PSI - Issue 64

Antonino Maria Marra et al. / Procedia Structural Integrity 64 (2024) 2117–2124 Marra et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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4. Ambient vibration tests The measurements were carried out through accelerometric sensors installed on five cross sections of the deck, named from A to E (see Fig. 2). Acceleration time-histories of different lengths were recorded (approximately from 30 to 50 minutes) under ambient vibrations. Both piezoelectric sensors and MEMS accelerometers were employed. Only the measurements obtained through the wired sensors are considered in this study. Deformation time-histories of the anti-lift bars were also recorded through strain gauges, synchronously with the acceleration time-histories recorded through the piezoelectric sensors. In this way, the deformation of the anti-lift devices due to traffic-induced vibration can be evaluated and correlated to the acceleration on the deck. Fig. 3 shows the accelerometric sensors installed for full-scale measurements. The accelerometers were installed on the lower side of the I-girders (see Fig. 2 for in-plan location). Sixteen accelerometers were employed in total: eight wired piezoelectric sensors (PCB Piezotronics) and eight wireless MEMS sensors (BeanAir). Five sections of the deck were equipped with accelerometers, two of them (A and C) with both sensor typologies, one of them (B) only with piezoelectric sensors and two of them (D and E) only with the MEMS. The wired sensors are monoaxial so that three of them are necessary to fully describe the motion of a cross section (one on each girder along vertical direction, one horizontal), while the wireless ones are triaxial and one per longitudinal beam is sufficient to identify the deck section motion for the vertical, horizontal and torsional degree of freedom. Fig. 2 also displays the sensor typology for each installation point: c and w subscripts refer, respectively, to sensors with cable (the wired ones) and wireless, while V and R, respectively, to vertical and radial (horizontal) directions. The results of full-scale tests were employed to determine the modal parameters of the structure (natural periods and frequencies, modal shapes). In this section, a few preliminary results are reported, in terms of natural frequencies identification. Fig. 4 shows an outline of the degrees of freedom related to the accelerations recorded in the measuring point of the cross section. The time history of the vertical acceleration of the deck is evaluated as the average vertical acceleration value of the two steel girders ((v 1 +v 2 )/2), while for the torsion the difference between the vertical accelerations of the girders is divided by their distance ((v 1 − v 2 )/d, with d = 4 m). Finally, the horizontal acceleration of the deck section, along the in-plan radial direction, is equal to r 1 , according to Fig. 4. The power spectral density of the acceleration related to the vertical and torsional degree of freedom for the deck midsection is reported in Fig. 5. The first two natural frequencies of the bridge are indicated in the spectrum of vertical (Fig. 5a) and torsional motion (Fig. 5b): ,1 = 5.1 Hz for the first mode and ,2 = 10.1 Hz for the second one (natural periods of 0.196 s and 0.099 s, respectively). According to the preliminary finite-element model, the first mode is expected to be related primarily to bending motion, while the second one to torsional motion. This seems to be confirmed by the full-scale measurements, since spectral peak at frequency ,1 is larger than the one at ,2 in the vertical motion spectrum, while the opposite result is found for the torsion spectrum. In any case, the intensity of two peaks is comparable in both spectra, without any clear prevalence of one of them, due to the curved layout of the deck under investigation and the consequent interaction between flexural and torsional motion.

Fig. 2. Plan view of the deck with the location of the sensors.