kN and micronuts

David Coley wrote: Hi, any chance of a link to those studies, please. I thought I'd read somewhere of some experiments a university did after a climber hit the ground in the USA which concluded the reason the second piece pulled was because the rope was semi-static by then.

Steve57 wrote:This is highly suspect just from a physics point of view. The condition of the rope (% elongation lost) would largely be dependent on the condition of the rope when the first piece pops and proximity of the next piece. If the next piece is 2 inches from the first, the rope won't have the same elongation properties if the next piece was 12 inches down. Btw, agreeing with David, the post David quoted didn't carry over

Steve, just to be clear, you don't provide any physics, just an unsupported opinion, which seems to be wrong---read the summary below.

David, I've only been able to locate one of the references I recall, Measurement of Dynamic Rope System Stiffness in a Sequential Failure for Lead Climbing Falls, J. Marc Beverly, BS-EMS, M-PAS Stephen W. Attaway, PhD, at tinyurl.com/p7oq9cz.

From the summary:

We were able to make accurate measurements of the system stiffness and show that knots play an important role in system stiffness. The figure-8 follow through knot absorbs an equivalent of nearly 1.5 m (5 feet) of rope for the first impact force. After that, the knot is “hardened” and has less absorptive ability.

Except for knot tightening, we did not observe rope hardening in the two impact drop. The rapid unloading after an anchor fails appears to reset the rope to its initial state. Testing on two impact drops showed that failing a piece of climbing protection absorbs energy and reduces the total fall factor. In the example studied, about half of the energy was absorbed on the first impact.

rgold wrote: David, I've only been able to locate one of the references I recall, Measurement of Dynamic Rope System Stiffness in a Sequential Failure for Lead Climbing Falls, J. Marc Beverly, BS-EMS, M-PAS Stephen W. Attaway, PhD, at tinyurl.com/p7oq9cz.

Thanks.
That was a very good read. And not the result I was expecting, I stand corrected.

Shockloading in a scenario where a dynamic rope is involved is a moot point in most scenarios. I did a fair bit of testing on this awhile back (posted the results somewhere on here I think), and found that in just about every case the "shockload" that occurs as one leg of an anchor rips has a (much) lower peak force than the peak force caused by the fall as the remaining leg(s) of the anchor attempt to arrest the fall.

There is, of course, exceptions to this, such as a FF2 directly on the anchor. I am mostly referring to a standard lead fall with a reasonable amount of rope out (e.g. FF 0.75 or lower).

rgold wrote: Steve, just to be clear, you don't provide any physics, just an unsupported opinion, which seems to be wrong---read the summary below. David, I've only been able to locate one of the references I recall, Measurement of Dynamic Rope System Stiffness in a Sequential Failure for Lead Climbing Falls, J. Marc Beverly, BS-EMS, M-PAS Stephen W. Attaway, PhD, at tinyurl.com/p7oq9cz. From the summary: We were able to make accurate measurements of the system stiffness and show that knots play an important role in system stiffness. The figure-8 follow through knot absorbs an equivalent of nearly 1.5 m (5 feet) of rope for the first impact force. After that, the knot is “hardened” and has less absorptive ability. Except for knot tightening, we did not observe rope hardening in the two impact drop. The rapid unloading after an anchor fails appears to reset the rope to its initial state. Testing on two impact drops showed that failing a piece of climbing protection absorbs energy and reduces the total fall factor. In the example studied, about half of the energy was absorbed on the first impact.

Well I read the article and it is addressing a different point than I was. I was hypothesizing about two pieces placed at a nearly identical vertical location, this test is utilizing two pieces placed like you would place them normally, the second significantly lower than the first failure point. Since this thread is about doubling up small pro, not the "unzippering" effect, I believe that my understanding of the forces at play still holds true. Or perhaps I just did not understand the topic of the thread in the first place, that has been known to happen from time to time :P