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A Question Of Force

Author: jmaher
Date: September 10, 2006
OK. Here’s a force issue for all of you “techies” out there.

When I go climbing in wild trees that have never been climbed before I usually try to get my rope over several limbs, perform a bounce test, then climb my entry pitch SRT. I have always told myself that this would be safer because I am distributing force among more than one anchor and in the event that an anchor breaks, there will be other anchors to keep me from creating an impact crater in the earth at the base of the tree.

I still believe this to be a safe procedure. However, a situation from this past weekend has given me cause for some re-thinking on the matter. Ron Reese and I were climbing a large pine on the property, mainly because Ron was interested in problems associated with climbing pine trees. Here is the scenario.

A throwline was fired into the tree using the Bigshot. The Bigshot wasn’t necessary, but Ron wanted to see it in action, so we used it to achieve the setting. The line went over two limbs, each five to six inches in diameter, and quite close to the trunk. The two limbs were not directly above one another. The upper limb was at about forty feet and two feet higher than the lower limb and about fort-five degrees to one side. The rope was hoisted and after it had passed over the upper limb, a figure-eight-on-a-bight was tied in the side of the rope going upwards, a delta inserted into the bight, and the throwline coming down was threaded through the delta also. Hauling resumed and as the end of the rope that had already passed over the limb continued downward it eventually passed downward through the delta as the delta traveled upward. We ended up having a setting consisting of a “pull-down” with the rope cinched around both limbs, and the delta and the figure-eight-on-a-bight snugged against the lower limb.

Here is the first question: How much force is being placed on that upper limb? Without having given it a whole lot of thought I simply assumed that the actual weight of the climber was the force being placed on the limb. That would be the case if the TIP was at the upper limb and the lower limb was not a part of the equation. “Not so”, says Ron. The lower of the two limbs is acting as the tie-in-point and therefore the force on the upper limb would be almost double the weight of the climber. We are, of course, speaking of theoretical rather than actual forces.

Next question: How much of the force is being “absorbed” by the lower limb? If the lower limb was directly beneath the upper limb there would be no force absorption at the lower limb. Here, however, the rope is making an approximate forty-five degree turn as it passes over the limb, so some of the force would be on the lower limb.

Next question: How much force would be in effect between the two limbs as the choking action of the pull-down “squeezes” the limbs toward each other? Would this influence the security of the setting?

Final question: If the upper limb was to break, how much of a dynamic force would be placed on the lower limb as it takes the weight of the two-foot fall from the upper limb? We will assume a clean sharp break that gives away totally and instantly.

Ron has suggested some experimentation here to determine just what is going on and to identify any security issues that would be in effect in such a situation. In the meanwhile I would like to see some of the rest of you run it through your brains and let’s have a little discussion.

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