Experimental studies on the global stability of concrete sandwiched double steel tubular columns

In improving the ductility and fire-resistance properties of framed structural systems, concrete sandwiched double steel tubular columns (CSDST) are found to be advantageous over concrete filled steel tubular columns (CFST), particularly when the columns are slender. However, a comprehensive understanding of the long column behaviour of CSDST is not fully developed. In this paper, experimental studies on the global stability aspects of CFDST columns is presented. Axial compression tests are carried out on t welve CSDST specimens with two CFST as benchmark specimens having non-dimensional member slenderness value around unity. Shape of the inner and outer steel tubes, hollowness ratio and concrete strength are selected as primary parameters for the experimental study. Comparing the experimental results of CSDST with CFST, it is found that the buckling capacity of CSDST is lesser than CFST. The degree of stability in CSDST is directly proportional to the hollowness ratio, irrespective of the cross-section shape and sandwiched concrete strength. The effective flexural rigidity of the specimens derived from the experimental results is reported here, which reiterates the inverse relation of hollowness ratio to the column instability. An average over-strength of around 12% for square and 20% for circular specimens is observed in the test axial capacities when compared to the code specifications (ANSI/AISC 360(16) and EN 1994-1-1 2004). Low initial global imperfection (≈ L/7000) in the specimens has led to the over-strength in test axial capacities. It is concluded that the code axial capacity equation for long column CFST (ANSI/AISC 360(16) and EN 1994-1-1 2004) can be adopted for CSDST long columns. However, these code equations are comparatively less conservative for CSDST with less than 50% hollowness.