Stress Concentration Factor of Concrete-Filled Steel Tubular T-Joints with Initial Stress in Chord Tubes

In concrete-filled steel tubular (CFST) truss structures, the welded joints that connect the CFST chords and hollow steel tube braces are particularly susceptible to fatigue cracking due to high stress concentrations. Additionally, as the dimensions of these structures increase and the demand for accelerated construction grows, significant initial stresses in chord tubes of the CFST welded joints have become a common occurrence. Consequently, both the stress concentration properties and the fatigue behavior of these welded joints may be affected. This study aims to investigate the influence of the initial stress on the stress concentration factors (SCFs) of CFST welded T-joints under axial loads in the brace. To this end, five CFST T-joint specimens with varying initial stress ratios were prepared and tested. Subsequently, refined finite-element models of CFST T-joints were established, and then expanded to encompass the geometric parameters of practical structures, enabling a comprehensive analysis of the SCF values and distribution in the CFST T-joints under varying initial stress ratios. The results indicate that the initial stress has the most pronounced influence on SCFs at the chord crown among all four feature positions of the CFST T-joints, including the chord crown, chord saddle, brace crown, and brace saddle. Specifically, the SCF at the chord crown under a brace tension scenario decreases nonlinearly as the initial stress increases, with a maximum reduction ratio of 74.5% observed in model tests. Therefore, it is anticipated that the presence of the initial stress would significantly prolong the fatigue life of the CFST T-joints. Furthermore, based on parametric analysis results, simplified prediction formulas for SCFs at all four feature positions of the CFST T-joints accounting for the effect of initial stress were proposed.