Cyclic Testing of Two-Story Steel BRBFs for Evaluating Performance of Moderately Ductile Built-Up Box Columns under Varying Axial Load

A built-up box column is commonly used for the seismic resistance in buckling-restrained braced frames (BRBFs) in Asia. This research aimed to study the lateral cyclic performance of different compact built-up box columns in BRBFs under varying axial load, caused by the overturning of the frame and axial forces of buckling restrained braces (BRBs). Two one-bay, two-story frame specimens with all-steel BRBs and moderately ductile or compact built-up box columns were designed and tested. The width-thickness (b/t) ratios for columns in Specimen S1 were 20.5 and 21.1, which were between the limits of highly and moderately ductile box column sections (λhd=13 and λmd=23.7) in AISC 341-22. The b/t ratios for columns in Specimen S2 were about 27.5, classified as compact box columns in AISC 341-22, but were close to the most seismic compact columns in AIJ. Specimen S1 with columns under varying axial load from 0.13Py to 0.61Py performed well at 0.04 rad drift, showing significant high-mode local buckling of the flange and web plates along the column height. Specimen S2 performed well at 0.03 rad drift but experienced fracture at the complete joint penetration groove welds between the flange and web plates near the column base when the frame moved to 0.04 rad drift. The work showed that the b/t requirement for highly ductile built-up box columns in AISC 341-22 is overly conservative, and the most seismic compact requirement in AIJ may be used for the limit of moderately ductile built-up box columns, not the highly ductile box column limit.