Building a bridge from one side

Question

Your people have been displaced by war, and have wandered for many months looking for a new home. Yesterday, you came across something truly marvelous. You are camped at the edge of a huge ravine, some two miles across and a thousand feet deep. At the bottom of the ravine runs a strait connecting two seas. Suspended magically in the midst of the ravine is a floating island, the top of which is level with the precipice upon which you stand, about a quarter mile away.

Your mission is to build a bridge from your location to that island, using the minimum amount of technology possible. Or in other words... how does one build a bridge when one has access neither to the ground below, nor the opposite side? Is there a way?

Answer 1

Hmmm, you've got problems. From the question text I'm assuming the island is at the halfway point and is ~1 mile across, so the bridge only has to be .5 miles long. I'm also assuming they don't have access to magic. If these assumptions are incorrect please clarify in the OP.

If they lack time and nearby resources they're screwed. If they have both:

One possibility is not building a "bridge" per-se, but rather extending a pole out to the island. Once they get one pole across it becomes easier to shuttle people along with, say, a basket and rope.

Now getting a half mile long pole across the ravine is no easy feat. If it's made of wood it's weight will make it bend down below the islands edge, so it will have to start at an upward angle or use some magic material that won't bend. It also can't be one piece, so these refugees will need to have the ability to make perfect connections between poles.

On second thought, a better idea: build a ballistia with a string attached to the bolt. Shoot the island. Now you have a rope extending across and you can either make a rope bridge or pull people back and forth in a basket. The first people to cross will have to crawl along the rope, so that will be dangerous, but also exciting if this is for a story.

Edit: So the longest range you can get on a ballista is ~800 meters. This isn't accounting for pulling along a bunch of rope capable of holding a man's weight. Don't wanna calculate that, so I'm just gonna halve it and say 400 meters (magic elven rope would of course help here). This is well short of .5 miles, so either the gap would have to be smaller, the ballista better, or they would have to build/find a higher place to fire from. For realism I would go for a combination of 1 and 3, but all would help.

Edit 2: Better Solution

Actually had another thought too. I'm assuming that if they can build a super long ranged ballista they can also build a boat capable of crossing the water to the other side. If they can, then send some guys to carry the end of a rope to the opposite cliffs (you'll need some good rock climbers). Then, pull the rope taunt so it's level with the floating island. Then shuffle someone along it to set up the pulley they'll use to cross.

This is probably a more feasible solution than the ballista, as the former would have its range vastly limited by the rope, and would be gambling that the bolt would be lodged strongly enough to hold both the rope and person.

Edit 2.5: Better Solution Maths

I'm guesstimating that the rope would weight 25 lb per 100 ft. This is mostly pulled out of my a$$, but rope capable of holding 100 lb on Amazon looks like it weights about 5lb per 100 ft. We need to hold a 200 lb man, plus its gotta be strong enough to hold together with just its own weight (not sure what the math is on calculating this strength), so maybe a thicker rope weighing 25 lb per 100 ft would be the way to go. Since you need 10,000 ft of rope, this brings the total weight to only 2,500 lb. This sounds heavy, but it could be a lot worse. They would need to set up some machinery on the other side to hold it taunt (and get the end up the cliff), but it's doable I'd say.

Or just go with the ballista.

Answer 2

Make a balloon. All you need is some decent silk/linen, ropes, some basket, and then something to heat up the air inside.

Heat the air, unstick the balloon, but take some rope with you so you won't fly away. Wait for right wind direction, fly over the island, land there (make a hole, or wait, or let someone rope down and anchor you and then wait).

Now you have rope connection between the mainland and the island. If you took only one rope over gap, all you now need is to tie on it another one at mainland side and pull it to the island. Then you will have two of them (and a big piece of rope on island). Alternatively you can make more flights by balloon.

When you have few ropes there, all you need next are just some planks to finish the bridge. Probably you will need a lot of ropes to carry such a long bridge (long? you should always use metric units, people all over world really don't know how long are your miles...), but it is quite easy to do if you have resources.

Answer 3

The answer lies in the question's premise.

Suspended magically in the midst of the ravine is a floating island, the top of which is level with the precipice upon which you stand, about a quarter mile away.

The premise of your question suggests there is technology available in this world from which one can construct the bridge: the technology which allows the floating island to exist.

Now consider this. The floating island is essentially a floating air pontoon. Instead of a water pontoon. The air pontoon floats in air. Now imagine stringing together a chain of similar but smaller air pontoons to form the bases of the bridge. Then lower an attaching walkway between each pontoon in the chain. Now you have a floating air pontoon bridge as requested.

Answer 4

Suspended magically in the midst of the ravine is a floating island, the top of which is level with the precipice upon which you stand, about a quarter mile away.

Ok, part of the locally available resources is something that allows magic suspension of huge static loads.

Use the same magic to make pontoons on which to construct the bridge.

Answer 5

2 miles for a group of poor refugees? We can barely do that with every bit of materials engineering and construction know-how that we have in the year 2017. The largest suspension bridge in the world is that size, and it took 10 years and $3.6 billion to build it.

Without magic or handwaving, it can't be done in the scenario you present.

Answer 6

Does the magic hold the island in place horizontally too, or just vertically? If the latter, then all you need to do is harpoon the island and reel it in.

Answer 7

A quarter mile is not that bad, 700 meter ziplines exist. The longest footbridge span I know of is the Kusma-Gyadi Bridge in Nepal which spans ~300 meters. The hardest thing is getting the first piece of cordage across the span, a balloon with the lightest cordage possible would be your best bet, although depending on the span a catapult might work. Then you use that to pull heavier cordage across. The biggest for you is getting a person on the other side to tie toe cordage to something, that's why I said a hot air balloon might be necessary.

Nepal is full of rope and cable bridges if you want to see how they make them. Many spans are just crossed by a single rope or cable and they hang from a basket and hand-walk along it, Although their classic three rope foot bridge is probably the best for constant traffic.

Oddly enough a suspension bridge (or more precisely half a suspension bridge), will work of a quarter mile, but it requires modern materials. The largest suspension bridge has a central span of 1991 meters (1.237 miles) that means you could make a nearly 1km long one sided bridge. that is Akashi-Kaikyo Bridge, Japan, but you need modern steel cables to do it.

Answer 8

Well, there's a way - bridging vehicles exist, and make bridges from 'one side':

Youtube video of a 'real' one'

And a LEGO one

It's pretty low tech - but moderately high engineering, as you need a folding span half the 'width' of your bridge, and a rather significant 'ballast' to stop toppling. On a quarter-mile span, that's going to be a rather impressive sort of a challenge - you'll need a bridge 'doodad' that's 2 lots of 200m, which isn't a trivial sort of a number. And enough 'counterbalance' on the bridging vehicle - you'd probably need a ballast on a lever, sort of similar to what you'd get on a gantry crane

But in a world where there's magic, it may be altogether more doable.

The bonus here is that you could - potentially - recover your bridge, and have a pretty secure fortress....

Answer 9

Use a kite

This will require amenable prevailing winds, so let's assume the wind typically blows toward the floating island from the settlers' side. Tie a 1/2 mile rope with an anchor to a large kite with a release mechanism. Fly the kite over the island, and release the rope and anchor, dropping it onto the island from above. Now you've got a single rope spanning the gap. Do this as many times as you like to start building a bridge, or just start sending people shimmying across the rope if your people have good grip strength.

Answer 10

Assuming access to the required resources here:

The bridge needs to be a quarter mile or roughly 400m across, and you only have access to one end.

You start by building an upright frame about 600m high. That's doable, even with stone age technology, assuming you have wood, stone and rope. A frame alone is sufficient. Now, ensure the frame is solidly anchored to the ground by tying long logs along the base, and driving pegs into the ground to hold them in place.

Next build a smaller frame inside the 600m frame, about 400m long, just longer than the distance from the 600m frame to the island. Loosely connect the upper end of the smaller frame to the main one. Strongly tie the base of the small frame to that of the main one. Once this is done, bridge the gap of the small frame with planks. I think you see where I'm going with this.

Once the smaller frame has been completely covered, attach a rope to the top end, loop it over the large frame and tie the other end to a large bucket full of rocks on the ground. Untie the top of the small frame from the large one and push the top end towards the ravine. The bucket should act as a counterweight, preventing it from crashing down. Remove a few rocks from the bucket, until the small frame starts to slowly fall under it's own weight.

Send a few people with ropes and stakes across your new drawbridge to secure the other end by tying it to trees or into stakes dug into the ground and your bridge is done.

If you haven't scrimped on the material or workmanship, it should be as strong as any castle drawbridge, but you might want to consider rails, both to reinforce the structure and to prevent accidents.

Answer 11

0.25 miles away is 1320 feet, which is a stretch, unsupported.

Ravine is 1000 feet deep. So the bottom of the ravine is closer than the edge of the floating island.

The top of the floating island is level with me. It does not state how low the bottom of the island is off the water.

Option 1: from below. I have at my disposal one large cliff, optimally placed, and as much gravity as I care to use. I propose with liberal use of explosives, the top of the cliff be liberated and sent downwards. Repeat. Liberation of this sort is the pastime of oceanside cliffs everywhere and there would likely be at the foot of this cliff the accumulated products of the cliff's own efforts over preceding centuries. A benefit is that material from the top of the cliff will tend to bounce and slide outwards, thus closing the distance between the edge of the cliff and where the flying island starts.

One can add to this pile additional material and so approach the bottom of the floating island via a pile of debris. Accumulated debris will probably be in a mound wider than necessary and human efforts can move and stack materials in a pyramid which more effectively approaches the bottom of the island.

Option 2: Reel it in. An intrepid, either climbing up from below route as per option 1 or flying across in a hot air balloon, lands on the floating island, finds the controls and then steers it closer to cliffside, to facilitate desired bridge. If this is the Miyazaki Castle in the Sky type island with no obvious directional control, one could exchange the balloon tether for successively stronger ropes, affix ropes to something solid on the floating island, and have your burly brethren reel it in. It is not like it will get stuck on something. Once it is close then put your bridge across.

I personally would be a little skeptical about the whole endeavor unless I were aware of other similar islands which had been colonized and which had not suddenly dropped from the sky when the float gubbins wore out.

Answer 12

A quarter of a mile is 400m. That's a long bridge span if it's supported both ends, but as you're only supporting it from one end during construction, that's one hell of a long cantilever!

My first instinct was balanced cantilever construction, to build a single-tower cable-stayed bridge. This construction method is often chosen when building cable-stayed bridges. Basically you build a tower, then add a piece of bridge deck to one side, connect it via cable to the tower, then repeat on the other side to balance it. In your situation, as you'd only have a cantilever on one side, on the other side you'd just be anchoring the tower down to the ground with cables.

However, this becomes a bit problematic when compared to today's technology. The Queensferry Crossing* took the record for the longest freestanding balanced cantilever. And that's for central spans of 650m. Which means that the length of cantilever from each tower was 325m. So the world record for this type of construction is less than your desired distance.

The other option I can think of for building a bridge from one side only is launching it. This is where you build a bridge on land, and then push it out.** This is really best suited for a bridge with multiple short(er) spans, so that for launching over the first span, there's a lot of bridge still on land to act as a counterweight. For use on only one span, you'd need a lot of counterweight! Add in that you're looking at a span right at the top of what's achievable without cables, and it's going to be very heavy and difficult to push.

In conclusion: What you're looking for hasn't been achieved in real life yet. I think it would probably be possible: there's no economic need to build a pedestrian bridge like you've asked for, so no engineers have tried: similar spans are always for heavy vehicles. But in order to build such a bridge you'd need state-of-the-art technology, not simple technology.

_{* Disclaimer - I was a Design Engineer for this bridge. :D Though I was working on the approach spans rather than the cable stayed ones.}

_{** This is how the approach spans on the Queensferry Crossing were constructed. Super cool videos: no1 and no2. NB Videos are probably only super cool if you like engineering.}

Answer 13

In this picture I removed most of the land so I did not give the impression the whole island is surrounded by land.

The person wanted 2 seas (1 & 2) and a river so now you don't have to imagine them. The river could obviously be much narrower or further away to allow as much dry land underneath as the person wants.

I made this drawing with sketchup make so I could show that it works for real.

Green is random ground, and upper area.
Blue is the air gap.
Brown is the floating island.
The pink area is the walk from 1 to 2, just follow the rim, say 20 feet away so no one falls in. 
Solid black line is rope, I had to made it super wide so you can see it, but it is actually a normal size rope.

**You still need LOTS of rope! approx 13100 feet **

However, nothing besides rope and wood.

• Group 1 is the starting point, and hold the end of the 13100' rope.

• Group 2 carries the other end around to point B. Then they pull it tight. The rope now crosses the island. Since this was done to scale the black rope line is hard to see.

  The rope is 2.5 miles long so physically carrying the rope into position should only take 1-2 hours maybe 3. Making said rope well that is another story. However, the rope has to be thick, but not super thick as at least 3800 feet will be on top of the island.

They may now cross the air gap by climbing along the rope.

You will still need large stone anchor points on each side to hold it in place.

• No magic.
• No hot air balloons.
• No shooting arrows with ropes.
• No hang gliders

Just a group of people carrying a tremendous amount of rope along the cliff side and pulling it tight.

Note one side is 5400 feet from island other side is 3700 feet. Between these positions you could attach a number of rocks say 20-40lbs along the 3800 feet it crosses the island. That way if the rope broke on either side the weight of the stones would hold the rope in place, allowing them to cross and repair the rope. Also you could put knots in the rope to keep the stones from moving to far to the left or right.

Fearless leader, cybernard , undaunted by the task ahead gives his 3000 followers time to rest. They have had no chance for groom, and etc for as long as can be remember. He has all of them cut off at much hair as possible. They contribute 5' feet on average for 15000ft, and another 1500ft in men's beards. They work tirelessly turning the hair into a rope for the task ahead of them.

Answer 14

I don't know your resources, but I would imagine a gently floating pontoon bridge. Simply align air-floating objects (like boats/rafts) and build a bridge across.

All you need is wood and rope. Where I am skeptical about this solution is the size of your body of water - it is very large, and susceptible to currents, et al. So brace it as well as you can.

Answer 15

Walk one or more ropes out to the island using the accessible land

Here's an opportunity for an epic, simple and totally believable solution - make a few extremely long lengths of rope, and find a way to have men walking it out over the ravine.

Ideally, get a man or two to that other side - whether they have to dive and swim/climb to the other side, or use hang-gliders, or use any natural features which may help them, enough epic perseverance will eventually get somebody to the far side of that canyon. And from that far side they can walk the other end of the rope across until it connects with the island. You could even then have the men on the far side die tragically and then give you a mystery or a later need to get to where they were.

If you're totally adamant on making the other side completely inaccessible, just reconfigure your ravine - make it just concave enough to allow such a feat without leaving your starting side.

---

This gives you options for later on

So you've created a simple, but fragile and temporary solution to getting to the island. This solution can go lots of ways - it can be built on, it can break, it can even provide unintentional access to an unknown entity from the far side of the rope. If you can loop the rope all the way around the island, perhaps your men (or somebody else) could even physically haul the floating island to land... or otherwise fail in their attempt to do so, for whatever reason.

Answer 16

http://www.lowtechmagazine.com/2008/11/tiles-vaults.html

Catalan Vaulting might actually be a useful technique here. It's a pretty cool technique that allows the construction of long spans that are very strong with relatively small amounts of material. The biggest concern for the completed span is that your floating island not move. I'm no expert, but it might be doable. Overlapping layers of tiles could be initially supported from below with supports. maybe add some suspension bridge rigging. This is an interesting problem, but you might be able to build out with this technique.

Answer 17

A kite is absolutely the most practical solution. This approach is actually quite simple and logical; it's probably the most practical solution yet.

It is also not without precedent. I'm shocked no one has mentioned the Niagra Falls bridge: https://www.niagarafallstourism.com/blog/flying-kite-contest-1847/

Of course the challenge here is the fact that no one has access to the other side, but this isn't as much of a problem as you'd think. The trick is to not tie, but loop the kite string through a ring attached to the kite, flying with the kite string "doubled".

Once the kite is well-lodged in a tree on the other side (yes, this may take a few tries, and perhaps a few kites), begin pulling the kite string continuously through the loop, attaching a stronger string, then a rope, and so on. As the rope gets stronger (and heavier), attach a stronger, heavier anchor (such as a grappling hook) -- this should also be attached with a ring so the rope can continue to be pulled freely through. When the anchor reaches the tree on the other side, it too should lodge.

You can repeat this looped-rope-and-anchor process as many times as needed to reach the desired strength. Finish by tying a final anchor to the rope and pull through until it catches firmly on the other side.

Now attach this rope to a tall tree or simple wooden tower on the local side and "zipline" across. The individual brave enough to test the zipline can tie down the anchor more solidly once they reach the other side. Naturally they should have also brought across 1-2 additional ropes, which can be manipulated from both sides to begin making a proper bridge.

Welcome to the floating island.

Answer 18

Wikipedia has a whole page on moveable bridges designed to allow ships and bridges to coexist. What you need is any of those a bridges that are anchored on just one side of the chasm plus some unobtanium or magic.

You could also build a pivot bridge entirely on land and then pivot it out over the chasm. This has the advantage of keeping all the builders work on solid ground.