The interaction of steel tube and concrete core in concrete filled steel tube columns

Concrete-filled steel tube columns have become increasingly popular in structural applications around the world. The concrete-filled steel tube system has a number of distinct advantages compared with ordinary steel or reinforced concrete systems. The enhancement in structural properties is due to the interaction between the steel tube and the concrete core. The confinement created by the steel casing enhances the material properties of concrete due to the triaxial state of stress. At the same time, the inward buckling of the steel tube is prevented by the concrete, thus increasing the stability and strength of the column as a system. In addition to these advantages, the steel tubes surrounding the concrete columns eliminate the need for formwork, which reduces construction time and costs. The main objective of this study is to investigate the load transfer by natural bond or by mechanical shear connectors. Furthermore, the interaction between the steel tube and the concrete core in concrete-filled steel tube columns under axial load as weIl as the effect of long-term load on concrete-filled steel tube columns are studied. Extensive experimental investigations as weIl as analytical and numerical modelling were undertaken in this study. The program involved testing 74 short column specimens under axial compression load applied to the steel tube alone, to the concrete core alone or to the steel tube and concrete core simultaneously at the top of the specimen, whereas the steel tube and concrete core are supported simultaneously at the bottom. The influence of a number of parameters on load transfer is studied including bond conditions, cross-sectional shape, strength and type of concrete, and length of the specimen. Two and three-dimensional nonlinear finite element models were developed to study the force transfer between the steel tube and the concrete core, the interaction between the steel tube and the concrete core and the performance of shear connectors in such composite members. The nonlinear finite element program ABAQUS was used. In the analysis, one and three-dimensional analytical models were derived to determine the distribution of the stresses and the strains in the concrete core and the steel tube under various loading conditions. Furthermore, an analytical model was developed to estimate the ultimate strength of circular concrete-filled steel tube short columns. The finite element models were then used in parametric studies to examine a wide range of parameters for the behavior of concrete-filled steel tube columns and the performance of shear connectors.