Abstract
Certain structures have the vibration modes of closely-spaced natural frequencies. The galloping behaviour of such a structure is studied. A cable-stayed bridge tower whose the natural frequency of the in-plane second mode is close to that of the in-plane first mode is employed as the case study. The structure is modeled as a linear two-degree-of-freedom system with proportional damping, and nonlinear quasi-steady wind force is assumed. An asymptotic modal analysis on galloping is conducted. The results indicate that the galloping of this tower is the steady-state motion either in one of the two modes, depending upon the initial disturbances, and that the coexistence of two modes in galloping, i. e. multi-mode galloping, is unstable. However, for structures with certain properties, e. g. unsymmetrically distributed mass, only the multi-mode galloping is stable. These results agree well with the observations in wind tunnel experiments.
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