Concrete-filled spiral-welded stainless-steel tube long columns under concentric and eccentric axial compression loading

Spiral-welded tubes (SWTs) are fabricated by helically bending a steel plate-coil and welding the resulting abutting edges. The cost-effectiveness of concrete-filled stainless-steel tube (CFSST) columns can be improved by using SWTs rather than longitudinally welded tubes as their fabrication is more economical and efficient. However, due to the high level of residual stresses and unique imperfection patterns present in SWTs, such concrete-filled spiral-welded stainless-steel tube (CF-SWSST) columns require separate consideration. Even though most practical columns are ‘long columns’, where the capacity is length dependant, only the behaviour of CF-SWSST short columns has previously been investigated. To address this research gap, twelve CF-SWSST long columns with effective length to diameter (L_e/D) ratios in the range 10.5–12.0 were tested under axial compression considering end load eccentricity ratios of 0, 0.15D and 0.4D and diameter to thickness ratios (D/t) ranging from 62 to 116. A global flexural buckling type failure mode was obtained for all the tested columns similar to that reported for CFSST long columns of other tube types. It was found that the guidelines of several existing design standards relating to mild-steel concrete-filled steel tube (CFST) columns are directly applicable to CF-SWSST long columns under eccentric loading. However, for concentric loading the predictions were non-conservative indicating that separate guidelines are needed for purely axially loaded CF-SWSSTs, including procedures to consider column slenderness effects of CFSSTs distinct from those specified for mild-steel CFSTs. In addition, fibre-element analyses considering unconfined concrete models gave conservative, yet close, predictions of the experimental eccentric axial capacities which was consistent with the experimentally determined negligible level of concrete confinement.