Behaviour and design of concrete-filled mild-steel spiral welded tube long columns under eccentric axial compression loading

Spiral-welded tubes (SWTs)are fabricated by helically bending a steel plate and welding the resulting abutting edges. SWT is a more cost-effective form of tube that can be used for concrete-filled steel tube (CFST)columns. SWTs contain larger residual stresses and distinct imperfection patterns compared to other tube types and hence warrant separate consideration. Previous research into concrete-filled spiral-welded steel tube (CF-SWST)columns has been mainly limited to considering short column behaviour. In contrast, most practical columns are ‘long columns’ where length dependant effects limit the column capacity. To address this research gap, 12 mild-steel self-compacting CF-SWST columns with diameter to thickness (D/t)and effective length to diameter (L _e /D)ratios in the ranges 56–118 and 10.5–11.9 were tested under axial compression. Load-eccentricities of 0, 0.15D and 0.4D were considered for the tests. A stable global flexural buckling type failure mode was observed consistent with those previously reported for CFST long columns of other tube types. Local buckling also occurred in the post-peak region. The spiral weld seam was observed not to be a preferential location for failure. On average, the experimental capacities were well predicted by the guidelines of six internationally used design standards. The scatter of the actual to predicted capacity ratios obtained was either equivalent to or more conservative than that reported for comparable CFST long columns of other tube types. The results suggest that greater strength enhancement than considered in the codes may be effective for sections with D/t < 87 in the tested L _e /D range. For eccentric loading, fibre-element based analyses using confined concrete material models gave closer predictions of experimental capacities than codified methods. The study provided evidence of equivalent behaviour of CF-SWST long columns to comparable CFSTs of other tube types and the applicability of existing guidelines for evaluating their strength.