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08-08-2010, 11:16 AM #1
Pole barn:concrete to perimeter details?
I've built a lot of buildings including pole barns, and poured a fair amount of concrete. So it was a surprise to realize I've never spec'd or poured concrete in a pole building.
Our EAA chapter received approval to build an addition to our 8,000 sf heated hangar. The purpose is to move the tools and machinery out of the original building into a ~2,000 ft addition (36 x 58) & fireproof it so we can also add welding & other "hazardous" activities on site.
The "old" hangar is a steel building with continuous massive footers and formed concrete piers with lots of engineered rebar. The new building will be offset, freestanding, and we got approved for a wooden framed pole building. There will be a short breezeway with firedoors & a firewall connecting them.
The question is, what's the best way to detail a free floating slab, to the poles and perimeter? We will probably add a zone to the boiler, insulate with 2" of blueboard (as in the old hangar) and pour 6 - 8" of concrete with the tubes buried in the slab (radiant heat).
However, this is potentially liable to some frost movement since there is (not intended to be) any footers except the concrete that sets the poles. We basically intend (have preliminary approval) to throw the building up over dirt, and then finish over time, including the floor pour. My best idea (feel free to shoot it down) is to set 2 x 12 PT lumber on edge at what will be finish floor grade on top, and lap the steel siding to that.
First, I assume with modern corrosive PT there ought to be some sort of isolation of the lumber from the steel so the steel does not corrode at the bottom?
Second, does there have to be a "slippery" separation between the concrete and the lumber, including the poles, so that if the slab would heave, it can't lift the building out of the ground?
Any links to "ideal" construction of this type?
This is upstate NY.
Thanks for any thoughts or shared experience!
08-08-2010, 11:38 AM #2
I've got 3 "Professional" built pole buildings. I'm in PA, not quite as cold as upstate NY, but gets close. 5" of concrete poured over enough stone to level the base. 2X12 PT bottom skirt board, steel screwed directly to board. 1/2" of fibre board around perimeter of slab,including poles, to provide seperation and expansion room.
10 years old now and no heaving or bottom rust............Bob
08-08-2010, 11:41 AM #3
I did the concrete floor in the pole barn thing for my shop. My pole barn was already there, so I had to pour the floor in it. Having the option, I think I would have poured the floor, THEN built the barn, probably with steel basement columns instead of 4x4s and anchored them to the floor. I have 22yrds of 4,000lb mix and 1500ft of #3 rebar in a 5" slab. I have about 20,000lb of machine tools on one end of the floor. Been there about 5yrs now and no cracks at all. Rebar was biased to the outer edges, on about 8" centers for the first three rows, then out to 10", then 12, then 15, etc... ends up about 24-26" centers in the middle. If it doesn't crack at the edge, it doesn't crack. Any excess bits of rebar I had were put in the bay doorway for unloading heavy machinery.
08-08-2010, 11:45 AM #4
08-08-2010, 11:48 AM #5
Pour all your piers first.
Then build your form between piers, and pour the slab separately. Leave the carboard on the tubes to separate the two.
If the slab and the piers aren't connected, any heaving shouldnt move the building.
Your siding doesnt need to be tied to the slab either, it just hangs on the purlins. Bottom skirt board used to form slab can be tied to piers, but not slab.
It is recomended to isolate PT, simply tacking some tar paper between the PT and siding would suffice.
08-08-2010, 11:55 AM #6
keeping the soil underneath well drained will reduce its frost movement.
08-08-2010, 12:07 PM #7
I have 3 pole barns I have had built over the last 15 years, and there are NO piers anywhere.
the whole advantage of a pole barn is they just drill the holes with a bobcat with a drill bit on it, stick in the poles, and fill em with concrete. All in one morning, usually.
All three of my pole barns just had the concrete poured right up to the treated 2x12 around the perimeter, and all have just had the sheet metal siding screwed right to the 2x12 as well. No rust yet.
We get a lot of rain, and it still seems fine.
08-08-2010, 12:37 PM #8
Sometimes sonotubes are used, sometimes not. I built one a few years ago where we had to bury the poles on one side, and put them on piers on the opposite side.
Nowadays its becoming frowned upon by inspectors to put any wood below ground, it all depends on whether or not its getting inspected, and the opinions/disposition of his local inspector.
08-08-2010, 02:28 PM #9
08-08-2010, 02:50 PM #10
Sounds like the basic construction approach is acceptable.
I'm going to go ahead and try to find "something" maybe just some rubber roofing tape to make it easy, to go between the metal and the perimeter board at the bottom.
I feel better about fiberboard to isolate the slab. We probably won't use any sonotubes. But I actually thing a sonotube would have enough bond between the 2 concrete surfaces to not provide a good shear surface, at least in the first few years. Appreciate the idea & the feedback though.
My experience with pole barns is to plant the poles, plumb and align them including partial construction, and then fill the holes with concrete. Sounds like it would be useful to check local practice, (Most of my pole barn construction experience was in Maryland a couple decades ago). Does anyone think it is important to pour concrete under the posts, as well as around them (2 pours)? I remember doing that for something on a commercial job. (as opposed to a farm).
Based on the input here, i feel good about drawing up a section to run by the inspector. I honestly can't remember if we had the steel hangar inspected when we built it in '96 or not. I think we may have had the electric service inspected, and our food service/water is inspected periodically. But can't remember on the building. So getting some ideas before I draw it to anyone's attention off the AP, if that proves necessary.
Thanks for getting me started!
08-08-2010, 06:11 PM #11
One thing I'd recommend for a cold climate is to trench the perimiter about 24" deep and put 1", or 1 1/2", rigid board insulation in the trench and then backfill. Will prevent frost from penetrating under the floor, and keep everything stable.....also saves on heat bills.
mfgbydesign liked this post
08-08-2010, 07:12 PM #12
You usally dig the holes and put a precast concrete pad in the bottom. Then put the pole on top of that, then pour your conrete. I would also put foam board between your rim borad and the concrete slab to prevent or at least slow down thermal transfer from the frozen ground to the slab. 2" is good but 4" is better. Proper insulation is very important in the long run.
08-08-2010, 07:54 PM #13
Why no Sonotubes? Aren't they the first line of defense against heaving?
08-09-2010, 08:31 AM #14
Stephen, a product has been developed that is replacing the old sisalcraft flashing around openings, commonly referred to here, as "Bitch-a-thane", though I believe it's spelling is Bituthane. It is much thicker than sisalcraft, conforms well to irregularities, (stretchable) and molds it'self around fastener shanks, making a very good barrier. Comes in rolls of 6" and 12" and probably other widths. More money than sisalcraft but a permenant barrier of uniform heavy thickness, maybe .060", that doesn't require that you bathe in solvent after applying it. Laps "weld" together naturally, a 1" lap more than enough. For this reason, it comes with a waxed paper separator between roll wraps.
An aside, in the early seventies, we of the San Diego chapter of the EAA were given the OK to erect some light metal hangers on the apron at Brown Field, an uncontrolled airfield very near the Mexican border. "But you can't change the surface, the asphalt must run through the buildings unchanged." Of course the asphalt was graded for run-off, so I pierced it with 1" (IIRC) all-thread, I hammer drilled through the asphalt and then drove the 3' allthread in with the hammer drill and shot, (transit level) elevating nuts to the grade of the asphalt high point of each hanger, the asphalt falling away from the walls for the remainder. One of these for each vertical structural member, perhaps 10' apart.
Doesn't rain much in San Diego so it worked out well, can't leave stuff like metal or cardboard on the "floor" though. The occasional critter seeks refuge within too.
Haven't seen them in close to 40 years, wonder if they are still doing the job. They were of course intended as a temporary installation.
Aha, here it is: http://www.metrowaterproofing.com/documents/BIT3000.PDF Spelled Bituthene. I noticed the PDF does not show the narrower rolls that I've used in the past.
08-09-2010, 07:24 PM #15
I'm getting a good education here!
That's something I had not considered, but it would be a real timesaver. Thanks for mentioning it!
Bruce- Tell me more. My (limited) use of sonotubes has merely been for cheap pier forms if the look was acceptable instead of square, carpenter build forms. Sounds like I may have missed the point that they break the "grip" of the soil? We always undercut, though. And if (square) formed, the area below the form is wider, so not been an issue. But you have got me thinking for this project. I'm still inclined to to do without, but interested in more info.
Robert: sounds ingenious. Bet you didn't have frost heave there either!
I'm trying not to be obsessive here. But want to cover the bases. Thanks for the link to the Bituthene, that looks like what my mind was going for when describing the "roof tape". Don't need waterproofing per se. Just a quick to apply reliable separation between the steel and the modern PT lumber. It may be the ticket depending on price and local availability.
I'm pretty sure "good" subslab & perimeter insulation will be used. We really did it right in the main hangar and it has paid off. OTOH, this building is a whole order of magnitude more "budget conscious" so sometimes stupid voting trumps good planning.
Thanks all, this has helped a lot and clarified my thinking on some of the issues.
08-10-2010, 03:42 AM #16
A note...short story,whatever.15 or so years ago designed/built trusses for an add-on pole barn section for a draft horse guy.They planted poles yadayada.My part was simply roof design and only had limited access to location of said roof.It was 75' long attatched to exist. bigazz barn.Anyhow a few years later the whole shebang went through a hurricane and was probabably exposed to sustained 75 mph winds over a period of 1/2 a day.To my delight,although horse boy only saw the problems,my trusses never batted an eye.Metal roofing got torn off maybe a 1/3rd.Trusses still hangin in.....the problem is they worked just like a wing and lifted 1/2 the poles out of the ground.The posts were 4' in the ground with concrete.Some were pulled out of conc. some pulled conc. with them.
I told him originally trusses came w/lifetime warrenty,so helped him rebuild structure for free.The design was 4/12 backside and around a 9 or so 12 front total span @24'.Poles were set back from fr side in the typ. fashion.Just beware of "lifting",windbreaks and in general which way the wind blows.BW
08-10-2010, 05:55 AM #17
I had a friend who was a civil engineer. We were discussing construction (back when I lived in NY) and the subject of Sonotubes came up. His explanation of Sonotubes was just as you suggest - they prevent heaving by allowing the soil to slip past the concrete. Prior to this, I thought that you were OK if you installed a pier so that penetrated past the frost line, but he assured me that without a smooth surface the pier would heave to some extent. He said he had seen many examples of incorrectly designed piers that heaved. He was also in favor of perimeter insulation to prevent frost under a slab.
Edit: I forgot to add the point about distortion of forms. Smooth walled forms can be OK except that the pressure of the concrete distorts the shape of the form. The resulting shape may or may not be OK depending on how well the form is built. Sonotubes distort as well, but they just increase in diameter (very slightly).
08-10-2010, 07:20 AM #18
There is a lot of advice here, and not all of it is sound.
I've been a concrete GC for 19 years now, and I guess you can say I've poured my fair share.
A couple of tidbits of advice:
After drilling all your pier holes (whatever you want to call them for the sunken posts of the building), you can use pre-cast concrete footers that drop right into the bottom of the hole. Before setting your posts on top, I always nail 2X4 cross anchors perpendicular to the posts so that they stick out about 3" in all 4 directions. Better anchorage.
Concrete is very corrosive, even to PT lumber. Over time, it will eat away at the timber. The cross members help anchor the posts in the concrete.
Insulation between the (base) rim PT perimeter boards and your slab should be considered critical. Besides the R factor, it also prevents the concrete from getting under your perimeter and heaving the building.
With a radiant heat system, you will not need to worry about heaving, as the heating system will prevent the ground from freezing, as long as it isn't turned off. The perimeter is at danger for frost heave, hence the need for insulation.
Rebar is very important, especially around the perimeter, and at entrance doors where heavy equipment will be on the slab.
My personal pole barn was poured with a grade beam slab. 12" perimeter, and 6" center thickness. Criss cross rebar around perimeter 8" OC, and 16" OC in the center of the slab. I park 2(two) 32,000 GVW trucks, always loaded full, inside the shop. 7 years and running, no cracks.
Stainless steel fasteners for todays PT lumber. I've seen galvanized eaten in 5 years to the point of easy fracture.
That's my .02. I'm in NW Illinois with a 48" frost line every winter. It's safe to say I've done a "few" of these.......never, ever a call back.