Capital projects:

In 2004 the Forth Road Bridge became the first suspension bridge in Europe to have its main cable opened up to check for signs of corrosion. Although the main cable showed no exterior signs of deterioration, the concern was that corrosion might be present inside – as had recently been discovered in the cables of older American long span suspension bridges.

When the cable was opened up, the bridge authority was surprised to find that 8-10% of the cable’s strength had been lost as a result of corrosion – despite the cable at that time being just 40 years old. Although such a loss is significant, it hasn’t necessitated traffic restrictions. It is crucial, however, that the corrosion is halted to prevent this happening in the future.

Engineers have now protected the cable with waterproof elastomeric wrap and fitted a dehumidification system in a bid to stem the progress of the deterioration. Dehumidification has already been used on the Forth Road Bridge in other areas and on in the main cables in of newer bridges in Japan and Sweden, but this is the first time it has ever been used inside a main cable that’s more than four decades old.

Three panels on the east cable were opened in early 2008 (two of which were opened previously in 2004) and, while the main cable was found to have suffered further strength loss due to corrosion, the rate of loss of strength was not as great as had been predicted in 2004.

This second internal inspection in 2008 indicated that the envelope of the predicted future strength of the cable, determined from the first inspection, was still valid.  However, the second inspection also indicated that the timescale when intervention, in the form of loading restrictions, might be required now extended from 2014 to 2021 as results showed that it appeared more likely that the rate of deterioration was predicted to follow the more optimistic line.

Therefore, from the 2008 results it was considered more likely that any restriction in loading would not have to be considered until between 2017 and 2021.

Further internal inspections have been allowed for in the Capital Plan, the third inspection is now programmed to start on site in summer 2012.  The process for carrying out this project will involve advertising in the Official Journal of the European Union (OJEU) to engage the services of a consulting civil engineer.  Following this, the procurement of the contracting services necessary to carry out the work on site will also have to be undertaken using the OJEU.  It should be noted that should the relative humidity within the cables fall more slowly than expected then the proposed date of the third inspection may have to be delayed.

The third internal inspection of the main cable will allow another point to be plotted on the cable strength loss versus time graph and be the first indication of the effectiveness of the dehumidification system, but further points in the future, post 2012, will need to be obtained in order to further increase levels of confidence in the capacity of the cables. It is almost certainly the case that some degree of uncertainty concerning the magnitude of future strength loss will remain and the cables will require to be continually monitored, and subject to a regime of internal inspections and strength evaluations, for the remainder of their service life.

Related documents:

• Main cable and anchorages update (17 February 2010)
• First internal inspection of the main cables - progress report (27 April 2006)
• Flint and Neill Partnership Technical Audit of the Main Cable Inspection and Assessment of the Forth Road Bridge: Final Report (7 March 2006)
• First internal inspection of the main cables - interim report (24 November 2005)
• First internal inspection of the main cables - update report (31 August 2005)
• Forth Road Bridge – First internal inspection, strength evaluation, acoustic monitoring and dehumidification of the main cables (Barry R Colford & Charles P E Cocksedge)