http://nyrampage.wordpress.com/2007/05/07/home-renovation-how-to-build-a-hearth/

You'll get some guidelines here

Easy plan here...

Under the proposed fireplace, run a simple 1'x1' footer and keep that particular slab area 8" thick... Outside of the FP area, run the same 1/1 footer with a 4" to 6" slab, provided you can have at least 3000 psi concrete poured. (Truck is cheaper than bags when you do this much concrete)...

3/8 rebar at 16" grid is sufficient outside of the fireplace, but under it, so 1/2 rebar or 12" on center. Mesh would not be a good idea for concrete support for under the FP, but you couldd use it elsewhere.

You should not be at 21000 lbs under the fireplace... But if those numbers are correct, make the minimum 3000 psi fiber reinforced pad under the FP 12" thick with 5/8 rebar 16 OC, and the footer while under the FP at least 16" x 16".. You can also land a small helicopter on that pad while you're at it.

JL Mealer
Mealer Companies LLC
http://mealercompanies.com