Where a haunch shows up at the edge of a slab
On a monolithic pour, where the footing and the floor go in together as one piece of concrete, the perimeter is not the same thickness as the middle of the floor. It gets formed with a thickened edge, steel set into it, so the wall of the building has something solid to sit on once it goes up. That thickened edge is a haunch doing the work a separate footing would otherwise do.
The same idea shows up at an overhead door. Every wheel that ever enters the building crosses that line, so the edge there gets formed thick and straight instead of left the same depth as the rest of the floor. Thin concrete at a door opening is usually the first thing to break down.
Thickened pads under a lift or heavy equipment
A concentrated load, something that lands its whole weight on a small footprint instead of spreading it across the floor the way a parked truck does, is what calls for a thickened pad. A few common examples:
Doing that during the original pour costs very little extra. Doing it after the floor is finished means saw cutting a hole in a slab that already looks right and patching it back in, and the patch never quite matches the rest of the floor around it.
- A two post or four post vehicle lift, on its column footprints.
- A hydraulic press or a stationary machine bolted to the floor.
- A lathe or another piece of shop equipment with a narrow base.
- A hoist column set into the slab.
Where a haunch shows up outside the building itself
The same idea applies to a driveway apron, the stretch where the drive meets the road, and the approach in front of a garage door. Both take more abuse than the rest of the drive, heavier turning loads, more freeze and thaw exposure at the edge, so thickness and reinforcement there pay off first when something is going to fail. It is not always a formal haunch in the structural sense, but it is the same logic: put the extra concrete where the extra load is, not spread evenly across the whole pour.