370 M. Motamedi and C. E. Ventura Fig. 41.1 Full scale wood frame specimen on shake table: (a) General view, (b) Wireless sensor installed at top for ambient vibration test Table 41.1 Characteristics of the ground motion used for shake table tests Ground motion Magnitude (Mw) Distance (Km) Focal depth (Km) PGA (g) PGV (cm/s) PGD (cm) Soil type Maule, Chile—2010 8.8 317 35 0.287 42.6 25.3 Intermediate soft Fig. 41.2 Power Spectral Densities obtained from ambient measurements: (a) Before test 1, (b) After test 1, (c) After test 2, (d) After test 3 Tromino ® velocity/acceleration sensors were used to carry out the ambient vibration tests. The Tromino sensors are suitable for high-resolution ambient vibration tests as they are fully portable, wireless, compact, and light instruments. Each sensor is equipped with two sets of three orthogonal high-resolution electrodynamics sensors (high gain and low gain velocity meter) and one set of three orthogonal digital accelerometers with a frequency range of 0.1–300 Hz [4]. For these tests the high-gain velocity data was used for the modal identification process. The collected records were time synchronized with a radio antenna and amplifier attached to each sensor. This allowed the synchronization of the recordings both within each measurement setup. The north component of the Tromino sensors were oriented parallel to north direction of the site. The sampling frequency of the recordings at each test was 128 samples per second. The total recording duration for each setup was about 20 min. 41.3 Modal Analysis The computer program ARTeMIS Version 4, was used to perform the modal analysis of the specimen. The software allows developing a 3D model of the structure and test points; the resulting mode shapes are displayed using this geometry. Two different techniques were used for modal identification: the Enhanced Frequency Domain Decomposition (EFDD) and the Stochastic Subspace Identification (SSI). These two modal identification techniques were used to cross-validate the results. The joint analysis of the signals measured in various strategic points of the structure made it possible to identify the modal configurations and the corresponding natural frequencies [5]. The Power Spectra Densities obtained from ambient vibration measurements before and after shake table tests are illustrated in Fig. 41.2. The peak values show the natural frequency of the specimen in each level of damage.
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