Three point anchor - The Saga Continues

Brady3 wrote: Would you mind sighting your sources on this? Because logic says otherwise. There is still only one strand going around each pin, why does that spot hold more with a bend as compared to the straight lengths between the pins? We know that adding bends to the cord/tape makes it weaker (this is why knots weaken cord/tape) so why do bends over a carabiner make it stronger? I did try searching for more technical information, but could not find any.

http://physics.stackexchange.com/questions/64526/tension-in-a-closed-loop-around-an-object-carrying-stuff

That does not answer the question. The link is looking at the tension (not the same as the force needed to break it), which is effected by the angle of the strands (our scenario is dealing with parallel strands). No where in that link does it talk about how much of the force is distributed to each strand (which still wouldn't answer the question).

The link was completely irrelevant.

Brady3 wrote:That does not answer the question. The link is looking at the tension (not the same as the force needed to break it), which is effected by the angle of the strands (our scenario is dealing with parallel strands). No where in that link does it talk about how much of the force is distributed to each strand (which still wouldn't answer the question). The link was completely irrelevant.

If tension is not the force needed to break it, then what force is that needed to break it?

The tension in the two strands is about equal.

This confused me quite a bit at first because it seemed like the narrower nylon webbing was stronger than the wider nylon webbing, but loop strength is different from strand strength for the reason presented in the link.

Brady3 wrote: The link is looking at the tension (not the same as the force needed to break it).

Tension in the loop is what's important here. So while that's a slightly different scenario, the principles still apply.

Tension is force, and tension is what break's the sling.

It's loaded like a pulley, and works the same way: ropebook.com/information/pu…

In the example here, even though the load on the pulley attachment point is 200kgf, the load on each end of the pulley is only 100kgf. The tension in the rope is 100kgf.

Proof why the tension isn't higher?
sparknotes.com/physics/dyna…

Like Jim said, the actual webbing used IS stronger than 11kN (15-16 seems right to me. Some of my slings do have min strength markings similar to UIAA requirements for tape). Running around the mandrel reduces the actual strength from a theoretical 30-32kN to a rated 22kN (which is a MINIMUM strength).

Brady3 wrote: Would you mind sighting your sources on this? Because logic says otherwise. There is still only one strand going around each pin, why does that spot hold more with a bend as compared to the straight lengths between the pins? We know that adding bends to the cord/tape makes it weaker (this is why knots weaken cord/tape) so why do bends over a carabiner make it stronger? I did try searching for more technical information, but could not find any.

Your logic says otherwise, engineering and physics says differently. It´s part of my job anyway, I don´t do a whole lot of textile stuff but for sure I´ve been to the tape weavers and specified stuff for projects a few times.

Look at a nylon sling like this one from Edelrid and you´ll see three colour contrast strands in the tape, these signify it is 15kN tape. When it´s sewn together it gets 22kN+.

Brady3 wrote:The link was completely irrelevant.

Not really. Let me try to paraphrase.

First of all, the parallel-strands case is a simple limit case of the one in that stackexchange question. If you figure out the general case, the special case is taken care of.

The tension in the sling must be uniform in the absence of friction, because otherwise some section of it would be accelerating, which is not. (Newton's second law of motion.) The guy who answered on stackexchange used the "little bump" to initially stand for this section of rope. It may help some to visualize, but it really isn't necessary. You can take your section of sling anywhere you want along the loop.

Since the tension in the loop is uniform, the forces the two strands apply to the load are the same in magnitude and together must balance it. So, the sum of their projections on the direction of the load must balance the load. For parallel strands, the magnitude of the projection is the magnitude of the force vector. Of course, the components of the forces orthogonal to the direction of the load must cancel out. For parallel strands they are null to begin with.

Brady3 wrote: There is still only one strand going around each pin, why does that spot hold more with a bend as compared to the straight lengths between the pins?

Ah, here's the hang up: There's two different loadings going on here.

The parallel sections are loaded in tension, and that's what's determining the tension in the loop. Like I said previously, in this case the ideal loading causes the force be split evenly, resulting in 11kN loop tension.

However, around the mandrel the sling is being loaded in compression, and being sheared. Around this portion the surface area of the sling in contact with the mandrel is the important factor. It's like the difference between loading it on a knife edge, and loading it on large rounded surface. But this loading is mostly independent of the loop tension (but dependent on the load being applied).

In this area the sling fails according to a different set of material properties.

It´s actually quite entertaining working on the problems around the pin, if you double a sling up one would imagine it gets 44kN but when one part lies over the other the outer one cuts the inner cleanly through and you get about 17kN from a new nylon sling. The slackliners have loads of problems with this and normally add a sacrificial layer between the wraps on a line locker.

Jim Titt wrote:if you double a sling up one would imagine it gets 44kN but when one part lies over the other the outer one cuts the inner cleanly through and you get about 17kN from a new nylon sling.

You're kidding, fascinating! 17kN is still stronger than any pro so it probably doesn't matter for the classic alpine draw, but that's good to know.

Also, I was not aware that the tape in the sling was actually 15-16kN. I assume the relatively tight curve around the 10mm pins is to blame for the strength reduction? It seems right in line with knot strength theory, now that I think about it- 32kN -> 22kN is just about a 33% reduction.

We probably don't normally think about the strength reduction around a carabiner, because it is no weaker than a knot elsewhere in the system?

Thanks for entertaining this discussion, always excited to learn more about approximating the strength of a given system. Very, very useful skill for building highlines.

OK, I was thinking of it wrong and essentially applying the force twice to contact point because the sling is in a loop. The pulley explanation was much more helpful.

Things get more confusing when you bring in circularity :P

Brady3 wrote: Would you mind sighting your sources on this? Because logic says otherwise. There is still only one strand going around each pin, why does that spot hold more with a bend as compared to the straight lengths between the pins? We know that adding bends to the cord/tape makes it weaker (this is why knots weaken cord/tape) so why do bends over a carabiner make it stronger? I did try searching for more technical information, but could not find any.

It's not that the spot around the carabiner is holding more weight, it is that physically forcing the material to turn in a sharp radius creates uneven loading of the fibers. Then in effect what you have is the fibers that are on the outside of the bend hold more weight than the ones on the inside of the bend. Once the fibers on the outside of the bend tear, the sling fails in what's essentially called cascading failure on a micro level. The lower the elongation of the material, the more of a problem tight turns become because the fibers stretch even less and so the loading between the fibers are even more disproportionate.

Slackliners address this problem by using what's called webbing locks, which in essence are like webbing drums made for slacklining. It holds the webbing over a much larder radius so each fiber is loaded more evenly.





If you were to place that 22kN sling around a 4" drum and pull test it, the strength might increase to over 30kN.

20 kN wrote: It's not that the spot around the carabiner is holding more weight, it is that physically forcing the material to turn in a sharp radius creates uneven loading of the fibers. Then in effect what you have is the fibers that are on the outside of the bend hold more weight than the ones on the inside of the bend. Once the fibers on the outside of the bend tear, the sling fails in what's essentially called cascading failure on a micro level. The lower the elongation of the material, the more of a problem tight turns become because the fibers stretch even less and so the loading between the fivers are even more disproportionate. Slackliners address this problem by using what's called webbing locks, which in essence are like webbing drums made for slacklining. It holds the webbing over a much larder radius so each fiber is loaded more evenly. If you were to place that 22kN sling around a 4" drum and pull test it, the strength might increase to over 30kN.

That´s it really, the fibres need to be loaded evenly. You get the same problems with wide tape in modern karabiners where the basket is intended for a rope and has a really tight radius, the tape sort-of bunches up and starts to rip long before it should break.

Jim Titt wrote: That´s it really, the fibres need to be loaded evenly. You get the same problems with wide tape in modern karabiners where the basket is intended for a rope and has a really tight radius, the tape sort-of bunches up and starts to rip long before it should break.

To add to the fact that many carabiners produced today are designed solely for use with Dyneema dogbones, and adding a nylon sling in there could reduce the strength of the dogbone below the UIAA mandate of 22kN. Further, using a nylon dogbone might decrease the strength of the biner, especially in the open gate mode, as it places the load further from the spine.

Jim Titt wrote: That´s it really, the fibres need to be loaded evenly.

Jim, would a sling sewn as a Mobius strip make any difference?
(Just curious, science is hard!)

There are slings sewn as Möbius strips for the purpose of providing an untwisted surface when is girth hitched.

What I want to know is, do these previous few posts build toward an argument for using accessory cord slings in lieu of webbing slings (for us nylon-lovers)?

I've read suggestions on the knot-tying forum ( igkt.net/sm/index.php?topic…) that accessory cord has more give than webbing (agent Smith says this).

Incidentally, agent Smith also presents some interesting three point anchors using Purcells, which I would like to discuss.

jktinst wrote:...

So I missed it but why not just use and eight on a bite?

Seth Jones wrote: Because Eric prefers complication and bulkiness to simplicity and effectiveness. That being said, I still think we might just be dealing with a master troll.

We all know the type!

Morgan Patterson wrote: So I missed it but why not just use and eight on a bite?

Bowline on a Bight is easier to untie?

20 kN wrote: Well, everyone has a different way of doing things. Many climbers carry a tether of some sorts, so I use my tether to build the anchor off of. I am a fan of completing tasks using equipment I normally bring on the climb anyway, so if I am going to bring a tether I am going to use it for more than one thing. In the case of bolted belays, I build my anchor using my PAS.

THIS and...

rgold wrote:I can't help but note that the rigging with the rope that I posted on page 2 is just dandy with 2 anchors (just end at point #3) and is within seconds of being as fast as using any kind of dedicated pre-equipped sling (if you are not belaying the leader directly off the anchor). One advantage is that you rig everything, bolts, trad gear, plants, and combinations thereof in exactly the same way, every time.

THIS...

Seriously you don't need to carry much to make an anchor. In fact apart I don't carry any dedicated anchor gear. I carry 1 nylon tether and a few multipurpose screwgates.

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Cons:
Way to complicated, heavy, bulky, a lot of links in the proverbial chain.

Pro:
Plenty of cord to use in self rescue when your climbing partner cuts all of that crap away and solos the rest of the the climb without you.

Pro:
Could be useful at a circus.

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