Footer Questions

[MMH]

I got a quote from 84 lumber for a 56x30 pole barn building. There will be a an overhang so the truss will actually be 40' wide. The 6x6 posts will be spaced every 8'. The snow load is 30 lb/ft2. So, each pole will take 4800 lbs. of load (30*20*8 = 4800). The plans show each pole footer as being 16" in diameter. This means that the the pressure on the footer will be 3462 lb/ft2.

When I stopped by the building department, they tell me that the soil only supports 1500 lb/ft2. At the same time when I showed them the plans that 84 Lumber supplied me with (which was fairly detailed), and I questioned only16" footers they assured me that these were big enough. If I go up to a 24" diameter footer the soil pressure will be down to 1538 lb/ft2. ive

I'm in western PA. The soil has tons of clay in it and being on the downward side of a big hill/mountain, always soggy from rain runoff. So I do not have any basis, but want to be conservative.

Also, the building department said that the bottom of the footer has to be 36" deep. I thought that the frost line was 36", so will probably go down to at least 42".

Sorry for the long winded post, but I really need some advice.

 

[Egon]

There are design parameters involving poured concrete piles. Your area may have existing codes.

Many concrete piles are belled at the bottom to increase bearing area. Soil conditions must also be taken into account for the design.

Not much help, I know but proffesional sevices may be required for the proper answer.

 

[Carl_NH]

I concur with Egon - belled at the bottom is typical - I would go 24" at the base of the pole. Also you have a 10' overhang is this supported or is it an unsupported overhang? If this is unsupported you will have a higher load on that side.

Also I presume you will use metal roofing? Most of the snow should slide off if this is the case - possibly a 4-12 or 5-12 pitch?

I havent been involved in pole barns since the 70s in PA (worked for Agway) and I am sure codes have changed some. I know there are others here that have built similar structures much more recently.

 

[JB4310]

When you say footer, I hear a slab under ground the a PT post will sit on?

If this is the case then definitely go with the larger footing, (that's what they are around here) and going a little deeper is not a bad idea either, though those frost line numbers are usually worst case and rarely does the frost get that deep.

If you mean a sonotube pier form that comes up above the ground with the post sitting on it then 24" sounds a little large and I would do something like was mentioned, just making it larger diameter at the the bottom 12" of the pier.

If you know your soil that well, I would not skimp on the footings/ piers even if it is a little overkill. It's just a little extra concrete.

I've heard with some of these pole buildings they just fill the footing hole with unmixed bagged concrete mix (dry), set the post on it and back fill. they say the concrete will set up under ground like that eventually ???
I guess that's so they can just come out and throw the building up ASAP and not have to wait for concrete to set.

JB

 

[GLyford]

I got a quote from 84 lumber for a 56x30 pole barn building. There will be a an overhang so the truss will actually be 40' wide. The 6x6 posts will be spaced every 8'. The snow load is 30 lb/ft2. So, each pole will take 4800 lbs. of load (30*20*8 = 4800). The plans show each pole footer as being 16" in diameter. This means that the the pressure on the footer will be 3462 lb/ft2.

When I stopped by the building department, they tell me that the soil only supports 1500 lb/ft2. At the same time when I showed them the plans that 84 Lumber supplied me with (which was fairly detailed), and I questioned only16" footers they assured me that these were big enough. If I go up to a 24" diameter footer the soil pressure will be down to 1538 lb/ft2.

I take it you're referring to the precast round footers, which you drop in the bottom of the hole, and the post sets on top and is backfilled with soil/gravel? Or are you going to pour your own at the bottom of your post holes? If your building department is happy with 16", that'd be the easiest to get. A quick search shows 12, 16, and 18 as common sizes.

Also, the building department said that the bottom of the footer has to be 36" deep. I thought that the frost line was 36", so will probably go down to at least 42".

Usually frost line is a worst case 100-year sort of scenario, so it's generally accepted as safe to go "right at" the code depth. It doesn't usually get that deep every year. A little extra won't hurt...if your auger will reach. 42 would mean that the top of your 6" thick precast footer is at frost depth, not a bad compromise. The biggest problem with frost is getting underneath and pushing up which is probably why they're not too worried about that last inch or two.

 

[MMH]

I concur with Egon - belled at the bottom is typical - I would go 24" at the base of the pole. Also you have a 10' overhang is this supported or is it an unsupported overhang? If this is unsupported you will have a higher load on that side.

Also I presume you will use metal roofing? Most of the snow should slide off if this is the case - possibly a 4-12 or 5-12 pitch?

I havent been involved in pole barns since the 70s in PA (worked for Agway) and I am sure codes have changed some. I know there are others here that have built similar structures much more recently.

The overhang would be supported. Not 100% sure about metal roof - may still opt to go shingle. The roof will be a 4-12 which is the same as my house.

 

[MMH]

I take it you're referring to the precast round footers, which you drop in the bottom of the hole, and the post sets on top and is backfilled with soil/gravel? Or are you going to pour your own at the bottom of your post holes? If your building department is happy with 16", that'd be the easiest to get. A quick search shows 12, 16, and 18 as common sizes.

I would be pouring my own. The bldg. dept. is happy w/ 16" but this is inconsistent w/ some of the other requirements. Less concerned about meeting code than making sure that I end up w/ something that is structurally sound.

 

[davedj1]

I'm not sure of your location but if you are that concerned with the total load I would make the roof something other than a 4/12 pitch. Even with metal roof the snow won't slide off. I would go with a minimum of 6/12 with 7 or 8 being even better.

 

[MotorSeven]

I'm with Dave, I'd up the pitch a little. That's a nice size, what did the quote you installed?

 

[Fixemall]

I'm not sure of your location but if you are that concerned with the total load I would make the roof something other than a 4/12 pitch. Even with metal roof the snow won't slide off. I would go with a minimum of 6/12 with 7 or 8 being even better.

I would look at 24" holes, pour your own and up the pitch as well. Around here they have upped some areas to 36" for pole barn holes, clay does funny things.