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Suspension cable corrosion strikes Severn Bridge

WEIGHT RESTRICTIONS could be imposed on the M48 Severn Crossing by the end of the year after severe corrosion was discovered in suspension cables.

Severn Bridge owner Severn River Crossing examined the main suspension cables earlier this year after engineers discovered similar corrosion in Scotland's Forth Road Bridge in 2004 (NCE 13 April).

The Severn Bridge opened in 1966, two years after the Forth.

The Severn Bridge's 508mm diameter main suspension cables comprise 8,322 wires bundled into 19 strands. These are encased in red lead paste, wrapping wire and paint.

'The emerging outcome is that [the level of corrosion in] the Severn looks worse than the Forth. In terms of how significant this is will require a few more months of testing the wires, ' said Highways Agency senior bridge engineer Martin Lynch.

Lynch formed his view after an inspection of the north side main suspension cable. Contractor C Spencer is half way through an inspection at a second location and about to start at a third. He said that preparatory work had begun to correlate the relationship between cable corrosion and weight restrictions.

'It will be a few weeks before we know how much the capacity of the cables has been reduced, ' said Lynch.

Consultant Faber Maunsell is advising the Forth and Severn Bridges owners. It recommended that the Forth be closed to heavy vehicles from 2013 unless corrosion is curbed (NCE 17 November 2005).

An Agency spokesman said that the Severn usually operated well within its weight limit as most traffic crosses the river on the M4 Second Severn crossing.

'But it's possible, ' he said, 'that we may need to consider restrictions on the rare occasions when the M4 is closed and traffic is diverted via the M48'.

Next in line

The Humber Bridge is the next UK suspension structure to have its main cables inspected.

It is 15 years younger than the Severn -- and celebrating its 25th birthday this Monday.

General manager Peter Hill told NCE this week that he would bring forward internal cable inspections by 10 years to take place in the next five years.

'We're obviously interested in what has happened on the Forth and Severn as we all share the same type of cables.

'We're looking at historical construction data. So far our external inspections haven't revealed any unexpected damage, ' said Hill.

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