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rgold
Jun 1, 2007, 6:44 PM
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The rope stretch accident report in the July 2007 Rock and Ice issue (page 36) has a number of errors.
EDIT: MUCH OF THIS IS THE RESULT OF AN INCORRECT READING ON MY PART. PLEASE SEE THE
CORRECTION POSTED BELOW
Error #1: The commentary says, "McGlynn's fall, however, was a relatively soft one, with a fall factor of just .66..."
The report says that McGlynn was 20 feet above a bolt that was 60 feet above the ground. This means that there was 60+20=80 feet of rope out for a 40 foot fall, so the fall factor was actually 0.5, not 0.66. (Of course, all these numbers are very rough estimates.)
Error #2: The same mistake is made when calculating the percentage stretch represented by 15 feet of absolute stretch. The article says this represents a 25% stretch. In fact, the percentage stretch is 15/80 X 100 = 19% (approx.) stretch.
Error #3: The article states, "...it is impossible to calculate precisely how much McGlynn's rope should have stretched."
Although calculations based on an ideal model of rope behavior cannot give results that agree precisely with what happens in the field, it is misleading to suggest that there isn't a precise ideal calculation, since there are well-known formulas describing what happens which are now at least 60 years old, calculations that have shown good although of course not perfect agreement with real-world results.
Anyway, assuming an 80 kg climber using a rope with a UIAA rating of 9 kN falling from 20 feet above a bolt with a total of 80 feet of rope out, the ideal model predicts a tad more than 17 feet of stretch.
Error #4: The article characterizes the observed 15 feet of stretch as being "on the high end for such a soft fall," but in fact 15 feet is right in the ballpark.
Error #5: The article says "McGlynn probably thought he was relatively safe 20 feet out from a bolt, 60 feet off the ground, but in fact he was practically free soloing." A climber falling from 80 feet above the deck, 20 feet above a bolt falls 40 feet, then experiences, say, 20 feet of rope stretch, which would leave them motionless and still 20 feet above the ground.
It is, of course, extremely difficult to estimate various distances with any accuracy, especially after a traumatic occurrence. The chances are that the numbers the article bases its analysis on are seriously erroneous. That said, for the data as given, it doesn't seem that this terrible tragedy had anything to do with rope stretch.
(This post was edited by rgold on Jun 2, 2007, 2:19 AM)
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majid_sabet
Jun 1, 2007, 7:09 PM
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Welcome to reality
This is why I only post cheap low level media reports , people are going to pull their calculator and start a flame with you over every fogging #s.
There they come.............
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retr2327
Jun 1, 2007, 7:12 PM
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Hi Rich:
Thanks for the analysis. I'd seen the R&I "analysis" this week, and thought it made no sense. Math errors aside, it seems to confuse the question of whether rope stretch would have allowed him to fall far enough to touch the ground with the very different question of whether it would allow him to "hit" the ground at a fatal velocity.
It seemed to me you're not going to get that much rope stretch without an awful lot of energy being absorped and dissipated; a sprained or fractured ankle maybe, but fatal head injuries? I have to wonder if the head injuries occurred from a glancing blow against the wall on the way down. I looked around on mountainproject.com and other sites on the web for more on this, and found surprisingly little.
Of course, it's not clear that anyone ever went up there to take exact measurements, so maybe that's a different possible explanation.
(by the way, Rich, it's Peter from the Valhalla gym. I look forward to discussing this with you next time you make it down)
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retr2327
Jun 1, 2007, 7:22 PM
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One other note. I though R&I's editorial decisions here were very bad.
McGlynn had written a prior letter to the editor in which he criticized their decision to print an article on soloing, and criticizing soloing as selfish and irresponsible, particularly for those with families, etc. Nothing that hasn't been written about -- and disagreed with -- at great length here on this site.
Then somebody wrote in criticizing McGlynn's letter, and pointing out that you are also taking a risk when you climb with ropes, etc. Fair enough, and the writer obviously didn't know that in the meantime McGlynn had been killed while roped up.
But then R & I chose to print the second letter to the editor, followed by a note that McGlynn had, in fact, been killed while climbing roped up. Whether accidentally or not, it came off as the ultimate "I told you so," and struck me as in very bad taste.
Anybody else have the same reaction?
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majid_sabet
Jun 1, 2007, 7:26 PM
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Recently, I was analyzing a report where a climber fell more than 100 feet (free fall on rope). He did hit his head in several locations on the way down and his helmet was broken in 5 different locations.
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trenchdigger
Jun 1, 2007, 7:33 PM
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majid_sabet wrote: Welcome to reality This is why I only post cheap low level media reports , people are going to pull their calculator and start a flame with you over every fogging #s. There they come.............
I think that's about the most asinine logic I've heard on this website. And that's saying a lot.
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flowin
Jun 1, 2007, 7:44 PM
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what i took from this was to be aware of your rope stretch...accidents will happen and the more we are aware the better.
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nnowinowski
Jun 1, 2007, 7:55 PM
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that definitely doesn't add up to a super hard fall even if the rope was halfway through its stretch you would think that it would absorb a substantial amount of force? However dynamic stretch in a rope is generally 20-40% I think?
Maybe a running belay was in order?
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rgold
Jun 1, 2007, 9:39 PM
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In reply to: This is why I only post cheap low level media reports , people are going to pull their calculator and start a flame with you over every fogging #s.
Majid, two comments.
1. I wasn't quibbling about minor details---the article gets virtually every aspect of the analysis wrong, including the hypothesis that gives rise to the title. From past experience, I have no confidence that R&I will publish letters with reasoned criticisms of their analysis, and so thought a post here might be more useful.
2. I think it is a good thing if people want to argue with my or anyone else's analytical conclusions. It often only through discussion that one arrives at an appropriate understanding, and anyone can make mistakes. Your solution of avoiding arguments by posting only worthless "low-level" accounts does no one any good.
In reply to: Math errors aside, it seems to confuse the question of whether rope stretch would have allowed him to fall far enough to touch the ground with the very different question of whether it would allow him to "hit" the ground at a fatal velocity. It seemed to me you're not going to get that much rope stretch without an awful lot of energy being absorped and dissipated...
Hi Peter. Yes, this is true. Rope stretch is the primary mechanism for absorbing fall energy, and to hit the ground with fatal impact, not much rope stretch could have happened yet. (And let's remember that with the figures given, rope stretch and all, the faller still ends up 20 feet off the deck.) However, it is true, as you say, that a fatal injury might have occurred during the fall---yet another scenario in which rope stretch is not a factor in the outcome of this tragedy.
I do want to make it clear that the purpose of my post was not to question what really happened or whether the reporting of it is accurate. R & I described a certain situation and analyzed it. Their analysis of the situation they described is so seriously flawed as to be, I think, worthless and misleading, and it seemed to me that some sort of correction was in order. As to what really happened, that is a different question and I have neither knowledge nor an opinion about that, other than to feel terribly sad for the victim and his family.
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fitzontherocks
Jun 1, 2007, 10:04 PM
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rgold, I hope you'll submit your posting as a letter to R&I. You're right, it looks like errors all around, and they should at least be made aware of it. Please don't take the attitude that "I have no confidence that R&I will publish letters with reasoned criticisms..." If you do, you only guarantee that outcome.
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retr2327
Jun 1, 2007, 10:18 PM
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"I do want to make it clear that the purpose of my post was not to question what really happened or whether the reporting of it is accurate. R & I described a certain situation and analyzed it. Their analysis of the situation they described is so seriously flawed as to be, I think, worthless and misleading, and it seemed to me that some sort of correction was in order. As to what really happened, that is a different question and I have neither knowledge nor an opinion about that, other than to feel terribly sad for the victim and his family."
Quite right. I read the article, followed through to the pictures of McGlynn (on mountainproject.com) and the tributes to him by people who knew him, and felt moved and upset. As so often happens, that translated into digging into the facts of the accident; it just didn't seem right that rope stretch could have been the critical flaw in what seemed to be an otherwise fairly reasonable exposure (20 feet over a bolt that was 40 feet off the ground). But that really is a separate issue.
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majid_sabet
Jun 1, 2007, 10:31 PM
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Rich
This happens all the time and no one ever said they are correct on their analyses. The best accident analysis is done when you get a group of experienced, climbers, riggers and climbing accident investigators together to do analysis based on the original report from the reporting parties and even then, you will always run it to some errors.
This is just the way thing work and it is never perfect.
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roy_hinkley_jr
Jun 1, 2007, 10:48 PM
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rgold wrote: A climber falling from 80 feet above the deck, 20 feet above a bolt falls 40 feet, then experiences, say, 20 feet of rope stretch, which would leave them motionless and still 20 feet above the ground.
Unless the belayer was tied in tight and using a Gri-gri, rope slippage through the belay device and the belayer getting lifted could easily account for the rest. Haven't read the article so not sure if they mentioned these factors. Agree that the tech articles (and gear reviews) in R&I are pretty lame.
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healyje
Jun 1, 2007, 11:23 PM
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roy_hinkley_jr wrote: rgold wrote: A climber falling from 80 feet above the deck, 20 feet above a bolt falls 40 feet, then experiences, say, 20 feet of rope stretch, which would leave them motionless and still 20 feet above the ground. Unless the belayer was tied in tight and using a Gri-gri, rope slippage through the belay device and the belayer getting lifted could easily account for the rest. Haven't read the article so not sure if they mentioned these factors. Agree that the tech articles (and gear reviews) in R&I are pretty lame.
I'd like to be excruciatingly clear here - a belayer, in a bad stance away from the base of a route, may get pulled off that stance towards the base contributing some slack to a fall, but the use of an ATC-like device by itself should not make a significant contribution to the distance fallen if you are actually belaying and locking off properly. Ever.
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roy_hinkley_jr
Jun 2, 2007, 12:27 AM
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healyje wrote: but the use of an ATC-like device by itself should not make a significant contribution to the distance fallen if you are actually belaying and locking off properly. Ever.
Try the Petzl belay rig in a gym sometime when it's in your area to see how incredibly wrong you are. They setup an easier fall (0.3 or so) than the one in this case with weights and a tensionometer and let people practise belaying with different gizmos. Even locked off with an ATC on a fat rope and wearing gloves, there is significant slippage. With a skinnier rope, hands in sub-optimal position, and no gloves on the belayer, the leader is in for a ride.
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stymingersfink
Jun 2, 2007, 12:47 AM
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roy_hinkley_jr wrote: healyje wrote: but the use of an ATC-like device by itself should not make a significant contribution to the distance fallen if you are actually belaying and locking off properly. Ever. Try the Petzl belay rig in a gym sometime when it's in your area to see how incredibly wrong you are. They setup an easier fall (0.3 or so) than the one in this case with weights and a tensionometer and let people practise belaying with different gizmos. Even locked off with an ATC on a fat rope and wearing gloves, there is significant slippage. With a skinnier rope, hands in sub-optimal position, and no gloves on the belayer, the leader is in for a ride.
luckily, i was on the belayer side of this incident, felt no rope slippage what so ever, and my leader touched down on the ice sheet at the base of the pillar just as light as cat, his crampons barely scratching the ice.
there was no immediate damage to the climber, perhaps other than a bruised ego.
in the incident I was a direct party to, the amount of stretch in the system was not surprising, as only one of the half-ropes contributed to the arrest of the fall, however in a fall of a similar distance utilizing a single-rated rope I would not have expected the leader to fall nearly as far.
I have not seen the article in R&I, but based on RGold's post I would suspect that something was overlooked in the analysis of the accident. Decking from 20 feet above a bolt at the 40 foot level would definitely cause me to search for other factors contributing to a fatality. Not saying that reaching the deck from such a location is a physical impossibility, just that the chances of such an impact happening at forces sufficient to generate fatal injuries seems outside the realm of probability.
I am curious if all of the variables involved in the accident were accounted for.
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paulraphael
Jun 2, 2007, 1:02 AM
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healyje wrote: but the use of an ATC-like device by itself should not make a significant contribution to the distance fallen if you are actually belaying and locking off properly. Ever.
Ever, except in a hard fall. ATC-like devices only provide friction. They act as a simple multiplier of your brake hand force. The amount of force multiplication depends on the device and on the qualities of the rope.
With the maximum brake hand force that even burly climbers can exert, these devices are rarely capable of generating more than 4kn braking force.
In most falls (low factor, with lots of friction in the system) this is enough to lock off the rope without any slippage. In a higher factor fall without much friction, a much the fall energy gets dissipated by the rope slipping through the device. In these cases belayers typically end up wishing they'd had gloves on.
I can find you multiple sources of real world tests that confirm all this, if you have doubts.
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stymingersfink
Jun 2, 2007, 1:49 AM
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paulraphael wrote: With the maximum brake hand force that even burly climbers can exert, these devices are rarely capable of generating more than 4kn braking force. In most falls (low factor, with lots of friction in the system) this is enough to lock off the rope without any slippage. In a higher factor fall without much friction, a much the fall energy gets dissipated by the rope slipping through the device. In these cases belayers typically end up wishing they'd had gloves on. I can find you multiple sources of real world tests that confirm all this, if you have doubts.
<aside>
When exposing new belayers to the concept of catching a fall, I will generally utilize a redundant system which will allow them to be lifted from the ground while providing me safety from a failed belay. On one occasion which comes to mind, the look on the belayers face as we passed each other [my 175lbs going down, her 115lbs going up] was priceless!
She arrested the fall before the secondary system came into play, btw. A softer catch I've never felt.
</aside>
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rgold
Jun 2, 2007, 2:13 AM
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I'm sorry to say that I have misread the article. The analysis part says that McGlynn fell "20 feet out from a bolt, 60 feet off the ground." Earlier it says, 20 feet out from his last bolt and 60 feet off the deck..."
I mistakenly read these (more than once) as saying the bolt was 60 feet off the deck, not 40 feet. So, looking back at my list of errors:
Error #1 is my error. The fall factor is indeed 0.67.
Error #2 is my error. The percentage stretch represented by 15 feet of rope stretch is 25%.
Error #3 still stands, but a new ideal calculation is in order. Assuming an 80 kg climber using a rope with a UIAA rating of 9 kN falling from 20 feet above a bolt with a total of 60 feet of rope out, the ideal model predicts just about 15 feet of stretch.
Error #4 still stands. 15 feet of stretch is even more precisely in the ballpark.
Error #5 Still stands but is no longer egregious. The rope stretch should have deposited the climber on the ground with little or no impact.
I think in view of this, my later statement that "the article gets virtually every aspect of the analysis wrong, including the hypothesis that gives rise to the title." is now out of line. The climber could easily have hit the ground with rope stretch, but shouldn't, in principle, have been killed by that impact.
The situation is now far more marginal, since anything else that puts slack in the system is going to just about guarantee the leader hits the ground. However, many things, like rope slippage, are themselves energy-absorbing and so do not necessarily imply a fatal impact.
(This post was edited by rgold on Jun 2, 2007, 2:31 AM)
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Dillbag
Jun 2, 2007, 2:28 AM
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RG... That's what happens when you do calculations after only one cup of coffee 
Good info the same though!
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kimgraves
Jun 2, 2007, 9:06 PM
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Let me just fess up to not ever thinking about rope stretch when evaluating fall consequences - other than the rope will stretch. The only guesstimating I do in my head is fall factor. But in fact I should guesstimate rope stretch as well. Rich, what's the ideal calculation you're using.
The R&I article and Rich's analysis (even flawed) has gotten me thinking. And that's all for the good.
Best, Kim
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rgold
Jun 2, 2007, 11:12 PM
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Kim,
I posted derivations of the standard equation here. In that article, Equation (7) gives the rope stretch for a particular fall factor and rope length.
The relation between relative rope stretch and rope tension is just Hooke's Law, so if you use the standard equation or, say, the Petzl calculator to get maximum rope tension, dividing that maximum tension by the rope modulus will give the relative stretch, and multiplying the relative stretch by the amount of rope out will give the absolute stretch.
Of course, this still requires that you know or calculate the rope modulus. Equation (11) tells you how to get the rope modulus from the UIAA fall impact rating.
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kimgraves
Jun 2, 2007, 11:26 PM
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Hi Rich,
Thanks for the paper. I WILL read through it. However I need something I can do in my head when I'm leading. Or a rule of thumb. E.g. in the accident report the rope stretched 25%. Is that a reasonable number to use to be safe. Any thoughts?
Thanks, Kim
(This post was edited by kimgraves on Jun 3, 2007, 12:02 AM)
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healyje
Jun 3, 2007, 1:21 AM
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roy_hinkley_jr wrote: healyje wrote: but the use of an ATC-like device by itself should not make a significant contribution to the distance fallen if you are actually belaying and locking off properly. Ever. Try the Petzl belay rig in a gym sometime when it's in your area to see how incredibly wrong you are. They setup an easier fall (0.3 or so) than the one in this case with weights and a tensionometer and let people practise belaying with different gizmos. Even locked off with an ATC on a fat rope and wearing gloves, there is significant slippage. With a skinnier rope, hands in sub-optimal position, and no gloves on the belayer, the leader is in for a ride.
Dude, I can belay a hard fall with nothing but a single non-locking carabiner (with no hitch of any kind) and no slack whatsoever is going to run through my bare hands let alone when using an ATC-like device. In thirty-three years of belaying endless falls I have never, ever, had rope run even inches let alone a significant amount of rope that would contribute to a fall. When that happens - I don't call it belaying, I call it failing.
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