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from the shape-of-things^Wbridges-to-come dept.
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
(Score: 4, Interesting) by MostCynical on Monday September 24 2018, @08:05AM (5 children)
if the picture of the proposed bridge in TFA is to scale, the upright components of this brige are over three kilometres tall.
Aeroplanes and helicopters watch out!
Suspect it will have interesting microclimate effects, as well.
"I guess once you start doubting, there's no end to it." -Batou, Ghost in the Shell: Stand Alone Complex
(Score: 4, Insightful) by choose another one on Monday September 24 2018, @09:47AM
They look to be about half-span long and 30degrees off vertical, 5km span would give only 2km high - still stupidly high though.
Bigger problem is "not yet consider dynamic forces arising from traffic or wind loading" - because as every engineer knows making bridges lighter is always the right thing to do...
(Score: 2, Interesting) by Anonymous Coward on Monday September 24 2018, @02:56PM (1 child)
This is the sort of thing where tunnels make a lot of sense. For places like this, you'd be better off building a tunnel underneath rather than trying to go over the top. Tunnels are expensive, but the technology needed is mostly there. They could likely even build a tube to put the tunnel in if they don't want to got to the absolute bottom of the body of water.
Around here, we've got a similar sort of problem where we've been using ferries to ferry people across the sound and the cost of tunneling has gotten to the point where the cost of just a couple ferries completely funds the building of a fairly long tunnel that would greatly improve the connection to the peninsula.
(Score: 2) by bob_super on Monday September 24 2018, @06:42PM
Today I learnt that the South end of the Gibraltar strait is actually part of the Eurasian plate.
I was going to point out that a tunnel that has to shrink over 2cm per year would be an interesting feat, but it turns out that it doesn't need to, since both end are indeed on the same plate, not on the Africa Plate.
(Score: 2) by deimtee on Tuesday September 25 2018, @01:34AM (1 child)
Wikipedia says the strait is between 300 and 900 metres deep. If your alternative is to build 3km towers, you may as well drop a couple of shorter ones in the in the middle and have reasonable spans.
If you cough while drinking cheap red wine it really cleans out your sinuses.
(Score: 0) by Anonymous Coward on Tuesday September 25 2018, @03:18AM
TBH, probably the easiest way of doing it, would be a floating bridge. It would be a challenge as this is connected to the ocean, and there's plenty of ships that are coming and going, but it would probably be easier to build a floating bridge than a suspension bridge.
The engineering on it would be pretty nifty as you could probably put the road surface on a gymbol and have a high rise section that's high enough to allow for large ships to pass under. Certainly easier to do than try to force a suspension bridge to do it.