Critical Failure Temperature Prediction of Axially Loaded Circular Hollow Section K-joints

The fire resistance of circular hollow section (CHS) K-joints sustaining axial loads was investigated comprehensively through finite-element analysis. To reproduce the mechanical behavior of axially loaded CHS K-joints under fire realistically, the critical temperature method was employed in the numerical analysis. The material properties of a Q355 steel tube at elevated temperatures were measured first and then defined in finite-element models (FEMs) of the CHS K-joints in the form of temperature-dependent true stress-strain curves. Based on a comparison of the simulated mechanical behavior and the tested behavior of the axially loaded CHS K-joints under fire, the accuracies of the established FEMs were confirmed. Employing the verified simulation method, a parametric study was conducted with the load ratio (n) and geometric parameters (β, γ, and θ) as the principal factors. The load ratio was found to play an important role in the fire resistance performance of axially loaded CHS K-joints. The critical failure temperature Tc of the joints decreased linearly with an increase in load ratio. The influence of the geometric parameter β on the critical failure temperature is more remarkable than those of γ and θ, and a higher β generally leads to better fire resistance. Based on the numerical analysis results, an equation for predicting the critical failure temperature was formulated for the fire resistance assessment of axially loaded CHS K-joints. Furthermore, the reliabilities of the temperature development determination method and the resistance method for CHS K-joints at elevated temperatures specified in some major international design codes were evaluated.