## 3 Bridge Description and Experimental Setting  
### Bridge Description  
The Nanjing-Dashengguan high-speed railway bridge (NDHRB) is located in the Nanjing section of the middle and lower reaches of the Yangtze River in China; it is the largest bridge with the heaviest design loading ever built in the world [2]. The main bridge consists of two continuous steel-truss arches and approach spans, with a span configuration of \(108+192+2\times 336+192+108\) m, see Figure 2. The arches feature three main trusses spaced 15 m apart in the transverse direction. The bridge girder uses an orthotropic steel plate; the railway track (ballast track) is supported by a continuous concrete trough, which is shear-resistant with the steel plate. The deck cross-section of NDHRB is shown in Figure 3. The bridge structure was built using three types of steel: Q345 qD, Q370 qE and Q420 qE.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**IBIS-S Parameters** \\ \hline Central Frequency/wavelength & 17.1 GHz/1.75 cm \\ Maximum distance & 1000 m \\ Maximum range resolution & 0.5 m \\ Maximum sampling rate & 200 Hz \\ Nominal displacement accuracy & 0.02 mm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main characteristics of the IBIS-S system.  
Figure 1: Scheme of interferometric radar for monitoring a bridge.  
The bridge includes six tracks: two tracks of the Beijing-Shanghai high-speed line, two tracks of the Shanghai-Wuhan-Chengdu railway lines and two tracks on the outer sides of the bridge deck for the Nanjing Metro, see Figure 3. The designed train speed is 300 km/h, and the designed load is more than 600 kN/m in the longitudinal direction. All the lines are nowadays operative.  
Fixed and movable bearings were mounted on piers to support the bridge and adapt to the displacement difference between each sections. Telescopic devices were installed at each expansion joint. Due to the large longitudinal displacements at both sides of the main bridge, eight SA60-1200B telescopic devices, with displacement adaption of +/-400 mm, were installed at four high-speed railway tracks in the two sides, see Figure 2.  
The bottom of the bridge exhibits a set of transversal steel beams every 3.0 m, which fit together with the three main steel trusses in the longitudinal direction and support all the corrugated steel plates. Thus, from the RAR viewpoint, the bridge appears as a composition of several high reflecting corners and edges along its entire bottom surface, which is good for radar reflection.  
Figure 3: Cross-section of the NDHRB deck showing different uses of the tracks. 1 Nanjing Metro Line S3 Upstream (S3-U); 2 Beijing–Shanghai (BJ-SH) high-speed railway; 3 Shanghai–Beijing (SH–BJ) high-speed railway; 4 Shanghai–Wuhan–Chengdu (SH–WH–CD) high-speed railway; 5 Chengdu–Wuhan–Shanghai (CD–WH–SH) high-speed railway; 6 Nanjing Metro Line S3 downstream (S3-D).  
Figure 2: Layout (above) and view (below, left) of Nanjing–Dashengguan high-speed railway bridge (NDHRB). Below, right: telescopic device mounted at locations ‘C’ and ‘D’ shown in figure above.  
Due to the remarkable characteristics of NDHRB, including long span of the main girder, heavy design loading and high speed of the trains, a long-term structure health monitoring (SHM) system was installed on the bridge [2].