Concrete filled steel tube composite beams and improvement of flexural performance

Concrete-filled steel tube (CFST) beams represent one of the most promising composite members within the CFST family. CFST beams are widely utilized in different structural applications including, bridges of several typologies, offshore structures, and high-rise buildings. CFST beams display several advantages over the conventional tubular steel or reinforced concrete members, including high flexural capacity, high stiffness, significant ductility, and energy absorption ability. The superior performance of CFST beams is thanks to the composite action between the steel profile and the concrete core. The concrete core delays and even precludes local buckling of the confining steel tube, and this significantly increases strength and ductility. On the other hand, the steel section provides continuous longitudinal and lateral reinforcement for the concrete core in addition to effective confinement. The essential goal of this thesis is to experimentally investigate the flexural performance of concrete-filled steel tube beams subjected to a pure bending load. To achieve this aim three different typologies of CFST beams have been studied comprising lightweight-concrete filled steel tube (LWCFST) beams, self-compacting-concrete filled steel tube (SCCFST) beams, and finally, SCCFST beams retrofitted using external bolted steel plates, all with various parameters and configurations. Five international pioneering design codes have been adopted to assess the flexural capacities of the tested CFST beams, involving the AISC 360-16, EC4-2004, DBJ/T13-51-2010, AS 5100.6-2004, and AIJ-2001. Furthermore, a theoretical model was developed to predict the moment resistance of the retrofitted SCCFST beams.