16 Effects of Seismic Retrofit on the Dynamic Properties of a 4-Storey Parking Garage 125 3rd Floor 2nd Floor 1st Floor Ground Level Basement N Reference Sensor Setup 1 Setup 2 Setup 3 Setup 4 Setup 5 Fig. 16.8 Sensor location and orientation, setup configuration Fig. 16.9 Singular values of the spectral densities, 2015 results (after seismic upgrade) The analysis was repeated also with the Enhanced Frequency Domain Decomposition (EFDD) technique, as to estimate the value of natural damping for each mode. This analysis confirmed the frequency values obtained by the previous analysis and the damping values were found to be between 0.2 and 3.3% for the different modes. In order to further confirm the results obtained via frequency domain, the data was analyzed with the Stochastic Subspace Identification (SSI-PC) method. The analysis in the time domain led to results which correlated very well to the ones obtained in the frequency domain in terms of natural frequency values. As for the damping values, the results agreed very well for the first fundamental mode, while they slightly diverged for the higher modes. The six mode shapes are illustrated in Figs. 16.11, 16.12, 16.13, 16.14, 16.15, 16.16 and the site period is illustrated in Fig. 16.17. A detailed summary of the natural frequencies, damping values and description of the modes is given in Table 16.1.
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