Effect of Column-Base Flexibility on the Seismic Response and Safety of Steel Moment-Resisting Frames

The effect of column-base flexibility on the response of steel moment frames is assessed through parametric simulation. The response of four frames (2-, 4-, 8-, and 12-story), designed as per current codes, is investigated through static pushover simulations and sophisticated nonlinear response-history simulations, including collapse simulation. For each frame, a range of base fixities is interrogated, including realistic values that are calculated from the designed connections. The results indicate that a reduction in base fixity alters the force distribution and the plastic mechanism, significantly reducing ductility capacity and strength, as well as collapse resilience, while increasing member forces. For the 4-, 8-, and 12-story frames, this trend suggests that the expected response of such frames is worse than is implied by simulations and design approaches that assume a fixed-base condition. However, the trend is beneficial for the 2-story frame, which is analyzed and designed assuming a pinned base.