Steel Special Moment Frame (SMF) is a preferred seismic force-resisting system for its architectural flexibility and high ductility. To meet the story drift limit specified in building code, design engineers prefer to use deep columns for their high in-plane flexural stiffness about the strong axis. While plastic hinging in wide-flange beams has been extensively researched, this is not the case for steel columns, especially deep-section (e.g., W24 to W36) columns that are subjected to both axial and flexural demands. Prior testing of shallow (W14) columns under varying axial load and cyclic bending showed excellent ductility capacity even when the axial load was high. Since deep columns usually have much larger section slenderness parameters (or width-to-thickness ratios h/t_w and b_f/2t_f) for web local buckling (WLB) and flange local buckling (FLB) controls as well as a larger member slenderness parameter, i.e., L/r_y, for lateral-torsional buckling (LTB) and flexural buckling (FB) controls, it was not clear if the same conclusion on shallow column behavior can be generalized to deep columns. This study intends to (1) generate an experimental database for analytical modeling of deep columns, and (2) evaluate the adequacy of design requirements for deep columns in AISC 341 and ASCE 41.
In Phase 1 of this study, twenty-five deep columns with five different W24 sections were tested. Each section varied in section and member slenderness. Phase 1 testing intended to investigate the effects of slenderness parameters, axial load levels, directions of bending, and lateral-drift sequences on the column responses.
Phase 2 study encompassed testing of additional twenty-three columns and was divided into two phases (2A and 2B). Shallow and deep columns including W14, W18, W24, and W30 sections were tested to further examine Phase 1 objectives and the effects of section depths, boundary conditions, and varying axial loads. Test results revealed that findings from Phase 1 could be extrapolated to deeper and shallower sections. Regarding the effect of boundary conditions, specimens with fixed-fixed end boundaries exhibited backbone curves that were similar to those of fixed-rotating specimens in term of maximum flexural strength, buckling mode, and axial shortening behavior. Results from varying axial load tests showed that first-story exterior columns behave very differently from the interior ones in SMFs.