## Hardware and Measurement System
### Smartphone Platform  
An Android application was developed by the authors in a previous work to assist the process of smartphone-based measurements [6]. The app offers a broad set of tooling options and features a user-friendly interface as shown in Figure 3. The app provides several options for measurements using the on-board accelerometer of the smartphone. Besides general settings (e.g., the sampling rate, the measurement duration, or the sensing axes), there are three different ways to trigger and store the recording of an acceleration signal:  
* a simple start-stop-measurement where the length of the signal is defined by the user activating and deactivating the measurement by hand,
* a buffered-measurement that runs continuously and stores only the data of a time-limited buffer into a file when deactivated by hand, and
* an event-triggered-measurement that runs continuously and stores only the acceleration data into a file when a defined signal threshold has been exceeded (e.g., during earthquake or explosions). Additionally, a delayed measurement after a defined time or for a simultaneous start on the next clock minute, as applied in [17], is possible. Data are stored temporarily in the working memory during data acquisition to avoid delays. After a measurement is completed, the data are saved to the internal storage of the smartphone within \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Product Name** & **Product Type** & **Vendor** & **Sensor Type** & **Hardware Interface** \\ \hline MPU-6050 & Sensor & InvenSense & MEMS accelerometer & I\({}^{2}\)C \\ MPU-9250 & Sensor & InvenSense & MEMS accelerometer & SPI \\ ADS1115 & ADC & Texas Instruments & Any analog sensor & I\({}^{2}\)C \\ CXL02 TG03 & Sensor & Memsic Inc. & MEMS accelerometer & I\({}^{2}\)C (ADS1115) \\ ADXL35 & Sensor & Analog Devices & MEMS accelerometer & I\({}^{2}\)C (ADS1115) \\ SI-11.S1.C-30 & Sensor & First Sensor & Inclinometer & SPI \\ HX711 & ADC & Avia SC & Strain gauge amplifier & Serial \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of implemented sensor driver modules. Figure 3: Screenshots of the user interface of the Android app. (**Left**) Visualization and settings dialog for the integrated signal of a captured blast impact operation; (**middle**) selection dialog of the signal range; (**right**) dialog for the semi-automated peak selection and tension force calculation. a defined tree structure of folders, and this structure is used as a database to manage projects and sub-elements. Each of the sub-elements in the folder structure holds a list of assigned measurements and enables the user to additionally create and store images, videos, voice memos, notes, sketches, location coordinates, or specific properties of building elements. This facilitates a comprehensive documentation of each measurement task and its environment at the investigated building element for the evaluation of analysis results or a detailed survey report. Furthermore, there are various in-app signal analysis tools to evaluate the measured data directly on the smartphone. As an overview, a list of meta data is attached to the measurement file, and the actual signal is visualized by a plot view. Another function applies a fast Fourier transform (FFT) on a section of the signal defined in an interactive plot view that enables one to cut off unnecessary or externally disturbed parts of the signal. Once the result of the FFT is computed, the frequency domain output can be used as input for a further option to calculate the tension force in structural elements such as bridge cables and external tendons. The taut string approach is used with parameters for stiffness and mass combined with an eigenfrequency obtained from a semi-automated peak-picking routine. An additional function provides the integration of the acceleration signal to the corresponding vibration velocity and displacement in the time domain applying an optional frequency filter for signal corrections.