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Main Cable Inspection Findings

Published on 25 Feb 2013

The Forth Estuary Transport Authority (FETA) today congratulated staff on inspection findings showing that work to tackle corrosion in the main cables of the Forth Road Bridge appears to be proving successful.

The Chief Engineer’s report on the inspection states that the strength of the cables has not materially diminished since 2008 and the cables are not expected to lose significantly more strength in future so long as the newly installed dehumidification system continues to function effectively.

FETA Convener Tony Martin said: “This is a success for the Forth Road Bridge team and vindication of the strategy they have developed to inspect and monitor the cables and prevent further loss of strength.

“It shouldn’t be forgotten that the Forth Road Bridge was the first major bridge to retrofit a dehumidification system after discovering corrosion inside its main cables. A bespoke system had to be designed from scratch.

“That the approach developed here is now being adopted as best practice by bridge operators worldwide is a measure of the unique expertise that we are fortunate to have built up here.

“Looking to the future, while this is good news the bridge still faces a number of maintenance challenges due to the weight and volume of traffic it is required to carry. It is therefore vital that the bridge’s programme of major capital projects such as the replacement of the main expansion joints and the upgrading of the acoustic monitoring system continue to be funded, in particular those projects that have been deferred until after the new bridge opens.

“As far as FETA is concerned nothing has changed with respect to the need for the new bridge. There have always been many reasons why a new crossing is required and the issue of corrosion only ever added urgency.

“I’m pleased that our engineers have opened up the options for the Scottish Government as to how the existing bridge could be used in future. We all want to see priority for public transport, but there is now nothing to prevent the structure also accommodating non-motorway traffic such as agricultural vehicles and learner drivers, as well as abnormal loads that cause delays to regular traffic.”

Chief Engineer & Bridgemaster Barry Colford said: “These findings show that the factor of safety in the cables has not materially diminished since the previous inspection in 2008.

“This gives strong comfort that the newly installed dehumidification system is slowing down corrosion of the 11,618 wires that make up each main cable.

“There are still existing cracks in some of the wires that might propagate from corrosion pits that existed prior to installation of the dehumidification system. These cracks may eventually lead to breakages but the rate of wire breakages is expected to slow significantly.

“Given this, we don’t expect the cables to lose significantly more strength in future so long as the dehumidification system continues to function effectively.

“A degree of uncertainty over the strength of the cables will always remain and they will need to be continually monitored and inspected for the remainder of their service life, however the results of this latest inspection are encouraging.”

The inspection was carried out by contractors C Spencer Ltd and was completed in October 2012 on time and under budget at a cost of £2,560,824. Specially designed access gantries were used that could be installed on the cables overnight, eliminating the need for traffic restrictions while work was in progress.

The inspection involved unwrapping the main cable at eight carefully chosen locations and wedging open the bundle of wires inside to make a visual assessment of the extent of corrosion and to take samples.

Following inspection, wire sampling and laboratory testing, an analysis of the cables was carried out by consulting engineers Flint & Neill Ltd and a review of that assessment carried out by Aecom.

The highest levels of corrosion were found at the lowest point of the cables and the Chief Engineer has recommended that future inspections be targeted in these regions, with the next inspection tentatively scheduled for 2017, depending on the results of ongoing monitoring.

This was the third internal inspection of the main cables following the initial inspection in 2004 and a second inspection in 2008.

After corrosion was first discovered FETA identified a package of measures to monitor the corrosion and minimise further deterioration. Dehumidification was identified as the best option to stop the corrosion, and an international team was assembled to design a bespoke system for the Forth Road Bridge.

Dehumidification of the main cables involved wrapping the cables in an airtight, waterproof sheath and injecting very dry air into the spaces between the wires with the aim of reducing the relative humidity to a level where corrosion cannot occur.

All the sensors installed as part of the dehumidification system are now showing that relative humidity within the cable has been reduced below the critical level of 40%.

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