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rgold
Nov 9, 2005, 7:36 PM
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Here are two quotes from that article that confirm Adnix's statement. The situation is one in which there is essentially no friction between rope and rock and, I think, just a single protection point.
"It was clearly shown that the maximum and average value of braking forces occurring during dynamic belay (not their duration) and consequently the load on the last runner are practically independent of the free fall height."
This appears to be a consequence of the claimed significance of the "slip ratio:"
"...the 'slip ratio' (length of rope slipping in the belayer's hand, divided by the total free fall height)...is the the single parameter determining the average belay forces." (No mention here of the relation between the slip ratio and peak belay forces.)
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xp
Nov 10, 2005, 5:20 PM
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We are digressing from the original subject. Maybe this should be moved to its own thread?
In reply to: In reply to: I know some Italians who have held more than that in the lab. It wasn't with natural gear but the conclusion was that fall factors don't matter really that much. The dynamics are same as with less severe (let's say ff 0,8) falls, only slightly more rope slippage. I've read some of the Italian stuff. I'm not sure this quote accurately reflects what I remember. I recall one point that said that because of the effect of friction through multiple biners, most falls with a bunch of pro in produce peak loads in a much narrower range than the fall factor would suggest. In fact, their analytic formulas produce an "effective" fall factor for various situations. These particular results don't apply to the case of a high fall factor fall onto the anchor, when the only source of friction is over the biner clipped to the anchor. The other part of the Italian results is that, of course, peak loads are mediated by rope slipping through the belay device, and I seem to remember a 20% variation in these loads caused by different belayer actions. I don't recall reading anywhere that peak loads for different falls with only one intervening biner are the same regardless of fall factor because of rope slippage---I'd be interested in a reference to that if there is one. Years ago when everyone learned the purposeful dynamic belay, I remember reading a contrary conclusion,in Summit Magazine, about some tests which concluded that dynamic belays could not reliably reduce peak loads on the top piece. Sometimes the belayer hung on tight enough for long enough for an almost static belay peak load to develop. (We're talking about fractions of a second here.) These tests were with hip belays, which may have been more effective than some of the belay devices in supplying friction, and so I don't know what they tell us about the situation today. At least until more information becomes readily available and is subject to verification, I'd say that supposing a factor 1.8 fall onto your anchor will produce no higher load than a factor 0.8 fall would be a potentially dangerous assumption.
The theory AFAIK is that beyond a certain (not too high FF), most belay devices don't provide enough friction to keep it "locked off". So with any fall with higher factor held by a common device, the peak force doesn't depend on the FF anymore, but on the point at which the device slips. That is why they talk about "slip ratio". More fall distance at 1 g acceleration means more slipping/stopping distance at "x" g decelleration. This can be a good model only if slippage plays the main role in stopping a fall. Sounds true for high FF fall: 12 m slippage on 30 m fall is very significant. Of course, it assumes the rope will run smoothly through the device and there won't be any accidental tangle or a knot on the 40' of slippage they measured in that particular test.
Friction below the top anchor increases the force necessary to start the slippage. In a way, it "aids" the belayer, whether they want it or not.
The 20% variation makes some sense, but I am sure all tests were with the same belay device, and the variation was in whether the belayer/belay was anchored. If they had eliminated the device completely and tied off the rope to the anchor as in the standard UIAA fall setup, there has to be more than 20% difference!
I've read about estimates that most belay devices increase friction somewhere by a factor of between 8 and 16. Even a reasonably strong hand will start losing grip on a rope when it is suddenly loaded with somewhere around 60 lbf. That is 0.3 kN. So the slipping should start somewhere between 2.5 and 5 kN at the belay. Of course it depends on the rope, hand, etc., etc., etc., but it is not hard to believe that if 4 - 5 people hung on the rope instead of just one, you won't be able to hold them even for a split second.
It may be dangerous to think that high FF don't produce higher loads. It seems also dangerous to think that a high FF fall can be caught without gloves.
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trapdoor
Nov 11, 2005, 1:43 AM
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John
I would like to see more examples (pictures) of how to incorporate an upward pull piece in to a gear anchor for multi pitch climbs. I know you talked about it alot but some more visuals would be nice.
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woodcraft
Nov 13, 2005, 6:13 AM
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John
I second the comments about "protecting the anchor" ie placing solid first pieces above the anchor.
Also, since the whole discussion of anchor strength is in relation to catching falls, it would be useful to include reports of falls caught- rope burns? pulled into the anchor? pieces shifted? Maybe a few sidebars...
Thanks for all your great writing, as a 'learn from books' type, you've helped me to be a safer climber, and I've been able to pass on info to non- readers as well.
Jay
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waynew
Nov 14, 2005, 5:26 PM
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John: Reading through your first Anchor (yet again!) book last night and kept coming back to the several photo series that showed multiple configurations in one location, with your evaluation. There's always more than one way to do things, and it's helpful to see some options.
And thanks for sharing your expertise.
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crotch
Nov 14, 2005, 6:44 PM
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I haven't read through the entire thread so this may have been mentioned, but it would be nice to remind folks that when setting up a belay at a station that also serves as a rappel point, it is polite to leave the chains/rings free so that other parties can rappel.
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adnix
Nov 14, 2005, 9:54 PM
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In reply to: It may be dangerous to think that high FF don't produce higher loads. It seems also dangerous to think that a high FF fall can be caught without gloves. |
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adnix
Nov 14, 2005, 10:02 PM
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In reply to: I've read about estimates that most belay devices increase friction somewhere by a factor of between 8 and 16. |
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cracklover
Nov 15, 2005, 12:37 AM
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Hi JL,
As others have said, your books have been a great help - so thank you!
Three things:
1 - Rappel anchor tricks, like the Texas Rope trick. Any interest in discussing these? Probably not, but just curious.
2 - Climbers operating on "anchor-faith". RGold hasn't mentioned it here, though perhaps he's PMed you, but he had a fascinating idea which he mentioned on another thread here on this site. I'm certain to remember it imperfectly, so RG, please feel to jump in - but here it is as well as I can recall.
Given the issue he highlighted regarding all of us having little to no experience with anchor failure - what about changing that? What about suggesting that people *do* create anchors that are borderline, building each with a good solid backup, to see what fails, and to learn from that. Just as a beginner excersize, you understand?
3 - Upward pull pieces. If I recall correctly, your anchor book was quite absolutist about upward pull pieces. I think all the photos in the back were shown to picture "faulty" anchors due to the lack of an upward pull piece. As you probably know, there are respected members of the climbing community who have very different views about the absolute need for an upward pull piece. Have you softened your stance on the matter? If not, I think it would be wise to answer the critics in your new book, as there are some pretty persuasive arguments out there.
Thanks again!
Gabe O
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glacierboy
Nov 15, 2005, 8:29 PM
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Hi everybody. I'm the editor working with John Long on the new anchors book. Thanks very much for all the great suggestions. We'll do our best to incorporate them into the book where appropriate. Because attribution would be next to impossible (few people use their full name when posting), we will assume that anybody who posted ideas on this forum has granted permission for their inclusion in the book. If you object to your suggestion being used, or have other concerns, please e-mail me directly at the address below.
FYI, Bob Gaines (co-author) has taken a lot of new photos, and any old photos will be re-scanned with much better equipment than was available in the mid-90s, when these books were first printed. Also, we will be adding illustrations in some areas to complement the photos. I mention this because the quality of the photos is a common concern.
At this point we're going to have to call an end to this discussion and get on with putting the book together (deadlines loom). Once again, thanks, and if you need to contact me, please don't hesitate.
Best regards,
John Burbidge
johnburbidge@hotmail.com
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philbox
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Nov 15, 2005, 9:23 PM
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Thanks John for dropping by and filling us in on where the book is up to.
I shall now lock this thread. If anyone wishes to start up another anchors thread then feel free but as far as input to the book goes, you heard the man, the deadline is at hand.
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philbox
Moderator
Nov 15, 2005, 9:24 PM
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philbox has locked this thread.
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Re: Dynamic equalization and no-extension
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