## 4 Applications
### Determination of Peak Velocities during Blast Operation at Schindgraphen Bridge  
The mobile measurement system was further employed in a different usage scenario at a highway bridge located in Central Germany. The considered box girder bridge has unique site conditions being partly founded in a quarry (see Figure 11) where every few months explosive excavations are performed. The monitoring task was to determine the maximum vibration velocity at the top of the tallest bridge pier due to ground shaking induced by these blasts. An allowable peak velocity had been previously determined from detailed studies of the structural performance under typical base excitation, thus serving as a threshold value to judge structural integrity on the basis of on-site measurements. The acceleration at the top of the tallest bridge pier was measured in three axes by an external MPU-6050 sensor connected to the RPi-based measurements system, two smartphones (Sony Xperia Z5, Nexus 4), and a tablet (Nexus 7) as well as a high-quality reference system. The recorded acceleration time histories were then integrated over time to compute the velocity time history and to determine the absolute peak velocity. Thereby, signal processing such as filtering and detrending of the individual signals was necessary. The peak velocities determined by the different systems are compared in Table 4. In comparison to a high-quality reference sensor system, the results obtained by the alternative mobile measurement systems are generally in good agreement. The superiority of the RPi linked to the MPU-6050 accelerometer over the smartphones in terms of sampling rate can be seen in the smallest deviations which are below 1.2% for all the axes, while the peak velocities found of the other systems are within the range of 3-9% and show notable variations between the axes. It should be noted that, shortly before the blast, the bridge had to be closed for traffic. The reopening of the bridge required the explicit approval of an engineer whose decision is based on the non-exceedance of the peak velocity threshold. In order to keep the interruption of the traffic on the highway as short as possible, it is crucial to ensure reliable, robust, and fast data processing. Since all the necessary data processing steps are implemented in the smartphone app, measurement and data analysis is integrated in a single mobile device, and the obtained results show satisfactory accuracy for judging the acceptability of the measured vibrations. The system employing a smartphone as a control unit for the RPi-based measurement system with MPU-6050 is a powerful solution for the engineer in charge. Knowing that the obtained peak velocities are typically well below the threshold value, it is also possible to employ the smartphone Nexus 4 as a standalone solution by defining the limit of accuracy. Since there is a trend of increasing sampling rates with which smartphones can record \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**System**} & \multirow{2}{*}{**Sampling Rate**} & \multirow{2}{*}{**Resolution**} & \multicolumn{3}{c}{**Peak Velocity [mm/s]**} & \multicolumn{3}{c}{**Relative Difference [\%]**} \\ \cline{5-8}  & & [Hz] & & **[bit]** & **x-axis** & **y-axis** & **z-axis** & **x-axis** & **y-axis** & **z-axis** \\ \hline Reference system & 2400 & 23 & 5.63 & 6.22 & 5.32 & - & - & - \\ RPi and MPU-6050 & 565 & 14 & 5.60 & 6.28 & 5.38 & \(-\)0.53 & +0.96 & +1.13 \\ Nexus 4 & 177 & 15 & 5.34 & 6.05 & 5.09 & \(-\)5.15 & +2.73 & +4.32 \\ Sony Xperia Z5 & 177 & 14 & 5.31 & 6.43 & 4.87 & \(-\)5.58 & +3.38 & +8.46 \\ Nexus 7 & 219 & 13 & 5.66 & 5.66 & 5.28 & +6.75 & \(-\)9.00 & \(-\)0.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of the different measurement systems in terms of their sensing characteristics and determined peak velocities along with the relative difference to the high-quality reference system. Figure 11: Highway bridge partly founded in a mining site.