Girth hitch strength

[Arborist1]

Does girth hitching a prusik around a carabiner increase/decrease the breaking strength of either the carabiner or the rope versus just clipping the biner directly on the loop?

 

[Z'sTrees]

I am not 100% but I would assume a girth hitch is sort of a choker, which would reduce the capacity by some amount.

 

[NorCalBrock]

I don't know the science or right answer- but I was under the impression a girth hitch actually increases the capacity because it spreads out the load at the friction points.


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[NickfromWI]

Are you wondering for curiosity? The real answer is if you're pushing gear to the point where this discussion matters, it's time to upgrade the gear!


love
nick

 

[Raven]

http://www.fdlake.com/rig-slng.html

Just a random search result, but should have the answer you're looking for. There IS a reduction in strength, at the point where the cordage bends around itself. Notice that the prusik hitch is also simply a triple girth or choke as well. I've heard it's roughly a 40% strength reduction depending on the size of the object being choked and the angle of pull. But that's why our gear is rated to lift a small truck.

If you want to choke the biner without adding bulk or strength reduction try using a Dogvine Loop instead of a standard Prusik Loop

 

[evo]

http://www.fdlake.com/rig-slng.html

I've heard it's roughly a 40% strength reduction depending on the size of the object being choked and the angle of pull. But that's why our gear is rated to lift a small truck.

But don't you need to account for it having two legs? something like 100# single leg break is = 200# basket and 120# girth? (random # used to make a point)

 

[Raven]

More searching reveals it's a question as old as climbing itself. Yes, theoretically the two legs will double the strength, but not before one of them is cut through by the friction at the "throat" or bend of the girth.

The diagram at the very bottom of this page seems to support that: https://www.lift-it.com/info_hitches_all.asp The "Double Choker Hitch"

Why do I feel like the construction riggers have a better grasp on this stuff than the rock climbers?

 

[DSMc]

....Why do I feel like the construction riggers have a better grasp on this stuff than the rock climbers?

That was a nice chart but keep in mind, there are many differences between sling and rope applications. Even on a girthed sling they need to calculate angle force multipliers because, as the chart explains, they are always there. With a rope on something as small as a carabiner, not so much. Due to the small bend radius involved with the op's question, I would be surprised to see much real-world measurable strength difference.

 

[NorCalBrock]

I agree - gear shouldn't be pushed to it's breaking point, especially with dynamic force.

But, it would be nice to know a best practice.

I've always used a girth / choker / prusik on loops at the attachment end because I thought it spreads the force throughout the rope.

A single loop seems to put all the force on the single bend of the rope at the attachment point.




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[evo]

Nahh, if you think about it the only distribution of force is on the object which its on, not the rope. The rope takes the same path around the hook, limb, carabiner or what ever, but with a girth there is a extra bend which futher distorts the fibers in the rope.
I'd venture to guess a girth made with a large spliced eye has no appreciable reduction, but girthing a sling compared to a basket has a large reduction.

 

[Arborist1]

Are you wondering for curiosity? The real answer is if you're pushing gear to the point where this discussion matters, it's time to upgrade the gear!


love
nick

Fascinating stuff guys thanks for the responses so far. I guess why I asked is out of curiosity on what the best way to attach a prusik loop to a carabiner for a 2:1 lanyard. Ive always girth hitched it to the carabiner but was wondering if it can just be clipped on to the carabiner instead? If the girth hitch does in fact decrease the strength of the rope than why does it seem like such a common practice; is it to keep it isolated on one part of the biner to decrease possibility of opening the gate?

 

[DSMc]

A prusic cord with two tight eyes clipped onto the carabiner would be the strongest.

 

[Arborist1]

http://efclimbers.net/wp-content/uploads/2013/02/Knot-and-cord-strength.pdf

Stumbled upon this which states loop strength around carabiner sized pin is ~80% of double the tensile strength. If I'm reading that right? From my understanding a girth hitch would be a little less than that but still would be at or above tensile strength of rope.

 

[NickfromWI]

...I guess why I asked is out of curiosity on what the best way to attach a prusik loop to a carabiner for a 2:1 lanyard. ...

Then the answer is, "strength, in this application, doesn't matter. Your hitch cord (beeline or the like?) is so strong that in even extreme climbing situations you'll NEVER create loads that would push things far enough where the loss from girth-hitching would ever matter."


love
nick