Material synergy and parameter optimization of axially-loaded circular UHPC-filled steel tubes

Abstract

This paper aims to investigate the material synergy and parameter optimization of circular ultra-high performance concrete (UHPC) filled steel tubes towards excellent axial bearing capacity and material utilization efficiency. 12 stub columns of UHPC-filled steel tubes (UCFST) were tested and analysed combined with 293 literature data sets under axial compression. The results show that increasing confinement index ξ and steel ratio α can enhance the ductility of the axial load-displacement curve and the failure mode of the column. Threshold values of ξ = 1.0 and α = 0.25 are proposed to distinguish the failure modes. Appropriately higher steel strength grade, lower D/t, larger α and ξ are recommended to ensure the good axial bearing capacity of the UCFST columns. To produce excellent material synergy and axial bearing capacity, the optimum steel and UHPC strength grades are recommended around C100 ∼ C160 and Q345 ∼ Q690 for the axially-loaded UCFST short columns, respectively. Furthermore, the optimum ranges of diameter-to-thickness ratio D/t, steel ratio α and confinement index ξ are <90·(235/fy), 0.10–0.30, 0.7–2.0, respectively. The current normal CFST design codes are inappropriate for the UCFST columns due to the parameter values exceeding limit values. Therefore, a reliable analytical model is proposed and well validated to predict the axial bearing capacities of circular UCFST short columns.