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Knots to reduce the maximum arresting load in rope systems...

Discussion in 'Tech Tips and Gear' started by Disruptive_Rescue, Jun 9, 2021.

  1. Disruptive_Rescue

    Disruptive_Rescue

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    Found this recently, usually on top of vertical related research, but this one slipped by me due to it being in a 2015 Journal of Dynamic Behavior of Materials publication, which I don't really follow...

    Not sure if this will be of interest to any, but the lead author is an engineer on the East Coast who is contracted by the Illinois Fire Service Institute to conduct research on some of their systems. Like much research, the motive behind it may be to counter someones or a company's claims...for this he went after the sewn eye claims concerning retaining the most efficiency of the rope, and he found a knot that beat the sewn eye - but also found some other hidden gems like the maximum arrest load (MAL) in a 100% aramid rope w/ a sewn eye was reaching ~80% of the ropes MBS when dynamically loaded with the force of a firefighter bailing out of a window (oddly enough in one of his previous studies he found that at 300 degrees C, the aramid fiber lost half of its strength, which makes this case for FF's a bit more alarming)... He has some interesting info on a knots ability to absorb energy. I know it is common for cavers to utilize a barrel knot on their cow tails - but this guy went straight to a quadruple overhand... anyway, although it is aimed towards firefighter bail-out ropes, it may have significance for some folks in here... seems obvious, but here's some data.

    Attached Files:

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  2. Kuenn

    Kuenn

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    Thanks for sharing. (For the record, I didn't read it all, just the abstract, intro and conclusion. All that other highbrow stuff is out of my league.)

    My 2 cents:
    An interesting study, although it does seem more applicable to industrial applications, for which I believe they have much more sophisticated gizmos for shock arresting than knots.

    So...does this mean we should tie an assortment of 'simple' knots in our rappelling ropes to protect us? o_O

    Arresting falls in canyoneering is, well, pretty much called rappelling. Every other kind of rope related fall is either unexpected or unplanned. And if, when it does occur I'm pretty confident an analysis of the knots used for shock absorption is going to be least priority in the retrospective...unless of course it was the failure point. (Feel free to disagree with that proposition.)

    Yep, my QAS (cow's tail) has a triple over-hand/strangle/barrel knot securing the handled ascender. It has been loaded and cinched so many times at this point any absorption capacity is -1 by now (#6 below). All those other knots are pretty much brick solid too.

    QAS.
    Last edited: Jun 11, 2021
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  3. Disruptive_Rescue

    Disruptive_Rescue

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    Kuenn - Great points and thanks for the detailed picture. The focus of the study was aimed towards firefighters performing an emergency egress situation, so the dynamic potential examined was an average firefighter in full bunker gear / PPE - getting their ass out a window - less than smooth. As you know their escape ropes thru the years have moved from 8mm Nylon / nylon...to aramid sheaths (technora, kevlar k-29 AP, Twaron) to 100% aramid escape lines between 7.5mm-8mm...now with manufacturers creating "escape systems" which in many cases have sewn eyes with either a spine locking carabiner or some derivative of a Crosby hook - So the author took a look at the dynamic behavior of multiple ropes containing various amounts technora with & without sewn eyes which are presented by many sales people as being superior to knots, and how this effects the maximum arrest loading of each rope set-up. The article did address the point you made concerning more sophisticated gizmos for arresting a shock - but due to the compact size of bail-out systems, many of the commonly used add-ons like screamers were irrelevant.

    That being front-Loaded... I threw this article on for really 2 reasons - 1) looking at some of the discussions in this forum, including the last topic concerning abrasion resistance, there appears to be quite a few folks using ropes containing the aramid fiber, at least in the sheath. This study did show the impact in a few configurations with bail-out ropes with a technora sheath and nylon core. I thought this may help give insight to those users on the behavior of these rope configurations if a small dynamic load occurs or has the potential for occurring.

    I agree that the utilization of this information is pretty limited for much of canyoneering as a whole - and to be honest I believe this information is contextual at best for firefighters... meaning it's good info to be aware of, but many of the anchors firefighters could potentially use in "extremis" under time-compressed constraints - may not benefit from any of the knots presented, although they may benefit from the information. IE - if I would be in a bail-out situation in a multi-story building and maybe kick in the drywall in a room and wrap my anchor around the base of wall stud - I then proceed to the window and straddle the window sill - give a little slack in my line to clear my DCD over the window sill and roll out "somewhat" smooth but guesstimated a bit too much slack and dynamically loaded my system...How much would any of those knots - either at my carabiner or anywhere in the rope between my anchor and the edge really help? Probably not much - since my anchor was low, I caught a little bit of an angle on the inside portion of the window sill and then the outside 90 degree angle of the sill can take up to ~70% my weight depending on the coefficient of the window sill material - so in this pretty common set-up, the edge is what will see my impact and all the knots I added would potentially see very little. In the end - all of our systems and rigging are pretty nonlinear...so with so many variable interacting on each other - one way to rig could maybe hep..or not so much. But in the above example I used a Crosby hook and hooked the inside of a window sill the knot would probably clear the outside of the sill and have the potential to assist in absorbing the dynamic hit...

    So, the #2 reason is...for those using ropes with aramids...or at least in the sheath, although the added knots may be of no realistic use (except in very specific situations), the information of the ropes behavior can make an end-user more mindful of rappel system loading, the importance of soft-start transitions when possible, and ways to assist less experienced people in their parties to perform dynamic limiting edge transition techniques if using aramid ropes. I am absolutely not "bagging" technora but as with any piece of equipment (or fiber) - understanding its behavior in any potential utilization just makes us perform more "mindful" rigging. As an example, we've seen a bunch of end-users within special operations that have received instruction from multiple companies on the use of dyneema slings...it has been presented as the "easy button" for all things vertical... wrap it as your anchor on a rooftop pipe in direct sunlight w/ temps over 100 degrees F... as a friction autoblock above their DCD for 200'+ raps, or even using in super-heated environments while operating breaching torches while tied off in a vertical environment and the dyneema sling is what is extending the DCD off of harness...People and instructors see the strength, weight, and small footprint of dyneema slings and don't research the material science - providing the knowledge of its behavior in various pathologies facilitating proper utilization. Obviously there are smart ways and environments to use dyneema slings...and there are times where mindless utilization occurs.

    That turned into a rant, Sorry...my adderall must have kicked in...Anyway, the second reason I posted was to give insight into ropes containing aramids and some of its behavior if a small dynamic event could occur...which may change the rigging technique, edge transition technique, or system evaluation. Also, no I don't disagree with your proposition, but I believe this may open the door to other research which may be more applicable to canyon mobility and/ or operational use - like various knots in webbing for fixed or releasable...or contingency anchors...multi-pitch raps with high tie in / rigging, knots for knot blocks vs carabiner blocks (FYI - I just threw all those examples out off the top of my head as an example - most may have no relevance, but you get what I'm saying).
  4. Kuenn

    Kuenn

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    Again from the article. I chuckled at this image at first, then disturbed by it.

    upload_2021-6-14_11-23-44.

    Really? Saddle clamps on a nylon/polyester rope?

    To top it off, they alternated the clamps. A big no no in cable rigging, My father-in-law (retired lineman 37yrs) would cringe at that image and then bop you upside the head and make you change it..."you don't saddle a dead horse". (Riggers vernacular)
    Last edited: Jun 14, 2021
  5. Jeff Randall

    Jeff Randall

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    Disruptive Rescue, thanks for the link to the publication. I enjoy these types of studies since they open the mind to other potentials.
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  6. Kuenn

    Kuenn

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    Jeff, I would be very interested in the "other potentials" you derived from this article. I'm at a loss to find any actionable items in it.
  7. Jeff Randall

    Jeff Randall

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    I just always keep an open mind when it comes to rope physics. Having done quite a bit of drop testing I’ve found that the initial dressed and set knot eye (bight) absorbs a bit more energy than a sewn eye on the same rope. Now, is this useful for most of what we do? Probably not, but for us rope geeks that like to understand how forces and friction plays out in a system it is fun to explore.
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