Effects of Section Compactness and SCWB Condition on Moment Redistribution and Plastic Hinging in SMF Built-Up Box Columns

Steel special moment frames (SMFs) with built-up box columns and I-shaped beams commonly are used for seismic design, especially in Asian countries. Modern seismic design codes assume that plastic hinging will occur near the base of columns and in the reduced beam section (RBS) region; a strong column-weak beam (SCWB) check is required to reduce the likelihood that column hinging will occur above the first story. This study investigated the effects of column section compactness (i.e., width-thickness ratio), axial force level, and SCWB ratio on the moment redistribution and plastic hinging in the bottom two stories of the columns. Two half-scale, two-and-one-half-story subassemblages with a column and RBS beams were cyclically tested; steel beams were included to simulate a realistic boundary condition to the column, especially after the beams started to form plastic hinges. The experimental observations of two subassemblages with varying column width-thickness ratios and SCWB ratios showed that meeting the SCWB requirement does not necessarily preclude plastic hinging in the second-story column. A finite-element parametric study with nine subassemblage models was conducted. A design criterion with an expression containing both the width-thickness and SCWB ratios is proposed to achieve plastic hinging near the column base, but not elsewhere along the columns.