Hi Dan. Ever sense I came to this site to try to get some real info on wood foundations I have been continually amazed at how useful this site is for people.
Dan, you need to tell us where you live and the type of soil conditions you have. This will help us with determining the expansive nature of your soils and the freezing and thawing aspect of your state.
You also need to tell us what kind of shop you are wanting to build. If you are doing wood working, that is one type of loading, but if you are bringing in combines and other heavy machines, or tractors and such, that is another. Also, the size of your shop and the size of the slab would be important.