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Corrosion reduces strength of Forth Road Bridge cables by 10%

The main suspension cables on the Forth Road Bridge have now lost about 10% of their strength due to corrosion, bridge owner Forth Estuary Transport Authority (FETA) revealed today.

This is less than anticipated and means that weight restrictions on the bridge will only apply from 2017 now rather than 2014.

Consultant Faber Maunsell carried out the first cable inspection in 2004 which revealed that the main cables had lost about 8% of their strength.

If this level of deterioration had continued, then lorries would be banned from crossing the bridge from 2014. With the second inspection concluding only a further strength loss of 2%, weight restrictions will now be delayed.

"The good news is that, although the cables are still losing strength, the worst case scenario of an HGV ban in 2014 now looks unlikely," said FETA chief engineer & Bridgemaster Barry Colford.

"The rate of deterioration appears to be at the more optimistic end of previous predictions."

The new inspection findings have also provided an updated benchmark against which to measure the success of efforts to halt corrosion.

A dehumidification system is currently being installed, which aims to prevent further corrosion by reducing the relative humidity within the cables to less than 40%.

A third inspection will be carried out in 2011/12 to verify the effectiveness of dehumidification.

"The bridge's long term structural integrity also depends on the outcome of precautionary checks on the bridge's anchorages," added Colford.

"We won't know the results of this work until 2011/12 either."

Consultant Flint & Neill will be carrying out an independent check of Faber Maunsell's findings.

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