Anchor bolts in pvc

There are three types of PIER foundations: 1) Friction piers, 2) End bearing piers, and 3) Combination of 1 and 2.

1. Friction piers rely on the soil to “grab” onto the side of the pier and keep it from sinking. The size (diameter) and length can be determined by calculating the resistance of the soil, load, etc. This is not your type of support to use.

2. End bearing piers are based on size, spacing, soil type, etc. Your foundation can be calculated based on the size of shed, size of floor beams and how much load they can span, etc.

3. Actually, this is somewhat experimental, so you’ll just need to estimate direct bearing capabilities and let friction on piers be a safety factor.

If you live in a high wind area, seismic active area, or deep frost area will add to the complexity of your design.

Your proposed 3’ spacing seems ok, but a 2x sill plate will not span 3’, unless the shed has a very light load (a place to store your push type lawn mower) and uses plywood siding to keep it from sagging. Also, your idea about extending the piers only 4” above ground is not acceptable. The code requires wood (siding, door trim, etc.) to be 6” above grade unless treated. Your sole plate can pressure treated, but it will only reduce its ability to support vertical loads. Also, most anchor bolts require 2” cover on its edges, so 4” seems too small. As someone has suggested, 6” would be more typical, but check calculations and recommendations from a place like Simpson connectors catalog.

I’d use 6” piers, with 4x6 at perimeter with 1/2” anchor bolts extending up through the beam and plate…at a minimum. If typical footings in your area extend 24” into the ground, I suppose that could work too, unless you have an extra heavy load… like a riding lawn mower.

Do you have termites in your area that could hide in the space between the pvc and concrete?