LATERAL VIBRATIONS OF STEEL CABLES INCLUDING STRUCTURAL DAMPING

The paper provides theoretical means of predicting structural damping of axially preloaded spiral strands undergoing lateral vibrations due, for example, to vortex-shedding. In line with the previously reported experimental observations and theoretical studies, it is assumed that frequency-independent cable damping arises from the frictional energy dissipation between the individual wires in contact. For pin-ended and axially pre-loaded spiral strands undergoing plane-section bending, it is now possible to show significant variations of the equivalent damping ratio with the type of strand construction details, length of the cable and mode of vibration. This is in contrast to the traditional approaches, which have invariably assumed a constant damping ratio based on rather limited (and sometimes even questionable) experimental observations. Frictional damping ratio is shown to decrease significantly with increasing length of the cable. The theoretical model also suggests that substantially higher damping ratios should be adopted for higher modes of lateral vibration of cables whose construction details (especially their lay angle) can also influence the damping predictions. Some previously published empirical formulations based on large-scale experiments on overhead transmission lines are found to provide encouraging support for the proposed theoretical model. The Paper also critically addresses the possible practical limitations of the model in the light of some related large-scale axial and/or torsional experimental observations on steel cables. The present theoretical model provides a more rational insight into the mechanism of damping in lateral vibrations of cables in various applications. In particular, the practical implications of assuming a constant damping ratio (irrespective of the mode of vibration and type of cable construction) for obtaining estimates of maximum amplitudes of vibration under vortex-shedding instabilities, are critically addressed.