# Fatigue Performance Test and Numerical Analysis of Composite Girders with CSW-CFST Truss Chords
## 2 Model Test
### Instrumentation Layout of Measurement Points and Inspection Procedures  
To analyze the hot-spot stress in the intersection area of the corrugated steel web and the chord, the extrapolated structural stress parallel to the weld was taken in the study as the hot-spot stress according to the relevant regulations in [21]. The extrapolation interval near the weld where the corrugated steel web intersected with the chord is shown in Table 4, in which \(L_{\text{r,min}}\) and \(L_{\text{r,max}}\) represent the minimum or maximum perpendicular distance of the unidirectional strain gauge on the gradient strain gauge away from the weld toe, respectively, and all the unidirectional strain gauges on the gradient strain gauge parallel to the weld. Figure 7 shows a gradient strain gauge specially made for this study.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Monitored Parts** & **Chord Side** & **Corrugated Steel Web Side** \\ \hline \(L_{\text{r,min}}\) (mm) & 4.0 & 4.0 \\ \(L_{\text{r,max}}\) (mm) & 10.0 & 14.0 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Interpolation intervals.  
Figure 6: Loading rig: (**a**) schematic diagram; and (**b**) photo of loading rig.  
According to the results of the fatigue performance test of the composite girders with CSW-ST truss chords reported in [11], the intersection of the inclined web of the corrugated steel web and circular arc transition section is not the only sensitive location for hot-spot stress, but also the key position for later fatigue crack initiation. Thus, the position was taken as the stress characteristic point of composite girders with CSW-CFST truss chords, where gradient strain gauges were arranged. In addition, two web wavelengths near the mid-span were taken as the monitoring range of hot-spot stress. Each monitoring point number was symbolically denoted as MNY-Z, where M corresponds to the chord (X) or the corrugated steel web (B), N corresponds to the left- (L) or right-(R) hand-side of the weld where the corrugated steel web and the chord intersect, Y is the gradient strain gauge number (1-8) counting in the longitudinal bridge direction, and Z corresponds to the number (1-3) of the unidirectional strain gauge arranged on the substrate of the gradient strain gauge. To monitor the changes in stiffness of the test model during the cyclic loading, displacement monitoring points were arranged at the bottom of the chord at \(L/4\), \(L/2\), and 3 \(L/4\) sections, and LVDTs were used to measure displacements of the composite girders with CSW-CFST truss chords. The layout of the strain monitoring points is shown in Figure 3.  
An elastic controllable synthetic wave was generated by a phased array ultrasonic testing system, which not only guaranteed high sensitivity and resolution for the detection of initial defects, but also facilitated the three-dimensional quantitative analysis of the initial defects and fatigue cracks [22]. According to the existing research results, the initial defects with the size of 0.05 mm \(\times\) 0.05 mm, which cannot be detected with the naked eye, can be detected by a phased array ultrasonic testing system. Thus, a phased array ultrasonic testing system was used to conduct non-destructive tests of the intersection welds between the corrugated steel webs and the chords of the composite girders with CSW-CFST truss chords following the requirements listed in [23]. According to the results of the initial defect detection, there was no initial defect at the intersection between the corrugated steel web and the chord, thus eliminating potential influence on the results of the fatigue performance test.