关键词: 矩形螺旋箍筋, 矩形CFST组合柱, 耐火性能, 箍筋间距, 约束

Abstract:

The internal stirrups significantly improve the fire resistance of concrete-filled steel tubular (CFST) composite columns, achieving compliance with stringent fire resistance rating requirements as per design specifications while eliminating the need for supplementary fire protection measures. This provides a new method for structural fire resistance design and enhances the structural safety service life. In this study, three full-scale rectangular CFST composite columns were designed and tested under standard fire coupling with axial loading. The influence of internal rectangular spiral stirrups and their spacing on the fire resistance was investigated. Based on the existing mechanical constitutive models of materials at elevated temperature, a numerical model was developed for such columns, and the mechanism by which the spiral stirrups constrain the core concrete to enhance the fire resistance was analyzed. 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%. Proper design of internal stirrups allows these axially compressed composite columns to satisfy the 150-minute Class II fire resistance requirement, and to eliminate the fireproof coatings on the steel tube surface. Increasing the stirrup spacing from 40 mm to 80 mm has minimal effect on the fire resistance. The developed numerical model can effectively predict the fire resistance of these columns, but shows limitations in accurately simulating their time-dependent deformation in fire.

Key words: rectangular spiral stirrup, rectangular CFST composite columns, fire resistance, stirrups spacing, confinement