Discussion Topic |
|
This thread has been locked |
rgold
Trad climber
Poughkeepsie, NY
|
|
Equalization was embraced as an essential ingredient in anchor construction until it turned out that our methods for achieving it don't even come close in many cases, and since then the discussion of better alternatives is burdened with the question about whether equalization is worth seeking at all. It was necessary when we though we had it, but it became unimportant when it turned out we didn't have it. Hmm.
""There seems to be a growing number of tests that suggest that the cordelette is almost never very good"
I have only heard of the Long et al tests. Are there any more?"
There was a series done by Attaway and reported in a Mountain Rescue conference. The results are not available online, but Attaway's conclusion that the cordelette did not meet their standards for equalization were reported in an online forum. Sorry, I don't have references.
"Finally, do we have reliable data on actual fall forces generated in factor 2 falls held with a belay device? My link above has some information."
A great link, thanks for it. But no useful data on factor-2 falls reported there, unfortunately
""Well, we know that belay anchors built by experienced climbers have failed catastrophically in the field. (I think I have heard of five such incidents in the past ten years.) In no cases were the anchors made up of pieces we would think of as marginal before they're placed."
Do you have any information of these cases? I have heard of an accident on the DNB but have zero info."
Sorry, I'm going on memory. There is the DNB case, the Tahquitz case, a case I read in the AAC accident reports a few years ago, a case Attaway mentions as the motivation for his report on fall impacts, and the case Werner referred to earlier in the thread. The random way this information comes to us suggests the likelihood of other cases we haven't heard about.
In general, there is very little information about what actually happened. In the Tahquitz case, the official conclusion excluded anchor failure as the cause.
""From an engineering perspective, equalization is the best strategy for guarding against such outcomes."
I disagree, knoweledge about the problems that could occur might be a better guarding."
Here I think you are wrong. As you say, peopel make misstakes, even in the presence of knowledge of the problems. They are sometimes wrong about which piece is good. The best strategies are those that minimize potential load to all pieces, are tolerant of off-axis loading, and redistribute the load to remaining pieces if one fails. These are the only available means of coping with the uncertainties that are part of real life.
"Do you really have enough information about the accidents so that you can draw your conclusions? After reading your post I get the impression that 5 good anchors, 3 good pieces in good rock that backup each other with no huge extension, for example using a cordellete, completely failed. Do we really know this?"
No. We know very little about these accidents.
But it is also the case that very few experienced climbers have ever had their anchor judgements subjected to a real test, much less a statistically significant sequence of tests. This means that most climbers really have no basis in reality for judging the strength of their anchors. The tragedy described by Werner earlier in this thread indicates that these claims are not simply hypothetical.
Once again, in the face of this really quite massive uncertainty, equalization is the best strategy, if it can be achieved.
"The reason for failure could be many things. Bad pro, pro that anly could take a force in one direction, an anchor with a large extension. The climber might have taken a 10 m fall onto the belayer. Not enough pieces.
But the effect of all of these things would be mitigated by having an equalized anchor that can adapt to off-axis loads and does not suffer from the possibility of large extensions. Your own analysis of the cases you linked to concludes an equalized anchor might have helped.
In any case, I don't know if there is exactly an "uproar" about all this. It seems to me that a relatively small group of climbers is interested in the intellectual challenge involved, and most others feel that the distributive (rather than equalizing) methods that we have are plenty good enough. It may be that those who are interested will eventually come up with a solution the mainstream will adopt, or it may turn out that the complexities of fabricating equalizing anchors and especially the associated problems of friction will ultimately doom the enterprise to a generally disregarded cult status.
|
|
murcy
climber
San Fran Cisco
|
|
Thanks for posting that, GO!
I'm puzzled, though. The two limiter knots freeze the sum of the lengths of the two outside legs; if they're taut, the powerpoint travels along an ellipse. But then the center strand's length, when taut) is constant, too, and so describes a circle. So it seems to me that apart from the one or two directions at which the circle and ellipse intersect, all the weight will be taken at the powerpoint either by the strand that connects the two knots directly (in which case the center piece is unweighted), or by the two other strands (in which case the center piece gets twice the load of the others, as seems to be the case in your photo). Or am I mistaken?
|
|
rgold
Trad climber
Poughkeepsie, NY
|
|
For those who are interested in such things (perhaps the empty set), I'd like to propose a reappraisal of the effects of a small extension in an anchor.
Conventional wisdom has been that extension is bad because of the "shock load" it would cause. As has been pointed out many times by now, the term "shock load" has never had a proper definition, but forgetting about the shock aspect, certainly the understanding is that the resulting load would be large in the context of possible anchor loads.
I'd like to suggest that maybe conventional wisdom has things exactly backwards; that in fact it is the fixed-arm cordelette that will impose a relatively large load on the remaining pieces, and that by constrast, an anchor that with a small extension will actually produce smaller loads on the remaining pieces than the cordelette would have.
Now "small extension" is ultimately going to need a definition too; my suggestion is that the ratio of extension to the length of belayer tie-in (assuming the belayer uses the rope to tie in)---in other words an H/L ratio for the fall caused by the extension and absorbed by the tie-in---is an appropriate measure of smallness.
Consider what happens if one piece of a cordelette anchor fails. With no extension, the load is immediately transferred the remaining pieces (actually, probably to just one other piece). The rope has no ability to recover during this transfer, precisely because the absence of any extension provides no moment when the tension in the rope is reduced. The net effect is that the remaining piece or pieces get the full maximum impact of the fall, the same impact that would have been delivered to the anchor had it remained intact, but now shared by fewer pieces. Put another way, no benefit is obtained from the energy needed to extract the first piece.
There is an instant in which a shared load is suddenly shared by fewer pieces; this is certainly a candidate for a "shock load" imposed by the cordelette.
Next, consider what happens when a piece fails and there is a small extension. During the instant of extension, rope tension drops to zero. When the load comes on the remaining pieces, they only have to arrest the remaining part of the fall, which clearly involves less energy absorbtion and so a lower peak load. In this setting, the energy involved in stretching the rope up to the point of extraction is removed from the total energy that must be absorbed by the remaining anchor.
This reduction effect is mitigated by the fact that the rope will have been "stiffened" by the first stretching and so will develop higher peak loads, and of course by the added effect of a (low fall factor) belayer fall. Perhaps the load on the anchor will be higher or the same, but it is also entirely possible that a lower anchor load will be observed, precisely as a result of the fact that a small extension allows the extracted piece to function as an energy absorber.
The fact that Wootle's tests essentially showed no "shock load" would be explained by the combination of a very low extension ratio and the energy-absorbing opportunity the moment of extension provided. This analysis would predict that the remaining piece in Wootles tests would have experienced higher loadings when the cordelette was employed. I believe, but am not sure, that some of his data does fit this pattern.
If testing were to bear this out, then small extensions would be viewed as possibly beneficial rather than always harmful, and yet another "benefit" of fixed arm rigging would become questionable.
|
|
rgold
Trad climber
Poughkeepsie, NY
|
|
Very nice insight Murcy. I agree that the CJMM rig never equalizes, in the sense that the probability you hit the two equalizing points is zero.
I don't have the time now to check, but one does wonder whether limiting knots in some of the other three-anchor rigs may also impose geometric constraints on equalization.
|
|
murcy
climber
San Fran Cisco
|
|
rgold writes:
"Consider what happens if one piece of a cordelette anchor fails. With no extension, the load is immediately transferred the remaining pieces (actually, probably to just one other piece). The rope has no ability to recover during this transfer, precisely because the absence of any extension provides no moment when the tension in the rope is reduced. The net effect is that the remaining piece or pieces get the full maximum impact of the fall, the same impact that would have been delivered to the anchor had it remained intact, but now shared by fewer pieces. Put another way, no benefit is obtained from the energy needed to extract the first piece."
I don't get it. If it took energy to yank the piece out, that energy is not going to create force on the remaining pieces. Imagine that it took nearly all the energy of the fall to yank the first piece; the falling climber would be slowed to nearly a stop, and the remaining pieces would have very little to hold. Of course, the rope might not be able to absorb too much more energy, but hey, it was going to have to absorb it all anyway.
|
|
rgold
Trad climber
Poughkeepsie, NY
|
|
"I don't get it. If it took energy to yank the piece out, that energy is not going to create force on the remaining pieces. Imagine that it took nearly all the energy of the fall to yank the first piece; the falling climber would be slowed to nearly a stop, and the remaining pieces would have very little to hold. Of course, the rope might not be able to absorb too much more energy, but hey, it was going to have to absorb it all anyway."
It doesn't take energy to yank a piece out. It takes force. (A very small amount of energy may be consumed by moving the piece a short distance and by generating some heat, but I believe this is negligible compared to the fall energy to be absorbed.)
The so-called "energy absorbed by extracting a piece" is the work done in stretching the rope until the rope tension (force) is enough to break or extract the piece. If that rope tension is not released, and I'm suggesting it won't be with fixed arm rigging, the force, i.e. the stretched rope, will simply be transferred to the remaining piece(s). Further stretching will occur to arrest the fall, at which point the total amount of stretch in the rope will be exactly the same as if the piece hadn't pulled, but now that load is applied to fewer pieces.
If you stretch a 2" rubber band by an additional 1", it doesn't matter how many times you stop while you're doing it. The total energy absorbed will be the energy it takes to stretch the rubber band by 50%, and the tension in the rubber band at the end will be...the tension in a 2" rubber band stretched to 3".
In the case you describe, I'm saying that although the additional rope stretch that occurs on the remaining pieces may be very little, the fact that there has been no release of rope tension means that the remaining pieces will still be subjected to the entire maximum force involved in arresting the fall.
Let's say there is only one other piece and the first piece held while most of the rope stretch was happening. Then the good piece experiences half the total load up until the failure of the bad piece, at which point the load on the good piece nearly instantaneously doubles. Isn't this what people mean by shock loading?
Now on the other hand, if the tension in the rope is released by a small extension in the anchor, then stretching and the build-up of rope tension (= force on remaining pieces) only has to absorb the fall energy remaining, with the ideal result that the remaining pieces experience lower load than they would have with the non-extending anchor.
It's all hypothetical, of course, but it seems quite plausible to me, unless of course I've missed something. I hope someone can test this.
|
|
GOclimb
Trad climber
Boston, MA
|
|
Murcy, that's correct. I said in an earlier post here, as well as in a response to CharlesJMM on the original rc.com thread that the CHJMM anchor only really equalizes between all the pieces within a narrow range, and for exactly the reason you stated so elegantly.
In reality, an elipse is close enough to a circle that within a range, the equalization of the CHJMM is pretty good. It's slightly better than the equalette, in which you immediately unweight one of three pieces completely as soon as you move the centerpoint at all. And of course it's far better than the cordelette, which really only ever weights one piece at a time as soon as you get off center.
This is exactly why the CHJMM anchor was not on the top of my list of anchors, despite its simplicity.
RG wrote: I don't have the time now to check, but one does wonder whether limiting knots in some of the other three-anchor rigs may also impose geometric constraints on equalization.
The Mooselette, multi crossed-sling, and Gordolette do not suffer from this drawback. However the Gordolette has a rather limited range and is awkward to set up.
GO
|
|
GOclimb
Trad climber
Boston, MA
|
|
RG wrote: Next, consider what happens when a piece fails and there is a small extension. During the instant of extension, rope tension drops to zero.
No it doesn't. It drops to the tension of stretching the rope X feet minus the number of inches of extension.
Removing for the moment a falling belayer from the equation (not unreasonable, since in many cases the belayer is on a stance) the force that then comes onto anchor should be very slightly less than the exact force at which the piece pulled out. What is key, however, is that in a two piece anchor that force is *shared* across the two remaining pieces in a dynamically equalizing rig, but is all on one piece in a cordelette.
To put it in simple terms: because it doesn't equalize, a cordelette makes it more likely that
1 - The first piece will rip, because in some cases it feels almost all the load.
2 - If the first piece rips, that the other pieces will rip, because they each feel more force than they would if they were sharing the load.
GO
|
|
rgold
Trad climber
Poughkeepsie, NY
|
|
""During the instant of extension, rope tension drops to zero."
How do you justify this assumption? Is Hooke's Law being thrown out? Or are you saying that the elongated rope shrinks back to its (near) original length during anchor extension? If the rope elongation is large compared to the anchor extension then how can this be? "
Hooke's law is still in operation. I'm saying the elongated rope shrinks some. Tension going to zero is an idealization, although I'm not quite sure how to think about a non-zero the tension in an unloaded rope. The small extension combined with the fact that the rope and climber are falling will limit the amount of recovery before the rest of the pieces engage. A shock-wave calculation made by Ken Cline some years ago on rec.climbing suggested that recovery occurs in extremely short time intervals.
It has been a matter of debate for years whether any real recovery actually happens. In addition to how much recovery might happen in the time interval of an anchor extension, there are the mitigating factors of rope stiffening (we know this happens) and the additional effects of the belayer falling.
|
|
GOclimb
Trad climber
Boston, MA
|
|
RG wrote: Let's say there is only one other piece and the first piece held while most of the rope stretch was happening. Then the good piece experiences half the total load up until the failure of the bad piece, at which point the load on the good piece nearly instantaneously doubles. Isn't this what people mean by shock loading?
Now on the other hand, if the tension in the rope is released by a small extension in the anchor, then stretching and the build-up of rope tension (= force on remaining pieces) only has to absorb the fall energy remaining, with the ideal result that the remaining pieces experience lower load than they would have with the non-extending anchor.
It's all hypothetical, of course, but it seems quite plausible to me, unless of course I've missed something.
I don't know whether you missed it, but it sure sounds like it:
If you have a 20 foot rubber band and stretch it 6 feet, you'll get tension X. Lower the top end 6 inches, and you'll get a tension of X minus a very little bit.
So in your scenario two, the remaining piece feels nearly double the force that extracted the first one.
Of course, that's assuming that the belayer doesn't fall directly onto the anchor. If she does, the anchor may feel significantly *more* force in scenario two than in scenario one.
By the way, if it's an equalizing three piece anchor instead of two piece anchor, when that first piece fails, the other two feel only 50% more force than the first one did, rather than double the force.
GO
|
|
GOclimb
Trad climber
Boston, MA
|
|
RG wrote: Tension going to zero is an idealization
Why? Take a rubber band stretched 50%, and reduce the stretch to 48%, and you say the ideal calculation of the tension in the rubber band is zero?
GO
|
|
rgold
Trad climber
Poughkeepsie, NY
|
|
"If you have a 20 foot rubber band and stretch it 6 feet, you'll get tension X. Lower the top end 6 inches, and you'll get a tension of X minus a very little bit.
Good points. I was thinking of the tension in the belayer's tie-in. What if I have a two-foot tie-in that stretches 8" and an 8" extension in the anchor?
|
|
Largo
Sport climber
Venice, Ca
|
|
I'm fiddling with the CHJMM right now. Once again, the devil is the limiter knots--the "geometric limitation" Rich mentioned. The CHJMM is an elegant idea but it doesn't work as well as hoped because once the limiter knots are set, it dynamically equalizes (within a limited but acceptable range) only in a vertical orientation. Meaning, so long as you are pulling straight down, you're okay within the given range. On the horizontal plane (meaning when you lean out on the rigging, as usually happens on all but hanging belays), the middle piece goes slack.
I'll have to start fiddling with the Moosealette and other two and see if there's not some way to simplify these ideas.
GO, can you be bothered to sketch out those other rigs as well. Thanks,
JL
|
|
cintune
climber
Penn's Woods
|
|
This is getting really interesting. I think this has been posted before in another thread, but here's some stuff that might possibly be useful to test out some of these ideas:
http://www.squid-labs.com/projects/erope/
"Squid Labs has developed an Electronically Sensed Rope - a rope or webbing with integral sensing capability which can be monitored electronically. Our technology can be used to sense wear and load conditions in rope and webbing. We currently work with customers to develop commercial and end-user solutions using our technology, including appropriate sensing elecronics integrated to the application."
|
|
the Fet
Knackered climber
A bivy sack in the secret campground
|
|
Murcy, that's correct. I said in an earlier post here, as well as in a response to CharlesJMM on the original rc.com thread that the CHJMM anchor only really equalizes between all the pieces within a narrow range, and for exactly the reason you stated so elegantly.
I missed this version of the CharlesJMM Anchor (Chuckolette?) on the rc.com thread and thought you (GO) were referring to one of his earlier designs. I played with it a little and it seems like if it's tied like this photo, with extra slack given to the strand with the twist, then it DOES load share good over a good range (if the strand with the twist is only weighted in one piece failure mode). It don't think it "equalizes" since the middle arm looks like it puts 2X the force of the outside arms (25/50/25) but that's ok, if it does everything else good. It equalizes 50/50 if one of the pieces fails.
The main issues I see are: if an outer piece fails a limiter knot pulls through the powerpoint biner, it's a little tricky and time consuming to tie (but not bad).
|
|
WBraun
climber
|
|
So ah mmmmmmmmm
What's behind the blue door?
|
|
GOclimb
Trad climber
Boston, MA
|
|
JL: Not sure how you're setting up your rig - it should make no difference whether you're pivoting around a vertical or horizontal axis.
GO, can you be bothered to sketch out those other rigs as well. Thanks,
Sure, if I have time, will do so tomorrow.
The Fet: I agree with everything you said regarding the CHJMM. I do not know if there is a more efficient way to break it down and build it again, as I have not used it much in the field. From my limited experience, the Mooselette seems much easier, since I just leave the two limiter knots in it. Then all that's required is to clip it into the three pieces, adjust the limiter knots, and I'm good to go.
Werner: What's behind the blue door?
Ah, but Werner, can't you guess? Another anchor, and another blue door, of course!
GO
|
|
raymond phule
climber
|
|
"A great link, thanks for it. But no useful data on factor-2 falls reported there, unfortunately"
The most important info was that you cant put a large force on a rope using an ordinary belay device. This is also true in a class 2 fall. The force on the anchor cant be high except in some special cases. The rope cant slip through the belay device, the climber falling on the belayer, the belayer falls an loads the anchor. Any more possibilities?
"Here I think you are wrong. As you say, peopel make misstakes, even in the presence of knowledge of the problems. They are sometimes wrong about which piece is good. The best strategies are those that minimize potential load to all pieces, are tolerant of off-axis loading, and redistribute the load to remaining pieces if one fails."
I agree
"These are the only available means of coping with the uncertainties that are part of real life."
I disagree about this. Assume that most belayas that fail is due to one problems mention above, rope cant slip, climber falls on the belayer and belyer fall and loads the anchor ( a rigid connection is very bad here). Then are a perfectly equalised anchor better than a non equalised but it might not be enough. Pointing out the main problem is important because it can make people more carefull.
"But it is also the case that very few experienced climbers have ever had their anchor judgements subjected to a real test, much less a statistically significant sequence of tests. This means that most climbers really have no basis in reality for judging the strength of their anchors. The tragedy described by Werner earlier in this thread indicates that these claims are not simply hypothetical."
I agree, but this make it important to try to analyze what actually happens.
"Once again, in the face of this really quite massive uncertainty, equalization is the best strategy, if it can be achieved."
Yes, but the problem is to achieve it. I am also afraid of the load from the belayer due to extension.
"It seems to me that a relatively small group of climbers is interested in the intellectual challenge involved"
It sure can be fun.
|
|
raymond phule
climber
|
|
Interesting thoughts rgold.
The reason for the result is that the maximum force happens when the fallen climber velocity is zero(or there about) because the rope elongation and thus the force is maximum at that time.
My quess though is that the rope is not going to be able to recover fast enough. Isn't a rope very stiff right after a fall? It sure is longer.
|
|
GOclimb
Trad climber
Boston, MA
|
|
JL: GO, can you be bothered to sketch out those other rigs as well. Thanks,
First, the 2 crossed-slings. I don't think you need a sketch, but I just want to reiterate that I think this is an excellent method for distributing the force over three pieces (note, I say distributing rather than equalizing).
Looks like this:
Of course if you wanted to use two anodized big biners at the power-point, you'd get the full non-binding characteristics of the equalette.
-*-*-*-*
To make the mooselette:
1 - Put two overhand knots in the cordelette. These should incorporate the section of line with the joining knot, so as to keep it out of the way. You can do this on the ground. These knots never need to come out.
2 - Place three pieces of gear, and put a loop of the cord through each biner, and pull the centers down to a powerpoint like if you were going to make a standard cordelette. Put the strand with the knots on the middle piece.
3 - Adjust the two limiter knots so that one is down near the powerpoint, and the other is up near the top.
4 - On each side, clip one biner (or draw, if it needs to be longer) between either of the outside strands, and either of the inside strands. Doesn't matter which one, but in the middle, clip it above the upper limiter knot.
That's it.
When I have time, I'll come back and add the Gordolette.
GO
|
|
|
SuperTopo on the Web
|
Let us know!