Mountain Project Logo

Unusual Girth hitch technique -ok?

Patrick Shyvers · · Fort Collins, CO · Joined Jul 2013 · Points: 10
Zacks wrote:Let me preface this by saying i'm not sure what the answer is. So i'm not trying to start flame war lol So in your picture, isn't the force on the rope where i put the red line the combined force of the 2 weights, or 100kg in the example. in a loop that point of the line would force equal to the total weight not half the weight. correct me if i'm wrong
Zacks, I can't see your picture, but...

- If you hang 100kg from one rope, the tension on the rope must be 100kg

- If you hang 100kg from both ends of a rope that has been wrapped around a pulley, the tension on each end of the rope must be 50kg- both ends hold half the load

- As for why the tension in the loop is not the combined forces, I'm not sure I have a good intuitive explanation why that isn't the case, but I can tell you that with a frictionless pulley the tension is always the same at all points on the rope. That's a rule, you can count on that.

Maybe it would be helpful to think about Newton's Third Law. When you pull on a rope attached to a wall with 100kN of force, the tension in the rope is 100kN right? But according to Newton's Third, we know that the wall is also exerting a force of 100kN on the rope in the opposite direction. That's why you don't move when you pull the rope. So you are pulling with 100kN, the wall is pulling with 100kN, and the tension on the rope is 100kN. If you've come this far, the rest is easy- the pulley is nothing but a change of direction, it does not change the tension on the rope.
Patrick Shyvers · · Fort Collins, CO · Joined Jul 2013 · Points: 10
Zacks wrote:If you ignore the knot the week point would be where it is only one rope ie the peak or point wear the rope/runner runs around a carabeiner or another rope. So a sling should only be as strong as 1 strand not 2, so the strength should be equal when run through a loop on a bite or girth hitched.
It's instructive to remember that most times the cord or webbing fails at the point of entry to the knot, rather than inside the knot.

Zacks wrote:Also I never understand when people want to say a girth hitch reduces the strength by x percent. You dont know all the variables!
You're right. Same thing goes for carabiners. Do carabiners rated for 22kN fail at 22kN? Mostly the answer is no. Most will fail at higher loads. Some rare carabiners will fail at lower loads. But a graph of all recorded carabiner failures is too much information. So we say, this carabiner is rated to 22kN.

22kN, much like "x percent strength reduction", is a pessimistic estimate to allow you to do the math on a napkin instead of a computer farm.
James Schroeder · · Fort Collins, CO · Joined May 2002 · Points: 3,166

Good god, the answer to the original question is that it is just fine from a safety perspective to do it either way, but neither is necessarily the most efficient.

Retracing an overhand around the tree/boulder generally conserves more webbing, and, more importantly, makes the length more easily adjusted when it comes time to equalize. There are times the single-strand girth hitch might be more efficient from a time standpoint while still preserving length. The looped girth hitch is almost never the most efficient because it does not save time over either method, is less adjustable than the other methods and takes more webbing than the other methods.

Greg D · · Here · Joined Apr 2006 · Points: 883
Patrick Shyvers wrote: If you hang 100kg from both ends of a rope that has been wrapped around a pulley, the tension on each end of the rope must be 50kg- both ends hold half the load - As for why the tension in the loop is not the combined forces, I'm not sure I have a good intuitive explanation why that isn't the case
Sounds like you are saying 100kg hanging from a rope will put only 50kg of tension in the rope? Do you want to revisit that?
Patrick Shyvers · · Fort Collins, CO · Joined Jul 2013 · Points: 10
Greg D wrote: Sounds like you are saying 100kg hanging from a rope will put only 50kg of tension in the rope? Do you want to revisit that?
If the rope is wrapped around a pulley and both ends are attached to the same 100kg load, then yes. 50kg tension.
Craig Childre · · Lubbock, TX · Joined Aug 2006 · Points: 4,860
Patrick Shyvers wrote: If the rope is wrapped around a pulley and both ends are attached to the same 100kg load, then yes. 50kg tension.
? Ummmm.... the pulley does nothing to spread the load?

Using your senario... if I cut one of those strands on either side of the pulley... how much weight will fall?

For the load to be spread over two strands, each strand needs to be independent.
Patrick Shyvers · · Fort Collins, CO · Joined Jul 2013 · Points: 10
Craig Childre wrote: ? Ummmm.... the pulley does nothing to spread the load? Using your senario... if I cut one of those strands on either side of the pulley... how much weight will fall? For the load to be spread over two strands, each strand needs to be independent.
We can go the other direction. The weight on the pulley anchor is the sum of the weight on the strands (that should be intuitively obvious).

So that means if both strands going to the one 100kg load each experience 100kg of tension, that means that the anchor is experiencing 200kg of load (even though we only have one 100kg weight)

It should be easy to see that the weight on the pulley anchor should be 100kg. It should also not be hard to see that the weight on the pulley anchor is the combination of the weight on the two strands. Therefor it should not be hard to see that the weight on each strand is less than 100kg- in fact, 50kg.
Greg D · · Here · Joined Apr 2006 · Points: 883

Um. What. So you have one weight attached to two strands with a pulley? What's the point of the pulley? It may be best for you to stick to the example diagram up thread. Two weights, one pulley.

Patrick Shyvers · · Fort Collins, CO · Joined Jul 2013 · Points: 10

Greg, it models a carabiner or other bend in rope/webbing. The question was about loading in that kind of scenario, e.g. why is a sewn sling 22kN instead of 11kN (its single-strand strength). But we can stick with the other diagram if people understand it better.

bearbreeder · · Unknown Hometown · Joined Mar 2009 · Points: 3,065
DAV

also from mammut

Hey Steph—this is a complicated answer, actually both of your readers are correct in a way. I actually made a diagram and sent it to the engineer I work with to make sure I had my ducks in a row. He did some testing to see if we could arrive at a specific rule for you—unfortunately that would take a lot more testing because the answer is complicated by a lot of variables that are almost never the same between any two “real” anchors, so it’s difficult to be too specific–but nevertheless there are some general rules of thumb we can keep in mind. Because of this, I’ve tried to generalize my own thinking on this without getting too caught up in numbers. Hopefully it’s helpful for your readers.

It is correct that using a loop increases the strength, and in theory it could even be doubled when compared to the strength of a single strand. The caveat is that in a climbing anchor the strength of this cord loop is never doubled, not even close. As part of discussing this, we did a semi-scientific experiment to illustrate the point. We took a loop of 7mm nylon cord (13kn tested breaking strength) and tied it into a loop with a double fishermans knot. When clipped into a carabiner at each end and pull-tested, this broke at the carabiner (picture 1) at roughly 35% higher load than the stated cord strength—not double, but still significant.


more at link ...

highinfatuation.com/blog/co…

so as a VERY conservative general rule of thumb

- single strand with knot ...~50% of cord strength

- single strand loop .... ~100% cord strength

- double strand loop ... ~200% cord strength

you should never count on a single loop to be double cord strength as the mammut test above shows

and to the OP ... in full strength webbing yr likely getting ~10 KN with that setup which works, but is not ideal (webbing might be well used and faded which means less strength) ... if it was a single strand with cord then the rating could well be below 6-7 KN especially if the cord is used well ... of course the knot itself will absorb a decent amount of force in a fall

thanks everyone else for the popcorn fun !!!

;)
Zacks · · Unknown Hometown · Joined Apr 2015 · Points: 65

Oh wow that's cool, thanks bearbreeder,

It seems to me that the guys at mammut are saying that the single strand is 50% strength due to the knot, and a loop is 100% since the knot is only on one side and is no longer the weakpoint, correct?

Kiri Namtvedt · · Minneapolis, MN · Joined Nov 2007 · Points: 30

When you're connecting a nut or cam placement to the anchor do you use a single strand or a doubled strand? I suspect most people use a single strand. The plan generally is to have three separate pieces, each attached to the master point with at least one strand of cord or webbing. Sure you could have a doubled strand (as with a cordelette) but if you have limited materials and your anchor points are not right next to your master point you will most likely run a single strand of webbing to your placement. This goes for trees and rocks as well as for nuts and cams. I don't think it's "unusual" to do this.

bearbreeder · · Unknown Hometown · Joined Mar 2009 · Points: 3,065
Zacks wrote:Oh wow that's cool, thanks bearbreeder, It seems to me that the guys at mammut are saying that the single strand is 50% strength due to the knot, and a loop is 100% since the knot is only on one side and is no longer the weakpoint, correct?
u got it

;)
mountainhick · · Black Hawk, CO · Joined Mar 2009 · Points: 120
Kiri Namtvedt wrote:When you're connecting a nut or cam placement to the anchor do you use a single strand or a doubled strand? I suspect most people use a single strand. The plan generally is to have three separate pieces, each attached to the master point with at least one strand of cord or webbing. Sure you could have a doubled strand (as with a cordelette) but if you have limited materials and your anchor points are not right next to your master point you will most likely run a single strand of webbing to your placement. This goes for trees and rocks as well as for nuts and cams. I don't think it's "unusual" to do this.
I guess I don't fit your theoretical group of "most people".

You state "cord or webbing", not the rope itself. The only time I commonly use a single strand is with the rope itself, whether anchoring to a tree or tying pieces together for a belay anchor. While I am not at all worried about the strength of that strand, I am concerned about using single strands of "cord or webbing" due to knotted strength.

It would be completely unusual that I would use single strands of cord or webbing in anchor building. Could happen, but I can't recall having been forced to do so in memory.
aikibujin · · Castle Rock, CO · Joined Oct 2014 · Points: 300
Zacks wrote:Let me preface this by saying i'm not sure what the answer is. So i'm not trying to start flame war lol So in your picture, isn't the force on the rope where i put the red line the combined force of the 2 weights, or 100kg in the example. in a loop that point of the line would force equal to the total weight not half the weight. correct me if i'm wrong
Zack, I think you’re mixing up two different forces (as your diagram suggests). And all that pull test, drop test, knot strength just served to muddle the water more. Let’s forget about knots and pull tests for a second and just look at a section of rope (or webbing, sling, etc): there are two basic ways to break a rope: 1. You can break a rope by pulling it from end to end until it breaks. A rope’s ability to resist this type of breakage is what we normally consider as the strength of a rope/webbing/sling (tensile strength). This type of end-to-end force placed on the rope is also called tension, and it runs along the rope. 2. You can also break a rope by applying a force perpendicular to a small section of the rope. If the force is great enough (high force falls with rope over a biner) or if the contact area is small enough (knife point), the rope can break at the contact point. Again, force in this type of breakage runs perpendicular to the rope. What we think of as “sharp edge resistance” is resistance to this type of breakage.

In the diagram where you drew the red line with two arrows pointing in opposite direction, that line is running along the rope. That’s “tension”. You’re correct that there’s a 50 kg worth of tension force (the force is actually 490 newtons, kg is a measurement of mass… but let’s not get too complicated here) pulling to the left, and 50 kg worth of tension force pulling to the right. But where you’re wrong is that you try to add the two together and say there’s 100 kg worth of force on the rope. In fact the two forces are opposite of each other in direction, so the sum of the tension forces is zero at that particular point in the rope (or any other single point on the rope). Which is logical, because the rope is not moving (Newton’s First Law). The tension acting on the rope from end to end is the weight of the 50 kg block (490 N).

I can almost hear you ask now, how can the tension be only 50 kg worth of force, when there are two 50 kg blocks pulling on it? To understand this, look at the first diagram with a 50 kg block hanging from the rope. Would you agree that the tension in that rope is just the weight of the 50 kg block? If you say “yes” you would be correct. But wait, at the top where the rope is attached to the ceiling, there’s actually a 50 kg worth of force pulling the rope up (Newton’s Third Law). Without this interactive force the rope and the block will just fall straight down. So there’s actually two forces pulling on that rope, the weight of the 50 kg block pulling down on one end, and 50 kg worth of force pulling up on the other end. In this sense, the ropes in diagram one and diagram two are experiencing the same force. If you drape the rope in diagram one over an anchor, it would look exactly like diagram two (Therefore the correct answer is: 3. Equal to).

Now, in the diagram where you drew the red line, if you turn that line 90 degrees so it points straight up and down and say there’s 100 kg worth of force acting on the rope that way, you’d be correct. The anchor in the roof is holding two 50 kg blocks of weights; therefore there has to be a total of 100 kg worth of force pulling down in the other direction. This is the second type of force I mentioned in the beginning, the one that acts perpendicular to the rope. What got you confused is that you thought this force is the tension, but it is not. This force is acting on the rope only where the rope wraps around the anchor (or call it pulley if you like), so it is spread across the contact area. Since the contact area is big in the diagram, it will unlikely to break the rope. But as the contact area gets smaller (carabiner, sharp rock edge, knife point…), it can break the rope with increased likely hood (cutting).

When we say a loop of rope is twice the strength of a single strand, we are talking purely about the strength in being pulled end to end, not its resistance to cutting. If we put all other factors aside, when we add one more rope to the first diagram I posted (hang the 50 kg block with two strands of rope), the tension in each strand of the rope is halved compare to using only one strand, so in effect you can hang twice as much weight before the rope will break. In other words, the strength of the two strands is the sum of the strength in the individual strands (when the strands are identical). But if we throw a knot into the mix to join the strands together, and place the loop around objects with different diameters, we are not just dealing with the tensile strength anymore, we’re also mixing in a rope’s edge resistance (a knot fails probably by a combination of reduced tensile strength and the rope strands cutting into itself). In the example above, the 7 mm nylon cord loop Mammut tested broke at the biner, this is more of a breaking due to “cutting” than due to insufficient tensile strength. What if they do the pull test with two biners (like at a powerpoint), around a large steel tube, or around a 2-ft diameter tree? If you could pull test against a large tree, I’d imagine the breakage will not occur at 35% higher load at the tree, but a higher number at the knot. What if the loop is not tied, but sewn? If they pull test a sewn runner looped over two large diameter objects, I suspect will we get very close to double the strength of a single strand.

In Mammut’s conservative general rule, you’re incorrect in assuming that the knot is only on one side and is no longer the weakest point. Mammut’s rule is precisely based on the assumptions that 1. The knot is the weakest link. 2. The knot weakens the cord by 50%; and 3. The total strength of the loop is the sum of the individual strands if their strength is identical. Basically, imagine a strand with a tensile strength of 10 kN. A single strand with knot is 50% of its strength, or 5kN. A knotted loop is the sum of the two strands (with the knot being the weakest link): 5 + 5 = 10 kN, or 100% of the single strand. A knotted loop doubled over is the sum of the four strands (again with the knot being the weakest link): 5 x 4 = 20 kN, or 200% of the single strand.

That’s more long winded than I intended, but hopefully that will help clear up your confusion.
patto · · Unknown Hometown · Joined Jul 2012 · Points: 25
bearbreeder wrote: u got it ;)
Oh dear. You've confused him again...
bearbreeder · · Unknown Hometown · Joined Mar 2009 · Points: 3,065
patto wrote: Oh dear. You've confused him again...
I think theres many a much more confusing posts on this thread than mine

At least i got a puuuurtay DAV pic and kool quotes from dead elephant

Though to be thorough i should have said that the weakness can be at knot OR the carabiner ... Which is why youll never see 200% in a single loop

;)
Zacks · · Unknown Hometown · Joined Apr 2015 · Points: 65

aikibujin. Id been playing through something similar to that in my mind, basically I guess the second weight acts as an equal and opposite force like any fixed piece would. So ill concede the physics. Good discussion sir!

Kiri Namtvedt · · Minneapolis, MN · Joined Nov 2007 · Points: 30

I don't mean to beat a dead horse re: the single strand of webbing, but in the midwest it seems like we end up having to find anchor points that are a ways away from the top of the climb, and thus it's often easier to run a single strand of webbing from each anchor point to the master point. That just seems normal to me, and has never seemed unsafe. At Shovel Point and Palisade Head along the north shore of Lake Superior, for instance, there are spots where the only anchors are a good 20 feet from the edge of the cliff.

Tom Sherman · · Austin, TX · Joined Feb 2013 · Points: 433
Kiri Namtvedt wrote:I don't mean...
Yes, keyword: Anchor(s), plural

Kiri Namtvedt wrote:anchors
I didn't read anything in like a page or two in here, but:

Your Life (not redundant)
Your Harness (not redundant)
Your Rope (not redundant)
--------- <From this point on make everything redundant
Rope Connecting Biners (use 2)
Anchor Legs (use 2)
Anchors (use 2)

^Looks like pretty simple way to look at it
Guideline #1: Don't be a jerk.

Trad Climbing
Post a Reply to "Unusual Girth hitch technique -ok?"

Log In to Reply

Join the Community

Create your FREE account today!
Already have an account? Login to close this notice.

Get Started.