International Guild of Knot Tyers Forum

General => Practical Knots => Topic started by: awtchi on March 06, 2014, 02:52:25 PM

Title: Looking for a knot that can be tensioned.
Post by: awtchi on March 06, 2014, 02:52:25 PM
Hi all,

First of all, thanks for taking the time to read my topic.

I am a complete newbie in the art of knots, my knotting knowledge is limited to tying my shoe laces ... This i why i came here, i need help with a project i?m working on.

I need two knots, here is a drawing (expertly made in Paint) to explain my need:

(http://s30.postimg.org/kvocr0qjl/rope_problem.png)

The poles that are going to be tied are much thinner than what i drew, maybe about 5/16" give or take. And the rope width will vary between 1/16" and 1/2".

The rope will be a Dyneema rope (i don't know if this will influence the knot much, but still, better mention this).

Fixed End: This is auto explanatory.

Tensioning End: This end will be used to tension the rope between the poles. I need a knot that can only be tensioned and stay at that tension for a long time, maybe forever if possible.

Both knots need to be able to handle a huge amount of tension.

Thanks for your time and your help!

Title: Re: Looking for a knot that can be tensioned.
Post by: roo on March 06, 2014, 04:00:17 PM
Hi all,

First of all, thanks for taking the time to read my topic.

I am a complete newbie in the art of knots, my knotting knowledge is limited to tying my shoe laces ... This i why i came here, i need help with a project i?m working on.

I need two knots, here is a drawing (expertly made in Paint) to explain my need:

(http://s30.postimg.org/kvocr0qjl/rope_problem.png)

The poles that are going to be tied are much thinner than what i drew, maybe about 5/16" give or take. And the rope width will vary between 1/16" and 1/2".

The rope will be a Dyneema rope (i don't know if this will influence the knot much, but still, better mention this).

Fixed End: This is auto explanatory.

Tensioning End: This end will be used to tension the rope between the poles. I need a knot that can only be tensioned and stay at that tension for a long time, maybe forever if possible.

Both knots need to be able to handle a huge amount of tension.

Thanks for your time and your help!

If the rope is slick, you may need to use a Slippery Eight Loop as a tensioner:

http://notableknotindex.webs.com/slippery8.html

...or a Versatackle:

http://notableknotindex.webs.com/Versatackle.html

(second diagram)
Title: Re: Looking for a knot that can be tensioned.
Post by: [Inkanyezi] gone on March 07, 2014, 09:50:51 PM
If it shall withstand lots of tension, maybe the versatackle at both ends might be an answer, but it is a tricky problem, as HMPE rope is not prone to stay knotted when tension is applied. There has been discussions on the board about loop knots that hold in HMPE, and I think thorough testing should be done before walking on a tightrope of the material. Sometimes I think the H is for Houdini and the E Escape, fill in for the letters between.

When approaching about half of the rope's breaking strength, most knots slip.

Spliced eyes might be needed to do the job, and spliced eyes with versatackle for tensioning might be the answer.
Title: Re: Looking for a knot that can be tensioned.
Post by: SS369 on March 07, 2014, 10:45:02 PM
Hi all,

First of all, thanks for taking the time to read my topic.

I am a complete newbie in the art of knots, my knotting knowledge is limited to tying my shoe laces ... This i why i came here, i need help with a project i?m working on.

I need two knots, here is a drawing (expertly made in Paint) to explain my need:

(http://s30.postimg.org/kvocr0qjl/rope_problem.png)

The poles that are going to be tied are much thinner than what i drew, maybe about 5/16" give or take. And the rope width will vary between 1/16" and 1/2".

The rope will be a Dyneema rope (i don't know if this will influence the knot much, but still, better mention this).

Fixed End: This is auto explanatory.

Tensioning End: This end will be used to tension the rope between the poles. I need a knot that can only be tensioned and stay at that tension for a long time, maybe forever if possible.

Both knots need to be able to handle a huge amount of tension.

Thanks for your time and your help!

Hell awtchi and welcome.

I think before a real meaningful answer can be given you should provide us with more information. What is the purpose of this construct? Will the rope just be a guide line? Will it have something else attached to it? Will this be a permanent thing?
How many poles in all?

Your poles are not all that substantial and your rope/cord is very substantial. I would think that any tension between the poles will bow them toward each other and you will be adjusting the tension till they touch!

I would not go so far as to say, as you have, that the fixed end is auto explanatory. You show an eye knot, but this can slide up or down and might need to be a tight hitch instead. Or the rod(s) may need a through hole(s).

We can go from here with some more data.

SS

Title: Re: Looking for a knot that can be tensioned.
Post by: Dan_Lehman on March 08, 2014, 07:57:54 AM

The poles that are going to be tied are much thinner than what i drew,
maybe about 5/16" give or take.
And the rope width will vary between 1/16" and 1/2".

The rope will be a Dyneema rope
(i don't know if this will influence the knot much, but still, better mention this).


...
I need a knot that can only be tensioned and stay at that tension for a long time, maybe forever if possible.

Both knots need to be able to handle a huge amount of tension.

I think before a real meaningful answer can be given
you should provide us with more information.
...

Your poles are not all that substantial and your rope/cord is very substantial.
...
Ditto/+1 to SS369's concerns.
And --goodness!!!-- what an odd set-up,
such as it's been so far presented : HMPE
super-static/-strong cord varying in size
from 1/16" to 1/2" :: that's about 30times
breaking strength (interpolating the missing
1/16" value from the table of strengths given
here : http://phillystran.com/product-catalog/12-Strand-Braids-Spectra-Dyneema (http://phillystran.com/product-catalog/12-Strand-Braids-Spectra-Dyneema)
(I tried to figure a rough factor from doubling diameter :
about 2.8 or so.)

And "forever" is a long time.

!!!! ?-?-?-?   :o

Cheers,
--dl*
====
Title: Re: Looking for a knot that can be tensioned.
Post by: SaltyCracker on March 11, 2014, 01:19:37 AM
There is a way, using the simple crossing knot, to pull effective tension between the two poles/spars. Don't have the time to do a video or sequential diagrams but essentially it involves forming a crossing knot, pulling excess slack out of the the line between the two poles by pulling the tailing/working end toward the "fixed end" pole then pulling back away from it, inverting the crossing knot over the top of the "tensioning end" knot's pole. This effectively cinches taut the "standing" line between the two poles/spars. Can then wrap around the tensioning end pole crossing knot, locking it in place by crossing over the crossing knot, pinning it in place, and using half hitches to secure. Wish I had more time to show but some of the knot wizards on the forum will probably understand, can clarify and improve upon... or, as is the custom, challenge the solution. Sometimes we complicate our solutions.
Title: Re: Looking for a knot that can be tensioned.
Post by: Dan_Lehman on March 11, 2014, 05:54:23 AM
Don't have the time to do a video or sequential diagrams but
... words can work, but we must sharpen them.
Quote
essentially it involves forming a crossing knot,
To clarify : the line runs around 2nd pole,
turns around itself,
goes *back* around pole,
Quote
pulling excess slack out of the the line between the two poles
by pulling the tailing/working end towards the "fixed end" pole
then pulling back away from it,
inverting the crossing knot over the top of the "tensioning end" knot's pole.
to be hauled upon (towards 1st pole)?

I'm lost, now, with "then pulling back away from it"
[= 1st pole]?!  For one thing, that seems like a LOT
of pulling back --i.e, maybe I'm e.g. a half-metre from
the 2nd pole in tensioning it by pulling towards 1st,
and now to "pull back" I must cover all that ground
(with slack!) and then again (the extent of material
in my grasp) before I gain tension in the opposite
direction. !??  --something's not right, here.

Also, I find that in the first pulling, if it takes enough
material --there's an issue re initial tension--, the
crossing knot will be capsized --a familiar sort
of thing, e.g., with rockclimbers belaying : they are
pulling IN for a following ("2nd") climber but if
she falls, she pulls OUT and flips the mezzo barcaiolo
(Munter hitch)
.

Quote
This effectively cinches taut the "standing" line between the two poles/spars.
Hmmm, weil, something I tried did work, around
my finger; but a smooth pole I think will see the
structure rotate, if one releases the tail.

.:.  I think another stab at words is needed,
but we should be closer, now.  Thanks,


--dl*
====
Title: Re: Looking for a knot that can be tensioned.
Post by: xarax on March 11, 2014, 08:02:24 PM
There is a way, using the simple crossing knot, to pull effective tension between the two poles/spars.

   Yes, there is, but we have to use TWO crossing knots, the one next to the other. See (1), and the attached picture.
I have not found any other way - and I bet there isn't any !  :)

1. http://igkt.net/sm/index.php?topic=4819.msg31391#msg31391
Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 13, 2014, 09:01:04 PM
Well, it doesn't look like Awtchi is coming back to answer the questions in a hurry, so in the meantime, how about we consider how we would do it if we were doing it for ourselves. 

Lets presume that those anchor poles are fixed to some rigid enough frame so that they can take the load that the Dyneema is capable of sustaining - then how would you fix the rope and how would you tension it?

Derek
Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 13, 2014, 09:14:40 PM
To start the solutions rolling, I will offer an anchor for the LHS pole.

First I would tie (yes, you guessed, a KC hitch) as the base of my anchor.  I would then lead the Dyneema into a multi turn around the pole.  The number of turns will depend upon the coefficient of friction between the Dyneema and the pole material but must be able to shed virtually the whole load into the pole at full rated Dyneema load.

The advantage of using multiple round turns is that a) the rope does not loose any strength in the fixing and b) sufficient turns can be utilised to guarantee that the Dyneema cannot pull out of the anchor knot.

Now I have to tension the rope and anchor it on the RHS pole.
Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 13, 2014, 09:34:34 PM
To tension the rope, I cannot expect any knot to function both as an anchor and as a tensioning device, so I would split the job into two.

Tensioning.  I would start by tying four VT friction hitches onto the end near the RHS anchor pole and connect each via pulley to a pulley anchor to create enough MA to allow the rope to be tensioned to the required force.  I would use a fifth VT as a 'third hand' to retain and sustain the tension while I then anchored the end of the tensioned rope.

Naturally, the first part of my anchor is going to be the same number of round turns I used to shed the load into the LHS post.  But now I need a tensioning hitch to supply the small load necessary to lock the capstan turns in place and allow them to generate the full loaded tension when the pulleys are slacked off.

Any ideas?

Derek
Title: Re: Looking for a knot that can be tensioned.
Post by: SS369 on March 13, 2014, 09:46:50 PM
Yes, disappointing that we've not read back from awtchi. Maybe he'll check in and contribute.

Since the poles won't come close to handling the tension we can induce using Dyneema, at any diameter indicated, I'll just throw in what I would do in any case.

Since we need an anchor knot that will have a right angle pull and needs to grip the pole so it doesn't slide up or down potentially, I would most likely use something that wraps the pole a couple of turns.
Maybe a Ground Line hitch (ABoK# 1680) and take it one step further with this slippery material and add a stopper knot.
Though a two round turn with half hitches knot and a stopper would probably suffice.

For the next pole, assuming there will not be anymore, I would consider using the double slipped prusik  ;) as presented here> http://igkt.net/sm/index.php?topic=4813.0 (http://igkt.net/sm/index.php?topic=4813.0). With the tail pulled through the last slip bight or with the lock suggested in that thread.

With the project in hand I might try other things too. (Of course.)

SS
Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 14, 2014, 01:20:52 AM
Hi Scott,

The OP required the RHS knot to be capable of tensioning the suspended rope.  How would you apply tension with the tie off you proposed?
Title: Re: Looking for a knot that can be tensioned.
Post by: SS369 on March 14, 2014, 02:58:18 AM
Hi Derek.

By hand of course.   ;)
Pull the cord/rope between the poles and at the same time take the slack with the other hand after the round turns and working part pass over. Like a Munter hitch and belaying, then complete the knot when the tension is adjusted.

With 5/16" poles I don't feel any force multipliers (versatackle, etc.) are needed.
Yes, I am still addressing the original post conditions, just in case.

SS
Title: Re: Looking for a knot that can be tensioned.
Post by: Sweeney on March 16, 2014, 10:40:24 PM
In terms of the fixed anchor at least the following would seem to fit the bill - being specifically tested in Dyneema: http://igkt.net/sm/index.php?topic=4756.msg30713;topicseen#msg30713 (http://igkt.net/sm/index.php?topic=4756.msg30713;topicseen#msg30713)

Barry
Title: Re: Looking for a knot that can be tensioned.
Post by: awtchi on March 17, 2014, 02:01:18 PM
Sorry guys, i got caught up with college and work stuff, so i didn't have the time to visit this thread.

Thank you so much for all the answers (i still didn't read them, i wanted to thank you all first).

To answer one of the first questions, here is another drawing about what i intend to use this rope for (attached).

Before explaining where i'm going to use the rope, i will tell you something about me. I am a Civil Engineering student and in my college, in order to graduate, you must submit a thesis. Mine will be about using ropes instead of steel wire in small prestressed concrete structures using

Now to the image. As you can see there is a concrete beam with a hole in the center. I will use the rope to apply a normal tension, compressing the concrete beam.

When i drew the first image, the poles i used were referring to the how i was going to tie the rope on each end. I was going to use a very short steel rod and loop the rope around it (I dont know how to draw this in paint).

Now i will read all your answers.

Thanks again for all the answers!

Title: Re: Looking for a knot that can be tensioned.
Post by: Sweeney on March 17, 2014, 10:37:42 PM
This latest diagram and explanation puts rather a different complexion on the problem. To restate what you want : the rope will run through a concrete beam down the horizontal axis. It will be attached to steel rods at each end of the beam to apply tension to the concrete. One end of the rope is therefore to be permanently attached but the other end needs to be attached in such a way that tension can be applied.  Where this differs markedly from the original proposal is the rope will be concealed by the concrete.

The answer would seem to be to attach the rope to both steel rods and then apply tension by forcing one (or both) rods away from the concrete beam using perhaps a hollow bolt with the rope running through the centre. The bolt fits partly inside and partly outside of the beam held in place by a nut and washer which when tightened acts to push on the steel bar to which the rope is attached creating tension.

The only other possibility I can see is to use the non-anchored rod as a windlass to apply tension but the inaccessibility of the rope in the concrete beam would make it difficult if not impossible to tie off the wound rope and stop it unravelling.

Barry
Title: Re: Looking for a knot that can be tensioned.
Post by: SS369 on March 17, 2014, 11:20:32 PM
Quote
I will use the rope to apply a normal tension, compressing the concrete beam.

I have worked concrete many years and I don't think I have ever compressed it with anything. Let alone a rope.
So I must be missing something.
What is the purpose of trying to compress the concrete beam? If it has rebar (reinforcing steel) in it you won't compress that either. And if you were to manage some how to compress this structure, what will keep it from bowing unduly?

Would you be talking about the form work to cast this beam in?

Curious.

SS
Title: Re: Looking for a knot that can be tensioned.
Post by: roo on March 18, 2014, 12:05:39 AM
What is the purpose of trying to compress the concrete beam?
I've seen pre-formed concrete slabs where they stretch steel cables or rebar, pour the concrete, let it cure and then let the steel retract back and compress the concrete.  The idea is to keep the concrete from seeing tension (say from bending) since concrete is much better at handling compression than tension.
Title: Re: Looking for a knot that can be tensioned.
Post by: SS369 on March 18, 2014, 03:36:22 AM
What is the purpose of trying to compress the concrete beam?
I've seen pre-formed concrete slabs where they stretch steel cables or rebar, pour the concrete, let it cure and then let the steel retract back and compress the concrete.  The idea is to keep the concrete from seeing tension (say from bending) since concrete is much better at handling compression than tension.


Yes, with preformed sections of overpasses for roads, but they don't use ropes and knots. They use multiple steel cables inside a conduit that get tensioned using a huge amount of hydraulic pulling. The cables are then wedged to keep the tension and the conduit is mortared completely.
I don't believe it is actual compression of the concrete as it is additional tension to resist deflection after cure.

Rebar may stretch, but I doubt very much it has any elasticity. Many of the bridges and parking structures use #11 which is of 1.25" diameter. I'd say that is high tensile strength, so it would resist stretching, especially with all the cross hatches formed in its surface to engage the concrete and unify it.

How much "retraction" the steel cables have after encased in cured concrete stretches my imagination.
Post cure tensioning with cables, sure and then fill the conduit.

So, have you got some knots for this?

SS
Title: Re: Looking for a knot that can be tensioned.
Post by: roo on March 18, 2014, 05:59:49 AM
What is the purpose of trying to compress the concrete beam?
I've seen pre-formed concrete slabs where they stretch steel cables or rebar, pour the concrete, let it cure and then let the steel retract back and compress the concrete.  The idea is to keep the concrete from seeing tension (say from bending) since concrete is much better at handling compression than tension.

Yes, with preformed sections of overpasses for roads, but they don't use ropes and knots. They use multiple steel cables inside a conduit that get tensioned using a huge amount of hydraulic pulling. The cables are then wedged to keep the tension and the conduit is mortared completely.
I don't believe it is actual compression of the concrete as it is additional tension to resist deflection after cure.

Rebar may stretch, but I doubt very much it has any elasticity. Many of the bridges and parking structures use #11 which is of 1.25" diameter. I'd say that is high tensile strength, so it would resist stretching, especially with all the cross hatches formed in its surface to engage the concrete and unify it.

How much "retraction" the steel cables have after encased in cured concrete stretches my imagination.
Post cure tensioning with cables, sure and then fill the conduit.

So, have you got some knots for this?

SS
Deflection=Force*Length/(Area*E).  So for steel at 30000 psi of stress, E=29E+6 and a length of say 240 inches, that's about a quarter inch of stretch.  It'd likely be more for cables.  I have no idea what kind of experiment the original poster has going, but it sounds interesting.  I don't know how practical it is.
Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 18, 2014, 09:50:38 AM
Hi Awatchi,  thanks for coming back and settling our inquisitiveness - what a sweet project - I hope you can 'pull it off'...

Amsteel Blue is as strong as steel wire rope of the same diameter, yet roughly 20% of the weight.  It consumes a tiny amount of energy to manufacture it in comparison to steel, and its crowning advantage - it doesn't rust !  Is it your idea to use it to pre-stress concrete?  If so, it seems we could have a genius visiting our board...

BUT, this rope does have some weakness that wire rope does not, and as far as fixing is concerned, its most important one is that in comparison to steel, it is easily compressible and deforms to rupture under point pressure.  Take a sharp knife to Amsteel Blue and press, the edge will cleave straight through - take the same knife edge to steel rope - well, you see what I am getting at.  This polymer is the opposite of concrete, it is massively strong in tension, but it is like jello in compression.  In using the rope for your project, your main challenge is going to be converting the rope tension into compressive force on the end of the concrete, because as you wrap the rope around the anchor point, the inner surface of the rope goes into compression against the anchor and you have to keep this pressure well below the deformation point or you will 'cut' through your rope.

The 'knot' for you anchor point is easy and is well defined for the use of this material - it is the spliced eye.  The safe working anchor radius is also well defined - but don't be surprised if it turns out to be greater than the diameter of the hole in your concrete spar, you may well have to arrange for the anchor to be well outside the end of the beam.  Also, the spliced section has roughly twice the cross sectional area  of the rope, you will need to allow for this to pass into the core of the spar.

But now comes your second challenge - the tensioning point.  The 'knot' is again going to have to be a spliced eye which will have to be passed down the core of the spar and be long enough to protrude the other end.  BUT, Amsteel Blue will stretch by ca 1% when under just 30% rated loading, so you are going to have to put the eye over the tensioning anchor, jack it away from the end of the spar to load it, then fit some form of concrete spacer between the tension anchor and the end of the spar.

If you can afford to sacrifice a little Amsteel blue, then there is an innovative grip hitch going crazy in the camping world at the moment called the 'Whoopie Sling' - it is effectively a giant VT grip hitch made by passing the rope through a piece of braid - much like the splice making the terminal eye.  By using a grip hitch such as this, you could have a means of pulling tension onto the rope from within the spar, until you drew the tensioning eye loop out of the spar, slide in the tensioning pin, then release the tensioning rope which unfortunately has to be sacrificed, because there is no way to retrieve it.

The final aspect of polymer ropes you must consider is creep.  Although the long polymer molecules have been pre stretched to align them during manufacture, they can still undergo plastic deformation under load.  I have no idea how significant this aspect of plastic string is, but if it happens to any significant degree, then the load set initially, will diminish over time -  Oops.

Hope this helps, and have fun.

Derek
Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 18, 2014, 10:14:40 AM
Just had a thought that might make tensioning easier.

Just like they 'spin' suspension bridge hawsers in place, strand by strand, so you could (with a little ingenuity) 'spin' your tensioning rope in-situ.

Take a much lighter Amsteel Blue rope and fix it with an eye splice loop to an anchor pin.

Now loosely pass the lighter rope 'n' times back and forth through the core around both the anchor points in turn, where 'n' is the tension multiplier you need to safely pre-stress the beam.

Then tension each turn to the desired load sequentially, temporarily locking each loaded line while you set up to load the next turn, until all the turns have been loaded.

Finally, for the last turn, fit a sliding grip hitch with a spliced eye over the rope (again the 'Whoopie' is ideal).  Pre load the last turn and slide the Whoopie hitch down in place and fit the eye over the anchor.  Then release the loading tension to allow the 'Whoopie' to retain the end under tension.  You might have to over tension the last turn in order to allow for the tensioning up of the 'Whoopie'

Derek
Title: Re: Looking for a knot that can be tensioned.
Post by: Wed on March 18, 2014, 11:01:13 AM
Is the rope going to be free inside the concrete element or molded into place?

If molded, how well will the concrete grip to the rope? If it won't grip properly, I suspect the tension will be compromised if the ends, or at least knots, are damaged.

Intriguing project none the less
Title: Re: Looking for a knot that can be tensioned.
Post by: awtchi on March 18, 2014, 01:36:44 PM
Quote
I will use the rope to apply a normal tension, compressing the concrete beam.

I have worked concrete many years and I don't think I have ever compressed it with anything. Let alone a rope.
So I must be missing something.
What is the purpose of trying to compress the concrete beam? If it has rebar (reinforcing steel) in it you won't compress that either. And if you were to manage some how to compress this structure, what will keep it from bowing unduly?

Would you be talking about the form work to cast this beam in?

Curious.

SS

Since concrete does not resist traction, and it resists compression very well, you can prestress it so that it doesn't suffer traction in any part of the beam, making it flex less and have less fissures. (I live in Brasil, and learn in Portugues, so i don't know how to explain this very well in English, sorry)

What is the purpose of trying to compress the concrete beam?
I've seen pre-formed concrete slabs where they stretch steel cables or rebar, pour the concrete, let it cure and then let the steel retract back and compress the concrete.  The idea is to keep the concrete from seeing tension (say from bending) since concrete is much better at handling compression than tension.

You got that right.

However, you can stress the concrete after it hardens, which is the way i am planning to go.
Title: Re: Looking for a knot that can be tensioned.
Post by: awtchi on March 18, 2014, 01:49:04 PM
Just had a thought that might make tensioning easier.

Just like they 'spin' suspension bridge hawsers in place, strand by strand, so you could (with a little ingenuity) 'spin' your tensioning rope in-situ.

Take a much lighter Amsteel Blue rope and fix it with an eye splice loop to an anchor pin.

Now loosely pass the lighter rope 'n' times back and forth through the core around both the anchor points in turn, where 'n' is the tension multiplier you need to safely pre-stress the beam.

Then tension each turn to the desired load sequentially, temporarily locking each loaded line while you set up to load the next turn, until all the turns have been loaded.

Finally, for the last turn, fit a sliding grip hitch with a spliced eye over the rope (again the 'Whoopie' is ideal).  Pre load the last turn and slide the Whoopie hitch down in place and fit the eye over the anchor.  Then release the loading tension to allow the 'Whoopie' to retain the end under tension.  You might have to over tension the last turn in order to allow for the tensioning up of the 'Whoopie'

Derek

I understood most of your idea, but i didn't understand what you meant by "Then tension each turn to the desired load sequentially, temporarily locking each loaded line while you set up to load the next turn, until all the turns have been loaded."

Is the rope going to be free inside the concrete element or molded into place?

If molded, how well will the concrete grip to the rope? If it won't grip properly, I suspect the tension will be compromised if the ends, or at least knots, are damaged.

Intriguing project none the less

The rope will be free inside the concrete beam, i suspect that molding it will not help much since i doubt that the concrete will grip the rope well.

As for the ends, that's why i'm here. If there is anyway to solve this problem, i believe that the guys here will have the answer.


Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 18, 2014, 11:36:03 PM

I understood most of your idea, but i didn't understand what you meant by "Then tension each turn to the desired load sequentially, temporarily locking each loaded line while you set up to load the next turn, until all the turns have been loaded."


OK awtchi, I will try to clarify by using a couple of examples.

When you put a new shoelace into a boot, you cannot just thread the lace, then pull on the ends to tighten it up,  the first few eyes will tighten, but after that, the force just does not travel past the eyes all the way down the lace.  This is due to the 'capstan effect' see http://notableknotindex.webs.com/friction.html (http://notableknotindex.webs.com/friction.html).  So, we can't just thread a rope several times around the two end anchors, then haul on the end and expect all the turns to tighten up to the same tension (unless the anchor bars have ultra slick surfaces such as hard anodised Al).  Every time the rope goes around an anchor, it looses force, so we have to tension every run in turn.

Let us presume, that the two anchor bars are sitting in recesses in the ends of the beam (to protect the tensioning rope from damage and abrasion.)

Take a length of rope that is long enough to pass through the beam the required number of times (n) to give you the necessary load multiplier.  Start the rope with a spliced eye and slide this over one of the anchor bars.  Then feed the rope down the channel in the beam, around the other anchor, then feed it back to the beginning, then around the anchor bar again alongside the spliced loop, then back down the channel, again and again 'n' times.  At this stage it is fixed on to one anchor by the spliced loop, passes back and forth from end to end 'n' times around both anchor bars, and at the moment it is hanging loose - untensioned.

Now an explanation on tensioning.  I was top-roping my climbing partner once when he hit a particularly difficult part of the climb and called to me to haul him up over the part that he was stuck on.  I leaned forward, took out all the slack in the rope and then leaned back, hauling the rope up 6".  But it wasn't far enough, he needed me to haul him up 18" to the next hand hold.  With 70kg hanging on a climbing rope, they go stiff as an iron bar,and only 1/2" thick, you can't grip them sufficiently to haul that 70kg up.  So what was I to do?  I let him down to his previous hold, then I took a loop and tied a Prussic loop onto the climbing line, it is an adjustable friction hitch.  I hauled him back up 6", then I slid the Prussic down the rope, put my hand through the loop which gave me a good grip on the rope and hauled him up another 6".  I then took the slack out of the rope above the Prussic and locked it into my belay plate.  I then slid the Prussic back down the rope and hauled him up another 6", and so on until he was climbing again.

You can utilise this technique of two tension points to tension the first length of rope passing through the beam.  Tie on a Prussic connections (or better a VT hitch), and slide it into the channel beyond the anchor bar.  Haul on the Prussic connection to tension the rope.  Maintain that tension while you work on tensioning the next rope.

Go back to the starting side of the beam and find the rope that comes back from the now tensioned line.  Slide a Prussic onto it and into the channel, beyond the anchor bar.  Haul on it to the required tension. Maintain that tension while you work on the next length.

Go back to the exit end.  The first Prussic is now doing nothing because the rope is tensioned both sides of it.  Remove it from the rope (using the VT hitch makes removal much easier) and fix it around the rope now running from the line under tension at the eye end.  Slide the hitch along the rope into the channel and haul on it to the required tension.  Repeat this tensioning end to end just as you would tension a shoelace all the way up the lacing.

The final run of rope has to be treated differently, because this run has to be tied off.  By now you should have found out how the Whoopie hitch works, it is simply a length of the rope braid, opened up and the other rope passed inside it, much like one snake swallows another.  Then when the opened braid is tensioned, it shrinks onto the inner rope, gripping it like a Chinese Finger Trap.  This is exactly the way the eye splices are made.

EDIT : see the termination tensioning in the next post.

And there you have it.
Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 18, 2014, 11:56:59 PM
Here is another refinement.

Make up a jig of timber with two anchor pins the appropriate distance apart. 

Slip an eye splice ove one pin, then run the rope around and around the pins the required number of times.  Fit prussic loops to them.

We will now make the Whoopie hitch in situ - perhaps we had better start calling it the Garter Hitch as Whoopie is probably a Trade Marked name...

Take the end of the rope one last time around the end anchor pin, then fit the end of the rope into a hollow Fid.  Bunch up a section of the Blue Amsteel long enough to give you the necessary bury length for a splice and pass the Fid into, through and out of the splice bury section - this is your Garter Hitch.  Make the end long enough to exit the beam and enough to grip onto to make the final loading haul.

Fit a length of split plastic pipe over the assembly and slide it off the jig pins.  This premade assembly is now ready to slide into the channel within the beam.

Slide the assembly into the beam, then fit an anchor pin through the eye and all the loops, finish sliding the assembly into the beam and locate the first pin into its slot.  Where the assembly protrudes from the other end, slide in the other anchor pin and tighten up the ropes to clamp the second anchor pin into place.  You will have to slacken off the Garter Hitch in order to have the Garter eye protrude sufficiently to fit the second pin.

Tension all the lengths then finally tension the Garter hitch end to lock the assembly in place.

A possible alternative would be to have a lubricated shim around the anchor pins so as to reduce friction and allow all the tensioning to be achieved just by hauling on the very end of the rope - job done...

Three things to watch out for - as mentioned before, you will need to establish the amount of relaxation the rope has under constant load.  Second, what is the impact of temperature on tension and upon relaxation rate, and finally what is the long term impact of oxidation or other chemical degredation on the rope?

NB: You may need to facilitate access in order to retension the rope over time.

Edit : 19 March 07:20 GMT change name to Garter Hitch and make into a single rope component.
Title: Re: Looking for a knot that can be tensioned.
Post by: Sweeney on March 19, 2014, 10:16:28 PM
Derek I think I can follow what you are trying to do here but I do think that the chances of a prusik or any other friction hitch gripping Amsteel must be slim at best - as I understand it HMPE just doesn't generate enough friction (a triple fisherman's knot has been shown to slip under load in Dyneema).

Barry
Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 20, 2014, 02:44:08 PM
Derek I think I can follow what you are trying to do here but I do think that the chances of a prusik or any other friction hitch gripping Amsteel must be slim at best - as I understand it HMPE just doesn't generate enough friction (a triple fisherman's knot has been shown to slip under load in Dyneema).

Barry

Hi Barry,

Yes, I have to agree that the low self friction of HMPE is a real challenge to sustaining knots.  However, Polyester to HMPE has a much better cf, giving us the chance of using polyester as the friction hitch components.  Also, with friction hitches, we have the flexibility of increasing the turns and therefore the gripping capability of the hitch.

But if the worst comes to the worst, and awtchi cannot get enough tension through the prusik hitches, then he can always go to the ultimate route of adding locked Brummel spliced tensioning lines into the HMPE.  These would be sacrificial in that they could not be removed after tensioning, but the cost would be trivial, and with appropriately buried tails, would give full rope strength to the tensioning line.

Derek
Title: Re: Looking for a knot that can be tensioned.
Post by: [Inkanyezi] gone on March 21, 2014, 09:55:19 AM
How about twisting?

Just the same way as you charge a rubber band motor.

This will cause an uneven stress through the rope, one set of strands, half of them, less tensioned than the other, although it may facilitate tensioning, if for example the anchor on one end is resting on an axial ball bearing, then rotated, maybe until the bearing collapses and locks from the compression load, otherwise locked by other means. Tension on the rope may be calculated from the torque and rope diameter.
Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 23, 2014, 03:19:08 PM
HI Injanyezi,

That is an interesting 'twist' to the solution.  And Amsteel has such a low elongation under load, a 5m length expands by only 5cm under 30% loading.  A double line shrinks by one rope diameter per turn, so in 11mm Amsteel, you would only have to give it five turns to tension it to 30%.  There would be ca 80 braid turns in a 5m length of Amsteel, so one group of fibres would be wound up to 85 turns, while the other six were 'unwound' to 75 turns.  So one group of fibres would be loaded to ca 60% while the other group remained essentially unloaded.

I wonder how a rope would perform under conditions like this, with essentially a whole load of redundancy built in against failure?  The rope would at first be stiff against load, yet as it failed, it would expand yet still retain the load...

Of course, in this application, the result would still lead to a failure of the concrete beam as the beam would only be prestressed so long as the cable did not elongate.

Derek
Title: Re: Looking for a knot that can be tensioned.
Post by: SS369 on March 23, 2014, 05:51:17 PM
Looking at this challenge, considering the material to be used, I hate to state this, but I think knots are the wrong way to go, so far.  :(

I would consider a spliced sling where one end could go around the metal anchor bar (of as large a radius as possible) and the other end of the sling around a opposing wedge device that incorporates similar radius at the contact to rope face location.
The low slope wedges can generate a great deal of force. I have used them to lift houses that have settled foundations.

Attached is a crude drawing of what I am offering for your consideration.

SS
Title: Re: Looking for a knot that can be tensioned.
Post by: DerekSmith on March 23, 2014, 08:10:55 PM
I have to agree Scott,

But I would ask, what criteria would be taken into consideration in making the choice between spliced sling vs eyspliced braid?

I note that there are a huge array of uses for Amsteel Blue that use locked Brummel eyesplices.  Do these have some advantage over spliced slings?

One aspect of solution which came up in the discussions was the protection of the tensioning medium from damage when the beam is being located.  If the anchors sit on the end of the beam as you have depicted, then some sort of protective cap needs to be fitted.  Alternatively, one end at least could be recessed into the end of the beam, but it might be difficult to engineer a system of recessing both ends that also allows the use of wedges.  Any thoughts?

Derek
Title: Re: Looking for a knot that can be tensioned.
Post by: SS369 on March 23, 2014, 10:51:52 PM
Hi Derek.

Actually I just thought that a spliced sling would only have one join whereas the other has two. Plus it is a double strength tendon.

Then there are splice-less slings and that would be even better. Round slings are generally of a continuous fiber wound around a sizing form and then outfitted with a braided sleeve to contain and protect the fibers. The fibers can then form to the load attachment needs and the shape of the item being hoisted.

Yes, the accessibility, both good and bad, to the tensioning method could be addressed by creative blockouts cast into the concrete forms, along with the thrust plates. Effectively making recesses where the ends and device will be located. 
More for the engineers to figure out.  ;)

It just seems to me that knots aren't the right tool for this job. There are so many factors, safety and longevity, not to mention cost, to be considered and rigorously tested before implementing.
They are already using carbon fiber tendons for these type of applications and use specifically engineered anchorages.

Re: OP. Knots have been shown to severely weaken this material (Dyneema likes straight line tension,  it has creep and amazing slipperiness) and their trustworthiness (knots) are still pending further exploration. So, I believe there will have to be a marriage of sorts between the rope and mechanical fasteners. Such as wedge type and mortar/epoxy style.

This has been very educational and I am learning some very interesting things.

SS
Title: Re: Looking for a knot that can be tensioned.
Post by: TheTreeSpyder on May 01, 2014, 01:41:24 AM
Reverse Paradigm:
(Though small experience with amsteel/dyneema slip/low stretch, applying rope stuff i use to scenario):

Don't put knots in highest tension parts, use leveraged angle sweat/swig to tighten line not inline pulls/jig compressions.
Pillars, slick for slide, but right enough amount of friction to quickly call up; and keep purchases taken from tensioned line length.
Finalize with tensionless hitch type strategy that doesn't deform the loaded tension part with own rope part, perhaps even spiral around 1st turn around spar for many nip  points w/minimal deforming main tension from pure inline, then just keeper/low tension eye around tension line w/minimal deformity.



This is an adaptation of swig/sweat line, gives 2x1 leverage  to bend tension line, by "Muenter"/backhand Turn(?) on spar.

 Made to work when was in tree, could only work end on spar, but this "Muenter" has a 2/1 built in to help the bending of the main line.  Main line will wrap further around host spar, then need enough slip to slide back around spar, while not losing tension, and then bend main line again w/2:1 bend of "muenter" rinse/repeat ratcheting more line from tension /standing part to working end, can leg push off spar, impact etc.

Can convert to timber, cow, improved half etc. easily and back if careful for temp/perm lockoff.
Or  hold nip to spar tightly as lacing working end thru, around back of spar, to pull nip deep around spar at convex point.
Finish with no/minimal deformation to loaded line with tensionless hitch, types of clove, constrictor/ground hitch etc.

Can get more nips to lock by main tension (for slippery use) by sliding bight of working end under where standing part hits tree, or poking bight thru near previous nip, would then spread nips so don't bridge tension off of each other.