Oscillations at Severn Crossing

The important frequencies of whole structures and of machines, in general, are mostly less than 50 cycles per sec and rarely more than 500 cycles per sec.

The lowest frequencies of a system can, in fact, be quite small. A clothes line slung between two posts and having plenty of sag, for instance, may oscillate freely at only one or two cycles per sec

An oscillation of this sort was observed during the autumn of 1959 in the grid system of the Central Electricity Generating Board at the Severn Crossing.

The frequency concerned was unusually low, being of the order of 1/8th cycle per sec. The crossing has two large pylons just over a mile apart, supporting transmission cables of 1.69 in diameter. It was found that, provided it blew from the right direction, a moderate wind would make the cables sway with low frequency and large amplitude in such a violent fashion that cables normally spaced 27 feet apart actually touched, leaving broken strands and burn marks, as well as short-circuiting the electricity supply.

A probable explanation of this behavior was eventually found and a cure effected by wrapping the conductors with thin plastic tape thus altering  the geometry of the surface presented to the wind.

(Excerpt from the book – Vibration by R.E.D. Bishop, Cambridge University Press, 1965, page 29)

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