Bridges--Vibration

When a vehicle crosses the grid section of an open grid or bascule bridge, a tonal noise is generated. The tonal character of the noise is a consequence of the periodic excitation of both the tire and the grid, that comes from the interaction between the vehicle tire and the periodic grid members. In this thesis, the parameters that control the level and frequency of the generated noise are investigated, with emphasis on understanding the contribution to the overall noise level from the vibrations of the grid. Field and laboratory measurements have been performed, together with analytical analysis, on sample grid designs. By determining the acoustic radiation efficiency of the grid, the noise contribution from the grid vibrations is estimated by combing the radiation efficiency with the field measured vibration levels. The results of this study show that the contribution from the grid is small compared to that which may be coming from the vibration of the tire. Without first reducing tire noise, structural modifications to the grid in the form of damping or acoustic baffles will not produce any significant noise reduction.

Wind loads on a bridge may be classified into two types: the buffeting loads and the self-excited loads. The research reported in this thesis is concerned with experimental determination of the self-excited loads in the frequency domain, in particular, their non-dimensional coefficients, called flutter derivatives. The experiments were conducted in a water channel with water substituting for air. Five bridge-section models of different shapes were tested, each of which was driven to move harmonically by linkages, and the forces on the linkages were measured to determine the fluid loads. A thin-plate model, simulating an airfoil, was also tested and the results were compared with those obtained from the thin airfoil theory. The setup of the experiments and data acquisition, processing and analysis are presented herein.