Modal coupling assessment and approximated prediction of coupled multimode wind vibration of long-span bridges

While single-mode analysis is simple and straightforward in the prediction of wind-induced responses of long-span bridges, it has adequate accuracy only for bridges with a weak modal coupling. With the increase of bridge span lengths and the adoption of streamlined cross-sections, aeroelastic modal coupling due to the interaction between the bridge and wind flow has become an important aspect of vibration. To more accurately predict wind-induced vibrations for modern long-span bridges, coupled multimode analyses of buffeting and flutter are usually required. The common approach to predicting coupled multimode response is to solve simultaneous equations with selected modes. However, quantitative information about modal coupling effects is not available in the process of selecting modes. Consequently, people can only choose much more than necessary modes to avoid missing any important ones in the coupled analysis. The objective of the present study was to quantify the modal coupling effect and provide a practical methodology to predict coupled multimode vibration without actually solving the coupled equations of motion. To achieve the objective, a closed-form spectral solution was derived by ignoring high order small coupling effects, while keeping the primary coupling effects in the solution. The modal coupling effect was then quantified using a so-called modal coupling factor (MCF). The MCF facilitates key mode selection and simplifies coupled multimode analysis. A prototype bridge was analyzed.