Abstract
Synchronous modal oscillations, characterized by unisonous motions for all physical coordinates, are well known. In turn, asynchronous oscillations lack a general definition to address all the associated features and implications. It might be thought, at first, that asynchronicity could be related to nonsimilar modes, which might be associated with phase differences between displacement and velocity fields. Due to such differences, the modes, although still periodic, might not be characterized by stationary waves so that physical coordinates might not attain their extreme values at the same instants of time, as in the case of synchronous modes. Yet, it seems that asynchronicity is more related to frequency rather than phase differences. A more promising line of thought associates asynchronous oscillations to different frequency contents over distinct parts of a system. That is the case when, in a vibration mode, part of the structure remains at rest, that is, with zero frequency, whereas other parts vibrate with non-null modal frequency. In such a scenario, localized oscillations would explain modal asynchronicity. When the system parameters are properly tuned, localization may appear even in very simple models, like Ziegler's columns, shear buildings, and slender structures. Now, the latter ones are recast, but finite rotations are assumed, in order to verify how nonlinearity affects existing linear asynchronous modes. For this purpose, the authors follow Shaw–Pierre's invariant manifold formulation. It is believed that full understanding of asynchronicity may apply to design of vibration controllers, microsensors, and energy-harvesting systems.
