**2. Description of the Bridge and Its Monitoring System**

The Egongyan rail bridge is a long-span suspension bridge located in southwestern China and across the Yangtze River. Figure 1 is a picture of this bridge, and Figure 2 shows its elevated view. It is a five-span self-anchored suspension bridge with double towers and double plane cables. The middle three spans are the main bridge, and the extended span with a span of 50 m is the approach bridge. The overall length of the bridge is 1650.5 m, and the main span is 600 m. In addition, the main girder is a closed single-box five-chamber steel box girder structure [39–41].

**Figure 1.** Egongyan rail bridge.

**Figure 2.** Elevation layout of the main bridge (Unit: m).

To monitor the operation status, a structural health monitoring system (SHMS) was installed on the Egongyan rail bridge [42] and has been in operation since 2020. A total of 267 sensors were installed at different parts of the bridge to collect various types of structural and environmental information. Among them, the temperature measuring points on the steel box girder of the main bridge were arranged symmetrically about the midspan, as shown in Figure 3. A total of 88 temperature sensors were installed at 11 sections (S1–S11) along the main bridge, and there were also eight measuring points (T1~T8) inside the surface of each section. Taking the mid-span section (S6); for instance, a plan view of the measuring point at each section is shown in Figure 3.

**Figure 3.** The layout of the temperature monitoring points. (S: Section number; T: Number of temperature sensors arranged on each section).

The surface temperature of the box girder was monitored using a JMZX-212HAT surface intelligent digital tandem strain gauge with built-in temperature sensors as the testing instrument [43], as shown in Figure 4. The main technical parameters of the sensor were as follows: Operating ambient temperature: −40 ◦C~+80 ◦C; Temperature measurement range: −20 ◦C~+80 ◦C; Temperature resolution: 0.1 ◦C; Temperature measurement accuracy: ±0.5 ◦C.

**Figure 4.** JMZX-212HAT surface intelligent digital string strain gauge.
