Auger Cast Ftg. for Cant. Wood Column (Pole Building)

[JasonPE]

I have a 34' X 120' pole building, where my client wants posts at 40' spacings. I designed non-constrained footings for lateral loads no problem.

However, what started as a 2' diam X 7' deep auger cast footing for lateral loads, has grown to a 3.5' diam. X 14' deep ftg. for vert. load. 3' diam. goes way too deep and 4' diam auger seems too large.

The huge increase came because of the weight of the footing. As I go deeper, the weight of concrete goes up as fast as the additional bearing capacity, theoretically. I am including skin friction {Fs=(K)(sigma prime v)(tan delta)}, which ended up about 12k of the total 42k capacity.

I have 25 psf snow, 8 psf dead load. Soil allow. cap. is 1500 psf, plus increase of 120 psf, per foot of depth, for soil overburden. My roof load is 22k. Footing wt. has reached 20k!

I am recommending going to smaller post spacings, but I am having a hard time convincing my client that we need this much footing. I know all this concrete won't sink to the middle of the earth, so my capacity increase for depth must not be linear like I am assuming. Any help? Sorry so long.

 

[JAE]

As you add weight from your footing (at 145 pcf for concrete) are you also subtracting the weight of the displaced soil (at 100 pcf or so)???

 

[civilperson]

Try belling or under cut the shaft for a greater area with less weight and volume. A six foot diameter bottom on a three foot diameter shaft for instance. Soil bearing pressure increases faster than the weight of overburden in most conditions. The old building code let the allowable pressure increase 500 psf for each foot of depth below two feet. Get shelby tubes of the soil at the elevation of the bottom of your footing and test for actual strengths.

 

[JasonPE]

JAE,

That is what I did in my 4th paragraph. Unless I get a geotech report, I am only aware that I can increase my allow. brg. pressure by the soil overburden I am removing. This is where I think I am being too conservative, but dont know what else I can do.

I could drill the ftg. I need for lateral, then add a spread footing at the top, just under the ground surface, like the head of a nail. Hmm.

CivilPerson,

Thanks, but I don't think my client would go to the expense of drilling a bell. It is supposed to be just a simple pole building, but his demand for 40' post spacing is making it not so simple.

 

[JAE]

JasonPE - I'm not talking about the bearing pressure - you say that as you go deeper, the weight of the concrete goes up as fast as the added bearing capacity. What I'm saying is that you should only be adding 145 - 100 = 45 pcf of added material as you go deeper.

For your building:

Area on each pole: (34)(.5)(40) = 680 sf.

Dead Load: (8 psf)(680) = 5440 lbs.
Snow Load: (25 psf)(680) = 17000 lbs.

Total Load at top of pier: 22,440 lbs.

Net Weight of 3.5 ft x 14 ft pier = 9.62 sf x 14 ft x 45 pcf = 6,061 lbs.

Total load on pier = 22,400 + 6,061 = 28,501 lbs.


Skin friction = 12,000 lbs.
Net required bearing pressure = 28,501 - 12,000 = 16,501 lbs

For the 3.5 ft pier = end bearing pressure = 16,501 / 9.62 = 1715 psf. This doesn't look to high.

 

[JasonPE]

JAE,

Thank you very much for your thoughtful input. Yes, that is basically what I did. I just thought that a 3.5' diam x 14' pier was too big for a simple open pole building, where a little settlement would matter little anyway.

I since spoke in depth with a Geotech Engr who said it is industry standard to ignore the weight of a auger cast pier since the wt. of conc. is close enough to the wt. of soil removed. Then still taking the allow. brg. increase of 120 sf/ft of depth due to consolidation at that depth. He said the actual strength you get with depth increase is FAR more than this, so the design is still conservative. Assuming a Class 4 or 5 IBC soil.

This gave me a 3' diam. x 9' deep ftg, which seemed more reasonable.

 

[JAE]

Sounds good - talking to the geotechs can always help.