The Scottish government’s Transport Scotland agency has hit back at press reports in the Scottish media suggesting that a technical problem identified five years ago but not acted on was linked to last week’s failure of a load-carrying truss end link on the Forth Road Bridge.
The 50-year-old bridge, connecting the Edinburgh area to Fife and the north east, was closed to all traffic last Thursday following identification of a weld fracture in a routine inspection on Tuesday.
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An article in The Sunday Post claims that a report identified problems with the truss end link system in 2010, saying that it was “significantly over-stressed during certain combinations of loading”.
The article also alleges that a £15m programme to replace the truss end links was proposed, but never acted on, as was a subsequent plan to strengthen the existing members.
The organisation in charge at the time was the Forth Estuary Transport Authority, which was funded by Transport Scotland from the ending of the Forth Road toll in 2008 to the dissolution of FETA in June 2015.
Last week’s routine inspection – and the proposed three-week repair operation — are the responsibility of Amey, which was awarded the contract to operate and maintain the bridge by Transport Scotland in December 2014, and took charge of it in June.
In an audio interview released to the press, Mark Arndt, a representative of Amey, explained that last week’s inspection “identified a failure to the truss end link, a structural load-carrying component that’s located immediately adjacent to the north east tower face”.
“The component comprises a sort of twin arrangement… of a pin-ended member. This is a vertical load-carrying component, and the failure we identified on Tuesday was a complete failure of one side of that component. That meant that 100% of that load, or a significant proportion of that load, was at least being carried by the adjacent pin.”
Arndt went on to describe further testing on the “non-broken element”, which identified a further problem. He said: “The conclusion of that testing was that… there was indeed propagation of a potential crack at a very similar location to where the crack had formed on the adjacent member.”
A weld fracture was found
The Sunday Post’s claims that shelved repairs were the underlying reason for the failure were based on comments made by John Carson, fomer head of Miller Civil Engineering, who led the team that built the Skye Bridge.
But a Scottish Transport spokesman said that FETA had been “fully funded” in 2008-15, and that the weaknesses identified in 2010 were not linked to last week’s failure.
He said: “Amey have informed us that the ongoing truss end strengthening works are to a different part of the truss end linkage system to that which failed earlier this week.
“The truss end link member, which transfers load to the pin linkage, and which has suffered a complete weld fracture near the pin joint, was not previously identified as requiring strengthening or to be at risk of failure.
“The unexpected nature of weld cracking leading to failure is highly unpredictable, and this issue is unrelated to the other strengthening works.
“A trial for the present truss end link brackets and works in the towers was completed on the northwest tower leg in May.
“Following assessment of this trial, Amey is now proceeding with design and strengthening works at the other three towers.”
In the audio interview, Arndt said that Amey is now midway through a week-long process to design a solution to the failure, likely to involve welding new steel plates onto the beam that had failed, as well as building temporary hanging scaffolding to access the site.
On-site repairs are then likely to last at least a further two weeks, although there are fears that storms and high winds typical of the Firth of Forth at this time of year could put this programme at risk.
He said that the structural calculations and design solutions would be checked by two independent firms, and that Amey and the design consultants were already in discussions with fabricators about the works.
The Forth Road Bridge was opened in 1964 and had a projected design life of 120 years. However, there have been fears that a doubling in the amount of traffic on the bridge, and the discovery in 2003 of corrosion in the steel cables, could significantly reduce its lifespan.
The bridge is due to operate alongside the new Queensferry Crossing road bridge which is due to open at the end of 2016.
