# Track-Bridge Interaction of CWR on Chinese Large-Span Bridge of High-Speed Railway
## 4 Special Problems of Track-Bridge Interaction  
### Earthquake Load  
The maximum longitudinal force of rail at the joint of the long-span bridge under earthquake action is much larger than that calculated according to the expansion condition of the current specification. It is suggested that the index of seismic force calculation of rail should be added to the CWR laid on the railway bridge passing through the earthquake area [61]. The analysis shows that the track restraint has an obvious influence on the longitudinal earthquake resistance of the railway bridge [62] and the low-order longitudinal natural frequency of the long-span bridge structure [54; 61], and reduces the seismic response of the structure [62]. The existence of the track can improve the fundamental frequency of the system to a certain extent, play the role of energy dissipation, reduce the seismic response of the bridge and pier [63], enhance the integrity of the bridge span structure along the bridge direction, reduce the traveling wave effect response of the beam end relative displacement, and increase the traveling wave effect response of the truss axial force [64]. It is suggested that the influence of track constraints should be considered in the seismic design of railway bridges crossing seismic areas, otherwise the bridge structure design will be too conservative [54; 61]. When studying the longitudinal seismic response of CWR on the long-span continuous beam bridge, the length of the calculation section on both ends of the subgrade should be 100 m [61], and the reasonable length of the subgrade on both ends of the simply supported beam bridge should be 90 m [65]. The spectrum characteristics of the seismic wave used in the analysis should be consistent with the spectrum characteristics of the soil at the bridge site. Within the range of a reasonable damping ratio, the damping ratio has little influence on the seismic response of the structure, which can be taken as 0.05 [54]. The seismic response of the structure is investigated according to the 8-degree design earthquake (Ag = 0.3 g). When determining the time history of ground motion acceleration, the normalized response spectrum is imported into the SIMQKE_GR program developed by UC Berkeley to generate artificial seismic wave W1. Figure 18 is the envelope diagram of the rail stress response of the 156 m New Yellow River Bridge under uniform excitation and the seismic passage effect (apparent wave velocity of 200 m/s). It can be seen that for the long-span simply supported beam bridge, the traveling wave effect increases the risk of falling beams and the possibility of collision, weakens the following ability of the track and the bridge in the middle of the beam span, and greatly improves the stress response amplitude of the rail at the end of the beam span, which has adverse effects on the seismic performance of the bridge and the track [64]. The traveling wave effect has a great influence on the seismic response of the long-span railway continuous beam arch composite bridge. With the increase of the apparent wave velocity, the bending moment of the arch rib increases, and the axial force first decreases and then increases [66]. For the continuous beam bridge, compared with uniform excitation, the longitudinal force of the rail increases, and the force of the brake pier decreases under the longitudinal traveling wave effect [63]. If the seismic passage effect is ignored or the seismic wave propagation velocity is assumed to be inappropriate, this difference may lead to significant underestimation or overestimation of the random vibration response of bridges and trains [67]. The viscous dampers with reasonable parameters can effectively reduce the vibration. The viscous damper has a significant damping effect on the bending moment and shear force at the pier bottom, relative displacement of the pier beam [68], displacement of the beam end [69], and longitudinal force [49] at the pier top, but has a relatively small effect on rail stress and rail deformation [48].