© 2013 DAR Publishers/University of Jordan. All Rights Dirasat, Engineering Sciences, Volume 39, No. 1, 2013 - 75 - Confinement of Concrete Filled Steel Tubular Columns - New Frontiers Abdelqader Najmi* ABSTRACT A new type of composite columns consists of tubular sections, filled with concrete has been investigated. Special link connectors are used to join the steel shell with the concrete inside. The method of connecting the two materials brings the level of confinement of concrete in this type of columns to values never reached before. The level of confinement reached surpasses by far the values obtained in spirally reinforced concrete columns when considering ultimate axial load. Concrete strength can be increased by more than 50%. These composite columns attain large axial strains at ultimate load of the order of 2% and more, such strains are well outside the plastic strains of steel and almost (6-10) times the concrete compression crushing strain of 0.3%. The resulting integrity of the cross section goes beyond preventing elastic local buckling of the steel shell; to sustaining the stiffness of the cross section in the plastic zone. Ultimate loads of connected composite concrete columns take place by the plastic buckling of the steel tube, and not by the crushing of concrete. Keywords: Composite columns, Concrete, Steel, Confinement, U-shaped links. INTRODUCTION Concrete in compression is usually characterized with a stress-strain relationship obtained from uniaxial standard compression tests. However, most concrete structural elements are subjected to a multi-axial stress state. A uniaxial stress state represents only one of an infinite number of multi-axial stress conditions to which an element of concrete in a structure may be subjected throughout the loading history of the structure; see Kotsovos (1987). The response of concrete varies widely for different stress states and it is therefore important to know how the concrete behaves for different multi-axial stress states. As an example, Kotsovos shows the variation of the peak axial compressive stress sustained by a concrete cylinder with increasing confining pressure. It was noted that a small confining pressure of about 10 percent of the uniaxial cylinder compressive strength was sufficient to increase the load- bearing capacity of the specimen by as much as 50 percent Both the strength and ductility of concrete are increased substantially under conditions of tri-axial compression. Richart et al. (1928) found the following relationship for the strength of concrete cylinders loaded axially to failure while subjected to confining fluid pressure 1 f . ' ' 1 (1) cc c f f f    The value of  , the lateral stress coefficient = 4.1, however, Balmer (1949) suggested a range between 4.5 and 7.0. ' cc f is the enhanced axial stress, and ' c f is the compressive cylinder strength of concrete. A new model was proposed for concrete confined by spiral reinforcement based on concrete-transverse steel interaction. The two main parameters were concrete strength and lateral stress lateral strain relationship that represents the response characteristics of the transverse steel to the lateral expansion of concrete. Assa et al. (2001) modeled a confinement mechanism and limited the lateral expansion of the confined concrete with the * Department of Civil Engineering, Faculty of Engineering and Technology, University of Jordan. Received on 6/2/2012 and Accepted for Publication on 28/6/2012.