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agdavis
Dec 22, 2011, 6:41 PM
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I've used this set up a couple times when I've been dealt questionable placements, but I've neve seen it in an anchor book or discussed, etc. Evaluate it:
Anchor. 4 pieces of pro (didn't feel safe only having three, given the placements). We'll call them pieces #s 1, 2, 3 and 4, arranged horizontally.
Connect 1 and 2 with a sliding x.
Connect 3 and 4 with a sliding x.
Then connect each of those pairs with one sliding x, to give you the master point.
Sort of like:
1 2 3 4
\ / \ /
\ /
\ /
Extension in the case of failure aside, any reason not to do this?
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billl7
Dec 22, 2011, 6:55 PM
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Have done similar.
If using one sling for each sliding X, there's loss of redundancy at the power-point sliding X.
If using single-length slings, the extension isn't going to be horrible - maybe a foot? A little less would be nicer.
Better hope those slings aren't needed on the next pitch! 
Bill L
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cardina16
Dec 22, 2011, 6:58 PM
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Well technically if there was a failure of the runner in the final master point sliding X it would be a catastrophic failure of the anchor. That said you could double up that runner to be extra safe.
Another option is you could use a single runner to join all of the legs instead of multiple sliding x-s. See page 36 of the Leubben book. (http://www.amazon.com/Rock-Climbing-Anchors-Comprehensive-Mountaineers/dp/1594850062/ref=sr_1_6?s=books&ie=UTF8&qid=1324580029&sr=1-6#reader_1594850062)
That said, I consider it safe to climb on, not the best setup perse, but good to know. I've used it when I accidentally forgot to bring a cordelette.
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colatownkid
Dec 22, 2011, 7:02 PM
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agdavis wrote: I've used this set up a couple times when I've been dealt questionable placements, but I've neve seen it in an anchor book or discussed, etc. Evaluate it: Anchor. 4 pieces of pro (didn't feel safe only having three, given the placements). We'll call them pieces #s 1, 2, 3 and 4, arranged horizontally. Connect 1 and 2 with a sliding x. Connect 3 and 4 with a sliding x. Then connect each of those pairs with one sliding x, to give you the master point. Sort of like: 1 2 3 4 \ / \ / \ / \ / Extension in the case of failure aside, any reason not to do this?
Nope. However, it's worth noting that with an odd number of pieces, the theoretical load distribution can be uneven. For example, if you have a 3-piece anchor, and you equalize pieces 1 and 2, and then connect that point to piece 3, pieces 1 and 2 each get 25% of the load while piece 3 gets 50%.
To use a similar diagram:
25% 25% 50%
\ / /
50% /
\ /
100% |
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njrox
Dec 22, 2011, 7:19 PM
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agdavis wrote: ...4 pieces of pro ...1, 2, 3 and 4, arranged horizontally. Connect 1 and 2 with a sliding x. Connect 3 and 4 with a sliding x. Then connect each of those pairs with one sliding x, to give you the master point. Extension in the case of failure aside, any reason not to do this?
This reminds me of a situation when I had 3 peices of pro.
1/2 were a sliding-x. And from 3, I created a cordellete (clipped to 1/2's X).
1 2 3
V /
\ /
M
The X was really only because I needed to extend 1/2 and all I had was a short sling. So, it was an X that really didn't "slide".
And I'm wondering how much the all the other sliding-x's in your set-up were really sliding, other than the X at the master point?
I'm also one of those people who doesn't think the sliding-x is the devil. I use a cordellete most of the time as my master point, but have put x's within the system (like mentioned above).
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rgold
Dec 23, 2011, 3:01 PM
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The net result of the rounds of discussion and testing is that sliding X's don't equalize very well. So the danger in setting up an anchor like this is that the theoretical appeal of equalization might lull you into trusting an inadequate anchor.
My sense of the testing that I've read is that no matter how you rig things, it is prudent to imagine that some single piece of your anchor is going to get half the load, maybe a lot more. In practical terms, if you don't think any two of your pieces would be, by themselves, adequate, than your anchor, no matter how fancy the rigging, probably isn't adequate.
I'm not saying you are necessarily supposed to retreat. But the leader and belayer should be fully aware that a fall directly onto the belay, either because there is no intermediate pro or because intermediate pro pulls, might extract some or all of the anchor pieces, and ciimbing decisions should be made with that in mind.
In particular, rigging, say, four marginal pieces with some sliding X's is going to result in a marginal anchor, and the party shouldn't lose sight of that by imagining that each piece is going to get a quarter of the load.
(A mathematical nicety of no practical consequence: rigging something analogous to the four-anchor case with other numbers of anchors requires that the number of anchors be a (non-zero) power of 2, i.e. 2, 4, 8, 16, etc. So it isn't that the method fails just for odd numbers of anchor pieces, as one might have concluded from colatownkid's post.)
(This post was edited by rgold on Dec 24, 2011, 2:55 AM)
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