The trouble started at 3:30pm on Tuesday when engineers making a routine inspection 40ft below the bridge deck found an “inner link support beam” had broken.
It forms the end link of the “truss” or framework of girders which support the southbound carriageway, on the east side of the north tower.
The carriageway was closed at 9:30pm, with traffic reduced to one lane in each direction in a contraflow on the northbound carriageway.
Specialist engineers returned to the scene on Wednesday, working into the night to make a more detailed assessment.
Further checks on Thursday included a structural assessment which indicated a “significant overstress” at the damaged area, and other locations.
A decision was taken to close the bridge to lorries - the heaviest and potentially most damaging traffic - shortly after 2pm.
Amey said further testing led to the discovery of a 20mm crack in an “outer link support beam”.
A meeting of the Scottish Government Resilience Committee was held, chaired by First Minister Nicola Sturgeon, at which the decision to close the bridge was made.
It was announced at 10pm and implemented at midnight.
Amey, the road maintenance contractor which runs the bridge for the Scottish Government’s Transport Scotland agency, said rge defect could not have been predicted and it happened quickly.
The area had been inspected in May and no damage was found, but a routine inspection of another element had brought the fault to engineers’ attention.
Amey described the fault as unprecedented and said it had occurred in an exceptionally hard-to-access section of the bridge.
Amey chartered engineer Mark Arndt said: “This is a complex engineering challenge. The component failure is in a difficult-to-access location and our response is also highly dependent on weather conditions.”