The Forth Road Bridge, connecting Edinburgh to Fife, has been closed to all traffic since 4 December after a crack was discovered in a truss under the carriageway.
Welding work began on 15 December, beginning with the assessment of the fault on scaffolding erected around the northeast main tower leg.
It is believed that preventative welding measures are being put in place on seven other locations on the bridge to avoid similar problems arising in the future. Other maintenance work such as painting and decking is also being undertaken at this time, reducing the need for future closures.
The work is estimated to cost approximately £2m.
It is hoped the bridge will reopen in the first week of January.