LAUSR

Abstract The damping ratio and frequency of vertically vibrating bridge deck section models under zero wind speed, extracted by the proposed piecewise fitting method from both vibration tests and fluid-structure interaction simulations, vary exponentially with vibration amplitude. In vibration tests, the damping ratio of a tri-box section model will be doubled when the top and/or bottom of the two gaps between the three boxes are covered by grilles. Air-induced damping is firstly identified as an important source of damping of a section model. The air-added mass also has a notable influence on the vibration frequency of section models. For three section models with shapes of tri-box, grille-covered tri-box, and closed single-box, the amplitude-averaged air damping ratios account for 48.5%, 67.5% and 55.1% of the total damping ratios of vibrating systems, respectively; and the amplitude-averaged added masses account for 3.8%, 4.3% and 12.8% of the model masses, respectively. Explanations for the dramatically different air damping and added masses of the section models are given based on the flow patterns of the surrounding air. A dynamic equation including nonlinear air damping and added mass is presented to accurately reproduce the whole procedure of vibration tests.