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The extreme engineering behind the UK's tallest bridge

Published 30th August 2017
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The extreme engineering behind the UK's tallest bridge
Written by Oscar Holland, CNN
There may have already been 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 now the UK's tallest bridge.
Expected to serve 24 million vehicles a year, the structure opened to traffic for the first time this week. It will be officially opened on Monday, marking the end of a decade-long engineering project involving 35,000 tons of steel, 23,000 miles of cable and 10 million man hours.

A bridge for the times

When Queen Elizabeth II cuts the ribbon at the Queensferry Crossing, it will have been 53 years to the day since she opened the neighboring Forth Road Bridge. Structural problems facing the latter -- including the corrosion of some of its steel cables -- were behind the decision to build a new crossing at a cost of £1.35 billion ($1.75 billion).
The original road bridge will continue to carry pedestrians, bicycles and public transport. The adjacent railway bridge, which was completed in 1890 and has been named a UNESCO World Heritage Site, also remains in use.
But with an estimated design life of 120 years, it is hoped that the Queensferry Crossing will outlast both of its neighbors. To combat the threat posed by corrosion, individual steel strands within the bridge's supporting cables can be replaced at any time, without requiring the bridge to be closed to traffic.
New sensor technology has also been installed to identify potential structural issues, according to the project's technical director Dr. Mike Glover, who believes that the bridge could stand for up to 150 years.
"About a thousand sensors have been installed on the Queensferry Crossing, carefully positioned to provide advanced warning of structural problems," he said in a press statement. "The data will tell the maintenance teams where to inspect and intervene to pre-empt potential issues.
"This is the future of roads and bridge maintenance -- combining sensor technology and cloud-based data analytics to predict problems before they happen."

A feat of engineering

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 boast 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 began in 2011 and was led by a consortium of companies from Scotland, Germany, Spain and the US. The bridge's central tower was built on rock beneath the water's surface, while the other two are founded on a giant steel cylinder beneath the river bed.
Innovations found on the structure include so-called "intelligent transport systems" that help manage traffic flow, and a line of 3.5-meter-high barriers to protect traffic from wind.
"The Forth Estuary is no stranger to blustery conditions," Glover said. "(We have fitted the structure with) permeable screens that allow some of the air through and scoops the rest up and over the bridge, preventing severe turbulent wind conditions across the deck.
"These wind shields will provide vehicles with protection from the frequent gale force winds -- reducing traffic incidents and keeping people safer."