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Cable driers to halt Forth Bridge corrosion

OPERATORS OF the Forth road bridge were this week considering whether to pioneer dehumidification technology to halt corrosion in its suspension cables.

This is thought to be the best short-term option for the Forth Estuary Transport Authority (FETA), ahead of complete cable replacement.

An interim report produced by consultants FaberMaunsell and Weidlinger, and published last week, recommends dehumidifying the cables.

This involves wrapping the cable in a neoprene membrane and pumping in dry air to remove moisture.

Engineers discovered severe corrosion on some of the wires in the 4km long suspension cables last year (NCE 6 January).

eading bridge engineer Jolyon Gill expressed doubts about the long-term effectiveness of dehumidification. He said halting corrosion this way would be 'virtually impossible'.

'Hermetically sealing something is virtually impossible.

Even a pinprick could begin to expand when air is blown through it, ' said Gill. He warned that creating an airtight seal around the junctions between cables and suspender clamps would also be difficult.

Dehumidification will cost £12M, but there is no certainty that it will work.

'It's never been done before and the air could just take one path and miss the rest of the cable. But FETA doesn't have many alternatives and will be the pioneers in the UK, ' said Gill.

He said that adding or replacing cables would have to be the long-term solution. This would cost up to £100M.

This Friday, FETA's board will consider the report, which also warned that if nothing is done by 2019 the bridge will have to close.

Ruby Kitching

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