Seismic Performance of Concentrically Braced Steel Frames in Multistory Buildings with Mass Irregularity

The influence of mass irregularity on building seismic response is examined for an eight-story concentrically braced steel frame with different setback configurations resulting in sudden reductions in plan dimensions and seismic weight along the height of the structure. Three locations of mass discontinuity were considered (25, 50, and 75% of the building height), together with two ratios of seismic weight (200 and 300%). A reference regular structure was also considered for comparison. The design of each structure was performed according to the proposed 2005 National Building Code of Canada NBCC provisions using two analysis methods: The equivalent static force procedure and the response spectrum analysis method. Although severe, the mass irregularity conditions considered in this study were found to have a limited negative impact on the seismic performance of the structures designed with the static analysis method. The performance of irregular structures exhibiting lower performance could be improved by using the dynamic analysis method in design, but not to the level achieved by the reference regular structure.