Innovative configurations for long-span suspension bridges

This paper reports the results of a piece of research about long-span suspension bridges with multiple-box girder steel deck characterized by low drag coefficient and high aeroelastic stability. For this type of bridges, by increasing the span length, the contribution to the stiffness of the suspension cables becomes dominant with respect to that of the deck, so that the ratio of the frequency of the first torsional mode to the frequency of the first vertical bending mode approaches unity, while they both decrease. As a consequence, if the deck cross-section geometry does not allow single-degree-of-freedom torsional flutter, one would observe the onset of two-degree-of-freedom classical flutter at a relatively low wind speed, depending on the dynamic and aerodynamic properties of the structure. The research presented herein, proposes an innovative approach to the design of long-span suspension bridges by studying deck configurations with frequency ratios of first torsional to vertical bending modes susceptible to couple lower than unity. The aim is to investigate, on the one hand, the possibility to obtain a total inhibition of the classical flutter instability mechanism in the wind speed range of interest and, on the other, the potentiality of such a design to allow significant savings in the costs of construction, which is an aspect of primary importance for this type of structures.