Knot Limited Sliding X - Failure Mode?

^^^^ doesn't matter how much the route wanders low down. The direction of pull is from the anchor to the last piece placed on lead.

Brian, although I've not seen a video of exactly the test you imagined, it is hard to believe that a cross loaded overhand with zero tail would not roll and fail if the applied force is great enough to snap the sling at the knot in the first place.

However....

When looking at most systems, when we say they are redundant, we only mean w.r.t. certain events.
In the case of a sliding X it is anchor failure. A rock landing on the X and cutting the sling will also make it fail. As might some sharp rock touching the sling when it is heavily loaded. (A sling can be very easily cut if it is under load.)

I think someone on MP posted a photo of a sling anchor that had been hit and cut by a falling rock - anyone remember that one?

cdec wrote:What if the sling breaks on the connection side of the knot? Seems the assumption is made that it breaks on the bolt side. Why? Sliding X is only made sort of redundant by using limiting knots. Think about using a quad if you want self-equalization and redundancy. Additionally rock fall is real, I had an anchor chopped and would not be here writing this if we used a magic X.http://www.mountainproject.com/v/rock-fall-results-in-chopped-anchor/110088639

I´d assumed it always broke on the central connection part since the tensions in the two strands are likely to be different there, the test I did earlier shows this is the case.

cdec wrote:^^^^ doesn't matter how much the route wanders low down. The direction of pull is from the anchor to the last piece placed on lead.

This is usually true, but not always, if the "last piece placed on the lead" is not near the top of the lead, and some wandering occurs after the second unclips from that last piece. This happens most on relatively easy ground of course.

rgold wrote: If there is no slack in the belay and the second falls, the peak load (if not mediated by rock and gear friction) will be twice the second's weight. This is because the second still falls some distance because of rope stretch.

Ok, that makes sense. Thanks for the correction.

rgold wrote:If the anchor point is lowered by 12", the second will drop 24".

Im not seeing this. In a simplified situation, a 24" sling clipped to two close pieces and you grab the middle to make the sliding x, the belay device would be 12" away from the two pieces. If a piece failed it would fall to the end of the sling, an additional 12".

rgold wrote:(1) Will the one-foot drop of the belay plate affect its ability to lock off, either because of a brake-hand position now above the plate or because of an impact with a rock feature?

As I said before, I usually belay a second in guide mode. No need for hands as it is auto locking. A weird impact with a rock might interfere with this. the odds of this are rare but admittedly not impossible.

rgold wrote:(2) If the belay plate anchors are relatively low, will a one-foot drop transfer some or all of the load to you? In this case there is an entirely different calculation with the potential for much higher loads on the third anchor if you should be pulled off your stance.

A good observation. Something to be aware of. Usually, for convenience, I keep the guide mode belay device above waist height for comfort while belaying. Again something to be aware of.

rgold wrote: Personally, I see little advantage in this situation to a sliding-X over independent anchor arms that do not allow for extension in case a piece fails. Perhaps if the direction of the load changes a lot as the second progresses and unclips pieces? But this is usually easily remedied by placing a directional piece at the top of the pitch before the belay.

I like this method because of its simplicity and speed of deployment. Although, if I'm gunna die I'll switch to something else.

I've yet hear a compelling argument against using two quickdraws at the anchor.

Don Ferris wrote: Im not seeing this. In a simplified situation, a 24" sling clipped to two close pieces and you grab the middle to make the sliding x, the belay device would be 12" away from the two pieces.

Oops, you are right---as long as the belay device stays locked off as it drops. I was thinking of the situation with just a carabiner (eg when a screamer deploys), in which case 12" of slack created goes through the biner as well as the biner losing 12" of altitude.

Rgold
I almost went back in and made that edit.
Absolutely right. There are several pitches here in the Wasatch that have big traverses into the anchors from the last bit of gear. As the second gets closer the angle changes dramatically.
That's when I will use a quad.

rgold wrote: I doubt the peak load being reached "slower" has anything to do with it. What matters is that the sling extension barely affects the fall factor, which means that in case of the failure of the top nut, the impact on the lower nut will be essentially no worse than if it had been the only piece, and may be a bit less because of the fall energy absorbed by stretching the rope enough to extract the top piece and the fact that when the rope is unweighted by the failure of a piece, it "snaps back" a little and so regains some of its former energy-absorbing capacity.

Isn't the problem with the X that, even though it seems to equalize perfectly when you tug on it, it cinches too tight under a heavy load to allow equalization? But high on a pitch, with lots of dynamic rope out, that initial impact is more of a gentle tug that the biner into an equalized position. That's what I meant by "slower".

This is likely the point where I should just stop worrying about math and just place as much good gear as I need to not crater.

Slartibartfast wrote: Isn't the problem with the X that, even though it seems to equalize perfectly when you tug on it, it cinches too tight under a heavy load to allow equalization? But high on a pitch, with lots of dynamic rope out, that initial impact is more of a gentle tug that the biner into an equalized position. That's what I meant by "slower". This is likely the point where I should just stop worrying about math and just place as much good gear as I need to not crater.

The friction doesn´t change with the force, when you tug it you unconciously move it to the position you think is best. Set one up with just a light weight (or none at all or hang yourself up on it )and slide the karabiner to the side and you´ll see there´s a position it stays put which isn´t the centre, same the other side.
The offset might not look much but that´s all it takes, to get equal loads on the legs it´s a question of millimeter adjustment.

Jim Titt wrote: Well yesterday I was at a meeting with the technical trainer of the area mountain rescue and took the opportunity to acquire two identical Wild Country 12mm nylon/dyneema slings, both with identical usage and with one year left before they are discarded after 5 years. This morning I broke them to answer your original question. As a normal sling the first broke at 14.28kN. The other I tied as a sliding X with overhand limiter knots and set up with an initial angle in the Vee of 60°. The first failure was in the exit of one of the knots to the centre section in the strand without the X at 13.06kN and then the remaining strand failed at the same point at 9.6kN

Thanks for doing this! So it sounds like in your test the sling failed in the X", with a single strand, then continued pulling cause the 2nd strand to fail (at a lower force). But the knot itself held (in this one test)?

Jim Titt wrote:So no, it isn´t redundant in the sense that if you apply enough force to break one strand the other will inveitably fail. Leaving Dyneema pre-tied is a bad idea. The knots weaken considerably with age and wear more than the rest of the material as the outside of the knot is a hard point which rubs, there were some tests done on this somewhere with tethers.

Thanks Jim. FWIW I would have used nylon slings for this.

Jim Titt wrote:For top-roping this loss of strength is probably irrelevant but safer and more convenient is two draws anyway with a massive increase in strength, reliability and redundancy. (Redundancy comes in different flavors, a single component used in a redundant way but with the possibility that it may have been damaged or contain a manufacturing defect is definitely less redundant than two completely different objects like two draws from different manufacturers).

I agree with you there. Generally my definition of redundant is if it remains safe when any one point in the system fails, regardless of cause. Seems the safest way to think about it, however unlikely one of the causes could be.

Llati Wonki wrote:I do not understand why all of this discussion for a top rope anchor.

The fuss isn't really about a TR anchor. More about the fact that I've seen this set up being presented as redundant in multiple places, and questioned the reality of that. Maybe they intended the redundancy to only be in case of gear failure, but it wasn't clear to me. and I can easily see someone judging this fully redundant on a cursory inspection, as it can easily SEEM like there's four independent part's of the system (the 2 loops on the anchor, and the 2 strands of the "X")

Marc801 wrote:Nope. Yes, you started it, but once posted it takes on a life of its own and you cannot control who replies, what they say, or if they stay on topic. It's the way net discussion forums work.

FWIW the same rule applies to me. I'm free to call people out on it, just as they are free to ignore that, or go off topic.

Ok, so I just had a thought, well two actually.

1) Doesn't the "equalette" have the same potential failure mode?

2) This set up would become fully redundant by adding an additional overhand, or figure of 8, knot to each leg of the sling to isolate the anchor side biners from the "X" limiting knots (so 4 knots total, spaced out). Any one failure point wouldn't be able to result in complete anchor failure. Obviously you'd need a longer sling than 60cm, though.

Then it's just back to the good old argument of "equalizing" vs "equalized"...which we've already been over.

Brian L. wrote:Ok, so I just had a thought, well two actually. 1) Doesn't the "equalette" have the same potential failure mode? 2) This set up would become fully redundant by adding an additional overhand, or figure of 8, knot to each leg of the sling to isolate the anchor side biners from the "X" limiting knots (so 4 knots total, spaced out). Any one failure point wouldn't be able to result in complete anchor failure. Obviously you'd need a longer sling than 60cm, though. Then it's just back to the good old argument of "equalizing" vs "equalized"...which we've already been over.

1) Sure, the equalette is essentially the same thing as a sliding X.
2) Lost me there, the one I tested broke at the exit of the knot for the sliding portion and adding more knots somewhere else won´t help.

Any of these systems you end up with all the load possibly on one strand of tape (this depends on your knot tying skills and how the knots tighten under load)spread out in a Vee so something like 10kN is all it´s going to hold. Seems barmy to me, I´d use a cordalette if I wanted to go for something like this since at least I´d know that bit wasn´t going to break but I´ve never bothered since just slapping a couple of draws or biners on is stronger and easier than anything else. Most of my local climbing areas there´s only one bolt anyway so how they are equalised is pretty irrelevant:-)

I'm thinking of a situation where a strand get's chopped (regardless of how), and the limit knot untied. With a secondary knot the anchor should still hold. Basically protecting against the "no tail" situation I was concerned about in the OP.

Maybe I'm misunderstanding your failure report above, but it sounds like the strands failed one at a time, on individual pulls - granted the 2nd strand failed at a lower limit (weakened by the first pull).

I see that as being no different than tying a master point with two 60cm slings, or one 120cm sling. In either case you're relying on 2 strands not to fail simultaneously in the loop. This test shows the loop failing on a 3 point cordellette anchor: youtube.com/watch?v=zfGYmsb… so it seems the failure point is basically the same.

If you had tied those same slings into a master point, what would they have held? Since the one sling itself failed at ~14KN, I would guess no more than that, and I'd expect similar weakening of the 2nd strand. The X failed first at ~13kn, and given the sample size of 1, I'd say that's roughly similar failure points.

I'm basing this on the fact that in BD's test, both the sliding X with limit knots, and the figure 8 master point failed at roughly similar loads. blackdiamondequipment.com/e…

Depends where the strand gets chopped whether an extra knot prevents the limiting knot from failing. In other words the safety of the anchor depends on guesswork/luck.

The sling is effectively doubled so should have held 28kN, reduce that a bit due to the Vee angle (it´s probably down to 30° by the time it´s all stretched and the knots tightened). The limiting knots prevent the two centre strands taking the load equally and weaken the material considerably. The load was probably shared unequally on the two strands until the first failed at 13kN, then the other had to carry the load on it´s own and let go at 9.6kN which is about what one would reasonably expect for a 14kN sling with a knot in it. I wouldn´t have thought the strand was weakened in some way.

Tying knots in anything weakens it so sure a cordalette or whatever is going to give the same sort of results though tying the centre knot with an 8 and using 8mm cord I´d hope to see a bit more.

The only knots I tie when I build a belay are in the climbing rope:-)

Jim Titt wrote:Depends where the strand gets chopped whether an extra knot prevents the limiting knot from failing.

I don't see that, unless the entire sling (2 strands) is chopped close to the limit knot. But a figure 8 master point has a same problem. Can you explain how you see it failing otherwise?

I guess at the end of the day, if you're really worried about hardware failure you should use independent pieces of hardware, regardless of what type of anchor you want to build. A figure 8 master point is more redundant with two 60cm slings, vs one 120cm sling.

I agree with you about using the rope when belaying multi-pitch (or from the top). Unless you're really concerned about time and speed (like every second counts), or really need that extra 10ft-15ft of rope, the rope is the easiest thing to use, and requires the least extra gear.

Really what I'm working on is finding a top anchor that can be pre-tied to work in many bolt situations (and hangs quickly), is "bomber" (because why not), and doesn't involve bringing up a large, bulky cluster on my harness when I lead the climb. I'm leaning towards just making a "mini-quad" from 8-10ft 5mm tech cord and calling it done.

Brian L. wrote: Really what I'm working on is finding a top anchor that can be pre-tied to work in many bolt situations (and hangs quickly), is "bomber" (because why not), and doesn't involve bringing up a large, bulky cluster on my harness when I lead the climb. I'm leaning towards just making a "mini-quad" from 8-10ft 5mm tech cord and calling it done.

As I and countless others have said for generations, two draws. Put lockers on them if you´re completely paranoid.

Jim Titt wrote: ...two draws. Put lockers on them if you´re completely paranoid.

+1 to this ^^^

Quick, easy, and already on you.

Jim Titt wrote: As I and countless others have said for generations, two draws. Put lockers on them if you´re completely paranoid.

eh, all those options are essentially the same.

FWIW I, nor my partner, are not fans of top roping on two draws (lowering so someone else can lead, fine). It's not that we feel it's specifically unsafe, obviously there aren't a bunch of accident reports out there about it, and it's very common to see, but our philosophy really is "why not" when it comes to safety, especially on a TR anchor that's out of sight.

Two locking draws is fine, but at that point it's still "extra", so not really a better option than a small quad, just different. Heck, even two extra regular draws for an anchor is extra, if you want to get pedantic.

Brian L. wrote: eh, all those options are essentially the same. FWIW I, nor my partner, am not a fan of top roping on two draws (lowering so someone else can lead, fine). It's not that we feel it's specifically unsafe, obviously there aren't a bunch of accident reports out there about it, and it's very common to see, but our philosophy really is "why not" when it comes to safety, especially on a TR anchor that's out of sight. Two locking draws is fine, but at that point it's still "extra", so not really a better option than a small quad, just different. Heck, even two extra regular draws for an anchor is extra, if you want to get pedantic.

If you've already decided what you want to do, why keep continuing the discussion? Why even ask the question in the first place?