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Tackling the cable corrosion conundrum

News - Engineers are to try dehumidifying rusting suspension cables on the Forth Road Bridge. But is this an effective solution?

A YEAR ago, engineers took their first peek inside the main suspension cables of the Forth Road Bridge to check whether any of its component wires were rusting.

The inspection was commissioned by bridge owner the Forth Estuary Transport Authority (FETA) after engineers discovered corrosion in the cables of bridges of similar age and construction in America (NCE 11 December 2003).

The results confirmed there was a problem 'Outside the cables looked dry, but inside we found corrosion, ' says FETA depute general manager Barry Colford.

Suspension cable corrosion can be caused by condensation or by rainwater entering the cable through cracks in paintwork or at clamp positions where there can be inconsistent cable wrapping. Water can also be locked in during the initial cable spinning operation.

At the Forth Road Bridge, the main cables consist of 11,618, 5mm parallel high tensile steel wires coated in red lead paste and wrapped with mild steel galvanised wire and a coat of paint.

Inspections showed that water has penetrated cracks in the paintwork, despite a rigorous maintenance regime.

The favoured solution to inhibit further rusting is dehumidification. This dries out the cable internally by rewrapping it in a neoprene material and pumping dry air through the wires.

The system was retrofitted on the Little Belt Bridge in Denmark in 2003. But the technique is new, and although the contract is now out to tender, Colford admits: 'No one will know how effective it will be until we open it up in 10 year's time.' FETA is also commissioning an investigation into the merits of replacing the corroded cables or adding extra cables.

Acoustic monitoring will be installed next spring to detect possible breaks in the wires before dehumidification equipment is installed in 2007. Tolls on the bridge will help pay for the £12.3M cost of the dehumidification and acoustic systems.

Cable corrosion problems are not limited to Europel Initial attempts to curb corrosion of suspension bridge cables in the US involved dousing the wires with linseed oil. This was done at the Mid Hudson Bridge in New York in 1990. But by 2003, water had found its way back into the cable.

While the humidification is now the accepted solution, there are shortcomings. There is no way of checking whether the warm air is reaching all wires, for example.

And if wires are already heavily corroded, dehumidification may not stop corrosion spreading, especially if the degradation is due to salt.

Many experts believe that it is virtually impossible to keep water out of a suspension bridge cable and that the key to dealing with corrosion is to monitor it regularly.

FETA's consultants Faber Maunsell and Weidlinger have warned that the Forth Road Bridge will have to close by 2019 if cable corrosion cannot be controlled.

But at least one US-based suspension bridge expert, who wanted to remain anonymous for commercial reasons, believes the warning is overcautious, as the prediction is based on a single cable inspection instead of monitoring cable deterioration over a period of years.

'Visual inspections can be misleading. A wire might look corroded but when tested, it has a lot of meat in it. At other times the wire looks fine but fails ductility tests, ' he says.

'People [engineers] do things in the name of conservatism, but you must qualify your statement - is it just that the safety factor has dropped from 4 to 3.5-' At the Forth Road Bridge only 31 of 11,618 wires were found to be broken in the worst affected section of cable (NCE 12 April).

US bridge engineering consultant Modjeski & Masters vice president Barney Martin says engineers should weigh up how corrosion affects the load carrying capacity of the structure.

It might be possible to reduce the weight of the deck by redesigning it to replace heavier elements with lighter ones.

He is also sceptical over the effectiveness of dehumidification. Saddle locations and positions of other connections mean that achieving a good seal against moisture is impossible. 'Some argue that vapour will actually seek out the dry air areas. I think the jury is out and only time will tell whether dehumidification is just a panacea'.

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