Long-span bridge flutter analysis in non-stationary downburst-like flows by Floquet Theory

Abstract

This study examines the research question whether bridge flutter instability can be triggered by thunderstorm downburst-like flows. The Floquet Theory is used to formulate a new analytically-based model, in which the loads are simulated using a transient periodic mean wind speed. This feature imitates the touchdowns of subsequent downbursts in the proximity of the deck at a relative distance $r$ corresponding to the maximum spatial intensification (equal to about $r/D_{db}=1.0$ with $D_{db}$ downburst horizontal core or “jet” diameter). The model accounts for the slow variations in the mean-wind flow velocity to evaluate flutter. Although this assumption may lead to a conservative estimation of the instability threshold, the study demonstrates that the critical flutter velocity is finite. Specifically, the model quantifies in closed form the minimum downburst intensity, i.e., the magnitude of the mean, horizontal along-wind velocity component, which causes the deck vibration to diverge. Two bridge models are used to illustrate the method: the Golden Gate Bridge and the Messina Strait Bridge (1992 design).