## 2 Conventional SHM Techniques
### Dynamic Monitoring Systems  
The key aim of dynamic monitoring is the identification of the structure's modal parameters (e.g., frequencies, mode shapes, damping ratios) that can be used to spot anomalies from the expected global behaviour and plan appropriate and timely interventions. Before installing a proper continuous dynamic monitoring system, preliminary dynamic identification tests are usually performed under forced or ambient noise conditions. In some cases, for economic reasons, for instance, only the first activity is followed by FEM simulations. When the devices are not continuously available, dynamic monitoring can  
Figure 3: Biaxial tiltmeters installed on eight capitals of the Milan Cathedral (from [37]). be conventionally achieved through the consecutive repetition of dynamic measurements over time [58]. Proper continuous dynamic monitoring comprises the instrumentation and acquisition systems that record the vibration response of the structure based on a set of sensors deployed at selected locations with sufficient spatial density and frequency resolution. Equipment based on accelerometers is typically adopted, such as force balance (Figure 4a), piezoelectric, and piezoresistive monitors, micro electro mechanical systems (MEMS) (Figure 4b), or seismometers. Concerning dynamic identification techniques, the ambient vibration test (AVT) and operational modal analysis (OMA) are among the first choices used by researchers for the analysis of heritage structures because of (a) the easy and non-destructive methods of testing, performed by measuring only the structural response under ambient excitation; (b) the sustainability of testing, which does not interfere with the everyday use of the structure; and (c) the multiple-input nature of ambient excitation, ensuring that the response includes the contribution of a certain number of modes [40]. The identified modal properties have proved to be well-suited for the optimisation-based calibration of numerical models [59, 60, 61, 62, 63, 64, 65, 66]. It is worth noting that vibration-based SHM techniques provide reliable results for slender or flexible structures, such as towers, vaults, domes, etc. The analysis of stiffer buildings, such as palaces and churches, preferably requires a mixed static-dynamic monitoring approach to obtain informative results [67, 68]. Similarly to static monitoring, one crucial aspect that must be considered when processing dynamic monitoring data is removing environmental effects. In particular, natural frequencies tend to exhibit daily and seasonal fluctuations, imputable to temperature and humidity variations, as well as, in the case of slender structures (e.g., towers), wind speed [41, 69, 70, 71]. The optimal location and the minimum number of environmental sensors that are sufficient to achieve about 90% of the statistical prediction accuracy of the natural frequencies was investigated by [PERSON] et al. [69]. Besides the well-known influence of temperature on natural frequencies, generally related to increments in frequencies with increasing temperature, [PERSON] et al. [22] reported a non-negligible effect of humidity on the dynamic response of masonry structures subjected to heavy rain events. According to the types of instruments involved, dynamic monitoring systems can be classified into two groups: conventional wired-based systems and wireless-based systems. The first system comprises measurement sensors, data acquisition (DAQ) systems, and, in some cases, remote connection systems. Although such systems are still widely used, the recent development of inexpensive wireless monitoring systems and MEMS has increased the interest in their adoption, particularly for historical structures [72]. One of the first publications on the dynamic monitoring of large-scale historical structures was carried out by [PERSON] et al. [73]. The authors investigated the dynamic behaviour of the Hagia Sophia (Istanbul). First, an ambient vibration survey was conducted by using four seismometers.