## 2 Conventional SHM Techniques
### Static Monitoring Systems  
Static monitoring continuously measures strain, stress, deformation, tilt, and displacement [46]. These parameters usually fluctuate slowly in the absence of dynamic events (e.g., earthquakes, tsunamis, etc.). Additionally, they are influenced by seasonal cycles and  
Figure 1: Static-dynamic SHM system installed in Consoli Palace, with crack meters (LVDTs), temperature sensors (T1, T2) and uni-axial piezoelectric accelerometers (A1–A3) (modified from [45]). environmental changes (e.g., temperature, humidity, rain, wind, etc.). For these reasons, a monitoring period of at least two years is usually required to deduce meaningful data. In historical structures, crack patterns can be one of the first indicators of deterioration or even a developing collapse mechanism that could be triggered by future events. Besides a first evaluation with a visual inspection, installing a long-term monitoring system aims to determine whether such cracks are active or dormant. Basic tools are based on manual and periodic controls using a plastic Tell-Tale fixed across the crack (Figure 2a), which, unlike the previously used strips of glass, provides information about the magnitude and direction of the movement. Another system consists of applying some fixed bases or anchor points (usually made of aluminium plates) on each side of the cracks and then manually measuring the distance between these points using a caliper or a deformometer. Continuous monitoring can be conducted using electric/mechanical displacement transducers, such as linear variable differential transformers (LVDTs, Figure 2b) or linear potentiometric displacement transducers (LPDTs). One of the most notable monitored crack patterns, in terms of extension, complexity and importance of the monument, is that of the dome of Santa Maria del Fiore in Florence, which is also one of the longest continuously monitored monumental buildings in existence (more than 60 years of data) [47]. A static monitoring system has been installed since 1955. Today, it inlcudes more than 160 instruments, such as mechanical and electronic deformometers, thermometers, and piezometers. The large amount of data collected across time allows for a good understanding of the actual behaviour of the structure. A similar example of crack monitoring is applied to the Basilica of Vicoforte's dome, the world's largest masonry oval dome. [PERSON] et al. [27] reported the results of data acquired from 2004 to 2014. The absence of significant trends in the crack opening extracted from the periodic measurements of the crackmeters proved the effectiveness of the strengthening intervention in 1987. Tilting is another severe structural problem that can affect the stability of CHS, especially in slender structures such as towers. The main reasons why buildings can develop a lean are usually related to (a) lack of foundation strength and (b) lack of foundation stiffness [49]. Tilt is typically measured through inclinometers, such as that used by [PERSON] et al. [50], for monitoring the inclination of Portogruaro Civic tower, and [PERSON] et al. [51], for monitoring the Torcato Church towers. The systems were based on pendulums hanging from the ceilings consisting of steel wires with a heavy mass attached. In the first example, two digital network cameras permanently record the position of the pendulum, and the position of the wire is calculated in real-time using image recognition software. Alternatively, for higher precision, uniaxial tiltmeters, such as those installed in the Church of the Monastery of Jeronimos in Lisbon [33], or biaxial tiltmeters, such as those installed for the monitoring of the Milan Cathedral [37] (Figure 3), can be used. Figure 2: Monitoring of cracks employing: (**a**) plastic Tell-Tale crack meter, and (**b**) an LVDT sensor (from [48]).