Abstract: A base for a heat treating furnace for annealing coils of steel strip. The base supports the coils in a sealed compartment through which an inert gas is circulated, and defines the bottom of the sealed compartment. The base comprises a top plate assembly, a floor, and side walls that define therein a sealed enclosure. The top plate assembly is supported on a plurality of upright, annular, concentric bearing partitions located in the enclosure that slidingly engage the bottom surfaces of the top plate assembly in a manner to accommodate radial thermal expansion and contraction. The spaces between the upright partitions are filled with a lightweight thermal insulating material. The top plate assembly comprises at least two coplanar plate sections located one-within-the-other and joined by an annular expansion joint. The joint has a "U" shape when viewed in cross section and accommodates radial thermal expansion and contraction of the top plate sections relative to one another.