How to support an unsupported tower

Connected Society 4 December 2017 Peter Curran

How do you make the world's longest three-tower cable-stayed bridge elegant and sturdy? By designing an innovative, world-class cable solution.

4 mins

The spectacular Queensferry Crossing in Scotland represents the pinnacle of bridge design and technical expertise. 

At its centre is a unique cable design that allows the bridge deck to be lighter and its three supporting towers to be more slender and needle-like. The northern and southern towers of the bridge are supported by cables that reach back to the bridge’s approach viaducts. However, the central tower stands unsupported, so a major challenge for engineers was to ensure its stability. 

The solution was a clever system of ten overlapping cables that provide extra support for the deck sections on either side of the central tower. By stiffening these segments with extra cables, the entire deck is stiffened sufficiently to stabilise the central tower. 

This also allows the deck to be lighter and the three supporting towers to have the same slender profile, significantly enhancing the bridge’s visual impact. 

One aspect which will not cause much of an impact is maintenance – closure and delays as a result of ongoing repairs are usually the bane of bridge users. 

The Queensferry Crossing has a unique solution to that too: the cables can be replaced strand by strand so that the bridge remains operational during any replacement process – a vital feature for such a major transport artery.

Speaking at the opening of the bridge, Scottish Council for Development and Industry Chief Executive, Mark Bevan said that “everyone involved in the opening of this majestic addition to the bridges over the Forth should be proud of their achievement.”

Cables to stay the distance

  • Length of the bridge: 2,638 metres
  • Length of cable-stay section: 2,090 metres
  • Two main deck spans: 650 metres
  • Number of cable stays: 288 – ranging in length
    from 94 to 420 metres
  • Amount of cabling: 37,000 kilometres of cabling -nearly enough to span the circumference of the Earth