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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Fig. 7. (a) FEM model, (b) first flex. horiz. mode at 1.29Hz, (c) second flex. vert. mode at 1.73Hz, (d) first tors. mode at 2.42Hz, (e) first flex. vert. mode at 2.47Hz, (f) flexural-torsional comb. mode at 3.43Hz, (g) third flex. vert. mode at 4.07Hz, (h) fourth flex. vert. mode at 6.65Hz.

Table 3. Experimental frequencies and FEM analysis comparison.

Metra Lab Feb.21 tromino

Metra Lab Feb.21 accelerometer

Metra Lab Oct.22 OMA

Mode description

FEM Feb.23

First flexural horizontal mode (translation along the Y-axis)

1,5

1,4

0,957

1,29

Second flexural vertical mode (rotation about the Y-axis)

-

-

-

1,73

-

-

2,510

2,42

First torsional mode (rotation about the X-axis)

-

-

2,600

2,47

First flexural vertical mode (translation along the Z-axis)

Flexural-torsional combined mode (rotation about the Z-axis)

3,48

3,4

3,402

3,43

-

-

4,426

4,07

Third flexural vertical mode (translation along the Z-axis)

Fourth flexural vertical mode (rotation about the Y-axis)

-

-

6,104

6,65

5. Results and discussion According to the indications provided in EN 1991-2, in paragraph 5.7: "In the absence of a significant response of the bridge, a pedestrian normally walking exerts the following simultaneous periodic forces: in the vertical direction, with a frequency range of between 1 and 3 Hz; In the horizontal direction, with a frequency range of between 0.5 and 1.5 Hz. Groups of joggers may cross a footbridge with a frequency of 3 Hz”. The dynamic tests carried out before this work showed frequency values close to the limit values in both the horizontal and vertical planes. The experimental graphs shown in Fig. 4(b) indicate that the introduction of constraints allows an increase in the frequency value in the horizontal plane without introducing significant changes in the vertical and longitudinal directions. In particular, the frequency in the horizontal plane increased from 1.5 to 2 Hz, while the critical limit was 1.5 Hz. It’s crucial to distinguish between vertical and horizontal deformability because the last one is associated with the pedestrian’s perception of oscillation when crossing the footbridge. The simulation of constraints was done in a simplified way, for sure with a deeper study of the constraint type, with the same behavior, a further increase in the frequency in the horizontal plane should be achieved. If the final decision is to keep the footbridge, also the behavior of the structure in the vertical and torsional plane should be considered to have a final solution, that fulfills all the requirements. 6. Conclusion and future developments The study aimed to highlight some issues resulting from the lack of maintenance, with a focus on the corrosion degradation and lock-in phenomena affecting the steel pedestrian bridge. The overall structure does not have critical issues. To extend the service life of the structure is possible the restoration of the protective coating and the