Study on restraint coefficient of the stirrups-stiffened square concrete filled double-skin steel tube axial compression stub columns

This paper proposes a novel structural design for stirrups-stiffened square concrete-filled double -skin steel tubular stub columns (SCFDST) that offers high capacity and superior advantages in terms of low consumption of concrete and steel, as well as economic efficiency. The study includes an axial pressure test on two sets of 8 SCFDST columns with varying hollow ratios and stirrup ratios, with a specific focus on the influence of stirrups on mechanical properties. Experimental results demonstrate that stirrups significantly enhance the mechanical properties of SCFDST columns with large hollow ratios, including stiffness, bearing capacity, and ductility, with a more pronounced effect observed as the stirrup ratio increases. A three-dimensional solid finite element (FE) model of the SCFDST columns is developed and validated using ABAQUS software and appropriate constitutive models. Parameter analysis is then conducted based on the FE model, revealing that the stirrups not only restrain the concrete itself but also provide additional restraint by limiting the deformation of the steel tube. This improvement effect was more significant in the middle of steel tubes section. When stirrup ratio was 0.015, the steel tube had the highest constraint efficiency on the concrete. The study introduces a constraint enhancement factor to represent the enhanced restraining effect of stirrups on the steel tube, which is incorporated into a fresh equation for determining the maximum bearing capacity (Nu) of SCFDST columns. The derived formula offers clear physical meaning and high accuracy, building upon the existing formula for CFDST columns.