Innovative reinforced concrete-filled steel tube system: experimental and analytical flexural investigations

Abstract

This study proposes an innovative reinforced concrete-filled steel tube (RCFST) system, wherein the steel tube is only extended over a limited span length. The flexural behavior of the proposed RCFST beams was experimentally investigated to consider two key design parameters — width-to-depth ratios (slender vs. shallow) and two steel tube thicknesses (1.4 and 1.8 mm) — with two control beams as the basis for comparison, resulting in a total of six beams. The steel tubes were 900 mm long, corresponding to 50% of the effective span length, and contained closely spaced rigid stiffeners to prevent buckling of the top flange; shear studs were also incorporated to ensure engagement and composite action between the concrete and the bottom steel flange. The RCFST beams were tested under a three-point loading configuration in displacement-controlled testing mode, with strain gauges and LVDTs instrumented to record the strains and displacements throughout the testing. The results showed that the improvement in the flexural capacity was more pronounced in the slender beams (43.6 to 51.7%) than in the shallow beams (24.0 to 33.9%). However, the flexural stiffness and mid-span deflection were substantially enhanced by 73.8 and 111.5%, respectively, in the shallow beams (1.4 mm thickness) compared to the control specimen. The ductility index and toughness for both shallow and slender beams exhibited considerable improvement, especially for the thicker steel. An analytical analysis of the RCFST beams using theoretical moment-curvature response and fiber sectional analysis gave a predicted load-carrying capacity close to that of the experimental results.