Cyclic behavior of square ultra-high-strength concrete-filled steel tubular columns with embedded steel bar-dissipators

The hysteretic behavior of concrete-filled steel tubular (CFST) columns deteriorates as concrete damage accumulates, especially when ultra-high-strength concrete (UHSC) is used. This study introduces a novel steel energy dissipator made of an assembly of vertical steel square bars embedded in the base of square CFST columns as a replacement of the concrete infill in the region of the bottom plastic hinge only. UHSC is used as a concrete infill material for the fabrication of the rest of the column. By utilizing the plastic deformation capacity of the steel bar-dissipators, a significant amount of energy can be dissipated more stably at large inelastic deformations, thereby improving the overall cyclic behavior of the column. To experimentally investigate the hysteretic behavior of square CFST columns with embedded steel bar-dissipators (ED-CFST columns), three ED-CFST columns and one reference CFST column were tested under combined axial and quasi-static cyclic lateral loads. The experimental results show that ED-CFST columns filled with UHSC exhibited enhanced deformation and energy dissipation capacities. Specifically, the ductility ratio increased by 34.1 %–112.3 %, the cumulative energy dissipation increased by 183.4 %–365.1 %, and the ultimate bending strength increased by 7.6 %–10.1 %. Furthermore, columns with smaller steel bar-dissipator section widths exhibited better hysteretic behavior. To elucidate the plastic deformation mechanism, the stress development of the steel bars was investigated through cyclic tension-compression material tests. Finally, a design formula is proposed for determining the configuration of steel bar-dissipators according to seismic deformation demands, accompanied by practical design recommendations for the detailing of the ED-CFST columns.