I'm thinking that I can use interior basement walls, kinda down the middle of the basement, to support a joint in the first floor's joist system.  The basement slab would be poured before these walls are constructed, a treated bottom plate would sit on the slab and the wall's studs (2x4s) would sit on the pt plate.  The problem I for-see is that the floor is not level in places with slopes to floor drains etc.  How does one get a flat & level first floor?  Is it common practice to build these walls say 1" or so short then add shims under each joist?   Any other ideas on how to cope with the floor's irregularities?

The guys that framed my basement cut each stud to the exact height needed to make the upper floor system level. First, the treated bottom plate was tap-conned to the basement slab (which was ~12" thick at the area of this load bearing wall). Then studs were cut to the necessary height at each end of the wall to support the top plate and make it level. The first top plate was installed and temporarily braced to keep it straight and level. Then individual studs were cut to keep the top plate level. These studs were then toe nailed to the bottom plate every 16" with a nail gun using galvanized nails and then nailed through the top plate into each stud. Then the second top plate was added after the intersecting walls were installed.

My upper floor is a modular so the marriage wall runs down the center (long length) of the house and is the primary load bearer. The upright supports consist of a combination of adjustable steel supports (3), and tripled 2X4 attached to a double bottom plate and double top plate (2). The steel posts are lag bolted to the marriage wall 2X10s at the top and sit on the concrete floor, which has been thickened to specs wherever there is a support. In the places where 2X4s were used instead of steel posts, the doubled pressure treat bottom plate is glued to the concrete floor with the 2X4s nailed to it and to the doubled top plate which is nailed to the marriage wall 2X10s. The 2X4's were cut to size. For the 46 feet there are 3 steel post supports and 2 2X4 ones.

-Rosalinda

Posted By Liebler on 12 May 2012 11:44 AM
I'm thinking that I can use interior basement walls, kinda down the middle of the basement, to support a joint in the first floor's joist system.  The basement slab would be poured before these walls are constructed, a treated bottom plate would sit on the slab and the wall's studs (2x4s) would sit on the pt plate.  The problem I for-see is that the floor is not level in places with slopes to floor drains etc.  How does one get a flat & level first floor?  Is it common practice to build these walls say 1" or so short then add shims under each joist?   Any other ideas on how to cope with the floor's irregularities?

If you are transferring floor and or roof load to the basement floor with this wall, there needs to be a footing. Almost always better to pour a strip footing c/w curb to carry this wall. The curb should be poured level so a conventional wall can be set up.

Posted By Liebler on 12 May 2012 11:44 AM
I'm thinking that I can use interior basement walls, kinda down the middle of the basement, to support a joint in the first floor's joist system.  The basement slab would be poured before these walls are constructed, a treated bottom plate would sit on the slab and the wall's studs (2x4s) would sit on the pt plate.  The problem I for-see is that the floor is not level in places with slopes to floor drains etc.  How does one get a flat & level first floor?  Is it common practice to build these walls say 1" or so short then add shims under each joist?   Any other ideas on how to cope with the floor's irregularities?

The proper way to build it is to run a string line across where the top plate would be and cut each stud to fit

You can pin a double treated plate at wall locations on the footing and then pour the floor, as a bonus the plates give the flatwork guys a good screed point. The wall is then "rough" flat and can be built quickly with 1 stud length, gauged and planed flat. Generally the fastest but you end up with 3 runs of bottom plate but then a load bearing wall is directly on the footing.