Oliver Riches MIStructE and Chris Hill MIStructE are structural engineers working on the Queensferry Crossing project, a new bridge over the Firth of Forth scheduled to open in August 2017. Here they discuss this impressive Transport Scotland project and their part in its design and construction.
The Queensferry Crossing is a unique project in a unique location. It stands alongside two other iconic bridges, each representing a different century and design approach: this is a stay cable bridge, as opposed to the Forth Road Bridge (a suspension bridge) and steel cantilever structure of the Forth Rail Bridge. When it opens it will be the longest three tower cable stayed bridge in the world.
L-R: Queensferry Crossing, Forth Road Bridge, Forth Rail Bridge (Image Transport Scotland)
It will provide a key strategic link from the East of Scotland to the rest of the country as well as a vital local connection for those working in Edinburgh and living in Fife (or vice versa). The closure of the Forth Road Bridge in December 2015 highlighted the need for a durable road crossing in this location, and the new structure is designed to have a life of 120 years.
We’re able to achieve this life span in large part due to the stay cables, which are replaceable components with a multilayer corrosion protection system - galvanized wires wrapped in wax within an HDPE extrusion, which prevents initial corrosion forming in the same manner as the Forth Road Bridge.
The Crossing features 288 stay cables in total, ranging in length from 97m to 419m. Each cable is made up of individual strands (between 43 and 106) and the total length of the combined cable strands is 23000 miles.
Construction of the Crossing started in 2011. As you would expect for a structure of this size, it has been a challenging project: 9 of 15 foundations were constructed from barges, with excavations taking place from 50m up in the air (to install the deck units) to 40m underwater - construction featured the largest ever underwater concrete pour (approximately 17000m3). It’s also a very windy location, and there were safety restrictions on many lifting operations, particularly for deck lifting and stay cable installation operations.
The first deck lift (Image Transport Scotland)
Fortunately, wind will not be such a big problem for drivers: the Queensferry Crossing will be able to stay open in much windier conditions than the Road Bridge thanks to its innovative wind shielding. It is installed on both sides of the bridge deck and stands approximately 3.3m high, with ‘post spacing’ which effectively slows down crosswinds and reduces over turning moments on vehicles. The cantilever deck and shape also contributes to reducing wind speed and direction, diverting the wind slightly upwards.
Wind shielding (Image Transport Scotland)
We’re very proud of the new Crossing, which not only addresses the engineering challenges, but also acknowledges the engineering heritage with which it shares the site. Aesthetics are hugely important on a project like this, and we’re delighted to have worked on such an elegant structure, a true engineering landmark for Scotland.