What is the longest bridge that a Bronze Age society could build across a bottomless chasm?

Question

My world is in the mid-Bronze Age, very loosely based on ancient Mesopotamia. Metal is scarce and generally of poor quality, and the only iron available is meteoric in origin. Gold is relatively common, and is used liberally for decorative purposes.

The continent is split in two by an enormous crevasse of magical darkness, within which tiny, distant pinpricks of light can be seen. Nothing that enters it ever comes back, be it animal, vegetable or mineral - it simply falls until it fades from view. Following a series of military successes, the Emperor has decided to establish trade and communications with the Distant People dwelling on the other side of the crevasse.

At the narrowest point of the fissure, the locally available materials are stone, clay, plant fibers, and a limited number of short trees. A river flows near enough to the city for it to be practical to ship materials there from further afield.

How far apart should the sides of the ravine be for the bridge to require an 'ancient wonder' level of effort to build, while not being completely impossible?

Answer 1

Let's see what we can find from reality, both present and historical. For reference with the dates in this post, the Bronze age lasted from very roughly 5,000 years ago to 3,000 years ago, though both start and end very much depend what region we're talking about.

The earliest bridges I can find: Tello bridge is 4,000 years old, largest span ''unknown'', I couldn't find any cite, but seems largish given the humans in the picture. (via https://www.britishmuseum.org/blog/worlds-oldest-bridge-being-preserved-iraq) I'm not 100% convinced it was even a bridge.

Umshian root bridge is only maybe 130 years old, but suggests another possible method of creating a bridge if there are vines that have evolved to grow long and strong enough to go a long way down into the crevasse: just make the vines grow along ropes, and over time, grow it bigger. Admittedly, a rope bridge may be more practical! Not sure the span, but he seems to be 150ish pixels tall, but is leaning forwards, so may be foreshortened in this position; the bridge seems to be 1700pixels wide, so we can guess it's at least 12 m (40 ft) wide, and one source (https://morelifechanger.in/firming-double-decker-root-bridge/) claims it's "about 50 meters long" (~160 ft). (via https://en.wikipedia.org/wiki/Umshiang_Double-Decker_Root_Bridge)

The Tarr Steps are old (perhaps from the bronze age, others think medieval) but have very short spans, just a few feet each. It's also super unlikely that these are the original flags, given what feet and river floods will have done over the last few thousand years, so it's been rebuilt many times. (via https://en.wikipedia.org/wiki/Tarr_Steps)

Arkadiko bridge has a TINY span. Just look at it. 1m (3 ft). And again, I'm a little skeptical of how original the stones are, but it's estimated about 3,000 years old. (via https://en.wikipedia.org/wiki/Arkadiko_Bridge)

The unfinished obelisk is not a bridge, but bear with me. This was a stone pillar, started about 3,500 years ago. It was planned to be the largest obelisk in Egypt. They stopped carving it when it cracked. BUT. It was 42 m long, and tapered. If it had worked, that'd make a nice big span!

If you have each side cooperating, and each one mines and pushes one of those out to the edge of the crevasse, you've got yourself a span that's about 40m of stone. More if you want to span the middle with a couple of trees. The danger is, of course, it's brittle, and likely to crack, and both sides are now near their tipping point and need counterweights. (via https://en.wikipedia.org/wiki/Unfinished_obelisk)

For comparison, Stonehenge's lintel stones (4,000 to 5,000 years old) are only 3 m (10 ft) long, and the unsupported length is even shorter. (via https://en.wikipedia.org/wiki/Stonehenge)

Convenient tall trees, or joined wood, would be better than stone. More flexible, lighter, easier to harvest, easier to counterweight, less prone to catastrophic cracking. Some of these, rarely, can grow to a height of over 100 m, making a bridge length of perhaps 80m entirely possible with both sides joining the trees. By binding a third tree tightly to join the two spans, you wouldn't even need to cantilever, at least not after construction. (via https://en.wikipedia.org/wiki/List_of_tallest_trees)

I still feel that the other answers are correct, though, and a rope bridge would be best, because at that point you're limited only by the strength and weight of the rope.

Queshuachaca bridge is at most one year old: it is normally remade annually, but collapsed in 2021 because it was not maintained in 2020. It was first built by the Incans about 650 years ago. Nobody knows when their practice of rope-bridge building started, though, other than that it goes back to at least Incan times. It has a span of 36 m (118 ft), but examples up to 45 m (147 ft) were once known. (via https://en.wikipedia.org/wiki/Queshuachaca)

Oh, here's one last idea: if the gorge is narrow enough, made of the right kind of rock, with the right faults, they could find a parallel faultline, and calve off a section to tip across and lean against the far side. Alternatively, they could try carving it out like the dolmen. Then once it's leaning against the other side, they can either carve steps in it, or just fill in the gap all the way up to ground level.

Answer 2

This is close to a duplicate of Building a bridge from one side. However, the limitation to the Bronze Age perhaps makes this a unique enough question. That question does make one point: somehow you need to cross the ravine before you can build the bridge. A tall tree, cut to guarantee both sides could handle someone's weight, could be one limit. But that would limit the bridge to maybe 30 meters. So, is there anything else we can do with Bronze Age tech?

I'm thinking your limit is a Bow and Arrow

For comparison, the Bronze Age was 3300 BC to 1200 BC. The catapult was invented in 400 BC, which means it's not a bronze-age tech. On the other hand, the bow and arrow has been in use since approximately 72,000-60,000 years ago, which establishes the bow and arrow as the only move-something-a-long-distance technology we have during the Bronze Age.

The Longbow world record is held by Jeremy Spencer, 379.51m. For practical math purposes, let's say that our initial limit is 380 meters.

But that was a perfect shot...

The oldest longbow specimen can be traced back to 2690 BC, so we're clear in that respect. But Mr. Spencer's world record shot was with, frankly, a perfect arrow shot either inside or on a perfect day... it's the proverbial long-shot even with the sacrifice of a couple of oxen. But it was also one more thing... unburdened.

Your emperor's workers would need to drag something across that abyss to start the process of bridge building. Now, I could drag out all the math about forces, velocities, mass, friction, and drag. But what it's going to basically prove is that the next limit is 75-100 meters.

EDIT: User @MartinModrák points out that a structure could be built on the Emperor's side of the abyss to improve the distance of the arrow. He's completely correct that the use of such a structure would greatly increase the potential distance of the arrow. It also increases the time that wind, etc. could affect the arrow, which has a limiting effect. But, in the best case, it's definitely an idea that would improve the credibility of this part of the OP's story and rationalize a greater distance. Thanks, Martin, for pointing that out! He also mentions tying multiple trees/shoots of bamboo together to create a longer traversable beam that wouldn't require help from the other side. While true, this substantially limits the total length of the bridge as the body weight sag will eventually pull it off the edges and an object that didn't have that sag would have a shatter limit as it must be dropped from a vertical position. In short, I think the arrow is still the best option. Heck, any solution that enjoys the help of the neighboring state is bound to be better than one that doesn't have it. Which brings us to...

You'll need help on the other side

Now, the pulley was invented at about 1850 BC. Yes, that's still Bronze Age, but it's a bit much to swallow that someone with a bow and arrow could attach a Bronze Age pulley to said arrow and get it any more than about three meters. Maybe ten. This means that you'll need someone from the civilization on the other side (or a very well-traveled team who sailed around the abyss to be on the other side) who could take the pretty light-weight thread/rope/cable the arrow pulled across, and pulled it... attached to a heavy enough rope (made of either plant fiber or woven leather) across the abyss and tied it off to something stout... like a big old tree.

After that, the bridge is just a matter of time and a few lost souls as people slip on the rope

We're basically done once you have the ability to move people and materials across the abyss. At that point you could build a rope bridge or, if you want to stretch the tech a bit, something even more impressive. But the ability to get those first few people and materials across, that's what limits the size of bridge you can build.

And I'm voting it's 75-100 meters based on suspension of disbelief. If you're talking realistic, then the historical limits of 30ish meters really is your limit. Bronze Age it might even be 20 meters on a good day. But I'm a fan of suspension of disbelief.

As for how long such a bridge might last... go rent a copy of Romancing the Stone and watch until you hear Michael Douglas say, "that's not a bridge... it's pre-Columbian art." Pause the movie, and stare at what he's talking about. Then remember that suspension of disbelief is your friend.

Answer 3

(Really a comment to JBH's answer but way too long)

You need to find a stout tree on the far side of the chasm. Build a kite (fabric on a wooden cross is enough), take two cords and fasten them to the corners (each cord is connected to two corners, crossing in the middle.) Station a crew there to wait for a big storm blowing across the chasm. With four people spread out properly it should be possible to fly the kite across the chasm even with no understanding of aerodynamics beyond recognizing that wind blowing under a tilted surface causes lift.

Fly the kite past the tree, crash it behind the tree so that the cords end up catching on the tree. Now you have two leaders and a pulley, you can pull across heavier stuff until you have something that can hold a person. Now the person crosses slung under the rope.

This is actually adequate to permit small scale crossing of the chasm--ride-underneath single-cable bridges have existed in the 21st century. You get to decide what plant fibers they have and thus the distance they can cross and note that they'll want to upgrade this if possible as this design will not last since they do not have a good wheel to ride on the cable.

Technologically, this requires rope and cloth. It also requires a decent amount of forest on both sides as they have no way of building abutments for the bridge, it will have to be anchored by tying off to enough trees to handle the load.

Answer 4

Incas lived practically in bronze age and they built rope bridges using grass fibers.

https://en.wikipedia.org/wiki/Inca_rope_bridge

The greatest bridges of this kind were in the Apurímac Canyon along the main road north from Cusco; a famous example spans a 45-meter gap

Answer 5

Frame challenge: Via Ferrata down and up

Assuming the tiny pinpricks of light at the bottom of the crevasse are glow worms, and not dragon eyes or lava...

Then the solution at least initially isn't a bridge across, it's a series of ropes going down. Pick the best line with some natural features for hand and foot holds, and chisel more into the rock as necessary. On the other side, do the same for a route up. Or if there are no features on the rock face, chisel sockets for wooden beams and build stairs (well within the capabilities of Bronze Age technology).

It's going to take a while to do it, sure, but that's the defining feature of all pre-industrial projects. Even a fully-funded cathedral took decades to build. The Emperor would surely know this, and not expect a quick win.

Perhaps even more importantly, the Emperor then knows the nature of the magical crevasse. If it's dark, you'd be lucky to get 10-20m of fall before you lose sight of a rock. It doesn't have to be a very deep hole for you not to be able to see something all the way down, and if the floor is covered with foliage then you probably won't hear it land.

Answer 6

The Chinese Woven Timber Bridge

The bridge can be made of shorter lengths of timber that lock together to form an arch

The longest timber arch covered bridge still existing is the Santan Bridge in Taishun County, Zhejiang Province. Net span: 42m

Longer ones could easily have existed but have all been replaced by modern construction bridges.

These bridges could be constructed completely from wood with no metal required

Answer 7

Okay, there are some good ideas with bows/arrows and kites, we might even get a boomerang suggestion. My solution involves the mountainous tribes just recently pacified who train and hunt with large raptor birds. Give them the task of sending over the supporting lines maybe three of four birds at a time. Might even shuttle young boys over and back and retrieve or guide those kites around the trees.

Answer 8

I think you could do at least 50 meters without any assistance from other side with BIG trunks

if you combine them like this

Answer 9

Frame Challenge: do the Distant People want a bridge? do they want to trade and communicate with the Emperor and the Near People? Possibly their immigration policy is similar to the folk on North Sentinel Island. The first visitor from the Empire may share John Chau's fate. I'm picturing the Emperor saying: I have just conquered everything on my side of the chasm; I want to bring you Distant People the benefits of democracy and free trade.

OTOH, contact is likely to be challenging linguistically: if the Near People haven't had dealing with the Far People, the languages could be as different as, say, Irish and Dyirbal.

Answer 10

There's a related problem of how much overhang can be achieved stacking dominos or bricks or similar.

It's fairly obvious that you can build a bridge simply by building two such towers, one on each side, so that they meet in the middle.

In principle you can build out to any distance, given a large enough supply of blocks. However the overhang is proportional to the logarithm of the number of blocks, so the limiting factors will be the compression & shearing strength of the material, and the precision of the stonecutters.

At some point it would require less material to fill in the chasm.

Answer 11

I think the thing people are forgetting is that a bridge's purpose is to make it easier to get from point A to point B. Looking at historical records is not going to give the full picture. In most non-fantasy situations there are alternatives to using a long bridge that would be much less difficult and expensive (for example using a boat or just going around). if those alternatives aren't available, the place is just left uninhabited. Its highly unlikely we have seen the full extent of a bronze age civilization's bridge building skills, because there wasn't a strong motive for them to actually use their skills to the limit.

You also have to consider the fact that its not just a pedestrian bridge, but something an entire army will have to cross along with their supplies, and it will see heavy foot traffic.

Considering these factors, I think this would be a massive military project and they would invest a large amount of resources in building a bridge that is beyond the capabilities of a bronze age civilization from our earth since there actually is no alternative.

Answer 12

I believe that ancient people could probably build an indefinitely long span if the cost/level-of-effort is relaxed. But only across shallow rivers. Stone arches require them to be able to boat to arbitrary points mid-river, and sink marine pilings. As they finish, they will create a bare area of the river bed upon which they can build a foundation of stone. They build a tower above this at a distance from the last that does exceed the limit of a stone arch (I don't know this number off the top of my head). Scaffolding is erected, and the archway is built. Then they move another 80ft or 100ft or whatever down the river and do it again.

It would be risky and expensive, but not beyond the means of some of the ancient civilizations. I think the biggest danger is that they did not have sophisticated understanding of things like thousand-year flood levels. I'd expect them to build it high enough to avoid floods that have happened within living memory, but as we're discovering even today, that's not nearly enough, you know?

For examples of what this might look like, see this:

https://en.wikipedia.org/wiki/Alc%C3%A1ntara_Bridge