Expert: Dan Griffin - 3/14/2009

Question
I would like to build an underground cold storage 20' x 60' using 12" wide block for the walls (concrete filled) and a flat clear span concrete slab for the roof so that it can be driven on with equipment weighing no more than 10,000 lbs.  I had a compaction test done and they said my soil could support a 10 story building with no problem.  Does a 36" wide x 12' deep footer with 5/8" rebar sound reasonable for the footer?

Answer
Carlton, something sounds a bit fishy.  A soil report usually indicates bearing capacity worded a bit differently that "can build a 10 story building".  This information would indicate you are on or at bedrock which will make digging this cellar almost impossible.

Let's assume for the moment that you have 3000# bearing capacity.
Let's assume you plan on the walls being about 10' tall.

160 LF of wall x 10'tall=1600SF of wallx 1'thick=1600CF x 155#/CF=248000#
Slab 20x60x1'thick=1200CFx 155#/CF=186000#
Vehicle load=10000#

444,000 #
160 LF of footing
2775#each foot of footing.

I think your 36" wide footing is overkill.  Please double check my math, I do not intend to make a structural decision for you.  There are certainly more calculations that need to enter the discussion ( I suspect you plan a doorway on at least one end).  You really need a structural engineer to help design a slab that will clear span 20' with that kind of load - I used a 1' thick slab which may or may not be adequate.