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For the Engineers Out There: Impact Forces on a Screw with Two Ropes


Original Post
doligo · · Unknown Hometown · Joined Sep 2008 · Points: 277

I was going through some ice condition photos and came upon this:

Two Draws/Ropes on One Screw

Looks like he clipped two draws into one screw and clipped two ropes independently. I've never seen this before, except for on traversing rock climbs. Not sure what's the thinking behind it (protecting against cutting one of his ropes?) But the engineers out there, do you think if he fell, the impact force is going to be greater with two ropes clipped into this screw in this manner? I'm assuming it's double ropes, since twins are not very popular in the US, plus with twins you can just clip them together...
rgold · · Poughkeepsie, NY · Joined Feb 2008 · Points: 525

The impact force will definitely be greater. How much greater is not so clear in the real world; the elementary theory says 40% greater but I doubt anything like this happens in practice, because modern halfs and twins meet UIAA standards for single ropes and so loads will not exceed the range assumed appropriate for a single rope.

Still, two strands are going to be stiffer than one and one would be adequate if the ropes are certified as half ropes.

kevin neville · · Somerville, MA · Joined Jun 2013 · Points: 15

Reduces rope stretch, less likely to hit a ledge/bulge/etc. Yes, it increases force, theoretically by a factor of square root of two if the ropes behave as ideal springs (they don't of course).

20 kN · · Unknown Hometown · Joined Feb 2009 · Points: 1,348

The load will increase to some degree. It is possible he clipped both strands because he was over a ledge. Who knows? There are times where fall distance is more important than impact force though.

Karl Henize · · June Lake, CA · Joined Aug 2013 · Points: 570

Commonplace for half rope technique, instead of twin technique. avoids rope burn, after alternating ropes higher up.

Guideline #1: Don't be a jerk.

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