Natural Frequencies of Long-Span Suspension Bridges Subjected to Aerodynamic Loads

We investigate the effects of steady aerodynamic loads on stability and natural frequencies of long-span suspension bridges through a simplified analytical model. The single (central) span suspension bridge model is considered, and the linearized integro-differential equations describing the flexural-torsional deformations of the bridge deck-girder are adopted as starting point. Thus, taking into account the second-order effects induced by a constant transverse wind in the bridge equations of motion, we derive a generalized eigenvalue problem in which all configurations intermediate between those of pure lateral-torsional buckling, pure torsional divergence, and pure free vibrations can be investigated. We show that the natural frequencies of a suspended deck-girder depend upon the mean (quasi-static) wind loading. As a consequence, the input parameters to the aeroelastic stability analysis result affected by that dependence, suggesting the possibility of modifying the dynamic stability analysis in order to take into account the mentioned influence. Based on this fact, possible implications for the flutter analysis of long-span suspension bridges are discussed.