The current ‘roller shutter’ or rolling leaf joints were developed, fabricated and installed by the German firm Demag and are believed to be the largest and oldest of their type in existence.

The main joints are located at the north and south main towers and Transport Scotland said that the replacement joints would be a modified version of the same type of joint.

Amey account director for the Forth Bridges unit Mark Arndt said: “The contract to replace the main expansion joints on the Forth Road Bridge is unrelated to the ongoing repairs to the truss end links. The main expansion joints are built into the carriageway directly underneath the main towers and allow the bridge deck to expand and contract in response to wind, traffic loading and temperature.”

The £9M contract is due to be awarded in autumn 2017, with a start on site in spring 2018 and work will last for eight months. The tender is valued above the £2M threshold where Amey would do the work under its five year management and maintenance contract. Amey is acting as the engineer, and the designer is Atkins.

Although the joints were scheduled to be replaced in 2009, a review ordered by the Bridge Authority concluded that the project could safely be delayed until the new Queensferry Crossing was completed, subject to five conditions. Delaying the work to allow for the new crossing to open would allow the work to be carried out at greatly reduced cost and with minimum disruption to traffic it said.

Conditions placed on the bridge included an increased monitoring and inspection routine, key components such as pins and springs should be replaced and in some cases modified to improve performance, and temporary failsafe devices should be installed as a precaution in case of failure.

“These joints have performed well and lasted longer than their designers originally anticipated. However, as moving parts, they are subject to wear and tear and have now reached the end of their service life,” said Arndt.

He said the original plan was to build temporary “mini bridges” over the joints to allow light vehicles to continue using the bridge during the replacement works. This would have lasted three months on each carriageway and HGVs would have been required to divert via an alternative route, he said.

“Now, thanks to the Queensferry Crossing, the joint replacement can be carried out with significantly less disruption to traffic and cost to the public purse, avoiding the economic cost of HGVs being diverted and avoiding the need for temporary “mini bridges” that would have cost £6M,” said Arndt.

“One carriageway at a time will be closed during the works, with a single lane of traffic in each direction on the opposite carriageway. Work on site is expected to begin during the 2018-19 financial year and will last approximately eight months, however, this will not cause delays to traffic with the Queensferry Crossing available and the Forth Road Bridge open to buses, taxis, cyclists and pedestrians.”

Transport for Scotland said that while the carriageway closures were in place for the replacement of the main expansion joints and the adjacent footway/cycleway joints, it is also proposed to undertake the replacement of the comb joints on the north and south approach viaducts, and the nosing joints at the north and south side towers.

The replacement of the footway/cycleway expansion joints on the approach viaducts will also be undertaken.

The main span of the suspension bridge is 1km long with side spans each of 408m. The deck is an open steel truss with an orthotropic stiffened plate on the main span and reinforced concrete slabs on the side spans. The truss is supported by two aerially spun cables each of approximately 590mm diameter.

It is the latest major engineering scheme on the bridge after a huge project to replace the truss end links after one fracturred in 2015.