Experimental study of rectangular CFDST beams with eccentric inner tubes under three-point bending

The structural performance of concrete-filled double skin tubular (CFDST) beams is examined in this study. Stainless steel rectangular hollow sections (RHS) were used as the outer tubes, while steel circular, square and rectangular hollow sections (CHS, SHS and RHS) were used as the inner tubes. The inner tubes were eccentrically positioned in the tensile zone of the cross-sections to enhance the bending moment capacity. A total of 30 CFDST specimens was tested under three-point bending. Three levels of inner tube eccentricity were investigated. The nominal compressive strength of the infilled-concrete ranged from 40 to 120 MPa. The full moment-deflection responses, bending moment capacities and failure modes of the investigated CFDST beams are reported and discussed herein. It was observed that increasing the inner tube eccentricity enhanced the bending moment capacity by up to 18% and increasing the concrete compressive strength from C40 to C120 led to a maximum improvement of 50% in bending moment capacity. Additionally, the experimental bending moment capacities were compared against predictions determined from current European, American and Chinese standards for composite carbon steel members. It was shown that the existing design provisions yielded conservative and scattered predictions for the investigated CFDST beams. Developments required in the current design provisions in relation to their application to the studied cross-sections include improved allowance for the strain hardening of stainless steel, concrete confinement and shear-bending interaction. These factors are to be addressed in future research. • 30 CFDST beams with eccentric inner tubes were tested under three-point bending. • Structural behaviour of beams was studied. • Increasing the inner tube eccentricity enhanced the bending moment capacity by up to 18%. • The applicability of the existing design provisions was assessed.