Monday, March 22, 2010

Maybe better to limit anchor extension after all...

Warning: Serious climbing geekery ahead. Short version: use bomber pieces in yer anchors...

I was out climbing on the weekend (rock, not ice), and at one point had to build a sorta odd anchor. A bomber cam, a very good nut, and a small nut that would be bomber if it weren't in Rockies limestone, all basically in a vertical line but spread out over about ten feet. I tied it all together with the rope to limit extension and felt good about it. But I knew that the stretchy rope would load each piece close to individually, and next thing you know I'm up for most of the night looking for information...

This is the most difficult to understand, broad and ultimately useful study about what happens when a single point in a multi-point anchor fails that I've read. It took me about an hour to start to understand what the results mean, and I'm still not sure I totally get it. The first interesting numbers really show why it's a bad idea to rig your anchors with your gear widely spread out unless you also use very long legs to hold it all together... This is why the combined force on each piece at the start of the test is higher than the weight hanging below the anchor (500 pound weight, but commonly 700 or so pounds total on the pieces...). To put it another way, it's not a good idea to have short legs and widely spread gear on any anchor system. This is why the "American Triangle" belay system had force issues.

The more I dig into the data at the end of this paper the more interesting the results become. No system does a very good job at equalizing the forces when a piece blows in the anchor, and this study was done in a perfect lab setting with a perfectly suspended weight. Tying marginal pieces together and equalizing them is bad physics if you want a good anchor. At least one piece in the anchor must be truly bomber; two or more is better!

Interestingly, total length of extension does appear to matter; this is in contrast to JE's tests. It doesn't matter (from the anchor's point of view, from the load's point of view it does...)whether you use one inch webbing, static cord, or 8mm nylon cord, high loads result when there is serious extension after an anchor piece fails. This is really interesting to me. JE's test results always seemed slightly odd to me, this makes more intuitive sense. However, Mr. JimE is a very smart guy, he likely has some ideas about what's going on. For now, I think I'll do a little more in my anchor building to limit extension. Not to equalize, but to limit extension. A cordellete, the rope, a sling, whatever, clip it all together so if something blows the extension won't be too dramatic. However, know that in the real world there will be extension and general weirdness, so make sure at least one and better two or three pieces are truly bomber in any pull direction that can be foreseen.

This is what's cool about climbing; just when you think you've started to figure it out something new or at least different comes along... I didn't learn anything about what I actually set out to learn (why does that happen so much on the internet?), but here I am writing about it like the nerd I can be...

If anyone comes up with something else out of this study please let me know, thanks.

Edit March 23rd: Some good comments below, thanks for that, be sure to read them.


Denny said...

The cordalette rules! Check out page 12, test B-1 summary. Anchor 8 is the cordalette, and the winner by a long shot. Why? Lowest extension, Page 31, 0.75-1.25".

James said...

Will, thanks for the post.

I agree that the study wasn't presented in a particularly clear format.

As a user of the Trango self-equalizing/load-extending device, I was initially bummed to see that equalization doesn't really happen -- at least in this trial -- but I'm glad to see it tested out.

I think we need a lot more testing like this; not sure who to do it.

One note about the cordelette; it still may be good, and I was quite surprised to see it equalize so well, despite being a static device -- but it was clearly at its best in this test design, because of the straight downward pull. In a real climbing situation, it's very easy to move the power point just a bit sideways, which could lump all the pull onto one piece.

Thanks again, Will, for giving this sort of stuff the visibility it deserves.


Brad said...

Very valuable info - thanks for posting this.

While it's true that Anchor #8 (the cordellete) has much lower MAF (maximum arrest force)values, there seems to be ta few problems with the study:

i) in the study, the direction of the force was straight down onto the cordellete -- which often does not happen in the real world. Thus, the real world equalization for the cordellete is probably significantly less than the authors present in the study.

ii) more importantly, the authors note that when one anchor blows with the cordellete, 87% of the load ended up on one anchor. Almost 90% of the load going onto one anchor seems unacceptable, even if the forces are a little lower.

iii) if you look at their photo, the sliding X (anchor #4) is set up with two big burly (steel?) carabiners, whereas the sliding X is often only set up with one carabiner -- and John Long's book on anchors made it clear that using a smooth round anodonized gives much better equalization for the sliding X. Thus, the actual equalization for a sliding X using a single smooth anodized carabiner may be much better than what the authors report.

iv) none of the tests measured the effect of minimizing extension in self-equalizing systems (i.e., using 'limiter knots' in the sliding X anchor). I suspect that if they had, the MAF would have been much lower for the sliding X anchor.

Finally, it's interesting that the authors did not include in their tests the 'equalizing figure eight,' which I suspect might have done better than any of the other systems tested:

While this study is interesting, I'll probably keep using something like the sliding X, with limiter knots, and one anodozed smooth locking carabiner.

Anonymous said...

Thanks Will for the post!

In reply to Brad:

The Equalette has stopping knots (essentially the same as a sliding X with stopping knot if not better) but it still has relatively high MAF.

MAF on other anchors are a few times more than the Cordelette (under 4kN)

Kim Graves said...


I don’t think you can consider the issues surrounding extension under anchor failure without first considering equalization before failure. Clearly the chance of any one piece failing are lower if there is equalization between multiple pieces. If there is no failure than extension is not an issue. The problem of course is that equalization is a problem because the equalization must function under dynamic conditions. And the traditional methods of equalization (clove hitches with the rope and use of the cordelette) don’t work. In addition, there are two failure scenarios you need to protect against when lead climbing: 1) the leader factor 2’ing onto the anchor where the load is down, 2) the leader falling after the first piece where the load is less severe but up or to the sided or one after the other as pieces above fail. (e.g.: if your anchor nuts are slotted down and the leader falls above the first piece, those nuts might well come out as the belayer is pulled up – equalization and extension being completely irreverent) The problem with the study you site is that it’s rescue oriented where loads are high, mostly static and directionally in one direction (down).

It seems to me that John Long’s equalette is a big step forward in dynamic anchor systems because “ideally” it both equalizes while minimizing extention if one piece fails. Its weakness is that it really is designed for two pieces (yes, you can make it 3 or more pieces using sliding x’s). How comfortable you feel with an anchor of two pieces depends upon the pieces and depends upon you. (This is a bone of debate between me and my partners). If the pieces are not multi-directional (i.e.: not pins, screws, bolts, threaded slings) I always to put in a piece below the anchor and attach it to the power point to protect the anchor from an upward pull. I compromise on the upward pull with only a single piece because the load is less. Put I always insist on a bomber piece going up. The bottom line I you need a minimum of three bomber directional pieces – two down, one up.

All this is “try:” life is full of compromises. Statistically there are relatively few accidents caused by anchor failure so all these systems work more-or-less. Rappelling is much more dangerous. AND driving to the crag is the most dangerous part of climbing.

Will Gadd said...

Denny: pretty much yes. Note that it didn't equalize well, but then again nothing does.

James: Pretty much yes too. The Trango system falls apart quickly as soon as things get "odd," which is where equalization might matter.. But I'm starting to think equalization is a myth in the real world, even with a sliding X on a smooth biner... The tests where there should be even less friction (equalette) don't show great results with three pieces. No better than a cordalette really.

Brad: Good thoughts. Yes, this is as "perfect" as it's ever going to get. Real world results likely to vary dramatically if equalizing systems don't even work in this simple situation.

On sliding X equalization: I'd think a nylon cord (page 29) sliding on a 'biner would be lower friction than webbing wrapped around even a smooth anodozed 'biner, or at least I would. So if the equalette doesn't equalize very well then why would a sliding X equalize better?

I'm not seeing any system that actually equalizes very well once one piece blows. The cordalette seems to at least limit the forces for the very fat belayer falling onto the anchor, or possibly the force of the belayer and falling climber in a factor two..

on your point IV, the equalette on page 29 does have load-limiter knots on it, and yet has relatively unequal forces...

Good thoughts form everyone, thanks!

Will Gadd said...

Kim, I broadly agree, check out the next post, thanks.

I'm not too worried about the "up" pull in general for anchor security for the simple reason that if you're getting yanked up that hard then the piece above you is likely holding... I am more concerned about where the belayer goes when yanked, there have been more bad accidents due that then belay failure for sure. Not many people think about what's going to happen if the leader falls and yanks them head-first into that roof above them... Good point

Simon said...

@Will: your last point is a good one. I find that there isn't enough discussion about what happens to the belayer under big, weird falls. Extension, for instance, can have all kinds of effects on the poor belayer. Thanks.

Anonymous said...

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Thanks for all the great posts, I love reading them. You're very good with words.

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