The collapse behavior of a 22-story steel building during the September 19, 1985 Mexico earthquake is investigated by studying hysteretic behavior, ductility factors of individual structural components, and overall instability of the building. The hysteresis models for truss-type girders, bracing members, and box columns to be used in the inelastic analysis of this building are developed. A series of 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 because most girders in the building have suffered large ductilities. Due to the load redistribut-ion effects from the ductile-failed girders, local bucklings developed at many columns on floors 2, 3, and 4. Therefore, most columns on floors 2 through 4 lost their load carrying capacities and rigidities which then caused the building to tilt and rotate. As a result, more columns on floors 5 through 7 developed local buckling and more bracing members buckled. It is believed that ductile failures of girders combined with the local bucklings of columns in the lower part of the building resulted in significant story drift, building tilt, P-A effect, and the failure mechanism.