2,000 nuts that hold up Forth road bridge must be replaced

FORTH Road Bridge officials have confirmed they will have to replace nearly 2,000 heavy duty nuts that help to hold up the structure, after The Scotsman revealed last year the repair was on the cards.

The problem, thought to be a world first on a suspension bridge, will take several years to rectify, but traffic will not be disrupted during remedial work.

More than £600,000 is expected to be spent on the new nuts after cracks were found in nine of the 1,888 that help secure 192 vertical steel-rope "hangers" connecting the bridge's main suspension cables to the vehicle carriageways. The bolts they are attached to may also have to be replaced.

The Forth Estuary Transport Authority (Feta), which runs the bridge, said the replacement work was precautionary and the structure remained safe.

The nuts secure cable bands to the suspension cables – cable bands are metal castings over which the hangers holding up the roadway are looped. Each cable band is held in place by a number of 35mm diameter steel bolts, pre-tensioned to a load of about 80 tonnes.

All the cable-band nuts and bolts were replaced ten years ago, but an inspection last year found the cracked nuts. They had been expected to last 30 years.

Feta then ordered an investigation, which found design and specification decisions and construction methods that may have contributed to the cracking.

The investigation found misalignment of washers may have led to uneven loading in the nuts, and the protective coating on the nuts may have allowed moisture to cause the damage.

Feta is seeking legal advice to find out whether the designer, or the contractor who replaced the nuts in the 1990s, is liable and can be sued for compensation.

Four of the failed nuts have been replaced on the west cable, using access platforms already in place. The five failed nuts on the east cable will be replaced this year.

Barry Colford, the bridgemaster and chief engineer, said: "Thanks to the vigilance of our inspection team, this problem was identified at an early stage and the bridge remains perfectly safe.

"Cracks have been found in only nine out of 1,888 similar nuts. Nevertheless, our investigation has concluded that any of these nuts could potentially fail in future, so the recommendation is to replace them all as a precaution. We're planning further tests in the laboratory before bringing a final report to the Feta board later this year."

Neil Greig, the Scotland director of the Institute of Advanced Motorists' Motoring Trust, said: "It is reassuring to hear the bridge is perfectly safe, but it is always worrying for drivers to hear of yet another new problem. The good news is that this problem has been caught early and can be addressed without causing extra congestion.

"In the long term, we remain most concerned about rust and other deep-seated cable issues."