Shockloading? Don't be so sure
The concern about shock-loading comes from a naive use of the impact force equation.
The latter overestimates forces for the short fall in the scenario
above, mainly because it assumes objects that fall are rigid and fall
without rotation and deformation. Assuming the forces are damped by 10'
of two-strand nylon rope used in the tests below, the impact
force equation predicts 844 lbs maximum force for a 2' fall (after the
blue webbing snaps), and 996 lbs for a 3' fall. The predictions are
more extreme for very static ropes; for 10' of double-strand 1/8" Amsteel Blue (Dyneema) we calculate 2982 lbs for just a 2' fall (using
the published elongation of 0.96% at 30% breaking strength). We're
assuming the major cordage in the system is the rope; including the
sling in the estimates would just lower the prediction of maximum
force, and make the problem non-linear.
People often find pre-existing slings at an anchor, and attach to several slings of unknown age or origin. Very often the pre-existing slings are of different sizes. More and more, peakbaggers don't carry sling material, hoping to use what's already there. However, the most popular sling material, nylon, is quite susceptible to UV, animal damage, rock cuts, and magic flying scissors**. I check a sling end-to-end before I use it, and prefer to use just one good sling. The most important advice here: If you don't trust the slings you find, use your own. Don't bitch about what you find-- you can carry 30' of black 5/8" tubular webbing for 9 ounces. The second important point: if you are worried about an anchor (and you can't make a new one), creep off it on rappel-- it is very easy to put a lot of force on the anchor in the first few feet.
I once put two Amsteel Dyneema slings around a
rock horn, as a rap
anchors for a 35’ rappel down a class 4 slope averaging ~70 degrees.
You could
actually downclimb that section, but I generally considered it safer to
rap. The second sling was another choice as a rap anchor, but was
mainly intended for a long-term exposure test.
(click on any picture to see larger images)
Picture above was from the first ascent; the two slings in this picture were eventually replaced with two new slings, approximately the same positions, but more carefully placed to avoid sharp points.
(click on any picture to see larger images)
In the drone view above, one person is just downclimbing the upper rap section.
The two new eye-to-eye slings were made of Amsteel and were coated with a urethane-based UV blocker (Maxijacket). These new slings replaced an uncoated Amsteel sling that had been left there 34 months*. One sling (1/8” Amsteel, 2500 lb bs) was 16” longer than the other (5/32” Amsteel, 4000 lb breaking strength). I use UV-coated Dyneema-based material because it is much more UV- and critter-chew-resistant than tubular nylon. Quicklinks were at the lower ends through both eyes of each sling; technically the strength of each might be double the breaking strength, but usually there was not enough pull along the frictional horn, to equalize the force. I really intended the thinner sling to be just another exposure test, which I would take down in 3 years; I mentioned that the sling was part of a test on the web page for that mountain. This was a very obscure summit, and I felt I could collect that coated 1/8” sling later.
Then the alternate reality of the climbing community judged me. The peak became a peakbagging goal. A climber claimed this was very dangerous situation, because if people put the rope through both slings, falling rocks could cut the thicker sling, then the thinner sling would fall 16” and shockload, snapping it. Before we discuss this climber's strange view of the world, let’s ask… what are real shock-loading forces likely to be, after a 16” fall?
Tests
First we use the exact same rope I used
for that very rap in the past: 8mm Sterling nylon, semi-static. There
are 10' of double-strand between loadcell (attached to the ceiling
anchor) and my harness.
Here's the force versus time (tops out at ~600 lbs versus the impact force predictions of at least 844 lbs): (click on any picture to see larger images)
The first 24" jump tightens the knot at the harness, absorbing much of the energy of the first fall; falling on a rap device, if you are lucky enough to control the fall, has a similar damping effect. While the 3rd fall is ostensibly 8" higher than the 2nd, you can see from the movie below that the 2nd jump was actually more aggressive.
The deviations from the simple theory are even more profound for hyperstatic ropes. Below is the testing of
dual strands of 3.2mm Dyneema Amsteel blue, with the last fall a
painful 24". We get to 500 lbs, not
the 2982 lbs predicted predicted by the impact force equation. Even
though the measured max force was less in this 2' fall, than it was in
my fall on the nylon ropes, it felt much more jarring. One could argue
that faster sampling might make sharper peaks; but the peaks are very
well-resolved, with 8 points at or above FWHM. In reality, the sampling error is less than 2%
and is less than other sources of uncertainty in the experiment. (Note
this experiment is not strictly comparable to the test with 8mm nylon,
as I was falling on a "no-twist" munter, not a stationary knot; but
that makes it closer to the purported scenario where one creates the
shockload during a rappel.)
The double amsteel cords
have such a low modulus (1/10th of PET), that
this was equivalent to falling on two 9mm PET rope strands. In truth,
my body absorbs a lot of the energy before it gets transmitted to the
anchor.
But I’m only 142 lbs; the people
who worried were about 150lbs, so maybe they could exceed 600 lbs. But the
breaking strength of each side of the thinner Amsteel sling (which was new and
UV-protected at the time) is 2500 lbs. They were safe, even if they decided to
jump backwards off the 70 degree slope. When there is more stretchy rope in the system, the "shock load" may be much less.
And now let’s consider the fantastical
circumstances they
imagined. You should always check an existing sling end-to-end before
it is
trusted (I often write strange things on
tags for slings, to get peoples' attention, and see if they actually remember). I have done tests trying to
cut Amsteel slings with a big sharp rock dropped directly onto them from 5’, 15x, and it was damn hard to arrange a geometry that would sever it. Consider:
in what scenario will
sharp rocks be crashing down and precisely hit and sever the thicker
sling,
while you are setting up to rappel? Do you possibly think that the
crashing
rocks might be a bigger danger than the 35’ rap on down-climbable
terrain? Or would you think, "well, there are rocks crashing down
around me. Instead of hiding behind the horn, I think it's safe
to rap!"
*I broke that uncoated sling at 70% of its original strength; it was definitely faded from our intense Nevada sun. The most idiotic thing is that they took down the coated Amsteel sling that I planned to take down this year, to prevent the unthinkable danger that someone might use it.
**It is very hard to cut Amsteel
Dyneema with normal scissors, but nylon cuts easily. That's why bear
bags are often made of Dyneema. I use special scissors to cut Dyneema.