Rope Bridge

Practical Knots
1

Hello All,

Several people mentioned this site to possibly answer a question I have. I’m building a rope bridge (~45’ long by 36" wide) and have successfully gotten the 2" main ropes secured and tensioned. I need to tie down the planks (see enclosed). I need to work from the top of the bridge extending it as I go along. What’s a good (easy way) to secure this? Seems to me like this has to have been done before but I can’t find any internet information. Happy to send you pictures when I complete the project. Note the Safety ropes from the handrail ropes to the 2" main ropes do not have to be tied into the planks themselves but could be.

2

Come on, folks. Here is a genuine practical problem. We currently have 124 replies in the Bowline thread that’s pure theory.

I don’t have any experience here, but I think I would find these links useful if I were building a rope bridge:
http://www.thetreehouseguide.com/bridges.htm
http://www.shepfarm.com/treehouse/bridge.html
http://www.mystrees.com/large_pages/Main_11.htm
http://glenn.cockwell.com/scouting/scouting_rope_bridge.html

3

Welcome to the forum GMellow. The following assumes that there is one safety rope for each plank.

I am thinking that by using 2 holes in the plank tie an overhand knot in the end of the safety rope then lead the other then down through the outer hole, round the support rope and back through the inner hole where a second overhand knot is added drawn tightly up to the plank. The rope is then led up and fastened to the top rope with say a buntline hitch. This should stop enough lateral movement of the plank to prevent it slipping off sideways and is pretty quick and simple to do.

If the safety ropes are less frequent I suggest you might try a short rope at each end of a plank without a safety rope (assuming that you don’t want to use metal fastenings such as a U bolt) taken down the hole in the plank around the support rope and back up through the second hole with an overhand knot at each end drawn tightly (in effect a rope U bolt) but with no continuation to the top rope

I have never tried this so the above suggestions are for you to try out rather than a solution but they help you to develop your own solution.

Barry

4

Attached are a couple of photos of a bird tray in my back garden. This is to feed birds who cannot use the cylindrical feeders above and also to catch the debris from the sunflower seeds used as feed. I set this up about a fortnight ago using rope around the arch side supports and the sideways movement if pushed is less than 0.5". It seems similar to the problem here which is why I suggested the possible solution above.

Barry

Bird Tray Top.jpg

5

I’m partial to Sweeney’s approach of incorporating the hand rail rope in the equation.

I might start off with a tight Constrictor Knot around the main rope using a relatively short smaller diameter rope. Keep the Constrictor Knot crossings on the bottom. Then, lead the excess rope through the two plank holes and hitch off the ends of the rope to the hand rail rope.

Constrictor Knot, second part of page:
http://notableknotindex.webs.com/boaconstrictor.html

Some final hitching options:
http://notableknotindex.webs.com/sailorhitches.html
Or a round turn finished with a Gnat Hitch.

You might choose to seize the rope ends to keep things looking tidy. Also, you could use a larger, single hole in the plank for both ropes if you want. If you make a wide V pattern with the lines traveling up to the hand rails, you’ll eventually end up with a net-like effect.

6

I see the important considerations to be:

  1. efficient use of cordage;

  2. redundancy;

  3. ease of tying;

  4. ease of adjustment / repair.

I can see some appeal to using a single rope (in a sense
–not to preclude that there are end-2-end knots to make
adequate length of pieces, overall), but am not sure of
the viability of this vis-a-vis durability vs. chafe in the
function of securing the planks.

I’ll presume that one cannot be matching plank-securing
to vertical hand_line-2-plank_line support --i.e., that the
vertical support lines will occur only about every 4 planks.
But that’s not a hard situation to accommodate. I’d see
using a line that satisfactorily enables twin vertical lines
(just as sometimes rockclimbers use twin (not “double”)
ropes vice a single rope --claimed to improve cut resistance).

Perhaps the running of such a single-line joint solution would
be:
a. from some anchorage at whatever point, assuming this to
be proximate to the initial plank, the line will run up and be
taken into the near interior (i.e. towards center, away from side)
hole,
b. down and around the support cable,
c. up through the exterior hole
d. and cross on the interior side of the -a- line,
which means one can tighten all (pulling on current working end);
e. which is led to go down into the 2nd exterior hole,
f. around under the cable and up out of the interior hole
g. and then tucked under the -e- part (mirroring the earlier such move)
h. and then over the plank down to secure to the cable.

To sketch the result (and help comprehension of the above words),
it’s essentially a girth hitch around the cable,
where the two SParts run (just in conceiving this structure) along
the planks, down through interior holes, around cables & up through
exterior holes, over the SParts as the collar.
IN USE, the ends/SParts will be pulled away from each other,
to run perpendicular to the planks, parallel to the cable.
I’m hoping that there is some sufficient nipping of the ends
by the collar should the line be abraded where it feeds into
this binding over either edge of the plank.
It might serve this structure to drill the exterior holes somewhat
closer together, to better strengthen the nipping collar.

Alas, it occurs to me that if one is using the “single line” approach,
there will be a lot of rope to be puling around each of these “moves”.

However, with some initial sizing of the work, measuring the amount
of line needed just to span one vertical support to the next, one can
chop the material into suitable lengths (do one per time, so to be
able to adjust length as needed); then the connection of one to the
next piece can be assuredly put in on the vertical support part,
and the amount of line in use for the next span will be reduced
to maybe a workable quantity.

Against the use of stoppers, which use does appeal in terms of doing
piece-by-piece attachments, building independence of each from the
others, is a fear that in time this binding tends to loosen --it’s not all
that easy to make tight. (Now, maybe nylon rope will tend to SHRINK
with exposure to the elements, and help counter knot compression?!)

Okay, there’s a start, one idea.

Cheers,
–dl*

7

To add to what has so far been suggested:

There is a need to prevent chafing of the smaller line used to secure the planks to the main rope,
There is a need to apply some form of tightening to ensure the grip of the planks to the main rope,
There is a need to provide for some readily examined structure to ensure regular maintenance may be carried out.

I suggest (and here it is up to you, GMellow, to ensure that what you do is secure) that you use some laid nylon seine twine, sized probably at about size 72 or larger, to form a lashing like ABOK # 2141 but made in multiple turns and finish across between the inner pair and the outer pair with frapping turns ABOK # 2136. Each of the turns of 2141 should be made tight using a heaver or marlinespike and the subsequent frapping turns also made with a heaver or 'spike. There then will be no turns made between the underside of the planks and the stout cable, there will be the opportunity to see what is working and replace it with relative ease and the tightening structure of the frapping turns may be readily applied at any time. Further, as has been stated elsewhere, the nylon will harden with sunlight exposure, gaining some resistance to abrasion. Do let us know what you achieve!

SR

PS Not to be churlish but why not just use some U-bolts with recessed acorn nuts? Do you need to keep this all as an all-rope bridge?

8

U bolts can deliver a mighty pinch.
Cord can be weakened by compression.

It is critical on a bridge, that nothing weakens the main load bearing cables, especially as they are strained to give as flat a bridge base as possible, and the weight of boards and passengers can create a massive tension in the horizontal rope. The last thing you want in this situation is a weakness induced by overtightly nipping the main suspension ropes.

Derek

9

Derek,

You are absolutely correct:

U bolts can deliver a mighty pinch. Cord can be weakened by compression.

and I did consider this before responding. The wood in the planks will give way long before the U-bolt is able to significantly deteriorate the cable, which I assume (rightly or wrongly) to be galvanized steel. That there may be abrasion 'twixt bolt and cable I do not doubt. It would therefore be sensible to wrap the cable in some friction tape several layers thick or to place a neoprene pad between the U-bolt and the cable to provide a reduction in the potential damage and to provide some greater frictive surface to the U-bolt and planks. Another assumption on my part is that a Structural Engineer or Civil Engineer has designed the cables, walking surfaces and anchors. Otherwise let the walker beware! :o

SR

10

I have seen a few rope bridges with wooden walkways and they were all slung as a slack line ie a gentle downward curve not a taut flat surface. But maybe I’ve been looking at the wrong bridges.

I would be surprised if the support cable were steel as thick as described in the original post - I don’t think that this is for driving a vehicle across!

Barry

11

Interesting then that the very first picture in the very first reference link is this-

http://www.thetreehouseguide.com/alnwick/photos/suspensionbridge.jpg

Of course, a flat bridge like this is only possible because the bed is suspended on the heavily curved suspension cables.

I agree that a normal bed suspended rope bridge as the OP is proposing would be gently curved, but it would be exactly that-‘gently curved’, because a steep curve turns into a ladder and the planks become impossible to grip in wet weather. Much more slope than ca 1 in 5 becomes hard work, and a 1 in 5 ‘flat’ amplifies the load of bridge + passengers by a factor of 500%.

Granted, the OP has sized the load cables at 2" which presumably have a massive safety margin, but the flatter he tries to get the bed, the greater tension he will create in them when a passenger(or three) adds to the weight of the bridge.

@Lindsey

I did not mean abrasion, but U bolts tightened so that they bite into the wood are well capable of delivering such a mighty nip that they will significantly weaken the suspension cable.

As a generality, the less elastic a cord is, the more susceptible it is to compression failure, and high modulus rope is rarely used as a bridge bed - who wants a ‘bouncy’ bridge?

The nip/compression effect I am referring to can be demonstrated by simply gently pinching a strand of cooked spaghetti between finger and thumb. A more dramatic example can be seen by pinching a large soft eraser across its middle, eventually, the centre of the eraser will split under the internal tension created by the nip and the eraser will break in two - In England we have candies called ‘Jelly Babies’ that demonstrate this effect very easily.

On a load critical construction, forces should be transferred by contact friction i.e. multiple round turns, not by high point pressure nip type connections, so for this application I would use round turns…

Bring the descender cords down from the rand rail cable , feed through one hole in a plank, wrap half a dozen times around the suspension cable and feed back up through another plank hole back to the hand rail cable. The round turns will provide the necessary grip of the main cable to keep the planks in place and will also act as an abrasion buffer between planks and main cable. Checking for wear would simply be a matter of lifting a plank up and sliding it a few inches up the side cables to inspect the turns.

For me, the greater challenge is how to attach the side cords to the hand rail so that you have adjustment to allow all the cords to be given the same tension ( you don’t want ‘baggy’ cords), and at the same time ensure that the will not slip/slide along the hand rail cord when passengers run their hands along the hand rail for support in transit. I do not at the moment have a solution for this one… a lockable/unlockable, adjustable, gripping hitch?

Derek