EVALUATION OF A ROAD BRIDGE DYNAMIC RESPONSE TO AMBIENT EXCITATION BY WAVELET AND OTHER ESTIMATION TECHNIQUES

摘要

In this paper, a dynamic test performed over a bridge located in northern Italy is reported: the dynamic measures are analysed by means of four different techniques to seek for the best algorithm within those appropriate for the identification of dynamic systems excited by unknown input. The bridge under test, in fact, is excited by the flowing traffic and by some other unmeasured excitations as wind or other ground micro-movements; the analysis based on the flowing traffic excitation has shown to be extremely advantageous for the road contractors, i.e. in the case of motorway, because it can be adopted for monitoring in-use roads, without the need of traffic stop. The accelerometers data have been used for the identification of the natural frequencies, viscous damping ratios and mode shapes of the bridge. The modal parameters have been extracted using four approaches (ARMAV, WET, CVA and BR) and a comparison within the performances of the adopted methods is also given. A FEM model has been set-up by using 8 nodes brick elements based only on a rough sketch of the bridge: this was used to optimise the position of the reference accelerometers. Due to the limited number of points measured along the bridge, the extracted experimental shapes have been expanded into a larger number of points to accomplish a minimum set of locations coincident with the FEM nodes; this expansion has been performed by means of the Guyan expansion based on the numerical modes. The non-symmetric distribution of the updated stiffness matrix AK has shown the differences with the original model and a few comments are reported as possible explanations of the bridge real behaviour.