New headache for Forth Bridge as 2,000 bolts may need replacing

Published on Sunday 12 October 2008 22:05

NEARLY 2,000 bolts which help hold up the Forth Road Bridge may have to be replaced after several were found to have cracked nuts.

Laboratory tests are being carried out to determine the cause of the problem, which is believed to be a world first on a suspension bridge.

Any repair would add to a growing list of work required on the bridge, which includes installing dry-air blowers in an attempt to halt corrosion of its main cables.

It also comes a week after officials revealed they faced funding problems because the replacement of worn-out carriageway expansion joints will cost 5 million more than expected.

In the latest headache, nine of 1,888 nuts on bolts fixed to the bridge's main cables have been found to be cracked, just ten years into their expected 30-year life span.

The bolts are attached to the top of vertical steel wire hangers, which connect the main cables to the bridge's carriageways.

The cracked nuts are attached to bolts in groups of four to six at the top of the 192 hangers. The four cracked nuts and their bolts found on the west cable have been replaced, while five on the east cable will be changed in the next year. There is no pattern to their location.

Barry Colford, the chief engineer and bridgemaster of the Forth Estuary Transport Authority (Feta), has already stressed that the bridge remains "perfectly safe" despite the problem.

However, he has now told the Feta board: "It may be that we have to replace all the bolts, in a rolling programme over the next years. As far as we know, investigations have shown we are the first (to have encountered this problem]."

Mr Colford said the cracks had been spotted by officials doing routine inspections. He said: "Staff were not looking for cracks in the nuts and had not expected to find them."

Mr Colford said news of the problem had sparked interest from staff of several bridges in the United States, whose bolts are horizontal, like those on the Forth Road Bridge. Those on the Severn and Humber – Britain's other major suspension bridges – are vertical.

Mr Colford said a 25,000 investigation of the problem was underway. It would consider whether more of the 3.5cm-long nuts were likely to crack. Each carries an 80-tonne load and weighs about 8oz (225g).

The bridgemaster said the cost of complete replacement was not yet known, but any work would not disrupt traffic.

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."

Mr Colford said the price of the expansion-joint work had increased because the cost of building bespoke ramps to take traffic over the work had been underestimated.

WHAT NEXT

REPLACEMENT of the cracked nuts has delayed other vital repairs on the bridge.

The work – along with wet weather this summer – has put back the installation of air-drying equipment in an attempt to halt corrosion of the main cables. Completion has been put back from October next year until the start of 2010.

However, officials said the project could be speeded up if a second work platform is bought from the Severn Bridge, which has installed similar equipment on part of its cables.