Concrete-encased CFST columns under combined compression and torsion : experimental investigation

Concrete-encased concrete filled steel tube (concrete-encased CFST) columns have been increasingly used in high-rise buildings and bridges in China in recent years. In practice, these composite columns may be subjected to combined compression and torsion under earthquake loads when they are adopted as columns in high-rise buildings or piers of viaducts. The corresponding behaviour of such composite columns thus needs to be thoroughly investigated to fill this knowledge gap. Results of a series of tests on concrete-encased CFST columns under combined compression and torsion are presented in this paper. A total of 26 tests were conducted on three types of specimens, i.e., concrete-encased CFST columns, hollow reinforced concrete (RC) columns without inner CFST components and conventional CFST columns. Based on the test results, effects of cross-sectional shape, axial load level and cross-sectional area of the inner CFST on the failure mode and the torque-rotation angle relation are investigated. Design formulae are then proposed which can reasonably predict the torsional strength of concrete-encased CFST columns.