The Queensfery crossing: an engineering marvel
There may already be two large bridges connecting Lothian and Fife in Scotland, but now there is a third. And this one is the biggest yet! In fact, towering 689 feet above the River Forth, the Queensferry Crossing is the UK’s tallest bridge.
Touted as “one of the world’s great bridges”, it will substitute the Forth Road Bridge as the main road route between Edinburgh and Fife. The new crossing should carry over 24 million vehicles every year. Widely considered as one of the largest Scottish infrastructure projects in a generation, the structure opened to traffic for the first time last week.
The name of the new bridge was decided in a public vote and pays homage to the Firth of Forth’s history and the two villages that sit on opposite shores: North and South Queensferry. Their names and that of the new bridge refer to a legend that St Margaret of Scotland, queen to the 11th-century king Malcolm Canmore, ordered the first organised ferry service to help pilgrims reach St Andrew’s.
A bridge for the times
Queen Elizabeth II cut the ribbon at the Queensferry Crossing on Monday, precisely 53 years to the day since she opened the neighbouring Forth Road Bridge. The need for another bridge emerged in 2004. Structural problems facing the latter – including the corrosion of some of its steel cables – were behind the decision to build a new crossing, rather than rejuvenate the old passage. The opening marked the end of a decade-long engineering project involving 35,000 tons of steel, 23,000 miles of cable, 10 million man hours at a cost of £1.35 billion.
The existing Forth Road Bridge was built in 1964 and has since been plagued by upkeep issues. However, it will remain used by bicycles, pedestrians and eventually buses. The iconic Century Forth Bridge, which was completed in 1890 and carries the railway over the river, also remains in use, lying only a mile away.
With an expected lifespan of 120 years, it is anticipated that the Queensferry Crossing will outlast both of its neighbours as it has been ‘designed for maintenance’. To fight the risk posed by corrosion, individual steel strands within the bridge’s supporting cables can be substituted at any time without having to close it to users.
Approximately a thousand sensors have been fitted on the Queensferry Crossing, carefully positioned to detect potential structural issues. The information will advise the maintenance teams where to check and intercede to pre-empt potential issues. This may well be the future of roads and bridge maintenance – merging sensor technology and cloud-based data analytics to predict problems before they occur.
While the bridge is significantly shorter than the world’s tallest – a title held by the 1,125-foot-high Millau Viaduct in France – it can claim two engineering records of its own. The new structure is both the longest three-tower, cable-stayed bridge in the world, and the longest free-standing, balanced cantilever in the world.
First announced in 2007, construction on the Queensferry Crossing started in 2011 and was managed by a consortium of companies from Scotland, Germany, Spain and the US.
The bridge’s central tower was built on the rock beneath the water’s surface, while the other two are founded on giant steel cylinders beneath the river bed.
Structural innovations incorporate the so-called “intelligent transport systems” that help manage traffic flow, and a line of 3.5-meter-high barriers to shield traffic from the wind. The structure has also been fitted with permeable screens. They work by letting some of the air through, whilst scooping the remainder up and over the bridge, thus avoiding severe, turbulent wind conditions across the deck. These wind shields will protect vehicles from the common gale force winds in the area, decreasing traffic incidents and keeping people safer.
The stages of building the Queensferry Crossing
Queensferry Crossing is one of the very few main developments of its type and scale that have been able to go from initial steps to completion in less than 10 years. Transport Scotland spent more than three years outlining, procuring and managing the project through to endorsement by the Scottish Parliament. The contractors arrived on site in the summer of 2011, with many engineering challenges ahead.
One of the most demanding elements was the underwater foundations of the three main towers and viaduct piers. Building these involved sinking massive steel caissons to the bed of the cold and huge river Forth, then excavating through the silt and till to bedrock. To worsen the situation, the Firth of the Forth is known for having tough and low visibility water, especially as it winds itself towards the North Sea.
Another difficulty was the production of the actual bridge. The project’s marine yard became a production line for fabricating the Queensferry Crossing’s deck segments, where casting the reinforced concrete deck for each steel box section trebled the overall deck section weight from around 250 tonnes to 750 tonnes.
These gigantic sections then had to be transported across the water to meet their respective towers. From the marine yard, these segments were barged out into the Forth to be elevated by special movable hydraulic lifting equipment called erection travellers. The process was highly sensitive to wind and tide, taking around four hours to carefully lift each section around 80 metres to deck level.
They were then held in place by the traveller before cable stays could be attached, tensioned and the section welded and ‘stitched-in’ with concrete. Each deck fan was then shaped with sequential north and south lifts that balanced the structure.
The last phase included completion of numerous operational and visual final touches to get the new bridge ready for its public opening. Until recently, the deck of the Queensferry Crossing showed exposed concrete – but road surfacing work added recently gave it recognisable road materials and markings. The wind-shielding technology was also added to complete the manufacturing.
The Queensferry Crossing is now finished. A moving roadblock was put in place to stop traffic driving across the Forth Road Bridge, with drivers being redirected to the new bridge. The Northbound carriageway opened first, with the southbound carriageway opened about 45 minutes later. On the weekend, the bridge closed again to allow 50,000 members of the public a unique opportunity to stroll over it. The bridge has finally permanently reopened earlier today to traffic. Are you looking forward to hitting the road?