Depth of footing for tractor shed?

[lennyzx11]

Unsure about a footing and wanted to get your thoughts.
Question is at the bottom after the information.

I have to build a retaining wall about 3 ft deep 20 feet long to hold back a difference in elevation for my shed I’m building. The lady at the code office of our small town seemed a little ditzy.

The shed will be pole barn style with 6x6 posts.

Code for frost depth is 50” here in Southern Vermont.

I wanted to set the posts on concrete but I’m told the whole footing for those back posts and retaining wall should be 5 ft deep all the way for 20 ft. A stem wall and set the posts on it.

The question is. Should that whole wall be built on a poured footing 5 ft deep. Or just sonotube the post locations that depth?
That depth more than doubles the height of the stem wall and seems excessive.
Thanks,
Lenny

 

[Have tractor did travel]

Depth is right - below the frost line. Can't speak to the continuous foundation vs sonotube

 

[KennyG]

For a pole structure, the sonotubes are the only footing required. Technically, you don't even need the concrete. If the poles are deep, they could be backfilled with stone. Some people prefer this for buildings without concrete floors.

The retaining wall is an interesting situation. Most places don't require permits or inspections for a 3 foot retaining wall. Depending on the soil situation, it's probably more important to have lateral stability. If it's all poured, I would want to have some type of tie back method into the bank of soil. I like the retaining wall block that fits together. That seems to resist movement well with only a shallow gravel bed.

 

[Jchonline]

Your county/city doesnt have a building code handbook or something that goes over this stuff? Mine is pretty specific in CO.

 

[s219]

Normally for a pole barn, you drill a post hole down below frost depth and either pour a footing (about 5" thick but it will depend on concrete mix and the bearing requirement) or drop in a pre-cast concrete cookie. Then set the post and backfill the hole. Backfill can be the spoils if suitable, or pea gravel, or dry-mix concrete. Normally pea gravel and dry mix are used when the spoils are not suitable, say if it's clay soil and cannot self-compact.

The benefit to this method is that you get tremendous side-load and bending moment resistance to the poles when set down and properly backfilled. This gives very good shear resistance against wind loads and other things that would make a wall want to move. Use diagonal lumber braces across wall girts to maximize the shear resistance.

The diameter of the hole you drill is going to be based on the soil bearing capacity and the size of the footing you need to transfer the post loads down to the soil. Normally I start with the roof area, calculate the snow load, figure out how much each post carries, then take the soil bearing capacity and back out the footing size. You can add 15-20% extra margin to be safe but it's usually not necessary. Often, regional snow load ratings have some extra margin built in already.

 

[s219]

I should add one thing -- for my smaller pole barn projects, I will sort of do the reverse for footing calculations. I have a 12" auger for my PTO digger. I can determine how much weight a 12" diameter footing will support. Then that tells me how many posts I need on the eave walls that will support the roof load. Same sort of calculation as above but instead of calculating footing size I calculate number of posts+footings needed. That's the lazy man's approach when you only have a 12" auger...

 

[jfh28]

if I understand this correctly.....the retaining wall will become part of the rear wall of your shed and not separate from the building.....if this is correct then the retaining wall will be considered part of your building and thus the foundation for it would need to be below frost line.......as a side note you sure wouldn't want the retaining wall frost heaving as your back wall......if the retaining wall is separate from the shed.......for instance like ten feet away from it then it would not be part of the building and at that point I would check your local codes for specifics and talk to local professionals familiar with your soil conditions but my gut feeling would be no the retaining wall would not need to be below frost line......Jack

 

[Carl_NH]

I agree with Jack above - put the retaining wall 10-15' from the building and then bring the grade up to the shed which may mean you need a lower wall potentially, Then backfill and get close to the grade/elevation you need for your pole barn.

Then as s219 and others have stated, do your calculations for load and poles, then go to the town with your design/layout for the permit (I assume they require one as most NE towns do). If you go in prepared you will face fewer issues in getting a permit.