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SRENE anchors may go the way of the dinosaur


nmonteith

9-Mar-2006
5:55:54 PM

Can someone post a photo example of a sliding X. I don't really understand how they work.


belayslave

10-Mar-2006
12:56:40 AM

Neil et al.
checkout
http://www.spadout.com/rock_climbing/wiki/index.php/Anchor
for some very rudimentary pics and explanation.

I think someone said you can only ever use 2 pieces in a sliding x setup, not entirely true as it's very
possible to (as i was showing someone just earlier this week) use 3 (or more) pieces. Though the more
pieces you add the less smooth the self equalisation is. Possible to avoid this by using two slings
attached to 3 or 4 pieces.
It's similar idea to a standard SARENE setup, but no knots, just two twists... hard to explain. i might get
around to taking some pics for people.

Edit : well got a bit excited and found another website
"2. The x-sling system (John Long) although still widely used are discouraged by authorities. Although is
equalises excellently any failure of an anchor will cause shock load on the other anchor. The cordelette
system is much safer. In fact a duo-cordelette tied in 7mm have a breaking strain exceeding 23kN and a
duo tied in 5.5mm Spectra approximated 30kN. A brilliant system if you consider that this anchor should
never experience anything above 12kN in any normal fall."


Phil Box

10-Mar-2006
9:00:21 AM

As far as one arm failing on a sliding x setup goes, it appears through research conducted by John Long et al, that shock loading may in fact be not very great. It was shown that the loads placed on the other arms only increase linearly and not exponentially. In other words there is very little shock load transmitted to the other arms.

bordo

10-Mar-2006
11:40:18 AM

I'd be more worried about the extension in terms of belaying than the anchors. Most belay positions don't lend themselves to suddenly being 1/2 a meter over the edge of a cliff. I imagine falling half a meter as the x extended and trying to hold a fall would be not that much fun if it meant you went from being on the edge of a cliff to hanging over a cliff.

One you get practised at using a cordalette it is pretty trivial to ensure that all your anchors are loaded when belaying.

Regards
Mark

uwhp510

10-Mar-2006
4:53:52 PM

Sure all the anchors get loaded a bit, but as soon as you have unequal lengths, even if the tie in point is perfectly positioned, then the cordalette is not going to be close to equalised, irrespective of the stretchyness of the cordalette material, and I think that this is the point (ie the non-equalisation aspect) of the whole hoo-haa.

It wasn't obvious to me at first, but if you think carefully about it you'll see why.

This is really only an issue in big-time loading I reckon (ie factor two falls).


anthonyk

10-Mar-2006
5:07:50 PM

well whatever, if you tie a proper cordalette anchor or a proper sliding X they're both going to work. there's so many other random things that could happen i think the details of this or whether your biner is rated to 2.5tonnes or 3 is all academic (although i would be cautious of using a single sling that can break anywhere and undo the whole anchor). as long as the set up is decent i'm happy thinking about the climbing instead. in fact the people i know who have had the most accidents are the ones that micro-manage their safety systems instead of focusing on themselves.

patto

10-Mar-2006
5:11:37 PM

On 10/03/2006 belayslave wrote:
>Edit : well got a bit excited and found another website
>"2. The x-sling system (John Long) although still widely used are discouraged
>by authorities. Although it equalises excellently any failure of an anchor will cause
>shock load on the other anchor.

Actually the point of the link thread was that real tests show that 'shock loading' is negligable and pretty much a myth if the belay is off the anchor. If the physics is considered then this shouldn't be a surprise because there is negligable mass in the extention.***

***Of course if there is mass in the system, eg hangin belay, the shock loading becomes a reality.

On 10/03/2006 bordo wrote:
>One you get practised at using a cordalette it is pretty trivial to ensure
>that all your anchors are loaded when belaying.
>

Again, the real world test have shown that in reality the forces are a long way from being equalised in most cordalette set-ups.

However the original thread has sensationalised the results as some ground breaking new discovery. Furthermore some people are running with their hand above their heads claiming cordelettes are dangerous and should be use. It's silly.

While cordelettes aren't particularly good at equalising, there still remains some equalisation. Furthermore equalisation isn't everything, after all, any piece I put in for an anchor I would expect to happily hold 10kN. I will continue to use a cordelette by preference because it a quick and simple way of making an anchor out of several bomber pieces. It covers redundacy well which I see is the most important aspect of anchor building.

uwhp510

10-Mar-2006
6:05:50 PM

Yeah I agree with all of that, but I think that the interesting thing about it is that even a PERFECTLY tied cordalette, which is loaded in the PERFECTLY right direction, if it has unequal length arms, doesn't come close to being equalised, which could actually become important.

For example, if you have a two armed cordalette with one arm twice the length of the other, then the load on the shorter arm is twice that on the longer arm, when its all perfectly set up. (ie the load F is distributed as 2F/3 and F/3).

Don't get me wrong, I'm no safety nazi. But the fact that a cordalette seems totally equalised when its not (if the arms are different lengths) is kind of interesting, and seems like a good thing to take acount of.

patto

10-Mar-2006
8:01:28 PM

On 10/03/2006 uwhp510 wrote:
>Yeah I agree with all of that, but I think that the interesting thing about
>it is that even a PERFECTLY tied cordalette, which is loaded in the PERFECTLY
>right direction, if it has unequal length arms, doesn't come close to being
>equalised, which could actually become important.
>
>For example, if you have a two armed cordalette with one arm twice the
>length of the other, then the load on the shorter arm is twice that on
>the longer arm, when its all perfectly set up. (ie the load F is distributed
>as 2F/3 and F/3).

How do you figure that? I think somebody has made a mistake with their triginometry.


mtnbear

11-Mar-2006
12:15:41 AM

On 9/03/2006 nmonteith wrote:
>Can someone post a photo example of a sliding X. I don't really understand
>how they work.

The deal with the sliding X is that by putting a twist in one side of the sling/webbing and passing the biner through it and the other strand of webbing, you're putting a wrap around the biner.

If one side of your anchor blows out with this, you will still be held by the webbing and biner because you have a loop going through the biner. more details on it can be found in Freedom of the Hills or John Long's "Anchors"(where as if the American Death Triangle were used the biner comes right off the webbing and so do you if one side blows)

I think I will be rigging a few "equalettes" to test them out myself (found on page 6 and 7) of the rc.com thread. Looks worth a try and would probably be just as fast as a cordolette once you get used to it.

I'm just happy someone decided to take a closer look at the way our anchors were set up and started what I hope will be an evolution in their construction design.

uwhp510

12-Mar-2006
7:54:53 PM

On 10/03/2006 patto wrote:

>How do you figure that? I think somebody has made a mistake with their
>triginometry.

There was no trigonometry involved whatsoever.

I did the analysis in one dimension. I can post it if you want, since its very simple.


plumb bob

12-Mar-2006
8:53:19 PM

I for one would be very keen to see your analysis as I can't see for a minute that unequal length yet
equalised arms can have the results you say.....

but you know, I have been wrong many times...

uwhp510

13-Mar-2006
9:31:59 AM

OK,

The crux of the matter lies in the fact that stuff (nylon webbing, cord, spectra, steel cable...) stretches, and provided we don't exceed the elastic limit of our material, stretches in the manner;

F = kx (where k is the spring constant, and x is the extension resulting from force F)

If we ascribe a length of cord (equating to one of the arms of the cordalette) the spring constant k1, then the spring constant of the other arm (which is twice as long) is k2 = k1/2 (which can be determined by treating the arm as two equal springs, of the length of the shorter arm, in series ie. 1/keq = 1/k1 + 1/k2).

So now we have two springs in parallel, being the two arms of the cordalette, one with a spring constant k1 and the other with a spring constant k1/2. Springs in parallel add, so keq = k1 + k2 = 3/2k. So the extension in the whole system, x, is due to the load F, which is the load on the belay and F = 3/2k1x

But the tension in each arm, which is each also extended by x, is given by T1 = k1x and T2 = k2x = k1/2x. Thus substituting for k1x, F = 3/2T1 = 3T2, ie T1 = 2T2.

The assumptions i've made are that
a) the two arms are directly in line, so that once loaded the system can't swing to the side to compensate, but since we are talking about the most ideal case (with no angle between the anchors, and a perfect loading direction, I think this is unimportant)
b) that we can treat nylon cord/webbing as a perfect spring. I think that this is at least a good approximation for my purposes.

If it all doesn't make sense I can draw a picture I guess.


anthonyk

13-Mar-2006
11:50:17 AM

On 13/03/2006 uwhp510 wrote:
>But the tension in each arm, which is each also extended by x, is given
>by T1 = k1x and T2 = k2x = k1/2x. Thus substituting for k1x, F = 3/2T1
>= 3T2, ie T1 = 2T2.

yes that is an interesting point, static ropes aren't really 'static' at all, just less stretchy than dynamic ropes.

i suppose you could argue that in a sliding system the tension is somewhat equal throughout the whole system, aside from the effect of friction on the carabiners.

from all that i'd say the nicest system on paper (according to my biases) is to use two big slings of equal length and use a double sliding-x on each, on 3 bits of gear. if you're happy with 2 pieces then great but i think if you're being picky about your anchor 3 bits vs 2 makes much more difference about whether it'll fail than anything to do with equalising. (the chance of a piece failing or not under any reasonable range of loads is a much more significant factor than whether a piece will fail under a given load or twice that load)

uwhp510

13-Mar-2006
12:02:06 PM

But this unequal tension could work to your advantage if you keep in mind that placing iffy bits in your anchor (if you've no other option) then putting them further away is better, and possibly better even than having them equalised. I wouldn't use this as my main, guiding principle in anchor building but it's something to keep in mind in less than ideal situations and something that I (despite my physics degree) had never thought about before.

The two arm thing in the analysis was for simplicity, but the principle could be easily generalised to any number of "arms" of the cordalette.


wombly

13-Mar-2006
12:48:49 PM

Do any of the physics types out there know if the rig setup will produce unequal forces in other ways apart from the spring idea? Ideally, to hold the max load you'd like to put more force on the strong anchors, and less on the weaker ones, as stated above. However, if you have to use dodgy placements you're not likelty be able to choose the locations of them. What do people think of trying to achieve the same goal by deliberately slackening the weaker anchors by the right amount ? This is not something you could do with the sliding x/equalette as the system decides for you.

jonorock

13-Mar-2006
1:39:17 PM

Has anyone ever had an anchor fail/partially fail on them?????

uwhp510

13-Mar-2006
1:56:58 PM

I reckon that would work from a physics point of view, because when you load up the anchor, you've put a bit of "preload" on the good bits before the weaker bits start to feel the load, but from a real life point of view I don't think you would have a hope of predicting the right amount of slack to leave, since there will be a very small margin between too slack and too tight.


wombly

13-Mar-2006
2:15:13 PM

Thought that might be the case - particularly if you're using short bits of "static" sling rather than the climbing rope.


Phil Box

13-Mar-2006
3:36:06 PM

On 13/03/2006 wombly wrote:
>Thought that might be the case - particularly if you're using short bits
>of "static" sling rather than the climbing rope.

Don't use static anything in an anchor. The test results so far indicate that the use of nylon climbing rope is preferrable to anything else for anchor building. In fact it looks like spectra is a decidedly inferior material to use as opposed to the nylon climbing rope. More to come in a few weeks time when Simon and I have started to get our teeth right into the testing.

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