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Holding power of ice screws
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qwert


Jan 24, 2009, 4:00 AM
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Holding power of ice screws
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Good news everybody!
Ice screws are much more secure than one might guess!

So i came upon this article by the german alpine club DAV, where they tested the holding powers of ice screws.
As you can see, the article is in german. So the friendly guy that i am, i am trying to give you the main points in english:

for this test they divide the ice the screws where placed in (in an actual frozen waterfall) into three categories:
Good: compact ice with a solid base
Medium: ice with 10 to 20% voids (either air or snow filled)
Bad: Dont know how to translate. that ice that is kinda made up of small tubes, with 30 to 50% void space.

So at first they did a rather small test in 2004, where they placed a 16cm screw in good waterfall ice, and found out that their puller cant do more than 22,8kN, whereas the screw and the ice could still take more. With further tries they managed to kill a biner and a bolt, but no screw.
Then they switched to shorter screws, where they finally had "success" with a 10cm screw, wich they placed in good, but only 15cm thick ice. At 10kN the ice failed, by breaking out large and cone shaped.
At medium quality ice they found an average holding power of 15kN for 16cm screws, and values of 11 and 18 kN for 12 and 13 cm screws (they dont tell how many they used, but i assume it was only two of the short ones).
At bad ice they give the two values of 1.2 and 13 kN ('but not how many, or how long screws).

So after this, they did a new and bigger test in 2005, to get a better database, this time only with medium length screws (BD 16cm, grivel 17cm and charlet 18cm) and some snargs. They say they destroyed 49 screws and 5 snargs.
They placed most of the screws 90 to the ice surface. In good ice they managed to break two hangers (at 13,2 and 15,6 kN) and one tube (at 18kN), but apart from this they just had the ice get some cracks and produce some sounds, before they had to stop at around 19kN because of their puller.
In medium quality ice they had an average holding power of 9,6 kN, however with some values like 4,3 and 18,3 kN.
They give some percentages about the ability to hold a fall for the screws, where they use 10kN as safe and 5 to 10kN as marginal, and everything under 5 as likely to fail.
So 56% of the screws in medium ice would have been safe, 13% marginal and 31% fail.
In bad ice they found the average holding power to be 5,6 kN with values from 1,2 to 13kN. Or in percentages: 13% would have been safe, 13% marginal and 56% would have failed. So there must be a typo somewhere.
I took the freedom to translate the diagram for it:

I guess its pretty self explanatory. The colored dots mean:
red:test stopped
yellow: screw failed
blue: ice failed

As mentioned they also tested snargs. They all failed under 10 kN, so they are not recomeded for use. (One placed at a hanging angle didnt even hold body wheight)

Also they had a look into the angle of the screw, where they found that srews placed in a hanging orientation (up to 30) generally hold more then screws pointing up.

Their conclusions are that medium length screws are fine for all purpose use, and the reccomendation to only use the longest screws for belays is not neccessary. Accoring to this article a 2 screw belay in solid ice can be treated like a 2 bolt belay.
Furhermore they conclude that even the shortest screws can hold hold a fall in good ice. Bad ice should be considered as what it is and if you dont have anything else, try to cluster together a few screws.

Hope that interesting for you.
They also did some tests in glacier ice, which i hopefully will provide you soon. But given that i am sick, my foot is still kinda busted and my boots are in italy for repair anyways, i should find the time for it rather soon.

qwert


(This post was edited by qwert on Jan 24, 2009, 4:01 AM)
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adatesman


Jan 24, 2009, 6:04 AM
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qwert


Jan 24, 2009, 8:40 AM
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Re: [adatesman] Holding power of ice screws [In reply to]
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How come that all my recent serious threads get moved? Nevermind, here is the second part on the holding power in glacier ice.

article
Again, it is in german, so here is my translations of the main points:

This time they did not place the screws in different ice qualities, since glacier ice is more or less of constant quality. They did however remove snow and rotten ice from the surface, to get to the good ice below.

In this test they used not just medium length screws, but different lengths.
What they found is the following:
13cm screws had an average holding power of 18,6 kN
16 cm screws had an average holding power of 19,6 kN (for comparison: average in good waterfall ice 19,8 kN)
22 cm screws had an average holding power of 20,7 kN.
They produced some nice graphs with the deviations, wich i have translated for you:



Furthermore they tested the holding powers versus the angles of placement with some 13cm screws, where the neutral one (0) had the best holding power, followed by the hanging one (-15). The one placed with the hanger pointing up had a significantly lower holding power.


Another thing they mention is how the screws fail. In the cold and brittle waterfall ice in the previous test they observed how the screws broke with the ice in a large area. In the softer and warmer glacier ice the screw simply got pulled out of the hole, leaving the hole almost intact. However when they did the test on glacier ice in cold conditions they observed a similar failure mode as in waterfall ice.

They conclude that glacier ice and good waterfall ice offer similar holding powers, so all tested screws can be considered safe.
They say that not the length of the screw is important for the holding power, but the number of threads on the tube.
Another thing they mention is the problem of "pressure melt out", when you weight the screw constantly, for example in a belay or toprope situation. This is a bigger problem in warmer ice, and with shorter screws, so you need a backup behind your main screw.

And last but not least they almost killed a biner toproping in a shockinly short time, since the rope did take up a lot of glacier debris and acted as a saw on the biner.

And there is even more stuff in this articles on ice threads, which i will bring to you soon.

qwert


(This post was edited by qwert on Jan 24, 2009, 8:42 AM)
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Partner angry


Jan 24, 2009, 9:00 AM
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Re: [qwert] Holding power of ice screws [In reply to]
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I was talking to a friend about this the other day. He's been climbing ice a decade longer than me but we've both placed our fair share of screws. Neither of us have fallen on one.

We both could think of times that we'd placed a super solid screw that we knew would hold. We'd also placed a bad one that we knew wouldn't. Somewhere in the middle is where most of the screws we'd placed would lie.

This study actually confirms that for me. A clearly bad screw will fail and a clearly good screw will hold. Other than that, it's hard to assess the one that pops at 4kn or 16kn. The standard deviation is very high.

That same friend, who got me into ice climbing, told me to think of all my screws the way I think of RP's on rock. You place them because they might hold but you don't fall on them if you can help it.

Still, crampons, tools, and racked screws might be the bigger problem in the event of a fall.


qwert


Jan 25, 2009, 2:59 AM
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Re: [qwert] Holding power of ice screws [In reply to]
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Another thing they did test was the holding power of V-threads in ice.
In the first article (the waterfall one) they mention that a drilled v thread with side lengths of 15cm is sufficient, and better than natural v-threads, but they dont give more data on that.
However in the second article (the glacier ice one) they have more data on v-threads.
There they used different screw lengths to drill v-threads (as equilateral triangles) and measured the holding powers compared to the areas of the triangles

They say that these holding powers are a bit lower than expected, and suggest 16cm screws as the minimum length for the construction of v-threads

qwert


(This post was edited by qwert on Jan 25, 2009, 2:59 AM)
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qwert


Jan 25, 2009, 3:29 AM
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Re: [qwert] Holding power of ice screws [In reply to]
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Ok, so thats what i would consider a roundup of the two articles.

So heres what i am thinking about it:
IT is quite amazing how much even the short ice screws can hold, and i think it is safe to use the values obtained in glacier ice for good waterfall ice.
So i happily will also include a few short screws in my rack, knowing that they are not only a "maybe better than nothing" piece for thin stuff, but that they can also offer safety, when placed in good and thick ice.
However i will not get them as main piece. I think it is quite an easy to forget to remove the upper, rotten layer in the ice, before placing a screw. With a 16cm screw the threads still are placed in the solid ice beneath, but with a 13 cm screw, part of the threads is in "worthless" ice. And according to the articles its the number of threads that counts, so that can affect the holding power.
Another thing that is quite interesting to note is how much the values are scattered in medium and bad quality ice. Even thoug the guys doing this test are experts, they still managed to severely misjudge the ice (thats how i see it). In all but perfect ice, the chances to have a total failure, or a real high holding power are quite good. There is still a chance to get over 10kN of a screw in bad ice, but on the other hand you could also be under 5 in medium ice.
So we are again at point zero in some way. We know that ice screws in good ice are equal to a bolt, but how do we know that the ice is good?
I can easily spot fucked up ice, and ice that is rahter good, but the high scatter in medium quality ice indicates in my opinion that the border between really good and really bad is really hard to find.
But that point is somewhat moot anyways, since falling on ice is dangerous enough, even with good screws.
I will still do all i can to avoid falling on ice, but should the situation arise, i can at least feel confident that there is a rather high chance that i will not be the screws in the ice i have to worry about on my way down.

On the v-threads: Even though 16cm screws seem to be ok for that job, i still will use my 22cm monsters for that. I dont know how it is for you, but i never manage to built the perfect triangle. Its always somewhat overlapping, and thus reducing the size of the v-thread. And especially in a retreat situation where one would mostly use those, i think it is nice to have more room for error, by using the longest screw possible.

I also still would opt for long screws for glacier travel. even though it is shown that a 13cm screw produces more than enough holding power for a crevasse rescue, i wouldnt suggest taking those with you.
I havent had it happen to me (and dont want to), but i guess if your are hanging on the ice, trying to stop yourself to get pulled into the crevasse your buddy has fallen in, you are not sitting there calmly digging through the rotten surface ice, making a nice ditch for your screw. You are rather scratching a bit on the surface in panick, and then get the screw in as fast as possible.
Again i think the longer screw has a much better chance of hitting solid ice, and being of actual value to you.

qwert


chossmonkey


Jan 25, 2009, 5:03 AM
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Re: [qwert] Holding power of ice screws [In reply to]
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I just skimmed this, but it seems like it just reconfirms the tests that BD and Craig Luebben have done in the past.


Hopefully I'll have time to read through this thread better.


Partner brent_e


Jan 26, 2009, 12:56 PM
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thank you, qwert!

nice of you to do the translation for us. the info is very useful. I still don't want to fall, though! lol


fresh


Jan 26, 2009, 1:47 PM
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good stuff. it seems that this confirms the common knowledge. I guess I'd like to see some research on how to determine good ice, since that's the critical variable.


Guran


Jan 27, 2009, 4:22 AM
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Re: [qwert] Holding power of ice screws [In reply to]
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Were any test done where they shockloaded the screw, rather than simply pulling on it? Sort of like an UIAA-fall?

Reason I'm asking is because ice is brittle and responds differently to impacts than to static pressure.

(part of the reason we can kick a front point in without much effort and then commit our full body weight to it.)


chossmonkey


Jan 27, 2009, 5:42 AM
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Re: [Guran] Holding power of ice screws [In reply to]
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Guran wrote:
Were any test done where they shockloaded the screw, rather than simply pulling on it? Sort of like an UIAA-fall?

Reason I'm asking is because ice is brittle and responds differently to impacts than to static pressure.

(part of the reason we can kick a front point in without much effort and then commit our full body weight to it.)
I think it was Craig Luebben who did some tests like that. He used a steel weight though and figured it made a more violet loading. I also heard of some live testing of screws at Ouray a couple of years ago where they used a live person as the test dummy.


chossmonkey


Jan 27, 2009, 7:01 AM
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Re: [qwert] Holding power of ice screws [In reply to]
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qwert wrote:

Also they had a look into the angle of the screw, where they found that srews placed in a hanging orientation (up to 30) generally hold more then screws pointing up.

By up do you mean the hanger up?

All the other tests I have seen say that in good ice you want to put the screws in at an upward angle so the threads are loaded more than the shaft since ice and screws both seem to have more tensile strength than shear strength.


Edited to ad:

Never mind I think I found what you meant a post or two down and it did mean hanger up and tip down.


Thanks for the summary.


(This post was edited by chossmonkey on Jan 27, 2009, 7:10 AM)


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