Load distributing anchor systems

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Postby fuzzy-hair-man » Nov 27, 2007 10:06 pm

kip wrote:This topic is old as the hills and caves themselves here are some threads if you want to sleep well but the wiki article (link 3) open the PDF is very good but read it well and make sure you understand it before you go running to the hills screaming a new prophecy. We have been doing some testing here in NZ over the last few years with multipoint for rescue and are doing some towards cave applications it supports the obvious but it is too early to release yet.

Thanks for the links! the last one is interesting :kewl: (I think I understood, don't get the math though :nuts:)

Two questions:
If you can, can you tell us what's the obvious? I can think that "load sharing is useful and effective but you need to know what your doing" is obvious but that might not be your point...

The second is if as the pdf suggests angle is not so critical (with reason) but leg length, including what is pulled out of the knot is important. Does this support a use where your rescue team or whatever takes in a pre-tied and weighted (pre-tensioned) cordelette (hence limiting the rope coming out of the knot) where all leg lengths are equal, your team then uses spectra etc (as recommended in the pdf) to extend the length of the longer two legs to the desired length giving you close to three (or 4) equal length legs and less variablilty because of reduced slippage out of the knot. It would probably make the rigging quicker too

The obvious downside is that your carrying a length of cord which now only has a specific job reducing the adaptability of your gear, but if required you could undo it assuming you can get it undone that is. :roll:

Or another approach: Tie these things out of spectra elimanting the leg stretch and length issue and then your only having to deal with rope or cord pulled out of the knot? But you have no shock absorbing qualities in the cordellete... but if all anchors are taking load there's not going to be any/much shock load right?

Am I missing something here? :neutral:

It seems the stretch is a double edged sword, it can make up for small errors balancing the load as the loaded leg stretches and pulls out of the knot until load is taken on the other legs but once the stretch runs out the main load will be back on that loaded anchor causing it to fail. Correct?
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the obvious but unfortunately not so

Postby kip » Nov 27, 2007 11:33 pm

I've been teaching rope stuff for a long time and have always gone for this rule of rigging multipoint anchors "same material in each leg- same number of strands in each leg- same length of each leg" with the words - "if possible" attached to that statement and you don't really know what you are doing - a rule of thumb in other words. With another guy over here we started to scratch our heads on the classic fig 8 and butterfly rig on SRT systems (which breaks the rule of thumb) and yes it screws with the anchor loading because you start to confuse the world of redirectors and anchor angles imm. Also leg lengths are often radically different and in some cases i've ended up with 1 F8 and 2 BFs and some times a double 8 and one butterfly and a 1000 other solutions.

Now where the hell has the load gone and which anchor has it? I dunno know but its there somewhere. So whats the relevance - well say you have 3 self drilling spit anchors average strength (doing mint placement and brand new) say 5kN approx (this is less then your average chunk of rock pro) so apportioning the load evenly is well "rather important" or at least understanding where the hell the force went becomes rather important if you just change a few factors on those bolt placements, age poor placement etc.

Do we have good answers on this yet unfortunately "no" so we cant get any reasonable rule of thumb out of it yet as the variables are too large when compared to a more standard rock set up.

And some contra to that PDF we are finding the angle is important as well as vertical and horizontal offset. No big suprises there, think pyramids and tripods same concept except anchors pulled rather than pushed (by gravity). So next time you head out - look at the rig and the offset of the rigging at the top and scratch your helmet a little and ask yourself where is the force and on which anchor? Be warned though soon you will have gone through your helmet and carved a wagon rut in in ya skull (if it is like some of the rigs i've seen over the years).

Now; have i changed the way i rig because of this and have a new radical solution "no" partly because all before has worked and i have no real evidence to contradict this but I still think a whole bunch before i climb onto the rope about how safe is it and whether the real issues are dealt with - the security of the anchors and if the biners are done up.

Well thats the little answer the rest is best served with beer in good pub. :laughing:
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1. the state of non existence
2. an empty area or space
Question: do caves really exist then?
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Postby knudeNoggin » Nov 28, 2007 4:05 pm

junkman wrote:Do you have an example of the Extension-Limiting Equalization Triangle?
A picture or diagram would be helpful.
...
Actually I did a quick search on google and found this


Egads, that bears no relation to what I described!

Look, it's a triangle, where, for a suspended (downward) load
the "V" 2 sides are composed by a single piece (over which the 'biners
clipping the load can run, to equalize), and the enclosing horizontal
3rd side of the triangle is what closes the loop and limits extension.
(In an equilateral triangle with 1-foot sides, the load will be roughly
10" or so below horizontal; releast one anchor and now the enclosing
loop of 3-foot circumferance hangs from the remaining anchor and
extends 3/2-feet, plus whatever pendulum shift contributes.) In the
OP case the anchor bolts are so close together that extension could
be made less, shortening the slack connector side bewteen bolts.

Here is an image of ELETs implemented in a single line, and in a
sling. Another way to implement an ELET with two short (24") HMPE
slings is to use one for the "V" and the other ties to the bight-ends
of the 1st with Sheet Bends (or Reverse Sheet Bends). Or, as I said
above, Cloves to anchor 'biners.

http://i2.tinypic.com/qwx3ro.jpg

*kN*
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Postby MessedUpMike » Nov 28, 2007 8:44 pm

I thought sheet bends were a distinct no-no as they could to easily come undone
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Postby NZcaver » Nov 29, 2007 11:53 am

MessedUpMike wrote:I thought sheet bends were a distinct no-no as they could to easily come undone

Hmmm, good point.

And for those people who count the numbers, even with a backup knot the Sheet Bend is at the bottom of the pile - at least in this round of testing. I can't say I've ever felt the need to use one in real rope for life safety applications.

Frankly, nor do I feel that an ELET is even vaguely worth the hassle for vertical caving or cave rescue applications. But I could be wrong. I know it's nice to learn new tricks and have plenty of options in the rigging "toolbox" - however there will always be a line somewhere between what is and isn't a functional AND practical technique.

If your anchors are so marginal that you really need rigging this elaborate to compensate for a partial failure... I would politely suggest you find (or install) better anchors. Just my opinion, of course.
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Postby knudeNoggin » Nov 29, 2007 3:24 pm

NZcaver wrote:
MessedUpMike wrote:I thought sheet bends were a distinct no-no as they could to easily come undone

Hmmm, good point.

No, it's not: somehow, one has to break through the 1-dimensional thinking
of "knot" being some ideal to which physical characteristics apply,
rather than realizing that it is to physical "knot"s that such things apply,
and these are denoted by such cumbersome, uncommon terms as
"sheet bend in <particular_cordage>". In this particular case (ELET),
one is tying off a sling, which is a closed loop, and which thus
cannot slip out the way Richards cites (for, what was his term for the bight
part--"fold back end"?), and would upon anchor-failure loading become
resp. of failed & holding knots be a Reverse Sheet Bend & Bowline (whose collar
is a closed-loop bight). Conceivably, another joint might be used. It
could be, as noted, Clove hitches to 'biners, or it could be Overhand loopknots
(which in the failed side would be the infamous EDK, which if it capsized
would become pure material, not a catastrophic failure). Now, these two
so-knotted structures apply to a single sling (Clove or Overhand LKs), as
forming them in doubled slings would be ungainly.

(In place of the Sheet Bend: tie a Reverse S.B., and make a 2nd, even 3rd,
final tuck down through the bight and wrapping that same side of it:
this should lock up well, but be forcibly untiable/loosened by pulling the
ends of the U-part/bight apart, to prise out some bit of mainline of the
other rope, which is secured by the coils overwrapping it.)

And for those people who count the numbers, even with a backup knot the Sheet Bend is at the bottom of the pile


... though, in the ELET, it comes in one side of the loop holding the load.

Frankly, nor do I feel that an ELET is even vaguely worth the hassle

For having in the tool kit for iffy protection, it makes (in HMPE tape) the
best-equalizing/-distributing structure, I think. One could have such a thing
pretied, using extra short cord as necessary to make the to-anchor linkage.

It is, in any case, a structure that limits extension, which for a Sliding-X
will take limiter knots, and an "X" more frictive contact w/'biner.

If your anchors are so marginal ...


Yeah, that will probably the general situation!

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