Development of Seismic Local Buckling Limits for Built-Up Box Columns in Seismic Moment Frames

This research aimed to develop new width-to-thickness (b/t) limits for local buckling control of the first-story built-up box columns in seismic moment frames subjected to axial and flexural loadings. Past research on isolated box columns was mainly focused on columns with b/t ratios ranging between the highly ductile limit (λhd) and the moderately ductile limit (λmd) per AISC 341-22. Six square box columns with b/t ratios significantly greater than those permitted in AISC 341-22 were tested in this study to supplement the available experimental database. Test results showed that column local buckling in the plastic hinge became significant after developing the peak strength beyond the plastic moment. Fracture occurred at 0.03–0.04 rad drifts for all specimens, being across or along the complete-joint penetration groove welds connecting the flange and web plates. The test database is further expanded by finite element simulation after a calibration of the numerical modeling technique with the test data; numerical simulation includes both square and rectangular box column sections. A modified procedure based on a methodology originally proposed for the development of similar limits for I-shaped columns was adopted. The proposed λhd and λmd limits are expressed in a single form for the first-story box columns, in which the axial load ratio, flange/web dimension ratio, height slenderness ratio, and material properties are included. The new forms indicate that both the λhd and λmd limits in AISC 341-22 are very conservative, particularly for box columns under a low-to-medium axial load. The λp limit, required to develop the plastic moment of a column per AISC 360-22, serves as the upper bound for the proposed limits.