Contents I. Introduction
Concrete-filled steel tubes (CFSTs) can be defined as composite structural elements consist of a hollow steel section (HSS) and infill concrete. The steel tube which can be in any shape (circular, rectangular, square, etc.) increases the strength and strain capacity of the concrete, whereas the infill concrete improves the global and local buckling resistance of the steel tube [1], [2]. In recent years, the applications of CFSTs in structures have increased significantly because of their structural and economical advantages. The steel tube eliminates the need for shuttering during the construction stage which reduces the construction cost and time. Also, the structural properties such as the strength, deformation capacity and the global and local buckling capacity are all increased due to the composite behavior between the steel tube and the concrete infill [1], [3]–[5]. Nowadays, the CFSTs are usually used as axial members due to their very large axial stiffness. To date, the use of CFSTs as flexural members in structures still very limited and questionable. Hence, the focus has been directed to investigate the flexural behavior of CFST beams to expand their applications as flexural members in structures [6]–[10]. The composite action of CFSTs depends on the bond stress between the steel tube and the infill concrete. In general, circular CFSTs are more preferable than rectangular and square CFSTs because they provide higher bond stress and better confinement to the concrete. The improved confinement of circular CFSTs will lead to prevent or delay the local buckling of the steel tube. However, rectangular and square CFSTs are used more in engineering practice due to the lack of design guidelines for circular CFSTs and the ease of handling of rectangular and square CFSTs [1], [3], [11].
