## 6 Experimental Test  
The following experiment was performed to verify the values of some of the calculated natural frequencies. The response of the bridge to the technical seismicity caused by railway traffic under the bridge was monitored. KB12 VD 15,040 accelerometers from Metra Mess and Frequenztechnik (MMF) were used to measure the response. This is a classic accelerometer that uses a piezoelectric effect. The sensors operate in the frequency range from 0.15 to 260 Hz. The signal from the sensor is amplified by the charge amplifier M68, also a product of MMF. The amplifier also serves as an integrating member and a low-pass filter. The signal is further digitized in an analog-to-digital converter (A/D converter National Instruments NI 9215) and enters the computer in numerical form, where it is stored and further processed. The complete set of instruments, including the computer, is shown in Figure 11.  
Signal digitization was performed with a sampling frequency of \(f_{V}\) = 1000 Hz. According to [PERSON]'s theorem [12], the highest signal frequency before digitization must be less than half the sampling frequency (Nyquist frequency \(f_{N}\) = \(f_{V}\)/2 = 500 Hz). To achieve the highest signal measurement accuracy in the time domain, the M68 amplifier manufacturer recommends setting the cut-off frequency of the low-pass filter to one-tenth of the sampling frequency. In our case, it is 100 Hz. The signal is digitized only after such filtering.  
The localization of the sensors on the second bridge field is shown in Figure 12 (top view on the left, side view on the right). Sensor 2 was placed above the support, sensor 3 at 1/4 of the span, and sensor 4 at 1/2 of the span. A total of eight measurements were carried out. The vibrations of the bridge were registered during the passage of eight trains under the first field of the bridge. Spectral analysis (Power Spectral Densities--PSD) was performed from the monitored records using the DISYS system [13]. Discrete Fourier Transform (DFT) was used to calculate the PSD. The length of the records ranged from about  
Figure 11: Sensor, amplifier, A/D converter, and operative computer.  
Figure 10: A simple schema of the Lafranconi bridge. Dimensions in m.  
60 to 200 s which, at a sampling frequency of 1000 Hz, represents 60,000-200,000 samples. The algorithm works with the number of \(2^{n}\) samples. The power of \(n\) was in an interval from 16 to 18. The detected values of frequencies with a dominant position (interval and average value) are given in Table 3. The experiment was realized in April 2016.  
It is interesting to note that in the experiment performed in August 2009, the experimentally verified value of the first natural frequency ranged from 4.042 to 4.048 Hz, which is practically identical to the result of the numerical calculation 4.04 Hz. The difference in the measured frequencies in 2009 and 2016 can have two causes. The first reason may be a change in the temperature at which the experiment was performed. In 2009, the air temperature was 28 \({}^{\circ}\)C and in 2016, it was only 8 \({}^{\circ}\)C. A change in temperature of 20 \({}^{\circ}\)C could affect the value of the first natural frequency by 0.08 Hz. The second reason could be that a completely different measuring string was used in the 2016 experiment. Relative deflections in the middle of the bridge span were measured. The natural frequencies were evaluated from the record of the free vibration of the structure after the vehicle left the bridge. However, a combination of both causes is most likely.