# Development and Field Validation of Wireless Sensors for Railway Bridge Modal Identification
## 1 Introduction  
The economic growth and development of a country depends significantly on the lifetime performance of its civil infrastructures. Among them, bridges represent one of the most vulnerable and crucial infrastructural connection nodes [1]. The degradation in time of this kind of infrastructures is due to several factors, among which aggressive environment, increasing loads due to higher traffic volumes, and lack of maintenance are reported [2]. For these reasons, the monitoring of bridges and viaducts is gaining more and more importance nowadays [3]. Structural health monitoring (SHM) focuses on controlling in time the loading conditions the structure is subject to, to assess its performance and to detect in advance damage and deterioration with the aim of performing predictive maintenance activities [4]. Recently, innovative approaches based on artificial intelligence and neural networks have been proposed for damage identification in complex structures [5, 6]. A more traditional SHM branch deals with vibration-based methods, which allow the identification of structural damages by detecting a change in the bridge dynamic behavior [7]. The key physical parameter to be measured in this case is acceleration. In recent years, the research in the field of monitoring activities on bridges has become more intense, due to the need for obtaining indications on the structural health to drive the maintenance operations and avoid failures with disastrous consequences [8]. The standard approach to carry out those activities is to instrument the bridge with monitoring systems composed of wired vibrations sensors (such as accelerometers orvelocimeters) positioned in suitable points of the structure with the aim of measuring the structural natural frequencies, reconstructing mode shapes, and detecting the damping value [9]. The variation in time of these two parameters represents a basic method to perform SHM of a civil infrastructure. Since it is important to observe the trend of these parameters during the life of the structure, the main goal is continuous monitoring in time [10]. The use of continuous monitoring systems for maintenance purposes is already established in many engineering fields, such as on railway vehicles [11] and wind turbines [12]. Nonetheless, in the case of viaducts and bridges, continuous monitoring is not an easy task to perform with the currently available instrumentation. Established monitoring systems are in fact characterized by the presence of wired sensors that present several limitations in this sense. Firstly, the permanent installation of wired sensors on railway bridges is not always feasible due to their structural conformation, and it can result very expensive in terms of both time and hardware cost. In most cases, the deck must be suitably equipped with ducts and other fittings devoted to the protection of communication cables. The installation time tends to be very long, and this issue can be even worsened by the fact that railway bridges are operative infrastructures which rarely can be closed for long time windows reserved to monitoring system installations. Secondly, the maintenance over a long time of the monitoring system can be costly, both from the hardware and the software point of view. Signal cables are subject to a constant degradation in time which require maintenance activities on them. Moreover, if a transducer inside the monitoring network presents some issues, it is usually difficult to find the root cause and fix the problem in the wired network. Thirdly, the large quantity of data collected by these systems requires a complex IT infrastructure to be managed in time. Big data analyses are often required to process data and obtain significant diagnostic information, resulting in an additional complexity to the problem. For these reasons, wireless sensors can represent a suitable alternative to wired ones when analogous measurement performances are achieved.