THE cost of investigating the strength of the Forth Road Bridge is to rise dramatically and could affect work on other parts of the structure, engineers warned today.
Bridge managers said an examination of anchorage points at the bridge’s base, which are vital to its integrity, would cost significantly more than anticipated after it was discovered they were buried deeper and at a steeper angle than recorded on official maps.
They said these factors would mean contractors spend longer on the investigation and would require resources over and above those provided by a £3.5 million budget for work on the bridge’s south end.
They also admitted the extra cost may mean a review has to be carried out of maintenance jobs elsewhere on the bridge to see if they can be deferred.
Barry Colford, chief engineer and bridgemaster at the Forth Estuary Transport Authority (FETA), said the investigation had been started to establish the condition of steel strands within the anchorage points which prevent them from breaking apart.
He told the News: “This investigation is about excavating and looking into structures which are crucial to the cables that provide the suspension in the bridge – without those you would have structural failure.
“The cables need to be anchored as they have huge tension in them. This is achieved by driving tunnels into the rock, filling them with concrete and connecting the cables to them.
“The concrete plugs are strengthened by steel strands, which are in ducts within the concrete and tightened to compress it.
“Because of friction between rock and concrete, the plugs aren’t ever going to be pulled out. But if steel strands within the ducts weaken then the concrete itself would just pull apart. And if you lose one of the anchorage points then you lose the suspension bridge.”
Mr Colford said it had not yet been possible to quantify the extra cost but added that he hoped to be able to report precise figures later in the year.
He said: “The challenge for us is that this has never been done anywhere else in the world – it’s also a unique type of anchorage that you have on the Forth bridge.
“We had tried various methods of determining the condition of the strands but an investigation of this sort hasn’t been done before.
“So we are having to develop techniques for excavating without damaging the strands and it costs money to do that. The viaduct piers are also nearby and we have to be careful that there’s no movement of those during the excavation.”
He warned that budgetary pressure could mean other work on the bridge has to be postponed.
He said: “If we suddenly get a very large variation in the cost of this investigation then we have to try to make sure that we can work within our budget.
“That would mean having to look at the maintenance programme as a whole and doing a risk analysis to see which work can be deferred.” FETA communications manager, Chris Waite, said further investigation would be needed before the final cost of examining the anchorage points is established.
He said: “The design of the anchorage is unique and it’s very difficult to estimate how much extra it will be at this stage.”
The west footpath and cycleway on the Forth Road Bridge is to open to the public every Saturday from this weekend to let people watch progress on construction of the new Forth crossing.
Since September, access to the footpath has been restricted to bridge staff and contractors while maintenance projects are carried out.
Bridgemaster Barry Colford said: “There’s some spectacular engineering going on beside the Forth Road Bridge at the moment. Work on the new bridge’s foundations is well under way and next year its three 207-metre towers will begin to emerge.
“We want the people of Scotland to have the opportunity to see this once-in-a-generation project close-up.”