Behaviour of axially loaded UHPC-filled double skin steel tubular columns

This paper investigates experimentally and numerically the axial loading behaviour of Ultra-high performance concrete (UHPC) (135 MPa)-filled double skin steel tubular columns. A total of 37 stub columns (450 mm long x 152 mm diameter) were tested, including totally filled control tubes and tubes filled with normal- and high-strength (NSC and HSC) concrete. The study examined the effects of outer tube diameter-to-thickness (D-o/t(o)) ratio and inner-to-outer tube dimeter ratio, which reflects UHPC wall thickness, on axial strength. A nonlinear finite element model using computer program LS-DYNA was also developed and verified. The study showed that the stubs with the thinnest outer tube (D-o/t(o) = 72) and largest inner void ratio (0.7) achieved equivalent strength to the NSC (41 MPa) totally filled thickest tube (D-o/t(o) = 32). Failure occurred by outwards local buckling of the outer tube and inwards buckling of the inner tube, along with UHPC diagonal shear failure. The peak load reduced nonlinearly as D-o/t(o) increased due to a transition from a class 1 to class 2 section, and the post-peak load drop increased with increasing concrete strength (f(c)(')) and (D-o/t(o)) ratio. As UHPC thickness reduced to a 50% volume reduction, axial strength reduced by 20-28%. The Canadian CSA S16:19 code provisions overestimated axial strength by 12-56%. A modification factor was developed and is recommended to be introduced in the code equation.