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

Abstract

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 show 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 is examined parametrically through 2160 Continuum Finite Element (CFE) simulations validated against physical experiments. Parameters interrogated 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.

Anchor‐yield exposed column bases for minimizing residual deformations in seismic‐resistant steel moment frames

“…This is about 50 % less when the inherent column base flexibility is acknowledged. This agrees with component level studies on embedded column base connections [16].…”

“…There are a number of solution strategies to address axial shortening. In a recent study, Inamasu et al 15,16 found that the flexibility of the column base connection can reduce the associated inelastic deformation demand of the column, thus the amount of axial shortening. However, first‐story drift concentration is likely to occur in steel MRFs in this case 17 .…”

Proposed methodology for building‐specific earthquake loss assessment including column residual axial shortening

“…In particular, All columns considered herein were assumed to be ideally fixed at the ground level. The inherent flexibility of exposed or embedded column base connections may significantly influence the column residual axial‐shortening Field observations from past earthquakes (eg,) along with numerical studies suggest that the inelastic behavior of columns in structures as well as bridge piers could be considerably affected by soil‐structure‐interaction, which was neglected in the present study. The paper findings suggest that recently proposed structural solutions may be further exploited to potentially minimize steel MRF column structural damage due to local buckling.…”

“…The inherent flexibility of exposed or embedded column base connections 60 may significantly influence the column residual axial-shortening. 61,62 F I G U R E 1 5 Variation in expected demolition losses with assumed median fragility parameters based on the long-duration set…”

Development of collapse‐consistent loading protocols for experimental testing of steel columns

“…The panel zone shear distortion is modeled with the Krawinkler model (Gupta and Krawinkler 2000b). Column bases are idealized as fixed; hence the column-footing interaction is neglected (Zareian and Kanvinde 2013;Inamasu et al 2019). Viscous damping is approximated with the Rayleigh model.…”