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Spoiling for success

Congratulations to David Piesold and his research team on their innovative mechanical solution to active aerofoil stabilisation for megaspan stayed or suspension bridges (NCE last week). This appears to be a much more direct and reliable system, with implied savings in construction and running costs, than the complex earlier proposal to fit individual aerofoil dedicated sensor, computer and actuator units.

I would add that Piesold's original application in 1992 to the ICE Research and Development Enabling Fund described an even more complex idea. Not only were the aerofoils to be retractable to provide leading and trailing edges according to the wind direction but 'strain adjusters' were also proposed for the cables or hangers to obtain an optimum distribution of load under various loading conditions. All these functions were to be computer controlled. There was even mention of spoilers and other devices used in modern aircraft.

I recall canvassing various bridge engineers at the time about these novel and complex proposals. One aerodynamics expert was dismissive of the prospect of a major bridge relying on computer control. He had just returned from a bout of long-haul flights and I took the opportunity to remind him that he had no such fears in flying in even more complex structures, which were computer controlled.

Several interesting questions are posed by the aerofoils. How would they interact with deck wind shielding - surely essential for megaspan crossings? Second, could they be used to benefit during the wind-critical erection stage, probably involving cantilevering, by erecting deck sections with the aerofoils already in service?

Third, could width and weight be saved by using continuous strings of aerofoils liked by flexible joints to form the walkways? These could be closed except for emergency use during the lively movements generated by high winds, or perhaps opened, at a fee, to seekers of funfair excitement.

Brian Pritchard (M), 44 Denmans Lane, Lindfield, West Sussex RH16 2JR

Flight deck

Your article on the David Piesold's aerodynamic bridge concept developed by David Piesold (NCE last week) was an excellent example of the 'creative swiping' of other technologies as encouraged by the recent ICE/DETR project on technologies for export success.

It was particularly welcome because in 1992 the ICE R&D Enabling Fund provided seed-corn funding for the initial research on aerodynamic bridges. I recall the Trustees' enthusiasm for Piesold's project as a piece of truly innovative thinking which held out the possibility of a significant advance in civil engineering technology. We must hope that a sufficiently enlightened client will now be willing to put the technology to work.

This is what the ICE R&D Enabling Fund is for.

Mike Thorn (F), chairman ICE Research and Innovation Committee, ICE, Great George Street, Westminster, London SW1P 3AA

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