# Research on the Fatigue Performance of Composite Girders with CSW and ST Truss
## 7 Conclusions  
The fatigue performance of the composite girder with CSW and ST truss was revealed by carrying out a test and FE analysis. The fatigue failure mode and the sensitivities of the hot spot stress to the geometric parameters of the composite girder above were studied. Furthermore, a fatigue life evaluation method was discussed. It could be concluded that: (a) linear extrapolation is suitable to determine the hot spot stress of the composite girders with CSW and ST truss. The fatigue performance evolution comprised three stages, i.e., initiation, expansion, and failure of the fatigue crack, with corresponding numbers of fatigue loading cycles of 0-1,266,000, 1,266,000-1,298,000, and 1,298,000-1,324,000, respectively. The fatigue crack initiation stage in the test accounted for about 95.6% of the fatigue life. The fatigue cracks were opening type I; (b) both the maximum hot spot stress and fatigue cracks of the composite girders with CSW and ST truss occurred in point S, which happened mainly because the inclined web of the CSW had a small out-of-plane bending stiffness but had to withstand an out-of-plane bending moment; (c) the hot spot stress in the composite girders with CSW and ST truss exhibited negative correlations with \(D_{\text{c}}\), \(T_{\text{c}}\), and \(R_{\text{b}}\), a positive correlation with \(T_{\text{b}}\), and a more complex relationship with \(\theta_{\text{b}}\). It was most sensitive to \(D_{\text{c}}\), followed by \(T_{\text{c}}\); (d) based on the hot spot stress method and the fact that intersecting welds were a type of T-shaped fully penetrable welded joint, the basic fatigue strength of 100 MPa after 2,000,000 cycles of cyclic loading was recommended to establish the _S-N_ curve of the joint. An empirical calculation model of the _S-N_ curve for the composite girder with CSW and ST truss was created, and to improve the representation and applicability, more fatigue performance tests on the composite girder with CSW and ST truss should be conducted to carry out further verifications and validations of the empirical calculation model of the _S-N_ curve put forward by this paper in the future; (e) due to the reason that the software of MIDAS/CIVIL was exactly suitable to conduct linear analysis, the results of the integral FE analysis on the composite girders with CSW and ST truss by the software of MIDAS/CIVIL were not very consistent with the test results. Thus, the simulation method based on the linear analysis of the composite girders with CSW and ST truss should be studied in further.  
[PERSON] and [PERSON] conducted the experiments and FE analysis; [PERSON] and [PERSON] processed the data; [PERSON], [PERSON] and [PERSON] contributed related tools; and [PERSON] drafted the paper. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key Research and Development Program of China (No. 2017 YFE0130300), National Natural Science Foundation of China (No. 52078137), and Natural Science Foundation of Fujian Province (No. 2019J06009).  
The tests were conducted at the Fujian Provincial Key Laboratory on Multi-Disaster Prevention and Mitigation in Civil Engineering at Fuzhou University. The support provided by the laboratory staff is gratefully acknowledged.  
This research was supported by the National Key Research and Development Program of China (No. 2017 YFE0130300), National Natural Science Foundation of China (No. 52078137), and Natural Science Foundation of Fujian Province (No. 2019J06009). The tests were conducted at Fujian Provincial Key Laboratory on Multi-Disaster Prevention and Mitigation in Civil Engineering at Fuzhou University.  
There is no conflict of interest for the authors.