Progressive collapse resistance of an actual six-story reinforced concrete frame structure is evaluated following predefined initial damage. The initial damage was caused by the simultaneous explosion (removal) of two adjacent exterior columns, one of which was a corner column. The mechanism of load redistribution and change in column axial forces (strains) are discussed. In the structure studied, the development of bidirectional Vierendeel (frame) action is identified as a major mechanism in redistribution of loads. Through careful instrumentation, the change in the direction of beam bending moments in the vicinity of the removed columns is demonstrated. In general, if such a change in the bending moment direction results in high tensile stress in bottom beam reinforcement at the face of a column, brittle local failure in the absence of proper anchorage can occur. This failure did not take place in this building. The nearly century old structure, without satisfying integrity requirements, resisted progressive collapse with a recorded maximum vertical displacement of only 1/4 in. (6.4 mm).