The Queensferry Crossing
Scotland, a picturesque country in the United Kingdom boasts the longest three-tower, cable-stayed bridge in the world, The Queensferry Crossing.
It surpassed many engineering challenges and has also achieved innovative structural design that makes it an engineering spectacle. The bridge opened on 30th August 2017 and formally opened by the Queen on 4th September 2017.
All in all, there are 13 main bridge foundationswith three excavated caissons, one precast cofferdam, five marine sheetpile cofferdams, one land sheetpile cofferdam, and three conventional pile or spread footings. The caissons contained large tremie placed concrete pours, one reaching almost 17,000m³ – the largest known underwater concrete placement.
Transport Scotland is the owner of the Queensferry Crossing and Ramboll was the leading Design Joint Venture (DJV) firm along with a number of engineering and design parties. Some of the firms include Sweco and Leonhardt Andra und Partners. The main contractor was the Forth Crossing Bridge Constructors (FCBC), which is a consortium of Hochtief, Dragados, American Bridge and Morrison Construction. Other civil engineering aspects of the project, such as the geotechnical investigations, were dealt with by engineering majors Arup and Jacobs.
Design Features
Challenging Design
During the process of detailed designing, 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, they were able to show that by some minor changes to alignment they 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 7000 tonnes of embodied carbon) and removed the lorry movements & its associated emissions that would have been required to dispose of the embankment material off site.
Offsite Construction, Onsite Assembly
Offsite construction, onsite assembly techniques were used entirely in the Queensferry project. The factory environment in which the 25 m to 30 m diameter circular steel caissons were created to the steel main deck sections before being lifted into place 50 m above the Firth of Forth to the sign and signal gantries used to manage traffic flows around the Crossing minimised embodied carbon. These factory fabricated components were also installed with all flooring, fencing and electrical equipment prior to erection over the road which also improved quality and reduced disruption during construction and into operation.
Predictive Asset Maintenance
During the design of the Queensferry Crossing they developed a new proactive predictive approach to asset management. Through the installation of sensors across the structure it was 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 enabling 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.
Stay Cable Design
One of the most striking features of the new crossing is its overlapping stay cable design. This aesthetic cable configuration provides extra stability to the structure and the Center Tower in particular, facilitating the sleek design of the towers.
The Deck
Suspended by these cables 50 m above high tide, the main deck offers 4 lanes of wind protected traffic, plus two hard shoulders.
The Towers
At up to 210 m high, the 3 slim towers make the Queensferry Crossing the tallest bridge in the UK, and 50 m higher than the earlier road bridge towers. They were constructed in stages using an innovative climbing formwork system.
The Foundations
The geology of the Forth estuary is complex and noted for different approaches to the foundations of each tower. All foundations bear on the top of the rockbed, which removed the need for expensive and delay due to time-consuming piling onto the hard rock that underlies the whole crossing.
Construction
Preparations for the new bridge began in September 2011 with works beginning at the southern end of the M90 to build the northern approach roads. 149 segments of bridge deck, each of 12 m (39 ft) long and 40 m (130 ft) wide, were constructed overseas then delivered by sea in October 2013. The approach steel bridge sections were manufactured by Cleveland Bridge & Engineering Company in Darlington.
The cable stay bridge erection cycle included segment lifting, rib bolt-up, welding, concrete stitch pour installation, cable stay installation and tensioning and gantry launching. A special narrow gantry was conceived and developed by the team that took full advantage of the center planes of cable stays and their attendant stay webs.
Records
The structure spans 1.7 miles (2.7 km) making it the longest three-tower, cable-stayed bridge in the world.
New world record in 2013 when achieved the largest continuous underwater concrete pour. The 24-hour non-stop operation successfully poured 16,869 cum of concrete into the water-filled south tower caisson.
Before completion of the final closure sections on the deck, the balanced cantilevers which extend 322 m north and south from the central tower, i.e. 644 m tip to tip, were recorded by Guinness as the longest ever.
Highest bridge towers in the UK (210 m)
Longest free-standing balanced cantilever in the world. (Centre Tower deck fan was 644 m wide prior to being connected to rest of structure)
Factual Numbers
15 days of pouring concrete 24/7 to achieve the Queensferry Crossing’s first world record for the longest continuous underwater pour. The concrete was poured to the foundations of the South Tower.
65 options were considered before the cable-stayed bridge design of the Queensferry Crossing was selected as the best to proceed with.
122 deck sections which make up the bridge deck. Each one of these sections can weigh up to 750 tonnes!
23,000 miles of cabling used which estimates to if laid.
35,000 tonnes of steel used in the bridge superstructure andpeople who voted in the name the bridge process.
150,000 tonnes of concrete poured over the course of the project.
10,000,000 man hours approximately that were involved in the construction.
Sustainability
As well as creating an iconic structure, which has come to represent the modern progressive Scotland, the bridge facilitates active and public transport and has reduced 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.
References
https://interestingengineering.com/innovation/the-queensferry-crossing-an-early-21st-century-engineering-icon
https://www.theforthbridges.org/queensferry-crossing/facts-and-figures#:~:text=World%20class%20%26%20world%20record%20 breaking,largest%20continuous%20underwater%20concrete%20pour.
https://ramboll.com/projects/ruk/queensferry-crossing-northern-europes-largest
https://www.americanbridge.net/featured-projects/forth-replacement-crossing-queensferry-crossing/
By -
Tuhina Chatterjee, Associate Editor - Civil Engineering and Construction Review
