MODELLING AND IDENTIFICATION OF THE DYNAMIC RESPONSE OF A SUPPORTED BRIDGE

This paper describes the dynamic tests performed on a simply supported bridge in Northern Italy under traffic excitation. The acceleration data have been used for the identification of the natural frequencies, viscous damping ratios and mode shapes of the bridge. Modal parameters have been extracted using the wavelet estimation technique, previously implemented by the authors of this paper. This work represents the first attempt in using the wavelet estimation technique directly on transient data and not on the impulse response estimates obtained via the random decrement technique. The capability of the wavelet estimation technique for extracting modal parameters from transient time responses has first been inspected by analysing a simulated set of data. The data have been obtained from the analytical continuous model of a three-span supported bridge. The bridge, excited by moving vehicles, has been modelled as a supported orthotropic plate and its response has been evaluated using the convolution technique. The vehicles have been modelled as multi-body systems, with linear suspensions and tyres flexibility, having globally seven degrees of freedom. An iterative procedure to include the dynamic interaction between the bridge and the vehicles has been implemented. The real bridge, 20 m long approximately, has been monitored using six capacitive accelerometers, measuring the accelerations in seven points of its north edge in two points of its south edge. In particular, the accelerometers on the south edge have been kept in fixed positions, acting as reference points. On the north edge, one accelerometer has been kept fixed at the mid-span location, while the remaining three have been positioned in two different set-ups. Each test has been repeated four times. The estimation of the modal parameters has been performed three times, using as reference point each of the fixed accelerometers. The results obtained from each estimation have been evaluated by means of a modal estimation `quality indexʹ introduced within the wavelet estimation technique.