outside walls.
Another option would be to use one column in the center. Then you could use a double 1 3/4 x 7 1/4 LVL beam. I would still make it continuous and install it one piece at a time and fasten together in place. Each 30' long ply of this 2 ply beam will weigh 108#. If you don't mind the column, this is what I would do.
Bill
