Would it be possible for the Roman Empire to build a bridge over the Bosphorus?

[JJ2161]

So, for my althist, I was thinking about Rome building a bridge over the Bosphorus after (fictional) Emperor Maximian transfers the capital to Romanova (New Rome, ATL Constantinople).

The bridge would be a grandiose work of engineering. It would be made of Roman concrete, in the typical arch design of the Roman bridges. It would also be taller in the middle to allow trading and military vessels to easily go through.

The bridge would be built after the conquest of Persia, by using the plunder to fund it, with the intent of facilitating the movement of troops between Europe and Asia. It would also have watchtowers and a suspension mechanism (think the Aztec bridges that connected Tenochtitlan to the mainland) to allow Roman troops to go through, but not enemies coming from the East.

Basically, because I plan for regular contact with China through the sea, I wanted the Roman Empire to have something that could be their version of the Great Wall. A work of engineering so great it would be viewed with awe even by the Chinese.

My problem is that I don't know if it is possible with pre-476 CE technology, or even technology that didn't exist at the time but could be realistically developed in the period. I know that the Romans could build monumental bridges. Additionally, in my research on the Bosphorus Strait (and the Dardaneles, the bridge could go over either), I'm struggling to find info on things like depth or topographical maps of the sea floor.

 

[Mildtryth]

So, for my althist, I was thinking about Rome building a bridge over the Bosphorus after (fictional) Emperor Maximian transfers the capital to Romanova (New Rome, ATL Constantinople).

The bridge would be a grandiose work of engineering. It would be made of Roman concrete, in the typical arch design of the Roman bridges. It would also be taller in the middle to allow trading and military vessels to easily go through.

The bridge would be built after the conquest of Persia, by using the plunder to fund it, with the intent of facilitating the movement of troops between Europe and Asia. It would also have watchtowers and a suspension mechanism (think the Aztec bridges that connected Tenochtitlan to the mainland) to allow Roman troops to go through, but not enemies coming from the East.

Basically, because I plan for regular contact with China through the sea, I wanted the Roman Empire to have something that could be their version of the Great Wall. A work of engineering so great it would be viewed with awe even by the Chinese.

My problem is that I don't know if it is possible with pre-476 CE technology, or even technology that didn't exist at the time but could be realistically developed in the period. I know that the Romans could build monumental bridges. Additionally, in my research on the Bosphorus Strait (and the Dardaneles, the bridge could go over either), I'm struggling to find info on things like depth or topographical maps of the sea floor.

From what i've heard, the maximum depth within the strait is 110 meters.

The length might be a problem - the longest extant Roman bridge around is 790 meters long, while the OTL Bosphorus Bridge is almost twice as long - and i'm using that as a reference because it seems like it's in the likeliest spot to build it - it's the spot with the shortest distance that's not too far away from the city.

But the suspension mechanism will probably be out of question.

 

[Mad Bad Rabbit]

Why not a pontoon bridge, like those of Xerces or Caligula? The Bay of Baiae is 3 miles across, so the Bosporus should be doable.

 

[Kellan Sullivan]

Didn't Leonardo da Vinci design a bridge over the straits for the sultan that was never built? When it was built by MIT a few years back, it was actually shown to be workable.

Now, bearing in mind Leonardo is nearly a millennium later, it might sound ASB, but I feel like Leonardo's design is probably still closer to what the Roman's had available than what it is to what we have nowadays.

http://e-casoc.blogspot.com/2019/10/leonardo-da-vincis-bridge-design-would.html?m=1

 

[Codae]

Didn't Leonardo da Vinci design a bridge over the straits for the sultan that was never built? When it was built by MIT a few years back, it was actually shown to be workable.

Now, bearing in mind Leonardo is nearly a millennium later, it might sound ASB, but I feel like Leonardo's design is probably still closer to what the Roman's had available than what it is to what we have nowadays.

http://e-casoc.blogspot.com/2019/10/leonardo-da-vincis-bridge-design-would.html?m=1

That record-breaking bridge they're talking about would just span the Golden Horn, though. To cross the Bosphorus the design would need to be scaled up by a factor of three or four, to say nothing about the challenges of the building process.

To get materials strong and cheap enough to make this bridge, as well as powering the necessary building methods, I think you need an industrial revolution. Even today we don't have any arches big enough to cross the strait in one span, and using multiple arches means having to construct supports with foundations deep underwater.

 

[CaekDaemon]

My problem is that I don't know if it is possible with pre-476 CE technology, or even technology that didn't exist at the time but could be realistically developed in the period. I know that the Romans could build monumental bridges. Additionally, in my research on the Bosphorus Strait (and the Dardaneles, the bridge could go over either), I'm struggling to find info on things like depth or topographical maps of the sea floor.

To get materials strong and cheap enough to make this bridge, as well as powering the necessary building methods, I think you need an industrial revolution.

Interestingly enough, I actually addressed something very similar to this question over in the Misc ASOIAF thread (this is the post - the relevant section is A, Bridging the Blackwater Rush; there's plenty of considerations in there that are ASOIAF centric, but that section focuses strictly on real world engineering considerations). Fundamentally speaking, Codae has it right: a bridge of this kind would need large cofferdams or some similar kind of structure to allow the river base to be exposed. The basis of how this is done hasn't really changed between the medieval and Roman era, so it'd look something like this...

..., but you can already see how little water that is actually working with, and asking for this bridge is going for a far, far grander scale. You're just not going to be able to use that technique in the face of depth this great. A quick google found this diagram of the depth profile of the Bosphorus, which gives a clear idea of the kind of depths involved.

Even getting to water depths of 50meters is a complete nonstarter for the kind of construction techniques that Rome would have available; they just can't get a foundation down there to actually build the bridge on. With the technology that they have, they'd need to somehow build a 50 meter tall waterproof box and then drain it, and long story short, that ain't happening with Roman technology. A modern bridge being built in those kind of conditions is more likely to just have the pillars built elsewhere and brought to the location and then sank (which is how the Kerch Strait bridge was built), but the Romans aren't getting that done. Even if you hand wave that, then you get to the kind of problems that I listed in that post regarding water pressure, which can only really be fought on this kind of scale with a pressurized structure akin to what was used to build the Brooklyn Bridge, else the cavity will end up flooding itself as water comes out of the now exposed floor of the Bosphorus. Without the means to either pressurize the build site for a caisson, or some other means to control the flow of water on a project that goes this deep, or some means to carry the full size pillars to the construction site and install them (a feat that is easier said than done even for modern tech), a classic arch bridge of Roman or medieval taste across the Bosphorus is basically unbuildable.

Fortunately, there is a way to bypass that particular problem of needing caissons or some kind of structural support, and it's...well, what they built

Just go over the chasm with no support, and you don't need to bother with the underwater part and everyone wins as the bridge becomes entirely possible...but I'm not so sure about the Romans building a massive suspension bridge

 

[Mad Bad Rabbit]

Xerces literally bridged the Dardanelles with a pontoon bridge and marched an army across. Granted it later sank in a storm, but during peacetime you could presumably keep upgrading until it didn't.

 

[Dathi THorfinnsson]

Xerces literally bridged the Dardanelles with a pontoon bridge and marched an army across. Granted it later sank in a storm, but during peacetime you could presumably keep upgrading until it didn't.

Pretty hard to allow traffic through with a pontoon bridge.

 

[Mad Bad Rabbit]

Pretty hard to allow traffic through with a pontoon bridge.

Build a long pier from the shore with a drawbridge section or high wooden arch, then use pontoons for the rest of the span. The bathymetry map shows extensive shallow banks of 10m or less, at various points along the channel: shallow enough for pilings, deep enough for Roman ships.

Or: have a swinging section. Anchor two ships firmly to the bottom, with two other ships moored in between them. To open the span, just pay out lots of rope until the two middle ships drift apart, downwind or downcurrent, and traffic can sail between them (being careful not to foul the submerged rope); then winch them back into place to close it.

Bonus: if an enemy fleet approaches, quickly summon rowers and untether ships, so your bridge transforms into your own fleet. Reassemble bridge after they are vanquished.

 

[CaptainCalvert]

Build a long pier from the shore with a drawbridge section or high wooden arch, then use pontoons for the rest of the span. The bathymetry map shows extensive shallow banks of 10m or less, at various points along the channel: shallow enough for pilings, deep enough for Roman ships.

Or: have a swinging section. Anchor two ships firmly to the bottom, with two other ships moored in between them. To open the span, just pay out lots of rope until the two middle ships drift apart, downwind or downcurrent, and traffic can sail between them (being careful not to foul the submerged rope); then winch them back into place to close it.

Bonus: if an enemy fleet approaches, quickly summon rowers and untether ships, so your bridge transforms into your own fleet. Reassemble bridge after they are vanquished.

I like the economy combined fleet/bridge option. I can easily see someone thinking it's a political winner until suddenly all the boats turn out to be in need of rebuilding and major overhaul just at a critical moment.

 

[Fabius Maximus]

Pretty hard to allow traffic through with a pontoon bridge.

There was a detachable section so ships could sail through when the bridge wasn't in use.