Pole Barn on the rocks..Literally!

[Smelt]

OK you Smart Guys,

Here's one for you. I'm trying to build a pole barn for my wife, er, um, I mean her sheep. It's going to be simple, 24' square, half on gravel with the other half enclosed and floored for hay storage. I'm going to use 6 x 6 pressure treats for the posts. Here's my problem: the place where I want to build said pole barn is pretty much on a slab of rock. Actually, our whole place is on rock, but that's another story. I can go down 1 or 2 feet before I hit big rock. Would this be deep enough for the poles? Would it matter if I drill the rock and stick in a length of rebar vertically, then drill a hole up through the post to sort of anchor it from skidding sideways. I will be running a 2 x 12 pressure treat "skirt" around the perimeter from post to post and double 2 x 8 (rough cut) inside and out at the top (8'). The roof will peak to 10'. Of course, I'll do the horizontal stringers and vertical sheathing.

I guess what I'm asking is; does a pole structure gain strength from being "in the ground"?

Your brillance would be appreciated.

 

[EddieWalker]

Is the rock solid? I don't mean if it's one rock, but is it solid to the point it could be your foundation? Can you drill it and put in concret anchor bolts?

If so, and I bet you can. I'd drill two holes on either side of you post and bolt an L shaped bracket to the rock. Then I'd bolt the bracket to the post.

One bolt on each side might work, but I'd consider that marginal.

Don't waste your time with rebar and drilling holes into the posts, that wont' hold anything. You need a solid anchor for any building, othewise, it's just something to fall down when the weather changes.

Eddie

 

[RobS]

As far as I know, most pole buildings rely on the poles in the ground for shear strength. This is unlike conventional stick buildings with diagonal bracing and such. On the rock, you'd probably be OK for vertical loads but may need to use sheathing in the walls for the shear.

BTW, I'm hardly an expert on this, but I do read TBN as often as I can (I did NOT stay in a Holiday Inn Express last night either)!

 

[junkman104]

I had to rent a jack hamer to bust through shale when I had mine put up. The builder had to have the holes at 3' minimum.

 

[bugstruck]

The rebar solution wouldn't be my choice either. If the rock is somewhere near level or can be chipped to level a properly connected post anchor (Simpson for example), with any voids between the rock and the anchor well grouted, would work. Still have to drill and connect the post to control uplift, not just shear. You could go two lifetimes and never need uplift control on that structure or you could need it tomorrow. We are in the early stages of a new commercial building. Wednesday evening a small twister took 20,000 SF of wire mesh and poly they just placed from the middle phase pour and flipped it over onto the phase one pour. Had that twister hit the building when the tilt-walls were up and braced (before the roof structure) it could have been ugly. And we seldom get twisters in Maryland. Twisters are vacuums in their own right and straight line winds over a peaked roof (from the right direction) turn the entire roof into an airfoil. That smallish twister and straight line winds are what the uplift control mitigates within reason, assuming the roof framing is properly connected too. Without uplift control I'd add a stabilizer and tail numbers The shear issue brought up earlier is valid but you can control that above grade with proper sheathing or bracing.

 

[CurlyDave]

I guess what I'm asking is; does a pole structure gain strength from being "in the ground"?

As it has been mentioned, the poles need to be set to the proper depth to take shear forces. With the rock you have this is going to be very difficult.

I would cost out a perimeter foundation and framing with 2x4s and sheathing. Maybe even a slab floor. I bet it would come out almost even with digging the holes properly in your rock.

 

[HappyCPE]

Without creating a big discussion regarding engineering [which I wouldn't be able to understand] I can tell you that anytime rock is encountered on a building site, it is removed [blasted, hoe ram, etc] to the frost depth in your area. Solid rock might eliminate bearing capacity issues such that you could place a perimeter foundation wall onto the rock without a footing, but you have to get down to that frost line. For bigger buildings we do rock anchors for the uplift resistance that has been discussed here. If any part of the sheep barn is open [including a big door] then wind uplift can be a serious issue. The specifics of pole building construction are somewhat new to me but I'm not thinking your site is a candidate for a pole building.

This website might be overkill but may be interesting:
http://www.dsiamerica.com/products/geotechnicalproducts.html

 

[bugstruck]

I can tell you that anytime rock is encountered on a building site, it is removed [blasted, hoe ram, etc] to the frost depth in your area. Solid rock might eliminate bearing capacity issues such that you could place a perimeter foundation wall onto the rock without a footing, but you have to get down to that frost line.

Happy, That is correct. That's how it is done now. Depending on the type of rock and the general stability though blasting and trenching rock just to pour concrete back is sometimes a waste on smaller structures. We are removing the rock on our building to the tune of thousands of yards just to pour a footer back into a full lock position within that trench. The first reason cited is to get to frost depth. Nonsense. There are some other better reasons for doing that on a tilt wall building but those reasons diminish a bit for a CMU foundation IMO, unless your in an area with perodic siesmic activity. CMU wouldn't be the preferred product in that environ anyhow. He's describing only a 24' square structure that could be modularized (2 modules) on a wood platform and set on a gravel bed with cable anchoring to the rock if the rock integrity was there. Not how I'd do it but it could be made quite acceptable for wind loading. Except that he is probably wanting an earth floor for his purposes. Dave's idea of a full frost foundation is a great solution until you consider cost against use. He can do what I recommended if the inspectors are understanding the design. The post are technically under the penetration requirements for shear at 1 to 2' but more is made of that than needs to be on this type of building. Adequately anchor it, connect it, and sheath it, then put a failure level wind load on it. It won't necessarily fail where one might think. With adequate post frequency he could build to 80-90 mph loading without too much effort and get it stamped. Ever notice that balloon framed homes are still standing after a major wind event while the modern homes (even the most recent vintages) are often destroyed? Mass and plaster add to that equation but if you've ever torn one apart, that framing method is pretty stout. The connections and shear are almost nonexistent at the foundation to baloon wall junction but they stand up well regardless. I poured a few footers on rock that didn't meet frost in the early days. They are still exactly where I poured them.