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Suspension cable corrosion strikes Severn Bridge

WEIGHT RESTRICTIONS could be imposed on the M48 Severn Crossing linking England and Wales 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 (NCEI May).

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 signicant this is, it 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 Bridge owners. It recommended that the Forth be closed to heavy vehicles from 2013 unless corrosion is curbed.

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 trafc is diverted via the M48'.

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