This paper presents the investigation of the collapse behavior of a 22-story steel building during the September 19, 1985, Mexico City earthquake by studying hysteretic behavior, ductility factors of individual structural components, and overall instability of the building. Extensive inelastic analyses have been performed for the building by using the multicomponent seismic input of actual Mexico City earthquake records. It was found that the structural response exceeds the original design ductility of this building, and most girders in the building have severe inelastic deformation. Due to the load redistribution that results from ductile girder failure, local buckling occurred in many columns on floors, 2, 3, and 4. Therefore, most columns on floors 2-4 lost their load-carrying capabilities and rigidities, which then caused the building to tilt and rotate. It is evident that ductile failures of girders combined with local buckling of columns in the lower part of the building resulted in significant story drift, building tilt, P-DELTA effect, and the failure mechanism.
