Client calls into my office at the end of the day Friday and says his Building Official will only accept his new pole building construction with holes 48 inches deep, with six inch thick concrete cookies in the bottom of the hole, and no concrete backfill around the columns.
Here is some background….
The building is a commercial pole building 40’ x 60’ x 14’. The client purchased engineered plans for the building, which includes all of the supporting calculations for the design.
While a hole for a pole building post might seem to be just a hole in the ground, lots of things are happening below the ground. The embedment has to be deep enough to put the bottom of the footing below the frost line. The footing beneath the column has to be large enough in diameter to keep the building from settling. The design must also provide for the resistance for uplift.In this particular building, the downward load on the footing is just over 5400 pounds. The uplift force is 1120 pounds.
Now it is 4:50 on Friday afternoon, so I ask for the phone number for the Building Department, and I quickly set my fingers to dialing, not expecting to find someone this late on a Friday….
Happily, I was immediately connected to a Building Official. The issue turned out to have absolutely nothing to do with any local requirements, and instead came from the client’s builder.
The builder insists upon digging the holes with the 12-inch diameter auger he has mounted on the back of his farm tractor. He refuses to set the posts in concrete, because if he doesn’t get a post in the right place, he wants to be able to move it around in the hole. His idea is to dig a four foot deep hole, and drop 12-inch diameter concrete cookies in the bottom of the hole!
I can foresee a myriad of potential problems coming up, even without breaking out my trusty stack of Tarot cards or my crystal ball. Assuming somehow these holes are able to pass the hole inspection (contrary to the engineer sealed plans) – a 12 inch diameter concrete cookie covers roughly 0.76 square feet of surface. Applying a load of 5400 pounds to it, means the soil bearing capacity would need to be somewhere in the neighborhood of 7000 pounds per square foot (psf). Table 1804.2 of the International Building Code gives a value of 4000 psf for sedimentary and foliated rock and 12,000 psf for crystalline bedrock. Neither of these types of soil would be touched by the 12-inch farm tractor auger. The probability of settling issues on one or more of these columns – right darn close to 100%.
The diagonal distance across a 6×6 (actual dimensions 5-1/2” x 5-1/2”) is nearly eight inches. Those 12-inch diameter holes better be pretty much spot on and perfectly plumb, or there are going to be some very interesting looking walls (as in not very straight at the ground line).
This builder does not want to backfill any of the holes with concrete to prevent uplift. With a hole this tiny, there is no way to even begin to attach an uplift cleat to the sides of the columns. There is also no way to adequately tamp compactable materials into the space between the column and the sides of the hole.
A registered design professional has designed this pole building. He has years of experience and has designed literally thousands of successful buildings. At his fingertips are the most powerful computer design programs. This design is nothing short of a work of art. His seal means “you can trust this building to be safe….and sound.” Ditch the concrete cookies – they are just not going to begin to do what is required by your new pole building.
Mike, I’m planning my pole barn and the the local building department has asked that I just “hand draw” the plans tobe submitted. My question is about hole size. My frost line is 36 inches, I’d like poles on 8 foot centers and my roofing material will be metal tiles at about 1.5 lbs per sq foot. The wind design is for 90 miles/hour gusts. My soil is sandy on the surface with clay below. (Eastern Colorado.) My question is this: Will an 18″ diameter work if the hole is 48″ deep with 10″ of concrete poured in the bottom? Thanks for all the posted research and blog.
Phil ~ Thank you for your kind words and for being a loyal and avid reader.
Keep in mind, when you submit “hand drawn” plans, you are now the designer of record and in the event of a structural failure (even years from now) you could be held liable. If you are going to piecemeal your building together yourself, I would recommend you consult a Registered Design Professional to do your plans – in many cases, it will also get you a discount on your insurance. With that said…..in order to determine the adequacy of any given hole to withstand the applicable forces is a very complex process. Before any calculations could be done, it would need to be determined if the materials being used for roofing and siding are adequate to provide diaphragms to transfer the wind loads to the ground. Excessive openings in endwalls, a tall narrow building, skylights, etc., could all cause otherwise adequate materials to not be able to deliver the needed strength. The difference in load to be resisted by the hole is a factor of FOUR between having adequate diaphragms and not.
Once past the first hurdle, all of the parameters of building dimensions (width, length, height and roof slope) need to be considered as well as the climactic loads (snow load, wind speed and exposure and seismic forces) and the dead weight of the structure itself.
The answer to your question – maybe.