Long-term experimental behavior of concrete-encased CFST with preload on the inner CFST

Abstract

Concrete-encased concrete-filled steel tube members (concrete-encased CFST) are innovative composite structural elements which are having gradually increasing utilization in high-rise buildings and bridge structures. In practice, the inner CFST component is generally erected first and thus subjected to constructional load before the outer reinforced concrete (RC) component is formed. Afterward, the whole composite cross-section is under long-term sustained load in the service stage. This paper studies the structural behaviour of such composite columns under combined preload and long-term sustained load through a series of experimental tests. Concrete-encased CFST columns under different loading conditions, together with their CFST counterparts, are tested and compared. The main parameters include preload ratio, loading age and sectional configuration. Influence of these parameters on the structural behaviour in terms of failure mode, bearing capacity, stiffness and ductility are evaluated. Finally, a simplified design method is recommended to account for the influences of combined preload and long-term sustained load on the ultimate strength of the concrete-encased CFST columns, where the combined effects can be reflected by a corresponding set of coefficients.