treated poles in concrete?

[ljmathias]

Need some advice- sons and I are about to build a large barn and a smaller workshop for their plumbing business.  First inclination was to pour slab for the large barn and set posts I cut from untreated pine, oak, whatever that I have available, putting these on 2X6 treated sill plates with ties to the posts, then make roof trusses for main part (30X60 with 30 ft clear span for assembling TF bents etc under shelter) plus two 12 ft wide sheds on either side for equipment storage, wood drying and a sawmill (hope to get a new/used one in the next couple of weeks to replace my '86 LT30). 

First thoughts on smaller workshop (plans from here for 24X20 made from all rough cut full dimension wood) were to use 6X6 posts set in ground first with slab poured around posts and down into post holes to fill and lock in place.  I've seen discussion on both ways- some say pouring concrete around even treated posts results in water held next to wood that leads to rotting, yet many of the articles on DIY show just this as the way to do it. Has anyone done this and found rotting to be a problem?  Any thoughts on concrete around treated posts in general?  Thanks for the help- this has been a fantastic forum for learning, something I don't plan on stopping doing for a long while, the Lord willing and the heart holds out...

Lon

[Dana]

It is my understanding that the treated post manufactures used to recommend pouring concrete around the post. I believe they now recommend pouring the crete in the bottom of the hole and placing the pole in the hole after the concrete sets.

If you are planning ahead for rot, pour a piece of angle iron into your floor next to each post. Then if the post rots you will have something solid to secure your post to when repairing it.

[Joel Eisner]

I am finsihing a pole barn for our mill and we poured a concrete collar around the poles as they sat in the hole.  We put intersecting #5 rebar about 10" from the bottom of the poles and placed them.  Once plumb we put about 12" of concrete around them.  The concrete is more for protection of uplift and the collar supposedly provides a better bearing surface that when placed under the pole.

[Don P]

I've got one cabin on poles we suspended in the holes and poured around. There are 3 pieces of rebar through each pole in the concrete. It has held up so far, about 17 years. I don't like it though. I have replaced rotten poles in a slab under a deck.

I've seen the collar specced on pole barns but have never done one. I usually pour a 2'x2'x12" thick footing at frost depth and go from there. Size depends on load and soil bearing capacity.

[ljmathias]

Thanks, you all- seems like a bit of divided opinion but I think I'll go with the ease of replacement approach and put the poles on top of a concrete footing... time for some excavator work this weekend.  Boys sure love their toys and my grandsons seem born to it- Nana (that's my wife) just made a John Deere pillow for the 3rd of 5 grandsons and we walked it over: he loved it!  "Green Tactor!" as he grinned and hugged it.  Went to sleep on it in the wifes arms.  Life don't get any better than that.

Lon

[scsmith42]

Lon, I think that the approach has more to do with the need to support either downforce or resist uplift, versus the longevity of the posts.

Here in Carolina, where hurricane force winds are common, resistance to uplift is a critical factor, so it's usually more important to place the concrete around the posts at a deep level in order to provide increased resistance to uplift.  If you have relatively poor soil, then adding a pad below the post will provide additional bearing area for download.

If you're concerned about uplift, then when you place your concrete you do not want to bring it all the way up to the surface.  The "cone of soil" above the foundation is what helps to provide uplift resistance.  Thus, you will have more uplift resistance if you have concrete around your posts at the lowest possible depth.

In a perfect world, if I had to design a foundation that would serve both needs, I would use Don P's footings under the posts, but I would find a way to attach them to the post so as to provide the uplift resistance.  Perhaps some galvanized angl such as Dana recommended poured into the slab and bolted to the post.

Usually though, when using posts for foundation support the hole is not large enough to work down in, and that's where Joel's method works well.

Scott

[logwalker]

I used a little different approach. I wanted to set 10" posts which were untreated. The wind load in my area is not more than about 100 mph every 20 years or so but the seismic is a 9.0 or greater every 250 to 700 years. It has been 305 years since the last one. So I knew I wanted a very stout base with some flexibility.

I designed these angle iron boxes that looked just like an old box kite. They extend 16" up the base of the post with a tight fit. It extends down into an excavated hole shaped like a inverted mushroom about 6' across. Below grade it had 4, 30" rebar pieces welded about 12" below the surface, 2 in each direction like a tic-tac-toe design. I got some large cardboard tubes from a steel roofing company. They are about 24" in diameter. I first put the box down in the tube and then put the rebar in from the sides through holes in the tube. I then reached in with the welder and ran good long beads . These would hold the box at a fixed level half out of the tube, half in. I put the tube with the fixture in the mushroom hole and poured it full of concrete mud. I could then pull it up and let mud out the bottom and bring the top of the tube to grade. Wit a stiff mix it would stay where you set it.

So now I have this angle iron box up out of the pour ready to accept the post. Later I peeled the tube off and poured the slab around each base fixture. The rebar extending out of the tube tied it together. I could then just drop the 16' post into the box fixture and through bolt it and it was done. No bracing of any kind was needed to then set the cross beams. I could even throw ladders against the post and work against them without braces. I was using a forklift to set the beams so it was important that I could drive around the posts. The beams I set were 32' long 7x22" recycled gluelams that are readily available in most markets. I did soak the bottoms in a copper-napthanate solution to deter bugs. But the do end up inside the building out of the weather.

I hope this is a clear explanation. If there is interest I could get some photos uploaded. It actually was very easy if not fast. It did work very well and allowed me to use my own untreated posts. All told I set 10 main bases and 4 smaller ones for a large stair landing.

Sorry about the length of my post, Joe