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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euros, including the stabilization of the portals with horizontal H profiles, stiffening of the deck with additional bracing both vertically and horizontally, and the application of a protective coating to the most damaged areas of the structure. The purpose of the investigation commissioned in February 2021 to Metra Lab Srl was to assess, through static and dynamic tests, whether the mechanical behavior of the structure ensures safety conditions for pedestrian and cyclist transit. The tests included the following: static load test, measurement of deformations during the load test using 20 strain gauges placed at significant points of the structure, and recording the fundamental frequencies of the structure in both vertical and horizontal planes (see Fig. 3(a)). For the static load test on the deck, six 6x3 m tanks were positioned for trial testing with a total load of approximately 300 kN. Considering the assumption of uniform distribution, this load was divided across each vertical cable, resulting in an intensity of approximately 18.75 kN for each vertical cable. Deformation measurements were made using digital centesimal levels and Invar digital stadia with barcodes. At maximum load, the deflection of the deck at the midpoint averaged approximately 43.5 mm, and the displacement at the hangers was approximately 34.4 mm. During unloading, at the step of 15.5 MPa (half of the maximum applied load), adjustments were made to 8 vertical cables to equalize the tensions. Considering the limited levels of traction detected experimentally on the vertical cable, the footbridge exhibited elastic behavior during the load test. For deformation measurement, a strain gauge was fixed at the lower end of each vertical cable, and four strain gauges were also fixed in the plates where the bearing cables anchor. All the measurements were acquired using an electronic signal conditioning unit with a Wheatstone bridge configured at 1/4 bridge. The tensile force recorded on the most stressed cable (Fig. 3(d)) was approximately 55.0 MPa. This cable is close to the truss column identified as A in the planimetric diagram, Fig. 3(a). The vertical cables, subjected to a tensile load of approximately 18.75 kN, showed a tensile stress of about 38.2 MPa, relatively low values. The use of strain gauges allowed adjustments during unloading, at the step of 15.5 MPa (half of the maximum load), to make the tensile stress on the hangers as uniform as possible.

Fig. 3. (a) Cycle-pedestrian planimetry with test ubication and photos of the tests, (b) OMA analysis, (c) seismograph spectrum, (d) spatial and temporal evolution of deformations detected with strain gauges.

This expedient certainly helped the structural rearrangement of the footbridge. To determine the fundamental frequencies of the structure in both the horizontal and vertical planes, seismographs and accelerometers were utilized. Measurements with seismographs (Fig. 3(c)) were conducted before and after the load test and the subsequent hanger adjustment. The observed frequencies were 3.48 Hz in the vertical plane and 1.50 Hz in the horizontal plane. In October 2022, an operational modal analysis (OMA, Fig.3(b)) was conducted to verify the frequencies observed during the previous investigation campaign (Metralab, 2022). Eight triaxial piezoelectric PCB accelerometers were connected to a Krypton dynamic data acquisition system. During the test, 8 accelerometers were applied according to two configurations. A frequency of 0.957 Hz was recorded, associated with the first mode of flexural vibration in the