Wood column backfill considerations and allowable soil bearing capacity 1

Old post and framed buildings I've looked at have often had a large flat rock at the bottom of the post and have been backfilled with compacted granular material. I've also seen some treated posts rot off at the interface of the concrete. Unless the concrete is carried a few inches above grade and the top sloped for drainage, I'd avoid concrrete.

Coating posts that go in the ground with the thought that you are protecting them may backfire when some leak develops or deterioration at the top allows moisture to enter and do its thing, held there by the coating.

For bearing - you do not say the size of the post. Wood piles with typical 7" tips are used in pile design. I am not sure that end bearing is the only criteria to consider; what about wind load? There was a few recent posts that you might want to search that discuss pole buildings.

First off, I would not treat the end of the posts, but bear the end on a gravel backfill for the bottom 6", then fill the rest of an augered hole with concrete. This will allow any accumulated moisture in the post to drain out the bottom of the hole.

This sounds like a pole structure, so it should be contructed as such.

What is the eave height and bay spacing?

Mike McCann, PE, SE (WA)

Thanks for the replies guys.

Mike, I appreciate the ideas. When you say "don't treat the end of the post", are you referring to just the end grain, or the entire section that is below dirt grade?

We have discussed the use of gravel for both the foundation below the column, and backfill of the hole. For use as the foundation, we don't do this stuff everyday and just don't have any data on how the gravel interacts and how to account for it.

Our proposed method is to use 5-6" precasted concrete cookies at the bottom, and 1000psi CLSM backfill. The only reason for CLSM is reducing costs since the full strength of concrete should not be required.

The rough dims of the building are 60x96, 16ft wall height, and columns 12ft on center. We have a 20psf live load on the roof that is driving the foundation design.

The code requirement is 75mph cont/90mph 3sec peak. We are designing around meeting that code for a unconstrained column condition, then shooting for about 130mph in a constrained case with a 6" concrete floor. Constraint would be on the windward wall only. We considered leeward constraining options but decided it would not be a good idea as locking the columns into the floor could cause problems.

Right now we are at 24" diam holes at 6ft deep, with 6" cookies in the bottom so column is 5.5ft deep.

This discussion is about totally different practices than agricultural pole buildings we have been building in Canada for who knows how long. We use a poured concrete footing in an augered hole, stand the CCA (or whatever) treated post on it with either nailed-on uplift anchors or drilled & cast-in-place anchors for larger uplift, and backfill with native soil or granular, again depending on what's required for uplift. The treatment of the post is your only defense against rot, you're dreaming if you think the backfill has some bearing on it. Around here, lumberyards stock treated 6x6, 6x8, & 8x8 up to 20' long for this. Canada Plan Service used to have a website with standard design charts that anyone could use, but it is no longer active. I'm pretty sure Google will find old copies for you, or at least lead you to the info. 60'x96' is a decent-sized barn, but I've done them 80' wide clearspan (more with interior columns) and anywhere up to 250' or 300' long and I'm not an ag specialist.

You have to remember that uplift is probably going to govern, even in Canada where we have snow loads.