After the Darwinday storms I went up the hill behind the house to look at the Coillte forest that was planted 35 years ago on Dreelan's farm. O'Manch had reported that there was quite a bit of wind-throw on the edge, including one tree that had blown down against the prevailing wind to lie across the path. I was surprised at how little damage there was - they say that the Sitka spruce Picea sitchensis plantation on White Mountain, 15 km South of our place, looks as if Hurakan god of storms had scythed it all down. This is predicable as native Sitka grows in extremely wet soils, and is genetically programmed for a very shallow root system: that's partly why it is so extensively planted on marginal land. What really surprised me however was the fact that several trees deep in the Dreelan forest had snapped off 5-10m up with no evidence of them having been struck by a falling tree. I suspected that this was due to harmonic resonance when the wind was gusting at just the right speed to treat trees of a particular height like an insufficiently elastic violin string. I've floated this idea past a number of tree chaps and nobody has said it is complete bollix.
thrilling newsreel footage of the collapse of the Tacoma Narrows Bridge in 1940. In that case, the wind was far less (about 65km/h) than the Darwinday storm but it was just the right speed to sustain and synergise the movement, like a child thrusting her legs back and forth to get a swing to reach the very sky. When the London Millennium Bridge was opened in June 2000 synchronous movement by large numbers of pedestrians caused a resonant lateral movement of the bridge which came as a total surprise to Arup the consulting engineers.
harmonic mass damper (see right hanging on its suspension cables) near the top of the building. This sphere is 5.5m in diameter! Dublin's Spire in the middle of O'Connell Street, known by local wags as the stiletto in the ghetto, although only 120 m tall, incorporates a mass damper so it doesn't bend itself out of shape in a breeze of the resonant frequency.
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