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sittingduck
Jul 23, 2007, 9:05 AM
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"Shock loading", is it still what it was hyped to be or only a myth like microfractures?
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microbarn
Jul 23, 2007, 12:22 PM
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Impact loads are much higher then loads applied slowly.
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sterlingjim
Jul 23, 2007, 12:59 PM
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The problem with the term "shock loading" is that it is not defined. If we say it is any rapid loading then we have to define how rapid.
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overlord
Jul 23, 2007, 1:18 PM
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shock loading... its not a myth but a physical rule.
if you fall and a piece of pro catches your fall, it must absorb your momentum. it does so by a thrust of force (thats a direct translation from slovenian, im not sure what if its called that in english).
so...
v*m=F*t
the shorter the time of the actual catching, the larger the force needed. so if you fall without some dampening in the system (like falling on a draw clipped directly onto pro and you your belay loop, or onto some webbing) the very short time means the force will be considerably larger than it would be if you had something to prolong the catching (like a dynamic rope, screamer etc.).
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sterlingjim
Jul 23, 2007, 1:27 PM
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Yes but how fast is the "thrust"?
Shock loading must be properly defined (force over time) before we can begin to discuss whether or not it exists or should enter it into discussions around forces generated in climbing situations.
(This post was edited by sterlingjim on Jul 23, 2007, 3:08 PM)
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dingus
Jul 23, 2007, 2:04 PM
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I think the pertinent question for climbers to think about is this:
is shockloading something climbers need to prepare for on EVERY ANCHOR?
DMT
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j_ung
Jul 23, 2007, 2:58 PM
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In the typical sense, I think the phrase "shock loading" most often refers to what happens to the remaining components of an anchor system when one or more other components fail. For example: "Dude, don't trust that magic X. If one piece fails it'll shock load the other."
I think what sterlingjim is getting at -- and he has a drop tower -- is that this isn't necessarily the case. And given I've never met any of you, I'm more likely to trust what the guy with the drop tower says. 
A more pertinent question might be: how does shock loading -- or the non-existence of it -- affect our climbing? This isn't an easy question to answer, I assume, since I'm hearing a lot from both sides of the debate.
(This post was edited by j_ung on Jul 23, 2007, 3:00 PM)
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shockabuku
Jul 23, 2007, 3:01 PM
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Impulse, as a single quantity, is not a completely adequate way to discuss shock loading since peak force is, at least initially, the dominating factor in anchor failure considerations. I think that impulse will be important when the discussion comes around to response times, in particular with the use of a device like a Screamer. However, that requires a pretty detailed analysis that's probably going to be hard to apply in practical situations.
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pastprime
Jul 23, 2007, 3:16 PM
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The idea is that if one piece pulls in a multi piece anchor or series of pro, it increases the force on the remaining pieces beyond the force that would have been applied had the failing piece not been there at all.
The reason it matters is that, if it were true, which it isn't, but that won't stop the clueless from adamantly repeating what they heard from somebody somewhere once; then when one is faced with a choice between placing a piece that might concievably fail, and not placing anything at all, one would have to stress and fuss and get all confused and bewildered, knowing that if the piece holds, you are better off, but if it fails, you are worse off.
Now let's all argue and do a lot of math and explain our convoluted reasoning, and completely ignore the results of drop testing that has been done, and reported, many times.
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moose_droppings
Jul 23, 2007, 3:48 PM
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j_ung wrote: I think what sterlingjim is getting at -- and he has a drop tower -- is that this isn't necessarily the case. And given I've never met any of you, I'm more likely to trust what the guy with the drop tower says.
No debating that IMO.
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sterlingjim
Jul 23, 2007, 4:11 PM
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pastprime wrote: The idea is that if one piece pulls in a multi piece anchor or series of pro, it increases the force on the remaining pieces beyond the force that would have been applied had the failing piece not been there at all. The reason it matters is that, if it were true, which it isn't, but that won't stop the clueless from adamantly repeating what they heard from somebody somewhere once; then when one is faced with a choice between placing a piece that might concievably fail, and not placing anything at all, one would have to stress and fuss and get all confused and bewildered, knowing that if the piece holds, you are better off, but if it fails, you are worse off.
Excellent post.
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boymeetsrock
Jul 23, 2007, 4:15 PM
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Please explain or refference why shock loading is a myth then.
-Boy
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cracklover
Jul 23, 2007, 4:59 PM
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It's quite similar to microfractures, in the sense that yes, they both exist, but they don't have the effect, or necessarily show up, where people seemed to think they would.
GO
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rgold
Jul 23, 2007, 5:21 PM
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I agree with Jim that shock loading is an undefined term. Although defining it as exceeding some particular threshold value of df/dt (sometimes called the jerk---I don't think, as claimed above, that impulse is the appropriate quantity) would be essential for mathematical modeling and certain types of testing, it won't be of any direct use to climbers to have such a distinction.
Nonetheless, it is clear what shock loading has always meant when used in climbing contexts: a shock load has always been the load imposed on an anchor by the arresting a free fall of some sort, and in this sense, there is nothing remotely mythical about the concept. According to this reasonable working definition, any time something drops and is stopped, there is a shock load to the arresting system.
Of course, once you have an actual definition like this, you have to give up the mythical uniform condemnation of all shock loads as "bad." A shock load is only "bad" if it breaks some part of the system or is the direct cause of an injury to a climber, and in this sense is neither better nor worse than any other kind of load.
However, system performance under shock loads (i.e. free-fall impacts) may not be the same as if the system is loaded slowly to the same maximum level. For example, there is some preliminary evidence that shock-loaded dynamic ropes behave like ideal Hooke's law springs (during the loading phase), but the same ropes, if slow-pulled to the same maximum tension, exhibit a load-elongation curve that is better modeled with a quadratic equation. In the sliding-X testing, opposite results on equalization have been obtained (by different testers) from slow-loading and shock loading tests. One reason for the discrepancies that have emerged to date may be differences in the role of friction, both within the rope itself and in the system, for the two types of loading (and it is here that the threshold value of the jerk might actually become definitive).
I think it is fair to say that the current characterization of shock loading as a "myth" traces its ancestry to Sterlingjim's tests about the effect of the failure of an anchor leg in a two-point anchor rigged with a sliding X. In these tests, after the simulated failure of an anchor point, the elongation in the anchor rigging extended the fall of the weight, resulting in what I think should be called a potentially higher shock load to the remaining anchor point (whether or not an anchor point fails, the system was shock loaded). The tests revealed no major increase in the expected load to the second anchor point after the first anchor point had failed, and it is this result that has been interpreted as busting the "myth" of shock loading.
Personally, and with all due respect to and gratitude for the amount of thought and work that went into the testing, work done by Sterlingjim and others on a pro bono basis with the expectation of nothing but hassles in the discussions to come, I think the tests themselves may have been misleading, because the amount of rope in the system was relatively large compared to the anchor extension. The result of that configuration is that the anchor point failure resulted in a very small increase in fall factor, which therefore should not be expected to produce much beyond the expected effect in the remaining anchor.
In a climbing situation in which gear failure produces anchor extension, the belayer takes a short fall whose energy must be absorbed by the tie-in. Even forgetting entirely about the load imposed by the falling leader, the ratio of anchor extension to tie-in length determines a fall factor for the belayer that could be high and so could result in a significant load increase on the remaining anchors.
Until it is possible to do anchor extenstion tests involving a belay weight tied in realistically close to the anchor as well as a falling weight, I'd be cautious about assuming anchor extension has a negligible effect on the load on the remaining pieces. This means that equalette users would be wise to minimize the potential effects of anchor extensions by making their tie-in to the power point as long as is feasible. (This is usually easy to do. For example, a three-foot tie-in to an anchor that has a potential three-inch extension gives rise to a tie-in fall factor less than 0.1.)
The most important single thing to do to keep potential small anchor extensions from being a problem is to use the climbing rope to tie in to the power point. Using nylon or, even worse, spectra slings or cords would significantly magnify the effects of the tie-in fall factor and could, in principle, result in extremely high loads if the anchor extends.
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microbarn
Jul 23, 2007, 5:24 PM
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pastprime wrote: The idea is that if one piece pulls in a multi piece anchor or series of pro, it increases the force on the remaining pieces beyond the force that would have been applied had the failing piece not been there at all. The reason it matters is that, if it were true, which it isn't, but that won't stop the clueless from adamantly repeating what they heard from somebody somewhere once; then when one is faced with a choice between placing a piece that might concievably fail, and not placing anything at all, one would have to stress and fuss and get all confused and bewildered, knowing that if the piece holds, you are better off, but if it fails, you are worse off. Now let's all argue and do a lot of math and explain our convoluted reasoning, and completely ignore the results of drop testing that has been done, and reported, many times.
If this is the direction you are going with shock loading, then I will have to agree with you. My earlier post was not thinking in this direction.
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coastal_climber
Jul 23, 2007, 5:29 PM
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microbarn wrote: pastprime wrote: The idea is that if one piece pulls in a multi piece anchor or series of pro, it increases the force on the remaining pieces beyond the force that would have been applied had the failing piece not been there at all. The reason it matters is that, if it were true, which it isn't, but that won't stop the clueless from adamantly repeating what they heard from somebody somewhere once; then when one is faced with a choice between placing a piece that might concievably fail, and not placing anything at all, one would have to stress and fuss and get all confused and bewildered, knowing that if the piece holds, you are better off, but if it fails, you are worse off. Now let's all argue and do a lot of math and explain our convoluted reasoning, and completely ignore the results of drop testing that has been done, and reported, many times. If this is the direction you are going with shock loading, then I will have to agree with you. My earlier post was not thinking in this direction.
Yes, because I've read in the book by Craig Leubben that forces applied slowly are more severe than than falls onto a piece(s).
>Cam
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weiman
Jul 23, 2007, 5:34 PM
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This is complete baloney. When one piece in a multi piece anchor fails the failing piece have picked up some of the energy from the load (climber) before it fails REDUCING the load on the remaining pieces. This is fundamental dynamic mechanics.
The above reasoning assumes the anchor is properly designed. For a poorly designed anchor (e.g. a two piece anchor with a large angle between the two arms) the addition of the second piece could add a huge extra load on the first piece with or without a failure of any piece.
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rgold
Jul 23, 2007, 6:30 PM
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weiman wrote: This is complete baloney. When one piece in a multi piece anchor fails the failing piece have picked up some of the energy from the load (climber) before it fails REDUCING the load on the remaining pieces. This is fundamental dynamic mechanics.
I think it prudent to back off from expressions of total certitude. I suspect that whatever "fundamental dynamic mechanics" suggests a load reduction doesn't also contemplate that the failure of a component produces a situation (in this case a longer fall distance) which would, even before failure, produce higher loads. Moreover, the discussion suffers from a lack of precision about what loads are being reduced or, put another way, how to recognize that a given load is less than "expected."
There are two questions here; the first is how much fall energy is "picked up" by the failing piece, but a more important one is whether the "pick up" mechanism allows for a decrease in peak load on the remaining anchors. In order to avoid the imprecision alluded to above, the decrease referred to here is a decrease from the load the remaining anchors would have felt if the piece that failed hadn't been there at all.
There have been debates on these issues in internet forums for about as long as their have been forums, including contributions by folks with alot of technical expertise. The issues have been clearly explained in multiple places; in brief, the answer to the second question comes down to whether or not (or how much) the rope can recover in the instant between the failure of one piece and the loading of the rest of the anchor.
Ken Cline gave a shock wave argument years ago on rec.climbing that suggested ropes should be able to recover. Recently, drop tests by Attaway et. al. seem to confirm that some recovery is indeed possible, but these test may not apply to the configuration of an extending anchor.
If I remember correctly, there were what I think should be called hints in Sterlingjim's tests that failed pieces had reduced expected anchor loads. Still, all things considered, I'd say we're a ways from the kind of certitude that would justify a finding of baloney just yet.
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weiman
Jul 23, 2007, 6:50 PM
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If the multi piece anchor is balanced then you will not get any significant additional fall distance by adding an extra piece. If the achor is not balanced then this becomes a completely different discussion.
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scrambled_legs
Jul 23, 2007, 9:13 PM
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rgold wrote: Ken Cline gave a shock wave argument years ago on rec.climbing that suggested ropes should be able to recover. Recently, drop tests by Attaway et. al. seem to confirm that some recovery is indeed possible, but these test may not apply to the configuration of an extending anchor.
So do ropes recover but not in the span of a sling extension or did the tests not take into account the force absorbed by the failed piece? Not sure what you are you referring to?
Thanks for all the info by the way. It's easy to be mislead when you see test results from a lab. I've always thought shockloading from a sling extension didn't make sense but never factored in the belayer component when considering it.
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weiman
Jul 23, 2007, 10:31 PM
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Well, balanced or equalized means the same thing in my mind. You can easily build an anchor that is equalized, in one direction, and at the same time doesn't extend. For most belay situations I would rather have an anchor that doesn't extend that one that is equalized in any direction. Also, I don't think it's critical to have all the pieces taking an equal amount of the load. The primary reason for using multiple pieces is not because one single piece cannot handle the entire force. Each piece should be strong enough to handle all potential load and then you use multiple pieces in order to make the anchor safe-fail. Note that the basic principle of all safe design in any area where a failure may be fatal the entire system must be either fail-safe (strong enough so that it cannot fail, (a single rope system is a good example here) or it must be safe-fail (the system must not fail even if any component fails (an anchor with multiple pieces).
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scrappydoo
Jul 23, 2007, 10:46 PM
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Shock loading is very real, but it's importance is situation specific.
Taking a factor 1.5 fall on a daisy clipped to an anchor will produce a shockload force on the daisy and the anchor (and your body). Repeated testing has shown that spectra daisys (very little stretch) fail at significantly lower peak forces than their stated breaking strengths because breaking strengths on climbing gear are determined by slow-pulling it to failure (except ropes).
A 20kN nylon runner will hold significantly higher dynamic forces before failing than a 20kN spectra/dyneema runner exactly because the nylon runner will stretch more, decreasing the amount of force applied per unit time (effectively lowering the height of the force curve by extending it out).
This is the whole reason we use dynamic climbing ropes rather than static climbing ropes-- to increase the amount of time the rope takes to decelerate the load, which decreases the maximum force the rope (and gear, and your body) is subjected to at any one period of time.
Throwing a rope and real-world friction into the equation (climbing situations) decreases a potential shockload to any part of the system quite significantly. However, a piece failure on a loaded multi-point dynamically equalized anchor using dyneema/spectra material (and clipped into directly) will produce a shockload (significantly higher temporary force on each remaining piece) than if the blown piece hadn't been incorporated at all.
This is still true with any materials in this situation and using a rope: the 'shockload' process still happens, but the materials, friction, and rope reduce the peak force so much that it becomes negligible compared to the baseline.
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