hickened and turned down slab thickness and width

huangyhg

thickened and turned down slab thickness and width
i am looking for a good reference or a few good tips regarding thickend or turned down slabs at interior bearing walls.  
i am working on a load bearing metal stud wall building with wood trusses for the roof.  i have an interior load bearing wall and was planning on thickening the slab at this location.
my only reference is a tm-809 which dictates thickening my 4" to an 8" for a width of 5 feet.  i think this width is a little excessive.  any ideas or thoughts or suggestions?
my wall loads are 945 lb/ft.  this building is located on shale mostly - engineered fill partly, mountain top location.
thanks!
jody
just design it like an isolated strip footing. 12"x24" will likely work all day long.  of course, use the allowable soil bearing pressure to size the width, concrete shear strength (without the slab) for the thickness, as usual.  this is probably a little excessive but i doubt it is a lot, plus it simple, known, and easily to calculate.  8"x16", 10"x20" and 12"x24" are common sizes around my area and usually work very well (typical soil capacity is 2000-3000 psf).  
the only other consideration that i'd add to ucfse's post is that with a thickened slab, the downward settlement due to load is a higher consideration than an independent footing where the wall and footing can move downward independently from the slab.
with a thickened slab, any downward movement will be usually reflected in cracks at the slab-thickened edge interface.  so using a bit more conservatism in your "footing" design (per ucfse above) would be a good consideration to minimize or avoid these cracks.