16 Effects of Seismic Retrofit on the Dynamic Properties of a 4-Storey Parking Garage 123 Fig. 16.3 Anchor bolts on internal shear walls, under construction Fig. 16.4 External shear wall and short columns, before upgrade as to capture the response of the structure at the most significant locations. Detailed illustration of the sensors locations and their orientation is provided in Fig. 16.8. The sampling frequency was kept as constant to 128 Hz. The recorded data was synchronized into ASCII file via computer program Grilla [1]. The natural properties of the structure, such as natural frequency, mode shapes and damping have been identified with different Operational Modal Analysis techniques. The analyses have been carried out with computer program ARTeMIS Modal v3.0 [2]. The data collected during the test were preprocessed and decimated to a frequency range of 0–16 Hz, considering ten projection channels and the resolution set to 1024. The Frequency Domain Decomposition (FDD) technique allowed to estimate the first 6 natural modes up to a frequency of 8.5 Hz. The plot for singular values of the spectral density matrix in Fig. 16.9. For sake of comparison, a similar plot obtained through the test run in 2014 before seismic upgrade is given in Fig. 16.10. The fundamental mode has been found to be a first bending mode in the longitudinal N–S direction and occurred at 3.05 Hz. The second natural mode was identified as a first mode in torsion and corresponded to a frequency of about 3.8 Hz. The remaining modes involve bending in both N–S and E–W and torsion, which sometimes occurred coupled to a bending mode. With the FDD method it was also possible to identify the site period, which corresponded to a frequency of about 0.6 Hz.
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