Seismic Behavior of Steel Moment Frames with Deep Columns

Steel Special Moment Frame (SMF) is a preferred seismic force-resisting system for its architectural flexibility and high ductility. Before the Northridge earthquake, shallow columns (section depth less than 356 mm.) were used generally in these seismic force-resisting systems. However, to achieve economy in design, there were growing trend to use deeper columns (section depth greater than 356 mm.) to satisfy the code-enforced story drift requirements in recent years. Despite the wide use of these columns, very few researches were available on the deep columns hinging behavior under axial compression and cyclic drift. Since a deep column has larger slenderness ratio and is more vulnerable to both local and global buckling, it is essential to investigate its behavior. These buckling modes can severely affect the response of the SMFs by producing undesirable effects such as axial shortening, which increases as the applied forces of compression becomes larger an as the ground motion used as seismic input becomes more severe.
In this study, total of fifteen four-story steel SMFs’ behavior was investigated using the finite element program simulations. Four key factors that affect the behavior of these frames were studied: 1) Column bracing; 2) Beam bracing; 3) Column stiffening; and 4) Strong Column Weak Beam (SCWB) ratio. Effect of the axial force level and the column section properties were also investigated for broadening the investigation. It is shown that deep columns can suffer local and/or global instabilities even at relatively low story drift levels. The findings indicate that the performance of SMFs can be improved by bracing deep columns at the top and bottom level of beam flanges and by adding stiffeners on the web of these columns. It is suggested that column shortening can be controlled by increasing SCWB ratio