Abstract
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 preloaded
spiral strands undergoing planesection
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.
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