concrete piers for large sheds

[illusioneer]

I'm designing a 16'x 20' work/storage shed and would like resources that will provide instruction on concrete pier dimensions and relation to strength. In particular I'm concerned that a suggested 8" diameter column buried 22" in the ground and protruding 12" above may not be sufficient without increasing the diameter and reinforcing with rebar. I will be placing the piers every 4' to support beams made out of two 2"x6".

I would like find a website or book that will take a do-it-yourselfer through the calculations for related issues on strength, size needed, forces to consider. I will be storing wooden theater props (200-300 lbs) and rehearsing with 3 people inside this "mini theater". So far all the build a shed books only give formulas for amount of concrete needed.

 

[RDK]

What you are calling columns buried in the ground are really piles. They will support the load by skin friction. Normal skin friction values are 250 to 300 psf of pile surface.


Your 22 ft pile at 8” diameter would have piD=3.14x(8/12)= 2 ft surface per ft length. Here we generally ignore the top 8’ of pile due to frost effects, your local practice may vary, so we get 14 ft effective length 14’ x 2 sq ft x 250 psf= 7,000 lbs load capacity. This is more load than the 2 ply 2x6 can carry.

The load will be the dead weight of the structure usually less than 5 psf for light wood frame structures. You will have to add snow, wind earthquake and rain loads as well as live loads, which are those due to use and occupancy. The building code will have these values for you.

For further information try searching for university courses that have on line notes and worked out examples. I have done this and found a wealth of good information freely available in all engineering topics.

You also could try talking to your local structural and geological engineers to get an idea of local practices, but generally if this is what experienced builders are using in your area it is usually sufficient.






Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[jimbo]

The original post said 22 inches into the ground, not 22 ft..

There is a problem somewhere there which needs resolution.

 

[ishvaaag]

Well, if the piles sounded more than enough, the stubs seem beautifully ... just? But well, there are houses in the trees as well, and scarce calculations seem to be made for them. It is on the awareness of owners to decide if they want thorough checks about everything they own.

To the eyes of an european living in an environment of RC and masonry buildings, these ways look flimsy. But we are not well placed to judge since in the same way construction people living out of carpentry have difficulty understanding the views of those living these more heavy environments, we neither have (but through calculation) proper way to indicate our preferences respect what contrarily in the USA, wood framing etc, is a way of life.

This said, I would go for a continuous perimetral foundation, and according to my preferences and for the temperate zones I live, no less than 90 cm deep for the structural concrete, plus somewhat more to provide a clean surface, and warn off the expected bad effects of ground freezing. This may look there to some to kill flies with cannons, but we do it customarily and not even blink. Our homes are sturdier -I don't necessarily mean safer!- even if we don't have, say (for the moment), the tornado risks that affect significant parts of the USA.

 

[SirAl]

My fence posts extend more than 22" below the ground surface! If it is a shed you are building, why not set it on pressure treated blocking and shim from time to time to level?

 

[Dogface]

What you are calling columns buried in the ground are really piles. They will support the load by skin friction. Normal skin friction values are 250 to 300 psf of pile surface. > RDK

Would not skin friction values decrease for increases in pile diameter? After all, if you double the diameter of the pier, you double the skin surface area but the mass increases four times. Soon the skin friction could not support even the mass of the pier.
If the above is true, does it follow that for the same mass of concrete in friction piers, longer and thinner is better?
Thank you