One efficient means of composite construction is that which utilizes high performance concrete filled steel tube (CFT) columns. The filling of the tube increases the stiffness and strength and overcomes potential local instability of high strength steel tube. The tube confines the high strength concrete, increasing its ductility. The combined system has the attributes of high performance structural steel (speed of construction, strength, long-span capability, lightweight) and high performance concrete (stiffness, damping, economy), while using the tube as formwork. It is imperative in seismic resistant design that the members be properly proportioned and the structural system be well balanced. Currently there is a lack of knowledge of the seismic behavior of CFT columns. Rational guidelines for the seismic design of CFT composite construction are needed particularly when high performance concrete and high performance steel are used. This research focuses on basic behavior of CFT members with application to moment resisting and braced frames. The objectives are to: (1) study the load-deformation response of non-circular CFT columns composed of high strength concrete and steel under combined gravity and lateral seismic loading; (2) develop and calibrate a analytical models and computational methods based on experimental results for predicting the response of CFT columns under seismic loading conditions; and (3) develop design guidelines and recommendations for CFT columns constructed of high strength materials. The investigator will involve pursuing analytical and large-scale experimental investigations. Short CFT columns will be first studied, in which the effects of various design parameters on a member's force-deformation and moment-curvature-thrust relationship will be investigated. The results from the experimental studies will be utilizes to develop and calibrate various analytical models for predicting the response of a CFT to combined axial and flexural loading. These models will include design-oriented strength prediction equations and fiber-based formulations for capacity and overall force-deformation response prediction. The second part of the project will investigate the behavior of long CFT columns subjected to combined axial and lateral cyclic loading, considering the effects of the design parameters. Using the models a parametric study will be conducted to expand the experimental data base. The results will be used to formulate the design guidelines. The knowledge acquired and design guidelines developed will lead to the practical and reliable design of this form of construction as well as develop the use of high strength concrete and steel materials in seismic resistant design. This project also involves close collaboration with Japanese and Germany Universities which will enhance the quality of research. This is a project supported under the second-year program of NSF initiative `U.S.-Japan-Cooperative Research on Composite and Hybrid Structures,` NSF 94-154.
