So sometimes I like looking into and investigating different climbing gear or situations because I'm curious, sometimes people ask me or email me a question, sometimes I see some sketchy stuff out at the cliffs and sometimes something happens out in the field that I end up hearing about and decide to spend some time looking into it to maybe answer some questions, or maybe even end up posing more.
Investigating joining two slings together and how strong they are is a combination of all of the above. I had kind of been wondering about all these skinny slings on the market; I had a student send in a comparison on different methods of joining slings together asking which way was the best; and there was an incident with John Sherman where he had a sling on his anchor break when he used two slings girth-hitched together—luckily no one was hurt. For details on John's incident you'll have to sift through info HERE.
Therefore, all of these things prompted me and my crack crew of QA Engineering guys to throw a quick list of experiments together and do some testing. Please note: This is NOT intended as a in-depth investigation into John's recent incident, rather just as information related to the joining of two slings together in general. So grab yourself a beverage of choice, because this one could get a bit long-winded.
INITIAL THOUGHTS
Personally if I have to join two slings together, I generally use the Strop Bend (close to a girth hitch, more later)—because it's clean and symmetrical. When it comes to forces, loads, etc., we engineering-types like symmetry. Also, I just make sure the two strands are the same width—no one ever told me that, I just thought it made sense. Think about wrapping a piece of fishing line around your finger and pulling—ouch—the different diameters really cut into you. But one finger wrapped around another and pulling—no bigs.
But I just did things that way because I did, and luckily I never really needed to test it out by accidentally taking monster whippers onto girthed together slings. I was hoping we'd learn some good info both for ourselves and to share with other climbers out there.
It's worth noting that surfing around the web for a while will bring up all kinds of info on this topic— some accurate, some not-so-accurate. One interesting tidbit of good information we found was a blurb by my predecessor here at BD, Chris Harmston.
It appears that he had already done some testing several years ago—but things have changed slightly since then—thinner and thinner webbing is now on the market. We would repeat some of his tests, and add some new ones to the mix.
So what did we do?
Well I have a spreadsheet about 20 tabs deep with all of the raw data, summary, statistical analysis, comparison, rankings, percentage differences, etc., etc.—as well as just as many more of photos pre testing, test set-ups, post testing, etc. But I'm not sure if we have enough bandwidth to post all of this data, plus I doubt anyone would look at it; so let me do my best to summarize.
THE TESTS
Tests we performed included:
Tensile Strength (static)
Pulling slings connected to failure in a tensile test machine—measuring ultimate load.
Tensile Strength (dynamic)
Using Black Diamond's drop tower, drop a mass onto two connected slings.
- Slings were girth hitched together for all tests
- Mass was 80 kg
- Fall factor was ~2
- Drops were repeated using the same rope per sling specimen, with loads increasing with each drop (because the elastic properties were being taken out of the rope)
- Loads on each drop, and number of drops recorded until sling failed
- New dynamic climbing rope (11 mm) per specimen
CYCLIC
Using a pneumatic cylinder, we cycled connected slings to a load of ~3.5 kN (~800 lbf). Note: This is approximately the load we could generate when performing a gnarly bounce test as if aid climbing, or a bit more than a typical toprope anchor sees during 'normal' toprope belay situations.
Note: It's worth noting that loads on toprope anchors can easily exceed this. For more, read Tyler Stableford's excellent article in Rock & Ice Issue #133, June 2004, "Climb Safe: Taking it from the Top."
THE SAMPLES
MATERIALS
Of course there are many many types of slings out there—we used the following materials:
- 11/16" nylon (think cam sling material)
- 12 mm Black Diamond Dynex
- New 10 mm Black Diamond Dynex
Note: Black Diamond currently sells 12 mm Dynex, but is coming out with 10 mm Dynex
Note: Several other manufacturers are currently selling 10 mm Spectra, 8 mm Spectra and even 6 mm Spectra slings. Some of these were tested as well.
Note: For our purposes, Dynex, Spectra, & Dyneema can be considered the same material
METHODS OF CONNECTING
We used three main knots to join the two slings:
Girth Hitch
Strop Bend
Climber's Hitch
Note: The girth hitch and strop bend are VERY close. I believe that most people use the term "girth hitch" loosely and it in effect covers both the true Girth Hitch as well as the Strop Bend. In these experiments we will treat them differently.
MATERIAL COMBINATIONS
We tested many different connecting methods in combination with material combinations in order to try to shed some light on the subject:
Note: Photo only shows some of the combinations and materials tested.
RESULTS
To try to give all our results would take pages and pages, here is the as-short-as-I-could-make-it version:
Note: For ease of comparison, strength values and reductions are compared to 22 kN—which is the CE minimum requirement for a NEW sling—so these numbers aren't actual reduction in strength of the slings, because it's possible that they are stronger than 22 kN when new—follow?
TENSILE TESTS
Chart shows percentage of when new, sling strength (i.e. 22 kN):
Note: the Strop Bend and Climber's Hitch are symmetrical, and therefore the results for using 10 mm Strop Bend to 11/16" Nylon is the same as using 11/16" Nylon Strop Bend to 10 mm, etc.
10 mm Dynex Girth Hitched to 11/16" Nylon—Static Tensile Test
10 mm Dynex Strop Bend to 10 mm Dynex—Static Tensile Test
8 mm Dynex Climber's Hitch to 11/16" Nylon—Static Tensile Test
OBSERVATIONS
- Joining two slings reduces the ultimate strength—and in some cases by up to and over 50%
- When nylon and a Dynex or Spectra material were combined, the nylon failed in all configurations
- In general terms, the narrower the material used, the greater the reduction in strength
- Also in general terms, mixing widths of materials when joining slings results in a greater reduction of strength
DROP TESTS (ALL SAMPLES JOINED USING A GIRTH HITCH)
8 mm Dynex STROP bend to 11/16" nylon—drop test— before failure
10 mm Dynex Girth Hitched to 11/16" nylon—drop test
8 mm Dynex Girth Hitched to 8 mm Dynex—drop test
8 mm Dynex Girth Hitched to 8 mm Dynex—drop test
OBSERVATIONS
- When webbing sizes are mixed, and under dynamic loading situations, the narrower strand typically fails (in all but one of our tests)
- During one test a girth hitch slipped to a strop bend—this sample ultimately went many drops more than as if it had been girth hitched
- More testing required to verify if a strop bend performs significantly better than a girth hitch in dynamic loading situations—we tested two more samples with intentional strop bends
- It appears that the slings joined with the strop bend performed significantly better in dynamic loading scenarios than slings joined with a girth hitch (compare to data above) held approx 50-75% more drops, as well changed the failure mode to the nylon sling
- In all tests performed, it took more than one relatively severe drop to induce failure into the system
CYCLIC TESTS
All configurations and samples tested (11/16" nylon to 11/16" nylon, 10 mm Dynex to 11/16" nylon, 8 mm Spectra to 11/16" nylon) using Girth Hitch, Strop Bend and Climber's Hitch all surpassed 5000 cycles at a repeated cyclic load of 800 lbf.
COMMENTS
Repeating these tests with a combination of an increased load and/or varying the rate of load may differentiate between stronger vs. weaker joining methods and materials combinations for repeated cyclic scenarios
CONCLUSIONS & FINAL THOUGHTS
As always, I must state a disclaimer that these findings are somewhat unofficial—just some information to think about. I'm not a climbing guide and don't even play one on TV. These experiments are NOT all inclusive or totally encompassing by any means—much more testing would be required in order to come to any firm conclusions. It is important that all climbers use their best judgment out in the hills.
First off, our results were very comparable to Chris Harmston's findings, and I agree with his recommendations—before you join two slings together think about the following:
- Is it possible to use a longer sling altogether?
- If you need to join to slings, using a carabiner is stronger
And in addition:
- If you must join two slings, use the same materials and width
- Symmetrical knots (like the Strop Bend and Climber's Hitch) appear to perform better than a standard Girth Hitch when joining two slings together
GENERAL
It's interesting to note that when webbing sizes are mixed and tested in slow static pulls, the nylon failed, however under drop tower dynamic situations, the thin webbing failed. That just verifies that the rate at which loads are imposed on a system can make a difference in ultimate failure load and mode.
SO WHY DID JOHN SHERMAN'S THIN SLING CUT WHEN LOADED IN THE WAY IT WAS?
Looking at the photo of the anchor set-up, I question how ‘equalized' this anchor was. It appears to me that the anchor point in question took the majority of the load in this situation, however, the loads seen in his toprope, rappelling scenario should still have been well within the limits of the material used.
John Sherman comments: "The anchor photo was shot the next day after I had re-rigged the anchor to finish my work the day before (the only re-rigging was to use the static line instead of the broken sling), then re-rigged it back to the previous (or close to) for the photo. I don't disagree that the failure side might have taken more weight (though I tried to avoid this), but the photo could be misleading as the clove hitch could be an inch or two off. Also when weighted the rope took a slightly different angle (the slings lift a few inches when the system straightens)."
It's interesting that his narrow web cut. As stated above, in all of our experiments, the only time that the narrow web cut was during the drop tests. This leads me to believe that perhaps his loading scenario was much more dynamic than originally suspected; or perhaps his girth hitch was not "dressed" (i.e. web folding over itself causing increased stresses).
"The girth hitch had a half twist in it—did this increase tension? Also the knot shows a distinct V-groove in the middle of the dyneema—it looks like there was extra tension along the center of the webbing making it act as if it were a smaller width. The knot pinched one length of dyneema against the other as the two strands exited the knot. One stand failed, the other was damaged as well at the same spot in the knot."
It's also worth noting that the cut of his narrow sling appears to be very clean—on all of the tests we performed, there was much fraying, etc of the ends after the breakage. Also, all of our narrow web failures went diagonally across the web, whereas John's appears to be very perpendicular to the web. Could it be possible that there was already a slight 'nick' in his thin web which allowed the break propagation during loading?
John Sherman's clean cut thin web
Typical frayed ends of webbing after breakage
So what is the exact reason, according to the laws of physics why John's sling cut the way it did? I don't know—but maybe someone has it out for him?? (Watch your back John.)
"BTW There's more than a few people who have it out for me—however only two climbers have ever been to the cliff where this happened and we were both on rappel at the time the sling broke. And any saboteur would have to be helluv clever to cut the dyneema then also nick the spectra on the backside of the knot. And why not just slice my rope? Furthermore my dog was chillin' atop the cliff and would have probably barked if a poacher or other person approached. (My dog is 12 and her teeth are worn down so she's not a suspect.)"
Hopefully his incident as well as the experiments and results described above will at least get you thinking a little bit the next time you need to join two slings together.
Climb safe out there,
KP
