Innovative solutions for long-span suspension bridges

The present work deais with fluid-structure interaction of long-span bridges with multipleebox deck. These flexible structures are very sensitive to wind-induced loads, within both serviceability and ultimate limit states. In particular, it is extremely important to guarantee a reliable safety margin with respect to the collapse due to flutter. This aim can be pursued by improving the system stability from the aerodynamic point of view: multiple-box girder decks are known to be an effective solution in this direction. Another common way to delay flutter instabilit is to increase the frequency separation, thus working on the structural dynamics side of the problem. An innovative solution would be the total inhibition of the flutter instability mechanism, at least conceming the lower modes, that is in the wind speed range of interest. This achievement could be very useful for future very-long-span bridges, for which any traditional countermeasure against flutter could have limited effects. The basic idea is to avoid classical flutter by inverting the vertical bending and the torsional natural frequencies of the lower modes with similar shapes (that is to obtain torsional-to-vertical bending frequency ratios lower than one). As a matter of fact, if this was possible and compatible with all the design constraints, the effect of fluid-structure interaction would be the reduction with the wind speed of the torsional frequency and, at the same time, the increase of the vertical bending frequency: the modes would tend to further separate instead of coupling and consequently they could not give rise to classical flutter. This result is theoretically well known but completely unexplored in practice. A feasibility numerical study has been the first step in this direction, in order to understand if it is possible to conceive a reasonable structure with these characteristics.