When is Post & Pad used?

[akce]

newbie here. I must place a 500sf cabin on 12' GP & GW-GM Alluvium (fair material in my opinion with allowable bearing pressure = 2000psf) in a cold climate. Tests confirm < 8% passing #200, so frost heave should be small (even if tests say moist to wet soils?). I'm looking for references regarding post/pad type foundation design, but not finding much. I don't understand what keeps them stable....sort of like a structure standing on toothpicks. If the posts were buried a sufficient depth (say 5') that would be different, but I see cabins with the pads sitting on the finished grade and simple blocking from the pad to the floor beams. I'd like to bury CIP concrete pads a foot or so and use sonotubes or masonry piers to say 18" above grade. I'm not sure an adjustable connection is req'd, but if it were adjustable (to correct frost heave) it likely would not have much moment resistance. (I guess the connection is more of a structural issue, but welcome your thoughts.)
QUESTION 1. Do you agree or disagree that this might be an effective foundation for this small structure?
QUESTION 2. What factors do I consider in choosing foundation materials (AWW, reinforced concrete, masonry, steel, etc)
QUESTION 3. IRC 03 doesn't seem to address this foundation style; would it meet IRC or IBC? (I struggle with the code)
QUESTION 4. Any reference material recommendations?
QUESTION 5. How is lateral resistance transferred from soil to pad, to pier, to structure (earthquake, wind, flood, etc)?
QUESTION 6. Should the piers extend to seasonal frost depth (6') even with the soils mentioned?
I look forward to your responses. regards

 

[oldestguy]

This maybe a late answer or comment.

This appears to be similar to supporting a house trailer, but at some height. In this area I see these setting quite high to get out of the flood levels. A number sit on barrels also. A number of them are on posts also, but it seems the posts are buried at the base to get lateral stability.

A neighbor here has an Amish built cabin like that and it sits on concrete blocks on end, but not a 5 feet. There also are cables to small caissons to hold it down in case of tornado winds. The caissons were installed before moving the cabin onto the site. Can you install some form of in-ground system first, such as a caisson with a hollow center to take the post?

If it were me, I'd provide some form of moment connection at the upper end of the posts. Take a look at what farmers do with a shed that has no walls, but a roof only on posts. The last few upper feet are secured with diagonal struts set into the post slightly and bolted there.

Many an old multi-story factory has the structure built this way. The diagonal struts at the top ends of posts provide the lateral stability along with the bending capability of the posts.

However, getting the cabin to that height probably means using a series of cribbing supports that will provide the lateral stability until the posts are securely fixed.

If the floor system will not take this form of strut, you can reinforce it with 90 degree planking on the underside of the joists.

If this is in an area subject to flooding, be aware that in addition to just water pressure, you may have log jams that will put much more horizontal load on the system.

Will this system permit diagonal cables, in addition to the struts? You can really do a job with a batch of diagonal cables and turnbuckles.