Seized pin which forced closure of Forth Road Bridge was "ticking timebomb" says engineer

The crack in the steelwork that forced the closure of the Forth Road Bridge in December was caused when a single metal pin designed to move had seized up, MSPs were told yesterday. Arup bridge engineer Richard Hornby told a Scottish Parliament inquiry into the crossing’s closure that it happened on a truss end link load-bearing beam that connects the bridge’s span to one of its pillars, adding that all the other similar links on the bridge were “ticking time bombs to greater or lesser extent”.

Transport Scotland now has plans in place to replace all the links. The 51-year-old bridge closed on 4 December 2015 and didn’t reopen until 23 December 2015, causing traffic chaos. The crossing is still currently closed to HGVs.

Hornby told Holyrood’s infrastructure and capital investment committee that the problem with the pin had been invisible and said it could have seized up years ago. There were no outward signs and nothing in the bridge’s inspection regime would have detected it. The failed beam was inspected 23 times from 2001 and nothing was found. “The failure of the member was because the pin had seized, and probably had seized for a number of years,” he said. “It’s only because the steel has been so good that it has lasted so long.”

Hornby said that only a hi-tech “structural health monitoring” system, using a network of sensors to check the bridge, could potentially have revealed the problem. MSPs were told that the latest monitoring will be used on the new Queensferry Crossing and is already in place on some sections of the Forth Road Bridge, but installing it on the entire structure would cost millions of pounds.

John Russell, of bridge operators Amey, backed up Hornby’s comments. “In the current circumstances and technology that we have on the Forth Road Bridge, that wouldn’t have been able to have been foreseen. If we have structural health monitoring on the bridge, that would be the way forward. If we had had that, it perhaps may have been picked up – perhaps.”

On the opening day of the inquiry on 20 January, several witnesses told MSPs that a 2010 decision to scrap plans to replace the truss end links was “reasonable” and “proportionate”. MSPs were also told that HGVs should be let back on the bridge in mid-February.

The new Queensferry Crossing, which will take much of the Forth Road Bridge’s traffic, is expected to open later this year.

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