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Wood column backfill considerations and allowable soil bearing capacity

Wood column backfill considerations and allowable soil bearing capacity

Wood column backfill considerations and allowable soil bearing capacity

Just about to break ground on a post frame building for out new shop and have to make some decisions.

1. The columns will be embedded in the ground approx 5ft. The columns are of course CCA treated. Our original plan was to coat the bottom of the column with something like spray in bedliner material to seal it from moisture, and backfill wtih concrete. I am getting mixed responses on backfill methods. Some fold indicate nothing but soil should be used, some say sand/gravel, some indicate concrete. This really comes down to what is going to promote degradation of the wood. Thoughts?

2. It seems that vertical soil bearing capacity will be the driver for our foundation design. We are using only published book values as it seems that soil testing could be all over the map and no real indication if that will lead to a higher rating. So far we are taking credit from the tables for embedment depth, surface areas for our concrete cookie to be placed under the column, and possibly a 2x factor for isolated columns due to the wide spacing.

I am curious if there are any additional considerations one might be able to take credit for on allowing bearing?

RE: Wood column backfill considerations and allowable soil bearing capacity

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.

RE: Wood column backfill considerations and allowable soil bearing capacity

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.

RE: Wood column backfill considerations and allowable soil bearing capacity

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.

RE: Wood column backfill considerations and allowable soil bearing capacity

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)

RE: Wood column backfill considerations and allowable soil bearing capacity

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.

RE: Wood column backfill considerations and allowable soil bearing capacity

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.

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