###### Abstract  
Currently, there is a growing concern about the conservation and maintenance of infrastructure. Within this context, bridges deserve special attention, given their technical complexity and strategic nature. To this end, modern technology provides the opportunity to implement systems for structural health monitoring (SHM), a field in which great advances have been made in recent years. In this sense, one of the fastest-growing lines of work in Civil Engineering is the early detection of incidents because of changes in the dynamic behaviour of structures. Throughout this paper, some of the most notable considerations that the authors have been appreciating in the latest structures studied are summarized. These may be of interest for the possible incorporation of dynamic parameters in SHM systems that could be implemented in other structures in the future. With this purpose, a review of the different issues that must be studied within the dynamic analysis of a structure is carried out, such as the structural typology, the type of instrumentation, the recorded accelerations, the analysis of the natural frequencies, the study of the modal damping ratio and the set of thresholds; issues that are also accompanied by examples observed in two real monitored structures. structural health monitoring; dynamics; bridges; natural frequencies; modal damping ratio +
Footnote †: journal: _Sciences_  
0000-0001-8991-4000-0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-0000-00000-0000-0000-00000-0000-00000-0000-00000-0000-00000-  
Several publications have studied SHM systems in-depth, especially in recent years. Among the most relevant, these can be highlighted: [PERSON] et al. on vibration-based SHM methodology using output-only measurements under changing environments [4]; [PERSON] et al. on the testing and long-term monitoring of a curved concrete box girder bridge in California (United States) [5]; [PERSON] et al. on the long-term monitoring and data analysis of the Tamar Bridge (United Kingdom) [6]; and the comprehensive review about operational modal analysis in bridges conducted recently by [PERSON] and [PERSON] [7]. Regarding the introduction of new technologies in SHM systems, the current panorama is in continuous evolution and fields of work that until now were unthinkable are opening. Thus, for example, a number of recent publications may be mentioned: [PERSON] et al. about the introduction of nanosensors [8]; [PERSON], [PERSON], and [PERSON] or [PERSON] et al. in relation to the use of systems based on computer vision to monitor the behaviour of bridges, even long spans [9; 10]; [PERSON] et al. on the methodology for identifying damage in spacecraft from measurements made on optical fibre [11]; [PERSON] et al. in the field of evaluation of reinforcement corrosion in concrete structures by means of embedded sensors [12]; [PERSON] and [PERSON] on the introduction of tools based on deep learning [13]; and [PERSON] et al. on the application of synthetic aperture radars [14]. Undoubtedly, the early detection of incidents because of changes in the dynamic behaviour of structures is an emerging line of study in civil engineering, as is also happening in other areas of technology. However, bridges are not repetitive constructions since each structure responds to the specific needs of the environment in which it is located, such as topography, geology, or the required functionality. Furthermore, among the problems that we encounter today are the excess of data obtained in the instrumentation, which must be analysed, interpreted, and selected to have a true image of the situation of the monitored structure, the relative lack of academic studies on real monitored structures in service, and the uncertainty about both the influence of environmental variables and the calculation methodology used in the case of the modal damping factor to propose systems that monitor the status of the structures.