The bridge is closed in both directions and public transport is being diverted via the Queensferry Crossing.
Traffic Scotland announced the closure yesterday and said that the road would remain closed until 6pm on Saturday, October 2.
Stagecoach has said buses will be diverted via the Builyeon Road, A900, before returning to the A90.
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Buses impacted include the X51, X52, X54, X55, X56, X57, X58, X59, X60, X61.