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Forth Replacement Crossing Bridge Sensors

5 Sep 2016

Designing solutions that extend the life of ageing infrastructure – thereby reducing the need for new construction – is one important circular approach in the built environment.

For the Forth Replacement Crossing, Arup proved the value of retaining the existing bridge for use as a dedicated public transport corridor for buses, pedestrians and cyclists, thereby reducing the project scope and resource use required by the new bridge. In addition to demonstrating environmental benefits and sustainable development, this approach resulted in an estimated £2bn saving for the client, Transport Scotland.

A system of integrated sensors will monitor the structural health of the new bridge to ensure the long life of this critical new piece of infrastructure, as well as making maintenance more efficient for the bridge operator.

Optimise / Exchange

Forth Replacement Crossing. Arup (£1.7 billion resource saving, Forth Crossing, Scotland, 2016)

Why was there a need for a replacement for the Forth Road Bridge?

Arup: The Forth Road Bridge (FRB) is one of the most vital economic arteries in Scotland and some 60,000 vehicles use the bridge every day. Despite significant investment and maintenance over its lifetime, the current Forth Road Bridge is showing signs of deterioration as a result of increased traffic and the influence of weather and climate overs its 50 years of service.

Given these issues and the impact major maintenance works would have, a comprehensive study into the future transport cross-Forth travel needs concluded that the FRB is no longer deemed viable as the long-term main crossing of the Firth of Forth.

(Courtesy of Arup)

What where the factors which lead you to propose retaining the current bridge?

Arup: The original design brief was to consider the Forth Replacement Crossing on the assumption that the existing Forth Road Bridge would be completely inoperable. The Forth Replacement Crossing Study (FRCS) had identified the requirement to provide sustainable transport modes across the Firth of Forth resulting in multiple functional requirements for the replacement crossing

The Jacobs-Arup joint venture took another look at the possible future use of the Road Bridge as part of a strategy that would promote sustainable development, by making the most effective use of existing infrastructure. A Managed Crossing Strategy was developed with the aims of providing greater reliability of the current service provision and promoting sustainable transport modes. It was identified that this was best achieved by two bridges operating in parallel so that:

  1. The maintenance backlog on the Forth Road Bridge could be completed more efficiently and without traffic disruption.
  2. Heavy Goods Vehicles could be removed from the Forth Road Bridge reducing he severity of maintenance issues and on-going maintenance costs.
  3. The shared functionality would allow a much less expensive new crossing whilst maximizing future operational flexibility.

How did you decide that there may be an alternative construction strategy?

Arup: By applying our design expertise we empowered Transport Scotland to challenge the original project brief. The original brief included a road traffic crossing with the potential to carry a light rail transit scheme in the future. We re-examined previous reports and established that the existing bridge could, with relatively minor changes, carry a light rail line. It could also be reused to act as a dedicated public transport corridor for buses, pedestrians and cyclists. Retaining limited use of the existing bridge in this way would reduce the weight of traffic on it and also extend its life whilst reducing the size of the new crossing.

Taken together with state of the art ITS (Intelligent transportation systems) technology this significantly reduced the need for new road construction, enabling a significant reduction in the original project budget.

Could you tell us a about the role of sensors in this project?

Arup: The current durability problems experienced on the existing Forth Road Bridge highlight the potential disruption that can be caused by the concealed deterioration of critical bridges.

Arup have developed a simple to operate, advanced and fully integrated structural health monitoring system that has been installed on the new Forth Replacement Crossing. This will give advanced warning of structural problems, and allow targeted inspection and intervention to ensure the smooth operation of the bridge.

Comprising approximately one thousand sensors, and integrated to comprise a resilient, real time, event driven system, the SHMS (structural health monitoring system) will ensure the continual surveillance of the applied loads, the environmental conditions and the structural response of the bridge.

(Courtesy of Arup)

What challenges were there in persuading the partners in the project that this was an economically, socially and environmentally effective option?

Arup: The FRC has set a new standard in engagement with directly affected communities and other interested parties on infrastructure projects in Scotland. A number of scheme options were assessed considering a range of criteria including functionality, cost, construction duration, aesthetics, environmental impact and maintenance.

Engagement was built into the heart of the project from the start and communications has always been one of the key elements of project delivery. Consultation and engagement with affected communities is therefore at the heart of the Forth Replacement Crossing project. Work with communities began with the early feasibility and option reports. It continued through developing the design, identifying land requirements and environmental impacts and minimising disruption to affected communities during the five and a half year construction period.

Scottish Natural Heritage has been consulted throughout the project, regarding the scope and methods of assessment and mitigation to avoid, reduce or offset potential impacts to habitats, species and designated sites from the proposed scheme.

Architecture and Design Scotland were consulted throughout the project development and agreed with project team that the mono-tower with a single box girder be recommended to Scottish Government as the preferred scheme. This was deemed to be an economical and low risk solution, easy to operate and maintain and the slim elegant towers achieved the desire for a modern icon without overpowering the existing bridges.

What were the monetary and environmental savings? To what extent did it impact on Arup’s own costs for the project?

Arup: The revisions and refinements achieved on this project mean that the outturn cost is now estimated at between £1.325 and £1.35 billion - with savings of around £2 billion on the original estimate with an associated improvement to the environmental impact of the project. (Please note we will not provide any comment on Arup’s own costs)

Is this perspective, the social and economic value of building less, something you are bringing to other projects?

Arup: Large civil engineering projects such as the Forth Replacement Crossing have a huge impact on society, the economy and the environment. Arup implement practices that promote economic security, social betterment and environmental stewardship and will strive for continuous improvement of performance in these areas on all projects.


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