Pieces pulling from falls

I met a guy at my local crag who fell on lead on a trad climb in NC. He said 4 pieces pulled before one held resulting in about a 60 ft fall and a SAR heli ride out since they were in the backcountry. I've also read about some similar incidents recently of multiple pieces pulling.  

Does each successive fall generate more force or are all the pieces that get pulled marginal?

Meaning, when you fall on the last pro with a marginal placement and it pulls, when you continue falling and reach the next piece is there a greater amount of force put on that one which would cause even a solid placement to pull? I'm just starting but it seems difficult to 4 marginal placements in a row.

1. Most of these "the piece pulled" stories you hear are almost certainly user-error.  Especially so if you're talking about zippering out a bunch of pro.  This is bad pro that pulls because it was never placed properly to begin with, which is very different than what most would define as marginal pro.

2. As I understand it, the force is reduced by some small amount as you pull a piece, so if you're zippering out a bunch of pieces in succession, in theory the force should be less on each successive piece.  Someone who better understands the physics might be able to explain whether the increased fall distance plays a role here, and how.

Meaning, when you fall on the last pro with a marginal placement and it pulls, when you continue falling and reach the next piece is there a greater amount of force put on that one which would cause even a solid placement to pull? I'm just starting but it seems difficult to 4 marginal placements in a row.

Rather than thinking of it as four separate falls, think of it as one fall to the piece that catches you with the four marginal placements dissipating some of the energy of the fall. Each piece of pro will experience a greater force acting on it than the piece above it, most of the time. You could fall 10ft to a piece that absorbs almost all of your energy before popping, and then fall another 2ft to a bomber piece that catches you. In that scenario the piece that popped could have experienced a greater force than the one that caught you.  

Dylan Barrett wrote:

Rather than thinking of it as four separate falls, think of it as one fall to the piece that catches you with the four marginal placements dissipating some of the energy of the fall. Each piece of pro will experience a greater force acting on it than the piece above it, most of the time. You could fall 10ft to a piece that absorbs almost all of your energy before popping, and then fall another 2ft to a bomber piece that catches you. In that scenario the piece that popped could have experienced a greater force than the one that caught you.  

False, lower pieces will experience less force than the higher pieces that failed.

from here:  http://mra.org/wp-content/uploads/2016/05/Beverly_Sequential_Falls2.pdf

eli poss wrote:

False, lower pieces will experience less force than the higher pieces that failed.

 

from here:  http://mra.org/wp-content/uploads/2016/05/Beverly_Sequential_Falls2.pdf

False, "Geometry, rope length, and anchor strength will cause these results to vary." 

Their experiment was constant with 7kN anchors and set distances between anchors. When you change these variables you will get different results. 

Possible that the lower pieces may have been pulled up when the first piece took the weight causing the lower pieces to get pulled out of placement (also user error).

Don’t go climbing in NC unless your trad game is on point. 

Dylan Barrett wrote:

False, "Geometry, rope length, and anchor strength will cause these results to vary." 

Their experiment was constant with 7kN anchors and set distances between anchors. When you change these variables you will get different results. 

Sure, but that's the best data I could find. Do you have any data that suggests otherwise?

eli poss wrote:

False, lower pieces will experience less force than the higher pieces that failed.

 

from here:  http://mra.org/wp-content/uploads/2016/05/Beverly_Sequential_Falls2.pdf

This is saying that pieces absorb energy when they blow, so a lower piece will experience lower force than it would if the upper piece was not there - a VERY different statement than the lower piece will experience lower force than the higher piece that failed, which was your interpretation.

For example, you fall 3 feet onto a piece and it blows. Then you fall 20 feet onto your next piece. Sure, the low piece experiences slightly lower forces than it would have if the top piece wasn't there. But it also experience much higher forces than the upper piece. Any time that less energy is dissipated by the upper piece than is subsequently accumulated during the fall to the lower piece, the lower piece will experience the higher force.

Dylan Barrett wrote:

False, "Geometry, rope length, and anchor strength will cause these results to vary." 

Their experiment was constant with 7kN anchors and set distances between anchors. When you change these variables you will get different results. 

The study shows that pieces will absorb force when fallen on. That makes sense, rope stretch and force from the piece will slow you down.

Now if your next piece is super far you will still accelerate due to gravity. The lower piece might experience a higher load due to distance (you keep falling) but does not experience more force because your first piece pulled.

Stephen D wrote:

This is saying that pieces absorb energy when they blow, so a lower piece will experience lower force than it would if the upper piece was not there - a VERY different statement than the lower piece will experience lower force than the higher piece that failed, which was your interpretation.

For example, you fall 3 feet onto a piece and it blows. Then you fall 20 feet onto your next piece. Sure, the low piece experiences slightly lower forces than it would have if the top piece wasn't there. But it also experience much higher forces than the upper piece. Any time that less energy is dissipated by the upper piece than is subsequently accumulated during the fall to the lower piece, the lower piece will experience the higher force.

I think you're comparing apples to oranges. I'm talking about a systemic effect of pieces failing and the rope stretching. You're talking about situational differences.

Sure, the next piece might be 20 feet down, but it also might be 5 ft down. Like everything, it depends. My statement was only meant to say that ripping a piece absorbs energy, resulting in a less harsh fall on the next piece, all else equal.

Kyle Tarry wrote:

Except, that's not what you said.  Here's what you said:

Maybe you meant something else, but you clearly stated that the lower pieces see less force than the higher pieces.  This is inaccurate, and there is no debate about that, it's simply wrong.

touche. I worded that response incorrectly. However, the point that I was trying to make stands.

And I'll take the word of somebody with a PhD over a high school physics class. In HS physics classes, they intentionally ignore things to simplify them. A commonly ignored thing in high school physics is friction, but I'm pretty sure that climbers know friction is very important.

The questions are usually framed in a way that is too vague to support any coherent answers.  In particular, the concept of a reduced load to a lower piece usually lacks enough description to know what the load was reduced from.  Here are two questions one could ask.

1. Does ripping a piece absorb fall energy? 

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.  An exception might be a nut or cam tracking through soft rock.  Even then it isn't going to be much.

2. If a piece rips, will the load to the next piece down be less than or more than it would have been if there hadn't been a piece to rip? 

It doesn't seem that extracting the piece absorbs any fall energy to speak of.  But 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.  But this possibility requires that the rope is able to "reset" during the very brief time interval between the failure of the first piece and the loading of the second piece.  If the rope stays tensioned, then the second piece down will experience the same or possibly greater loading than it would have without the failed piece above, depending on how much the rope "hardened" during the first extraction.  This was the subject of some long and, in places, very erudite discussions on the old rec.climbing site.

What the Beverly paper established is that the rope does indeed snap back, and so can begin absorbing energy anew when the second piece is loaded.  This means that  having a bad piece that is extracted can reduce the load to the next piece below what the load would have been without the extracted piece.  How much effect you get depends on how long the bad piece stays in before blowing, as this determines how much rope stretch you'll get to reduce fall energy.

It seems plausible that there is going to be some rope hardening due to extraction, i.e. the rope will not snap all the way back to its previously unhardened state.  In this regard, it is worth noting that half rope clipped alternately completely avoid the hardening phenomenon, to whatever degree it might occur, as the rope doing the catching on the next piece down was never loaded by the extracted piece.

As for the original question about pieces pulling in falls, there isn't much one can say in general.  The placements themselves might not be up to the imposed loads, or some or all of the lower pieces zippered from lower to higher because a (bad) rope path.

rgold wrote:

climber pat wrote:

It seems to me the arguments for rope hardening while pulling the first piece is somewhat dubious if you are also arguing that the pulled piece does not absorb any energy to speak of.   The rope hardening should be accompanied by dissipation of energy.

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."

climber pat wrote:

The rope hardening should be accompanied by dissipation of energy.

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.

Ipso facto, only place pieces that hold.

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. 

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.  

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. 

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?