# Development and Field Validation of Wireless Sensors for Railway Bridge Modal Identification
## 3 Field Test on the Bressana Railway Bridge  
An experimental campaign was arranged to test the developed system in the field. The purpose of this test was to verify the efficiency of the energy harvesting system equipping  
Figure 3: Schematization of the acquisition procedure. the sensors, to check the wireless communication in the real monitoring context, and to understand the significance of the vibration data collected. Moreover, a conventional monitoring system based on wired velocimeters already mounted on the bridge was used to perform a benchmark between the results obtained from the two different solutions. The Bressana railway bridge on the Po River is located near the city of Pavia, in Lombardy. The bridge crosses the river on three spans approximately 77 m long each (Figure 4a). The bridge is constituted of a truss steel structure with a double deck; the lower deck is devoted to the railway traffic while the upper one is reserved for the vehicular traffic (Figure 4b). First, the permanent monitoring system, already installed on the bridge for monitoring activities, is briefly described. The schematic setup of the system is shown in Figure 5. This system consists of 15 wired velocimeters, 5 for each span. The upstream velocimeters (placed at the bottom of the top view in Figure 5) are biaxial sensors that measure the lateral and the vertical velocity of the bridge in the application points. The downstream velocimeters are monoaxial sensors that measure the vertical velocity only. In this way, the natural frequencies related to the vertical and lateral modes can be estimated, as will be shown in Section 4. Figure 4: The Bressana bridge on the Po river: (**a**) Lateral view of the bridge crossing the river with heavy vehicles running on the upper deck; (**b**) internal view of the lower deck reserved to railway traffic. Figure 5: Schematic set-up of the wired velocimeters forming the permanent monitoring system. Concerning the developed wireless monitoring system, it was easily installed on the bridge. Two sensor nodes were mounted on the pillars in the middle of the central span by using a suitable structural adhesive (Figure 6a). The gateway was instead placed in a balcony of the railway deck (Figure 6b). The main features of the two different monitoring devices installed on the bridge are reported in Table 3. As can be noted, the adoption of wireless sensors for this kind of a monitoring campaign presents several advantages in case the performance of the two systems is comparable in achieving the monitoring goal. Among them, wireless sensors are easier to be installed (compact and lighter devices), with no need for cables for power supply and data transmission and a lot cheaper than wired sensors. During the months of the field test on the Bressana bridge, the sensor nodes did not exhibit any communication issue. The energy harvesting system was able to recharge the Li-Po battery when the sun was present, allowing a very long autonomy. An example of the functioning of the PV panel in recharging the battery during a typical winter day is represented in Figure 7. The graph represents data related to the transmission phase of the acquired time histories, which is the most expensive one in terms of energy consumption. In the time period between approximately 6 am and 3 pm, the PV panel mounted on the sensor node was hit by the sunlight, producing an inflow energy characterized by a positive input voltage on the board. If the focus is switched to the current incoming to the battery (where the minus sign means a recharging current), it is possible to notice that the energy