# Development of a Structural Monitoring System for Cable Bridges by Using Seismic Accelerometers
## 2 Structure of the Monitoring System  
The system is composed of four parts each of which corresponds to the major process of the structural health evaluation (Figure 1). The first part is the format conversion process where the compressed format of the acquired acceleration data is converted to text format for numerical calculations (Figure 2). The Standard for the Exchange of Earthquake Data (SEED) and its subset with reduced metadata (miniSEED) are widely used for recording the raw data from seismic accelerometers. These formats are maintained by the international Federation of Digital Seismograph Networks (FDSN) and additional details can be found elsewhere [23]. Since the SEED and the miniSEED are data formats devised mainly for the archival and exchange of seismological time series, format conversions are needed for processing of data. The compressed format of the raw data needs to be converted to text data for further analysis procedures. Physical factors are needed to make text data from the raw data, which is the record of changes in voltage acquired from the accelerometer sensors. The physical factor is calculated from the sensitivity value for the recorder and the response value for the accelerometer. The physical factor is calculated from the sensitivity value for the recorder and the response value for the accelerometer and acceleration in gal is obtained by multiplying the physical factor to the count values in the raw data, where, gal is defined as 1 cm per second squared (1 cm/s\({}^{2}\)). For instance, the physical factor is calculated as \(2.336\times 10^{-4}\) from Equation (1), if the sensitivity and the response values are given as 1.1921 \(\upmu\)V/count and 0.5102 V/(m/s\({}^{2}\)), respectively. \[\text{Physical Factor}=\frac{1.1921}{0.5102}\frac{\frac{\upmu\text{V}}{\text{ count}}}{\frac{\upmu\text{V}}{\text{sec}^{2}}}=2.336\,\times\,10^{-4}\text{gal/count}. \tag{1}\]  
Figure 1: Major processes of the monitoring system. The second step is the noise filtering process. Since there are various sources of noise in the measured accelerogram, processing data without noise filtering will mislead the analysis result, as shown in Figure 3. Details on the noise filtering schemes for seismic accelerometers are given elsewhere [24]. Raw data of acceleration records include noise from various sources such as instrument's installation conditions and environments. In general, low-frequency noise below 0.1 Hz is called microte  
Figure 3: Displacement response histories with/without filters and comparison of noise filters. Figure 2: Conversion process from miniSEED to ASCII data formats. While damage indicating methods have been traditionally developed based on relative displacements of the structure [29], studies on relationship between the degree of structural deterioration and the changes in natural frequencies have shown that the pattern of changes in natural frequencies is affected by the location and the degree of damage. The change of natural frequencies can be used as a damage indicator since it is sensitive to the stiffness change of a structure [30]. In the study performed by the National Disaster Management Institute [20], a method to identify the structural damages based on the change in the natural frequencies is presented. Earlier studies on the measurement of dynamic responses showed that the detection of a structural damage is feasible by observing changes in natural frequencies due to damages on the structural members [30; 31]. Based on the recent development on sensor and measurement technologies, advanced studies on the relationships on vibration modes and structural damages have been performed to identify the location of damages and show that a stiffness decrease due to a structural deterioration causes the change of natural frequencies in lower modes [32; 33; 34; 35; 36]. The fourth step is the condition evaluation of the bridge structure.