Progressive Collapse Resistance of an Actual 11-Story Structure Subjected to Severe Initial Damage

Progressive collapse resistance of an actual 11-story reinforced concrete structure following severe initial damage is studied experimentally and analytically. The initial damage was caused by simultaneous explosion (removal) of four first-floor neighboring columns and two second-floor perimeter deep beam segments. The structure resisted progressive collapse with a maximum permanent vertical displacement at the top of the exploded columns of only about 56 mm (2.2 in.). The response of the structure is evaluated analytically using different modeling methods. Beam growth and, in turn, the development of the beam axial compressive force are modeled and discussed. It is demonstrated that such axial compressive force can significantly affect progressive collapse resistance of the structure. The shortcomings of nonlinear modeling with commonly used plastic hinges are quantified and discussed. It is shown that such a modeling method ignores axial and flexural interaction in beams and can underestimate the resisting element internal forces and in turn progressive collapse resistance of the structure. By using fiber hinges in an analytical model, such interaction is accounted for and the local and global experimental data are closely estimated. The progressive collapse-resisting mechanisms primarily include the axial-flexural action of the second-floor deep beams and Vierendeel action of the flat plate system in floors above.