I suggest you go to the forum at hearth.com. There you are likely to get the kind of expert advice you are seeking from enthusiasts who live and breath fireplaces.

Good luck.

Here is the lates International Mechanical code one and two family dwellings code concerning concerning footing requirements

'R1003.2 Footings and foundations. Footings for masonry
chimneys shall be constructed of concrete or solid masonry at
least 12 inches (305 mm) thick and shall extend at least 6 inches
(152 mm) beyond the face of the foundation or support wall on
all sides. Footings shall be founded on natural undisturbed
earth or engineered fill belowfrost depth. In areas not subjected
to freezing, footings shall be at least 12 inches (305 mm) below
finished grade.'

If you want more details about reinforcement requirements please post your questions on my forum Hearthtalk.com or use the provided link

Here is a link that might be useful: Hearthtalk