## 7 Conclusions  
In this paper, 1:2-scale fatigue tests were carried out on a built-in double-fixed steel anchor cable-girder box on a high-speed-railway cable-stayed bridge, and a finite element model based on the test verification was established to analyze the force characteristics, the force-transmission mechanism of the built-in double-fixed steel anchor cable-girder box, as well as the influence of different design parameters on its mechanical properties. Furthermore, a dataset was established, and a mechanical-property-prediction-and-evaluation system for the built-in double-fixed steel anchor cable-girder box was built based on a PSO-BP neural network. The conclusions are summarized as follows:  
1. By analyzing the mechanical properties of the main force-transmission welds in the original bridge model and the test model, it was shown that the test model effectively simulated the force state of the real bridge.
2. Under fatigue loading, the stress distribution in the built-in double-fixed steel anchor cable-girder box was homogeneous, and that the cable force was mainly transmitted through the weld between the support plate and the web of the main beam, where the stress-concentration problem was more prominent. The weld can be considered as the main fatigue-sensitive feature of the built-in double-fixed steel anchor cable-girder box.
3. The fatigue-test results showed that most of the stresses calculated through the finite element analysis were in good agreement with the experimental measurements. The stress measured in the whole structure remained stable after 2 million fatigue-load cycles. After 3 million fatigue-load cycles, no surface cracks were observed, indicating that the fatigue performance of the built-in double-fixed steel anchor cable-girder box was good and met the designed load-bearing-capacity requirements.
4. The influence of the key design parameters on the force-transmission mechanism showed that variations in the thickness of the support plate had a greater effect on the shear-stress distribution in the steel anchor box than the variations in the thickness of the pressure plate, and that the designed length of the weld seam had a more significant effect on the force characteristics of the anchorage structure of the built-in double-fixed cable beam than the plate thickness.
5. The PSO-BP neural-network-based predictive evaluation system for built-in double-fixed cable-beam steel anchor boxes can be used to explore the intrinsic relationship between the designed cable force, the plate-geometry parameters, and the mechanical properties of the main force-transfer welds during the iterative training process, and that it can generate reasonable predictions of the mechanical properties of the main force-transfer welds, providing a new and effective evaluation method for the design phase.  
Conceptualization, [PERSON] ([PERSON]) and [PERSON] ([PERSON]); methodology, [PERSON] ([PERSON]); software, C.Z. and L.H.; validation, [PERSON] ([PERSON]); formal analysis, [PERSON] ([PERSON]) and C.Z.; data curation, [PERSON] ([PERSON]); writing--original draft preparation, [PERSON] ([PERSON]) and [PERSON] ([PERSON]); writing--review and editing, [PERSON] ([PERSON]); visualization, [PERSON]; supervision, [PERSON] ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Data Availability Statement: Data sharing not applicable.  
The authors would like to thank the Editor-in-Chief, Editor, and anonymous reviewers for their valuable reviews.  
The authors declare no conflict of interest.