# Application of Sonic Tests and Modal Identification in the Characterization of Masonry Walls
## 4 Environmental Vibration Tests and Modal Identification  
Dynamic response is an intrinsic property of structures which depends mainly on the material properties, geometry, and support conditions, and it will remain constant unless there are any structural modifications or damages [25]. As natural frequencies are sensitive indicators of structural integrity, an analysis of periodic frequency measurements can be used to obtain information on the global dynamic behavior of the structure, and to validate and update the numerical model of a structure [9]. The environmental vibration test method is based on natural noise and low-frequency vibrations from the environment and can be applied without direct excitation of the structure [26]. In order to apply the Environmental vibration test method, there is a need to make a careful choice of sensor positioning, that will enable to make correlations between natural frequencies and vibration modes of direct measurement. The assessment of the structure response in the time domain is performed by means of a combination network of high-frequency force-balance accelerometers. To assess the natural frequencies and modal shapes of the structure, it is possible to apply the frequency domain decomposition (FDD) technique in the frequency domain and the data-driven stochastic subspace identification (SSI) method in the time domain, which allows the modal parameters to be extracted from the environmental data [27]. Modal identification provides reference values of modal parameters useful for the calibration of numerical models, based on the Young's modulus of the masonry, so that the model can represent the actual dynamic behavior of the structure in its current state [28]. The dynamic characterization tests were performed to obtain the modal shapes and natural frequencies of the walls. The tests were done using four uniaxial accelerometers and the records were taken with one setup, acquiring 600 s with 1500 samples/s. Figure 7a shows one setup at the time of the test, while the sensors layout is shown in Figure 7b. Accelerations were measured in both directions at some locations in order to detect both possible in-plane and out-of-plane mode shapes. The modal estimation was carried out using ARTeMIS software, which allows analyzing the results from all test setups simultaneously, and the representative singular value plots provided by (traditional) FDD are shown in Figure 8 with all picked modes. The peak values of frequency were selected using FDD analysis, to later compare the results and determine the natural frequencies of the wall obtained with the numerical model. Figure 9 shows the first three identified modes of the walls in terms of mode shape and natural frequency. This first mode consists in the expected out-of-plane vibration of the facade and presents a slight difference in the walls, as seen in Table 3. The second mode is a torsional mode, and the third mode is in the plane of the wall. The differences observed are most likely related to the construction issues; however, the third mode associated with the in-plane response of the wall was the one where most differences were observed; however, no apparent cause was identified.