Forums: Climbing Information: The Lab: Re: [patto] Theory about forces in a 3-legged cordelette: Edit Log




jktinst


Jul 16, 2012, 9:06 AM

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Registered: Jun 29, 2010
Posts: 88

Re: [patto] Theory about forces in a 3-legged cordelette
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It seems to me that these predictions of doom if any extension is allowed on a blown pro are based on the premise that the belayer is attached to the belay anchor through a static link and that s/he is yanked off his/her stance and goes straight into free fall without opposing any resistance.

Nowadays, I am using as much as possible belay-setting and belaying approaches that aim to either avoid altogether the possibility of the leader falling directly on the belay or severely limit its impact. As a result, the extension vs. equalization debate affects me a lot less than it once did. However, even with these approaches, failure of the first few progression pros (however unlikely) could still result in the leader falling on the belay so the question remains relevant.

With respect to the scenario above: I use a dynamic cowís tail and, although I am reluctant to use stancing as a primary means of limiting the impact of a leader fall on the belay anchor, I would like to think that, if one of the anchor's pros were to blow, I would offer some resistance to being pitched into free fall by the few inches of extension that this would cause on my anchorís central point.

It seems to me that most proponents of dynamic equalization accept that it must be balanced with extension limitation. It follows that equalization cannot be expected to distribute the load of a leader's fall evenly to all the anchorís pros regardless of where this fall may occur with respect to the belay. As a result, some anticipation of, and allowance for the likely offset should come into the planning of a dynamically equalized anchor (as it does with a static one) to try and allow optimal distribution of the load despite the extension limitation.

In addition, however much one may dislike the Sterling/Long/Gaines tests on extension, their equalization tests demonstrated very clearly that dynamic equalization is essential to allow load sharing between the anchorís longer arms and its shorter ones. Without this, you get most, if not all of the load on the shortest arm and, if it blows, on the next shortest one, etc. in a potential cascade failure scenario. So, despite the increasingly strident warnings that Iím going to die for allowing any extension on my belay anchors, I continue to see problems at least as dire with the absence of dynamic equalization.

In building an equalized anchor, I take into account not only the potential offset of the fall, but also the length of the arms of the system, the height of the central point and the quality of the stance (as well as other things that I won't clutter this discussion with) and set my extension-limiting knots close enough together to minimize the risk of getting yanked right off while still allowing the essential long-to-short equalization to occur.

Of course there is no preventing the yanking off and freefall situation in a hanging belay so, in these situations, why not 1) have the belayer hanging well below the anchor on a long dynamic cow's tail (which is one of the approaches mentioned above), 2) clip the leader's rope into the central point, and 3) limit extension somewhat more, looking mainly to take care of the long-to-short equalization ?

(This post was edited by jktinst on Jul 17, 2012, 1:31 PM)



Edit Log:
Post edited by jktinst () on Jul 16, 2012, 9:12 AM
Post edited by jktinst () on Jul 17, 2012, 1:31 PM


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