## 3 Numerical Modelling  
Three-dimensional geometric models of measuring bases were prepared in Ansys Space Claim software (19.2, Ansys, Inc., Canonsburg, PA, USA, 2019). Numerical analyses aimed at determining the modal base parameters were carried out using Ansys Engineering Simulation and 3D Design Software (19.2, Ansys, Inc., 2019) (based on the Finite Element Method--FEM). Verification of numerical analyses was carried out on the latest version of Ansys software (2020 R1, Ansys, Inc., 2020). For the needs of numerical analyses, all steel elements (sections: \(50\times 50\times 5\) mm, \(30\times 30\times 4\) mm, \(5\) mm and \(10\) mm thick flat bars welded to them, bolts, welds were matched with typical parameters adopted for steel (a Young's modulus E = 210 GPa, a Poisson's ratio v = 0.3, a density \(\rho\) = 7850 kg/m\({}^{3}\)). Additional mounting devices necessary to screw the accelerometers to the measuring bases were attributed with material parameters as for \"Aluminum 2024-T4, 2024-T351\" (E = 73.1 GPa, v = 0.33, \(\rho\) = 2780 kg/m\({}^{3}\)). During the three-dimensional modelling of individual measuring bases, efforts were made to reproduce their construction and real work as accurately as possible. Therefore, welds between the angle bars and angle bars and flat bars welded to them were additionally modelled. The discretization of individual geometric models was carried out using solid finite elements type Hex20 and in places where this was not possible solid elements type Tet10 were applied. After initial discretization of individual models of the measuring bases, the mesh of nodes was densified until it was found that further increasing of the mesh density practically did not affect the change in the value of the results obtained. The effects of discretization of individual models in the form of a summary including the final number of nodes, the number of finite elements and the maximum size of the finite element are given in Table 1.  
Contacts of individual elements of the measuring bases were modelled in most cases as \"bounded\". The exception are the contact surfaces between the screws and the mounting head as well as the mounting head and flat bar, in which \"no separation\" linear contacts were used. Unfortunately, due to the planned type of analysis (modal analysis), it was not possible to use non-linear \"frictional\" or \"frictionless\" contacts, because all these contacts will be reduced to their linear counterparts anyway. In order to obtain reliable results, numerical simulations in Ansys Engineering Software (19.2, Ansys, Inc., Canonsburg, PA, USA, 2019) were carried out for measuring bases with attached point masses in the places of screwed accelerometers. In the case of the measurement bases No. 1 to No. 3, analyses were carried out for three types of IEPE/ICP transducers: Bruel & Kjaer 8340-type (Narum, Denmark) uniaxial seismic accelerometer with a sensitivity of 10 V/g and a weight of 0.775 kg (identical in terms of measuring sensitivity and weight as uniaxial accelerometers model 3191A by Dytran Instruments, Inc., Chatsworth, CA, USA), low frequency uniaxial accelerometers model 3187D from Dytran Instruments, Inc. with sensitivity 0.5 V/g and weight 0.122 kg, as well as miniature triaxial accelerometers model 356B18 by PCB Piezotronics, Inc. (Depew, NY, USA) with sensitivity 1 V/g and a weight with a magnetic base of 0.041 kg. The final discretization of the individual measurement bases is shown in Figure 2 (in the case of measuring bases No. 1 and No. 2 with mounted lightweight aluminum heads for mounting accelerometers). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Measuring Base No. 1** & **Measuring Base No. 2** & **Measuring Base No. 3** \\ \hline Nodes & 358,834 (222,670) & 209,982 (111,552) & 193,124 \\ Elements & 164,467 (74,512) & 103,594 (35,509) & 48,137 \\ Max body sizing & 3 mm (2 mm) & 3 mm (2 mm) & 2 mm \\ \hline \hline \end{tabular} The number of nodes and finite elements without mounting head are given in brackets.  
\end{table}