Formulae for hot-spot stress concentration factors of concrete-filled CHS T-joints based on experiments and FE analysis

Fatigue of welded concrete-filled tubular joints is a new subject to be investigated, compared with fatigue of welded tubular joints without concrete which has been studied well. Fatigue cracking is usually located at the hot spot stress of a welded concrete-filled tubular joint. A method of calculating its hot spot stress is essential for fatigue strength assessment. Hot spot stress concentration factors (SCF) of welded concrete-filled circular hollow section (called as CFCHS) T-joints respectively under axial force and in-plane bending are investigated experimentally and numerically in this paper. SCF testing was carried out on ten CFCHS T-joints with different non-dimensional geometric parameters β (ratio of brace to chord diameter), 2γ (ratio of chord diameter to thickness) and τ (ratio of brace to chord thickness). The experimental findings shows in most cases, a remarkable decrease in SCFs of CFCHS T-joints, compared with circular hollow section T-joints without concrete. Finite element (FE) modelling with 3D quadratic hexahedral solid element was developed and verified by the experimental data. It is found that the conversion relationship SCF= 1.2 SNCF (strain concentration factor) can be adopted for CFCHS T-joints. Using the reliable FE modelling, parametric study was conducted to know the effect of non-dimensional parameters on SCFs of CFCHS T-joints. Based on multiple regression analysis, a series of formulae are recommended for calculating SCFs in the chord and brace of CFCHS T-joints subjected to the axial force and in-plane bending in the brace and in the chord respectively, which are validated by both the experimental data and FE modelling.