Hi Irving2008

I've done some work with footings, walls, and a new driveway but I've always just followed best practice. Collins complete DIY manual (Jackson & Day) is a good read but might not go into the detail you are after.

I don't know anything about building calculations with concrete but believe it is very good in compression and very poor in tension. So depending upon how your soil movement puts loads on the base would dictate how to best reinforce it.

For a cantilever, where only one of the footings was taking the load (and the floor was not supported by the clay if it moved away) then the tension would be on the top surface. This would suggest reinforcement at the top.

For a simply supported beam where both footings are taking the load but there is also alot of weight on the floor and the clay has moved from under it then the tension would be on the lower surface. This would suggest reinforcement at the bottom.

I don't know what reinforcement in the footings would give you, if anything.

I remember seeing a programme about applying and holding a pre-load onto the reinforcement whilst the concrete set. This would then apply a permanent compression on the concrete enabling it to be used in a situation where it would experience some tensile loads (the limit being where the tensile load equalled the built in compression load). In effect a DC offset towards the compression side.

These are just my thoughts on it - best get proper building advice!