Multimodal flutter of a long-span suspension bridge in service and during construction

The flutter stability problem is addressed based on a realistic case study, namely the Julsundet Fjord Crossing, having a main span slightly longer than 1600 m. Both the service configuration and two different construction sequences are investigated. An aerodynamically efficient closed-box stiffening girder is chosen, and the preliminary design of the structure is carried out accounting for the available global and local loads as well as for the actual project constraints. In the construction phase, the evolution of the dynamical properties of the bridge that are relevant for the flutter stability assessment is closely inspected. The flutter results reveal that a multimodal approach is mandatory for an accurate prediction of the critical wind speed for this type of bridges. Indeed, bimodal calculations lead to nonconservative flutter estimates, and the work clarifies the reason for that. Although the study is limited to a null wind angle of attack, the required high safety factor against flutter instability is just barely guaranteed in service, while it is not during construction if the common strategy of deck erection proceeding symmetrically from midspan towards the pylons is followed.