Analytical Study Of Square Concrete-filled Steel Tube Stub Columns Under Axial Compression

Based on the concept of confined concrete, the Concrete-Filled Steel Tube (CFT)structure is developed as a new composite system which takes the advantage ofinteractions between internal concrete and steel tube to obtain higher capacity, betterseismic performance and etc. Therefore, such type of structure has been widely usedin practice.Recently, a lot of efforts have been made to investigate the behavior ofconcrete-filled square steel tubes. However, most of them are mainly based on theobservational approach rather than establishing a complete theoretical system toexplain the interaction mechanism between two composite materials. In addition,based on limited experiments, the widely used fiber models also encounter the limitedapplications. Given this, a large number of previous tested data were collected andthey were unified by using conversion formula for mechanical parameters of concreteincluding the size effect coefficient. At the same time, statistical analysis ofgeometrical and material parameters in the experimental database was also done.The performance of columns made of concrete-filled square steel-tubes underaxial compression heavily depends on its compressive strength and the monotonicbehavior. By evaluating the design formulae of the bearing capacity available in bothdomestic and foreign codes as well as published simplified models to calculate thecompressive strength, a new model with considerations of the buckling of steel tubeas well as the interactions between concrete and steel tube was proposed, which hadmore clear mechanical meaning and higher reliability. For monotonic behavior,collected load-displacement curve were classified into different groups according totheir different softening behavior. The applicability of exsisting fiber models wereevaluated by comparing with experimental results at the feature point. Based on therelevant assumptions, which have been confirmed by finite analysis, a modifiedsemi-empirical fiber model including the interactions of concrete and steel tube wasestablished and the parameters of this model were obtained through a statisticalanalysis of the experimental curves. At last, three classic concrete constitutive modelswere evaluated and their critical parameters with respect to the mechanical properties of columns made of concrete-filled square steel-tubes were clarified by finite elementanalysis.