Owing to the complex nature of flow over bluff bodies, aerodynamic forces and moments on structures can only be expressed empirically in terms of coefficients which are usually measured in wind tunnel tests on models. Substituted in the equations of motion of a bridge, they enable some insight to be gained into the behaviour of the bridge when exposed to the action of wind. Three such theoretical treatments are considered at some length in this report. In the first, the conditions required for the occurrence of various forms of aerodynamic instability are obtained and also the response to buffeting or forcing by turbulence. In the second, a number of contributions to stiffness and the corresponding natural frequencies are derived for suspension bridges. In the third, turbulent forcing is the main theme of the treatment. A simple account of the various forms of aerodynamic instability of bridges is given in an appendix which also contains explanations of principal terms used in the text.

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