bathtubframing
Replacing a bathtub with a new one pictured. It will go into a 3-walled space. Durock and tile will be attached to wall to finish the job.
I've been reading many articles and they mention a ledger board for tub support.
I drew a picture. Is the ledger board supposed to be under the flange side (as pictured in black) or do you nail the flange to the ledger board (which means I'd have to shim all the studs to be flush)?
I was and still am in complete agreement with paperstreet's answer, buy another tub. I've never used this system. These look like a fail waiting to happen. Available from schluter.com, asked about at johnbridge.com calling them field installed flanges.
No. Your joists must bear on the beam directly, using hangers or actually resting on it. Even if you'd bolt the ledger up to the bottom of the beam you'd be relying on cross-grain strength. Not a good situation.
As to your beam sizing question, that's for an engineer to answer (with drawings in hand). We don't have enough information about the structure as a whole to properly answer that. I can speculate, since 2x8 lumber isn't used for floor systems anymore, that even tripled you wouldn't meet most codes.
Best Answer
Your tub should have clear and specific instructions. It varies by model and material.
However, if a ledger board is used, it'll be under the flange, providing support from below. There's no point to a ledger if you have a typical framed wall and can simply nail the flange to the studs every 16".
The only tubs I've ever installed using a ledger are large spa-type tubs, and those are usually set into a full deck. Typical shower tubs simply get fastened to the wall framing. Do follow the instructions, though, or you'll void your warranty.