The invention relates to the structure of the bottom area of a metallurgical furnace (1), comprising a bottom part (2) and a wall (3), both of which include a steel structure (6, 21) forming the external surface, and a refractory inner brick lining (12), the bottom part (2) and the wall (3) being joined together; the wall (3) of the furnace (1) is divided into a lower part (4) and an upper part (30), whereby the steel structure (21) of the lower part (4) is a separate ring consisting of parts (23), joints (19) being provided between the parts (23) and arranged to expand. The invention further relates to a method for controlling the expansion of the furnace (1).