PSI - Issue 78

Fabio Micozzi et al. / Procedia Structural Integrity 78 (2026) 1451–1458

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3.3. Measurements extracted from the video recordings Video recordings for computer-vision monitoring were made using an industrial global shutter monochrome camera (Teledyne FLIR BLACKFLY S BFS-U3-23S3M-C) equipped with a fixed-focal lens (Tamron 23FM50SP). Acquisition frequency was set at 200 frames per second. Video post-processing for displacement extraction used the same methodology and software in Micozzi et al. (2023). Selected frames of the video footage are reported in Fig.9 while the obtained vertical displacement of the upper front tube is reported in Fig. 10. Please note that the acquisition of the strain gauges and the acquisition of the video footage started at different times, hence the abscissa of Fig. 10 has a different zero compared to the abscissa of Figs. 7 and 8. The interruptions of the experimental curve in Fig. 10 is a consequence of the steel ball moving in front of the monitored target after bouncing back, thus, temporarily interrupting its visualization (see the last three photograms of Fig. 4). The curve clearly shows the initial loading phase due to the impact that brings the deflection from zero to the maximum value of about 150 mm in around 0.1 seconds, followed by a partial recovery and a permanent plastic deflection of nearly 120 mm. However, it must be remarked that this value is the maximum deflection of the outer frame of the school deck while the inner frame resulted in a permanent deflection of about 55 mm, causing a minor decrement of the shelter space.

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c

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Fig. 9. Extracted video frames at: (a) before impact; (b) impact start; (c) maximum deflection; (d) bouncing back.

Fig. 10. Vertical displacement of the upper front tube as extracted from the recorded video.