Engineers told MSPs the fault could not have been foreseen despite regular inspections of the area beneath the bridge’s carriageways.
They also said a decision in 2011 to scrap repairs for a separate problem, which might have also covered the fault, had been correct as it was not classed as “safety critical”.
The first hearing of the inquiry by the Scottish Parliament’s infrastructure and capital investment committee follows the bridge being shut for nearly three weeks last month.
It is expected to remain closed for heavy lorries until mid-February, pending further repairs.
The fault was caused by the seizure of a pin which allows free movement of the truss –girder framework – which supports the carriageways.
Scott Lees, head of network maintenance for the Scottish Government’s Transport Scotland agency, said: “We believe the defect was unforeseen.”
Richard Hornby, director of engineering consultants Arup, which has worked on the bridge, 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. There was no evidence these pins were not moving from the inspections that were carried out.”
John Russell, operations manager for bridge operator Amey, said that since 2001 the failed section had been inspected 23 times and no fault found. He said sensors – which have now been installed – might have detected the fault.
Mr Russell said: “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.”
Several engineers giving evidence agreed that the scrapping of the previous £15 million repair had been “reasonable” and “proportionate”.
The fault it was aimed at fixing has now been addressed.
The move, by previous bridge operator the Forth Estuary Transport Authority, came at a time of other major work on the bridge.
Mark Arndt, of Amey, said: “The member which failed was never identified as at risk. We have to prioritise what we do.
“The capital plans was a wishlist – engineers want to fix everything as if it is brand new, but you cannot do that.”