Life-cycle based analytical theory of concrete-filled steel tubular structures and its applications

Abstract

<p indent=0mm>Concrete-filled steel tube (CFST) is a composite structure consisting of a steel tube filled with concrete. The steel tube and its concrete core work together to carry external loads applied to the structure. Due to the interaction between the two kinds of materials, i.e., the confinement effect of steel tube on the core concrete, CFST structures can effectively delay/avoid the local buckling of the steel tube and the brittle failure of the concrete core. The superior structural properties of CFST include high strength and stiffness, good ductility, excellent seismic and fire performances, and easy construction. Therefore, CFST structures have been extensively used in large-scale constructions including industrial plants, subway stations, transmission towers, high-rise buildings, and bridges, etc. CFST structures have also offered substantial benefits on architectural appearance and engineering economy. The confinement effect in CFST throughout its service-life essentially contributes to the structural advantages, whilst it has, at the same time, brought about enormous complexities in the corresponding load-resisting mechanisms. Firstly, an in-depth understanding and accurate characterization of the confinement effect, and the development of some feasible constitutive models for the core concrete that facilitate the analysis and design of major infrastructures, becomes the key issue to be addressed. The salient loading factors in the service-life of CFST structures need to be considered in the constitutive models, such as long-term sustained load, low-cycle cyclic load and full-range fire. Secondly, the establishment of damage mechanism analysis and strength prediction models for CFST structures considering long-term sustained load, complex loading conditions, earthquake and fire becomes another key issue that needs to be resolved for the safety design of major CFST constructions. Life-cycle based damage mechanism and analytical theory of concrete-filled steel tubular structures have been investigated and established based on nearly <sc>20 years</sc> of persistent and systematic study. The design methods for the ultimate loading capacity of CFST structures were further proposed. The major research outcomes can be summarized in the following three aspects: (1) The interaction between the steel tube and its core concrete as well as the passive confinement mechanism in CFST were revealed. A general constitutive model for the confined core concrete under various stress/action conditions was established with the “confinement factor” as an essential variable. The considered actions include long-term sustained stress, cyclic load and elevated temperature. (2) Theoretical models were established to account for the damage mechanism of CFST structures under long-term sustained load and complex loading conditions. The ultimate strength of CFST members under various loading conditions was analysed, including compression, tension, bending, shear, torsion, and the combinations of different loads and load paths. The creep and shrinkage of concrete as well as the deformation of CFST under long-term sustained load were accurately simulated. (3) Theoretical models were established to simulate the damage mechanism of CFST under earthquake and full-range fire. The hysteretic properties and ductility of CFST members under low-cycle cyclic loading were revealed. The model for calculating the fire resistance of CFST columns was proposed, in which the coupling effects of loads and full-range fire were taken into account. This paper briefly reviews the development of CFST structures and outlines the life-cycle based analytical theory for CFST structures. Finally, the potential development trends of the CFST structures are discussed.