## 2 Acquisition Frequency and Reasonable Acquisition Frequency  
When the health monitoring system has been deployed, discrete data extraction from the system's continuously generated signals constitutes the majority of the work required to obtain bridge structure information. The frequency at which this acquisition occurs serves as an indicator of the effectiveness of signal capture to a certain degree. Therefore, a reasonable acquisition frequency is very important to judge the bridge state by using the change in live load response, and different acquisition frequencies will lead to different integrities of monitoring data [16; 17]. In view of the sensor's performance, data storage capacity, field installation, and other practical conditions, and for the sake of cost, it is usually desirable to obtain the response of the bridge structure with a smaller acquisition frequency [18]. Consequently, in order to facilitate the dynamic monitoring of bridge structures, it is necessary to investigate the feasibility of capturing the true response process of the structure with a lower acquisition frequency and the determination of an appropriate acquisition frequency. The acquisition frequency of a bridge structure can be defined by: Measuring the value of the continuous responses in units at intervals. The unit acquisition process produces a series of numbers, called samples. The response of the original structure is replaced by samples for analysis. Among them, the reciprocal of the acquisition interval (1/T) is the acquisition frequency (\(f_{s}\)), and its unit is sample/second, that is, hertz (Hz). According to the Nyquist sampling theorem, the reasonable acquisition frequency is defined as twice the cutoff frequency of the structural response [19], as shown in Figure 1.  
The traditional cutoff frequency selection process is that the original time series is Fourier transformed to obtain the frequency spectrum, and then a position between the two poles is found in the frequency spectrum and the frequency value corresponding to this position can be used as the selected cutoff frequency [20; 21]. The traditional selection method of cutoff frequency depends on the spectrogram. Once the spectrogram is distorted or too complicated, it is difficult to find the accurate cutoff frequency [22]. For small- and medium-span, simply supported beam bridges, [PERSON] [23] put forward a method based on modern dynamic signal analysis theory to extract the maximum static response of the bridge. By approximating the effect of moving vehicles on the bridge as the superposition of the moving constant force and the moving harmonic force, this method carries out spectrum analysis on the dynamic response of the bridge, and selects the right valley value  
Figure 1: General amplifier or filter frequency response curve. of the first main lobe of the power spectrum as the cutoff frequency, thus determining the frequency band of the static component in the bridge vibration response; [PERSON] [24] regarded the fast response pressure sensitive paint as a \"dynamic pressure sensor\". In order to obtain an effective working frequency, a series of studies have found that if the signal amplitude gain \(>-3\) dB, the corresponding frequency is the cutoff frequency, which can better reflect the dynamic change in pressure, otherwise, the measurement result will be distorted. [PERSON] [25] monitored a long-span PC beam in Guangzhou in real time by total station, and obtained the midspan deflection time-history curve. According to the probability analysis method in the reliability theory, the acquisition frequency at a 95.0% assurance rate was determined as a reasonable value. In conclusion, this article defines the cutoff frequency of the structural response signal of a bridge. The cutoff frequency is defined as the extent to which the deflection response efficiently functions within a particular frequency range, contingent upon the structural response undergoing a specific amplitude change and meeting the effective resolution criteria.