Torsional behaviour and working mechanism of concrete-filled galvanized helical corrugated steel tubes

Concrete-filled galvanized helical corrugated steel tubes (CFCSTs) are newly proposed for strengthening the corrosion resistance, ductility, and constructability of traditional RC structures. Their composite effectiveness and basic compressive, bending, and shear properties have been investigated previously. However, the unique spiral shape and lock-seam connections of the helical CST may lead to complex torsional responses, which have not been studied. This paper, therefore, attempts to present static experiments for the CFCSTs subjected to pure torsion and combined compression-torsion loads. The main test variables are the member types, loading directions, boundary conditions, and axial compression ratios. The static loading test set-up and instruments have been introduced in detail. The torsional working mechanisms are discussed carefully with the analysis of failure modes and torques versus torsional angle curves. The direct and indirect contributions in torsional resistances of the helical CST have been analysed and calculated through elastic-plastic stress/strain analysis. Finally, the applicability of the existing design methods for the torsional bearing capacity of CFCSTs is examined.