Experimental investigation on fatigue vulnerability of integral joints in double-deck steel truss girders under out-of-plane loads

To investigate the fatigue behavior of integral joints in double-deck steel truss girders under out-of-plane loads, this study took the Shiziyang Bridge as a case study and conducted full-scale model tests on the upper and lower integral joints. The nominal stresses, hot spot stresses, and stress concentration factors (SCF) at member connections under out-of-plane loading were measured and analyzed. A numerical analysis of the SCF at the connection between the crossbeam top plate and the gusset plate was conducted using response surface methodology (RSM). A set of parametric formulas was developed to calculate the SCF for this structural detail. Based on the analysis results, the fatigue performance of the Shiziyang Bridge main truss joints was evaluated. The results indicated that under out-of-plane loading, the stress concentration effects at the connections between the crossbeam flanges (top and bottom) and the gusset plate were significantly greater than those at the connections between the chord and the gusset plate. Moreover, the lower joints exhibited a substantially higher stress concentration effect than the upper joints. Under the experimental loading conditions, the maximum SCF at the connection between the crossbeam and the gusset plate reached 2.37 and 4.67 for the upper and lower joints, respectively. The crossbeam top plate thickness, crossbeam web thickness, gusset plate thickness, and weld hole diameter all significantly influenced the SCF at the connection, and the proposed parametric formulas provided a good fit. Under fatigue load model II, considering the out-of-plane loading on the joints, the most critical detail in the main truss joints of the Shiziyang Bridge was the connection between the crossbeam top plate and the gusset plate, where the maximum hot spot stress range reached 41.31 MPa. When the hot spot stress S–N curve corresponding to fatigue detail category 90 MPa was applied to the fully welded integral joints, the fatigue performance of the main truss joints met the relevant code requirements.