Outbuilding Slab Thickness

[optionguru]

I'm looking to build a new work shop for my business with one of those pre-fab steel buildings 40' x 70'.  My existing shop has a slab that has deep cracks and broken concrete.  I'm in NH and the building will not be heated full time.  

How thick does my slab need to be or what do I need to do special to avoid the busting up of the slab?
Thanks,
Peter

[glenn kangiser]

Many times cracks like that are caused by expanding clay soil.  On one project I did in that type of area the engineer had us raise the pad some with sandy soil  then trench around 3 feet deep to seal moisture out and keep it from getting under the slab.  We had a couple #4 rebars around the perimeter and 6x6 10x10 remesh in the entire slab.

Note that you do not excavate the pad into the clay soil as it will make a bath tub -fill the sand with water.

Heavier shop slabs we went 6" but most were 4"