Stability of multi-loop knots under partial failure

[The Ex-Engineer]

Hi there,

Hopefully someone on here can point me in the right direction.

I'm trying to find any data on the strength and especially the stability of common multi-loop knots under partial failure. Basically, which knots are still intrinsically stable if one out of two (or two out of three loops) are cut through whilst both standing ends are loaded and not parallel.

I'm trying to investigate what is the best multi-loop knot (if any) to tie to improve the abrasion resistance of an emergency mountaineering abseil anchor. For example, assume we have SINGLE loop of 5mm cord tied around a large spike (with a double fisherman's) and will be threading the abseil ropes directly through this.

It would seem sensible that tying a multi-loop knot in the loop and having the abseil ropes pass over multiple thicknesses of cord might improve the abrasion resistance in the case of untoward rope movement. However that obviously assumes that:
1) The knot doesn't significantly weaken the set-up.
2) In the case of a double loop knot, the knot doesn't still fail if one strand abrades.

[Obviously using thicker cord, multiple strands plus leaving behind a rap ring, maillon or karabiner is a better option, but in the real world that doesn't always happen.]

My gut feeling is that the Double Bowline might not be stable enough and I don't know whether other well known options (Double Figure of Eight Loop, Triple Bowline) are significantly more secure or whether other better options exist.

Many thanks in advance.
Mark

[roo]

I have a feeling that your verbal description could be open to misinterpretation.  Do you have a diagram or picture of the setup you have in mind?

[agent_smith]

Hello Ex-Engineer,

One knot that I can quickly vouch for is #1085 (Double F8 eye knot).

I prefer to be more technically accurate in describing what you call a 'loop' as being an 'eye'. Think 'eye bolt', 'eye splice', etc. You wouldn't call these items 'loop bolts' or 'loop splices'. The term 'loop' is being confused with 'eye'.

If you lose one of the 'eyes' - the knot remains stable and secure under load.
A lot of testing has been done to verify this (eg rope test lab)... Richard Delaney (owner of rope test lab) has cut one of the eyes of #1085 while it was under heavy load - and nothing happened (it remained stable and secure despite the loss of 1 eye).

As for #1080 (Bowline on-a-bight) - the situation is less clear-cut, because it depends on whether there is a 'tail' or not.
There is a vulnerability with #1080 when there is a tail - that is, the knot is tied so that there is only one SPart (standing part) which corresponds to a single nipping loop. If #1080 is tied so that there are 2 SParts, this corresponds to 2 nipping loops.

In terms of recreational abseiling - I would surmise that #1080 and #1085 are typically used to build anchor systems.

Mark G

[Dan_Lehman]

The OP's hypothetical scenario strikes me as being
highly unlikely/impractical :: that such a supposed
bolstered structure would have the same two strands
of 5mm cord tied to/through the anchor, but with the
added dble-eye knot now 4 strands for better treating
threaded-through rap. rope --and yet we're to worry
that one of these quarter-force-loaded strands might
break?

I think that an overhand knot (in the doubled lower end
of the 5mm loop) with the tail bight brought up between
the *spine* and then *backflipped* into collaring position
(as done for the other knots mentioned) might be a good
solution (to this unseemly problem) :: the cut-eyed SPart
strand will nonetheless be tied in the twin overhand, and
so be guarded against slipping out of the knot unlike with
the bowline on a bight (where it would be making just a
turn in the knot, the cut tail end unsecured).


The more likely concerns are where double-eye knots
are used for conjoining two anchors (with the knot's
two SParts), and one anchor fails; or sometimes the
eyes will take on non-twin, separate duties, and then
one might become unloaded.

--dl*
====

[JD]

Totally off topic, but why @Dan_Lehman are your posts not as wide as others?

[Dan_Lehman]

Totally off topic, but why @Dan_Lehman are your posts not as wide as others?

Because I want them to be read,
and often in with particular arrangement,
not ticker-run across the screen.
 

[alpineer]

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[Dan_Lehman]

!thgir yltcaxe