Time-vs. frequency-domain analysis of single/multiple-box girder bridge deck sections

Flutter instability is a phenomenon due to fluid-structure interaction and it is one of the problems of major concern in long-span bridge design since it can lead to the collapse of the structure. In case of not-very-bluff or quasi-streamlined sections the instability involves two modes (a vertical bending and a torsional one) which tend to couple when the wind speed increases, leading to divergent oscillations (classical flutter). Conversely, in case of bluffer cross sections usually the undamped vibrations concern a torsional mode only (single-degree-of-freedom torsional flutter). In both cases, at the present state of the art wind tunnel tests are absolutely necessary to correctly address the aeroelastic stability problems for bridges.