New deck configurations for long-span suspension bridges with high aeroelastic performance

摘要

The present works deals with the fluid-structures interaction of long-span suspension bridges with multiple-box decks. For this kind of structures, to guarantee a reliable safety margin with respect to the collapse due to aeroelastic instabilities is a design priority. As a matter of fact, by increasing the span length of modern suspension bridges, the ratio of the frequency of the 1st torsional natural mode to the frequency of the 1st vertical-bending natural mode with similar shape is very close to unity in still air. In these cases, if the cross-section geometry of the deck does not allow one-degree-of-freedom torsional flutter (e.g. in the case of streamlined multiple-box girder deck), one would observe the onset of self-excited divergent oscillations due to the coupling of the 1st torsional and the 1st vertical-bending modes (i.e. two-degree-of-freedom coupled flutter) at a more or less high wind speed, depending on the dynamic and aerodynamic properties of the structure. In this paper, an innovative attempt to achieve the total inhibition of coupled flutter mechanism in the case of long-span suspension bridges is presented. The basic idea is to avoid coupled flutter instability by inverting the critical frequencies (that is to obtain 1st torsional to 1st vertical frequencies ratios lower than one) after revising some geometric parameters of the structure (i.e. the suspension system and the deck). If it were possible and compatible with all the design purposes, the torsional frequency would continue to decrease as long as the wind velocity increases, and so it would depart from the vertical frequency further, instead of approaching it, causing coupled flutter instability. The preliminary numerical and experimental results seem to lead to the conclusion that the bridge solution proposed herein is feasible and can imply a significant reduction of construction costs of the deck.