double figure 8 question

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double figure 8 question

Postby Dangerjudy » Aug 30, 2013 1:14 pm

If you have a rope tied to two adjacent anchors with a double figure 8, and one of the anchors fails, will the knot fail? This is assuming a reasonable load on the rope, well within its specs.
What if one of the loops in the figure 8 is cut? Will the knot undo itself?
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Re: double figure 8 question

Postby NZcaver » Aug 30, 2013 1:45 pm

Dangerjudy wrote:If you have a rope tied to two adjacent anchors with a double figure 8, and one of the anchors fails, will the knot fail? This is assuming a reasonable load on the rope, well within its specs.
What if one of the loops in the figure 8 is cut? Will the knot undo itself?

Assuming the knot is tied in regular caving rope and has been properly dressed and tensioned, and is only subjected to 'normal' loading, most likely no on the first question but the second one could be sketchy.
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Re: double figure 8 question

Postby Scott McCrea » Aug 30, 2013 1:46 pm

It depends. Maybe. Probably not. The only way to know is to try it. Do some backyard/garage testing and report back with your findings.
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Re: double figure 8 question

Postby Caver John » Sep 1, 2013 12:19 pm

The loops can be used independantly. If one is cut, the other will still function.
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Re: double figure 8 question

Postby Dangerjudy » Sep 3, 2013 9:14 am

Well it's not exactly the answer to my question but it's cool, check out this knot fail blog:

http://www.peakinstruction.com/blog/tag/knot-failure/
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Re: double figure 8 question

Postby bradyfl » Mar 4, 2014 8:50 pm

an 8 will fail in the knot, unless a loop is cut. in a static pull they fail at the knot (hence the reason they do not require a back up safety). on a double, if one loop is failed the other will remain intact and will not slip, but the issue is the possible shock load when the rope finds it's natural plumb after the load is no longer distributed. the main reason, aside from not having a bomb proof anchor, to use multiple anchors is to change the departure point. with that in mind make sure you aren't setting up unnecessary anchors just for some cool looking rigging. also remember a double loop 8 is no stronger than a single loop, they both fail at the knot.
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Re: double figure 8 question

Postby PeterFJohnson » Mar 11, 2014 3:40 am

Caver John wrote:The loops can be used independantly. If one is cut, the other will still function.


bradyfl wrote:on a double, if one loop is failed the other will remain intact and will not slip


I know this is generally accepted wisdom, but does anyone have any testing to back it up? For instance, I have heard people say the same thing about a bowline on a bight and the above video seems to suggest that that knot doesn't function that way. I have always been leery of the independence of the loops on a double eight since the knot isn't often put to the test in that way. The structure of it doesn't seem to isolate the loops that much.

I tend to build my Y-hangs using a butterfly so it might be a moot point. But from time to time it is nice to use a double eight to allow the approach traverse line to head straight for the Y-hang knot/pitch head. A butterfly leaves the traverse line on the wall.
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Re: double figure 8 question

Postby William Tucker » Mar 11, 2014 12:09 pm

I have some anecdotal testing that seems to prove that "if one is cut, the other will still function" is not true. On a drop with two bolts, a double-8 was used; but, because of an oblique touch on a shelf below, the rigger tried to make one loop bigger to offset the knot to avoid the touch. I told him that it would not work and it did not (no big deal for a slight touch, anyway). After cycling with rappel and ascent, we found the loops to be equalized at the end of the day. This says that should one of the loops be cut, it could continue to "equalize" until there was a failure when the cut end worked its way through the knot. This is intuitively obvious as the loops are easily resized in relation to each other before the knot is loaded. I believe the double-8 is not safe if one of the loops is cut but will (after some extension) survive the failure of one of the anchors, just not the loop itself.
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Re: double figure 8 question

Postby caver.adam » Mar 11, 2014 1:42 pm

I like William's anecdotal testing. Looking at how the knot is tied you can see that the two loops are not independent. Alter the length of the loops without untying the figure 8 and you will see this.
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Re: double figure 8 question

Postby bradyfl » Mar 12, 2014 6:22 pm

hey William I was thinking about your post about the loops equalizing, and im trying to visualize the rigging. when the loops were attached were they not equally tensioned, was one loaded more than the other? if the offset sizes of the loops were equally loaded I don't see why one would slip. if they were equally tensioned to begin with and they slipped did one loop lose tensions? I have never offset the size of my loops and am curious if offset they will always have a tendency to try to equalize. im going to play around with it some more and see if I can recreate this as well as try to fail the entire knot by failing a loop by either failing an anchor or cutting a loop. the majority of my knowledge and experience with rigging is in a rescue scenario, I have been trying recently to look more into the mountaineering and climbing fields to be more well rounded. in doing this I have found that a load distributing/self adjusting double loop 8 will fail if an anchor is failed or a loop is as well as doing this with a triple loop load distributing/self adjusting.
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Re: double figure 8 question

Postby William Tucker » Mar 12, 2014 9:13 pm

bradyfl: The two loops were purposefully unequalized in an attempt to cause the knot to be off center: one loop was carrying almost all of the load while the other was somewhat slack to begin with. The knot worked its way toward equalizing the two loops as it is wont to do. This tendency for the knot to equalize the two loops is why it is unsafe if one of the loops is cut. It could work its way to the point where the cut end is pulled all the way through.

I think that you are right about the knot also being unsafe in the loss of an anchor. I took the time to tie a few and it was quite easy to get it to completely collapse and fail if one of the loops is allowed to pull back through the body of the knot. It did depend upon which loop was pulled; but, I was able to cause it to totally fail. That surprised me and will make me look at the knot differently from now on.

To be certain that we are all talking about the same knot, I am speaking of the Double Figure Eight described on page 46 of On Rope, New Revised Edition. Smith and Padgett describe its load sharing and self adjusting tendencies; but offer no guidance as to redundancy. I am now leaning toward the view that it offers little to no redundancy.
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Re: double figure 8 question

Postby Anonymous_Coward » Mar 13, 2014 3:05 pm

This is a great discussion, but I feel like some things are starting to get confused. A normal double figure eight is not usually considered a load distributing/equalizing knot. Yes, it will slowly try to equalize under cyclic loading and unloading (like someone climbing the rope), but it is not a true self-equalizing knot. The legs are more or less "fixed" and the loops must be adjusted properly in order to equalize the load. I do agree that the knot would be in danger of failure if one of the loops were cut and then the knot was subjected to cyclic loading. But....

How many times have you ever seen the loop of a double figure eight get cut? I have used these knots hundreds of times and I have seen it exactly zero times. It is a nice learning exercise to talk about it, and realize the failure modes, but it is very unlikely to happen in reality. If you set two bolts, and then one or both of the double eight loops have to bend over a sharp spot, then you have either set the bolts improperly, or you are using the wrong knot, or both. The reason to use a double eight is to share the load between two anchors, usually bolts. The redundancy is there to protect against anchor failure. If we routinely worried about knot loop failure, then all knots would have two independent loops.

bradyfl, could you point us to the reference that says a self-adjusting double eight would fail because of anchor failure or loop failure? The self-adjusting double eight that I am familiar with would fail because of big loop failure, but not failure of one anchor. I don't think they are referring to a run-of-the-mill double eight here, but a true self-adjusting one. The one I know has a carabiner involved that joins the big loop and the little loop.

I do not agree that a (normal non-equalizing) double figure eight knot would fail in the event of one anchor failure. If that was true, then the whole operation of setting two bolts and using this double loop knot would be for no reason. Think about a double eight Y-hang off of two bolts. Imagine this rope is loaded, and then one of the bolts pops out of the wall. With the knot still loaded on the other bolt, what possible circumstances would cause the now unloaded loop with maillon, hanger, bolt, and probably chunk of rock attached to pull back through the loaded knot? I don't see how it is possible.
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Re: double figure 8 question

Postby Scott McCrea » Mar 13, 2014 3:16 pm

:exactly:
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Re: double figure 8 question

Postby bradyfl » Mar 13, 2014 3:39 pm

towards the end of my last post I was getting off topic... I some times can start rambling lol. the self adjusting double loop 8 is a completely different knot than the double loop figure 8 we started talking about. my reference to it failing if an anchor is failed is from personal experience, in a controlled setting. when I say fail, it created a shock load on the rope, it remained attached at the one anchor, but with the shock load on the rope (in a rope rescue environment) the rope would be out of service. as long as you maintain a static system safety factor of 6:1 or greater nothing should break... it is not really a feasible knot for an anchor in the rescue field when there are many other options available that are simply better options. im trying not to go back to so much rope rescue but that is where the majority of my rope experience is...

with a loaded double loop 8, with each end equally loaded, I can not make it pull through... I did it a few times and couldn't make it happen I used roughly 250lbs on 7mm for it.
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Re: double figure 8 question

Postby trogman » Mar 13, 2014 3:54 pm

Anonymous_Coward wrote:
I do not agree that a (normal non-equalizing) double figure eight knot would fail in the event of one anchor failure. If that was true, then the whole operation of setting two bolts and using this double loop knot would be for no reason.


Not entirely for no reason. Yes, part of the reason for two bolts is so that in case one fails, you will have a second one. The other (and in my mind, the more important) reason is that it distributes the load, so that each anchor is supporting <100% of the load. This makes it less likely that either anchor will fail in the first place.

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