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Dehumidifiers slow Forth Road Bridge cable corrosion

Corrosion of the Forth Road Bridge’s main suspension cables has significantly slowed down following the installation of a dehumidification system, according to an official report published this week.

Bridge operator the Forth Estuary Transport Authority (Feta) now believes it can avoid imposing weight restrictions from 2021 as had been planned.

“The results of the inspection appear to demonstrate that the rate of deterioration of cable strength has been reduced and the factor of safety against failure of the cables has not materially diminished,” said Feta chief engineer and bridgemaster Barry Colford in the report.

“This is giving strong comfort that the newly installed dehumidification system is retarding the corrosion of the bridge wires.”

Inspections in 2004 by consultant FaberMaunsell - now Aecom - revealed that corrosion induced wire breaks within the main cable could lead to a ban on lorries using the crossing by 2014.

Feta installed the dehumidification system on the bridge in 2009 but still anticipated that it would have to impose weight restrictions from 2021.

The latest report reveals that Feta does not expect to impose any weight restrictions during the serviceable life of the bridge.

“It’s a great success story as it shows dehumificiation can work on existing structures,” Colford told NCE.

Colford insisted that controlling corrosion on the existing crossing did not undermine the business case for the neighbouring Forth Replacement Crossing, which will bypass the existing bridge.

“The issues [with the bridge] are not just with the cable,” said Colford. “The deck joints and surface need replacing, which would require extensive closures.”

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