# Research on the Fatigue Performance of Composite Girders with CSW and ST Truss
## 6 Fatigue Life Evaluation Method  
Using the parameters of the test model, Figure 15 has compared the theoretical fatigue life obtained from the evaluation method for the fatigue performance in AASHTO, CIDECT, and API with the test results, where \(\Delta\sigma_{\text{h}}\) represents the hot spot stress range. It was found that the deviation between the theoretical fatigue life obtained from the evaluation method for the fatigue performance in AASHTO, CIDECT, and API and the test results reached 21.0%, which illustrated that the evaluation method for the fatigue performance in AASHTO, CIDECT, and API was not suitable for the evaluation of the fatigue performance of the composite girder with CSW and ST truss. The following basic computational model for the fatigue design \(S\)-\(N\) curve based on the hot spot stress range of the composite girders with CSW and ST truss was formulated according to the literature [32]:  
\[(\Delta\sigma_{\text{h}})^{m}N=C_{0} \tag{6}\]  
where \(N\) represents the fatigue life, \(m\) is the slope of the fatigue design \(S\)-\(N\) curve, and \(C_{0}\) represents the constant of the fatigue design \(S\)-\(N\) curve, which is determined as follows:  
\[C_{0}=2\times 10^{6}\times\left(\Delta\sigma_{0}\right)^{m} \tag{7}\]  
The fatigue failure of the composite girders resulted from the fluctuating tensile stress range, and fatigue cracking was found at the intersecting welds (Figure 9). The fatigue details of the composite girders with CSW and ST truss were shown in Figure 16, and it could be seen from Figure 16 that the welded joint of the composite girders with CSW and ST truss was composed of the welds between the CSW and the chord, and the construction details of the welds was the full penetration weld. Thus, it belonged to the T-shaped fully penetrable welded joint. Meanwhile, under the action of the load, the welded joint of the composite girders with CSW and ST truss played a vital role in transferring the load, namely that the welded joint of the composite girders with CSW and ST truss needed to bear the load. Therefore, according to the literature [32], the strength grade of the fatigue details of the composite girders with CSW and ST truss could be classified into D type weld joint. Based on the relevant regulations [32], the slope \(m\) of the fatigue design \(S\)-\(N\) curve of the composite girders with CSW and ST truss was taken as 3. Considering the fact that the fatigue failure belonged to the brittle failure, the \(S\)-\(N\) curve used to evaluate the fatigue performance of the composite girder with CSW and ST truss, which should have a certain  
Figure 15: Comparison of the existing \(S\)-\(N\) curves. security surplus. Therefore, by referring to the literature [33], a cut-off limit for the fatigue design was not adopted, and the slope \(m\) of the fatigue design \(S\)-\(N\) curve was still taken as 3 after 10,000,000 cycles of fatigue loading. Only the structural stress concentration in the intersecting welds was considered, and the influence of the shape of the intersecting welds on the hot spot stress was ignored. Therefore, according to the relevant provisions for the fatigue design \(S\)-\(N\) curve [32], the basic fatigue strength \(\Delta\sigma_{0}\) after 2,000,000 cycles of loading could be determined based on the basic fatigue strength of the T-shaped fully penetrable welded joint. As per the requirements [10], the intersecting welds of the composite girders with CSW and ST truss belonged to the T-shaped fully penetrable welded joint and should meet the stipulation of full penetration groove butt welding. Meanwhile, the weld toe should be ground after welding. Hence, the basic fatigue strength of the intersecting welds was taken as 100 MPa. According to the literature [10], \(T_{\text{b}}\) should be between 9 mm and 40 mm.