Curved composite twin-girder bridges are suitable for mountainous areas, due to their advantages of light self-weight, excellent mechanical performance, and fewer construction requirements. It has been found that many composite twin-girder bridges collapsed due to fatigue failure. However, the literature review showed no relevant studies on the fatigue performance of curved composite twin-girder bridges. Because of this, the specimen of 1:2 scale curved composite twin-girder bridge in accordance with the design scheme of Xizhen Bridge in China was designed and tested. Three possible fatigue details were selected: cruciform connections, transverse attachments, and transverse splices named Class I, Class II, and Class III. For the test data of nominal stress (NS), equations were proposed to convert the strain value into the internal force of the fatigue detail position. The stress caused by torsion accounts for 2.8% of the total stress, which is almost negligible. The fatigue evaluation process based on the hot spot stress (HSS) S-N curve method is presented. The HSS method is more conservative than the NS S-N curve method in predicting the fatigue life of complex structures with high-stress concentrations.

A CHS-to-CFRHS Y-joint that consists of a circular hollow section (CHS) brace and a concrete-filled rectangular hollow section (CFRHS) chord by welding has a simple and smooth weld profile that saves time and cost for the fabrication of CHS-to-CFRHS Y-joints and leads to a superior fatigue performance, compared with other welded tubular joints. This investigation presented an analysis of the stress concentration factors (SCFs) of CHS-to-CFRHS Y-joints subjected to axial tension loading of the brace. First, a finite element (FE) modelling method, which was validated with the experimental results cited in the reference, was utilised to establish the FE models of CHS-to-CFRHS Y-joints. Then, a parametric analysis was conducted to investigate the influences of the significant non-dimensional geometric parameters on the SCFs of CHS-to-CFRHS Y-joints. It is found that the intersection angle of the brace and chord has an important influence on the magnitudes of the SCF values. An increase in the intersection angle of the brace and chord will increase the values of the SCFs at the 60° location and saddle. The values of the SCFs at the 60° location and saddle reach the maximum value when the intersection angle of the brace and chord reaches 90°. Furthermore, on the basis of the large database of the SCF results, empirical design equations were established to calculate the SCFs at the crown toe, 60° location and saddle via multiple regression analysis. A safety factor was applied to the empirical design equations to ensure safe and reliable results of SCF calculations for the fatigue design of CHS-to-CFRHS Y-joints in a composite truss structure. Ultimately, a comparative analysis of SCFs was conducted with the FE models of welded tubular joints with rectangular hollow section (RHS) chords and CFRHS chords. The results reveal that infilling concrete in the chord leads to a reduction in SCFs along the weld profile of more than 11% on average, and the peak SCF decreases by more than 15%.