Originally Posted by AndyGump
For many years now most rooms other than bedrooms have been required by code to support 40 lb. "live loads" and 5-10 lbs. of "dead load" per sq. ft.
More that likely your existing joists are designed to handle these loads, though you have not said what the present joists sizes and spans are and I do not expect that you could know this.
In all probability you will be fine in installing a normal sized tub.
I do know that the rooms above the kitchen are supported by 2x10 16" O.C. that span 11 FT bearing on an exterior wall and a center wall most of the length of the house. The wall ends at the living room, however which sits underneath where the tub would be, and unless there's a beam in there somewhere that the joists are hung on, or perhaps change direction, the span could be double that. The floor doesn't seem excessively bouncy so who knows.
Can i stiffen the area up by Framing a floor system where the tub would sit where the joists in the new floor system sit perpendicular to the existing floor system effectively transferring the load over several floor joists at one time instead of just sitting over a single joist and subfloor?