Truss end links repair on the Forth Road Bridge - one year on

Forth Road Bridge closure on 12/12/15. Pic by Scott Louden.
Forth Road Bridge closure on 12/12/15. Pic by Scott Louden.

One year on from the re-opening of the Forth Road Bridge after an unprecedented three-week closure for repairs to fractured steelwork, considerable progress has been made.

The bridge re-opened to 91 per cent of traffic on December 23 last year after the completion of emergency repairs. Work then continued around the clock through storms and freezing weather to design, fabricate and install temporary brackets and cables to allow the bridge to re-open to HGV’s and abnormal loads up to 150 tonnes on February 20, 2016.

State of the art structural health monitoring systems were installed, providing engineers with live data on stresses and strains in the steel. These systems have since been reviewed and enhanced.

At the same time engineers pushed on with design of a permanent replacement for the components that had failed. Option appraisal was complete by April, with the chosen design incorporating a sophisticated permanent sliding bearing arrangement to be fixed to the tower beneath the existing truss – a world first.

A full-scale mock-up trial of the strengthening works required inside the main tower was carried out in August, involving the construction of a six-metre high replica on dry land, using 12 cubic metres of concrete. This drew on the latest developments in material technology and provided Amey’s engineers with invaluable information on the challenges faced whilst working within a confined space.

Work on site began at the end of August on a trial of the permanent truss end link replacement at the North East main span – the location where the original fracture occurred. The detailed design is now complete and fabrication of the steelwork and bespoke bearings is in progress.

Thanks to the temporary repairs already completed, this trial can be carried out without any significant disruption to traffic, allowing Amey’s engineers sufficient time to work through and assess each detail of this unique and challenging project.

Work on site is now expected to be complete in March 2017 and lessons learned will inform similar permanent replacements planned for the other seven truss end link locations beginning in June 2017.

Mark Arndt, Amey’s account director for the Forth Bridges Unit, said: “The team did a great job to get the bridge reopened to traffic last winter, but we’ve kept the momentum going to implement a permanent solution. Thanks to the work already carried out, this next phase of works will not cause significant disruption to traffic, however we’ll continue to keep the public updated on progress.

“We now have better data than ever before on the stresses in the bridge thanks to the structural health monitoring systems that have been installed. This, combined with the ongoing proactive investigation and analysis carried out by our engineering team, will be invaluable as we replace the truss end links and apply lessons learnt to other areas of the bridge. ”

Transport minister Humza Yousaf said: “A year since the reopening of the Forth Road Bridge is the right time for Amey’s Forth Bridges Unit to provide an update on their work to design, test and install the permanent repair to the truss end links. This is a technically challenging project, but crucially, due to the temporary repairs completed last year, is one that can be undertaken without any significant delays to traffic using the bridge.

“Engineers are now in a much better place to understand the ongoing health of the bridge. This is due to the improved structural health monitoring and studies of key elements of the bridge over the course of the year. As normal, general traffic will continue to use the bridge until the Queensferry Crossing opens in May 2017.”