Enhancing Effect of Rectangular Spiral Stirrups on the Fire Resistance of Concrete-Filled Rectangular Steel Tubular Columns

Internal stirrup confinement can significantly enhance the fire resistance of concrete-filled steel tubular (CFST) columns. Even without any fire protection, this approach still enables the columns to meet the design requirements for the corresponding fire resistance rating, providing a novel method for structural fire protection design and enhancing the safe service life of structures. This paper designed and fabricated three full-scale rectangular CFST columns and conducted fire test on them under the combined effects of standard fire exposure and axial loading. The study investigated the fire performance enhancement of such members via internal rectangular spiral stirrups, and examined the influence of stirrup spacing on their fire resistance. Based on the existing mechanical constitutive models of materials at elevated temperature, this study developed a numerical model for rectangular CFST columns and revealed the mechanism by which the spiral stirrups constrain the core concrete to enhance the fire resistance. The research results indicate that, with equivalent or slightly lower total steel usage, the internal rectangular spiral stirrups can significantly improve the fire resistance of such columns, with an increase in fire resistance of up to 104.1%. By reasonably designing the stirrups parameters, this type of axially compressed column can meet the Class Ⅱ fire resistance rating requirements for 150 minutes without the need of a fire protection layer on the steel tube surface. When the stirrup spacing increases from 40 mm to 80 mm, its effect on the fire resistance of such members is relatively small. The established numerical model can effectively predict the fire resistance of rectangular CFST columns with internal rectangular spiral stirrups, but its accuracy in predicting deformation at different heating time is less satisfactory.