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Arthur T Knackerbracket has found the following story:

Newly identified bridge forms could enable significantly longer bridge spans to be achieved in the future, potentially making a crossing over the Strait of Gibraltar, from the Iberian Peninsula to Morocco, feasible.

[...] A bridge's span is the distance of suspended roadway between towers, with the current world record standing at just under 2km. The most popular form for long spans is the suspension bridge form, as used for the Humber Bridge, though the cable-stayed bridge form, where cables directly connect the tower to the roadway -- such as used in the recently constructed Queensferry Crossing in Scotland -- is becoming increasingly popular.

As bridge spans become longer, a rapidly growing proportion of the structure is needed just to carry the bridge's own weight, rather than the traffic crossing it. This can create a vicious cycle: a relatively small increase in span requires use of significantly more material, leading to a heavier structure that requires yet more material to support it. This also sets a limit on how long a bridge span can be; beyond this limit a bridge simply cannot carry its own weight.

[...] Professor Matthew Gilbert from the University of Sheffield, who led the research, said: "The suspension bridge has been around for hundreds of years and while we've been able to build longer spans through incremental improvements, we've never stopped to look to see if it's actually the best form to use. Our research has shown that more structurally efficient forms do exist, which might open the door to significantly longer bridge spans in the future."

[...] The mathematically optimal designs contain regions which resemble a bicycle wheel, with multiple 'spokes' in place of a single tower. But these would be very difficult to build in practice at large scale. The team therefore replaced these with split towers comprising just two or three 'spokes' as a compromise that retains most of the benefit of the optimal designs, while being a little easier to construct.

For a 5km span, which is likely to be required to build the 14km Strait of Gibraltar crossing, a traditional suspension bridge design would require far more material, making it at least 73 per cent heavier than the optimal design. In contrast, the proposed two- and three-spoke designs would be just 12 and 6 percent heavier, making them potentially much more economical to build.

[...] The team emphasise that their research is just the first step, and that the ideas cannot be developed immediately for construction of a mega span bridge. The current model considers only gravity loads and does not yet consider dynamic forces arising from traffic or wind loading. Further work is also required to address construction and maintenance issues.

-- submitted from IRC

Journal Reference:
Helen E. Fairclough, Matthew Gilbert, Aleksey V. Pichugin, Andy Tyas, Ian Firth. Theoretically optimal forms for very long-span bridges under gravity loading. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science, 2018; 474 (2217): 20170726 DOI: 10.1098/rspa.2017.0726

Original Submission