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Severe corrosion found on Forth Bridge cables


OPERATORS OF the Forth Road Bridge said this week they were about to install acoustic monitoring equipment on its two cables after discovering extensive corrosion damage.

At least £3M of remediation work is likely to be needed to repair the 4km of steel main suspension cables on the 2.5km long, 1km central span structure.

Bridge operator, the Forth Estuary Transport Authority (FETA), ordered a £1.2M examination of the 40 year old bridge's 600mm diameter cables in March last year.

It became concerned after American engineers reported corrosion problems with cables in suspension bridges in the US (NCE 11 December 2003).

FETA and consultant FaberMaunsell have already examined cables at four low level locations and will open four high level locations in the next three months.

'You can see cracks in the paint [in the upper parts of the cables] which have let water in. It has worked its way down to the low points, ' said FaberMaunsell technical director Charles Cocksedge.

The study involves removing eight 5mm diameter high strength internal wires from the 11,618 which make up each cable, and splicing a replacement wire at each location.

Rusting has occurred even though the cables are continuously maintained and repainted by FETA.

'We expected it [the cable] to be in excellent condition and were disappointed with what we found. A few [broken wires] have been found - we could be teetering on a very slippery slope, ' said Cocksedge.

FaberMaunsell has recommended that FETA install tiny microphones along the cables to detect breaking wires. The £300,000 contract is out to tender.

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