Natural rope bridges

I have thought about making a natural rope bridge or traverse rope for a while. Floods here make it difficult to get around in winter and spring. Of course, such ropes would have to actually be strong and sturdy enough to hold a person, probably two for security. I don’t have enough experience I think to make a permanent bridge, but anyway my philosophy points towards the most frugal and simple options, and a simple log or two across the river would be swept away easily in a flood.

I have heard about zip lines in China that were made from twined bamboo plants. This makes me think that maybe reed canary grass, if twisted into quite a thick rope and tarred, might fulfill that purpose with enough strength. I have experimented with making rope from them and it is good and sturdy, though I haven’t measured the strength.

Is this something you have done? What are your materials, your experiences? Have you ever had a bridge fail unexpectedly, etc?

It's an interesting challenge!

Pardon me, I can't help but conjure an image of Indiana Jones crossing a decrepit and sketchy rope bridge over an impossibly deep gorge for dramatic effect. He always makes it over of course, after a few close calls.

The safe working load of a rope or cable is generally considered to be around 10% of the breaking strength. For a hand-made rope that safely supports humans we are talking about a big, fat multi-strand rope. That is a lot of work.

If you want to test thinner versions for strength, stringing over a benign pool or pond could be a lot of fun. Young men will take on the challenge to impress girls. Roll your eyes all you want, it generally works.

Personally, if we are talking seriously about crossing a river in flood, I want a substantial wire rope cable with three clamps on either side, and a higher safety cable on a different tree with the same arrangement. Cables and clamps are cheap and effective. So are PFDs. My 2c.

Oh! So if I want to test the finish rope, I will have to apparently find over a thousand pound weight. Or ten people to hang on to the rope. I am laughing at the image…

A quick search is saying that breaking strength is predicted based upon the diameter squared. So say my half inch reed canary grass rope breaks at fifty pounds—only a guess.

Some quick calculations:

K*1/4=50
200*1/4=50
200*d^2=1024
d √200=32
d=32/√200
Coming out to about three inches? Three and a half for a little excess? That would make sense.

Also thank you for the advice about the wire ropes—that is also good to look into.

Sounds like the Incan bridges : https://www.thearchaeologist.org/blog/the-rope-bridges-of-the-incas-the-ancient-technology-that-united-andean-communities-fades-into-history

John Ochsendorf wrote a paper on Inca bridges as a senior undergraduate engineering student at Cornell:
https://www.loc.gov/item/2021687736/
Unfortunately, for an archival video from the Library of Congress, the videographer wasn't totally up to snuff.

Ochsendorf's now a full professor at MIT, and fairly famous for having written "Guastavino Vaulting: The Art of Structural Tile", a beautiful coffee table picture book with an extensive historical and technical discussion of structural tile in North America (and some earlier precedents in Spain and South America, when that was New Spain).  "Subway tile" is called that because of Raphael Guastavino, Seniors use of the glazed tile to finish (among other things) the vaults in the New York subway system.

Anyway, Ochsendorf documented both the manufacture of the heavy grass ropes and also surveyed and described the stone anchors.

It also might be worth checking out TM 5-279, a WWII era tech manual on constructing expedient suspension bridges in the field, using wire rope.  As I recall, there is good technical info in there, especially regarding anchorages.

Try here:
https://archive.org/details/TM5-279

As a Boy Scout, we built bridges using a set of lashed shears at each end, with a foot rope and two hand ropes.  They never crossed anything though, just hung over a grassy field, but it was a good experiment.  I'll see if I can find a reference in the Pioneering Merit Badge or Field Books.  It was a little precarious, but worked.  My understanding is that the vertical distance between foot rope and hand ropes is critical to stability.

On edit:
Here's a link to an online version of the Pioneering Merit Badge book, with instructions for a slightly different version from what I remember:
https://pioneeringmeritbadge.org/double-a-frame-monkey-bridge/
The style I've built only had a single set of shear legs at each end, whereas this one uses a double shear (more like a W than an X).  This version looks more substantial than what we did when I was a kid, and is probably better for a semi-permanent installation.

A possible alternative might be a cable way of some sort.  There was a very crude one across a creek at the old fish hatchery where we sometimes stayed as Boy Scouts.  No longer a fish hatchery and having been passed to the local university's forestry department, there was still a cabin and sauna and other appurtenances, including the cable breeches buoy to cross the creek.  I know of at least one other such setup near-ish to me, across a much larger river and gorge.  One can also be seen here:
40°24'31.4"N 109°14'05.5"W
(Just put that in Google Maps, and drop "peg man" on the bridge - you'll see the tower on the west side of the Green River.  I think this is the Chew Ranch's cable way, independent of the Cub Creek Road bridge.  I saw the cable way this past June when we visited Dinosaur National Monument.  This is on the road to the historic Josie Morris Ranch.)
YouTube user Michygoss bought a "Sky Mule" and set it up to get her cabin building supplies down a steep hill from the road to her cabin site.
https://www.wyssenseilbahnen.com/en/plantation-cableway-small-cableway/plantation-cableway-skymule/
Not exactly your application, but it might be an alternative style of crossing gizmo.

However, the challenge of building your bridge with natural materials would not be met with a steel cable!

I heard about the Inca bridges some time ago. They are a big community project and if I recall correctly, most of them were rebuilt yearly.

Have you seen pictures of the "live vine" bridges in India? They are equally cool, and I've read in the past of people making similar out of basket willow. If your area is wet, I suggest you at least have a look at that approach. Essentially, you poke willow whips into the ground on either side of where you want the bridge and they will root and provide the base for the bridge. The rest of one of the projects I read up on, was a mix between live and dead wood. I believe the eventual plan was to have most of the wood live, however, I think using some dead wood where people are walking, protects the live supports and helps the bridge last longer. These too require considerable pruning to keep them working, but might be safer in the situation you're describing.

In Boy Scouts we would go camping by a small ravine, 100 feet across and a good 50 feet deep.
We worked on the pioneering merit badge one year.
Make a rope bridge to cross the ravine.  The catch?  We had to make all the rope on the spot.  
1 inch rope was the goal.  It worked.  We used 1/8 inch sisal to make the rope.
The 3 rope bridge was left up for future campouts.
It lasted 4 years until someone vandalized it.
Fun times.

I do agree that a cable bridge would be best.