Seismic Stability of Wide-Flange Steel Columns Interacting with Embedded Column Base Connections

The rotational flexibility of column base connections is commonly assumed to be detrimental to the seismic response of steel moment-resisting frames (MRFs) because it increases story drift demands and promotes soft-story formation by lowering the point of inflection, thereby exacerbating the moment demand at the first-story column top end. However, recent experimental research indicates that wide-flange steel columns with flexible boundary conditions may have enhanced response because base connection flexibility delays the plastic hinge formation and local buckling progression within the column cross section near the base. Local buckling greatly reduces the lateral and torsional restraint at the column ends such that the delay in local buckling also delays member instabilities, which often control the deformation capacity of wide-flange steel columns. This effect was examined parametrically through 2,160 continuum finite-element (CFE) simulations validated against physical experiments. Parameters examined include the applied axial load ratio, the column length, and the lateral loading protocol for the column sections, along with five levels of column base connection flexibility. The results indicate that base connection flexibility has a significant (and positive) influence on key aspects of column response, including (1) lateral deformation capacity, which is on average 40%-50% greater for a realistic value of flexibility, relative to a fixed-base condition; and (2) residual column axial shortening, which is important from the standpoint of retrofit and reparability (a 30%-60% decrease). These benefits are more pronounced in cyclic (versus monotonic) loading histories and for members with cross sections more prone to out-of-plane instability. The implications of these findings are outlined for existing buildings as well as prospective design methodologies. Limitations are discussed.