Auger size for 8" tube piers?

[markert2523]

Howdy folks,

City finally issued my zoning permit (no building permit required for <200SF) for my 12x16 modern manshed.  I'm renting a fancy post hole digger tomorrow and wondering which size auger to use for 8" tubes.  The shed will sit on two beams with four piers under each beam.  I plan to pour about 4 inches of concrete at the bottom of each 2 feet deep hole and sink a vertical piece of rebar at that time that will go up the center of the tube.  When that dries, I will put the tubes in, backfill around them, and fill them with concrete.  I was thinking of using the 12" auger and maybe reaching down with a garden trowel and enlarging the bottom diameter of each hole a little.

Think that will work?  I kind of want to avoid using the 18" auger (the next biggest size) and minimize the dirt work.

Thanks

[DirtyLittleSecret]

What kind of soil, where is it, and what's the frost depth? 

[markert2523]

Frost depth not very deep here.  I believe the city requires footings to be 18" below grade on regular construction.  Soil is usually a sort of loamy clay without much rock.

Thanks

[MountainDon]

The question really is what size of footing is required to support the anticipated loads; building weight plus contents and wind forces that might be transferred through to the foundation.

I don't have a 'for certain' answer for that. Here, our building department usually specifies 16x16" poured concrete pads 8 inches thick, with reinforcing steel. That may be a size that is fail safe for our conditions and hence prescribed whether or not it's overkill or not.