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#28529 - 04/24/07 09:43 PM Re: cordelette-attention RG [Re: Dillbag]
rg@ofmc Offline
Pooh-Bah

Registered: 12/25/99
Posts: 2472
Loc: Poughkeepsie, NY
Have the guys over at Sterling done any tests on 2 gear placements "equalized" -vs- 3? It seems that the load between 2 pieces would be more evenly distributed than 3 pieces using a cordelette.

You would think. Actually, I think all their testing was done on two-point anchors, but I would have to check at home to be sure about that. My memory is that cordelettes (meaning fixed arm anchors) with equal-length arms don't do as well as you'd expect. In addition to the fact that folks can't tie them perfectly, the way in which the knot emits slack as it tightens is a source of inequality in loading that is hard to predict and replicate.

Once the arms are of unequal lengths, physical theory tells us that the ratio of arm tensions will be roughly the inverse of the ratio of arm lengths (this would be precisely true if the pieces are all in a vertical line), and so very unequal arm lengths will lead to considerable disparity in the load on each piece, no matter how perfectly tied the rig is. It is my understanding that such testing as has been done has confirmed at least the rough outlines of these predictions.

when leading in blocks or doing all the leading yourself, i find anchoring with the rope becomes a hinderance.

I certainly don't want to unearth the cordelette are/are not useful arguments, but there is no question that they make life much simpler when a single person will lead every pitch, either for part or all of the climb.

Building an anchor that's guaranteed to hold a FF2 fall every time you build one might not really be the right answer, even if it were possible.

I completely agree, but I think there are many people who aren't willing to hear this. So here's a hypothetical question: if you understand that your anchors can't always withstand the ever-so-rare factor-2 fall, then what kind of probability of failure are you comfortable with, remembering that the event itself is so unlikely. 1% ? 5% ? 10% ? I suspect, with nothing but gut feelings to back it up, that 5% of gear anchors wouldn't withstand a factor-2 fall.

I just set up such an anchor a few days ago. Three small nuts and, I think, a blue alien. No opportunity for anything better. Each nut was well-placed, but they were small (not brassies). The leader had to traverse straight right off the belay on a vertical wall, with no additional pro available for about 10 feet. If a fall happened before the first pro was placed, the initial impact would have had a lateral component that could initiate a cascade failure. This anchor had all the ingredients, but of course we'll never know whether it was one of the 5% or not. I suspect it might have been.

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#28533 - 04/25/07 02:38 AM Re: cordelette-attention RG [Re: rg@ofmc]
dalguard Offline
veteran

Registered: 03/22/00
Posts: 1515
Loc: CT
 Quote:
So here's a hypothetical question: if you understand that your anchors can't always withstand the ever-so-rare factor-2 fall, then what kind of probability of failure are you comfortable with, remembering that the event itself is so unlikely. 1% ? 5% ? 10% ? I suspect, with nothing but gut feelings to back it up, that 5% of gear anchors wouldn't withstand a factor-2 fall.

Sounds generous to me, depending somewhat on where and whether by gear you mean no trees or pins either. If a FF2 fall is so unlikely that most people don't experience one in their climbing career and the chance of anchor failure is only 5%, that's a really rare occurence. I'd be comfortable with that.

It would be nice to believe that we know when an anchor-busting fall is likely to happen but I'm guessing they're usually real accidents in the "oops" sense.

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#28534 - 04/25/07 02:40 AM Re: cordelette-attention RG [Re: empicard]
dalguard Offline
veteran

Registered: 03/22/00
Posts: 1515
Loc: CT
 Originally Posted By: empicard
 Originally Posted By: dalguard
the cordelette doesn't even come into play until the shit has already majorly hit the fan.

how's that?

If you redirect. If you belay off the anchor or your harness, the situation is different.

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#28541 - 04/25/07 04:52 PM Re: cordelette-attention RG [Re: talus]
rg@ofmc Offline
Pooh-Bah

Registered: 12/25/99
Posts: 2472
Loc: Poughkeepsie, NY
Still in response to the question from Talus, I went home and dredged up the following commentary, based on the Sterling tests of a cordelette on two anchors.

In principle, in a fixed-arm system with all pieces in a vertical line, arm tension is inversely proportional to arm length. So if one arm is double the length of a second arm, the tension in the first arm will be half the tension in the second arm. These results are simple consequences of the assumption that the rigging material obeys Hooke's Law, but are strictly valid only for the situation when all anchor pieces are in a single vertical line.

Now predictions based on idealized behavior may or may not be observed in practice. In the field, the arm lengths can never be perfect, and even if they were, the way in which small amounts of slack may or may not be released by the knot is unknown. Moreover, neither my simplistic formulation nor the Sterling drop tests consider the effects of arms radiating from the power point at various angles.

In the Sterling drop test, the long arm was about twice the length of the short arm. Ideally then, the load measured at the long arm should have been about half the load measured at the short arm. This ratio was apparent in about half the trials, but way off in the other half. The lack of exactitude isn't suprising when one looks at the discrepancies from 15% to 35% in the loads obtained from equal-armed cordelettes, which indicate just how hard it is to actually tie functionally equal arms, even when there are just two anchors.

What I get from the combined theoretical and experimental results is that equalization is unobtainable in principle when the arm lengths are unequal, but in any case the climber's best efforts to tie a correctly proportioned fixed-arm rig will nonetheless lead to unpredictable and perhaps significant inequities in load distribution.

I have gone so far in the past to suggest that for fixed-arm rigging, you should probably assume that each piece will get the full load in turn. This rather bold assertion turned out to be more appropriate than I imagined, with 6 of 15 attempts at unequal fixed-arm rigging ending up with more than 80% of the load on one piece.

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#28544 - 04/25/07 05:33 PM Re: cordelette-attention RG [Re: rg@ofmc]
Cornell Climber Online   content
journeyman

Registered: 07/12/04
Posts: 63
I would feel much more comfortable with a decision to use/not use a cordalette if I understood the mechanics of a cascade failure. Does ripping the first piece absorb some of the energy of a fall?

Imagine a simple two piece anchor with cordalette. Leader takes a factor 2 fall. Because of different arm lengths and knot slippage, one anchor piece absorbs 80% of the energy of the fall and that piece fails. Does the remaining piece now have to absorb 20% of the energy of the fall, or 100%, or something between? I understand there is a bit of extension when the first piece fails, but I don't think it is enough to significantly increase the total force of the fall (imagine a 20' fall with 10' of rope, the extension in the above example can't be more than a few inches).

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#28547 - 04/25/07 05:57 PM Re: cordelette-attention RG [Re: Cornell Climber]
rg@ofmc Offline
Pooh-Bah

Registered: 12/25/99
Posts: 2472
Loc: Poughkeepsie, NY
I would feel much more comfortable with a decision to use/not use a cordalette if I understood the mechanics of a cascade failure. Does ripping the first piece absorb some of the energy of a fall?

This is one of the $64,000 questions. The answer depends on how much recovery the rope can achieve in the moment between the impact that extracts the first piece and the impact on the second piece. This has been debated for years, with creditable arguments on both sides but virtually no experimental data that I know of. Although I can't locate the data right now, I recall that some of the Sterling drop tests suggested some energy absorbtion may have taken place.

For two-piece anchors with unequal arms, it seems to me that the Equalette is a superior method and not at all hard to set up with cordelette materials. But for three-piece anchors, I think Dawn's comments about the unsatisfactory nature of the options still holds.

I understand there is a bit of extension when the first piece fails, but I don't think it is enough to significantly increase the total force of the fall (imagine a 20' fall with 10' of rope, the extension in the above example can't be more than a few inches

I think the extension issue may be more complex this argument and the Sterling tests suggest. The reason is that fall energy of the belayer has to be absorbed by the belayer's tie-in; the amount of rope out to the leader doesn't matter. If the extension in the rigging is significant in relation to the belayer's tie-in, or if the belayer has made what I consider to be the mistake of anchoring with low-stretch materials rather than the rope, then it seems possible to me that the effects of anchor extension could be significant.

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#28549 - 04/25/07 06:24 PM Re: cordelette-attention RG [Re: rg@ofmc]
dalguard Offline
veteran

Registered: 03/22/00
Posts: 1515
Loc: CT
 Quote:
Does ripping the first piece absorb some of the energy of a fall?
My understanding is that it depends somewhat on when and how the piece fails. If the piece blows instantly because it sucked, then it probably absorbed nothing and if it blowing adds extension or distorts the angle of pull for the remaining pieces in a bad way, then it did more harm than good. But if it slowly dragged through the rock, deforming itself or the rock as it went, there's no extension in the anchor, and the angle of pull doesn't change, such that the load is gently transferred from piece A to piece B, then it did more good than harm.

Obviously reality lies somewhere in the middle.

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#28556 - 04/25/07 08:27 PM Re: cordelette-attention RG [Re: rg@ofmc]
Mike Rawdon Offline

Carpal Tunnel

Registered: 11/29/99
Posts: 4276
Loc: Poughkeepsie
 Originally Posted By: rg@ofmc

In principle, in a fixed-arm system with all pieces in a vertical line, arm tension is inversely proportional to arm length. So if one arm is double the length of a second arm, the tension in the first arm will be half the tension in the second arm. These results are simple consequences of the assumption that the rigging material obeys Hooke's Law,

<snip>

This rather bold assertion turned out to be more appropriate than I imagined, with 6 of 15 attempts at unequal fixed-arm rigging ending up with more than 80% of the load on one piece.


Hookes's Law may represent rope behavior fairly well. Or maybe it doesn't. As we talk about the relative effect of using non-stretchy cordage, consider that, as a limiting case for ZERO stretch, the arm with slack in it will take ZERO load. That is, irrespective of which leg is long or short, the taut leg bears the total load. And as you pointed out, it's hard to tie a perfect knot. Again, this model represents a limit, and I think Hooke's represents the other extreme. I suspect , as Dawn said, that reality lies somewhere in the middle.

AS for the test data, Hooke's suggests a 2:1 distribution i.e. 67:33 %. So 6 out of 15 drops were 80% on the shorter strand. Once again, I don't expect real numbers to match up to the mathematical model, and the difference between 67% and 80% isn't going to keep me awake at night. Our ability to assess placement strength is nowhere near this good anyway. ("Will it hold 6.7 kN or 8.0 kN??")

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#28562 - 04/26/07 12:05 AM Re: cordelette-attention RG [Re: Mike Rawdon]
rg@ofmc Offline
Pooh-Bah

Registered: 12/25/99
Posts: 2472
Loc: Poughkeepsie, NY
Hookes's Law may represent rope behavior fairly well. Or maybe it doesn't.

Most of the evidence I've seen, and some other I've been told about, suggests good representation, especially if one also introduces some kind of damping effect.

As we talk about the relative effect of using non-stretchy cordage, consider that, as a limiting case for ZERO stretch, the arm with slack in it will take ZERO load. That is, irrespective of which leg is long or short, the taut leg bears the total load.

This is true for the pieces in a vertical line, but not in general in other cases.

And as you pointed out, it's hard to tie a perfect knot. Again, this model represents a limit, and I think Hooke's represents the other extreme. I suspect , as Dawn said, that reality lies somewhere in the middle.

I don't understand what the common phenomenom is that has these two "extremes" as limiting cases, and so I don't understand what the middle is the middle of.

AS for the test data, Hooke's suggests a 2:1 distribution i.e. 67:33 %. So 6 out of 15 drops were 80% on the shorter strand. Once again, I don't expect real numbers to match up to the mathematical model, and the difference between 67% and 80% isn't going to keep me awake at night.

We agree that the difference between 67% and 80% load on a single strand of an unequal two-arm anchor is within the type of variation one can expect from the uncertainties of tying up the rigging as well as the failure of real materials to behave like ideal springs, and I'm glad you're getting proper rest. But Talus asked about cordelette two-point rigging being more or less equalized, and I don't think anyone would say that 80% or more of the load on one strand occurring in half the unequal-arm trials is anywhere near equalized.

Our ability to assess placement strength is nowhere near this good anyway. ("Will it hold 6.7 kN or 8.0 kN??")

I wonder how reliably we can tell the difference between a 4 kN placement and an 8 kN placement. As far as I know, John Stannard was the first, last, and only person with this type of knowledge, and he didn't have to contend with the much more mysterious holding properties of cams.

The fact that we aren't good at estimating real holding powers is an argument for trying to find better equalizing methods, not for ignoring them. Or at least for understanding how to manage the imperfect methods we have in the most effective way.

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#28564 - 04/26/07 01:14 AM Re: cordelette-attention RG [Re: Dillbag]
paulraphael Offline
enthusiast

Registered: 01/22/02
Posts: 321
Loc: New York, NY
Something I've been curious about is the effect of f2 falls on the belayer. Since evidence (and common sense) show that belay devices slip after 2 or 3 kn, with the amount of breakhand force people can typically create, then this can add up to a lot of rope slip. In the especially frightening event of a long f2 fall, the high forces could be sustained enough to pull yards of rope through. Without burly gloves, how would someone hold on at all?

It's yet another reason for the leader to get that first bomber piece is SOON. but i wonder how much rope slip to expect even in a short fall right onto the anchor.

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