Lots of questions need to be answered. If you go over to www.johnbridge.com they've got both structural engineer and tile experts that can help you with this. You need to do some detective work first, though, they will need to know how far apart the piers are, what the beams sizes are, how thick the subfloor is and what it is made of and probably some other things! The part you make the shower on and the part you tile may need different prep. To tile, you can't do it over dimensional lumber, it needs to have something to decouple the seasonal movement of the boards from the tile. You can do this with a mud bed, or plywood and cement backer unit (cbu) or a decoupling membrane. Anyway, go over there and you'll get some good answers. It will probably take a lot longer to get the info here - they specialize only on tiling, so it will go faster.