Anyone Familiar with Pole Sheds? 4

[zstructural]

I have been asked to design the foundation for a pole shed. They said they typically dig an 18" diameter hole, drop the 6X6 wood post in, and pour it solid with concrete half way up.

I have a couple of concerns.
1) is it okay to bury treated wood in concrete?
2) will the concrete bond to the wood to resist uplift? Or, should I have them leave some bolts sticking out of the side of the post to "grab" the concrete?

Also, are there any other issues I should be concerned about? My plan is to design the post and footing as cantilevers, cantilevering up from the ground, and the framing will be by the building supplier (I was not asked to review the framing above grade)

Thanks so much for your input!

 

[msquared48]

I commonly do provide a couple of rebar throuh the post, extended into the concrete for uplift.

As for the concrete, I commonly pour the concrete to the top of the grade, and yes, treated lumber is embedded in pole footings all the time.

Just use the equations in the IBC to determine the proper depth, based on the soil type, and whether or not there is a slab present.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MiketheEngineer]

I have done a bunch of them.....commonly referred to as Pole Barns or Post Frame construction.

Yes - treated posts are typically buried about 48'' into ground with and without concrete. I usually use a couple #4 rebars drilled through the post to "grab" the concrete or earth. Note that the post might deteriorate at 20-30 years.

These building are not meant to last a lifetime.

Size of hole depends on post(truss) reaction, wind and soil values.

The cantilever design is very conservative but good. You might be disappointed with your results - depending on building size and height. You can(might) get some shear help with the metal cladding.

Check out National Frame Builders (NFBA.org) I think and anything written by Frank Woeste or Stan Suddarth.

There a many companies that do this every day - like Morton, Wicks, BBL(Missouri), Borkholder (Indiana), EPS(Iowa) etc.

 

[zstructural]

Thanks Mike and Mike! The building is open on 3 sides (one of the three butts up to an existing building, so only two sides are truly "open"), so I only have cladding on one side to help out with transferring the wind loads. I do have a concern with the totally open end that butts up to the existing building. I don't know much about the existing building yet. Perhaps I can tie into it with the end truss, or talk them into installing posts/footings along that wall (it might not be getting opened up).

Thanks again for your help, I will look into the references you suggested. If you or anyone has any other input/opinions, I'd love to hear them.

 

[Brad805]

I personally think pole buildings should be left on the farm. I have never followed the lateral restraint mechanism engineers use to justify these to meet codes.

Brad

 

[Splitrings]

ASAE 486.1 Shallow Post Foundation Design and an ASAE paper named "Uplift Resistance of Post Foundations" are a good place to start.

I typically specify my embedded posts be treated to AWPA Category UC4C.

 

[a2mfk]

Brad- they building these things in the city now?

I have a feeling that many a client has been unhappy with a structural engineer's actual design of such a structure. I imagine the initial client meetings are like- "well, what we usually do is _____, and that's been working for ____ years now.... Why the hell you need so much darn concrete and all these fancy connections?"

 

[boo1]

We usually specify kiln dry before treatment .60 pcf (CCA or ACQ) or 2.5 pcf CCA for projects that can afford it. Provide a means to hold the butt off the ground to maintain cover. Consider hot dipped galv spikes or #4 rebar for uplift. I have seen coatings systems used for the inground portion.

 

[zstructural]

Splitrings, do you know if the UC4C is much more expensive than the UC4B (0.80 vs. 0.60)? I'm reading that there should be a minimum of 0.60 for typical pole-shed posts - so I'd like to specify the 0.80 especially because I will probably be burying it in concrete, but I don't want to give the contractor a heart attack either. A2mfk is right - what they said they "always" do is quite skimpy. Through my research in the last day (my budget is BLOWN, but it's a learning experience) I've realized the pole building community has devised their own way of calculating their unconventional methods. I'm just glad I only have to design the foundation and not the whole building.

I think I'm going to have to bite the bullet and buy their ASAE 486.

If anyone has any other input, I'd love to hear it. There are a ton of different foundation types so it's helpful to hear what people have had actual success with in the past, and perhaps not-so-good success with.

Thanks!

 

[Splitrings]

zstructural,
If you are treating southern yellow pine, there is no difference in treatment levels between UC4B and UC4C other than ACQ is not allowed for UC4C. The posts I have been using have all been special order, (ever seen a 14"x14"x18' SYP post? Maybe that isn't a post when they get that big!) so I am guessing specifying a different treatment level other than 0.60 pcf of CCA is likely not going to be considerably more money. If you are trying to use something locally available, you are likely going to be stuck with UC4B, likely ACQ preservative.

ASABE 486 is free on the ASABE website. Here is the link:

http://asae.frymulti.com/azdez.asp?search=1&JID=2&AID=29222&CID=s2000&T=2

Most of my foundations have been for 40-60' pole buildings. Most posts are 8x10 SYP (snow loads in Vermont). I have been using a 30" diameter footing and 1-2' deep collar around the post with rebar pins extending into the post. Embeddment depth is typically 5-6'.

 

[zstructural]

Thanks Splitrings.
I'm not a member of ASABE, so I don't have access to that.

Thanks for your input. One question - Do you get the bearing to check out for the rebar embedded? I haven't plugged through my calculations yet, so I'm not sure what kind of uplift I'll have, but it will probably be fairly significant because I have open walls on 2 adjacent sides. Or, is there some sort of calculation for the bond the concrete might have with the wood?

 

[Splitrings]

My service level uplift forces are typically less than 2000# per post. These are easily resisted by a couple of 5/8" dowels. The dowel fastner calculators on the AWC website don't do double shear with concrete, so I assumed two pins in single shear. I am actually using two pins (pass completely through the post into concrete on both sides) that are oriented 90 degrees apart.

 

[spats]

I have two fairly old documents on pole building design that were produced by the American Wood Preservers Institute. You might give that a try.

 

[boo1]

Sliderruleera had 2 pdfs on polr barns at:

http://www.slideruleera.net/miscellaneous.html