A seized pin is what caused the crack in the Forth Road Bridge, according to an engineering consultant.
Richard Hornby told a Holyrood inquiry that the pin had probably been seized for years, leading to the closure of the bridge.
He also told the infrastructure and capital investment committee it was only the quality of the steel that stopped the crack from appearing earlier.
MSP’s are currently holding an inquiry into the fault which led to the closure which lasted almost three weeks at the end of last year.
Mr Hornby explained that there had been nothing in the inspection which would have discovered the seizure and that it was near enough impossible to lubricate the bearing.
It is believed that only hi-tech structural health monitoring could have determined the fault.
The latest monitoring equipment is to be used on the Queensferry Crossing and is in place on some sections of the Forth Road Bridge.
However, to install it along the full bridge would cost up to £10m.
Since 2001 the failed section had been inspected 23 times and not once was a fault found, showing how difficult it was to detect it in the first place.