Design equations for maximum stress concentration factors for concrete-filled steel tubular K-joints

Stress concentration factors (SCFs) for welded tubular joints can be decreased by filling the chord with concrete leading to a longer fatigue life. However, there are currently no design formula available in guidelines to predict the SCF of concrete-filled circular hollow section (CFCHS) K-joints, thus limiting their applicability in bridge design. To address this gap, finite element models for CFCHS K-joints were developed and compared against test results to ensure their accuracy. Then, a comprehensive parametric study was conducted to establish relationships between maximum SCFs and four variables: brace-to-chord diameter ratio ( β ), chord diameter-to-thickness ratio (2 γ ), brace-to-chord thickness ratio ( τ ), and the angle between braces and chord ( θ ). A total of 480 FE models were examined under three loading conditions including brace and chord loading: balanced axial force, chord axial force, and chord bending. Design equations to predict the maximum SCF for CFCHS K-joints were established by multiple regression analyses of the numerical results. A comparison of maximum SCFs between circular hollow section (CHS) and CFCHS K-joints was made, and it was concluded that average reductions of 42% and 33% in maximum SCFs in CFCHS K-joints at the locations of the chord and brace were found compared to CHS joints for balanced axial force, respectively. Finally, a case study illustrating how to use the proposed equations for fatigue safety verification was presented. • Studying the concrete-filled steel tubular (CFST) K-joints. • No stress concentration factor (SCF) design equations available for CFST K-joints. • FE modelling, validation and parametric study conducted under three loading conditions. • Design equations proposed to estimate the maximum SCFs. • Filling the chord with concrete reduces SCF by 3%–42%.