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Capital projects:

Main cable replacement study

Following the discovery of corrosion within the main cables of the Forth Road bridge, consulting engineers WA Fairhurst were appointed in September 2006 to examine possible options for the replacement or augmentation of the main cables.

Fairhurst were assisted by the Danish company Cowie Consult and the American company Amman and Whitney, both of whom have extensive international experience in cable supported bridges. Edinburgh consultant SIAS carried out traffic modeling and economic consultant Roger Tym & Partners assessed the economic impact of traffic restrictions on the bridge. The work was peer reviewed by consulting engineers Flint & Neill and Transport Scotland.

The consultant has now completed Interim Stage 2 of the study which, through a process of review and concept development, has identified the most appropriate options for either replacement or augmentation of the main cables, should this become necessary.

The study also showed that replacement or augmentation, although technically feasible, would result in significant delays to the strategic road network over a period of 7-9 years if it had to be carried out without an alternative crossing in place.

The study concluded that major maintenance works on Forth Road Bridge over a sustained period, involving lane and carriageway closures, would potentially result in:

  • economic output falling to a level in the order of £1 billion below that anticipated were the bridge to be operating normally
  • a drop in turnover in excess of £1.3 billion below that anticipated were the bridge to be operating normally
  • a loss of around 3,200 jobs, some of which may turn out to be permanent.

It should be noted that replacement or augmentation of the main cables will only become necessary if the current scheme to dehumidify the cables fails to prevent further loss of strength.

Related documents:

As has been previously reported to Members, dehumidification is a well-tried system of 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.  Such 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.  Whilst there is good reason to have confidence that dehumidification can slow down or halt corrosion there is no body of evidence yet available to allow an unconditional assurance to be given that this will prevent a further reduction in strength loss in the main cables at Forth.

The Bridge:

Facts & Figures

Opened 1964, 2.5 km long, Main span 1006 metres
  • Roadworks on bridge both directions (03:21 BST 24/05/15)