Do a search on this website, this has been discussed countless times. I believe the title of the most popular book is Design of Floors on Ground but I don't have a copy handy and don't remember the author.
That being said, for sheds and exterior mechanical pads there is a lot of leeway typically. A lot of the engineers I know ask the client what is the acceptable end result of the building, if some seasonal movement is acceptable then it's a quick easy slab-on-grade, if no (or minimal) movement is allowed then the design becomes trickier.
The key to most slabs-on-grade is in the base preparation. You can put all the concrete and steel you want but shitty base soils equal shitty foundation performance.
Thank you for the response. I did do a quick search on this site for the topic, but none of the thread titles looked specifically about the design of slabs-on-grade, but moreso just problems encountered in various situations. I'll check this forum again for previous threads on the topic. Thanks for the book reference.
Thank you for the helpful input, and that makes sense how there's a lot of leeway in the design and how some of that is up to what the client cares about in terms of the acceptable end result of the building. I can see how base preparation is important, too. If the base material is pretty crappy, would it be typical to replace the soils with some better fill...and if so, how would it be best to determine the depth of excavation?
Second everything Jayrod has said. I'd like a reference for these isolated outdoor pads too but I'm not aware of one myself. For small, moderately loaded units I tend to treat them more like raft footings. My default is 300 thick with two light mats of reinforcing. If it's a large floor plan, I'll spend the time to refine the design. I'm curious to hear what others use. I've seen 150 with one mat used successfully.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
I have Concrete Floors on Ground by Tarr and Farny. It is published by PCA. I discusses most things related to SOG design (base preparation, concrete mix, durability, placing, finishing, etc.) I like it.
My slab on grade folder includes:
ASCE 32-01 Design and Construction of Frost Protected Shallow Foundations (for exterior equipment pads)
Ringo's book, mentioned above,
ACI 302 Guide For Concrete Floor and Slab Construction
ACI 360 Design of Slabs on Ground
PCA Slab Thickness Design for Industrial Concrete Floors on Grade
PCA Concrete Floors on Ground
WRI Design of Slab-On-Ground Foundations
Army TM5-809-12/AF 88-3
British TR-34 Concrete Industrial Ground Floors
A structure magazine article, "Concrete Slabs on Grade Subject to Concentrated Loads (Some of my coworkers love this one, I`m a lot more skeptical
and Euclid Chemical Concrete Floor Systems Design Guide.
If I had to pick one - it's Ringo. Its a great summary of many of these resources and easy to use.
I downloaded my copy from Scribd for $10 (I think) and printed it.
