eli5: How did people build bridges over deep and/or dangerous water if they didn’t have the equipment to go under water?
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There are many different ways to do this depending on the available technology, cost and the ground conditions. For example we know the ancient Romans had pile drivers on barges which they could use to drive long piles into the river floor that would support the bridge. Some rivers would also dry up for some parts of the year, at least enough to build a bridge. Or the rivers could be partially drained for the construction. But even with these different techniques available the most common was still ferries at river crossings. Bridges are relatively new most places, most of them less then a century old.
There are many different ways to do this depending on the available technology, cost and the ground conditions. For example we know the ancient Romans had pile drivers on barges which they could use to drive long piles into the river floor that would support the bridge. Some rivers would also dry up for some parts of the year, at least enough to build a bridge. Or the rivers could be partially drained for the construction. But even with these different techniques available the most common was still ferries at river crossings. Bridges are relatively new most places, most of them less then a century old.
There are several answers here that are correct, but they used a different method from any of them when the built the Brooklyn Bridge. They built a wooden “room” larger than the bridge supports, and submerged it, and essentially just made two big square holes in the river to work in.
There is some really cool engineering tricks involving a thing called a caisson.
https://en.wikipedia.org/wiki/Caisson_(engineering)
Basically it is a box to keep water out while they build the footings.
Build the box, pump water out, work in the relative dry while hoping pumps keep working and the box doesn’t leak.
Another tool they used was driving pylons or piles deep into the ground. These could be put in place from barges or other floating platforms.
Another kind of bridge is like a rope bridge.
Send a small rope across with a projectile (or a person travelling the distance by boat and/or on foot) Use this rope to pull a larger rope across. Keep on bringing rope/cable across the gap until you have enough strength / support to build a bridge.
There are different types of bridges for different kinds of challenges.
There are several answers here that are correct, but they used a different method from any of them when the built the Brooklyn Bridge. They built a wooden “room” larger than the bridge supports, and submerged it, and essentially just made two big square holes in the river to work in.
There are several answers here that are correct, but they used a different method from any of them when the built the Brooklyn Bridge. They built a wooden “room” larger than the bridge supports, and submerged it, and essentially just made two big square holes in the river to work in.
There is some really cool engineering tricks involving a thing called a caisson.
https://en.wikipedia.org/wiki/Caisson_(engineering)
Basically it is a box to keep water out while they build the footings.
Build the box, pump water out, work in the relative dry while hoping pumps keep working and the box doesn’t leak.
Another tool they used was driving pylons or piles deep into the ground. These could be put in place from barges or other floating platforms.
Another kind of bridge is like a rope bridge.
Send a small rope across with a projectile (or a person travelling the distance by boat and/or on foot) Use this rope to pull a larger rope across. Keep on bringing rope/cable across the gap until you have enough strength / support to build a bridge.
There are different types of bridges for different kinds of challenges.
There is some really cool engineering tricks involving a thing called a caisson.
https://en.wikipedia.org/wiki/Caisson_(engineering)
Basically it is a box to keep water out while they build the footings.
Build the box, pump water out, work in the relative dry while hoping pumps keep working and the box doesn’t leak.
Another tool they used was driving pylons or piles deep into the ground. These could be put in place from barges or other floating platforms.
Another kind of bridge is like a rope bridge.
Send a small rope across with a projectile (or a person travelling the distance by boat and/or on foot) Use this rope to pull a larger rope across. Keep on bringing rope/cable across the gap until you have enough strength / support to build a bridge.
There are different types of bridges for different kinds of challenges.
Depends on how big the bridge is and what type you need.
A suspension bridge can be built by pulling ropes across the water. You don’t even need to pull the main ropes across first, you can pull a lighter rope across then use that to pull the larger rope or join multiple smaller ropes together to make the large bridge.
You can also [launch](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjl57f4zYD-AhW9m1YBHQ8mBCcQwqsBegQIChAE&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DNgGL5-3dkVw&usg=AOvVaw32Ili4AiyW2cTvgYgj8BGY) a bridge, that is, build it on land then push it out over the water. This is still an option for building bridges, sometimes by building a segment, launching it out then building another segment and repeating the process. You can do this with piers in the middle or as a single span.
You can lift the bridge out into place – this may need larger lifting gear but it can also be done in parts and then assembled in place. You can also [use a crane on a boat](https://www.engineeringnz.org/programmes/heritage/heritage-records/auckland-harbour-bridge/) to float the bridge out then lift it up.
All of the above can be done to completely bridge over the deep water if you can make the bridge long enough between supports. If you can’t, then you’ll need to put supports in. A coffer dam is a fairly common way of doing this which I see has been explained already. You can also use a drilling rig to drill a pier into the ground without needing to dive down but both of these methods wouldn’t be achievable in very deep water without reasonably modern technology (post industrial revolution at least). Before that, they were limited in where they could build bridge piers to areas where they could build dams deep enough or where they could drive piles into the riverbed from the surface
To add to the previous explanations – here’s an animation how a 13-meter tall bridge in Prague was built in roughly 60 years (Charle’s Bridge). It’s an official reconstruction of the local historic society how it was built. Should answer some questions. IIRC it started 1348 and was opened 1407. (not sure about it and too lazy to check it)
[https://www.youtube.com/watch?v=nJgD6gyi0Wk](https://www.youtube.com/watch?v=nJgD6gyi0Wk)
Edit> 1357 – 1402, meeting was less important so I actually checked it 😀
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