CORROSION of the main cables of the Forth Road Bridge is being successfully slowed down by blowing dry air through them, its operators announced today.
The latest inspection gives “strong comfort” that the dehumidification system is “retarding the corrosion of the bridge wires”, according to the Forth Estuary Transport Authority.
The check follows previous inspections in 2004 and 2008, which led to ministers ordering a new £1.6 billion bridge to be built because of fears that lorries would have to be banned from the bridge if its loss of strength caused by the corrosion was not stemmed.
Chief engineer and bridgemaster Barry Colford said today: “A degree of uncertainty concerning the magnitude of future strength loss of the main cables will always remain and the cables will require to be continually monitored, and be subject to a regime of internal inspections and strength
evaluations, for the remainder of the service life of the bridge.
“However, the results of this latest inspection, albeit reduced in scope, are encouraging.
“The dehumidification system applied to both cables appears to be slowing down the rate of deterioration.”
The results confirm a Scotsman exclusive a year ago that corrosion was being stabilised because the dry air pumped into the 2ft thick cables since 2009 was coming out dry. Mr Colford said then: “I am expecting that we will find a situation no worse than in 2008.”
However, he also stressed the new crossing was required to avert major traffic disruption on the Forth Road Bridge during increasingly-frequent repair work.
The latest check, made last October and costing £2.6 million, involved unwrapping the cable, which comprises 11,618 pencil-thin wires.
It found a total of 55 broken wires in six of the eight sections of cables inspected, with the greatest corrosion at the lowest point of the cable.
Today’s report, which will be discussed by the bridge authority on Monday, said: “The results of the inspection appears to demonstrate that the rate of deterioration of cable strength has been reduced and the factor of safety against failure of the cables has not materially diminished.
“This is giving strong comfort that the newly installed dehumidification system is retarding the corrosion of the bridge wires.
“Although wire corrosion appears to have been slowed down, 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 the wire fractures, but the rate of breakages is expected to slow significantly.
“This should result in a further slowing down of deterioration of the cables and lead to a reduction in the loss of magnitude in the factor of
The 2008 inspection pushed back the expected imposition of a lorry ban if the corrosion had continued to get worse from 2014 to 2021 - which is after the planned opening of the adjacent Forth Replacement Crossing in 2016.
A spokesman for the Scottish Government’s Transport Scotland agency said: “While the dehumidification process wasn’t the key reason for going ahead with the new Forth crossing, we welcome the fact that FETA’s efforts appear to be working and congratulate those involved. This means we can continue to plan on the basis of the existing Road Bridge being used as a dedicated public transport crossing.
“As the chief engineer and bridgemaster has repeatedly said, a degree of uncertainty will always remain. Cables will need to be continually monitored and inspected for the remainder of their service life, and damage already done to the wires inside the cables cannot be repaired.
“The most disruptive maintenance project in the bridge’s history has already been put on hold [replacement of carriageway expansion joints], and without an alternative crossing this would have caused unprecedented disruption to traffic and the economy over an extended period.”