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.

Specialist bridge engineer Richard Hornby, of engineering consulting firm Arup, told a Scottish Parliament inquiry into the crossing’s closure that it happened on a load-bearing beam called a truss end link that connects the bridge’s span to one of its pillars.

Mr Hornby added that all the other similar links on the bridge were “ticking timebombs to greater or lesser extent”.

Transport Scotland said plans were now in place to replace all the links.

The 51-year-old bridge closed on December 4 and didn’t reopen until December 23. The crossing is still closed to HGVs.

Mr 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.

He said: “The failure of the member was because the pin had seized, and probably had seized for a number of years. It’s only because the steel has been so good that it has lasted so long.”

Mr Hornby added that only a hi-tech “structural health monitoring” system, using a network of sensors to keep tabs on the bridge, could potentially have revealed the problem.

The committee heard the latest monitoring will be used on the new Queensferry Crossing and is in place on some sections of the Forth Road Bridge, but installing it on the entire structure would cost millions.

John Russell, of bridge operators Amey, also said that only structural health monitoring could have “perhaps picked it up sooner”. He said the failed beam was inspected 23 times from 2001 and nothing was found.

He said: “In the current circumstances and technology that we have on the Forth Road Bridge, that wouldn’t have been able to have been foreseen.

“My opinion would be 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.”

The committee are looking into the causes of the closure.

On the opening day of the inquiry yesterday, several witnesses told them a 2010 decision to scrap plans to replace the truss end links was “reasonable” and “proportionate”.

The 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.