PSI - Issue 62

Massimiliano Bregolin et al. / Procedia Structural Integrity 62 (2024) 916–923 M. Bregolin et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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horizontal plane, a torsional mode with a frequency of 2.510 Hz, two predominantly flexural modes in the vertical plane with frequencies of 2.600 Hz and 3.402 Hz, respectively (mode 2 and mode 1). A second torsional mode of 4.426 Hz and a flexural mode of 6.104 Hz were recorded. 4. The new experimental campaign Based on the previous investigations conducted by Metra Lab, the critical aspect of the footbridge dynamic behavior was the first natural frequency, 1.5Hz in the horizontal plane. To increase this critical frequency, constraints were introduced to horizontal displacement on February 2, 2023, at the space that separates the deck from the hangers (approximately 20 cm). For this purpose, wooden blocks of adequate length and a section of about 15x15 cm were used (Fig. 4(a)).

Fig. 4. (a) Restraint with wooden blocks; (b) Comparison of horizontal displacement spectra before and after the introduction of the restraint.

Recordings were made using two seismographs, comparing the two configurations with and without the application of the wooden blocks as constraints. During the recordings (duration 10 minutes), pedestrians were advised to walk calmly to avoid influencing the measurements. The two seismographs were positioned at the midpoint of the footbridge, on the upstream and downstream sides. The recordings showed an increase in the frequency of horizontal displacements from 1.5 Hz to 2 Hz (Fig. 4(b)).

Fig. 5. Relevant points where measurements have been performed: numbers indicate thickness measurement locations, arrows the hardness ones. (a) downstream lateral side; (b) upstream parapet.

Subsequently, a non-destructive testing campaign on the steel was carried out to assess the corrosion status of the footbridge based on thickness and hardness measurements (Fig.5(a-b)). The thickness measurements were performed using the Phasor XS from General Electric. The instrument was used in conventional mode cabling it with an ultrasonic transceiver probe with a central frequency of 4 MHz for longitudinal waves. The instrument provides measurements with a sensitivity of tens of microns. To investigate the uniformity and homogeneity of the steel, we carried out about a hundred thickness measures. Thereafter, about thirty hardness tests were conducted on the same elements using the Leeb hardness test with an Equotip portable hardness tester and a type D probe. This test aimed to obtain an estimate of the tensile mechanical resistance of the steel based on the measurement of Leeb dynamic