There are two conditions under which the metal frame of a building or structure can be judged suitable for use as an individual grounding electrode

The Code: Section 250.52 (A)(2): Grounding Electrode

(2) Metal In-ground Support. The metal frame of a building/structure can serve as a grounding electrode when it meets at least one of the following conditions

  1. At least one structural metal member is in direct contact with the earth for 10ft or more, with or without concrete encasement
  2. The bolts securing the structural steel column are connected to a concrete encased electrode [250.52(A)(3)] by welding, exothermic welding, steel tie wires, or other approved means.
Examples of both conditions are shown. (1) is on the left and (2) is on the right

Notes

This connection is not required to occur at more then one building column or structural member, as long as there is electrical continuity through the entire metal frame of the building. The single connection to the earth allows the entire structural building frame to be used as a grounding electrode.