Circular concrete filled thin-walled steel tubes under pure torsion: Experiments

This study experimentally explored the torsional behaviour of 33 specimens, which comprises of 18 concrete filled steel tube (CFST), 9 steel tube (ST) and 6 concrete (C) specimens. The length to diameter (L/D) ratio was 7.14 while the ratios of diameter to thickness (D/t) were 60.9, 46.7 and 40.0. The compressive strength of the infill concrete was 24.2 MPa (normal) and 33.3 MPa (relatively high). All specimens were tested to failure and the obtained data was analysed. The results indicated that buckling failure of CFST specimens was prevented due to the presence of concrete infill. Diagonal cracks of concrete infill observed on CFST specimens after cutting steel tubes are the evidence for the collaboration between steel tube and concrete. This evidence also reveals that the concrete infill contributes to increase the torsional moment capacity of CFST and helps steel tubes to work effectively. The superposition method fails to apply for CFST members under torsion because it does not take into account the mutually beneficial collaboration and interaction between steel tubes and concrete infill which resulted in the improvement on the failure mode, mechanical properties and behaviour of CFST specimens under torsion. Details of the test results and above-mentioned findings are reported in this paper. <comment>Superscript/Subscript Available</comment