I am planning on installing a 12 x 16 shed on a sloping site. The low end of the site will be ~3 ft below the high end. I am planning on using sonotubes but was not sure on what kind of spacing and diameter to go with. Should I just use specs for decks - for example 3 tubes spaced at 8 ft and 2x8 joists spanning the 12 ft?
any input is appreciated....
Check out John's little cabin.
http://www.jshow.com/y2k/listings/3.html
It's smaller and lighter than what you are planning, though.
Seems like one of the foundation pier sites does the math for you--big-foot ?.
Try this one--Footing Tube .pdf file.
http://www.foottube.com/FootingTube_Cat04.pdf