Wind-Induced Lock-In Excitation of Tall Structures

Circular and square tower models were tested in a boundary layer wind tunnel. At close to the critical reduced velocity and particularly at low values of structural damping, displacement dependent lock-in excitation was found to cause large increases in cross-wind displacement response, especially for the circular tower, in a suburban type wind model. Critical cross-wind response amplitudes were determined from the response characteristics. These response amplitudes represent the level of cross-wind response of a structure above which the response can not be accounted for by the random wake excitation process and that lock-in excitation is significant. A prediction procedure, which consists of a random excitation model and a sinusoidal lock-in excitation model, is described. Cross-wind response predicted by these models agreed well with the measured response.