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BetaRock


Jun 25, 2012, 4:29 PM
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Theory about forces in a 3-legged cordelette
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The Sterling tests that John Long's book on Climbing Anchors summarized showed that a 2-legged cordelette with unequal legs knotted on a bight experiences an average difference of 3.5 kN between the two legs. This shows that cordelettes are miserable at equalizing even when properly "pre-equalized".

What I'm wondering is what the behavior is with a 3-legged cordelette knotted on a bight?

My theory is that a 3-legged cordelette can be thought of as two, 2-legged cordelettes:

Cordelette 1: (Leg A) + (Leg BC)
Cordelette 2: (Leg B) + (Leg C)

"Leg A" in this case is simply whichever leg experienced the extra 3.5 kN of force. The remaining two would split the remainder of the load the same way a normal 2-leg cordelette would. That is, if there is a 7 kN impact force on the 3-legged cordelette, then Leg A would see 3.5 kN more than B and C combined, and B would show some proportionally large difference to C.

The resulting distribution would be something like 70% on Leg A, 20% on Leg B, and 10% on Leg C.

Have there been any tests on the 3-legged cordelette that might give me some insight on this? I feel like it would have some implications on the mini-debate on whether an equalette is more effective than a cordelette on 3 placements.


patto


Jun 25, 2012, 4:48 PM
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Re: [BetaRock] Theory about forces in a 3-legged cordelette [In reply to]
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I believe John Long did do tests with the 3 legged cordalette. But there was so much wrong with John Long's 'research' that it is hard to know where to begin.

The two legged cordalette is trivially easy to equalise well as long as there a proper 'V' angle formed. A three legged cordalette is almost impossible to get perfect but in my experience I can get rough equalisation without too much difficulty.


The two biggest flaws in John's research were:
**the presumption that near perfect equalisation is necessary.
**the false conclusion that shock loading doesn't matter



(This post was edited by patto on Jun 25, 2012, 4:49 PM)


BetaRock


Jun 25, 2012, 4:59 PM
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patto wrote:
I believe John Long did do tests with the 3 legged cordalette. But there was so much wrong with John Long's 'research' that it is hard to know where to begin.

The two legged cordalette is trivially easy to equalise well as long as there a proper 'V' angle formed. A three legged cordalette is almost impossible to get perfect but in my experience I can get rough equalisation without too much difficulty.

First, I don't think John Long himself was involved in the Sterling tests to which I'm referring. Those tests were performed by Jim Ewing, R&D manager at Sterling Ropes. The statistics were completed by Dr. Lawrence Hamilton and Dr. Callie Rennison.

Second, according to the test summary, they did not perform tests for a 3-legged cordelette rig. Only the 2-legged.

Would you mind detailing the flaws in Ewing's tests? The details of the experiments are laid out in the book and determined that the load distribution across the two legs were dismal: almost 800 pounds difference between the legs in a Factor 1 fall.

This difference in force seems significant to me. Is there debate surrounding how they performed those experiments?


(This post was edited by BetaRock on Jun 25, 2012, 5:03 PM)


patto


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BetaRock wrote:
First, I don't think John Long himself was involved in the Sterling tests to which I'm referring. Those tests were performed by Jim Ewing, R&D manager at Sterling Ropes. The statistics were completed by Dr. Lawrence Hamilton and Dr. Callie Rennison.

Second, according to the test summary, they did not perform tests for a 3-legged cordelette rig. Only the 2-legged.

Would you mind detailing the flaws in Ewing's tests? The details of the experiments are laid out in the book and determined that the load distribution across the two legs were dismal: almost 800 pounds difference between the legs in a Factor 1 fall.

This difference in force seems significant to me. Is there debate surrounding how they performed those experiments?

Only a little bit of debate Wink Laugh

http://www.rockclimbing.com/..._reply;so=ASC;mh=25;

The fourth post and many others are by John Long. As you can see John WAS involved in the tests.

I can't speak for all the details on John's tests but basic trigonometry ensures close to equal loading if you create a decent 'V' in a two leg anchor. It seems he aligned the pieces vertically so naturally equalisation is quite poor particularly with static cord.

Your 800pound difference is meaningless unless you give it as a percentage. I would consider 20%, 20%, 60% equalisation still adequate as far anchors go. The goal is more about redundancy rather than perfect equalisation.

John Long was also incorrect in his conclusion that shock loading doesn't matter. It very much matters if there is a load at the anchor such as in a direct belay or a hanging belay.


(This post was edited by patto on Jun 25, 2012, 5:32 PM)


majid_sabet


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are you talking about three point anchor with equal distance between each leg to anchor ?


curt


Jun 25, 2012, 6:49 PM
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patto wrote:
John Long was also incorrect in his conclusion that shock loading doesn't matter...

I don't believe that was his conclusion. His conclusion was that when one point of an anchor failed, the shock loading measured on the remaining leg(s) of the anchor were not as significant as one might anticipate.

Curt


Express


Jun 25, 2012, 7:00 PM
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A large part of my work includes structural design for cable-stayed architectural cladding systems. Perhaps I can shed some light on the mechanics of the problem we're looking at:

For a two-anchor system, you should be able to resolve the forces using basic free-body principles from physics or engineering statics --- IF the segments were perfectly unstretchable cables in a known geometric configuration. Depending on this configuration (i.e., the angles between the bolts) - the resultants at each anchor bolt would be easily calculable - but already not necessarily equal.

But things are even more complicated because you need to evaluate everything based on the geometry of the system after loading. The slings, webbing, rope, etc. stretch under load (according to a stress-strain relationship for the material) and that changes this geometry. And, what's more, the longer the sling is, the more it stretches under load.

So, to determine that final geometry under your design load, you would need to know (a) the elastic modulous of the rope or webbing in each segment, (b) it's cross sectional area, (c) the EXACT unstretched length of each segment, and (d) the exact magnitude of the loading at the masterpoint. It starts to get a little hairy when you see that the geometry, tension, and amount of stretch are all inter-related.

This is all just for a system with two anchor bolts.
For a three-point system, it you're introducing additional angles, additional internal deflection, and more uncertainty (for a human approximating everything in the real world) in the final [tensioned] geometry.

We still haven't accounted for the fact that even the best "self-equalizing" knots still deal with some amount of internal friction, the error in measurement of sling length, the assumption of linear behavior in what are typically anistropic nonlinear materials, error in measurement of applied loads, et cetera et cetera.

It starts to become a fairly difficult and complicated problem, and it's not surprising to me at all that we should see such a large differential in the loads at each anchor point.


(This post was edited by Express on Jun 25, 2012, 7:03 PM)


patto


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curt wrote:
I don't believe that was his conclusion. His conclusion was that when one point of an anchor failed, the shock loading measured on the remaining leg(s) of the anchor were not as significant as one might anticipate.
Curt

Which were totally false conclusions. As has been discussed numerous times here and on MP.

His test setup showed minimal shock loading as would be expected where there is no mass in the anchor system. As has been stated numerous times when there is mass at the anchor this conclusion goes out the window.


JimTitt


Jun 26, 2012, 12:15 AM
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Marc Beverly has done a lot of work on this, also covering in great detail the effect of assymetrical placements and the resultant stretch differential in the legs mentioned on the thead.
His results are predictably grim with large variations in the forces on the various legs. Other work shows the same or worse results but with the imbalance in the legs shifting which indicates the human factor tying the central point can be the dominant factor.
These tests were done in laboratory conditions where orientation to the direction of load was easy to see and with very experienced guides and rescue riggers tying the knots so we tested a random selection of climbers on the cliff and got even worse results, in some cases no load at all being placed on one of the pieces.

The dynamically equalising systems fared slightly better, only one of these failing to load one piece at all but of course the effects of extension if the belayers weight is included in the system are not to be ignored, sometimes reaching alarming proportions.

I would build the anchor attempting to equalise two pieces (which we can do fairly well) and then add in the third approximately equalised as a back-up. Our experience is that one can better equalise using clove hitches than tying a central knot and so the resulting anchor tends to end up looking like a traditional anchor built using the rope. Whether one uses the climbing rope itself or a dedicated length of cord is a personal choice and also depends on the circumstances. I would not use a dynamically equalising system in a belay.

Uneven leg lengths should be adjusted using as much low-stretch material as possible, a doubled or tripled Spectra/Dyneema sling being fairly low stretch at the loads we are considering, karabiners even more so.
Vertically orientated anchor pieces are more difficult partly because the leg lengths are considerably different, either one joins them all up in series with the rope/pord and accepts this or equalises all the legs with low-stretch material to one point.

Marc Beverlyīs paper can be read here:- http://www.caves.org/section/vertical/nh/51/Multi-point%20pre-equal%20anchors.pdf


shockabuku


Jun 26, 2012, 4:08 AM
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Do you think clove hitches equalize better because they have some inherent slip that causes equalization when loaded or that, for some reason, it is a better method for the human in the system to utilize?


JimTitt


Jun 26, 2012, 4:42 AM
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They are easier for us to judge the tension and more importantly the rope path through the knot is always the same. With an overhand or 8 in a load of strands itīs anyones guess which strand goes where so even if you got everything perfectly equal to start with there is differential slip in the knot which screws things up considerably.
Beverly covers this in his paper, both practically and theoretically but in the end to no real effect on getting better equalisation which isnīt suprising.


shockabuku


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Sounds reasonable, thanks.


Partner rgold


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I've been arguing for the conclusions of the Beverly paper for about ten years now, mostly without any particular effect. Of course, I only had theoretical considerations (combined, however, with lots of practical experience) to base things on, and in the absence of experimental confirmation, the amazingly unexamined SERENE dogma prevailed.

Even now, the practical superiority of clove-hitch based anchor rigging, the relatively minor role of arm angles, and the more critical issue of arm length seems little recognized (in the US), at least from what I can observe out in the field. Part of this is because it doesn't seem to matter that much; anchors aren't failing (but it is hard to know what to make of this, since anchors are so very rarely tested).

The oft-cited tests about the irrelevance of anchor extension missed a critical point by setting things up so that anchor extension made an insignificant contribution to fall factor. Practically speaking, the model tested a solo climber falling directly onto the anchor, with anchor extension insignificant relative to fall length. Not modeled was the very different situation of a belayer being pulled off the stance, in which case that fall energy has to be absorbed by what might be a very short tie-in, in some cases (unfortunately) fabricated with static material rather than the rope.

As for modeling three-point anchors, the problem is complex because, as the civil engineers will tell you, the three point anchor is statically indeterminate---you cannot determine the arm loads from the vector equilibrium equations because you end up with more unknowns than equations and so infinitely many possible solutions. In the case of a rigid truss, extra equations are obtained from the fact that there is also a torque equilibrium, but in the case of ropes, no torques are present and the additional equations come from the elongation of the anchor arms, which depends on the rope modulus as well as the specific geometry of the anchor. A consequence is that the power point will not, in general move straight down, and its final position has to be calculated in order to calculate the anchor arm tensions. (Beverly, by the way, punts on this one in his analytical model and just assumes that the power point moves straight down. This assumption boils down to constraints on the anchor arm tensions (or restrictions on the anchor arm geometry) that are not, in general part of the picture, meaning that the model will not, a priori, account for a certain amount of anchor arm load inequality in the field.

A situation to watch out for with three-point anchors arranged horizontally is that a piece on one of the two outer arms is relatively weak. This should be avoided if possible. If an outer arm blows with the standard symmetrically rigged configuration, all the load will transfer to just a single piece, the middle piece, and the third arm will not be loaded unless that middle piece also blows, setting up the cascade failure scenario that seems the most likely way for a multi-point anchor to fail. (The fact that there is no momentary relaxing of tension in this scenario means that the extraction of anchor pieces will not reduce fall energy to any significant degree.)


(This post was edited by rgold on Jun 26, 2012, 8:20 AM)


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Somewhere on Supertopo Jim and Chiloe posted a bunch more figures, but I can't find them right now.

Anyway, your theory is way off. To find the force on each leg, you must treat it independently for stretch, and then also see how those stretches would interact with each other. It is not a simple problem.

GO


JimTitt


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Äs you say modelling 3 point (or more) is going to be difficult at best. The issue of whether the power point is considered to move straight down or move to a new position is one that not only makes the theory difficult but in practice means one has to take two different scenarios into account when testing. Is the load constrained in its direction or can the load move across to below the powerpoint? I can get considerably different results by using a free hanging or dropping weight or by pulling from a pre-determined point, both of which it is reasonable to assume could occur in a real situation.
The outer point failure problem is why I would prefer the belay set up as a two point anchor with the third then added in. Most experts agree that you concentrate on two good pieces as a minimum and treat everything else as a bonus which might pleasantly suprise and this seems to be the general practice evolved over the years.


jktinst


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I believe that this is the thread that cracklover is referring to.
http://www.supertopo.com/...1/Equalizing-anchors

You have to scroll a fair bit to get to the figures.

Do try and read up on the various anchor building, equalization vs extension threads (like that one) before making up your own mind about this and keep in mind that more recent threads (like yours) tend to have fewer contributors and less debate due to fatigue over the topic.


patto


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It is odd though. That we have an entire WORLD of climbers, many of which are engineers or other such professions, yet we have so little real insight into the discussion.

The same couple of names always reappear to bring rational thought into the discussion.


Thankfully in the country that I mostly climb in, all these dynamic anchors have never been in vogue. Its cordelettes and rope anchors all the way!


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jktinst wrote:
I believe that this is the thread that cracklover is referring to.
http://www.supertopo.com/...1/Equalizing-anchors

You have to scroll a fair bit to get to the figures.

Do try and read up on the various anchor building, equalization vs extension threads (like that one) before making up your own mind about this and keep in mind that more recent threads (like yours) tend to have fewer contributors and less debate due to fatigue over the topic.

Exactly. Thanks for finding that. BTW, perusing through that thread I notice that a lot of the pics and figures I posted (as GOclimb) seem to be gone. If there's anything you want to see of mine from over there, let me know, and I'm sure I can dig up the originals.

GO


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rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:



It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.
Attachments: 3-arm_stretch.gif (7.48 KB)


JimTitt


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Certainly itīs a concept but exactly how well we can do this is going to be the problem, trying to get the tensions equal is virtually impossible so controlling the unequality is going to be equally as hard if you see what I mean.
I tried a couple of other ideas such as using a mix of dynamic and low stretch material, different numbers of strands on a cordalette and various diameters of cord but for the typical belay setup it all seemed as bad as ever and sometime worse! For example thin cord should stretch more but the amount of slip through the knot is less so it ends up not extending as much as the thicker rope.
All of which presumes that you actually do want to get the forces equal which assumes you know all the pieces have the same strength which is another matter altogether!


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Jun 29, 2012, 12:07 AM
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Well, I agree you're never going to get it perfect (probably not even close to it), but you can definitely get it closer than with the "initially equalised powerpoint case". Here's my reasoning, in a slightly more formal form.

Assume for the sake of argument that each leg of the anchor is perfectly elastic (that is, assume that it follows Hooke's law, F = -kx, where F is the force, k is the spring constant (equal to the elastic modulus divided by length) and x is the change in length). Further assume the simplest case: that all three anchor points are in line both with each other and with the direction of fall. What we want to do is optimise so that at some moderately severe fall (say 6-7 kN?) F is equal for all three legs - say, 2 kN for each.

Plugging in a reasonably realistic rope modulus (20 kPa), it comes out that under these circumstances you want each leg of the rope to have stretched by 10% at the one point in space. This is what this idealised system looks like for a 3-leg anchor with legs 10, 50 and 100cm long:



At low forces, the load is entirely taken by the longest leg. Not ideal, but at least this is under the lowest forces. The nice thing is that as the force increases towards your "optimum" level everything gets closer and closer to equal. Once you get past that it all diverges again with the force in the shortest leg very quickly growing to dominance - but in the standard case that divergence starts from zero force.

Obviously non-ideality in rope behaviour and good old trigonometry complicate the situation somewhat, but there is a basic truth here: when building an anchor make your longest arm tight and leave a little slack in the shorter arms, and you'll have at least slightly better equalisation under any substantial load.
Attachments: anchor.gif (9.08 KB)


JimTitt


Jun 29, 2012, 1:18 AM
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I shall play with this in the morning on the test rig and see how it works out. In the meantime we will need an i-phone app to cope with more random anchor pieces!


blondgecko
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Cool! I'll look forward to seeing what happens.


patto


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All this is grossly violating the K.I.S.S. principle.


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It is going to be hard to know "what happens" unless Jim can do a very large number of trials.

I like theory as much as anyone, but I doubt, given the probabilistic nature of the arm-slack choices, that there is much likelihood of achieving a better load distribution by guessing how much slack to put in the shorter arms. I think the time spent fiddling with arm lengths would be better spent climbing and belaying, but I'll be delighted if Jim's results suggest otherwise.

Moreover, considering the "cascade failure" potential for rigs with horizontally oriented anchor points and long outside arms, it might actually be better to have lower loads on the outside arms as a bit of insurance against having one of them blow first.

Unless something definitive emerges, I think the best operating assumption is that one of your pieces is going to get at least half the load.


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blondgecko wrote:
rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:



It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.

Yeah, but if you get it only slightly wrong (by as little as a half an inch) then the long arm of your three-piece anchor gets zero force.

I think a better approach to "fixing" the standard cordelette would be to tie a large (like an extra two or three wraps) central knot, and leaving it loose. This gives you both the benefit that it will equalize itself a little bit, and also absorb some force. Unfortunately for hanging belays, you'll probably tighten it a bit already in the direction you're hanging. And, of course, as Jim Titt suggested earlier in the thread, all those strands in the knot will tighten themselves different amounts, so the force the pieces feel will never be properly equalized no matter what.

But really, the big issue with all non-self-equalizing solutions is that if you're even just a little bit off in anticipating the direction of force, you can wind up with all of the force on one piece. Not good.

This can happen very easily. Maybe you have equalized it perfectly for you, the belayer, standing on a ledge. But the leader falling past you will not have her rope run in the same direction - the ledge is in the way. It will instead pull more in an outward direction. If there is any vertical component to the pieces, the bottom one will take all or most of the load first, and if it pops, then the middle one, and if that pops, then the top one.

The same thing can happen from a horizontal perspective - if the falling leader's rope goes off to the side relative to the direction of anticipated force.

Edited to add - it is certain that leaving the knot loose, with a lot of wraps, would help solve this issue. How much? Who knows.

GO


(This post was edited by cracklover on Jun 29, 2012, 8:58 AM)


degaine


Jun 29, 2012, 9:26 AM
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All of these rigs and theories focus on getting some form of equalization between three pieces. Why three pieces?

What if the equalizing cordalette only had two arms and if one uses three pieces, two of those pieces are rigged to produce only one arm? Sure, 50% of the force/weight is on one piece, but if it's bomber...

I've been paranoid enough in one or two situations over the years to rig an anchor with more than three pieces, but when tying the cordalette or using the rope I just used sling craft or a carabiner to combine pieces so that my cordalette/rope only had two or three arms.


Partner cracklover


Jun 29, 2012, 10:02 AM
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degaine wrote:
All of these rigs and theories focus on getting some form of equalization between three pieces. Why three pieces?

I can't speak for others, but I'll say that for me, the three-piece anchor is best for discussion for several reasons:

1 - My most common trad anchor incorporates three pieces.

2 - When the anchor uses less than three pieces, those piece(s) are usually so bomb-proof that the anchoring method and importance of good equalization probably makes no difference.

3 - IMO it's very easy to get reasonably good load-sharing for two-piece anchors.

4 - My four piece anchors typically either 1 - use the same rigging methods as three piece ones (so the same theory applies) or 2 - have two weak pieces joined by a crossed sling to jointly form one "arm" of a three piece anchor.

In reply to:
What if the equalizing cordalette only had two arms and if one uses three pieces, two of those pieces are rigged to produce only one arm? Sure, 50% of the force/weight is on one piece, but if it's bomber...

That's fine too. Probably better if you have one piece that is definitely much better than the other two.

In reply to:
I've been paranoid enough in one or two situations over the years to rig an anchor with more than three pieces, but when tying the cordalette or using the rope I just used sling craft or a carabiner to combine pieces so that my cordalette/rope only had two or three arms.

Precisely. Which is why almost all the discussion has been around two- and three-point anchors.

GO


JimTitt


Jun 29, 2012, 10:15 AM
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I would be climbing and belaying tomorrow but right at the moment itīs pushing into the 90īs here and I put a saw into my finger anyway so playing and gardening are on the schedule!

Thinking about how to best test this without too much work today while doing something utterly boring and was struck by some of the peculiarities of the climbing world. I was killing two birds with one stone testing a batch of bolts and at the same time calibrating the hydraulics on a new portable bolt tester, The 12mm stainless quick links kept breaking at about 68kN without bolt failure or getting to the designed pull for the tester (80kN) so every time I had to re-rig everything with my normal tester to break the bolts. So on one hand I was getting frustrated by these recalcitrant things that wouldnīt break and on the other hand thinking about how to assemble a load of weak gear to make a strong enough anchor.

Anyway the test concept is (initially) to work backwards and set up an unequal leg length belay, load it and then adjust the legs under tension until the loads are equal, release the load and see what the differences in lengths are. Whether it will be of any practical use is anyones guess!

First Iīm going to water the beans!


billl7


Jun 29, 2012, 2:23 PM
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Just want to chime in that I appreciate knowledgable folks who have written down their thoughts here.

Bill L (one who has progressed from the common knotted cordalette, to John Long's equalette, to a modified equalette, back to the knotted cordalette, and now sometimes climbing with a partner who prefers to use only the rope for anchor rigging.)


ptlong2


Jun 29, 2012, 3:28 PM
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JimTitt wrote:
Äs...

Umlaut?


blondgecko
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Jun 29, 2012, 4:37 PM
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cracklover wrote:
blondgecko wrote:
rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:



It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.

Yeah, but if you get it only slightly wrong (by as little as a half an inch) then the long arm of your three-piece anchor gets zero force.

Oops - I was going to make this explicit, but then forgot. All this is predicated on the use of dynamic rope for the anchor - the more stretchy, the better (within reason). As the cord becomes stiffer, the margin of error gets smaller and the range over which you get decent equalization shrinks. With standard cordelette it will be utterly impossible.

Yes, it violates KISS, but I still think it's worth thinking about - if only because understanding this leads to real understanding about exactly why the original cordelette rig is impossible to equalise. It's also fun to speculate on outside-the-box ideas - imagine making each arm mostly static, but with the last foot or so of each arm made of bungee cord. I suspect you'd get much better weight distribution that way.


JimTitt


Jun 29, 2012, 11:00 PM
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ptlong2 wrote:
JimTitt wrote:
Äs...

Umlaut?

I use a German keyboard which has all sorts of wierd stuff like ÖÜÄß. And the letters are in different places compared with an English one.
Since the A and Ä are completely opposite ends of the keyboard obviously careless as well!


JimTitt


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cracklover wrote:
blondgecko wrote:
rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:

[image]http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?do=post_attachment;postatt_id=6303[/image]

It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.

Yeah, but if you get it only slightly wrong (by as little as a half an inch) then the long arm of your three-piece anchor gets zero force.

I think a better approach to "fixing" the standard cordelette would be to tie a large (like an extra two or three wraps) central knot, and leaving it loose. This gives you both the benefit that it will equalize itself a little bit, and also absorb some force. Unfortunately for hanging belays, you'll probably tighten it a bit already in the direction you're hanging. And, of course, as Jim Titt suggested earlier in the thread, all those strands in the knot will tighten themselves different amounts, so the force the pieces feel will never be properly equalized no matter what.

But really, the big issue with all non-self-equalizing solutions is that if you're even just a little bit off in anticipating the direction of force, you can wind up with all of the force on one piece. Not good.

This can happen very easily. Maybe you have equalized it perfectly for you, the belayer, standing on a ledge. But the leader falling past you will not have her rope run in the same direction - the ledge is in the way. It will instead pull more in an outward direction. If there is any vertical component to the pieces, the bottom one will take all or most of the load first, and if it pops, then the middle one, and if that pops, then the top one.

The same thing can happen from a horizontal perspective - if the falling leader's rope goes off to the side relative to the direction of anticipated force.

Edited to add - it is certain that leaving the knot loose, with a lot of wraps, would help solve this issue. How much? Who knows.

GO

That an angular offset on the load means all the load comes on one piece isnīt in fact the case in general, only in the vertical situation and even then only if you used a non-stretch material.
If you consider a classic two point anchor with an angle of 90° between the two legs simple geometry tells us that as the load moves away from the centre axis the load on one side decreases progressively until the load offset is 45° when the one piece has no load.
Stretch and knot slip have an effect on this but generally it is beneficial, a bit complicated for here though as it would take me hours to post it all!
As a rough rule for most anchors having a dynamically equalising system has no benefit until the load offset is more than 15° which in real life is an enormous amount and relatively unlikely to occur. Friction over whatever causes the load to be offset reduces the force on the belay anyway which should be taken into consideration.
In your ledge example the advantage of a static equalised system is that the belayer can anticipate the angle of the rope over the ledge and build the belay to suit, if he used a dynamic equalising system the belay would equalise to suit him and then the friction in the system prevent equalisation occuring under load. This is the theoretical aspect, experience tells us the rope would probably get cut through on the ledge and load on the belay become irrelevant!


JimTitt


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blondgecko wrote:
rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:



It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.

Well I took the quick and easy approach and worked on a two-piece anchor (at least for now).
The setup was a fairly narrow angled anchor with 30° included angle, two lengths of 9mm rope with fig 8 on a bight at one end as one would tie-in and clove hitched into karabiners at the pieces. Basically how one would belay if you were on half-ropes.
One leg was 1m (1000mm) long and the other 50cm (500mm) so half the length and the knots pre-loaded to 10kg to tighten them about the same as by hand.

The central point was lightly loaded to 20kg and the tension equalised by adjusting the piece position. A load of 200kg was applied (from a fixed direction) and the difference in the tensions was 24% more on the shorter leg.
To re-equalise the loads the shorter leg piece needed to be moved down 4mm which indicates how accurately we will have to judge things.

Clearly the rope stretches and slips in the knots, the longer leg stetched 130mm (12%) and slipped 120mm, the shorter leg stretched 80mm (16%) and slipped 130mm.
So we could correct for this at the set-up stage:- If the higher loaded leg slips 10mm more due to the increased force then we can add this to the initial length. Then we could correct the stretch for the different load which is 60mm not 80mm so we need to add in 20mm to get things equal so adding a total of 30mm into the short leg.
This actually moved the load imbalance over to the wrong side (the long leg got a higher load) so clearly too much.
After a fair number of attempts I found needed to tie the short leg 18mm slacker than it would normally be if they were initially equalised or 3.6% of the length.
Having a poorly dressed fig 8 or even using one round stock karabiner and one I beam karabiner on the pieces made more difference to the load imbalance than I could manually correct, the I beam karabiner locking the clove hitche locked up much faster, the slip on a 12mm HMS being over 50mm at these loads.

For a typical belay judging the lengths exactly enough is going to be a hopeless task but probably for something like a very long line from an anchor well back maybe a something could be gained.


blondgecko
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Jun 30, 2012, 5:28 AM
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JimTitt wrote:
blondgecko wrote:
rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:



It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.

Well I took the quick and easy approach and worked on a two-piece anchor (at least for now).
The setup was a fairly narrow angled anchor with 30° included angle, two lengths of 9mm rope with fig 8 on a bight at one end as one would tie-in and clove hitched into karabiners at the pieces. Basically how one would belay if you were on half-ropes.
One leg was 1m (1000mm) long and the other 50cm (500mm) so half the length and the knots pre-loaded to 10kg to tighten them about the same as by hand.

The central point was lightly loaded to 20kg and the tension equalised by adjusting the piece position. A load of 200kg was applied (from a fixed direction) and the difference in the tensions was 24% more on the shorter leg.
To re-equalise the loads the shorter leg piece needed to be moved down 4mm which indicates how accurately we will have to judge things.

Clearly the rope stretches and slips in the knots, the longer leg stetched 130mm (12%) and slipped 120mm, the shorter leg stretched 80mm (16%) and slipped 130mm.
So we could correct for this at the set-up stage:- If the higher loaded leg slips 10mm more due to the increased force then we can add this to the initial length. Then we could correct the stretch for the different load which is 60mm not 80mm so we need to add in 20mm to get things equal so adding a total of 30mm into the short leg.
This actually moved the load imbalance over to the wrong side (the long leg got a higher load) so clearly too much.
After a fair number of attempts I found needed to tie the short leg 18mm slacker than it would normally be if they were initially equalised or 3.6% of the length.
Having a poorly dressed fig 8 or even using one round stock karabiner and one I beam karabiner on the pieces made more difference to the load imbalance than I could manually correct, the I beam karabiner locking the clove hitche locked up much faster, the slip on a 12mm HMS being over 50mm at these loads.

For a typical belay judging the lengths exactly enough is going to be a hopeless task but probably for something like a very long line from an anchor well back maybe a something could be gained.

Well, as Huxley said, many a beautiful theory was killed by ugly reality! I had, of course, neglected to take into account knot slippage, which is at least on a par with stretch in those lengths and as you say will vary too much. I will point out, though, that if you were to load the rig to represent a more severe fall (say, 400 kg) then the amount of slack you'd have to add to the short leg to get equalization will grow.

... which is the whole problem, in a nutshell. No matter how you tie the rig there's only ever going to be at most one "magic" fall force at which you get perfect distribution between legs, with everything diverging on either side of that. If you deliberately tie the longer legs a little short, you're at least ensuring that they'll get a look-in. Tie them all coming to a common point at zero load, and there's a very strong chance that the longer legs will only see substantial forces in the event of the shortest leg failing. Like you say, though, it probably only really comes into play for very big differences in length.

Definitely beats a fully static rig, though.


JimTitt


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Well thereīs always the Shunt trick.
The Shunt has the interesting characteristic that if you have two strands in it but only one is loaded it will slip down the loaded strand until the tensions become roughly equal. If you tie the two ropes a bit slack into the pieces/bolts and fit a Shunt connected to your belay loop it actually equalises quite nicely.
With a 9mm rope the difference in load on the legs is about 20kg. And of course if the load comes from another angle it re-equalises the same.
The downside is that if one piece fails the Shunt slides down the other strand but thatīs why you only leave a bit of slack or tie an extra knot and at least it is a slide not a drop.
Some people (me included) tie into the anchor as usual with a bit of slack and then fit the Shunt on the other strands coming down from the karabiners as this removes any worries about the Shunt damaging the rope though with two strands fitted the loads have to be very high for this to occur as it will slip a bit first. Personally I donīt find this worrying but it seems easier to this way.


knudenoggin


Jul 13, 2012, 11:14 AM
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BetaRock wrote:
The Sterling tests that John Long's book on Climbing Anchors summarized showed that a 2-legged cordelette with unequal legs knotted on a bight experiences an average difference of 3.5 kN between the two legs. This shows that cordelettes are miserable at equalizing even when properly "pre-equalized".

What I'm wondering is ...

Why not improve on the knotted cordelette (meaning "cord", of nylon)
with something that DOES equalize, and spare all of this long-winded
debate about forces per length & angle?

As I wrote in a nearby thread:
knudenoggin wrote:
Frankly, it strikes me
as a too-rigid and somewhat clumsy structure. Especially in contrast
to one I've sketched as an implementation of an "ELET" --2-legged
anchor ("extension-limiting equalization triangle"). It seems to me
that using the common, handy materials of 7mm (twinned) cord
(or other) and a 24"/60cm HMPE (Dyneema, mostly, these days)
tape sling, one gets simplicity, looow friction (for the *equalization*),
and greater flexibility (thinking that UNtying this will be possible,
and so the materials can do other tasks as needed; and the particular
tying --length of legs-- can be set per need, not pre-set & fixed).

[image]http://s14.postimage.org/v1w7tdw1p/knots_ELET_of_cord_sling_M888_1.jpg[/image]

[image]http://s13.postimage.org/xtxcw22df/knots_ELET_of_cord_sling_M888_2.jpg[/image]

Full images (+/- 130kb ; 888pix long side):

http://s14.postimage.org/...ord_sling_M888_1.jpg

http://s13.postimage.org/...ord_sling_M888_2.jpg


These have yet to meet a test device, but seem sound IMO.

*kN*


patto


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knudenoggin wrote:
Why not improve on the knotted cordelette (meaning "cord", of nylon) with something that DOES equalize
Because dynamic equalisation and no extension are an impossibility. Extension can pose significant dangers whereas the lack of in general equalisation does not.

(I'd have to be absolutely desperate to be building an anchor where I thought I needed to equalise the pieces for me to be safe)

JimTitt wrote:
Well thereīs always the Shunt trick.
Interesting... Are you actually advocating this as a useful way of constructing an anchor/belay? Or more as a novelty?


JimTitt


Jul 14, 2012, 12:19 AM
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Your system does nothing better than any of the other common ones, merely adding some more complication and using more gear. And it will equalise just as badly as any of them as well.
And as Patto says, you canīt have equalisation without potential extension which we prefer to avoid to put it mildly.


JimTitt


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patto wrote:
knudenoggin wrote:
Why not improve on the knotted cordelette (meaning "cord", of nylon) with something that DOES equalize
Because dynamic equalisation and no extension are an impossibility. Extension can pose significant dangers whereas the lack of in general equalisation does not.

(I'd have to be absolutely desperate to be building an anchor where I thought I needed to equalise the pieces for me to be safe)

JimTitt wrote:
Well thereīs always the Shunt trick.
Interesting... Are you actually advocating this as a useful way of constructing an anchor/belay? Or more as a novelty?

Well Iīd advocate bolting the belays anyway! And certainly wouldnīt advise anyone to use a piece of equipment outside the instructions from the manufacturer.
However it is a good way of achieving exactly what we want (in fact by far the best way) and I certainly know a couple of people who use this system. Itīs actually quite cool the way it releases the higher loaded rope until the tensions become equal. All at your own risk of course and back it up!


knudenoggin


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JimTitt wrote:
Your system does nothing better than any of the other common ones, merely adding some more complication and using more gear. And it will equalise just as badly as any of them as well.
And as Patto says, you canīt have equalisation without potential extension which we prefer to avoid to put it mildly.

Not so fast (and noting that we've not seen test restuls):

1) Re "complication", I submit that it's far simpler to tie
cord (twinned 7mm nylon, perhaps preferably --or 6mm)
to the bight ends of an HMPE sling than to fashion appropriate
knots per cordelette or other, more complex structure;
1.b) ... and, so, morEasily adjusted

2) The "extra gear" is but one 60cm/24" HMPE sling
--hardly going exotic, with this. (No extra hardward,
no clever fashioning of a single bit of cordage into
multiple functional parts.)

3) And given that that particular material --low-friction HMPE--
is at the point where 'biners must slide to equalize,
yes, it does equalize better than others
(this, I'll note, is Craig Connally's observation).

4) Finally, re extension, note the full extent of that :
about a half foot (with 60cm/24" sling; and not counting
pendulum effects of a blown anchor, of course).
What I took from Sterling's testing was that extension
was far over-rated (and, Patto, where's the testing
that proves otherwise, body in system?).

*kN*


JimTitt


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Dyneema hybrid slings arenīt much better than pure nylon in the friction stakes and adding the second karabiner you show negates almost all of the difference.
Tell me exactly what you want to test and how and I will test it for you, if I designed the test protocol I already know the answers from other tests and can assure you it is no better than any of the conventional systems.
Your system is already more complex by using two slings to do the job of one. Knotting cord and Dyneema together is already dubious and you will have problems untieing the knots afterwards
You are making the basic beginners error of assuming the extension is only half of the length between the limiter knots/length of sling. If one is introducing an equalising element you are by definition saying that the direction of load is unknown as if it is known you could use a static system. Therefore the possiblity exists that the load can come from one side and if the other piece fails the extension is the full length you allowed for equalisation.
With the belayer directly attatched to the belay even a limited extension can be disastrous and will anyway ALWAYS give a higher load on the remaining piece than if no extension occured, therefore any system allowing extension is worse.


patto


Jul 14, 2012, 12:11 PM
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knudenoggin wrote:
What I took from Sterling's testing was that extension was far over-rated (and, Patto, where's the testing that proves otherwise, body in system?).

Those Sterling tests have been the worst thing to happen to this debate as time and time again they are brought up to suggest extension barely matters. The conclusions were wrong, extension matters.

It matters when there is a mass at the belay such as when the belayer is hanging on the belay. As many other tests have shown a body weigh mass falling onto static slings produces very high forces.


JimTitt


Jul 14, 2012, 12:43 PM
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Certainly worth noting that even with a simplistic system doing extension tests McKently had to abandon testing when the sling broke at over 20kN though with a heavier weight than we would normally use but which would reasonably refelcts the forces we would encounter. He was using 9" of extension.
With the same set-up but more extension (12") Iīm seeing 32 times the suspended weight alone (the belayer) on one piece and 25 times on the other without even considering the falling climber.
And there are worse scenarios out there!


patto


Jul 14, 2012, 3:48 PM
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Those test results need to be retold over and over and over again. The conclusions from the Sterling have been a damaging disservice to the climbing community.


knudenoggin


Jul 15, 2012, 10:21 AM
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JimTitt wrote:
Certainly worth noting that even with a simplistic system doing extension tests McKently ...

Thanks, URLink/citation for Mckently (among his many testings)?

*kN*


patto


Jul 15, 2012, 11:18 AM
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knudenoggin wrote:
JimTitt wrote:
Certainly worth noting that even with a simplistic system doing extension tests McKently ...

Thanks, URLink/citation for Mckently (among his many testings)?

*kN*

Google worked for me, this was the third link. I think this is what Jim was meaning.
http://files.meetup.com/1324053/A_Look_at_Load-Distributing_and_Load-Sharing_Anchor_Systems.pdf

But even without test results simple high school physics analysis leads to the same conclusions. The Sterling tests didn't have any mass attached to the anchor so there was nothing to cause the shock load. (Their mass was only the falling climber which is attached with a DYNAMIC rope.)


JimTitt


Jul 15, 2012, 12:22 PM
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Dropping a weight on a rope and allowing extension is naturally a very selective scenario and in fact it represents a situation which I have never encountered in my entire climbing career which is someone using a direct belay for a lead climber off a trad anchor. It reflects neither the worst case nor the usual situation where the belayer is directly in the system.

Regrettably when the belayer is attatched to the belay and one piece fails under the impact of the falling leader (which is the most likely event) the belayer is accelerated downwards not by gravity but the force imposed by the leader through the belay device.
Traditionally one was "plucked from the stance" to oneīs doom but nowadays we can see that the acceleration is probably around 4g looking at the typical forces we can generate through a belay plate. So simple FF1 thinking is incorrect when reviewing extension because really you would get a belayer taking a FF4 equivalent onto the sling plus of course the still regrettably falling leader so we need to add maybe 4 to 6 kN as well. The smart and fast reacting belayer will of course drop the leader and grab the belay to reduce the combined impact!

All in all a somewhat worrying prospect which leads me to think that potential extension has no place in a belay, especially if you involve low-stretch materials.


patto


Jul 15, 2012, 11:44 PM
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JimTitt wrote:
Dropping a weight on a rope and allowing extension is naturally a very selective scenario and in fact it represents a situation which I have never encountered in my entire climbing career which is someone using a direct belay for a lead climber off a trad anchor. It reflects neither the worst case nor the usual situation where the belayer is directly in the system.

Regrettably when the belayer is attatched to the belay and one piece fails under the impact of the falling leader (which is the most likely event) the belayer is accelerated downwards not by gravity but the force imposed by the leader through the belay device.
Traditionally one was "plucked from the stance" to oneīs doom but nowadays we can see that the acceleration is probably around 4g looking at the typical forces we can generate through a belay plate. So simple FF1 thinking is incorrect when reviewing extension because really you would get a belayer taking a FF4 equivalent onto the sling plus of course the still regrettably falling leader so we need to add maybe 4 to 6 kN as well. The smart and fast reacting belayer will of course drop the leader and grab the belay to reduce the combined impact!

All in all a somewhat worrying prospect which leads me to think that potential extension has no place in a belay, especially if you involve low-stretch materials.

This needs to be made a sticky in the LAB if not the front page. For half a decade I have been arguing this but the notion that extension doesn't matter has been entrenched thanks to John Longs book.

Talk about 4G acceleration an equivalent FF4 fall is down right scary. (That said the physics can be comparable to seat belts, its about ensuring even deceleration.)

Thankfully all this discussion is mostly academic because belay failures are extremely uncommon. However if we are going to start trying to reinvent things like some people are, we better damn improve things!


(This post was edited by patto on Jul 15, 2012, 11:47 PM)


jktinst


Jul 16, 2012, 9:06 AM
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It seems to me that these predictions of doom if any extension is allowed on a blown pro are based on the premise that the belayer is attached to the belay anchor through a static link and that s/he is yanked off his/her stance and goes straight into free fall without opposing any resistance.

Nowadays, I am using as much as possible belay-setting and belaying approaches that aim to either avoid altogether the possibility of the leader falling directly on the belay or severely limit its impact. As a result, the extension vs. equalization debate affects me a lot less than it once did. However, even with these approaches, failure of the first few progression pros (however unlikely) could still result in the leader falling on the belay so the question remains relevant.

With respect to the scenario above: I use a dynamic cow’s tail and, although I am reluctant to use stancing as a primary means of limiting the impact of a leader fall on the belay anchor, I would like to think that, if one of the anchor's pros were to blow, I would offer some resistance to being pitched into free fall by the few inches of extension that this would cause on my anchor’s central point.

It seems to me that most proponents of dynamic equalization accept that it must be balanced with extension limitation. It follows that equalization cannot be expected to distribute the load of a leader's fall evenly to all the anchor’s pros regardless of where this fall may occur with respect to the belay. As a result, some anticipation of, and allowance for the likely offset should come into the planning of a dynamically equalized anchor (as it does with a static one) to try and allow optimal distribution of the load despite the extension limitation.

In addition, however much one may dislike the Sterling/Long/Gaines tests on extension, their equalization tests demonstrated very clearly that dynamic equalization is essential to allow load sharing between the anchor’s longer arms and its shorter ones. Without this, you get most, if not all of the load on the shortest arm and, if it blows, on the next shortest one, etc. in a potential cascade failure scenario. So, despite the increasingly strident warnings that I’m going to die for allowing any extension on my belay anchors, I continue to see problems at least as dire with the absence of dynamic equalization.

In building an equalized anchor, I take into account not only the potential offset of the fall, but also the length of the arms of the system, the height of the central point and the quality of the stance (as well as other things that I won't clutter this discussion with) and set my extension-limiting knots close enough together to minimize the risk of getting yanked right off while still allowing the essential long-to-short equalization to occur.

Of course there is no preventing the yanking off and freefall situation in a hanging belay so, in these situations, why not 1) have the belayer hanging well below the anchor on a long dynamic cow's tail (which is one of the approaches mentioned above), 2) clip the leader's rope into the central point, and 3) limit extension somewhat more, looking mainly to take care of the long-to-short equalization ?

(This post was edited by jktinst on Jul 17, 2012, 1:31 PM)


JimTitt


Jul 16, 2012, 12:00 PM
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Realistically we have to consider that the belayer can provide no resistance himself, firstly because this is the worst-case scenario and secondly because nowadays with the general rise in standards of climbing the belay stances are becoming smaller and smaller. When I began climbing belay ledges were nice big things you could have a party on, nowadays I consider a foothold half as big as my rockboot a nice spot. This is particularly noticeable as we trend to ever steeper routes on limestone which at least in Europe is the way things are going.
With the increase in standards and improvements in equipment allowing harder routes to be climbed it is inevitable that more people are exposed to stances and worse falls and belay technology has to be designed around this, at least if we are going to preach a "one size fits all" dogma.

The difficulty is that less experienced climbers have seized upon the equalisation concept as being a solution to everything without it being made clear that they are exposing themselves to enormous potential dangers to no or little benefit. From the not-inconsiderable numbers of posts about equalising top-rope anchors for example this is clearly the case.

In the hands of an expert climber who understands the limitations and the potential downsides and makes every effort to mitigate their effects then a dynamically equalising system may have some benefit (though this is generally doubtful). For the general climbing population and general use there are no benefits, only potential negatives.

Your approach of concentrating more on avoiding a catastrophic fall on the belay is the correct one, combined naturally with getting loads of the best gear you can. No matter how you package it a bag of crap is still crap.
Equalising the parts of a belay is a tacit admission that one doesnīt think any of the pieces are in fact strong enough but one is still prepared to use them which cannot be a desirable way to approach belaying or climbing in general, I would abandon the route at this stage in preference.


patto


Jul 16, 2012, 12:25 PM
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For those that dislike reading big blocks of text:

JimTitt wrote:
Equalising the parts of a belay is a tacit admission that one doesnīt think any of the pieces are in fact strong enough but one is still prepared to use them which cannot be a desirable way to approach belaying or climbing in general,

As I've mentioned before I equalise RP and other micro gear that I place on lead. It makes sense in such situations.


tomcecil


Jul 16, 2012, 8:02 PM
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 "No matter how you package it a bag of crap is still crap.
Equalising the parts of a belay is a tacit admission that one doesnīt think any of the pieces are in fact strong enough but one is still prepared to use them which cannot be a desirable way to approach belaying or climbing in general, I would abandon the route at this stage in preference."
That is sound advice--

There are no magic systems, it's all about the individual pieces.
The method you use to build an anchor hardly matters if the gear is good.
Practice placing gear not worrying about which jackalette to use--


blondgecko
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Jul 21, 2012, 5:31 AM
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patto wrote:
Those test results need to be retold over and over and over again. The conclusions from the Sterling have been a damaging disservice to the climbing community.

Just to play devil's advocate here, there's another way of looking at this. So limited-extension systems lead to dangerous forces if you have the belayer anchored straight into them and a piece fails. There's two possible ways to respond:

1. Limited-extension systems should not be used;
2. What can be done to counteract this and make it safe again? If you're talking about a maximum extension of 12", then about 6" of shock cord between power point and belay would provide more than enough shock absorption, with minimal increase in complexity.


avalon420


Jul 21, 2012, 7:16 AM
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patto wrote:
All this is grossly violating the K.I.S.S. principle.
No it doesn't, we all just have to start climbing with an accurate measuring device, and a calculator. OR YOU WILL DIEEEEE! Crazy


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