Posts Footings: What do you prefer?

[LD1]

I will probably wear everyone out over the next months designing (in my head) me polebarn. This thread will talk about posts, and how everyone sets them, and different methods of backfill and uplift prevention.

I have done some researching on the best methods, and just end with more questions than answers. About the only thing that is universal is they need a solid concrete base under them. In my case, ~18" or so and 6" thick. But beyond that, there is many different techniques.

1. Backfill with dirt
2. Backfill with gravel
3. Concrete encasement.

I dont think I want to do concrete encasement. At least not with modern PT posts.So that leaves dirt, and gravel. But it would seem to me that gravel wont offer much in the way of uplift protection. So searching that, there are all sorts of ideas.

1. Nail, rebar, or whatever around the bottom of the post then pour another 6" of concrete.
2. 2x4 blocking around the base
3. brackets

the brackets cost almost as much as the post

Here is one Idea I have tossing around in my head...Instead of post holes, dig a trench. I already have a BH with 18" bucket, and would have to rent a SS w/PHD (~$300). And I plan on digging a trench anyway at least on one side to run a tile (build site is at the bottom of a hill). So just dig the trench, pour 18x18 pads for the posts, then attach all the bases of the posts together (below grade) with a PT 2x4. Lay a tile down beside the posts, Then backfill the entire trench with gravel. With a 2x4 connecting all the bottoms, it should offer good uplift prevention. And again, I am digging a trench anyway. The cost of 2x4's rated for GC to tie all the posts together is ~$80. For 18 posts, uplift brackets would be about 5x's that or more.

And my main reason for thinking of backfilling with gravel is to prevent settling so I can pour concrete ASAP.

So lets hear some thoughts. What are you all's prefered methods of setting polebarn posts? Am I nuts for thinking about setting the posts in a trench, bracing, then filling? (remember, I am digging the trench anyway)

 

[murphy1244]

They dug 40" holes and used concrete pucks then filled with dirt then concrete on top.

 

[LD1]

Wow, 40" holes are pretty big. How far is your post spacing? Based on my loads and soil, I need about 2-2.5 sq ft for the footings.

 

[murphy1244]

Wow, 40" holes are pretty big. How far is your post spacing? Based on my loads and soil, I need about 2-2.5 sq ft for the footings.

40" deep X 24" wide. Spacing was 8' apart except at doors

 

[LD1]

I dont quite need a 6-2/3 square foot under each post in my area.

What was the reason for going with such a large pad? 2-story building? 100PSF snow rating? 80' span trusses? or just wanted a 5x's safety factor?

 

[EddieWalker]

Pole barns are a proven building design that has been around for centuries. You put the pole in the ground and then build your walls on to the pole in the form of purlins. Then you set your trusses or rafters on top of the walls and cover it all up with siding and roofing material. The reason it is so effective is the amount of strength you get from the poles in the ground. Kind of like how a tree stays up right. If you start with a pad that is well drained with the water moving away from the building and the posts are protected by the walls, there is no reason regular PT posts wont last a hundred years.

Over engineering and creating opportunities for water to gain access to those posts is almost as bad as allowing standing water at the base of the posts. Gravel in a hole will hold water. Just think what a septic drain field does. You surround the pipes with gravel so the moisture coming out of the pipe will have some place to go and then over time it will soak into the surrounding soil. The same thing happens when you put anything with a void into the hole with your post. Don't do it.

Every soil has a load bearing rating. If you are building on soil that is considered weak, like sand or loam, then using the concrete disk or pouring a pad at the bottom of the hole is needed. I'm not aware of any significant negatives to using concrete at the bottom of the hole or the filling in the entire hole except the expense. If you have highly expansive soil, like black clay, you need to create a bell at the bottom of the hole to anchor the posts in the ground. From what I understand, extreme frost heave needs this also. But going a lot deeper solves that problem as well. It just depends on your climate.

I personally use the soil from the hole t backfill my posts with the post resting on the bottom of the hole without any concrete. I have red clay and it is a very stable soil.

If you are going to dig a trench under the wall to create your 2x4 anchor to hold the posts down, I would worry about compacting all that soil you will be putting back in there and the amount of moisture that will leach into that area. In my opinion, you will be creating a week area until the soil compacts and dries enough to not hold water. This will take years. You will also have to worry about frost heave due to the extra moisture in the soil in these trenches. On top of the additional time and cost to do this, it sounds like an area that will lead to movement and future problems.

Keep it simple. Have you ever heard of a post coming out of the ground in a pole barn? Worst case tornado/hurricane scenario and all the problems are with the roofs and siding coming off. Posts set in the ground do not come out of the ground.

Eddie

 

[doxford jim]

Check out this company:

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[s219]

When we did our pole barn, we had to backfill the post holes with dry concrete mix per engineer who signed off on the footings. The soil/spoils from digging the holes would not have enough resistance for wind loads, letting the posts move sideways and tilt over. Uplift wasn't the issue. So I'd be very cautious about digging trenches or backfilling with gravel unless you knew the posts were going to have proper lateral support.

I would not worry about rotting issues one bit if the area is properly graded and drained, as Eddie mentions. If you bury a piece of PT wood in the ground, it will last a long time. Where it can rot is at the surface, if water is allowed to collect there. So if you can avoid that issue, you don't need to worry. If using concrete to backfill the posts, dome the top of it so that it can't trap water next to the post. Put in gutters and/or make sure the grading carries water away from the building.

 

[GPintheMitten]

I would either drill holes for posts or trench for concrete wall or block foundation.

A trench for poles seems to weaken the structure for reasons stated above.

 

[nybirdman]

Built one two years ago for my son-in-law.Per code for our area.We dug a trench because it was easier than doing large holes;18"x18"x6" reinforced pads,brackets for each post and back filled with removed dirt.
This is in northern NY,so similar to your area.The building has not moved.
This summer he poured concrete in half of it(PT boards around bottom against the walls)

 

[LD1]

Built one two years ago for my son-in-law.Per code for our area.We dug a trench because it was easier than doing large holes;18"x18"x6" reinforced pads,brackets for each post and back filled with removed dirt.
This is in northern NY,so similar to your area.The building has not moved.
This summer he poured concrete in half of it(PT boards around bottom against the walls)

If you had to do it over again, would you go the trench route? or rent a machine with 18" auger?

 

[LD1]

Pole barns are a proven building design

snip

Eddie

So if I understand you, you dont like gravel OR a tile running next to the posts?

My concerns were the exact same actually. But kept reading everywhere online that said gravel allows for better drainage. But in my mind, it would seem to create a little french drain around the post. Thats why I tought about the addition of a tile.

As to the posts, as per tables and my soild type, I am guessing 3000-4000PSF bearing capacity. At max truss loading plus dead load, each post should see ~6000lbs. Thus a 2 sq ft or a little better pad. Certainly not just a post set in the ground, at least not in my area.

 

[EddieWalker]

Undisturbed soil is going to be the best at shedding water and have the highest load bearing ability. Once you dig that soil, everything changes. Getting it compacted to being as solid as it was before you started digging is impossible. When I worked construction in CA, we had to get it to 98 percent to pass inspection before anybody could build on it. That meant getting the moisture level perfect and vibrating it in lifts with heavy equipment.

You will never get the soil in a ditch or even around the post close to what the soil was before you started digging no matter what you use or how long you work at it. Not a criticism, it's just a part of building and something that has to be dealt with. With just dirt in the hole, compacted as good as you can get it, you will still get more moisture in that hole then the surrounding soil.

Adding rock to that hole now allows it to hold even more water. Rock mixed with soil means lower compaction and more voids for moisture. You are in fact creating a place to hold water. More rock, the more moisture you will have in there. This means less strength on the post, greater opportunity for rot and an attraction for termites.

Overbuilding is one thing, over engineering is another thing. I feel your trench idea is over engineering a simple method of building that has been proven to be very effective just the way it is.

Eddie

 

[LD1]

I certainly wasnt trying to over engineer it. Just trying to save time and money.

I need a trench along the north side anyway for tile as it slopes toward the building. I could have that 72' trench dug in an hour with equipment I already own. So was mainly wondering the feasibility of setting posts in there with the tile and be done.

I dont own a PHD bigger than 9". With our average soild bearing capacity of 3000psf, that leaves each colum only able to support ~1300lbs if I poured a 9" round pad in the bottom. With my building span and 8' center posts and snow load, I need to support ~6000lbs per post.

Thus would leave me spending $200+ and half a day with travel just to rent a PHD and SS. On top of still having to dig a trench, lay a tile, and back fill. I was just trying to kill 2 birds with one stone and be done.

But looks like I will be renting a PHD

 

[2manyrocks]

I agree with Eddie that loosening the soil with a trench and then trying to make post construction work is going to create more work and the loose soil will be a problem. However, it shows foresight on your part to be thinking about controlling water run off on the site before you start construction. If it is primarily a surface water run off issue you have to deal with, sometimes the most effective means is to re-grade and cut a berm/ditch to divert water flow from your building site.

A picture of the site could be helpful to the discussion.

 

[nybirdman]

We had access to a backhoe and the trench made it easier to work the forms ect.Back filled with the tractors we had on site.We wouldn't have done it any different if a large post hole digger was available.
As stated per code we had to have a reinforced pad with fixed dimensions an a very large post hole digger would have been required.

 

[farmer2009]

I am not advocating this as I don't have any experience with it. But one thing I have seen builders in my area do is on the new PT post wherever they contact concrete they put atleast a 1/4" of roofing tar on the post. They say it will seal the water away from the wood. You have to remember concrete is also porous. Not saying it's a supplement for proper drainage or nothing like that.

 

[bucktaker]

On my Pole building we put in large 36" holes double concrete pucks, then backfilled with crushed limestone..its hard packed. Its not gravel that is loose it

I have started redoing my old wood fence posts this way they set tight and hopefully will last longer than the locust post in clay dirt

 

[LD1]

We had access to a backhoe and the trench made it easier to work the forms ect.Back filled with the tractors we had on site.We wouldn't have done it any different if a large post hole digger was available.
As stated per code we had to have a reinforced pad with fixed dimensions an a very large post hole digger would have been required.

You you did a pole bard like I am describing by digging a footer? Any tips on how to pull it off successfully or how you did it? the whole back filling and compacting without damaging or shifting the poles thing?

 

[Oldgreybear]

Our 40 yr old pole barn was installed using the following tools
Strong arm post hold digger
GroundWork® Post Hole Digger with Wood Handle - Tractor Supply Co.
Drive shaft from a Model A for a tamping bar
A shovel
and a saw.

it is 40 years old built using used utility poles for the uprights. They were dug down a 4 feet and tamped solid. The barn is 40x64 feet. with a full length loafing shed. The only posts to be replaced in that time have been on the loafing shed. This is really the best way in blackland to build a hay barn.