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In my book series, there are 2 main continents on the planet Aurea: Koumaris and Louzi. There is a spot here where the 2 continents come very close to touching, separated only by about 2,500 feet (760 metres) of water at the strait's narrowest point. However, the strait sits on a transform fault and is just over 375 feet (115 metres) deep here. The civilization on the planet, is based primarily on Komnenian-era Byzantium, and as such they would have the architectural technology of around that place and time. Could they bridge this body of water, and if so how?

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  • $\begingroup$ Separatrix may be right; the only plausible way that comes to mind would be if the "bridge" was floating... and the water conditions probably make that impractical if not impossible. $\endgroup$
    – Matthew
    Mar 25, 2020 at 19:35
  • $\begingroup$ Not only a bridge but a tall bridge; I have to think this would be an important waterway for ships. $\endgroup$
    – Willk
    Mar 25, 2020 at 20:33
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    $\begingroup$ Leonardo da Vinci's bridge design might be relevant here: livescience.com/da-vinci-bridge-never-made.html $\endgroup$
    – jamesqf
    Mar 26, 2020 at 0:01
  • $\begingroup$ @jamesqf - I was just reading about that bridge and came back here to post an answer. But you were here first. Why don't you put that up as an answer? I have to think if it could be done in 1504 the Byzantines could have done it. $\endgroup$
    – Willk
    Mar 26, 2020 at 2:17
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    $\begingroup$ @Willk: Yes, but the Leonardo design is only a bit over 900 ft. The other problem would be building scaffolding to hold it during construction - once it's up, it's self-supporting, but until then it's unstable. Still a problem to this day: there's a recently-built bridge near me (en.wikipedia.org/wiki/Galena_Creek_Bridge ) which would have collapsed if a strong windstorm (not uncommon hereabouts) had happened during construction. $\endgroup$
    – jamesqf
    Mar 26, 2020 at 17:36

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Yes, technically

The thing here is that a bridge isn't required to have supports all 375 feet deep if it doesn't need supports. Or, that is to say, so long as it can just float on the surface, then the bridge could work. And now I introduce pontoon bridges. Pontoon bridges can, given the right equipment, be built very quickly and over wide stretches of water.

Unfortunately, there are two downsides. The first is that it can't be used to transport large quantities of heavy objects, given that it's a floating bridge. And the second is that, given the fragile nature of it, honestly all it takes are a few medium sized waves and the whole bridge is gone. So if this gap is anything but quiet waters, you can have a bridge last a few hours or something, and then it's gone.

But it's possible to build a pontoon bridge.

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    $\begingroup$ Xerxes used temporary pontoon bridges across the Dardanelles for an invasion of Greece. $\endgroup$
    – Patricia Shanahan
    Mar 25, 2020 at 21:12
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    $\begingroup$ There actually are a number of floating bridges in use today, particularly in the Seattle/Puget Sound area. For instance en.wikipedia.org/wiki/Lacey_V._Murrow_Memorial_Bridge $\endgroup$
    – jamesqf
    Mar 25, 2020 at 23:55
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    $\begingroup$ And a third one: ships can't pass trough/under the bridge $\endgroup$
    – Kepotx
    Mar 26, 2020 at 7:51
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    $\begingroup$ The comment on fragility seems wrong. Boats are not "fragile" and sunk by "a few medium sized waves". A properly designed pontoon bridge is basically a line of boats fastened together. $\endgroup$
    – alephzero
    Mar 26, 2020 at 9:40
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    $\begingroup$ @Kepotx Actually, all three of the long floating bridges in the Seattle/Puget Sound area (one carries Interstate 5) have high-rise sections for (limited) ship passage; they used to have drawspans before they were rebuilt over the past twenty years or so. Drawspans are practical for a floating bridge even with medieval technology. $\endgroup$
    – Zeiss Ikon
    Mar 26, 2020 at 11:08
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It's a fairly short answer, but historically solid.

No

Or they'd have bridged the Bosphorus that's 2450ft wide at its narrowest point.

In practice the Bosphorus wasn't bridged until 1973, which tells you quite how hard a job it is.

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    $\begingroup$ And the Bosporus isn't anything like 375 feet deep. $\endgroup$
    – Zeiss Ikon
    Mar 25, 2020 at 19:27
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    $\begingroup$ @Matthew suspension bridges need tensile steel at that size. And tensioning with care, unless you want Tacoma Narrows at the first breeze $\endgroup$
    – Adrian Colomitchi
    Mar 25, 2020 at 22:51
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    $\begingroup$ Although the Bosphorus was bridged by Darius I in approximately 513 B.C. That bridge was by boats (I originally heard it was done by filling and sinking boats to make pylons, but Wikipedia suggests they were used as pontoons, just as Xerxes did later at the Hellespont.) Of course the bridge only survived a few months. $\endgroup$
    – Paul Sinclair
    Mar 26, 2020 at 16:57
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    $\begingroup$ It is very much possible to have a pontoon bridge which has a part that opens. The Koninging Emma bridge, Curaçao is one (rare) example: en.wikipedia.org/wiki/Queen_Emma_Bridge $\endgroup$
    – Willeke
    Mar 27, 2020 at 11:20
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    $\begingroup$ "They would have built a bridge if it had been possible" is not (on its own) completely convjncing. It is sometimes useful to not have bridges. That is why bridges tend to be blown up during wars, and why rivers that divided important travel roads sometimes have been intentionally left without bridges. E.g. the Oi river in Japan during the Tokugawa shogunate. $\endgroup$
    – Jan
    Mar 27, 2020 at 14:36
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No

Even with today's technology building a bridge over a transform fault** is a perilous endeavor - non-trivial displacements over time preclude a rigid bridge and the presence of the fault will trigger major earthquakes frequently. At the best today one may try a suspension bridge, but it won't last long if it ever get to be build at all.

As for a suspension bridge during Byzantium times, some problems of the cables:

  1. rope won't do, not tensile enough, tensile steel is well in the future

  2. any cable/rope used is going to be heavy at those distances - tensioning them correctly is a huge problem at that time (don't tension the correctly and you have Tacoma Narrows)

  3. corrosion - a google search for Golden Gate Bridge corrosion see for yourself

** San Andreas fault is one such a transform fault. If moves on an average of 30-50mm/y

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They could, but they wouldn't.

From technical point of view there is nothing that could stop them from building such structure. Justinian build 430 metres long bridge around 500 AD.

There are few reasons why not from logic, logistic and need point of view.

Logic - is there a need to "waste" time, effort and materials to build such structure. Are there no other more imporant need on both sides (like fortification, roads, coast line).

Logistic - Is the movement of people and goods so intense it require a bridge (that would need to be high enough to let ships pass or have bascule part (which again require even more materials). So if the X amount of uses are so big that it cannot be fulfilled by ferry with Y capacity taking of each set of time? So if the ferries would need to take of so often they would form a floating bridge by itself.

Need - bridges over rivers where very strategic points, they created natural bottlenecks. The downside that it was created on both ends. Do you need to spend a lot of money and work on service just for someone on the other side to say "nah, you can't pass. untill you pay us one million dollors!"

What could be done, much cheaper and faster, is to expand existing harbor into to sea. Just 100 metres from each side shorten the distance by 200 metres. Giving you on both sides 200m metres of side ferries, ships and other can attach to. And more space to load, unload good and people.

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    $\begingroup$ The ferry argument reminds me of another point: economics. The Remagen bridge over the Rhine is something that existed, but has not been reconstructed after WWII. It's function was replaced by ferries nearby - who would meanwhile strongly resist a reconstruction that would render them unemployed ... $\endgroup$
    – Hagen von Eitzen
    Mar 27, 2020 at 20:45
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If by bridge you mean "giant pile of rocks", then yes you could build a bridge, though it would take many decades. A trapezoid with a height and width of 115 meters, a length of 760 meters and an upper width (walking path) of 10 meters would have a volume of 5.46 million cubic meters. The great pyramid had a volume of 2.5 million cubic meters, even more if you include its 2 neighbors, so it is within the realm of possibility that a civilization could spend 50 years dropping slabs of stone into the ocean to very slowly create a bridge.

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    $\begingroup$ That would be a dam, not a bridge, and I'd be amazed if the dimensions you're describing could hold back the current that's likely present in the channel. $\endgroup$
    – Salda007
    Mar 26, 2020 at 8:36
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    $\begingroup$ @Salda007 in fairness it wouldn't be trying to hold back the current - what is described could be quite porous. That said, as the construction neared completion, the speed of the current passing through the largest remaining gap would still increase with inverse proportionality to its size, meaning that finishing such a bridge would be difficult (read: impossible). However, mixed methods - i.e. building a traditional bridge for the last stretch, might be more feasible (than this method alone) depending on the exact situation.Even so other suggestions are more practical. $\endgroup$
    – Isaac
    Mar 26, 2020 at 11:12
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    $\begingroup$ @Isaac, it might start out quite porous, but between dirt and sea life, it'll very quickly clog up. $\endgroup$
    – Mark
    Mar 26, 2020 at 22:23
  • $\begingroup$ I didn't think of this - great idea ! $\endgroup$
    – Fattie
    Mar 28, 2020 at 17:49
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Absolutely. Technically, it's no problem. You just make the strait less deep. You start dumping fill (largish rocks, mostly - no soil) at one end, and when the level of the fill gets to within about 10 feet of the surface you start building a "normal" bridge, which gradually gets extended as the fill ridge extends. Assuming a 45 degree slope for the fill ridge, you're talking about 10 million cubic meters of rock, which is "only" about 4 times the volume of the Great Pyramid of Giza.

It won't last, of course. Transform faults aren't stable, so the bridge is going have problems - but that's an issue for later generations to deal with. It is not a surprise that most of the 7 wonders of the ancient world are no longer with us.

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An alternative to bridge may be to make a mole like the one constructed by alexander the great for the siege of TYR. [wikipedia link]1

In your case, this would required extensively large amount of raw materials and hours of works, but it does not seem impossible if your empires are able to commit on this task for a very long period of times.

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I once enjoyed a very decent fish dinner on a restaurant on the Galata bridge in Istanbul, a floating bridge which spans two continents as described. Pontoon bridges have been around since at least Roman days.

Sadly, I see it was damaged by fire in 1992 and replaced by a far more boring structure.

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    $\begingroup$ The Galata Bridge spans the Golden Horn, therefore it's completely on Europe continent. $\endgroup$
    – user73560
    Mar 27, 2020 at 20:54
  • $\begingroup$ I stand corrected! But the engineering challenge is identical. And the dinner was great. $\endgroup$
    – David Hambling
    Mar 29, 2020 at 7:38
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Any technology that is distinguishable from magic is insufficiently advanced...

With that level of tech, I would think it unlikely. However, it may be possible to use some form of cable ferry.

These were widely used at one point. The ferry was attached at each end to a cable spanning the channel. The ferry had a large flat keel. By extending the tackle at one end of the ferry, the relative angle of keel and current could be changed, propelling the craft across the water with only the slight sound of burbling water.

This could also be done, with sails, with the advantage that the boats don't need a heavy keel to keep from being blown to leeward.

The down side: Like elevators, only one per shaft/cable. Unless you figure a way to make a one way grid of cables, and shunt the north bound boat onto the south bound cable for the return trip.

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Traveling Bridge

I'm sure that they could figure out that they could make a series of traveling floating bridges that could depart from one side of the straight and travel to the other using some form of locomotion. Making a large piece of cloth to catch the wind might do, or they could fashion some sort of stick that they could put in the water to push the floating bridge in the right direction.

If only we had a good name for such an amazing traveling bridge...

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    $\begingroup$ I got the... ummm... drift, not sure if others did. In any case I can't stop but notice the answer is sorta blown off the course of the question, which seems to insist on the traditional meaning of the word bridge. $\endgroup$
    – Adrian Colomitchi
    Mar 26, 2020 at 23:14

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