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blondgecko
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Nov 26, 2008, 12:29 AM
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So, reading John Long's "Equalette" thread up in general, in particular the people mentioning that they usually make their anchors with the rope anyway, got me to thinking. How much does dynamic vs static rope affect the directionality-dependence problem in a classic "powerpoint" or cordelette-style anchor? My gut told me it'd be quite a bit.
So, anyone who's read the "improved sliding X" thread knows the problem. A cordelette, lacking any real stretch, invariably puts all or most of the weight on only one arm as soon as the load goes even a little off-axis. What happens when you add stretch?
I did a few preliminary calcs:
Numbers:
Young's modulus of rope: 0.6 GPa
Cross-sectional area per arm: 120 mm2 (about the area of a doubled over rope)
Geometry: 3 placements, spaced in a straight line 0.5m apart. Powerpoint initially 1.5m directly below the middle placement.
I went and closed the spreadsheet I used without saving it, but here's my general findings:
Stretching the powerpoint straight down by 5cm yielded a load of about 2.5kN in the middle leg, and a little over 2kN in each of the outer legs (being longer, their spring constant, modulus*area/length, is lower). Keeping that 5cm down and shifting over to one side by 10cm (about a 5 degree deflection; note that I was too lazy to do the calcs to keep the total net force of the fall constant) increased the force on the middle leg about 50%, doubled the force on the opposite side leg, and reduced the force on the near-side to around 0.5kN.
This might sound bad, but it's a darn site better than what you'd get on a static set-up, where 100% of the force would be on the opposite leg.
Worth doing some real-world tests? Or has it already been done? And would these results carry over into further improvements in equalization in sliding configurations? Why don't we use dynamic material for cordelettes, anyway?
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adatesman
Nov 26, 2008, 12:34 AM
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blondgecko
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Nov 26, 2008, 2:08 AM
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Cool! I'll look forward to seeing what comes out of it.
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philbox
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Jan 30, 2009, 12:55 AM
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Make your own strain guages.
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johnwesely
Jan 30, 2009, 1:31 AM
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I can't wait to see. I am a big fan of all of your pull test shenanigans.
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adatesman
Jan 30, 2009, 1:31 AM
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philbox
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Jan 30, 2009, 1:59 AM
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adatesman wrote: philbox wrote: Make your own strain guages. Um, if you mean potting the strain gage into a chunk of steel, sure (although I'm probably too lazy to do it). But I was actually short the strain gage indicator, which does the signal conditioning, sampling, etc., and is somewhat less simple a thing to build. Fortunately thanks to Ebay and a very understanding wife I almost have everything together and debugged. Supposedly I'll be picking up the test weight tomorrow (waiting on directions), at which point I just need to make arrangements for the shipping of the spool of cord. 
I've got 4 of those strain guage indicators, plus the strain guages plus the computer program to run them. I'm just waiting for one of my mates to finish his thesis and then we will start to dynamically test various anchor configurations. This set up has been sitting here for a couple of years since another one of my mates died. I ended up with the setup cos' it was my idea. I can't wait to get out to the cliff and start testing.
We've already done some preliminary testing on a 3 point anchor in a vertical crack before Simon died. Very fun watching others taking falls and then checking out the graphs. Highly illuminating.
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philbox
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Jan 30, 2009, 2:02 AM
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What I am short of is a reasonable facsimile of the human body as a test dummy. I aren't at all happy to simply use a metal test weight. I want to do the testing with something that resembles the human body with all its spooginess. That or simply convince someone to do some factor two falls for me, ouch.
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adatesman
Jan 30, 2009, 2:58 AM
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philbox
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Jan 30, 2009, 3:13 AM
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PM to come.
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adatesman
Jan 30, 2009, 3:36 AM
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robdotcalm
Jan 30, 2009, 4:06 AM
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Well, I am serious. Historical note. When Craig Luebben was designing the BigBro as a senior honors project, I was on the faculty committee overseeing it. He suggested, I fall on it for some realistic testing. I told him he flunked. Anyway, he settled for dropping 75 lb. sandbags, which have some spoonginess. However, I think a dead calf would be closer in spoonginess to a live human climber.
Gratias et valete bene!
RobertusPunctumPacificus
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hafilax
Jan 30, 2009, 8:30 PM
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The load doesn't even need to go off axis. You just need the arms to be of different lengths and in that case the result is independent of the Young's Modulus since the spring constants are inversely proportional to the length (I'm picturing a vertical crack scenario where the arms are all roughly parallel).
I don't think a reasonable experiment could be made since it's an ill defined problem. There are an infinite number of setups and force directions. That's not to say that your intuition isn't right and maybe dynamic chord would be good for building anchors with. I've heard of more than one person cutting an old twin or double for just that purpose or for a Purcell Prussik.
The benefits will be partially defeated at hanging belays.
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acorneau
Jan 30, 2009, 9:36 PM
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hafilax wrote: That's not to say that your intuition isn't right and maybe dynamic chord would be good for building anchors with. I've heard of more than one person cutting an old twin or double for just that purpose or for a Purcell Prussik.
BlueWater makes a 6.5mm, 7mm, and 8mm dynamic accessory cord specifically for that:
http://bluewaterropes.com/...ord&CategoryKey=
The other rope manufacturers may as well.
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philbox
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Jan 30, 2009, 11:18 PM
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What would be good is to do some modelling to determine if it is possible to create a metal weight that has an added spring that aproximates the spooginess of the human body. This way you can get results that are based on a predetermined set of values as far as the crash test dummy/weight goes.
I just think that the standard of letting an 80kg hard weight fall for testing is not accurately giving us real world results. The human body is spoogy and that should be reflected in how we test. The tests should allow us to see exactly what happens in the real world. The hard weights do not do this.
I reckon a spring and a shock absorber within the system but at the end of the rope and connected to the drop weight may be the answer. Just guessing though. Once Rodney and I start to develop the system further we will know better.
I just got word that Rodney now has lotsa free time to devote to the project of continuing to develop the dynamic load testing system. Woohoo, it's been a couple of years on the backburner.
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hafilax
Jan 30, 2009, 11:42 PM
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How about a CNC hydraulic system that you could program in the applied force as a function of time?
I think that in most situations the impulse is controlled by the elongation of the rope and that the effect of the compression of the human body would be a minor perturbation. I'm just thinking of the distance over which a rope stretches versus the amount a body could compress. The body could do a few cm's at most. It might have an effect in low stretch material tests however.
Are these experiments scalable? Why does it have to be done at climbing gear fail limits? If the same info can be gleaned from dental floss and cheap strain gauges then why go to the expense and hazard of high forces.
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philbox
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Feb 1, 2009, 7:23 AM
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I really want to see what happens in the real world of actual climbing situations. Lotsa people do tests using static rigs but the rig we have set up is portable and can be used out at the crags to test any scenario up to three point anchors and one power point. In other words we have 4 points from which to take data.
I think we have as close to the perfect test rig to demonstrate what happens in a fall scenario out at the cliffs. Could be good to est any scenario in the new John Long Anchors book.
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rgold
Feb 1, 2009, 5:59 PM
Post #19 of 25
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philbox wrote: What would be good is to do some modelling to determine if it is possible to create a metal weight that has an added spring that aproximates the spooginess of the human body. This way you can get results that are based on a predetermined set of values as far as the crash test dummy/weight goes. I just think that the standard of letting an 80kg hard weight fall for testing is not accurately giving us real world results. The human body is spoogy and that should be reflected in how we test. The tests should allow us to see exactly what happens in the real world. The hard weights do not do this.
I agree with Halifax about this---it is hard to see how the relatively deformations in the human body could absorb significant amounts of fall energy.
It is by now received wisdom that hard weights give results that are too high, but I wonder whether anyone has actually tested this assertion side by side with a human subject. I suspect the CAI group has some knowledge about this, since they did extensive testing with falling weights and at least some with human subjects.
It seems conceivable that lower maximum impact values might result from the fact that a human body doesn't stop all at once. Perhaps a torso weight with appropriately massive chains for arms and legs might give more realistic results (if it turns out that the hard weight really isn't realistic). But in such a model the chains flop around without any muscular resistance and so some work done moving resisting limbs would be lost.
There are a number of anchor set-ups I'd love to see tested. It should be clear, especially after Jim Titt's superb account of the forces generated by belay devices, that dynamic testing, as opposed to slow pull testing, is essential.
1. We need a proper test of the effect of extension in sliding systems, since the original tests seem very unlikely to reflect what happens in a belayed factor-2 fall system. This is absolutely critical to the evaluation of such systems.
2. With regard to extension, it would be nice to know whether the extraction of an anchor piece reduces the subsequent load to the anchor by the work done stretching the rope up to the extraction tension. This depends on whether the rope has the chance to "snap back" or not, something Attaway has shown does happen when a protection piece pulls. In the case of and extending belay anchor, it is not at all clear whether there will be an analogous "snap back" opportunity.
3. We need to find out whether three-point sliding systems equalize any better than a tied cordelette under dynamic impact load, or whether the predicted friction in sliding systems negates their theoretical equalizing abilities.
4. Tied cordelette systems obviously respond poorly to off-axis loads. We need to find out if significant off-axis loading occurs for factor-2 falls that are to one side of the anchor.
5. Karl Baba proposed, on Super Topo, using screamers with a tied cordelette to produce full equalization under impact loads on three-point systems. This seems quite plausible in principle, and ought, I think, to be tested, especially if the sliding systems don't perform.
6. I have suggested that the problems in load distribution that occur when tied cordelette arm lengths are very different might be mitigated by using the combination of a dynamic cordelette and low-stretch slings on the most distant anchors, so that the dynamic arms are all approximately the same length. If this worked, it would be a practical low-tech solution to the unequal load distribution when anchor points are arranged in a vertical line.
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adatesman
Feb 1, 2009, 8:30 PM
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philbox
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Feb 2, 2009, 7:28 AM
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Rodney is free not this weekend butthe weekend after and the one after that. Beyond tha I am able to devote some whole weeks to this proj. Our first order of business is to get the test gear up and running again. After that we will consider what is the most apropriate use to which we can put the gear.
I have always been very interested in the extension question and equalisation of sliding x configurations. Dynamically testing these is high on my priority list.
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JimTitt
Feb 2, 2009, 8:57 AM
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The problems with not using a solid dummy or adding a (another) "sponginess factor" is that a) the results are not easily compared to other drop tests and b) the sponginess factor is already included in most drop testing such as the tests on fall arrest systems and by the military.
You can look at http://iatselocalone.org/...ultiplier%20Test.pdf for a good exmple with some useful data.
For a real in-depth review of the history of various dummies and humans in drop testing there is http://www.hse.gov.uk/...df/2002/crr02411.pdf which should keep you busy for a while!
Jim
(This post was edited by JimTitt on Feb 2, 2009, 2:21 PM)
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knudenoggin
Feb 4, 2009, 6:45 AM
Post #23 of 25
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rgold wrote: It is by now received wisdom that hard weights give results that are too high, but I wonder whether anyone has actually tested this assertion side by side with a human subject. I suspect the CAI group has some knowledge about this, since they did extensive testing with falling weights and at least some with human subjects.
folks to take some deliberate drops with a system with measuring
devices in place, and then to compare that to values got with a solid
mass? (Okay, so volunteerism might be low, but we could *vote*
people in. I know I've been making a mental list of posters whom
I'd like to see dropped, though until now I hadn't been thinking of
using a rope.  )
Or, someone could check w/CAI.
motto: several wknds in the laboratory can save precious minutes
from research!
In reply to: 1. We need a proper test of the effect of extension in sliding systems, since the original tests seem very unlikely to reflect what happens in a belayed factor-2 fall system.
and not merely that of the falling climber?
In reply to: 3. We need to find out whether three-point sliding systems equalize any better than a tied cordelette under dynamic impact load, or whether the predicted friction in sliding systems negates their theoretical equalizing abilities.
Craig Connally asserted--and it seems reasonable, to me--that the
least friction comes with metal on HMPE (translated to a 'biner on a
Dyneema/Spectra sling). And the structure that uses this well is the
ELET--no twists around the metal, just the V of material for whatever
given angle it assumes. ELETs equalize two anchors, so naturally
expand to four rather than three; but one could make an imbalanced
3-point anchor with one pair of anchors ELET'd into a master ELET
--after all, in any single-point failure there will be 50% of force
on each surviving anchor anyway, ideally, so starting off 50-25-25
vs. 33-33-33 shouldn't be a big deal.
In reply to: 4. Tied cordelette systems obviously respond poorly to off-axis loads. We need to find out if significant off-axis loading occurs for factor-2 falls that are to one side of the anchor.
In reply to: 6. I have suggested that the problems in load distribution that occur when tied cordelette arm lengths are very different might be mitigated by using the combination of a dynamic cordelette and low-stretch slings on the most distant anchors, so that the dynamic arms are all approximately the same length. If this worked, it would be a practical low-tech solution to the unequal load distribution when anchor points are arranged in a vertical line.
Although if the ELET gives the needed equalization, it should be
simpler, and not have to toss away any of the dynamic inches.
*kN*
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knudenoggin
Feb 4, 2009, 7:14 AM
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acorneau wrote: hafilax wrote: That's not to say that your intuition isn't right and maybe dynamic c[]ord would be good for building anchors with. I've heard of more than one person cutting an old twin or double for just that purpose or for a Purcell Prus[]ik. BlueWater makes a 6.5mm, 7mm, and 8mm dynamic accessory cord specifically for that: http://bluewaterropes.com/...ord&CategoryKey=
Hmmm, "will hold a higher load before slippage" doesn't sound all so
shock-absorbing. And as far as building anchors goes, BlueWater goes in
the opposite direction re elasticity, with their 7mm
In reply to: 7mm cordelette cord is manufactured with a polyester sheath with a nylon core. Designed specifically for use as a cordelette it has less stretch and higher strength than traditional accessory cords.
while causing greater force by reduced elasticity--the body and
anchors will prefer lower forces even via "weaker" cordage.
*kN*
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kingbiscuit
Apr 7, 2009, 5:02 AM
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Any progress on this?
I am anxiously awaiting.
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