Fire resistance of square double-skin concrete-filled steel tubular columns and concrete-filled double steel tubular columns

Square double-skin concrete-filled steel tubular (DSCFST) columns and concrete-filled double steel (CFDST) columns possess excellent load-carrying capacity and ductility. However, there has been limited research on the responses of DSCFST and square CFDST columns exposed to fire. This paper presents a series of tests on such columns loaded concentrically to determine the effects of the steel tube thickness, the strength of materials, load ratio, and boundary conditions on their fire resistance. Additionally, finite element (FE) models are established using ABAQUS, in which the concrete's transient creep strain is considered through the development of the usersubroutine UEXPAN. A parametric study is undertaken by simulating 156 FE models to further study the behavior of DSCFST and CFDST columns under fire exposure. The fire test and numerical results suggest that both the load ratio and strengths of the internal tube and concrete core play significant roles in determining the fire resistance of DSCFST and CFDST columns. The developed FE models are demonstrated to capture well the fire behavior of loaded DSCFST and CFDST columns. The proposed design model predicts the ultimate loads of DSCFST columns with good accuracy; therefore, it can be employed in the practical design of DSCFST columns.