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Pieces pulling from falls


rgold · · Poughkeepsie, NY · Joined Feb 2008 · Points: 525

Actually, the take-away from the Beverly paper is that because the rope recovers, there is in principle some advantage in having a poor piece that fails as opposed to none in that position, in terms of the peak load to the next piece down.  So only placing pieces that hold may not actually be the best strategy. 

What we really have no way to guess is how much load reduction to the piece below you'll get from an extracted piece.  If the piece pulls easily you won't get any real benefit, and if you expended energy placing it, you might be more likely to fall.  So athough the physics arguments and Beverly's result are interesting and settle a recurring argument, they really have very little to tell us about whether or not to place marginal pro beyond what most people have aways said, namely that it won't hurt. 

climber pat · · Las Cruces, NM · Joined Feb 2006 · Points: 215
rgold wrote:

I thought that's what I said.  "...the stretching of the rope while the failed piece is still in place does absorb fall energy, which makes it possible that the next piece down could experience a lower peak load than it would have experienced without the failed piece above."

You also said "The answer, as far as I know, is that the amount of energy absorbed by a failed piece is tiny and probably insignificant, as very little work is done."  Taken to the extreme 0 energy is absorbed and the rope is unaffected.  Taken to the other extreme all but 1 joule is absorbed. The whole analysis of the situation depends heavily upon how much energy is assumed absorbed by the failing piece.  It can be a lot or a little.  I have heard stories of people who were almost stopped (thought they had stopped) by a piece that then failed.  I have seen piece fail without being able to feel any impact on the rope.

A couple of separated sentences in your original post taken together could allow one to derive that the rope is unexpectedily hardened by pulling a very marginal piece.  I don't believe this is the case (or what you were trying to say) and just tried to point it out.  I failed to be clear previously and I think I am still being unclear.  

climber pat · · Las Cruces, NM · Joined Feb 2006 · Points: 215
Slartibartfast wrote:

1) That's what she said and 2) it is, just much less than we would like to think.

I've always understood that the confusion stems from the difference between actually slowing down and just slower acceleration. It's like when you're accelerating fast in your car and ease off the throttle; you didn't slow down, you're just not speeding up as fast as you would have been. I also understand that I'm probably going to use the wrong term at some point and people will yell at me for that.

When you fall, you're gaining speed with every moment you're in the air. When a piece blows, then sure, it slowed your acceleration for a quick sec, but you're still falling, accelerating, and generating more force. So, say you take a fall that, theoretically, should have generated 12kN onto a piece that blows at 10kN. Now you'll only put 2kN on the piece just below that one, right? Wrong. At the moment that piece blew, you were generating 10kN, and you still have that much force behind you (plus whatever you made between the two pieces). The piece didn't "absorb" 10kN, it just slowed your acceleration for the brief moment it was loaded.

Chaining three 10kN pieces together, for example, doesn't make a 30kN anchor like we imagined in the olden days, just a 10kN anchor that will fail three times. Allegedly, this is why we started equalizing anchors to make them stronger. Then we found out equalization isn't a thing, which is why yer gonna die.

I said nothing about the magnitude of the forces absorbed or the effect on the fall.  Just that I believe that the effect on the rope of pulling the marginal piece should be proportional to the energy absorbed. 

Velocity is the integral of acceleration.  In other words the velocity at a certain point in time is the sum of the acceleration over the duration of the fall.  If the acceleration is lessened for an instant then the velocity is less than it would have been without the lessening of the acceleration from that point on. 

Michael Schneider · · Unknown Hometown · Joined Apr 2014 · Points: 785

Isn't the reduction of forces momentary? and a passing repeating phase sort of thing, where each, successive ( sucky-badly placed)- pulled piece absorbs some energy, then the forces return/gain, as the length of the fall increases?

Marco Velo · · Unknown Hometown · Joined Feb 2017 · Points: 0

There may be another mechanism at work here that we have not accounted for. I once asked an IMFGA guide who happened to have a B.S. in mech. Engineering this same question. He explained that  in a longer fall there is more rope to absorb force and that  this absorption also takes place over a longer interval of time than in the case of a shorter fall. This longer time  interval  adds a complication to the calculation of force on what would be the second piece. My recollection is that the point was  that the longer time nterval lowers the effective force. Perhaps someone with a stronger background in physics can illuminate this a bit. 

rgold · · Poughkeepsie, NY · Joined Feb 2008 · Points: 525
climber pat wrote:

You also said "The answer, as far as I know, is that the amount of energy absorbed by a failed piece is tiny and probably insignificant, as very little work is done."  Taken to the extreme 0 energy is absorbed and the rope is unaffected.  Taken to the other extreme all but 1 joule is absorbed. The whole analysis of the situation depends heavily upon how much energy is assumed absorbed by the failing piece.  It can be a lot or a little.  I have heard stories of people who were almost stopped (thought they had stopped) by a piece that then failed.  I have seen piece fail without being able to feel any impact on the rope.

A couple of separated sentences in your original post taken together could allow one to derive that the rope is unexpectedily hardened by pulling a very marginal piece.  I don't believe this is the case (or what you were trying to say) and just tried to point it out.  I failed to be clear previously and I think I am still being unclear.  

Well, perhaps I wasn't as clear as I could be.  The actual extraction of a piece absorbs almost no fall energy.  What energy is absorbed is absorbed by the rope stretching while the piece is still in place. So there are potentially two sources of fall energy absorbtion when a piece fails: one is the work done extracting the piece, and the other is the work done in stretching the rope.  My comments are that the extraction work is negligible and virtually all the energy absorbtion that does occur is because of work done stretching the rope.  This means that the event corresponding to the the gear failure has an associated energy absorbtion (which is all that you are saying), but that energy absorbtion is not produced by the actual extraction but rather by work done in stretching the rope.

Put another way, I was trying to explain the mechanism for energy absorbtion when a piece fails, and you took my comments as saying there is no energy absorbtion (which, in my defense, I never said).

Brian Abram · · Celo, NC · Joined Oct 2007 · Points: 498

No one who leads unrehearsed routes at their limit places perfect gear every placement, and anyone who claims otherwise is either a full of shit armchair climber or is oblivious. Even superficially good placements occasionally fail. If you fall (sit/rest/take ≠ fall) on gear often enough, you're going to eventually have a piece blow out on you unless you stick to Indian Creek-style splitter crack climbing forever (and even then...) 

Mike Slavens · · Houston, TX · Joined Jan 2009 · Points: 35

To make sure I'm understanding this correctly, whether or not a piece ultimately holds or fails the force on that piece is reduced (if ever so small) by a piece failing above it?  The piece may or may not experience more force than the piece above that did blow, but the fact that a piece above it did blow inherently lessens (once again if ever so mall a reduction) the force on the next piece?

rgold · · Poughkeepsie, NY · Joined Feb 2008 · Points: 525

When people say things like "lessens the force on a piece," the question is less than what?  The lower piece will, in principle, get a smaller load than it would have gotten if the fall had been arrested without a higher piece failing.

Don B · · Gardena (South Bay) · Joined Sep 2017 · Points: 20

Thanks for all the responses. Here are some of the main points I'm getting (I think). Let me know if I'm wrong though.

-The lower pieces will experience less force if the piece(s) above blew than if it would have if there was no piece above. Meaning the difference in the force it receives in the two scenarios is relative to itself.

-Lower pieces will receive a greater force than the piece above it.

-How much force the piece below a blown one is also affect by how much the rope has restored itself between pieces. "Hardness"

It seems incrediblethat in the experience I was told that after all those pieces blew that one finally held. The piece that finally held must have been really bomber because the fall was so high.

Andrew Krajnik · · Plainfield, IL · Joined Jul 2016 · Points: 277
Don B wrote:

Thanks for all the responses. Here are some of the main points I'm getting (I think). Let me know if I'm wrong though.

-The lower pieces will experience less force if the piece(s) above blew than if it would have if there was no piece above. Meaning the difference in the force it receives in the two scenarios is relative to itself.

-Lower pieces will receive a greater force than the piece above it.

-How much force the piece below a blown one is also affect by how much the rope has restored itself between pieces. "Hardness"

It seems incrediblethat in the experience I was told that after all those pieces blew that one finally held. The piece that finally held must have been really bomber because the fall was so high.

You can't really make that second one as a blanket statement. It's likely true in a lot of scenarios, but not in all. As in so many physics and engineering questions, "it depends". As someone stated earlier, if the upper piece nearly arrested the fall, meaning the falling climber nearly came to a stop, then it's entirely possible that the lower piece does see a lower load than the piece that blew. However, as also previously stated, marginal pieces or those that have shifted might fail at a very low load, and with very little work, so it's likely that in the majority of cases the lower piece does see a higher absolute load than the one that failed.

The most imporant takeaway is your first -- that the lower piece will see a lower load relative to what it would have seen otherwise. As rgold stated, the upper piece "doesn't hurt".

Of course, if you expended a significant amount of energy placing that sketchy piece, instead of climbing higher to find a better placement, it could still contribute to the fact that you fell in the first place.

GabeO · · New Haven, CT · Joined May 2006 · Points: 306
Don B wrote:

It seems incrediblethat in the experience I was told that after all those pieces blew that one finally held. The piece that finally held must have been really bomber because the fall was so high.

No.  A slightly less than factor 1 fall (meaning a fall so severe you'd almost hit the ground on a single pitch route) would generate on the order of 8-9 KN.  A well placed medium sized nut in good rock should be expected to hold more than 10 KN.  So no, it doesn't require extraordinary gear, or extraordinary placements to hold a severe fall. 

Cheers,

GO

ckersch · · Unknown Hometown · Joined Apr 2013 · Points: 150
Don B wrote:

Thanks for all the responses. Here are some of the main points I'm getting (I think). Let me know if I'm wrong though.

-The lower pieces will experience less force if the piece(s) above blew than if it would have if there was no piece above. Meaning the difference in the force it receives in the two scenarios is relative to itself.

-Lower pieces will receive a greater force than the piece above it.

-How much force the piece below a blown one is also affect by how much the rope has restored itself between pieces. "Hardness"

It seems incrediblethat in the experience I was told that after all those pieces blew that one finally held. The piece that finally held must have been really bomber because the fall was so high.

I'd say the following:

-A piece that blows will reduce your velocity compared to not having that piece

-The velocity reduction is proportional to how much energy is dissipated by the system during the time between when the piece starts to become loaded and when it fails

-You will accelerate between the time your top piece blows and the time the bottom piece becomes loaded to 1G, at which time you will decelerate

-If the net change in velocity due to acceleration from gravity exceeds the net change in velocity due to deceleration from the top piece, you will apply more force to the second piece than you did to the first, regardless of how much the rope recovers

-If the net change in velocity from the deceleration you experience at the first piece exceeds the increase in velocity you experience as you accelerate towards the second piece, and the rope recovers between the two impacts (which elipos's source on the first page indicates is likely), then the top piece might experience more force, though if it failed significantly below its peak load this might not be the case

Lowest case force scenario on the second piece would be a piece that blows at peak loading when it's reduced your fall velocity to close to zero, with enough space between the first and the second piece for the rope to recover. A succession of multiple pieces that do this will progressively reduce the force applied to each lower piece. Highest case force scenario would be a first piece that blows immediately before any fall energy is absorbed.

Will Handy · · Unknown Hometown · Joined Aug 2016 · Points: 0
Marco Velo wrote:

There may be another mechanism at work here that we have not accounted for. I once asked an IMFGA guide who happened to have a B.S. in mech. Engineering this same question. He explained that  in a longer fall there is more rope to absorb force and that  this absorption also takes place over a longer interval of time than in the case of a shorter fall. This longer time  interval  adds a complication to the calculation of force on what would be the second piece. My recollection is that the point was  that the longer time nterval lowers the effective force. Perhaps someone with a stronger background in physics can illuminate this a bit. 

A longer fall does not always mean there is more rope to stretch... that's the whole point behind fall factor. In this case there is exactly the same amount of rope in the system as you pop each subsequent piece of gear (unless your belayer let go!). The only thing that may result in more rope stretch in this scenario as you fall beyond the first piece of gear that popped (other than higher force) is that there is less gear in the wall and therefore less friction in the system so the rope can stretch more easily.

Briggs Lazalde · · Unknown Hometown · Joined Sep 2017 · Points: 0

Don b, you have made my people war with each other... Kill me, for I have nothing left..

Michael Schneider · · Unknown Hometown · Joined Apr 2014 · Points: 785
Briggs Lazalde wrote:

Don b, you have made my people war with each other... Kill me, for I have nothing left..

for making fun of your name, I am sorry and apologize, that is a  -stand up and be counted- offer, that you made on the 'Sweet Deals thread , Thanx again.

I am all for the drift of this one too, but 1st 

and no one else has said this?

please learn to set your 'pro' to protect from  forces from what I call Rope Work. 

That means take a minute to check how upward & outward forces 'work' against the placement, back it up leave another piece if you have any doubt.

Cory Furrow · · San Francisco, CA · Joined Jun 2016 · Points: 25

What about Kinetic and Potential Energy?  Would Potential Energy be reduced if you're below the piece when it pops (Assuming all gear placement is at the same distance from each other?

Ryan Dirks · · Washington, DC · Joined Jul 2012 · Points: 5

Personally I'd be hesitant to make any definitive statements about this topic - it seems highly situational to me.  In theory, when the first piece pulls it should slightly reduces the load on the next piece, since it momentarily decelerates the falling climber.  But what about the effect on the rope?  Does the first impact stretch the rope, thereby increasing the impact force on the piece that holds?  I'm not sure if there are studies on this, but it seems to me that there would be a lot of variables involved - namely how good the first piece is, initial fall distance, distance between pieces, the rope, behavior of belayer, etc.

I also have seen slow motion videos of pieces pulling, and it seems more often than not the rope whips around immediately after it pulls.  This seems likely to increase the odds of compromising the next piece, but again, I have no definitive information.  I do recall hearing of a bad accident a few years ago where a climber pulled the first piece or two, and the next piece (or two?) unclipped themselves from alpine draws.  Again, this seems highly situational.

In my personal experience, I've taken roughly 15 falls on gear, and pulled one piece (a small cam).  The belayer and I felt a slight tug as it pulled, and an instant later I was caught by the next piece.  Maybe if we got a big database of personal stories we might be able to make some conclusions, but in my opinion the subject is too complicated.  Place good gear and it hopefully won't be a problem!

physnchips · · Boulder, CO · Joined Jan 2016 · Points: 0
Cory Furrow wrote:

What about Kinetic and Potential Energy?  Would Potential Energy be reduced if you're below the piece when it pops (Assuming all gear placement is at the same distance from each other?

Gravitational potential energy stays the same, but the overall potential energy can be reduced if the first piece is able to stretch the rope. If the overall potential energy reduces then the next piece has less energy that it has to cancel (the hope is that you'll have no kinetic energy after everything) in order to bring you to a stop. As rgold mentioned, you want your rope to recover from its previous stretch otherwise the you won't have the most significant energy mitigation factor (your rope stretch) once you hit the next piece (in fact, as he mentioned, if you're truly bottomed out and the rope doesn't recover then you're worse off). 

Rather than circlejerking too much, I think the big takeaway is don't rely on marginal pieces. If you're truly screwed placing, and you don't see anything further up then back off and safely get to where you can shake-out, regroup, and make rational decisions for either totally bailing or powering through.

Barry M · · Unknown Hometown · Joined Nov 2017 · Points: 0
Michael Schneider wrote:

and no one else has said this?

please learn to set your 'pro' to protect from  forces from what I call Rope Work. 

That means take a minute to check how upward & outward forces 'work' against the placement, back it up leave another piece if you have any doubt.

+1, having your second come up and saying you had a piece just hanging on the rope means your doing it wrong. 

Guideline #1: Don't be a jerk.

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