The thickness of a slab on grade is controlled by the wheel loading of the vehicles and the spacing of the vehicle wheels. You might want to check out "Concrete Floors on Ground" by Portland Cement Association. Also, ACI 360R has a summary table of other methods of designing slabs on grade. It lists the methods by Portland Cement Association, Wire Reinforcing Institute and the Corp. of Engineers. Each of these groups has their own method of designing slabs. Keep in mind that the steel in the concrete in the floor is only used for temperature and shinkage and not for flexure.