ABSTRACT
In recent years, concrete filled steel tube (CFT) columns have become widely accepted and used in multistory buildings as well as bridges. These elements provide the synergetic advantages of ductility and toughness associated with steel structures and high compressive strength associated with confined concrete components. The advantages of CFT columns over other so-calledmixed or hybrid systems (fully encased or partially encased systems) include the fact that the concrete prevents local buckling of the steel tube wall and the confinement action of the steel tube extends the usable strain of the concrete (Wu et al., 2005; Wu et al., 2007). In addition, CFT columns have improved fire resistance and significant cost reductions in comparison with traditional steel construction. Composite CFT columns are especially efficient as the vertical elements in moment resisting frames in high seismic areas because they have a high strength to weight ratio, provide excellent monotonic and dynamic resistance under biaxial bending plus axial force, and the concrete provides additional damping (Tsai et al., 2004; Tsai et al., 2008; Tsai & Hsiao, 2008; Hu and Leon, 2010). A typical moment connection that was part of a composite braced frame consisting of steel I shape girders and either circular or rectangular (CCFT or RCFT) columns tested by Tsai et al. (2004) is illustrated in Fig. 3.1.
