Building rope bridge between trees

nino1

I am building a small rope bridge between two trees for the kids. (See attached pics)
I have used a small diameter pulley rope that i got from woodies but the bridge has turned out heavier than i thought so im not sure it will withstand the pressure.

Was thinking of using a wire rope instead but thinking that although it would be sturdier that it would be harder to secure at the ends to the tree.

I have two pieces of timber at each end. I was going to bore a hole in them, stick the rope through and put a big knot to secure but couldnt do this with wire rope.

Any suggestions would be much appreciated.

CJhaughey

Firstly that cord for sash weights will not last, it will rot and fall apart very quickly, you need to use at least 10mm Ø Polypropylene rope to be sure of it lasting, the next thing is having the treads attached by a hole, to ensure stability so they don't all slide down to the middle you should fix each tread to the rope by means of a clamp. so the rope sits under the tread.
U bolts are probably the easiest way of clamping the rope to the board you may need to counter sink the holes so the threaded part of the U bolt doesn't stick up proud of the tread.

To be safe you will need handrails as well, those bridges can easily flip.
Personally I think a 3 rope setup that you walk across the rope while holding on to the handrope is safer.
Google Burma bridge for better ideas.

bonzodog2

Agreed re the rope. You could put the rope through small lengths of pipe between each step as a spacer to keep them apart.

TheBoyConor

Definitely. Those chords are far far to weak. It would be deadly dangerous and could result in serious hurt if they snap. I would even say 10mm PP rope is too light. Sure, it might handle the tension no problem but you must remember that with use and wind the places where the rope contacts the timbers will start to gradually fray reducing the strength over time. Thicker, 25 or 20 mm rope will be only slightly more expensive and will be vastly stronger. Aslo, it is impossible to get a decent grip on narrow rope with your hands.

The rope shouldn't bear directly on the timber edges as you are concentrating the stress there. Rout rounded grooves out of the timbers to hold the ropes at the bearings.
Don't use u-bolts for holding the ropes as they could pinch the rope and weaken it. You can get clamps specifically for gripping rope.

Also the handrail ropes should be just as big and as tightly tensioned as the ropes on the bottom with ropes between them also to stop falls out out between them. It also will stop the walkway from "flipping" which would be incredibly dangerous if it were to occur. Whatever you do don't go creating hazards. You don't want a neighbours child falling off of it and the parents suing you.

my3cents

You need to do a bit of maths before you do anything further just to check your rope sizing.

Force on each rope (F) = 0.5 * W / sin(A)

where W is the weight of the child (or children plus an allowance for the bridge itself)) and A is the angle made by the defection of the rope from horizontal. The times 0.5 is because the load is between two points so half it for the load at one end.

What that all means is the tighter the rope and lower the rope defection the bigger the rope you need.

So lets say you make a nice tight rope bridge with a 10 degree deflection with 2 kids on the middle of it and the kids total weight is 50kg. F = 0.5 Times 50/1.70.17 which means you rope has to be good for at least 150kg. Two ropes so you'd think you could half that but all the weight can easily go to one side and one rope so you can't. Then you need a safety factor for when the kids jump up and down on the rope.

If you really tighten the rope so you have a 5 degree deflection (that would be really tight and probably impossible) then the maths says the load on the rope will be close to 300kg.

So my advice is don't use any pulleys, upgrade you rope a bit and tighten it by hand so there is at say 15 degree defection with kids on it (100kg force/pull on rope).

Personally I'd allow at least a ten times safety factor so I'd be looking for rope that can hold a tonne or more and half inch polypropylene has a breaking strain around 2,000 tonnes so should do the job.

No worries if anyone wants to correct my maths

Edit> Maths typo kindly pointed out by Calahonda52 sin 10 is 0.17364817766 not 1.7 (end result correct).

Lumen

Tree structures are hard.

You need non-elastic rope, sometimes called "climbing rope" but not "dynamic" sort of climbing rope used for...climbing, because that is designed to stretch in the event of a fall.

Other good tips here:

http://www.thetreehouseguide.com/bridges.htm

TheBoyConor

M3C, that's a pretty good initial analysis.
One thing I'd add though is that the trees will deflect inwards under loading, so the tension in your ropes will drop and will vary depending on the load.

While 1/2" rope might work from a numbers POV, practically the issues highlighted in my previous post would suggest that an even thicker rope would be a good idea. Thicker rope will only be a very small additional cost.

my3cents

TheBoyConor wrote: »

M3C, that's a pretty good initial analysis.
One thing I'd add though is that the trees will deflect inwards under loading, so the tension in your ropes will drop and will vary depending on the load.

While 1/2" rope might work from a numbers POV, practically the issues highlighted in my previous post would suggest that an even thicker rope would be a good idea. Thicker rope will only be a very small additional cost.

I think you'll find that thicker rope is a good bit more expensive but tbh you need to design for a 200kg adult simply because when your back is turned some adult will be less than adult and try and show off.

I had the 10mm anchor bolts of a swing I built snap and it was only later that I found out the 4 adults had tried to get on a swing built for one child :rolleyes:

Another issue with the OP's design might be that without spacers between the wooden treads any gaps that form due to the timber warping may become pinch points so one child's weight could crush an others fingers.

I think I would go with some sort of monkey bridge design and cut out the wood treads altogether.

Plenty more monkey bridge info at scoutpioneering.com.

Alun

Think about the wood too. Is it pressure treated? How slippery will it get under the trees like that?

Lumen

Alun wrote: »

Think about the wood too. Is it pressure treated? How slippery will it get under the trees like that?

The slipperiness is easily solved with griptape.

Calahonda52

OP:
wire top can be fixed using small u bolts, double the wire and use a steal washer against the face of the horizontal pice in the tree.
However it is a high risk idea for all the reasons mentioned

Lumen

Am I the only one who disbelieves the whole "for the kids" story?

Dog Man Star

That's a hanging waiting to happen.

TheBoyConor

Tbh, yeah.

If a neighbor's child gets hurt off it you'll be in a world of hurt with a claim.

listermint

Looks to me to over 8 foot off the ground.

You really need to do more sums on this. This could go from dangerous to fatal. Halt the works and start again

monkeynuz

nino1 wrote: »

I am building a small rope bridge between two trees for the kids. (See attached pics)
I have used a small diameter pulley rope that i got from woodies but the bridge has turned out heavier than i thought so im not sure it will withstand the pressure.

Was thinking of using a wire rope instead but thinking that although it would be sturdier that it would be harder to secure at the ends to the tree.

I have two pieces of timber at each end. I was going to bore a hole in them, stick the rope through and put a big knot to secure but couldnt do this with wire rope.

Any suggestions would be much appreciated.

The sash cord at woodies depending on which diameter you got has a breaking strain of 20/30kg.

Peregrinus

CJhaughey wrote: »

Firstly that cord for sash weights will not last, it will rot and fall apart very quickly, you need to use at least 10mm Ø Polypropylene rope to be sure of it lasting . . .

Polypropylene rope is not ideal for this job; it degrades in sunlight. I wouldn't use it for any permanent outdoor fixture. Plus, it stretches.

As others have said, you need a rope with minimal stretch; otherwise your bridge sags hugely when someone walks on it. I'd go with polyester rope - very strong, minimal stretch, won't degrade in sunlight or moisture.

Lumen

How much sideways force can a tree take? Do they snap or uproot?

All trees break at wind speed of around 42 m/s, apparently.

http://www.sciencemag.org/news/2016/02/trees-regardless-size-all-break-same-wind-speed-here-s-why

Peregrinus

Lumen wrote: »

How much sideways force can a tree take? Do they snap or uproot?

Depends on the tree, and of course on how you have fixed the rope to the tree. The important question may not be "at what force will this tree fail?" but "at what force will this branch fail?", which may be a much lower force.

A rope-bridge should be secured to the main trunk of the tree, preferably below the point at which the major branches diverge from it, but of course depending on the species and shape of the tree that may not be possible. The tree should be mature, well rooted and (obviously) healthy. Generally you pass the rope right around the tree, so that the rope bridge is not, so to speak, trying to pull itself out of the tree, but to pull the tree over.

Lumen

Peregrinus wrote: »

Generally you pass the rope right around the tree, so that the rope bridge is not, so to speak, trying to pull itself out of the tree, but to pull the tree over.

In the rope bridge zipline experience thing I did in Italy, the cables passed around the tree and over vertical wooden members to spread the force.

It might be worth a visit to the nearest zipline place to check out what proper engineering looks like.

Alun

Lumen wrote: »

It might be worth a visit to the nearest zipline place to check out what proper engineering looks like.

Sounds like a good idea. There's one especially for kids at Kilruddery House here in Bray if that's any use.

https://www.squirrelsscramble.ie

These guys have places in Dublin Mountains, Roscommon and Cork

https://www.zipit.ie/

nino1

Thanks for all the advise.

Way more to think about than i had originally thought!

I live close to farran woods zipline so will pop over to have a look

jack of all

Lots of useful advice here already, only things I can add is suggest using plastic spacers between the the treads/ boards, maybe some HDPE (something like "Hydrodare" pipe) sleeves, with edges on the pipe sleeves and holes in the timber chamfered/ deburred with a chamfer bit- to avoid fraying of the rope over time. Also the securing of the rope ends/ terminal detail will be critical (knots weaken ropes?), where ever the ropes loop around the tree or branches try and sleeve them in some garden hose or similar to avoid damage to the tree and rope. Some good quality hardwood decking (you won't need much) would make a better tread/ walkway, just double up the thickness, route the required grooves and use a good polyurethane glue to join both halves together. A laminated board adds in some factor of safety. Use the best materials you can afford and good luck with it, the kids will love it!