PSI - Issue 62

Francesco Mariani et al. / Procedia Structural Integrity 62 (2024) 955–962 Mariani et al / Structural Integrity Procedia 00 (2019) 000 – 000

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3.2 Ambient vibration test The dynamic behavior of the viaduct was experimentally investigated in 2022 through Ambient Vibration Tests (AVTs) (Chen et al 2017). Five distinct measurement setups, depicted in Figure 3(a), were considered separately to accurately determine the modal parameters of the structure, including natural frequencies, damping ratios, and mode shapes. The instrumentation comprised high-sensitivity (10 V/g) uniaxial accelerometers PCB393B12 and a data acquisition system, NI cDAQ-9188, equipped with four NI 9234 modules, providing a total of 16 channels. This configuration enabled handling acceleration measurements with a sampling frequency of 1653 Hz. To ensure a comprehensive estimation of the modal features, the duration of each record was selected to be significantly larger than 2000 times the expected fundamental period. Specifically, each acceleration recording lasted about 35 minutes. Parameter identification was conducted using MOVA software (García-Macías et al 2020). The acquired acceleration signals from the five different setups were detrended and resampled at 40 Hz before applying the Enhanced Frequency Domain Decomposition (EFDD) method and combined by the least squares method to extract the global modal features of the entire viaduct (Au, 2011). Five vertical global modes of vibration were identified as shown in Figure 3(b), and their mode shapes compared with the results of the Finite Element (FE) model. The consistency between numerical and experimental mode shapes were investigated through the definition of the Modal Assurance Criterion (MAC) matrix, shown in Figure 3(c). Specifically, these modes were retrieved into the model as modes number 3, 4, 6, 14 and 16.

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Fig. 3. (a) Setups of the AVTs campaign; (b) Results of the modal identification.

4. Numerical Finite Element model MidasGen software (CSPFEA Engineering Solutions) is employed to model the case study viaduct, utilizing approximately 320 Timoshenko beam elements to define its geometry. The structure incorporates two concrete classes, as outlined in the design report, one for the piers and abutments with nominal cubical strength of 30 N/mm 2 and another for the box girder with nominal cubical strength of 40 N/mm 2 . These strength values are converted according to the Eurocode 2 (European Committee for Standardization, 2005) to assign the concrete classes as inputs of the model. Specifically, a concrete class C25/30 is assigned to piers and abutments while a C32/40 is chosen for the girder. As highlighted in the previous Section 3.2, the results of the compression tests on the cores are more than 20% lower than expected. Consequently, a statistical sample is established by analyzing the model with consideration for 12 different values of the Young’s modulus , summarized in Table 2. These values range from a maximum equal to the design value to a minimum equal to the lower one determined by the experimental tests. The prestress system is incorporated into the model by considering the information available from the design drawings.