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Our most iconic bridges can be categorized in two basic types:

Suspension...

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...and cantiliever.

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Here is the scenario in question:

There is a straightforward (as in radiating out in cardinal directions) network of bridges that connect city to city in the whole of Europe (can't think of a continent with a flatter enough terrain to start.) Each bridge is 200 feet wide and 200 above street level, enough for four railroads, 18 traffic lanes and four pedestrian walks. Per 6,000 feet stood a massive stone-and-concrete tower 1,125 feet tall. Above city grounds, the towers would double as apartment complexes.

Using the detail listed above, which type would better suit this kind of challenge, suspension or cantilever?

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    $\begingroup$ Qualify "better". Cheaper? Stronger? Cooler looking? $\endgroup$ – intrepidhero Oct 7 '16 at 20:19
  • $\begingroup$ What materials are available? $\endgroup$ – JDługosz Oct 8 '16 at 9:14
  • $\begingroup$ The scale of the project is ridiculously big! why would they want to do that in the first place? Just the width of the bridge is larger that the width of many rivers. $\endgroup$ – Vincent Oct 8 '16 at 15:22
  • $\begingroup$ @Vincent untappedcities.com/2013/07/25/… $\endgroup$ – JohnWDailey Oct 8 '16 at 22:25
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Suspension bridges are not typically used for very heavy loads. Suspension bridges are for spanning large gaps (2,000 feet +) cheaply. In your scenario, a cantilever bridge would be the way to go.

Edit: I didn't understand the "Per 6000 feet" bit. As others have pointed out, no bridge meets your requirements. Cantilever bridges more than a mile long are for all intents and purposes are mechanically impossible, and a suspension bridge that long will not have the strength to hold that much weight. You can build whatever bridge you like if you use Unobtanium

"Cheaply" means the amount of resources (both men and material) used to span the gap.

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  • $\begingroup$ "Spanning large gaps cheaply." How do you mean by that? $\endgroup$ – JohnWDailey Oct 7 '16 at 21:47
  • $\begingroup$ Without knowing much about the differences between the two types of bridges, I would guess that he means suspension bridges are much less expensive to construct than cantilever designs. He's saying that your scenario would require these bridges to bear extreme amounts of traffic (constantly loaded) without much in the way of large gaps (wide rivers, etc.) to span. $\endgroup$ – Palarran Oct 7 '16 at 21:59
  • $\begingroup$ untappedcities.com/2013/07/25/… So this idea would have been a failure from the getgo? $\endgroup$ – JohnWDailey Oct 7 '16 at 22:15
  • $\begingroup$ That is likely light rail, but yes it pretty much would be doomed. Look at the longest suspension bridge.It only has 6 lanes traffic lanes and 4 emergency lanes (and at the toll they charge to cross, it will pay for itself in ~10000 years). Now look at railroad suspension bridges. Can I build a mountain bigger than Everest? Yes, but why?? $\endgroup$ – Tim Oct 9 '16 at 19:46
  • $\begingroup$ After doing more searching, that bridge would have been a 3000 foot main span and would have carried some heavy rail. Based on the 1930 estimated cost of $180 million you're talking about $2.4 billion after adjusting for inflation (but not real estate prices). Some other projects for perspective: $\endgroup$ – Tim Oct 9 '16 at 20:16
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Neither one will work with current technology.

The longest cantilever bridge span in existence is the Quebec Bridge at 1800 feet -- far short of your 6000 feet. Suspension bridges can handle the span (barely: the world's longest span, the Akashi Kaikyō Bridge, is 6532 feet), but the engineering calculations for a multi-span bridge are so complicated that nobody's ever built one. The few "multi-span" suspension bridges in existence (eg. the Bay Bridge) have artificial islands in the middle, making them structurally two bridges connected end-to-end.

What might work is a cable-stayed bridge. The Russky Bridge has a longest span of 3622 feet, more than half your desired length, and calculations for a multi-span bridge are simpler than those for a suspension bridge. Stabilizing the central spans is still a bit of a challenge, and you'll need to make your towers taller.

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  • $\begingroup$ "Stabilizing the central spans is still a bit of a challenge, and you'll need to make your towers taller." The only problem is, there's no bridge tower on Earth taller than 1125 feet, so I can't imagine such a comparison. $\endgroup$ – JohnWDailey Oct 8 '16 at 1:00

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