Chapter 23 Dynamic Testing on the New Ticino Bridge of the A4 Highway Elena Mola, Franco Mola, Alfredo Cigada, and Giorgio Busca Abstract The dynamic tests on the two separate viaducts constituting the bridge over the Ticino river offered the chance to start a thorough identification process consisting in numerical simulation, experimental activity and model updating, to gather a deeper knowledge of the bridge behavior. In addition, this series of tests has offered a good benchmark to prove the actual strengths and weaknesses of the Italian standards for dynamic testing. The two viaducts are identical and made up of a series of equal length spans, making modal identification a more complex task, since it has to be performed in the presence of dense spectra with a number of closely spaced peaks, belonging to quite similar mode shapes. Operational Modal Analysis (OMA) was performed and is scheduled to be repeated in the future to assess any eventual on-going damage. The paper will present the results of the experimental activity compared to the numerical predictions and the model tuning activity. Keywords Operational modal analysis • Composite bridges • Eigenvalue analysis • Model tuning • Monitoring 23.1 Introduction The study of the dynamic performance of civil infrastructures, such as bridges and viaducts, is a very important matter both in the academic research community and for the Institutions, since their security and reliability are deeply related to casualties, resilience and strategic performance in case of events such as earthquakes. The importance of correctly assessing the dynamic behavior of strategic infrastructures is reflected in most European normative documents, with different levels of detail, depending on the Country. In Italy, there are prescriptions enforced by the Italian Code, i.e. D.M. 14 Jan 2008 [1], for strategic flyovers, bridges or otherwise significant structures that need to guarantee increased performance levels in case of an emergency. For these cases, the design and construction validation protocol needs to include dynamic experimental analysis of some sort, through which at least the first natural frequency of the structure can be estimated and compared to that predicted by the numerical analysis during the design phase. Unfortunately, the current version of the Code does not specify what kind of dynamic tests must be carried out, but the only requirement is the identification of at least the first vibration frequency, and its comparison to the one predicted by the numerical model. For modal identification and model tuning to be effective, though, it is definitely necessary to identify and compare a full set of modal parameters and mode shapes, rather than one single value of frequency: in this way, thorough correlation and cross-correlation tests can be performed between the experimental and the numerical results. In the case at hand, the Owner of the highway A4, SATAP Spa, required a thorough experimental modal analysis of the Ticino Bridge, which was carried out by the staff of Politecnico di Milano. The experimentally dynamic properties were then compared to the numerical analyses carried out by the staff of ECSD Srl, in charge of the final design and construction validation. It is well known that modal identification can usually be performed in two ways: Operational Modal Analysis (OMA) [2] and Experimental Modal Analysis [3]. If the dynamic tests are mainly meant as a design validation tool, then operational modal analysis is often preferred. It provides sufficiently accurate information on the modal properties of the infrastructures with reduced global costs with respect to an experimental modal analysis requiring forced excitation. When using OMA, E.Mola ( ) ECSD Srl, Via Carlo Goldoni, 22 20129, Milan, Italy e-mail: elena.mola@ecsd.it F.Mola ABC Department, Politecnico di Milano, Piazza Leonardo da Vinci, 32 20133, Milan, Italy A. Cigada • G. Busca Mechanical Engineering Department, Politecnico di Milano, Via Giuseppe La Masa, 34 20156, Milan, Italy © The Society for Experimental Mechanics, Inc. 2017 J. Caicedo, S. Pakzad (eds.), Dynamics of Civil Structures, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-54777-0_23 187
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