Seismic performance of steel moment frame office buildings with square concrete-filled steel tube gravity columns

A dual lateral-force resisting system consisting of a primary lateral-force resisting system and secondary concrete-filled steel tube (CFT) columns placed in the gravity framing is presented in this paper. The dual CFT system concept relies on the primary lateral-force resisting system to supply the main lateral strength, while additional lateral strength and robustness is provided by the CFT columns. To explore the viability of the concept, the predicted seismic performance of 1-story, 2-story, and 4-story office conventional buildings, with perimeter steel moment frames and wide-flange gravity columns, was compared to the performance of the same buildings but employing square HSS columns filled with unreinforced concrete. The analyses predicted that, compared to conventional buildings, buildings with the dual CFT system were 20-83% less susceptible to seismic collapse, depending on the strength and ductility of the primary moment frame, the orientation of the wide-flange columns in the conventional building, and the number of stories. Using high-strength, thick, or slightly larger CFT columns did not significantly improve collapse safety. Buildings with the dual CFT system generally had improved seismic performance, depending on the moment frame design, the number of stories, and the intensity of the ground shaking. Buildings with the dual CFT system had up 45% lower repair costs, up to 64% shorter repair time, and a lower probability that the building would be deemed unsafe.