Our shop floor is 8" thick, was poured in '83 and it has very, very few cracks in it. It was done well. We don't put anything nearly as heavy as you guys in the midwest would put onto this floor - we don't have any machine that goes more than 20K lbs.

In addition to the thickness of the floor, remember that you should be compacting the soil under the pad before you pour. I certainly wouldn't be going with any floor less than 6" thick, and given that the price of concrete will go only one direction (up), I'd think about getting it at least 6", better at 8". Look back at the last 30 years of farm equipment -- wagons, tractors, combines - they're all getting bigger and heavier, not smaller.

While you're at it (ie, now, before you pour) think about whether you want any heat in the floor.