The shear stress in the 2x6 top plate is what usually controls how much load you can deliver to it from the 2x12.
If you have a single top plate (double top plate is a standard code requirement in almost all codes), with an 85 psi allowable shear stress in the 2x6, the 2x6 will have an allowable load from the 2x12 of about 234 pounds, the further the 2x12 moves to the center between the 2x6 studs the higher the load can be, if the 2x12 is exactly in the center between the two studs, the allowable load can go up to about 468 pounds (2x234)
If you have a double top plate the allowable load will be 468 pounds (2x12 close to the studs) and 935 pounds (2x12 close to the center between the two studs)
Using the worst case scenerio of single top plate and 2x12 close to the stud, the allowable uniform load on the 2x12 joist with an 8 foot span 234 pounds times 2 (shear on 2x12 is half the total uniform load) = 468 pounds divided by 8 feet = 58 pounds per linear foot of joist, with 16 inch spacing would be ~ 45 pounds per square foot total allowable dead plus live load on the 8 foot joist.
If the top plate is doubled, this translates to 90 pounds per sqaure foot allowable dead plus live load on the 8 foot span.
I hope this helps.
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