Posts Footings: What do you prefer?

   / Posts Footings: What do you prefer? #1  
LD1

LD1

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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 :eek:

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)
 
   / Posts Footings: What do you prefer? #2  
They dug 40" holes and used concrete pucks then filled with dirt then concrete on top.
 
   / Posts Footings: What do you prefer?
  • Thread Starter
#3  
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.
 
   / Posts Footings: What do you prefer? #4  
LD1 said:
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.
Click to expand...

40" deep X 24" wide. Spacing was 8' apart except at doors
 
   / Posts Footings: What do you prefer?
  • Thread Starter
#5  
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?
 
   / Posts Footings: What do you prefer? #6  
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
 
   / Posts Footings: What do you prefer? #8  
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.
 
   / Posts Footings: What do you prefer? #9  
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.
 
   / Posts Footings: What do you prefer? #10  
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)
 
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