The definition for a structure in article 100 is simply "that which is built or constructed." So couldn't a footing or column qualify as a structure? Then the ground ring would only have to encircle a footing, and not the entire building.
Article 100 also has a definition of a building; "A structure that stands alone or is cut off from adjoining structures...". The NEC handbook states that a "building" can also be a pole, a billboard sign, or a water tower.
