Aeroelastic Instability of Suspension Bridges and the Contribution of Lateral Flutter Derivatives

The present study highlights the decisive influence of lateral float derivatives in triggering the aeroelastic instability of long-span suspension bridges. Indeed, these derivatives play a central role in the evolution of self-excited aerodynamic forces affecting the transverse dynamic behavior of the deck, particularly in cases of multimodal coupling. The analysis is based on an enriched modal formulation that takes into account vertical, lateral and torsional displacements, and enables precise estimation of the critical floating speed. Extensive parametric studies have been carried out to identify the most influential parameters on intermodal coupling and added aerodynamic damping. The role of each flutter derivative has been examined in detail, with particular attention paid to lateral derivatives, whose combined effect with self-excited drag is discussed. The methodology adopted is based on solving a nonlinear eigenvalue problem using an iterative numerical algorithm, simultaneously estimating bridge vibration frequencies and critical wind speed. The approach is applied to the case of the new Tacoma Bridge, where the most significant modes for multimodal instability have been identified. Finally, the influence of main span length is discussed, based on a bimodal solution base. The results are compared with reference data available in the literature, providing a robust validation framework for the proposed formulation.