Where a building is partially or completely demolished or removed, all floor area which is constructed to replace that which is demolished or removed from the building or structure shall be subject to the taxes imposed by this article. For the purposes of this article, floor area shall be considered as being demolished or removed when the structural floor materials are removed or when fifty percent (50%) or more of the exterior walls surrounding a floor in any story is demolished or removed. (1962 Code § 8-9.06)