PSI - Issue 57

Qingyang Wei et al. / Procedia Structural Integrity 57 (2024) 262–270 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Proposed strategies

3.1. Local vibration modes

In dynamic analysis of bridges, usually the lower-order modes (global modes) reflect the global information of the bridge, and the higher-order (local modes) modes reflect the local information of the bridges. The lower-order modes are easily obtained while the higher-order modes are difficult to excite. Many studies have shown that lower-order modes of structures are insensitive to local damage,while higher-order modes are more sensitive if a certain higher order mode is related to the local damage (Wei et al (2021), Huth et al (2005)). Therefore, the local cracks in the vertical ribs will not have significant effect on the global dynamic properties of the entire system, whereas it will have a larger effect on the local vibration properties of the vertical ribs. The more feasible method to obtain the local vibration information related to the vertical ribs by the combination of the local impact and local measurement on vertical ribs. In this case, the local structure, the vertical ribs are emphatically investigated, and the global frequencies of the vertical ribs are examined, which are also the local frequencies of the entire steel bridge. The investigated structural detail is subjected to normal and shear stresses under operation. The stress fields around the crack will be changed once the crack appears and propagates. Moreover, the stresses on two sides of the crack will be highly irrelevant due to the discontinuity caused by the crack. Therefore, it is a feasible method to focus on the relationship between the opposite sides of the crack surfaces, i.e., the above and below sides of the vertical ribs. Potentially, long-term monitoring of the correlation between stress variations on opposite sides of cracked sites will be useful for monitoring the development of existing cracks. 3.2. Local strain responses

4. Results and discussions 4.1. Local vibration modes

As shown in Fig. 3, the vertical rib is impacted by the instantaneous force alongthe normaldirection ( x -direction ) of the plate. An arbitrary node on marked with ‘node 1’ the vertical rib is selected and the acceleration responses of this node in the process are extracted. The acceleration responses in the x , y , and z -directions are sampled by 1000 Hz, as shown in Fig. 4 (a) – (c), respectively.

Fig. 3. Layout of the excitation and measurement.

The free vibration acceleration in the x -direction decays faster than the accelerations in the other two directions within the same 0.3 s duration, indicating that the free vibration acceleration in the x -direction contains the higher frequency vibration information, while the other two directions contain more low-frequency vibration information induced by secondary vibration.