*3.2. FEM Model Validation*

The primary FEM model, including a fixed support and the initial material parameters, produced the first pair of natural frequencies with the values of 6.96 Hz and 7.01 Hz, in the *x* and *y* directions, respectively. Such values were approximately three-fold higher than the measured ones. Thus, the boundary conditions were modified by adding vertical springs, and the model was manually calibrated to the natural frequencies and mode shapes obtained with the PP-CA technique in the combined signal variant. The following parameters of the model were validated: material parameters (elastic modulus and mass density) and the supporting spring stiffness. Four supporting springs were considered. The bottom surface of the base was divided into four areas, in which nodes were kinematically bounded in the *z* direction to the spring situated in this area (see Figure 11).

The initial values were updated to obtain a sufficiently high compliance between numerical and experimental frequencies and mode shapes. The initial and final material parameters are summarized in Table 2. The initial stiffness of the supporting springs was *kx* <sup>=</sup> <sup>1</sup> <sup>×</sup> 1010 <sup>N</sup>/m and *ky* <sup>=</sup> <sup>2</sup> <sup>×</sup> 1010 <sup>N</sup>/m. The final stiffness of the springs was *kx* = 3.75 <sup>×</sup> 10<sup>9</sup> N/m and *ky* = 3.75 <sup>×</sup> 1010 N/m. Table 1 compares the numerical and experimental frequencies of the lighthouse.

**Figure 10.** The comparison of the mode shapes obtained using different methods together with numerical results. PP-CA—the results based on SC signals, NExT-ERA—the results based on SA signals.

**Figure 11.** Supporting springs layout.

**Table 2.** Initial material parameters and parameters obtained by FEM model updating.

