J. M. Cuschieri
Ctr. for Acoust. and Vib., Dept. of Ocean Eng., Florida Atlantic Univ., Boca Raton, FL 33431
When vehicles travel across the grid section of an open grid or bascule bridge, noise is generated which for most type of grid configurations has a very strong tonal characteristic in the frequency range between 150 and 250 Hz. To investigate the mechanism by which this noise is generated, and identify possible noise mitigation procedures, a series of measurements of both sound and vibration levels have been performed on 12 bascule bridges with different types of grid. The grid types can generally be classified into three configurations, two of which are of a rectangular or square shape, while the third configuration has diagonal members and is generally referred to as four-way grid. For the rectangular-shaped grids, both the vibration and the sound spectra exhibited a dominant peak at a frequency that scaled with the speed of the vehicle and a typical length of the grid spacing. For the four-way grid, the spectrum did not show a very strong peak, but was more broadly distributed in frequency. The general overall sound level was not much different from one bridge grid type to another. The grid is an open structure and its radiation efficiency in the 200-Hz region is very low. This makes the tire a potentially significant source of noise. [Work sponsored by FDOT.]