THE RACE to save the Forth Road Bridge’s corroding cables got under way yesterday when a system to pump dry air into the crossing’s supports was switched on.

Bridge bosses have fingers crossed the £7.8 million project will help preserve the 44-year-old structure, which each year bears more than twice the weight of traffic it was designed to carry.

Forth Estuary Transport Authority (FETA) hopes that drying out the cables will stop the thousands of wires which take the weight of the bridge from deteriorating.

More than 100 of the 23,236 five-millimetre thick steel strands stretched between North and South Queensferry are known to have snapped. Breakages are detected using an acoustic monitoring system.

If the dehumidification system fails, motorists would face the nightmare scenario of prolonged bridge closures.

FETA has looked into the option of strengthening or even replacing the main cables and the subsequent disruption to the roads network could cost the local economy in the region of £1 billion.

However, chief engineer and bridgemaster Barry Colford was confident the dehumidification equipment would work.

He said, “Dehumidification systems have been installed as a preventative measure on a number of newer suspension bridges in Japan and Sweden, but this is the first time dehumidification has been retro-fitted in an attempt to stop on-going corrosion on this scale.

“This is trail-blazing work, but I’m confident we’ve got the right team and the right technology in place to get the job done successfully.”

Dry air is now being pumped through about a quarter of the total length of the bridge’s two main suspension cables. The aim is to prevent further corrosion occurring by reducing the relative humidity within the cables to below 40%.

Since April last year, contractor C. Spencer Ltd has been wrapping the west cable between the two main towers in an airtight membrane.

Councillor Tony Martin, FETA’s convener, switched on the first of three dehumidification plants that have been installed on the bridge.

He said, “The full system will not be up and running until late 2009, and we will then need to give it about 18 months to work before we can measure how effective it has been.”