Research on CFT Column Systems

Paper No. 2618Concrete filled steel tubes （CFT) offer numerous advantages for seismic resistant design. CFT provides large axial stiffness and load capacity. The steel confines the concrete and significantly increases stiffness and resistance. The concrete fill restrains local buckling of the tube, and increases member ductility, while permitting more slender steel elements. Unfortunately, there also are severe limitations in the application of CFT, because the behavior of CFT elements is quite different than steel or concrete elements. Further, connections between CFTcolumns and steel members are also difftrent than those used in ordinary steel construction. The US-Japan Cooperative Earthquake Research Program on Composite and Hybrid Structures was started within this cotext, and CFTwas one of 4 focus areas in this program. The goals of the research program were to develop practical guidelines so that structural engineers in both the US and Japan can understand and economically use CFT for seismic design. The Japanese research emphasized the strength, ductility and behavior of CFT menbers and the internal diaphragm connections. Jpanese studies included development of a database ofpast research results and testing to evaluate 3-dimensional behavior and biaxial bending of CFT elements. However, the bulk of theJapanese rescarch considered CFT columns under concentric and eccentric loading of both rectangular and circular tubes under a wide range of material properties and colum slenderness ratios. US research emphasized connection details which are practical for US construction. In addition, studies of the resistance and ductility of CFT members and the bond stress transfer and composite action for CFT elements were completed. Several typical connection systems, which appear to be most practical for the US and which provide good seismic performance, are noted. Design guidelines are being developed in both countries. These are discussed, and the economic consequences of using CFTcolumns are noted. Some differences and similarities between the US and Japanese guidelines and results are explained