Flat base furnace bottom is easy to be constructed, and it needs less firebrick types than dish furnace bottom, so the flat base furnace bottom is very widely used in America.
Dish furnace bottom needs more types of firebrick than flat base furnace, and it costs more time to be made than the flat base one, while it has two imporatnt advantages as follows.
1. The dish furnace bottom is designed according to furnace casing size ......

http://www.yilongrefractory.com/news/Design%20of%20AOD%20Furnace%20Bottom.html