Girth Hitch

[szymiec]

So I know a girth hitch weakens as the point at which it is hitched around increases.

A biner or master point is fine, but what about the BFT!? Big Tree

At which point is the girth rendered useless? Mike?

C

[chossmonkey]

Fine for what?


The friction between the rope and whatever you girth hitch will likely make more of a difference than the diameter. If you have a tree and a metal light pole of the same diameter I'd think they would slip at much different loads.

I'd assume that why it holds so well on a biner is the friction of the rope against itself rather than on the biner.

[szymiec]

I think you might be thinking of a clove hitch man.

I nearly failed an acmg course because one of my 3 anchor points was a sewn sling girthed around a tree. The guide told me it is significantly weekend by the large diameter of the tree. Im 99% sure it has nothing to do with friction

Chris

[chossmonkey]

Ooops


It thought the big no no with girth hitches was connecting two slings? One sling could potentially cut the other.


I've never heard anything about one being weakened by going around a tree. Have you posted to any other boards?

You could try asking KP. --->http://www.bdel.com/scene/beta/qc_kp.php

[chossmonkey]

Why would you girth hitch a biner?

What are you referring to a master point? If you aren't slinging trees or other natural features I can't really think of any place you would girth hitch other than maybe into a belay loop. Or GASP!!! two slings, which normally ends up being that other hitch I can't remember the name of.


Good luck on the rest of your tests if you have more.

[szymiec]

I use a girth hitch on occasion.

1) Daisy on my belay loop. (Always inspect after and before each use!, read Skinner Accident)
2) Tie off screws & pitons.
3) On my ascender setup for the big jug fests belay loop-->sling-->Ascender. and belay loop-->5mm spectra cord-->2 wire gates.

I think that as the diameter of the tree or whatever increases, the angle on the sling where it meets increases creating an outward direction of pull that will stress the union on the girth hitch.

chris

[granite_grrl]

I see what you're getting at. I'm no master at mechanics (I'm sure the rest of the engineers on this board will pipe in here pretty quick), but it has to do with the angle which the webbing makes around the tree from the point of the actual hitch.

The more inline the webbing around the tree is to the songle peice coming off the less force it'll generate. This same concept is why the death triangle is a problem and you want to extend a mutli piece anchor. (here's hoping that someone will come along and explain this better than me )

All this being said, I don't think there's too much difference between a medium tree and a large tree...and really, how small would you want to go. I guess it would also make a difference with the shape of the tree, like if it had a big flat spot or something.

I guess what it comes down to is how much force is actually going to be generated. I wouldn't think twice about bringing a second up using a clove hitch and a big ass tree, they're just not going to generate that much force when the fall. But if its really a concern you could always toss a longer sling around the entire tree and clip into both ends (with a knot near the end to maintain redundency).

Gosh, all this would be easier explain with pictures.

[PaulB]

I nearly failed an acmg course because one of my 3 anchor points was a sewn sling girthed around a tree. The guide told me it is significantly weekend by the large diameter of the tree. Im 99% sure it has nothing to do with friction

According to the June 99 version of the ACMG Technical Handbook, "The girth hitch can create a pulley effect on large objects."

Which I think is the same idea as what you said:

I think that as the diameter of the tree or whatever increases, the angle on the sling where it meets increases creating an outward direction of pull that will stress the union on the girth hitch.

[szymiec]

yeah totally.

So back to my original question. When is big too big?

C

[STeveA]

girth.GIF (2.31 KiB) Viewed 2920 times

This is a fun physics problem although I doubt it would ever be a problem in a real climbing situation. There are many variables involved such as the friction around the tree and the angle of the girth on the loop. I am guessing that the large tree would have less force per square inch and thus less friction on the sling. The largest forces on the girth hitch would occur in the theoretical situation of no friction. In this scenerio if we treat this similar to the American death triangle the worst scenerio would be as shown in the sketch with a small angle between the main part of the sling attached to the rope and the loop on the other end of the sling.

In the death triangle this can create forces much larger than the strength of the anchor. However this only occurs when the anchor points in the system are solid, such as bolts. With the girth hitch if the force on the sling gets too great it would pull the climber up the cliff. There are certainly large forces on the sling due to the girth hitch, however I do not think they would ever be large enough to break the sling in a real climbing situation. I have broken slings in the lab and it takes a massive amount of force. I cannot see a sling breaking in a static situation. If there is some motion involved in the system with heat being generated then there may be problems. I would be interested if anyone has more insight into the problems with the girth hitch. Is there an actual incident that has caused this concern?

[szymiec]

NICE! Thanks man.

Those damn institutional anchors are enough to make me crazy. doing that around a tree in an ACMG exam is an immediate fail on the rock guide exam! Even if it is only used in 1 of 3 points.

c

[STeveA]

Interesting article.

http://www.raichle.ch/mammut/uploadedFiles/Full%20Report%20on%20Sherman%20Sling%20Break.pdf

[Adam]

steve, in your diagram the hitch is hanging away from the tree, towards the load. isn't the point to put the hitch towards the back of the tree to prevent slippage and to avoid the 'death triangle'? in this case, would you fail the exam?

girth.JPG (7.08 KiB) Viewed 2810 times

[STeveA]

I tried to visualize where the sling would end up under a heavy load. I think my diagram is correct. The load is F2. If you put the hitch towards the back of the tree it will rotate until it ends up where I show it. The only thing keeping it in place at the back of the tree would be friction, which may well be enough, however I think the greatest forces on the Girth hitch would occur under the theoretical frictionless system in which case the sling will rotate until the forces equalize.

[Adam]

I tried to visualize where the sling would end up under a heavy load. I think my diagram is correct. The load is F2. If you put the hitch towards the back of the tree it will rotate until it ends up where I show it. The only thing keeping it in place at the back of the tree would be friction, which may well be enough, however I think the greatest forces on the Girth hitch would occur under the theoretical frictionless system in which case the sling will rotate until the forces equalize.

gotcha.

[Murph]

http://en.petzl.com/ProduitsServices/C06180%20FINANNEAU%20C06503-B.pdf

I’m not sure if this is will make things clear?… but the strength varies when slings are girth hitched in different positions…

[STeveA]

I'm glad they warn you not to use a saw on the sling. I guess I will leave mine home from now on.

[Adam]

interesting that the position i have in my diagram holds less weight than the 'slipped' diagram of steve's. so steve it looks like while your resulting orientation might create a triangle it is still better (for shear load purposes) to be in that position. what that may not take into account is the 'slipped' orientation can have the webbing rubbing against itself which is a recipe for disaster.

i think i'm gonna stop using girths all together.

[Fred]

I think the tight girth is worse because of the sharp bend in the sling at the "knot". Knot strength is usually a function of the tight bends. I think...

[mathieu]

If you look at the one that only holds 8kN, could it be because of the 180 degree the bend in the sling, at that point the ring that holds the bend is subject to two forces (one from the load, the other from the friction the load places on the object). I'm no physicist but it looks like that ring is now subject to two loads.

[Murph]

When the girth hitch is in the weaker position, mechanical advantage (2:1)? is created. The loaded strand also pulls where the girth hitch joins. A 2:1 would create twice the load and explain why this position is only rated half as much. If you are confused try girth hitching your hand and see how it is easier to tighten the 8 Kn as compared to the 16 Kn position.

[STeveA]

This discussion is a good reminder that we should all be aware of the physics involved in climbing. I have no problem using the Girth hitch, but I will pick the strongest setup for the given situation, and will avoid it if I don't need to use it. I also make sure that I have a backup.

I still have not found any accidents that point to the Girth hitch being a cause. Lots of info on how it weakens the setup but nothing to show that it has been a problem.

[Fred]

I use it all the time.

[chossmonkey]

I use it all the time.

Me too, unless something else seems like it would work better.

Its fast, easy, doesn't use much gear, has few if any knots to tie wrong or try to untie, and no biners to get cross or tri-loaded. Even if a sling is reduced to 8kn, that is still as strong as if not stronger than an improperly loaded biner.

To bad the ACMG doesn't see it that way.

[mike]

I didn't see this one 'til now.

The 8kn load is explained by the fact that both legs of the sling comming and going from the girth hitch have equal weight- so each leg gets to hold 1/2 of the 16kn that the girth hitch does.

The 2x 22kn sling, is basketed and holds twice the weight of the sling rating- kind of, but that's an over zealous rule of thumb. The basket has to be upside down U-shaped over a relatively large diamete round thing to actually double the sling rating.

As for the 16kn setups... it's like the other pics. If you turn your head righthand sideways when you think about this it'll make it easier. The girth hitch becomes a 'the load'. The actual load (your head upright for a second) is the same on both sides of the girth hitch. (sideways again please). The angle created by the deflection the girth hitch causes goes into anchor angle type calculations. So you end up with 3 equal loads radiating out from the girth hitched point- equal loads on each of the three legs.
So what happens on big trees is that you get up around the dreaded 160 degree anchor setup where the force of the load is multiplied by three on each of the 2 vertical legs- the upper of these legs is goes around the tree (minus friction) and multiplies the whole thing into a bad situation. I know this is an incomplete answer- but it's complicated and I explain things best with my whole hands rather than just my 2 index fingers- it does give the basis for the problem though.

I'd suggest not using girth hitches too much. An option would be to tie clove hitch through a biner into the place where you were going to set the girth hitch and clip the biner to the end of the sling. Same shape- different forces. This set up, taking for granted, the use of a good angle at the biner- would be rated the same as the sling. Basketing the tree would be almost twice the rating of the sling (or maybe the rating of the biner- if you're sporting '70s gear)

Mike