## 2 Bridge Description and Longitudinal Displacement Monitoring  
The Nanjing Dashengguan Yangtze River Bridge is the longest six-track high-speed railway bridge in the world. As shown in Figure 1, the bridge consists of two continuous steel-truss arches and four approach spans, without expansion joints over the total length of 1272 m. The heights of the truss ribs in the main spans vary from 12 m at the crown to 96 m at the spring line. The bridge has six tracks, including two tracks on the downstream side for the Beijing-Shanghai (B-S) high-speed railway, two tracks on the upstream side for the Shanghai-Wuhan-Chengdu (S-W-C) railway and two tracks on the outer sides of the bridge deck for the Nanjing Metro. The design load of the six tracks is greater than 600 kN/m along the longitudinal direction of the bridge. As of 2018, the train speed on the two tracks of the B-S high-speed railway have been as great as 350 km/h, with all six tracks now in operation. The bridge structure features three main trusses spaced 15 m apart in the transverse direction. Three specially designed ball-steel bearings are used on each pier. At Pier 4, the center truss rests on a fixed bearing, and the bearings for the two side trusses allow for transverse motion; on the other piers, the bearings for the center truss allow for longitudinal motion, while the bearings for the side trusses allow for both longitudinal and transverse motion. A total of eight specially designed expansion devices are installed at the expansion joint of the girder ends at Piers 1 and 7 (Figure 2), where each railway track (B-S and S-W-C) is installed and with one device at the Beijing/Shanghai girder end. The stock rail and the fixed side of the expansion device are located at the main bridge, while the switch rail and the free side of the expansion device are located at the ramp bridge, with the stock rail spanning the expansion joint. On the free side of the expansion device, the expansion box and sliding support, the sleeper and rail, the sleeper and guide-rail can slide relative to each other in the longitudinal direction of the bridge. The ramp bridge on the Beijing side uses a 30-meter-wide steel-truss bridge (and a 30-meter-wide steel girder), while the ramp bridge on the Shanghai side uses two independent 15-meter-wide concrete girders.  
Because of the unusual characteristics of the Nanjing Dashengguan Yangtze River Bridge, including the long length of the girder without any expansion joints, and the fact that this bridge is used by high-speed trains, a long-term SHM system was installed in 2012. Considering the significant need for safety in high-speed train systems, contact sensors cannot be used on the rails and rail-related devices. To monitor the service state of the bridge, magnetostrictive displacement sensors are employed to measure the longitudinal displacements of the bearings and expansion joints. The locations of longitudinal displacement measurement are shown in Figure 3. The downstream and upstream bearings on Piers 1-3 and 5-7 are measured relative to the longitudinal displacements between the upper and lower plates of the bearing. The expansion joints on Piers 1 and 7 are measured relative to the longitudinal displacements between the main bridge and the ramp bridge. The sampling rate of the magnetostrictive displacement sensors is 1 Hz. The magnetostrictive displacement sensors were installed on the expansion joints in 2017. As shown in Figure 3, all of the sensors are 1 m from the center line of Truss 1, 2, or 3. Moreover, an atmospheric thermo-hygrometer is employed at the  
Figure 1: Bridge and its bearings and expansion devices (units: meters).  
Figure 2: Expansion devices and bearings at Pier 7.  
arch foot between the southern middle span and the southern side span to measure the atmospheric temperature and humidity, with the sampling rate of the atmospheric thermo-hygrometer being 1 Hz.