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Heavy weather

AIRPORTS:

Calculating the forces the mesh covering would put into the supporting structure was far from easy.

Designer Robert Benaim & Partners director Brian Bell explains: 'To calculate wind loading we consulted every available authority, to no avail. So we made intelligent guesses, then carried out wind tunnel tests to confirm them.'

Wind loading turned out to depend very much on the angle between the wind and the net - it became more 'opaque' as the angle increased beyond 90degrees. Benaim also assumed that a 3m height of litter would build up around the perimeter, turning the blocked mesh completely opaque. For snow loading a different approach was adopted.

'The standard cable ties that connect sections of the mesh together act as snow fuses', says Bell. 'In the rare event of a significant snowfall they would fail, dumping the snow and relieving the load on the structure.'

Hales is quite happy with this solution, suggested originally by cable- net subcontractor Bryden Ropes. 'If the fuses blow it would be a simple job to refix the ties,' Shutes points out.

Final calculations showed 32mm steel cable would be needed for the main cable net, with 12mm secondaries. 'This represents a factor of safety of 1.5 rather than the 8 or 10 that would be used for crane ropes and the like,' Bell adds. 'But with no fatigue problems to worry about, because our cables aren't constantly running over small diameter pulleys, this is acceptable for what is essentially a short life structure.'

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