Queensferry Crossing, cable design

Ramboll: Queensferry Crossing. Image courtesy of Transport Scotland

Ramboll: Queensferry Crossing. Image courtesy of Transport Scotland

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Centre-stage of the bridge’s aesthetic and structural elegance are the glistening white crossover stay cables covering a 160m length of the deck at mid-point along the two main spans. In total there are 288 stays with each individual cable stay comprising of between 45 and 109, 7 wire, 15.7mm diameter strands. These stay cables range in length from 94m to 420m.

With two main spans being situated over navigation channels, a major challenge to the three-tower design was the stability of the central tower. The required stiffness was obtained by extending the length of the stay cable fans beyond mid-span so that they overlap in the central region of each span. By supporting these segments with the extra cables the effective structural depth of the deck mid-span is increased. This stiffens the entire deck sufficiently for it to stabilise the central tower.

The ten extra overlapping stay cables in each of the two main spans double the number of cable supports for each of the ten mid-span deck sections both sides of the central tower. As the vertical load is shared by four cables the force per cable is relatively low.

The innovative solution to the structural challenge of stabilising the central mid-estuary tower created a virtual truss system involving 23,000 miles of stay wire stretched between the towers. This solution allowed all three towers to have the same slender profile tapering to a narrow top. This enhanced the cable-stayed crossing’s visual impact.

The cables are anchored at the top and bottom and are sheathed in HDPE to provide protection against corrosion. The novel plastic sheathing features a ribbed design, intended to reduce any resonance in the wind. The cable-stayed design will also be easier to maintain, allowing individual strands within the cables to be taken out and replaced if necessary, without adversely affecting the traffic capacity.

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