Operational modal testing and analysis of a long span footbridge

Within the reception tests of a long span footbridge, over-passing the A13 highway near Bologna (Italy), dynamic tests were performed, with the excitation being mainly provided by the micro-tremors, wind and traffic in the neighbouring highway. Although no serviceability issues were observed, the vertical dynamic characteristics of the bridge turned out to be quite complex, with several couples of modes exhibiting closely spaced natural frequencies. The paper mainly focuses on the application to the data collected on the footbridge of different output-only identification techniques, ranging from the classic peak picking technique to the more recent frequency domain decomposition and stochastic subspace identification methods. Hence, the reliability of the different techniques, especially in the correct identification of the dynamic characteristics of the closely spaced modes, is investigated.