Ambient and free-vibration tests to improve the quantification and estimation of modal parameters in existing bridges

The correct estimation of modal parameters has an important role in ensuring the structural reliability of existing bridges. Operational modal analysis provides rather accurate extraction of natural frequency and mode shapes, but the corresponding damping estimates are subjected to higher uncertainties. This paper reports the results of ambient and free-vibration tests performed on five different typologies of road and railway bridges: steel trusses, steel box, multi-girder reinforced concrete, masonry and reinforced concrete arch bridges. In one case, data acquired by a continuous structural health monitoring system over a period of 1 year are available. In another case, the passage of trains is exploited to perform free decays. The processing of ambient vibrations is performed through several frequency and time-domain identification techniques, whereas the measured free decays are analyzed by the logarithmic decrement method applied to the autocorrelation functions of the signals. Outcomes are compared, evaluating the quality of modal damping estimates and the accuracy of results. The influence on modal parameters extraction of structural typologies, length of acquired time histories and ambient noise, loading and environmental effects are studied. At the end, it is demonstrated that the estimation of modal damping is more reliable for flexible structures when SHM and free-vibration data are available.