## 7 Conclusions  
Based on the above investigations, the following conclusions can be drawn:  
* The impact factor reaches the maximum values when the vehicle speed is 40 km/h in both the single line and double line loading cases. The largest impact factor is 4.2% in the double line case and 2.7% in the single line case, both achieved by the arch rib. Whether single line or double line loading is applied does not significantly affect the impact factor of the bridge.
* Both the vertical and transverse amplitudes do not change significantly with the train's speed when the speed is no more than 30 km/h. When the speed is increased to 40 km/h, the vertical amplitude is increased and a peak appeared, likely due to the resonance between the bridge and the train. The transverse amplitude is insensitive to the change of the train speed. When the loading case is changed from single line to double line, the vertical amplitude increases while the transverse amplitude of the bridge reduces. The transverse amplitude of the bridge approaches zero as two trains run on the double line, revealing that the transverse vibration of the bridge is mainly caused by eccentric train loadings.
* The dynamic accelerations of the bridge approximately linearly increase with the train's speed. In the single line loading case, the discrepancies of the amplitudes of the vertical and transverse accelerations of different parts of the bridge are no more than 18%. In the double line loading case, the amplitudes of the vertical and transverse accelerations of the quarter of main span and the middle of the Y-shape frame are still very close to each other, but the amplitudes of the vertical and transverse accelerations of the middle of the main span of the bridge are significantly greater than those of the quarter of the main span and the middle of the Y-shape frame. Their difference increases with the train's speed and is up to 44% as the train's speed is increased from 5 to 75 km/h. The acceleration of the track beam is much greater than that of the arch rib. The reason can be that the track beam is subjected to direct loading and the vehicle is coupled with the track beam, while the arch rib is subjected to indirect loading.
* The braking load mainly leads to larger longitudinal vibration of the bridge but its influence on the transverse and vertical vibration is less. When the braking load acts on the bridge, the longitudinal displacement and the vibration phase at each calculated position of the steel truss girder is consistent, that is, the steel truss girder makes longitudinal integral translational vibration. When the temporary impact occurs, the longitudinal accelerations of each calculated position varies greatly. As the distance from the braking position increases, the longitudinal acceleration of the steel truss girder becomes smaller.
* The acceleration of the vehicles increases with the vehicle speed, not the number of vehicles. The transverse acceleration of the vehicle is significantly greater than the vertical one, and the transverse comfort is worse than the vertical comfort. The maximum transverse acceleration of the vehicle is 1.01 cm/s\({}^{2}\). The grade of the vehicle running stability is excellent.  
The research described in this paper was financially supported by the National Natural Science Foundation of China (Grant No. 51508474 and 51108382), the Fundamental Research Funds for the Central Universities (Grant No. 2682015 CX07), and the Science and Technology Research and Development Plan of China Railway Construction (Grant No. 2014-C34). The visiting scholarship for the first author to visit the Department of Civil, Architectural and Environmental Engineering at the Missouri University of Science and Technology (Rolla, MO) was made possible by the Chinese Scholarship Council (Grant No. P-1-02556).  
The authors have contributed equally to this work.  
The authors declare no conflict of interest.