Is Shock loading a myth?

I don’t understand the actionable takeaway...

Al Pine wrote: I don’t understand the actionable takeaway...

I think it is more like a myth busters episode. We simply now know that we arent going to explode if we are subjected to a high fall factor at the anchor. I think the fact that we don't have any major takeaways is a good thing. If we did, then the results would have been very different, and probably a lot scarier, and Jenks may be in the hospital with a broken back.

My gripe with the shock loading is that the term doesn't mean anything anymore.

Two people will talk about rope jumps with a dynamic rope and climbing above the anchor while tether with a PAS as shock-loading, implying they're remotely related.

Ma Ja wrote:

I think it is more like a myth busters episode. We simply now know that we arent going to explode if we are subjected to a high fall factor at the anchor. 

please don't jump to this conclusion, as this is not at all what the video shows. all that is shown is the maximum loads for the specific scenarios they tested. this does not mean that you cannot generate higher forces that will harm the human body in other rope systems.

i find this article interesting because it relates potential impact forces to the various parts of the human body (obviously more geared towards industrial rope access, but you get the idea): https://www.hse.gov.uk/research/hsl_pdf/2003/hsl03-09.pdf

curt86iroc wrote:

a shock load is defined as an impulse, which is a function of force and time (ex. the area under the force/time curve). 

so yea, the video does not show an impulse (which has the units of newton*sec). it shows measured loads, which is not the same thing...

Source for this specific definition of "shock load"?

Kyle Tarry wrote:

Source for this specific definition of "shock load"?

same place i got the definition for gravity...high school physics... :)

So here is my question about the sliding x. Used as pro on lead. You find two little placements near each other that take little brass nuts. At best they hold 4kn each. You are looking at a fall that may have 5-6kn potential. Quick draw to each? Ponytail em together? Sliding X? Willl there be distributed force in each method enough to hold? Will some force be dissipated by the upper quickdraw blowing out? Should I just down climb and let someone else try? Slam in a pin?

curt86iroc wrote:

same place i got the definition for gravity...high school physics... :)

but seriously, if we need to ask this question...

There's no need to be condescending.  If it's taught in high school physics, it should be pretty easy to find a good reference.

I have been able to find several references on Google Books which imply that shock load is a force, not an impact.  For example:
https://www.google.com/books/edition/Formulas_for_Mechanical_and_Structural_S/CTJMUt_7KnYC?hl=en&gbpv=1&pg=PA1&printsec=frontcover&bsq=shock%20load

When speaking of a shock load, engineers usually have in mind a load that is of a large magnitude, but with a very short duration.

Of course, I understand that lots of different fields use different terminology, or different definitions for the same terminology, so I'm very willing to accept that in another field "shock load" may specifically refer to impulse alone.  I'm just looking for more information on that, as it is not the convention that I'm used to.

Ian Bales wrote:

Lol seriously its great that you remember a definition, but it's clear you don't understand what's going on here. What's important to study is in fact the maximum force during a shock load situation. Impulse doesn't break things, force does (or more specifically, pressure). So what's happening during a shock load situation is that you have an object with momentum (falling human body) that is shock loaded (impulse happens in a short amount of time) leading to a higher force. Impulse = F*T, small T means large F. It is true that if you calculate the impulse and measure time you could calculate the maximum force on the anchor, but you could also just measure the force - there's no need to measure both.

i've addressed all your concerns in my previous posts. i've agreed to several of your points already AND linked to an article that reviews impulses to the human body and possible injuries. 

and yes, force does break things...but the time over which that force is applied also matters. think about change in momentum and acceleration (jerk).

Kyle Tarry wrote:

There's no need to be condescending.  If it's taught in high school physics, it should be pretty easy to find a good reference.

I have been able to find several references on Google Books which imply that shock load is a force, not an impact.  For example:
https://www.google.com/books/edition/Formulas_for_Mechanical_and_Structural_S/CTJMUt_7KnYC?hl=en&gbpv=1&pg=PA1&printsec=frontcover&bsq=shock%20load

Of course, I understand that lots of different fields use different terminology, or different definitions for the same terminology, so I'm very willing to accept that in another field "shock load" may specifically refer to impulse alone.  I'm just looking for more information on that, as it is not the convention that I'm used to.

fair point. being stuck inside is making me surly. my apologies..

​http://freeit.free.fr/Structure%20Engineering%20HandBook/21.pdf​​​

Kyle Tarry wrote:

Source for this specific definition of "shock load"?

Just look up mechanical shock, impact and impulse on Wikepedia. All is explained and yes, the video shows the peak force, not shock anything. Incidentally I'd want to see some specs on the strain guages to be convinced they are actually recording the true peak force, most of that type don't as they are filtering at a brutal level to make then usable for weighing stuff, the lifting one I have is sampling at 50hz but logging every 10th second. My other two sample faster AND log vastly faster at 4800 per sec and normally you want to then produce a plot to get near the true peak. Or buy a better strain guage!

That is a good text (it came up during my search too).  However, I didn't see anywhere in it that specifically defined "shock load" as an impulse (force-time units).  I very well could have missed it though, is there a particular section that clarifies that?

Note that figure 21.2 illustrates my misgivings about trying to call a shock load specifically as an impulse value.  They show 2 different scenarios with the same kinetic energy absorbed, but the peak force differs greatly due to the deflection of the beam spreading out the force application over time.  If we call shock load the area under the curve, as you suggest, these two scenarios would have the same "shock load", which seems like the exact opposite of what we want; we want to identify the fact that the peak force (and rate of force application/removal, which is the really important too) is significantly higher for case 2.  Depending on an impulse number to indicate this will not work, you need to look at peak force and application time, as distinct entities.

To be clear, I am not disputing that the time component isn't relevant; it's super important, because if the time is very long and the load application is very gradual, it's not "shock loading" at all.  What I am disputing is the specific allegation that shock loading is defined as impulse and not as force.  I would suggest that "shock load(ing)" doesn't have a strict numerical/unit definition at all, and simply refers to a situation where load gets applied and/or removed quickly, and therefore creates a bunch of other considerations (energy absorption, strain rate sensitivity, vibration, etc.).  The definition at the bottom of that paper seems to agree:

Shock loading: The application of an extremely high force over a very short duration of time. 

Jim Titt wrote:

Just look up mechanical shock, impact and impulse on Wikepedia.

Wikipedia defines "mechanical shock" as "Shock is a vector that has units of an acceleration."  So that disagrees with the claim that it is an impulse, and opens its own can of worms.  In any case, we should be careful about claiming that "mechanical shock" means the exact same thing as "shock load" anyway.

 Incidentally I'd want to see some specs on the strain guages to be convinced they are actually recording the true peak force, most of that type don't as they are filtering at a brutal level to make then usable for weighing stuff, the lifting one I have is sampling at 50hz but logging every 10th second. My other two sample faster AND log vastly faster at 4800 per sec and normally you want to then produce a plot to get near the true peak. Or buy a better strain guage!

I had the same thought.  It's not just about sampling rate too, right, because there is going to be low-pass filtration (software and/or hardware) that's really helpful in a lot of applications, but will potentially chop off the peak if it's sharp enough.  Without measuring this scenario with a high-rate gauge and plotting the results, it's impossible to know what the minimum frequency requirement would be.  On one hand, that could be part of why the forces didn't go up very much when they reduced the amount of rope in the system; on the other hand, the "squishy bag of water" effect of the human body will serve as a low-pass mechanical filter on the whole system, so you might not need as high a rate as you'd expect.  Interesting for sure.

I"ll have a look in the morning at some of my plots and see what I log at.

Jim Titt wrote: I"ll have a look in the morning at some of my plots and see what I log at.

Jim, if you have any data for a human body taking a fall onto a static attachment point in a harness, I'd be really interested to see it!

Most of the tests and conversations around "static" materials and "shock loading" involve testing with rigid bodies (i.e. steel weights), which are appropriate for long falls on dynamic ropes, but not so appropriate for simulating a human taking a short fall onto a static tether.  Of course, nobody wants to volunteer to factor-2 onto a chain for science. :)

Is a high rate of sampling to record the peak impulse applicable when using a dynamic (rope) versus a static connection?

I wouldn't think so in the case of a force absorbing connection but this isn't my area of expertise so am genuinely curious.

Kyle Tarry wrote:

Jim, if you have any data for a human body taking a fall onto a static attachment point in a harness, I'd be really interested to see it!

Most of the tests and conversations around "static" materials and "shock loading" involve testing with rigid bodies (i.e. steel weights), which are appropriate for long falls on dynamic ropes, but not so appropriate for simulating a human taking a short fall onto a static tether.  Of course, nobody wants to volunteer to factor-2 onto a chain for science. :)

We just need some test pigs to volunteer for us.

hillbilly hijinks wrote: Is a high rate of sampling to record the peak impulse applicable when using a dynamic (rope) versus a static connection?

I wouldn't think so in the case of a stretching connection but this isn't my area of expertise so am genuinely curious.

The required sampling rate is a function of the stiffness (or response frequency) of the mechanical system.  Of course, complex systems don't just have one simple resonant frequency, but the concept still applies.  As the system becomes "softer" (such as with a long section of dynamic rope), the frequency (and/or rate at which load is applied/removed) decreases, as does the need for high speed sampling.  In theory, a system with a static tether is much stiffer, but the human body is the dominating factor in that case and it's kinda floppy.

​https://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem​​​

You can get into all kinds of academic shenanigans with this, such as measuring (or simulating) the system with an extremely high sampling frequency, then doing a Fourier transform to identify what the signal looks like in the frequency space, which will indicate what the minimum measurement frequency might be.  This is also one way to identify mechanical resonant frequencies of a structure, such as a building or a machine tool frame, to make sure you don't accidentally excite it in use.

curt86iroc wrote:

please don't jump to this conclusion, as this is not at all what the video shows. all that is shown is the maximum loads for the specific scenarios they tested. this does not mean that you cannot generate higher forces that will harm the human body in other rope systems.

i find this article interesting because it relates potential impact forces to the various parts of the human body (obviously more geared towards industrial rope access, but you get the idea): https://www.hse.gov.uk/research/hsl_pdf/2003/hsl03-09.pdf

Could you propose to Ryan how they could have done a more specific test to gain the knowledge that wasnt gained from his test?

Kyle Tarry wrote:

The sampling rate is a function of the stiffness (or response frequency) of the mechanical system.  Of course, complex systems don't just have one simple resonant frequency, but the concept still applies.  As the system becomes "softer" (such as with a long section of dynamic rope), the frequency (and/or rate at which load is applied/removed) decreases, as does the need for high speed sampling.  In theory, a system with a static tether is much stiffer, but the human body is the dominating factor in that case and it's kinda floppy.

https://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem

You can get into all kinds of academic shenanigans with this, such as measuring (or simulating) the system with an extremely high sampling frequency, then doing a Fourier transform to identify what the signal looks like in the frequency space, which will indicate what the minimum measurement frequency might be.  This is also one way to identify mechanical resonant frequencies of a structure, such as a building or a machine tool frame, to make sure you don't accidentally excite it in use.

So, is the take away that in a climbing application when dynamic connections are used there is no peak impulse being missed in the posted set up and the extra forces when one piece fails are low?

Keep in mind I took very much to heart Ser Titt's example of the belay failing from bad placements etc being just altogether bad...but I think that other than that the take away from the video is that this idea of extension being a deal breaker is over hyped?

And we are then back to Jim's iconic statement of just "get strong placements and tie yourself to them" as being the effective holy grail in anchor building.