AbstractThis study presents a numerical assessment of the behavior of seismically designed steel frame buildings against ground floor column loss. In the designed prototype buildings, moment frames with beam-to-column rigid connections and concentric X-bracing frames resist the lateral force, whereas the steel-concrete composite floor slabs resist the gravity load. Macromodels are used to capture the building response when removing ground floor columns. The macromodels are built with a reduced modeling approach, in which the concrete damage and the local steel fracture behavior are accurately considered. The macromodeling approach is calibrated by high-fidelity models and validated by composite floor test. The validated macromodels are used to investigate the effect of column loss location, total number of floors, floor slab, beam-to-column connection type, adjacent span, and steel brace on the collapse resistance of prototype buildings. To account for sudden column failure, an energy-based approach is used to convert the quasi-static response curves to dynamic response curves. The structural robustness is derived by comparing each column failure case’s dynamic ultimate capacities with corresponding design requirements. Structural robustness enhancement strategies for steel frame buildings under progressive collapse scenarios are summarized and discussed. Moreover, a retrofitted moment-resisting connection with steel strands is proposed to enhance the steel frame buildings’ robustness by providing a second line of defense.