Drying the Forth Road Bridge

The Forth Estuary Transport Authority (FETA) has awarded C Spencer and Bennett Associates a £7.8m contract to install dehumidification equipment on the main cables of the bridge.

C Spencer and Bennett Associates were involved in the unwrapping and original internal investigation of the cables of the ForthRoadBridge in 2004. The inspection revealed that the cables had suffered significant corrosion. The current loss of strength, as verified by a Scottish Executive-commissioned independent audit, is believed to be in the region of 8 to 10 per cent.

The discovery of corrosion in older suspension bridges in the USA prompted FETA to become the first European bridge authority to carry out an internal cable inspection.

A package of measures was put in place by FETA to monitor the corrosion and minimise further deterioration. Dehumidification was quickly identified as the best line of defence against further corrosion.

C Spencer and Bennett Associates will install equipment that will inject dry air into the main two cables of the ForthRoadBridge. Dehumidification works on the principle that if the relative humidity of the air within the cable is reduced to 40 per cent, then corrosion cannot occur. This particular method is already used in Japan and Sweden where dehumidification systems are fitted to newer bridges as a preventative measure. This is the first time that a dehumidification system will be retrofitted in an attempt to stop ongoing deterioration of the cable.

Bennett Associates will design the temporary works and gantries to enable equipment to be moved around the structure whilst works are being carried out. They will also provide the mechanical design for the injection system exhausts.

C Spencer and Bennett Associates are currently working on preparatory design work on the dehumidification system and access platforms. The work itself is due to begin in April 2007 and installation is expected to be complete by September 2009.