Main cable dehumidification

Works to install a dehumidification system on both cables were successfully completed before the target end date of 31 October 2009. Dehumidification of the west cable commenced in March 2008 and is producing the expected slow and steady fall in the relative humidity within the cable.  The dehumidification of the east cable commenced in September 2009 and it is expected to produce the same fall in relative humidity levels within the cable.

The first indication of the system's effectiveness became available in 2012 when the cable was re-inspected.


Following the discovery of corrosion within the main cables of the Forth Road bridge, engineers have now protected the cable with waterproof elastomeric wrap and fitted a dehumidification system in a bid to stem the rate of deterioration.

Dehumidification works by blowing very dry air into the cable at a very low pressure, with the aim of reducing the relative humidity to a level where corrosion cannot occur. This is a well-tried system for preventing corrosion of steel and is already in use in the anchorage chambers of the bridge. However, its application to main cables of suspension bridges is relatively new.

Similar systems are being fitted to new bridges to protect them from corrosion and retrofitted to existing bridges in Japan, Sweden and Denmark where corrosion has been uncovered but at an earlier stage than on Forth. A dehumidification system has also now been fitted to Severn Bridge following the work at Forth.

Whilst there is good reason to have confidence that dehumidification can slow down or halt corrosion it cannot of course restore strength loss and, in addition, there is no body of evidence yet available to allow an unconditional assurance to be given that it will prevent a further reduction in strength loss in the main cables at Forth.

The reasoning for this is that individual wires that have already suffered from corrosion are likely to contain some micro cracks along their length.  The number and depth of these cracks, and their potential to grow within the dehumidified cable is unknown.  Therefore, the determination of how many of them will in turn lead to wire breaks is unknown and, as loss of strength is directly related to wire breaks, the future strength of the cables is also unknown.  The data from the dehumidification and acoustic monitoring systems will be kept under review and along with future intrusive cable inspections, will  help provide evidence of the effectiveness of the dehumidification system.