PSI - Issue 78

Daniele Sivori et al. / Procedia Structural Integrity 78 (2026) 481–488

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SHM procedures when evaluating structural degradation phenomena and potential damage (see Gentile et al. 2019, Kita et al. 2019 for application on cultural heritage structures). 3.1. Sensing technologies The technological solutions adopted in the design of the monitoring system should satisfy, primarily, the technical requirements posed by the measurements of the operational and the seismic dynamic behavior of both the bell tower, the church and their internal and external dynamic interactions. The operational regime exhibits small random vibrations in the low-frequency band starting at around 1 Hz, with the fundamental mode of the structure — the first flexural mode of the bell tower — identified at around 1.57 Hz (Degli Abbati, Sivori et al. 2024). Proper modes of the church can be found in a higher frequency range, from around 3 Hz to 5.5 Hz, with very low-amplitude accelerations, in the order of µm/s 2 at the lowest heights. Conversely, seismic excitation potentially causes a response in the nonlinear regime, with large displacements and accelerations in the orders of g at the highest elevations. These two aspects underscore the necessity of employing sensors and acquisition systems with a very large dynamic range, at least 140 dB. To satisfy this requirement diverse sensing solutions have been analyzed:  PiezoElectric (PE) seismic accelerometers: characterized by good sensitivity and low noise, inexpensive but bulky in the multi-axial version;  Capacitive Micro Electro-Mechanical Systems (MEMS): miniaturized accelerometers with typically higher noise levels compared to PE, but offering a larger frequency band and full scale range;  Force-Balance (FB) accelerometers: generally, the largest and heaviest, offering the lowest noise on the market but limited by bandwidth. Table 1 presents the technical specifications of commercial accelerometers pertaining to each sensing solution, in their standard and High Sensitivity (HS) versions. Table 1. Technical specifications of commercial accelerometers considered for the monitoring system, with different sensing technologies (PE: PiezoElectric, FB: Force-Balance, MEMS: Micro Electro-Mechanical Systems, HS: high-sensitivity version). Accelerometer Frequency range ( Hz ) Sensitivity ( V/g ) Spectral noise @1Hz ( µg/√Hz ) Full scale* ( g ) Dynamic range ( dB ) PE 0.7–450 10 0.5 0.5 100 MEMS 0–1000 0.5–1 (*5g) 7 2/5 90 FB 0-200 10 (*1g) 0.03 0.5–4 160 HS PE 0.1–200 10 0.06 0.5 110 HS MEMS 0–500 0.5–1 (*5g) 0.2 3/5 110 HS FB 0–320 5 (*4g) 0.003 0.25–4 166 Figure 2 illustrates the spectral noise of each sensing solution presented in Table 1 and compares it to the spectral density of ambient vibrations acquired at different heights of the church and bell tower, where L2 corresponds to the roof of the main nave, L3 to L5 to the soaring sections of the bell tower. Regarding operational vibrations, it can be observed that:  only the first two flexural modes of the bell tower in both directions (i.e., the peaks at 1.57 and 2.04 Hz) exhibit spectral densities exceeding 10 µg/√Hz ;  all other modes of the church, located above 3 Hz, exhibit significantly lower vibration levels, with spectral amplitudes mostly below 10 µg/√Hz . In this respect, measuring ambient vibrations at low heights requires the employment of FB accelerometers or PE accelerometers. In this regime, the dynamic response of the church can be effectively captured by HS MEMS or PE sensors, whereas the noise floor of conventional MEMS often masks operational vibration levels.