Behaviour of CFDST composite beam-column with circular outer and square inner tube

Concrete Filled Double Skinned Steel Tube (CFDST) composite column offers a promising alternative to Concrete Filled Steel Tube (CFST) owing to its effective cross-sectional utilization. While CFDST composite column and beam-column with circular as well as square outer and inner steel tubes were studied extensively, behaviour of circular outer and square inner steel tubes cross-sections of CFDST (CS-CFDST) is not yet fully explored. The paper experimentally investigates behaviour of slender CS-CFDST column and beam-columns in terms of strength, deformation, strain profile along the length, failure modes, moment-curvature relationship, confinement effect and load-moment interaction curves. Strength results, when compared with corresponding CS-CFDST columns with remaining three possible combinations of cross-sections, indicate that CS-CFDST column performs at par and thus, can be used as practical composite column. Axial capacity of CS-CFDST column is found to be 16 % higher as compared to circular CFST column establishes effectiveness of CFDST cross-section. The failure modes of slender CFDST column yields global buckling when the outer steel tube is circular and local buckling for square outer steel tube irrespective of inner steel tube shape. A theoretical strength estimation of CS-CFDST beam-columns derived extending the EC-4 design code, exhibit over conservativeness in predicting the axial capacity signifying the contribution of concrete confinement effect. The strength enhancement factor proposed in the present study offers more accurate assessment of concrete confinement effect of CS-CFDST beam-column with higher load eccentricity ratio, e/Do up to 0.45 and thus, showing improvement over e/Do of 0.1 specified in EC-4 design code.