# Smart Sensing of PSC Girders Using a PC Strand with a Built-in Optical Fiber Sensor
## 2 Fabrication and Test
### Test and Measurement of Full-Scale BH Girder Bridge  
For the measurement of the dynamic properties, accelerometers and displacement sensors were disposed vertically at the center on the bottom of each segment. As presented in Table 3, the values measured by the FBG sensor at mid-length of the OSPC strand in tendon one were used. Consequently, the accelerometers measured the vertical vibration, and the OSPC strand measured the transversal vibration. Figure 10 illustrates an installed accelerometer and the actuators used to load the bridge. Figure 9: Full-scale BH girder bridge prototype: (**a**) Placing of top concrete; (**b**) At completion and installed for loading test. The forced vibration test using the actuators applied loading with gradual increase of the exciting frequency from 1 Hz to 5 Hz, and the corresponding responses of the bridge were observed. The observation revealed that the largest amplitude of vibration occurred around 4 Hz. As shown in Figure 11, resonance happened between 4.2 Hz and 4.3 Hz in both data from the accelerometers and FBG sensor. In order to analyze the frequency contents of the vibration test data from the accelerometers and FBG sensor, the Welch method [17] was adopted to estimate the averaged periodograms of the overlapping segments in the signals, and these averaged periodograms were used to perform the analysis of the two different data in the frequency domain. Since the resonant frequency occurred below 30 Hz for the bridge, the sampling frequency was set to 100 Hz to secure a sufficient margin. The frequency band computed by the Welch method fell within the Nyquist frequency between 0 and 50 Hz. The Hanning function was used as the window function needed to conduct the segmental averaging of Welch and minimize the leakage effect. The window function involved a total of 4095 data, and \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Anchor Number**} & \multirow{2}{*}{**Maximum Prestress Force (kN)**} & **Prestress Force after Removal of** & **Loss Ratio with Respect to** \\  & & **Hydraulic Jack (kN)** & **Maximum Prestress Force (\%)** \\ \hline One & 173.1 & 153.1 & 11.6 \\ Two & 164.0 & 153.0 & 6.7 \\ Three & 157.3 & 144.7 & 8.0 \\ Four & 166.5 & 154.2 & 7.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Immediate loss of prestress force per anchor. Figure 10: Loading test of full-scale BH girder bridge prototype: (**a**) Installed accelerometer; (**b**) Actuators. the segments were generated by moving the window according to the time sequence. Fast Fourier transform (FFT) was executed for each segment to compute the power in the frequency domain, which was averaged to provide the final value. Figure 12 plots the natural frequencies of the accelerometer data and FBG sensor data obtained by this method. The computed natural frequency of the first mode appears to be identical for both types of data. This result indicates that the FBG sensor embedded in the OSPC strand that was arranged transversally in the bridge could accurately measure the acceleration of the structure. The dynamic analysis of the bridge structure was performed. To that goal, a three-dimensional model was established and reflected the track structure to achieve precise analysis. Two-node beam elements were used for the girder and cross beams, and four-node shell elements were adopted for the deck. The natural frequency analysis was executed using an in-house program developed at KICT in 2008. Figure 13 depicts the analytic model and the first-mode shape of the structure [13]. From the analysis, a natural frequency of 4.2783 Hz was computed for the first mode, which is slightly smaller by about 0.1279 Hz compared to the frequency of 4.1504 Hz obtained from the test. This small difference can be credited to the boundary conditions, which assumed simple supports (hinge and roller) in the analysis, whereas the prototype was supported actually by two rollers at its ends.