knotted dyneema for anchors

[ian]

i've been using a double length dyneema runners (10 mm from wild country) knotted (with a figure eight) a couple of times and then girth hitched to my harness at the tie in point to use to anchor myself while setting up an anchor or rapple. the recent post about john sherman's anchor failure has made me a little concerned about this set up. I was curious if people thought that it was the sawing motion of the dyneema runner that was girth hitched to the other runner that caused it to fail (if this is the case maybe i don't have to be concerned?) or something else? would a figure eight knot present the same potenial for the sling to fail?

 

thanks....

[pms]

Let's wait to hear more details from the Sherman report. I believe Edelweiss gives a 12kn strength rating for their 7mm Dyneema slings slings when girth-hitched.

[selkirk]

I've done the same thing though only for easy alpine stuff and for glacier. (building a personal anchor out of 8mm dyneema runners for, my rock climbing/cragging anchor is one of the Metolius PAS systems.). Where I'm not likely to need it for anything, so I figure go as light weight as possible.

 

After the Sherman failure I won't be doing this anymore. My guess is that some of the damage may have been do to a sawing action, but likely the majority was due to sharp bends in the dyneema.

 

It's pretty common knowledge that knotting reduces rope or runner strength (some knots more than others, with the girth hitch and overhand being the worst, and figure 8 being better). I have a feeling the sharp bends caused by the girth hitch caused the failure, and that because of the nature of dyneema, this strength reduction due to knotting in this case (tight bends due to small diameter) is much worse than with larger chords.

 

I would be very very very loathe to take a short fall onto

a personal anchor like this. The girth hitch into the harness might be ok (semi protected due to friction with the harness but has a very sharp bend where the chord wraps around itself) but the figure 8's in the middle would make me very nervous.

 

If your set on this configuration you might try passing the dyneema through the waist/leg loops so that it isn't knotted, and then using a bit of tape or something to create your loops. Short clipping is no longer an option though.

[Farrgo]

I've done the same thing too, for years. I think there needs to be a lot more research done about the limitations of spectra type materials. Everyone know that knots reduce the strength, but the reduction is relative to the material. We don't worry about a figure-eight reducing a rope to failure. Should we worry about spectra runners strength reduction when knotted?

 

As for girth hitching to your harness, I would highly doubt that simply using a dynemee runner girthed to your harness would put it in a situation where failure is likely. I mean they make daisy chains out of spectra/dynemee/etc. and you girth those through your harness does this mean aid climbing set ups should be suspect?

[Blake]

It's my understanding that the Sherman failure didn't fail due too much weight/strength overload but jut because the skinny runner cut through the other runner. It's wasn't too heavy, it was too 'sharp' and sawed through on multiple rappels.

[ian]

thanks for the replys so far....i defientally don't plan on falling on these runners....in fact the only time the are weighted is at a hanging belay....i'm also not that concerned about the girth hitch part as this is the same as a an aid climbing set up....i am concerned, however, about the knots, as Selkirk pointed out, because they end up being quite small due to the small width of the runner and i wonder what the potential is for any 'sawing' action. i don't think the knots themselves weaken the runner that much....the runners start out at 22 kn and daiseys are like 3 kn in the pockets and people hang on those all the time....so the knot would either have to weaken the runner to less than a seventh of its strength or there is some kinda 'sawing' going on....

Edited November 2, 2006 by ian

[selkirk]

The picture of the sherman failure that I saw had the failure point below the knot. I really can't imagine that there was much sawing action.

 

As memory serves the strength reduction due to knotting is directly tied to how sharp the bends in the rope are.

Climbing ropes have a relatively large diameter so the bends don't end up being very sharp for the knots used (figure 8 is almost full strength, overhand have sharper bends so the strength reduction is larger). Most of the Daisy chains I've seen are still relatively wide (1/2 inch or wider?) So any knotting or girth hitching ends up with relatively broad radius turns.

The 8mm and 6 mm slings though have such a small radius that it seems like the bends would be much tighter and could cause additional weakening. Especially when combined with the spectra.

 

I could be talking out of my ass, but aren't Kevlar and Spectra incredibly strong but also very stiff fibers that weaken more significantly than nylon due to repeated knotting as the individual fibers actually break and not just bend?

[dmuja]

So WC dyneema 10mm starts out new at 22kn rating.

 

If you keep it for a while, at 2 years it *may* lose aprox 33% of that according to some experience around here.

 

Now we're down to 14.75kn.

 

Girth hitching further reduces strength - *maybe* from 30 up to 50% according to some. The tighter and smaller the radius the greater the weakness - frm what I hear.

 

Now your down to maybe about 7 to 10kn.

 

If your second falls, could it generate (depending on your belay set up) a 7kn force on your (the learder/belayer) cows tail/daisy chain?

 

It depends on the route and your belay set up etc, id think (guess) it might be so if there was slop in the system or some user error.

 

thanks for the replys so far....i defientally don't plan on falling on these runners....in fact the only time the are weighted is at a hanging belay........

 

 

BTW - my opin is that it might be better to keep the dyneema for intermediate pro (knot free) and use the beefier stuff for the belay anchor. I would NOT want my leader belaying me up while hanging off a single dyneema runner - with girth hithes and knots especially.

 

I use the beefy PAS girthed to my beefy belay loop or use the rope. Those daisies with the weak tacked pockets should be taken off market imho.. but hey, free solo'rs get away with alot less than that.

 

http://www.metoliusclimbing.com/pas.htm

[ian]

yea i don't ever use the runners when i'm belaying, i just use them either before the anchor is set up when i'm leading or to anchor myself when i'm rappeling....

 

even at 7-10kn that still seems like a lot to me--especially when i'm only using it to hang statically (its double what daiseys are) i'm more worried, i guess, about potential cuting when the runner is knotted.

[catbirdseat]

I could be talking out of my ass, but aren't Kevlar and Spectra incredibly strong but also very stiff fibers that weaken more significantly than nylon due to repeated knotting as the individual fibers actually break and not just bend?

I believe that Kevlar is much more susceptible to fatique from repeated flexing than is Spectra. This is an effect distinct from the weakening caused by bends at knots and hitches.

 

In general, the more static a cord is, the greater the strength reduction for a bend of a given radius. This is because the fibers on the outside of a bend take most of the force, while those on the inside radius take less.

 

In the case of dynamic ropes, the outer strands are able to stretch, thus allowing more force to be borne by the inner fibers.