footing for pole barn posts

[NoTrespassing]

This info comes from a very nice pole building book I have. It has tons of info in it and I refer to it along with my framing book when I build.

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Contrary to popular thought, placing a concrete pad under the pole is not the most effective way to increase its bearing strength. According to pole engineers, a necklace of concrete poured around the pole increases its bearing strength considerably more. This is because the majority of the vertical load is transmitted to the soil by skin friction with the pole. Thus, increasing the surface area contact with a concrete necklace improves both the pole's vertical and lateral loadbearing ability.
For poles in average to good soils at the proper embedment depth, a 12inch concrete necklace poured below the frost line should be used. Lag screws should be embedded 6" from the bottom of the pole to help tie the pole and concrete necklace together.
After the concrete necklaces have been poured and allowed to set up for several hours, backfill with sand or crushed stone.
For poles in below average soils or poles that are at less than ideal embedment depth, a full necklace of and earth/cement mixture can increase the bearing capacity of the pole. Soil should be screened at 1 inch to remove large rocks and mixed 5 parts soil to 1 part concrete. This mixture is almost as strong as concrete for the purposes of a necklace and is considerably cheaper. Fill the entire hole up to within several inches of the surface with this mixture, using lag screws along the pole to secure it to the necklace.
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Keep in mind, the above is for MAXIMUM RIGIDITY. Many of the buildings in this book support an entire 2 story+ house on poles. Just thought you might be interested in it.

Kevin

 

[bebster]

Cowboydoc, you're probably right for most pole barns. 4" is pretty thin cookie if the load on a pole is large, but then again that's probably not the case with most pole barns, since there are usually plenty of poles to spread the load, and typically not that much weight in the walls and roof.

If the barn has a few really beefy posts and has a loft and extra materials in it, 4" won't be enough -- but again that's not your typical pole barn -- I'd call that a timber frame barn instead. But in the interest of simplicity, you could always use 2 cookies for a total of 8" deep footing and that would carry quite a bit of weight for heavier pole barns, and would be cheap insurance if you had doubts.

It's really important to make sure the cookie is below the frost line (which is a local factor known by all builders and building inspectors in the area). This is a function of how deep the hole is and has little to do with how the base of the hole is treated.

 

[SPIKER]

hi guys:

the "Concrete Cookies" they used for my barn were bigger than those stated above, they werte about 16" across and 6~8" thick. solid and weighed alot... anyhow no lags into the poles were used and only clay/rock backfill was used (basically what came out of holes) then water settled and tampped in after 3 months of natural setteling with a 2" spud bar.

anyhow that is typically how they build them around here.

my barn is 38x50x10, 2 story with 10' x 50' x 8' porch on one side. see here at http://www.bright.net/~ispike/home2

Mark M /forums/images/graemlins/smile.gif

 

[yooperdave]

<font color="blue"> </font> Contrary to popular thought, placing a concrete pad under the pole is not the most effective way to increase its bearing strength. According to pole engineers, a necklace of concrete poured around the pole increases its bearing strength considerably more. This is because the majority of the vertical load is transmitted to the soil by skin friction with the pole. Thus, increasing the surface area contact with a concrete necklace improves both the pole's vertical and lateral loadbearing ability.

<font color="black"> </font> WOW - I must respectfully disagree.

I hope we have some geotechnical engineers here to back me up on this. Cowboydoc is appropriate.

Skin friction takes place when you drive a pole or pile into virgin soil. Usually you need many feet of earth to engage any amount of usable skin friction. The first couple of feet are usually ignored due to top soil and/or debris.

A pole building foundation is usually drilled or dug. The soil around the pole has been replaced, and is not virgin. The depth of the pole is usually determined by frost depth.

The size of this pole barn is very small, and a precast concrete cookie is more than structurally adequate.

This is a very easy building to build and analyze - do not make it harder than it is.

Good Luck

Yooper Dave

 

[mopower440]

the 'concrete cookie' method would be nice to do, but ive looked at lowes and home depot and havnt seen any, they only have the rectangle shaped blocks that are maybe 17 3/4inches long and 7 3/4 inches wide and maybe 3 3/4 inches thick, unless thats what you guys are talking about? They usually use these under skids on sheds and stuff, is this what you are referring to and are they strong enough? they seem to be made the same strength as cinder blocks..

 

[hockeyhead]

Find a local lumber yard that creates pole barn kits. They will be able to supply them or get you a source. If not, and I cannot imagine that, make them by drilling your holes and pour them 10" thick. Set poles on top after hardening. Around me there is Manns, Carters, Wicks, and Bannana lumber that all have standard kits or build you a kit to your requirements or sell you only what you need, again good luck with your barn building adventure /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif

 

[rambler]

Kevin, you are the only one to mention what I thought was code for pole bildings, at least in some parts - that the concrete foundation needs to tie into the pole with lag bolts of some type. Just setting the post on top of some footing won't do in some locations. Wasn't going to mention it, as I don't know all that much about it, but your method is my understanding of it as well.

Around here, small little carsheds are made with 6x6, but big machine sheds are often built with basically telephone pole sized poles. We have to go 6' to get below frost line, and this clay packs pretty tight. Lots of side-friction in that setup.

--->Paul

 

[hockeyhead]

42" here in S.E. Michigan is code to get below frost line, 6x6 posts on corners 4x6 posts are all the rest. I was required to have three inspections, 1) hole and post, 2) framing 3) final.

 

[NoTrespassing]

Yooper Dave,

I'm not sure why the author used the term "skin friction" in his explanation. I believe you are correct, that term should be reserved for driven pilings.
I also believe however that the authors design greatly increases the end bearing capacity of a pole and eliminates the posibility of frost heave.
That being said, I have never found it necessary to use this method and would probably only consider it for a pole that was going to support an elevated floor.

Heres a link to the book that the quote came from:
http://www.civilbookstore.com/index/book/0913589160.html

This is the civil engineering section of Amazon.com. It looks like a great place to find info.

Kevin

 

[bebster]

If it were me, I'd put those 3 3/4" thick pads 2 deep in the bottom of the hole just to be safe, especially on the corner poles. 7" wide is a bit narrow, so I'd also put them side by side for the first course, and then at right angles for the second course.

But in any event, put them below the frost line. Attached is a map from the EPA that shows approximate frost depths in the U.S.