Changes to my 16x16 foundation plan

[ovidgsd]

So I dug my footing holes this weekend as follows:
Po----<8'----Po----<8'-----Po
/               /                 /
/               /                 /
<8'             <8'             <8'
/               /                 /
/               /                 /
Pc----<8'----Pc----<8'-----Pc
/               /                 /
/               /                 /
<8'             <8'             <8'
/               /                 /
/               /                 /
Po----<8'----Po----<8'-----Po

Po = the row that will support the roof weight (end of pitch)
Pc = the center row of the building (Pc on outside are gable ends)

For all Po holes, I was able to get about 4 feet depth for my sonotube posts - down to some seriously compacted gravel/stone/clay.  However, for the Pc holes, I could only dig to about 2 feet.

Thanks to good advice from MOUNTAINDON, it seems that with a box of 2x12 girders at 3 pieces thick, I really dont need the holes labled Pc for footings where I plan to use 16 foot 2x10 floor joists.  However, I still would like a girder up the middle along the line of Pc footings - I like the idea of a really SOLID floor - as I stomp around all 230lbs and friends LOL

I plan to insulate all my footings using the Frost Protected Foundation Guidlines that I found on this site.  I'm thinking top layer of 12" loam, 3" gravel, 3" high density foam, 3" gravel, then regular clay soil under - all under the building and going out 4 feet to each side - thus a 20' x 20' insulated area.

If I do this insulation, here is my idea for the footings/piers.  For the load bearing Po holes, 5 foot sonotube posts with the "big foot" at the bottom.  For the Pc holes, how about just gravel compaceted base, with 4 12" x 16" cement blocks, stacked 2 high, and cemented together, with PT posts on top.  I'm figureing that if I use all that insulation, and the Pc footings are not "needed" then I don't need to do deep posts on the Pc row? 

My only concern is what would happen if there was heaving of any of the Pc row shallow footings?  if that hapens, would the floor in the middle of the building heave? I'm thinking no, but what do you think?

Best thanks for any opinions!
(property is in Northern Maine)

[John Raabe]

If the frost depth in your area is 48" then the deeper outside piers should be fine - and insulating the footings would not be needed. As Don mentions you can span the full 15' distance between your beams using almost any 2x10 @ 16" without the interior beam and piers. If 2' is shallower than your frost depth then why not eliminate the potentially troublesome interior row and the foundation insulation and go with a few more 2x10s and put them at 12" o/c if you think 1/480 for deflection is somehow too bouncy?

Spend the extra foundation money on better insulation in the floor system where it will produce some real fuel savings.

[MountainDon]

What John said, with the caveat about the frost depth. If the frost depth is 4 feet or less, but more than 2 feet you could run into problems with center beam movement. Insulated foundations do have their place but if the 4 foot depth meets local frost conditions there is really no need to go that deep and do the insulation.

Spacing the floor joists at 12 inches would give a more solid feel to the floor. I seriously doubt you would find any problems with that and no center beam. Not installing a center beam saves some money that should pay for the extra joists. With the triple layer and plywood spaced beams under the walls the actual span is more like 15'1"

[ovidgsd]

John and Don,

Thanks for your time and advice - you are saving me alot of money and time  - check is in the mail!!!