Stability of Steel Columns in Steel Concentrically Braced Frames Subjected to Seismic Loading

Lateral loads primarily induce axial loads in members of steel concentrically braced frames but columns may also be subjected to drift induced bending moments that may affect their strength and stability. The article discusses three examples of CBF columns that are subjected to combined axial compression and bending moments in structures under severe seismic ground motions: columns at an intermediate level where soft-story response develops in an existing seismically deficient X-bracing, fixed base columns at the first level of a well-proportioned multi-story X-bracing system, and columns in multi-tiered braced steel frames. Nonlinear dynamic analysis of the columns is performed to examine their stability response under axial and bending demands. In the first two cases, the results indicate that bending moments have limited effects on the global stability of the columns. Conversely, columns in multi-tiered frames must be designed to resist the expected combined axial and flexural demands in order to prevent column instability.