Super long span bridges: wind-resistant design developments and open issues

Suspension bridges achieved spans exceeding 1500 m only in 1998 with the completion of the Akashi Kaikyo Bridge (Japan) and the Great Belt Bridge (Denmark). The most recent record has been obtained by the Canakkale Bridge (Turkey), with a main span of 2023 m. A slightly longer bridge is currently under construction in China and significantly greater spans are in planning, such as the one of Messina Strait Crossing, aiming for an impressive 3300 m span. In this discussion, we address three critical challenges associated with such ambitious designs. The first challenge centres on the significance of vortex-induced vibrations, particularly in multi-box girders. The prediction of these vibrations before construction relies on wind tunnel tests with scaled-down models characterized by inherent uncertainties. The second issue involves non-synoptic winds, such as downbursts. While these events are relatively small and short-lived, they have a non-negligible probability of occurrence on large structures and may induce damages and loss of serviceability, due to their intensity and the presence of vertical velocity components. Finally, we underline the importance of robust design practices, emphasizing the potential risks of relying solely on aerodynamic cross-section optimization without providing additional structural stiffness and damping resources in these critical structures.