I am building a structure with large frame timbers. The structure rests on 16 8x8 posts. There will be a foundation with a perimeter footer, but I want to know if the posts on the interior of the structure should have a thicker foundation as well. Should there be some sort of pier or footer under these interior beams poured with the foundation?
At a 3000psf bearing capacity of the soil underneath, each 8x8 post without footing can bear a paltry 27 sqft at 50psf live load.
(3000psf x 64 sq-in / 144 / 50psf) = 27 sqft
If this is all you need, you still have to worry about post tilt due to an uneven bearing capacity or hardness gradient under such a small post.
You need a soil analysis and a load calculation by a structural engineer.
Square footings are checked for “vertical shear” (the force of a post being pushed through the footing) and “bending” ( the force that causes the footing to bow and break).
Thickening the footing AND adding rebar horizontally in the footing helps both vertical shear and bending.
The size of the footing depends on the soil bearing capacity resisting the applied load. In order to resist all loads, the footing is thickened and rebar added to resist the loads.