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Anchor Safety Margin
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clmbnski


Mar 12, 2006, 5:52 PM
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Anchor Safety Margin
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What do you think is the minimum safety margin needed with a trad anchor?

I think the most common belief is that an anchor should be able to withstand a factor two fall. But building an anchor with that in mind is not as straight forward as it seems depending on if you think there will be slippage through the belay device ect. vs. the measured impact force with a static belay.

I think it is reasonable starting place to use the rope's measured impact force as a baseline for the estimated effect of a factor two fall. An average would be around 9kN.

Lets say you use a cordelette with medium sized gear rated at 10kN. The force more or less goes on one peice, it will hold but there is not much of a safety factor. On the other hand you do have back up pieces to increase the odds that your anchor holds. Or maybe you feel that slippage in the system will limit the impact force to ~5kN (with belay device) giving a bit more of a safety factor. Is this an acceptable situation from a philosophical point of view?

I know people have various opinions on the cordelette but what I am really looking for is how much of a safety margin do you think there should be in the anchor strength (1*the max force, 2*the max force,...)?
Also what is your baseline for determining the max force in the worst case scenario: static belay vs. slippage?

Edited to clarify question


jackflash


Mar 12, 2006, 7:11 PM
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Re: Anchor Philosophy [In reply to]
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What is your philosophy when building an anchor?

Methodological reductionism. An anchor is best explained by its parts.


nedsurf


Mar 12, 2006, 7:35 PM
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Stop using philosophical in this way. You are thinking people like myself with this type useless degree are similar to engineers or something. When one is doing philosophy, they are turning a critical eye about what is, or how to think about what is. You might be thinking of the word theoretical, as in theoretical yield, forces or some other more practical thing.
Aside from this post, having a philosophy such as a corprate philosophy or some kind of world outlook hippie philosophy is totally different from doing philosophy. We might not be able to get jobs, we might nit-pick you to death on details and definitions, but we're fun at parties and settle for nothing but the truth with a capital T. No BS. 8^)


sspssp


Mar 13, 2006, 8:54 AM
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What do you think is the minimum safety margin needed with a trad anchor?

I think it is reasonable starting place to use the rope's measured impact force as a baseline for the estimated effect of a factor two fall. An average would be around 9kN.

Lets say you use a cordelette with medium sized gear rated at 10kN. The force more or less goes on one peice, it will hold but there is not much of a safety factor.

I think youd would have a pretty hard time generating 9kN force even with a factor 2 fall. But be that as it may, a piece (cordelette or cam) that is rated at 10kN is not going to fail at 10.01 kN. In reality that 10kN has a large safety margin on it. It would probably take 20~30kN before it actually failed. But before that happened there is a good chance you would start ripping the pieces out of the rock (depending on rock type) and if your leader actually managed to generate that much force on the rope/system (which means his/her harness is generating 20~30kn of force on his waist/legs), it is probably going to be a pretty grim day anyway.

cheers


reg


Mar 13, 2006, 9:45 AM
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In reply to:
Stop using philosophical in this way. You are thinking people like myself with this type useless degree are similar to engineers or something. When one is doing philosophy, they are turning a critical eye about what is, or how to think about what is. You might be thinking of the word theoretical, as in theoretical yield, forces or some other more practical thing.
Aside from this post, having a philosophy such as a corprate philosophy or some kind of world outlook hippie philosophy is totally different from doing philosophy. We might not be able to get jobs, we might nit-pick you to death on details and definitions, but we're fun at parties and settle for nothing but the truth with a capital T. No BS. 8^)

no offense - but - what?


healyje


Mar 13, 2006, 9:57 AM
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Given when you build an anchor you can't determine any aspect of its strength all you can do is build the best anchor you can. There really is no such thing as "margin" as it implies some known baseline that the margin exceeds and there is no such baseline. Again, even if there was such a thing, you couldn't determine either - just build the best anchor you can with the combination of what you've brougt and what you've been dealt in the way of placements. I understand you're intent, but this really isn't a necessary discussion and you'd be better served just reading and re-reading the discussions in the "sliding-X" and "solution" threads...


narcan


Mar 13, 2006, 9:58 AM
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Although somewhat overkill for trad anchors, as a reference, the safety factors used in Mountain Rescue are 10:1, and the factors used by Fire Rescue standards is 15:1....it is my opinion that a minimum factor of 5:1 should be employed for trad anchors, after all, why risk yours and your friends life to anything less.....


vegastradguy


Mar 13, 2006, 10:10 AM
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In reply to:
Although somewhat overkill for trad anchors, as a reference, the safety factors used in Mountain Rescue are 10:1, and the factors used by Fire Rescue standards is 15:1....it is my opinion that a minimum factor of 5:1 should be employed for trad anchors, after all, why risk yours and your friends life to anything less.....

sure, this is ideal, but.....

In reply to:
Given when you build an anchor you can't determine any aspect of its strength all you can do is build the best anchor you can. There really is no such thing as "margin" as it implies some known baseline that the margin exceeds and there is no such baseline. Again, even if there was such a thing, you couldn't determine either - just build the best anchor you can with the combination of what you've brougt and what you've been dealt in the way of placements.

because the real truth is that while some anchors can be built with a higher safety factor than 5:1, most of us have been in plenty of situations where the safety factor has been considerably less than 5:1...not that we were happy about it, but we had to deal with it anyway.


clmbnski


Mar 13, 2006, 3:09 PM
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a piece (cordelette or cam) that is rated at 10kN is not going to fail at 10.01 kN. In reality that 10kN has a large safety margin on it. It would probably take 20~30kN before it actually failed.

Really? I havent seen evidence for that.

In reply to:
but before that happened there is a good chance you would start ripping the pieces out of the rock

That I agree with

In reply to:
Given when you build an anchor you can't determine any aspect of its strength all you can do is build the best anchor you can. There really is no such thing as "margin" as it implies some known baseline that the margin exceeds and there is no such baseline.

You can estimate strength. Sure there are a lot a variables that make it uncertain but it is still useful. I think a lot of people just throw in the standard 3 peices and call it good. But 3 microcams is obviously much weaker than three hexes. So how do you know when you have put in enough gear? Most people probably use experience and intuition, but why not estimate anchor strength? Yeah you might be forced to make due with something that is not ideal in the real world but at least you will know it is not ideal.

I bet that a safety margin of 5 to 1 is going to be hard to achieve in a trad anchor. (gear not trees) So is that a problem?


healyje


Mar 13, 2006, 3:29 PM
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There is absolutely no way to judge the numerical strength or strength ratio of an anchor when you're climbing. While such numbers and ratios exist, they are unknowable, pointless, and irrelavent when climbing. But, you can and should make an intuitive and informed judgment about how "solid" your anchor is. This again goes back to your skill and craft with protection. Part of being skilled in the craft means being able to judge the relative performance of your work and if you can't tell a good placement from a bad one you're in trouble regardless. Knowing how solid an anchor is means a step up in skills and experience relative to being able to assess how best to utilize, assess, and optimize the placements available for one.

You also have take into account the purpose for the anchor - belaying, portaledge, hauling, rapping, speed ascents - all may entail you making different decisions relative to the same available placements. Hans Florine trying to break his time on the Nose is unlikely to take the time to build the sort of anchors someone would build to bivy in a ledge and haul. How much is enough? As good as possible for the task at hand...


vegastradguy


Mar 13, 2006, 3:32 PM
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Here's your question:

In reply to:
why not estimate anchor strength?

Here's your answer:
In reply to:
Most people probably use experience and intuition...

the real truth is that experience will tell you whether its ideal. for instance, i'd pick three micro-cams (or four) as a belay in solid rock over three large hexes in sugary soft sandstone.

more often than not, the ledge you're on (assuming there is one), the rock quality, and the availability of pro directly contributes to how secure you think the anchor is. after that, its a crap shoot anyway for how strong the gear actually is, how strong the rock is, what sort of forces can even be generated (is that ledge big enough to prevent a factor 2 fall?), etc, etc.

plus, taking time to figure out how strong your anchor is by actually computing the math in your head seems like a good way to slow yourself down, which, given the right circumstances, can kill you.


g-funk
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Mar 13, 2006, 3:48 PM
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Re: Anchor Philosophy [In reply to]
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Stop using philosophical in this way. You are thinking people like myself with this type useless degree are similar to engineers or something. When one is doing philosophy, they are turning a critical eye about what is, or how to think about what is. You might be thinking of the word theoretical, as in theoretical yield, forces or some other more practical thing.
Aside from this post, having a philosophy such as a corprate philosophy or some kind of world outlook hippie philosophy is totally different from doing philosophy. We might not be able to get jobs, we might nit-pick you to death on details and definitions, but we're fun at parties and settle for nothing but the truth with a capital T. No BS. 8^)

I hope English is not your first language.


sspssp


Mar 14, 2006, 1:34 PM
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In reply to:
a piece (cordelette or cam) that is rated at 10kN is not going to fail at 10.01 kN. In reality that 10kN has a large safety margin on it. It would probably take 20~30kN before it actually failed.

Really? I havent seen evidence for that.

I came out of graduate school in civil engineering. Manufactures put a safety margin [on] on anything that might endganger life or limb. Whether you are talking about giant steel columns or a tiny fastener. Any climbing gear is going to fall into that category.

When you do tests on materails, they don't all fail at the exact same strength. So typically, they find an "average" that the thing fails at and then the actual rating might will be anywhere from half to one-fourth that amount (so if some individual piece is below "average" it is still comfortably above the rating).

Climbing gear is the same way. Now if you leave your cordelette out in the sun for 3 years, its strength will suffer. But fresh out of the shop, a cord rated at 10.0kN won't fail at 10.01kN. I would happily wager you on that.


jercech


Mar 14, 2006, 1:43 PM
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The American Concrete Institute Building Code Requirements for Structural Concrete (ACI 318-05) Appendix D covers structural anchors in concrete including expansion bolts just like the ones used in climbing.

Sorry, I was just perusing the chapter for work. I wonder how various rocks perform compared to reinforced concrete?


antiqued


Mar 14, 2006, 3:29 PM
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="sspssp"
I came out of graduate school in civil engineering. Manufactures put a safety margin [on] on anything that might endganger life or limb. Whether you are talking about giant steel columns or a tiny fastener. Any climbing gear is going to fall into that category.[\quote]

This is true for civil codes, factors of 10X or higher for life critical pieces are quite common. To some degree, this is because they are not inspected, stay out in the rain, grease, salt ...., and threaten naive users who cannot monitor or understand the critical factors.

It is not true for airplanes or climbing gear. No plane takes off with 10X fuel required to reach Tokyo; no wing can withstand 30+Gs.

Your climbing gear is safe only because it is used, inspected and maintained by an expert! You!

Your climbing rope is spec'ed to an impact force of ~9kN, for a FF 1.7, 80kg climber. It will break aroiund 20kN (PMI, 9.7mm data). This is a safety factor of 2.2, unless you happen to weigh 100kg and take a FF2. If your rope passes across an edge, you may be damn lucky if it holds 6kN!

If the open gate rating of 7kN on old carabiners meant that they really had an open gate strength of 70kN, then no one would have been breaking them!

There is a certain paperwork inertia. UIAA minimum for 7mm cord is 7kN. Manufacturer's sites quote: Mammut - 13kN, Bluewater 11.6, PMI 9.3, Beal 10.3. Chances are that one or more of the weaker ones is actually stronger than listed, but the rating hasn't been upgraded due to paperwork expense.

Because of weight constraints (like airplanes), climbing gear has no specified safety margin - just (usually) a manufactuing margin. If you want to climb with that civil engineering margin, an interesting first step would be to replace one of your shoulder slings with a chain rated to 22kN, or to use 10 slings on each piece of gear.


clmbnski


Mar 14, 2006, 3:39 PM
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sspssp said:
In reply to:
Manufactures put a safety margin [on] on anything that might endganger life or limb. Whether you are talking about giant steel columns or a tiny fastener. Any climbing gear is going to fall into that category.

Ok you are right there is some built in margin. Saw that black diamond rates their gear 3 standard deviations below the average strength in each batch tested. I had heard about their 3sigma thing before but thought it was something else.

Now I wonder what an average standard deviation is for pro...


glyrocks


Mar 14, 2006, 3:57 PM
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Don't worry Nedsurf, I understood it. It was no literary artwork, but I followed you.


jercech


Mar 14, 2006, 4:17 PM
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the term "safety factor" is really a nebulous term defined by whoever wants to define it.

In the ACI code for structural concrete I mentioned above, the required strength of an anchor must be greater than 1.2 times the dead load plus 1.6 times the live load. And then the anchor must be designed such that you statistically have a 90% confidence that there is a 95% probability of the actual strength exceeding the nominal strength...and that's simplified, the code is actually 27 pages long followed by another 77 pages of notes and commentary.

Who's going to mess with that when building a climbing anchor?

edited for typo


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