Footers

[htmt6612]

I am looking to buy a prefab shed that is 12 feet by 30 feet, however, since it will go in a flood plan i need to elevate this structure 12 feet.  I was thinking about using an individual peering system  where i would dig down 36" and fill a concrete tub and three add 3 pieces of rebar and then put either 6*6 or 8*8 on top of that and connect them at the top with two 2*10  using bolts.  Can anyone tell me how many post i would need and if this type of footing would work.  Thanks for any help you can supply.  I am not sure if it matter but the structure is going in Hampshire County in WVA.

[peter nap]

Welcome to the forum.

IMHO and only IMHOm three feet isn't enough. I'd go 5 and fill 1 foot with gravel and put the post in concrete and rebar to ground level.

I think you could go 12 feet between posts but again, to be safe, I'd space at 10 or less.

These posts will all need to be tied together and I'd use a 2x10 splash board at the bottom just to be safe.

This is bigger than what your doing but it gives you an idea:

[John Raabe]

I would check to see what is being done by the professional pole builders in your area. There are lots of options and trade-offs depending on your layout, climate, soil and local materials and equipment.

Setting your poles deeper as Peter suggest will add stiffness and bracing against sideward forces such as wind and earthquake. It may also be required in cold climates to get down to frost depth - but I expect 3' may be plenty for your WVA - at least in terms of frost depth. You may still want to go 5' for the wind resistance.

The pier spacing is determined by the bearing capacity of the soil and the size of the beams spanning between the poles. In good solid soil you can have fewer poles with heavier beams carrying the loads between them. You will want someone to do a load trace on your design so that you don't waste money building structure you won't use, or, even worse, undersize something you really need to depend on.

[peter nap]

Johns the expert and pretty much the absolute word in these things.

I tend to be a little over cautious. The pole barn is one I'm building for myself with some changes...like the roof.

The designer specified 6X6 poles every 12'. I changed it to allow a 4X6 in between every 6X6. It's my tender body in that thing during windstorms.

[John Raabe]

Always a good idea to overbuild if you're not sure.  8-)

Peter, thanks for your comments but I'm not a real engineer so my opinion shouldn't be counted any more than anyone else's. When I do understand the general forces at work, I try to explain what I see. The specifics can vary enough that it can be difficult to tell someone exactly what to do based on a general description like these questions necessarily are.