I spent some more time geeking out on the data last night. Here are my conclusions:
1. All of this is equalization/extension theory is primarily relevant in only three situations: A factor-two fall directly onto a belay, catching a second who somehow takes a relatively hard fall onto the belay, and when building a two or more piece protection point (this happens a lot on sketchy trad climbs and also on ice). These are situations where it would be real nice if the anchor equalized well under load, and then didn't shock-load the other pieces if one failed.
2. Unfortunately, it's about impossible to get any sort of realistic equalization out of a multi-piece anchor (with the gear we commonly use as climbers). If you go to page 29 of the study there's a low-friction equalization situation (equalette) shown there. Even in this perfect situation the pre-drop load totals per piece are obviously different (and could be improved by adding another sliding X etc.,), but even looking at the total "per leg" it's clear it isn't anwhere near perfectly equalized. The rest of the sliding X stuff etc. are worse (with the exception of page 28, but the extension problem is horrendous). A big smooth anodized aluminium 'biner might improve things a little, but even with knots etc. the real problem is what happens when one piece fails and the anchor extends. Yes, you cold tie limiting knots etc., but it looks to me like any extension is violent, which makes sense if you think about it (relatively static webbing or limited cord on a sliding X--bang).
3. In this study, and this is only one study, extension in the anchor is more problematic than poor equalization in terms of the max forces generated on the anchor. That's a real big departure from previous studies I've read.
All of this may and likely will change with the higher forces involved in a factor two or other high-impact situation with a lighter belayer and a larger fall force...
My basic idea that one piece in the belay must be capable of handling very high forces hasn't changed. I want one absolutely for sure bomber nut, cam, screw, whatever. Two absolutely bomber pieces are better, hell throw a third one in for grins. Two or even three or ten "maybe" quality pieces just aren't good enough. If I'm "equalizing" a stubby, an icicle and a shit pin for a piece before punching it up a difficult bit of alpine terrain I'm going to assume that the entire piece is only as good as the strongest individual piece.
I remember a helicopter pilot explaining the term "Jesus nut" to me. He didn't mean a super-religious person, he meant the nut that held his main rotor on. If that broke the only thing left to do was pray to Jesus. In a belay I want one super-solid "Jesus nut" that will hopefully hold any impact I can foresee and then some. And, because I'm not into the whole one-god thing too much, I'll put in another Jesus nut... And still try to limit extension to some point, and even roughly equalize it all.
And this may all change again once JimE gets some more research done, or I see another study done differently. I doubt that the basic concept of having one "for sure!" piece and preferrably two is going to change. And if I can't get that level of security then I'm gambling with two lives.
2 comments:
Will,
Great stuff - thanks for updating your thoughts on this. I've reviewed the study a bit more as well -- and as I'm having trouble sleeping right now, here's the results of my pre-bedtime thoughts:
I noticed some possibly important differences between this test and the testing done by J. Ewing (and reported by J. Long):
i) the J.E. study used an actual factor 1 fall, whereby the dynamic load (100 kg) was dropped on 0.5m of 10.2 Sterling lead rope. In contrast, the J.M. et al. study simply used 595 lbs of force pulled straight down from directly below the anchors. The J.E. study therefore seems much more realistic of what would actually happen in a 'real' life fall and the forces subsequently placed on the anchors.
ii) the J.E. study also actually had .5m of dynamic rope built into the system, whereas the J.M. study was just measuring the force on the anchors with the force transmitted straight down through a piece of static cord or webbing. Once again, the 'real' world belayer typically has tied into the belay anchors with at least .5 m of dynamic rope (and often is tied in with more than that) between him/her and the anchors. Once again then, the J.E. tests (factor 1 fall with dynamic rope in system) seem much more representative of a 'real' climbing situation than the J.M. tests (slow-pull, static rope testing) -- a point that J. Long makes a a few times in his book.
iii) the J.E. study used two anchors; the J.M. study used three. Not sure if this is important or not.
iv) Perhaps significantly, the J.M. study cordelette appears to have three EQUAL length arms (from the photo). The J.E. study confirms that equal-arm cordelettes actually have pretty good equalization -something the J.M. study also subsequently found. However,the J.E. study also showed that UNEQUAL arm cordelettes -- which are far more common in most real life climbing anchors -- have pretty terrible equalization (3-4 times worse than sliding X's, according to J.E. data). Thus, if the J.M. study had used unequal arm cordelettes, I would suspect they would find their equalization far inferior to self-equalizing systems.
What do all these differences equate to? Possibly nothing. However, I would propose that might mean the following:
a) in terms of pre-failure equalization, the J.M. study made the cordelette seem much better than it probably actually is, because they used equal length arm cordelettes, rather than unequal arm length ones
(b) the spiked load on the self-equalizing systems ("shock loading" after anchor failure would likely decrease significantly (disappear?) if dynamic rope was in the system (the more, the better) -- this was confirmed by J.E.'s testing when they did leg-failure tests of the equalette.
I can see the J.M. study being applicable if the belayer is tying into the anchor with a daisy chain or something equally 'static,' but otherwise, perhaps the J.M. study isn't as relevant to 'real' belaying situations as it first appears.
I completely agreee with the necessity of having one bomber (or more) piece in the anchor. However, when one or more of the pieces are less than stellar (and you have no other choices), it seems that J.E.'s testing gives a potentially more realistic picture of how well the load is going to be equalized between the various sketchy pieces than does the J.M. testing. The J.E. study also seems to suggest that if forces exceed the ability of a self-equalized system to absorb them - and one piece blows - that the risk of 'shock loading' remaining anchors in a self-equalizing system is largely mitigated by the presence of dynamic rope in the system.
Finally, it's interesting to note that Anchor #1 (twin of the Trango Alpine Equalizer) actually equalizes extremely well -- just slightly below the "ideal" sharing of the load prior to any anchor failure (Test A, pg. 12).
OK, now that got me all nice and sleepy! Time for bed! :)
Thanks again!
Brad
Nice one Brad, thanks, hope you had a good night in the end!
Yes on the belayer always using the rope to tie into the focal point. Critical. I often use the rope for the entire belay system just for this reason...
The big factor in the JE tests that's unknown right now is this: What will change with the belayer being accelerated faster than just gravity as pieces blow? It adds a big variable, and cold lead to ugly fall factor multiples. Hard to explain but I'll bet you get the idea pretty quickly...
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