I am planning on building a new cabin next spring utilizing 12" concrete piers as the foundation system. The cabin will be 24 feet deep by 30 wide. I will place 3 rows of piers, with 4 piers per row spaced 8 feet between each pier within the row. There will be a span of 15 feet between each row of piers. By question is this, what size of wood beam should be place on top of the concrete piers. I plan on using 2x12 floor joist on 16" centers placed on top of the wood beams. The floor joist will have a 15 foot span between the wood beams. The cabin will be a single story structure. Any advise will be greatly appreciated.