The floor is not carrying the building
Most tractor sheds are post frame construction, and the posts already carry the roof and walls on their own footings before the floor ever goes in. That means the concrete only has to carry what happens on top of it, a tractor, a loader bucket, hitched implements, not the weight of the structure above it.
It also means the floor can go in on its own schedule. A shed that has been standing on gravel for a season or two, waiting on a real floor, is a common job for us, not an unusual one.
The base is what actually holds the floor flat
Concrete is strong under compression and weak in bending, so a floor sitting on soft or uneven ground will crack and settle no matter how thick it is poured. Around here that usually means red clay, which holds water and moves with the freeze thaw more than a sandy or rocky base would.
Anything soft comes out of the pad first, topsoil included, until what is left will hold. Crushed stone brings the elevation back, and it goes in shallow and gets packed as it climbs instead of being filled to grade in one shot and rolled once across the top. That compacted base is the real answer to what holds the floor up, more than the concrete sitting on top of it.
This matters more inside a shed already standing than it would on an open lot, because the posts around the edge are fixed points. The floor has to be built up or cut down to meet them at the right elevation, and the base work has to happen in a space the posts already occupy.
Thickness sized to what rolls across it
Five inches is standard where a tractor and its implements will actually roll across the floor. Going an inch past ordinary flatwork changes what a point load can do to a floor. A bucket set down hard, or a narrow tire under a loaded grain buggy, gets carried across a wider piece of slab before that weight ever reaches the base, and the added depth leaves the steel somewhere useful to sit.
Four inches is enough only where nothing heavier than foot traffic crosses the slab, a dry corner used strictly for storage. Where one part of the shed takes something heavier than the rest, we thicken that section in the forms rather than pouring the whole floor to the worst case in the building.
Reinforcement placed where the loads actually are
At five inches there is room for rebar or wire mesh to sit where it actually does work instead of resting against the bottom of a thin slab doing nothing. The mix gets chosen for the load, usually a 4,000 PSI air entrained design on a shed floor. An open bay means winter weather reaches the concrete, and the entrained air is what carries it through the freeze thaw that follows.
An open shed with no doors on one or more sides sees weather almost like an outdoor slab does, rain blown in, snow tracked across it, the same freeze and thaw a driveway takes. That is one more reason the mix and the base underneath it do not get scaled down just because there is a roof overhead.
The door opening takes the worst of the wear
Every tire and every implement in the building crosses the same opening on the way in and out, so that edge takes more punishment than any other few feet of floor. We form a straight, thickened edge at the door rather than letting the slab thin out toward the opening, because that is the first spot to break down if it is built light.
Control joints get cut on a grid once the floor is placed, and we keep the main travel lane between the door and the back of the building in mind when we lay that pattern out, since joint edges take the beating from a rolling wheel more than the open panel around them.
None of that stops the floor from shrinking. It just decides where the shrinkage shows, in a straight cut we placed on purpose instead of a crack that wanders wherever the slab happens to be weakest that week.