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ptone
May 27, 2003, 4:15 AM
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Just a question for any old pros out there...
I am in the habit of using self-equalizing anchor configurations, I think this started while clipping 2 point bolt anchors, but I've used it (with a longer sling) for three point trad placements. I'm wondering what the pros and cons of this versus tied-off anchor equalization...
Shoot--as I write this I see one thing--with tied off equalization, it is easier to set restricted placement angles to protect from pro pulling right?
Anything else about the sims/diffs that would be useful to consider??
Thanks for your help,
peace
-p
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bouldertoad
May 27, 2003, 4:30 AM
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One problem comes to mind when using a self equalizing system on traditional anchor systems. The first is that each anchor is not independent. By this i mean if one piece blows out then the others are shock loaded increasing the potential for them to fail. On a two bolt anchor it is probably not as bad since the anchors are hopefully up to snuff.
Whenever I set up a trad belay i always use three pieces tied of using a cordelette with at least one upward pull piece tied into the main anchor point. This system ensures an even distribution of force on the anchor.
Hope this helps a little.
One more thing check out john longs anchor book........pretty good stuff.
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jt512
May 27, 2003, 5:20 AM
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The reason we equalize anchors is because we believe that there is a chance that one of the pieces could fail. This applies even to beefy bolt anchors; afterall, if we didn't believe that a bolt could fail, why would we install two of them.
Now, if we are equalizing the pieces with a self-equalizing knot (eg, a sliding x), then if one piece does fail, the remaining piece(s) will be shock loaded, which could cause a total anchor failure. On the other hand, if we approximately equalize the load on the pieces using, say, a cordelette, then if one piece fails, we avoid shock loading the remaining pieces. The question is: which is better, approximate equalization with no possibility of shock loading, or perfect equalization with the risk of shock loading.
The answer is that almost always it is better to forgo perfect equalization in favor of eliminating the risk of shock loading because the consequences of shock loading are just too great. The only time that it would be more important to perfectly equalize the pieces would be when the individual pieces are so poor that if one blew, the remaining ones would also blow, even if they were not shock loaded. This is not the kind of anchor that most users of this site will ever have to build.
-Jay
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rgold
May 27, 2003, 7:02 PM
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This issue has been debated over and over, usually with the same concensus: No-extension anchor systems are better. Although I agree with this conclusion for almost all situations, I do think that the claims made for no-extension systems are overly optimistic.
1. Equalization is a misnomer. Once there are more than two pieces, there will be a distribution of forces but rarely if ever equalization. Even with two pieces equalization is not guaranteed.
2. If one part of a tied-off "equalized" system fails, it is entirely possible that the entire remaining load will be transmitted to just a single anchor of the remaining system. This means that a "cascade" failure is a possibility, in which each piece takes the full remaining load and fails in turn. For example, if you have three pieces horizontally spaced and "equalized" with a cordalette and one of the outer pieces fails, the entire remaining load will be transferred to what was the center piece; the piece on the other side will contribute nothing unless the second piece fails. By constrast, a sliding equalized system will cause a drop but will then distribute the impact to the remaining two anchors. Neither of these scenarios is pleasant to contemplate, and it isn't clear that one will always be preferable to another.
3. Tied-off equalized systems don't work in directions that are different from the direction they were tied to resist. For example, if a climb involves a horizontal traverse off the belay ledge to a protection piece, and the leader falls, the belayer could be subjected to a sideways impact if the piece holds and/or a vertical impact if the piece blows. A cordalette tied for one of these directions will transmit the entire load to one piece in the other direction. In this situation, a sliding equalized system will adjust to either direction of pull, again at the risk of allowing a drop if a piece pulls (although the probability of a piece pulling is less becuase of the sliding system's ability to distribute forces.)
We do know of the complete failure of four-anchor systems, and it is hard to imagine a worst-case distributed load of 2 1/4 kN blowing each of the four anchors. The one-at-a-time cascade failure seems far more likely. There is no way to know whether a sliding system, with its extension-induced shock loads but ability to redistribute, would have been any better.
The only general conclusion to draw from this is that their are no automatic answers and judgement is called for. One not entirely obvious specific conclusion is worth stating: If you have are setting up a horizontally spaced three-piece anchor with one of the outer pieces questionable, you may well be better off just using a tied-off "equalizer" on the two good pieces rather than incorporating all three and risking a cascade failure.
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crotch
May 27, 2003, 7:44 PM
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gone
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brutusofwyde
May 27, 2003, 8:19 PM
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Jeeze, folks, if your anchors are that sketchy, slap some screamers into the system. As one screamer blows apart, it extends until the other screamers start taking up the load. Max force on any piece is 2 Kn. Worse than that? slap in some scream aids, and back them up.
Worse than that?
Drill some bolts.
Last time I had a bad anchor set-up (Calaveras Dome, Banzai) I used a combination of screamers, Sliding X and tied Cordelettes for the belay. Eight pieces total. After one of the pieces popped while I was hauling, I confess, I drilled.
Brutus
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gthornberg
May 27, 2003, 8:43 PM
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Think cordelette. It equalizes without the chance of complete failure. Here is a really good link I found on the topic
http://www.gunks.com/rock/cordelette/page2.htm
Hope this was helpful
GT
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jt512
May 27, 2003, 8:58 PM
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Registered: Apr 12, 2001
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In reply to: Think cordelette. It equalizes without the chance of complete failure. Here is a really good link I found on the topic http://www.gunks.com/rock/cordelette/page2.htm
If I understand rgold's post, then the set-up pictured in the above site illustrates precisely the risk he is talking about. If the left piece were to fail, the entire load would be transferred entirely to the middle piece, which, in the worst case scenario, could lead to a cascade failure.
-Jay
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dirtineye
May 28, 2003, 2:50 AM
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I used to use the sliding system, now I use the knotted system. I've never had any anchor piece blow, but I have had the pleasure(?) of catching a 235 pound friend in a 20 foot fall on one of my anchors.
I agree with Rgold that should a piece in the tied off system blow, the force can be felt by just one piece, allowing the possiblity of sequential failure.
If you want to keep using the sliding system, you have to guard against something cutting whatever you used for the sliding part. I got partially around this by using two equal length cordalettes to get redundancy.
The extension problem bothers most people more that the cascade problem. Why this is I don't know, maybe it is because of the SRENE method espoused by Long. Most people accept SRENE as gospel. Maybe that is wrong, but that's how it is.
It could be that since in the sliding system, if your cordalette breaks or cuts you have total failure, and in the tied system you don't have total failure, the tied off system is better.
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