# Should I have thicker areas of foundation under posts?

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?

• Normally "yes" but it's between you and your engineer depending on the soil loading and slab details. If not engaging an engineer, err on the side of doing more than you need to (including plenty of rebar.) – Ecnerwal Mar 22 at 17:51

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.

• Why do you say “at a conservative 3000 psi bearing capacity of the soil”? Do you mean 3000 psf? Btw, the Code uses 3000 psf as its largest bearing capacity without a soils test. If you’re saying 3000 psi concrete, then 2500 psi concrete is the highest without an independent concrete breakage test. What are you saying? – Lee Sam Mar 23 at 4:19
• @LeeSam yes 3000psf of course, fixed it. (And "conservative" was not the right word here) We don't know what OP is building or on what. – P2000 Mar 23 at 4:39

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.