An experimental study of the vortex-induced vibration of a slender box-girder bridge deck section

Abstract

This thesis examines the vortex-induced vibration (VIV) of a very slender box-girder bridge deck section. In particular, the dependance of the response and excitation mechanism on the geometry of the bottom knuckles is investigated. This study extends previous research into VIV of bridge decks by considering a very slender box-girder with chord to depth ratio of 11.2. To the best of the author's knowledge, this study is the first to investigate experimentally the VIV of such a slender section.

The behaviour of a slender box-girder in VIV was investigated experimentally, using small-scale physical modelling techniques in a wind tunnel. The excitation mechanisms are studied using pressure taps around a cross-section and several spanwise locations. Accelerometers measure the response. The findings demonstrate that the excitation mechanisms for heave and pitch responses are different (for values of wind speed, divided by frequency and chord, ≤ 1.6). The heave is excited by trailing edge vortices and shows a significant sensitivity to the geometry at the bottom knuckle. For shallow angles ≲ 20 degrees, between the base panel and the lower inclined panels, the trailing edge shedding is sufficiently weakened such that no response is observed. In contrast, the pitch response is excited by a combination of trailing edge vortices and motion-generated impinging vortices, where the frequency of shedding is double, triple and quadruple the oscillation frequency. The mode of the leading edge vortices is sensitive to the bottom knuckle angle and higher modes produce larger vibrations. A critical angle of 20 degrees results in minimum response for the bare deck (only beating angular displacement is observed) and no lock-in.

The difference in excitation mechanisms for the degrees-of-freedom results in different reactions to geometrical and structural modifications, including the bottom knuckle geometry, road furniture and structural damping.