Study on seismic behavior of concrete filled steel tube columns under pure torsion and compression-torsion combined action

Based on the quasi-static test on eight CFST columns subjected to pure torsion and compression-torsion combined action, the torsion behavior of CFST columns with various section types, steel ratios and axial load was studied. The test results show that the hysteresis curves of CFST columns under pure torsion and low compression-torsion are very plump. The unloading stiffness of CFST columns was equal to the initial elastic stiffness. The torsional capacity of CFST columns could be improved by the low compression load, and the ductility was also good, but the torsional capacity of CFST columns could be reduced by the high compression load. When CFST columns were subjected to pure torsion, spiral diagonal compression struts would be created in the infilled concrete, and the diagonal compression force was equal to the axial tension force of the steel tube in order to satisfy the axial load equilibrium condition on the section, so the axial strain may be produced in the steel tube. The shear strain has a good linear relationship with the rotation angle of the section when CFST columns are subjected to pure torsion and compression-torsion combined action. Based on the test results and literatures available, the torsion mechanism of CFST columns was analyzed in detail, laying a foundation for the development of member system element of CFST columns with torsion effect considered.