Rebar Mesh Footing Details

Quite.

GA

Plus, the shape and loading of the concrete will have some bearing on rebar use, assuming this question comes from observation of a structure in front of you. You may have offset or moment loading on the footing, surcharge on one side, seismic or uplift forces, any number of factors that the structural engineer considered when coming up with the design you see. With the limited data you have provided, it is difficult to even guess at the reinforcing design considerations.

There is no rule of thumb that relates the top reinforcement steel to the lower reinforcement steel. The only likley relation that I can think of off hand is the requirement for a minimum amount of reinforcement steel that is required for temperature steel.

The amount of steel in the top or bottom of the foundation depends on so many variables. These include tension stresses, compression stresses (these are all related to loading and the dimension of the foundation), concrete strength, thickness of the foundation, bearing material strength, etc.

If this is a foundation of consequence and you utilize a rule of thumb, you would be extremely foolish. If, however, this is a foundation for something like your deck or perhaps even a house, then the use of a standard detail or rule of thumb is probably acceptable. That said, most locals in the US require that foundation plans for inhabited structures be stamped by an engineer.

So please consider what you are supporting with the foundation before you prepare a foundation using some rules of thumb.

You can do almost anything but your thicker rebar mat is supposed to be on the bottom. The center spacing will be dictated by your specs if you are using double mat on significant loads.

I worked many concrete jobs and county requirements are very minimal and too minimal in my opinion. Federal/state requirements are much tougher and realistic.

I always say rebar is cheap insurance. Double or triple it if you can on home projects for footings. They had a pathetic 3" heel and toe footing on my 6 inch wide 3 foot wall that met county requirements. Now there is significant cracking. You cant easily go back and make it right.