A team from Spencer Group has carried out works to replace truss end links that connect the bridge deck to the towers.
The project has its origins in the discovery, in December 2015, of a major fault on the crossing, which spans the Firth of Forth, connecting Edinburgh to Fife. One of the eight truss end links on the bridge was found to have fractured. The bridge had to be closed for nearly a month while urgent repairs were carried out.
A temporary splint was applied to the fractured link, followed by a permanent structural repair to that area. Bridge owner Transport Scotland subsequently decided to replace the other seven key links and Spencer Group won the tender for the works.
The £10m scheme has involved cutting out the truss end links and replacing them with a unique new sliding bearing system. It has required a series of complex works to be delivered to a part of the bridge which has limited accessibility and was never designed with replacement in mind.
To deliver the works Spencer Group installed temporary stairs and platforms below the bridge deck, allowing access to each bearing arrangement.
Challenges included working in confined spaces within the main towers and manoeuvring large new structural components to precise tolerances.
The replacement bearings were delivered to the bridge from a workboat, so the works had to be co-ordinated with tide times within the busy Forth estuary. Ensuring effective communication via two-way radios was crucial, with engineers working below the bridge deck having to contend with significant noise and vibration as traffic passed above them.
Spencer Group bridges projects manager James Barnes said: “This project has been particularly challenging in relation to access because the truss end links and the new bearings we have fitted are below the bridge deck, about 40 metres above the water. No permanent access was in place, so we installed a temporary access platform to allow our personnel to reach the location of the works.
“With our supply chain we have fabricated over 200 tonnes of new steel and joined it to the existing structure in some very inaccessible areas. The pieces that we cut out to replace were 7-8 tonnes in weight and some of the heaviest lifts have been up to 16 tonnes. All the pieces have had to be lifted onto the bridge deck or lowered to the workboat, so there have been major lifting, hoisting and access challenges that had to be overcome.
“A key factor in the success of this project has been the collaborative approach between everyone involved. It’s been a real team effort to come up with the right solutions and undertake the works in the most efficient manner, with safety the number one priority.
“Long span bridges have a very long design life, meaning the quality of the work produced must be to the highest standard.”
As well as installing the new bearing arrangements, Spencer Group has introduced modifications that will make future works easier, including new permanent access stairs and platforms.
Amey, the long-term maintenance contractor for the Forth Road Bridge, represented Transport Scotland on the project.
Angus Bruce, Amey’s major bridges manager, said: “A key requirement of the project was for the contractor to have previously worked on long-span, cable-supported bridges and Spencer Group is one of only a handful of contractors to have this experience. The quality assurance and technical requirements were extremely high, so we needed a highly competent contractor who would work diligently, look at all aspects of the project, ensure everything was risk assessed and that method statements were created and agreed with all parties before the works were carried out.”
Spencer Group managing director Gary Thornton said: “We’re delighted to have completed this important project to maintain the structural integrity of the Forth Road Bridge and to add it to our extensive track record over many years of delivering critical, high-level works on landmark crossings.
“The project showcased the expertise we bring to works such as this, which pose a range of major challenges. The solutions we put in place ensured critical components were replaced without having to interrupt traffic flows at any time.”
The company is currently delivering another maintenance project, on the Humber Bridge, close to its headquarters in Hull. This project involves the design and construction of a bespoke access platform to enable engineers to access, inspect and replace the bridge’s hangers – the high-tensile steel ropes that suspend the bridge deck from the main cables.
The hangers are exchanged by locally lifting the whole bridge deck to remove all load on the target hanger. Works are done at night to avoid disruption to traffic.