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Load off your mind


Re your feature on Stonecutters bridge in Hong Kong (NCE 12 May), the vortex shedding phenomenon, which rightly is a resonance problem, has nothing to do with strong winds or 'large wind loads'. The opposite is the case as vortex shedding generally occurs at moderate wind speeds with a more regular and steady wind.

The forces themselves associated with the 'slipping' vortices are of no concern structurally as these forces are very small. The concern is one of resonance - the vortices are sent off at regular intervals the frequency of which will depend on the wind speed. If the frequency coincides with one of the vertical frequency of the deck then, even for the small forces encountered, the deck can start vibrating in the relevant mode.

Although these vibrations generally are of no structural consequence, they are unacceptable from a user's point of view and might cause discomfort as quiet large amplitudes can be reached.

Also the guide vane is not trying to 'guide the wind away from the structure' rather the opposite, to guide the wind along the structure ie we do not expect vortices to form with the guide vane installed.

Klaus Falbe-Hansen

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