Queensferry crossing: huge carbon saving

The Queensferry Crossing links Edinburgh with the county of Fife, and stands proudly alongside its illustrious neighbours, the world famous Forth Bridge, now a UNESCO world heritage site which opened in 1890, and the Forth Road Bridge, a classic suspension bridge opened in 1964.

Construction on the Queensferry Crossing began in 2011, to replace and enable essential maintenance works on the existing Forth Road Bridge, which was no longer deemed viable and is now used by pedestrians, cyclists, emergency services and public transport.

A £1.35bn investment by the Scottish Government, Queensferry Crossing was officially opened to traffic by the late Queen in 2017. It has come to represent the modern progressive Scotland, facilitating active and public transport, whilst reducing emissions through improved traffic flows. Reducing carbon during design and construction was also an important focus. Through challenging the design and employing innovative approaches, many tonnes of carbon have been saved.

One of the most striking features of the new crossing is its overlapping stay cable design, covering a 160m length of the deck at mid-point along the two main spans. This beautiful cable configuration provides extra stability to the structure, and the Centre Tower in particular, facilitating the slender design of the towers.

Suspended by the cables 50 metres above high tide, the main deck, with an overall width of 40m and 2.7km in length, offers 4 lanes of wind protected traffic, plus two hard shoulders. Each of the deck sections was prefabricated before being shipped to the Forth estuary to be fitted out prior to installation.

Computer models were created to analyse the whole bridge, with additional three-dimensional models for load cases applied to the model to simulate the self-weight of the bridge, various traffic loadings and climatic conditions such as temperature effects and wind.

At 210m above Mean High Water level, the three slim towers are critical to the unique appearance of Queensferry Crossing and make it the tallest bridge in the UK, and 50 metres higher than the earlier road bridge towers. They were constructed in stages using an innovative climbing formwork system.

The geology of the Forth estuary is complex and called for different approaches to the foundations of each tower. All foundations bear on the top of the rockbed, which eliminated the need for expensive and time-consuming piling onto the hard rock that underlies the whole crossing. Ramboll engineers conducted field mapping of the geology of the area along with inspection of rock cores recovered from below the seabed to identify good quality rock across the Firth of Forth to produce a consistent philosophy for the design of the foundations.

During the detailed design of the scheme Ramboll identified that the Ferrytoll Viaduct on the approach to the northern end of the main crossing could be re-engineered to offer significant benefits in cost, programme and embodied carbon. The original design utilised a much longer viaduct however we were able to demonstrate that by some minor changes to alignment we would be able to replace a significant proportion of the concrete structure with an embankment formed from site won material. This change simplified construction, reduced traffic disruption, reduced construction materials (saving 7,000 tonnes of embodied carbon) and removed the lorry movements and its associated emissions that would have been required to dispose of the embankment material off site.

During the design of the Queensferry Crossing the opportunity for a new proactive predictive approach to asset management was developed. Through the installation of sensors across the structure it has been possible to create a digital twin of the Crossing. This digital twin takes data from the sensors and provides real time data on its structural health. It also enables its performance under normal conditions to be modelled alongside emerging trends or anticipated changes in loading to predict problems before they happen. This not only reduces operational costs and enables maintenance work to be planned well in advance, it ensures work is only undertaken when necessary, thus reducing disruption to the Crossing’s users and associated carbon.

Ramboll is proud to have led the Design Joint Venture (DJV), which includes Sweco and Leonhardt Andra and Partners. The DJV worked for main construction contractors Forth Crossing Bridge Constructors (FCBC), a consortium of Hochtief, Dragados, American Bridge International and Morrison Construction, helping deliver the crossing and surrounding road approaches.

The stunning Queensferry Crossing forms the centrepiece of a major upgrade to the important cross-Forth transport corridor in the east of Scotland. The 22km footprint includes major improvements to the surrounding road networks and connects the M9 and M90 together for the first time. The project also integrates smart motorway traffic management technology, the first such implementation in Scotland.

The project to design and construct the bridge has been recognised within the industry for its innovation and efficient delivery, collecting the following major awards.