# On Site Investigation and Health Monitoring of a Historic Tower in Mantua, Italy
## 3 Preliminary Operational Modal Tests and Dynamic Characteristics of the Tower  
Two series of ambient vibration tests (AVTs) were conducted on the tower. The first test was carried out between 31/07/2012 and 02/08/2012 [6] with the objectives of evaluating: (a) the dynamic characteristics of the tower; (b) the possible effects of the poor state of preservation of the upper region on the global behavior; and (c) the impact of temperature changes on the natural frequencies. The second test, preparatory to the continuous dynamic monitoring, was performed on 27/11/2012 with the two-fold objective of evaluating the possible effects of added wooden roof and scaffoldings (Figure 5) and providing reference values of the modal parameters for the subsequent monitoring. During both tests high-sensitivity accelerometers (WR model 731A, 10 V/g sensitivity and 0.5 g peak acceleration, Wilcoxon Research, Germantown, MD, USA) were used to measure the dynamic response of the tower in 12 points, belonging to four pre-selected cross-sections along the height of the building (Figure 6a). Since access to the inner walls of the tower was not available during the first survey, the accelerometerswere mounted on the outer walls making use of the same movable platform employed for visual inspection. During the second test, performed after the installation of wooden roof and provisional scaffoldings inside the tower (Figure 5), inner mounting of the accelerometers was preferred (Figure 6a). Figure 6: (a) Instrumented cross-sections and layout of the accelerometers during the preliminary tests (November 2012) and the continuous dynamic monitoring; and (b) vibration modes identified applying the data-driven stochastic subspace Identification algorithm (SSI-Data) to the data collected on 27/11/2012. The modal identification was performed considering time windows of 3600 s and applying the data-driven stochastic subspace Identification algorithm (SSI-Data) [17; 18] available in the commercial software ARTeMIS Extractor [19] (Release 5.3, SVS, Aalborg, Denmark, 2012). Since the second test was aimed at estimating the modal parameters used as reference values in the continuous dynamic monitoring, only the results of this test will be presented herein. The application of the SSI-Data method to the datasets collected in the test of November 2012 (about two hours of acceleration data recorded with an outdoor temperature of 10 \({}^{\circ}\)C-11 \({}^{\circ}\)C) allowed the identification of five vibration modes in the frequency range of 0-10 Hz. Figure 6b summarizes the identified dynamic characteristics of the tower and allows the following comments:  
* two closely-spaced modes (B\({}_{1}\) and B\({}_{2}\)) were identified in the frequency range of 0.90-1.00 Hz and are associated to dominant bending in the two main planes of the tower, respectively. The modal deflections of first mode (B\({}_{1}\)) involve bending in the N-E/S-W direction, whereas the second mode (B\({}_{2}\)) is dominant bending in the orthogonal N-W/S-E direction;
* the third mode (B\({}_{3}\)) is dominant bending in the N-E/S-W direction with slight components also in the orthogonal direction;
* the fourth mode (T\({}_{1}\)) is characterized by dominant torsion until the height of 46.0 m and coupled torsion-bending of the top level. The local behavior of the upper region of the tower alters what would otherwise be classified as a pure torsion and was not observed in the previous dynamic test so that it can be conceivably associated to both the poor structural condition of the upper level and the effect exerted by the wooden roof, directly resting on the weakest region of the tower; and
* a local mode (L\({}_{1}\)) was identified at 9.89 Hz and involved torsion of the top levels of the tower. The presence of a local mode provides further evidence of the structural change in the masonry quality and morphology highlighted in the upper part of the building by the visual inspection.