Dynamic instability of suspension bridges

Suspension bridges are long, slender flexible structures which have the potential to be susceptible to a variety of types of wind-induced instabilities, the most serious of which are divergence (due to stationary wind forces) and flutter (due to aerodynamic forces). Flutter occurs at certain wind speeds where aerodynamic forces acting on the deck feed energy into an oscillating structure, so increasing the vibration amplitudes. If this situation is approached the basic safety of the bridge is threatened. This paper describes a computational method for predicting flutter speed based on a modal technique. A selection of the lowest vertical and torsional natural mode shapes is included with the aerodynamic forces in an interaction analysis which yields an unsymmetric matrix eigenvalue problem, the roots of which indicate if flutter is possible. The paper addresses the question of how the degree of refinement of the basic structural model and the number of natural modes included in the interaction analysis affect the flutter speed predictions.