Abstract
Vibration modal testing was carried out on a 130-year-old metallic riveted bridge, which was taken out of service, to estimate its modal properties under two different boundary conditions. A crane was used to lift the bridge, allowing it to move freely under the excitation load. An impulse hammer was used to excite the structure and a total of nine accelerometers were used to record the acceleration time histories. The response of the bridge, which is comprised of two main girders and seven cross girders, was estimated with a roving hammer technique, where one impulse hammer was used to excite the bridge at multiple locations, thus leading to the estimation of modal properties for the whole bridge, as well as individual girders. The set of modal properties obtained from free-free testing was compared with that estimated from identical testing of the bridge while resting on end supports. This comparison allows the quantification of the effect of the support conditions both on the estimates of the modal properties and on the associated noise levels. Ultimately, the results of this study are aimed at the refinement of bridge numerical (typically Finite Element based) models.