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Forth cable dehumidification system fully operational

The operators of the Forth Road Bridge last week announced that the drying out process has begun on all sections of the bridge’s corroded main cables, after installation of a dehumidification system was completed last Thursday.

The dehumidification system aims to prevent further loss of strength in the cables by reducing the relative humidity inside them to a level where corrosion cannot occur.

The project has been completed on schedule by contractor C Spencer Ltd supported by Forth Road Bridge’s in-house maintenance team. Work began in April 2006, using specially designed platforms to move up and down the cables and wrap them in a watertight and airtight membrane. Dehumidification equipment was then installed to blow very dry air into the spaces between the 11,618 individual steel wires that make up each main cable. Sensors monitor the exhaust from the system in order to measure the fall in humidity within the cables.

Corrosion in the main cables was discovered during the first internal inspection of the cables in 2004. This inspection was carried out following the discovery of similar problems in older American bridges and was the first such inspection to be conducted outside the USA.

Chief Engineer & Bridgemaster at Forth Road Bridge Barry Colford said: “This was a major project, made more complicated by the need to work at height above the carriageway without restricting traffic for any length of time. C Spencer Ltd have done an excellent job to complete the project on schedule.

“The cables must now be given time to dry out. Experience from Japan, where the technology was developed and has been installed on newer bridges, suggests this could take up to 18 months.”

The first indication of the system’s effectiveness will be available in 2011/12 when the cable is re-inspected. However, the Chief Engineer has warned that there will always be some degree of uncertainty as to how much strength the cables will lose in future, as a result of the corrosion that has already occurred.

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