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trenchdigger
Aug 5, 2004, 8:18 PM
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Based on the recent threads about Death-O-Lettes, I thought I'd address a related, but more general issue that might help answer the question there.
Simply put:
What is the minimum single arm strength you're willing to accept in an anchor?
There are two possible answers I would expect to hear:
a.) At least as strong as the piece of pro it's attached to.
b.) At least XkN.
What would you answer? If you answered b.), what value did you put into the X variable? 5kN and rely on equalization to build strength? 10kN? 20kN?
As many have illustrated in various ways, true, perfect equalization in an anchor a myth. Still there are ways to maximize the likelyhood for equalization. One way to impact that is by adjusting the angle between outermost anchor arms.
Although for most it would depend on the situation, but generally, what do you feel is the optimal angle between outermost arms in an anchor?
5 degrees? 10 degrees? 30? 60? 90? (And no, you can't choose the obvious but impossible ideal of 0 degrees)
Minimizing the cordelette angle between pieces has been proposed by some to reduce load multiplication and minimize the load on a single arm of the system. However decreasing this angle also greatly decreases the room for error in equalization and increases the likelyhood that a single piece will be forced to sustain all of the load. Example: Assume a 2 piece anchor tied with a cordelette with an angle of 10 degrees. Mis-equalize or change the direction of pull by 5 degrees (commonly encountered in climbing situations) and all of the load is tranferred to one piece in the anchor.
So it seems safer to increase that angle to help with equalization. But not too much or forces will be multiplied. The ideal anchor angle range should lie somewhere between 30 and 90 degrees. The wider the arms, the more the angle of pull must change before a single piece sustains the total load. And even at 60 degrees, two ideally equalized pieces bisected by the direction of pull will each sustain only 58% of the total load. Decreasing that angle to 30 degrees, each piece sustains only 52% of the total load. Now let's throw in a 5 degree error from the ideal direction of pull in this 30 degree anchor just like we did with the anchor with 10 degrees between arms. In this condition, the arms of the two-piece 30 degree anchor must sustain only 35% and 68% of the load. Now remember that with a 10 degree anchor, one arm will sustain 100% of the load in this situation.
I'd like to hear some simple answers to the questions I posed in bold.
~Adam~
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trenchdigger
Aug 5, 2004, 8:31 PM
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... And I'll start.
What is the minimum single arm strength you're willing to accept in an anchor?
I prefer answer a.) At least as strong at the pro it's attached to, but in an effort to address the topic I'll also answer for b.):With at least 3 pieces of (pseudo)equalized pro, I'll accept a minimum of 10kN per arm .
Although for most it would depend on the situation, but generally, what do you feel is the optimal angle between outermost arms in an anchor?
Ideally I prefer 60 degrees to be the angle between the outermost arms of my anchor. I feel this provides the best combination of load distribution and equalization error compensation.
~Adam~
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alpnclmbr1
Aug 5, 2004, 8:32 PM
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In reply to: What is the minimum single arm strength you're willing to accept in an anchor?
15kn: 70% of a 22kn runner = two slings girth hitched = tied 9/16 supertape (I try to minimize or eliminate 9kn crossload failure points)
In reply to: Although for most it would depend on the situation, but generally, what do you feel is the optimal angle between outermost arms in an anchor?
60 degrees max.
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alpnclmbr1
Aug 5, 2004, 8:35 PM
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Note: the 15kn leaves some room for wear and tear.
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rrrADAM
Aug 5, 2004, 8:56 PM
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1) 8Kn, which is the same as some of my smaller gear. This would be in one arm only, and not all 3 points.
2) As small an angle as achievable... But I also am a believer in the don't protect in just one weakness mantra, as in if another seperate weakness is available, I'll put a piece in that, even if it increases an angle.
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tedc
Aug 5, 2004, 9:37 PM
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1) 10KN. My anchors must have minimum of two (3 generaly prefered)legs who's strength is determined (by me) to be 10KN or better. One leg good to 20KN doesn't count; neither does 3 legs at 7KN. If any of my anchor components are weaker than 10KN I will necessarily need more than two. i.e. Four 7KN placements could become two 12KN legs(sliding x, sliding x, tie off the x's to a powerpoint.(strengths derated from the theoretical 14KN, due to the inability of even the sliding x to perfectly equalize them). This, to me, would be a borderline but acceptable anchor.
2) I am pretty happy with angles between 45 and 60 degrees.
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jt512
Aug 5, 2004, 9:49 PM
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In reply to: I'd like to hear some simple answers to the questions I posed in bold.
I'll give you a really simple answer: If the rock and your available gear allow it, you want each individual arm to be able to withstand a factor-2 fall all by itself, and you want at least three pieces just like that. Then do your best to equalize them. If your individual pieces really are Solid, then you can live with imperfect Equalization. Looking at Alpnclmbr's quantitative answer, I think we are saying the same thing.
-Jay
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tradklime
Aug 5, 2004, 9:58 PM
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In reply to: What is the minimum single arm strength you're willing to accept in an anchor?
I suppose as strong as the gear it is attached to, but 8 kn should do. Most gear you use to build an anchor will likely meet or exceed this. Although, I can't help to think that this is rather arbitrary because it's the rock you should worry about, not the gear breaking or what you attach to the gear with.
In reply to: Although for most it would depend on the situation, but generally, what do you feel is the optimal angle between outermost arms in an anchor?
Less than 45 degrees.
Like everything else this is very situational. What you have available, rock quality, etc. etc. Hard and fast rules... i dunno. I like to think of good gear and a good anchor similar to obscenity, "I know it when I see it". Or at least I like to think I do.
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vegastradguy
Aug 5, 2004, 10:13 PM
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15kn, <60 degrees, or what Jay said.
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trenchdigger
Aug 5, 2004, 10:29 PM
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In reply to: I'll give you a really simple answer: If the rock and your available gear allow it, you want each individual arm to be able to withstand a factor-2 fall all by itself, and you want at least three pieces just like that. Then do your best to equalize them. If your individual pieces really are Solid, then you can live with imperfect Equalization. Looking at Alpnclmbr's quantitative answer, I think we are saying the same thing. -Jay
Agreed. Good point. Always build the anchor as strong as possible. But what is the minimum you'll accept as an anchor to lead above? At about what general point (and yes, as I and others mentioned, it's situational) will you deem an anchor insufficient based on the strength of the weakest arm?
~Adam~
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jt512
Aug 5, 2004, 10:35 PM
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In reply to: In reply to: I'll give you a really simple answer: If the rock and your available gear allow it, you want each individual arm to be able to withstand a factor-2 fall all by itself, and you want at least three pieces just like that. Then do your best to equalize them. If your individual pieces really are Solid, then you can live with imperfect Equalization. Looking at Alpnclmbr's quantitative answer, I think we are saying the same thing. -Jay Agreed. Good point. Always build the anchor as strong as possible. But what is the minimum you'll accept as an anchor to lead above? At about what general point (and yes, as I and others mentioned, it's situational) will you deem an anchor insufficient based on the strength of the weakest arm? ~Adam~
I would never consider a multi-arm anchor insufficient based on the strength of the weakest arm. That's why you have multiple arms.
-Jay
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rrrADAM
Aug 5, 2004, 10:45 PM
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Once again... Very astute answer Jay. Good job.
Now that I think about it... I'm gonna have to place whatever I can in a given situation. It's not like I have a bolt kit to drill and bolt in bomber anchors when the rock doesn't offer the most optimum choices. I've had to build a nest of 6 pieces before, that took a lot of cord and slings to get right.
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tedc
Aug 5, 2004, 11:05 PM
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In reply to: I would never consider a multi-arm anchor insufficient based on the strength of the weakest arm. That's why you have multiple arms. -Jay
Really :?
Top of pitch. One 3/8" stainless bolt of unknown depth but looks pretty good (guess at least 15KN strength). Only other gear is two pretty well fixed stoppers (a #4 probably good for 6KN, but I'd derate it to about 5KN due to age and the quality of placement; and a #2. looks good but still only 2KN :shock: ) So you have three legs and one of them is at least equal to your 15KN "spec."
Is this anchor "insufficient"?
You can determine my answer by reading my reply to the original poster's question.
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jt512
Aug 5, 2004, 11:18 PM
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In reply to: In reply to: I would never consider a multi-arm anchor insufficient based on the strength of the weakest arm. That's why you have multiple arms. -Jay Really :? Top of pitch. One 3/8" stainless bolt of unknown depth but looks pretty good (guess at least 15KN strength). Only other gear is two pretty well fixed stoppers (a #4 probably good for 6KN, but I'd derate it to about 5KN due to age and the quality of placement; and a #2. looks good but still only 2KN :shock: ) So you have three legs and one of them is at least equal to your 15KN "spec." Is this anchor "insufficient"?
I think you need a little work on your deductive reasoning, but as to the specific anchor: Basically, you're asking whether I would climb above an apparently good bolt and a #4 nut, since the #2 doesn't add much to the anchor. I'm guessing I would, if the moves above the anchor weren't too bad and/or I could get a good piece in close above the anchor. Then again, I've climbed above a single-bolt semi-hanging belay in sandstone, so maybe I'm an accident waiting to happen.
-Jay
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ralphwolf
Aug 6, 2004, 2:37 PM
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OK. Good.
jt512 will use tedc's anchor. Kinda.
He will accept that setup with (whatever the calculations show to be) it's minimums. Multiple 2 factor anchors aside for the moment...
Minimums are pretty subjective. As jt512 deduces. Anchors to match the falls at risk...
He seems to be assessing 1) the risk of a fall, 2) chances of placing additional protection of a fall (presumabley protecting the integrity of the the anchor?) and then climbing past it 'cause he feels he will not fall on it or downclimbing 'cause tedc's anchor is (going to be tested in real life and jt512 is concerned that it's) too sketchy. That is the questions point, right? Acceptable - Climb up or Unacceptable - Go down?
Seems reasonable. Beyond the anchor physics it's not quantifiable but damn reasonable! That is jt512's lesson of course. Use your well thought out judgement based on your training and experience. Wish for 2 fall factor anchors first and foremost but when you're actually on the rock, climb like a climber.
Mark
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tedc
Aug 6, 2004, 3:36 PM
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In reply to: I think you need a little work on your deductive reasoning, ...
Do tell. Seemed like a reasonable analysis to me.
I would also use that anchor in the unique conditions you describe but I would NOT attempt 5.10 ish moves directly off the belay, nor would I attempt 20' of 5.8 friction slab off the belay if this were the best I could do for an anchor.
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trenchdigger
Aug 6, 2004, 3:45 PM
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So I think what we've proven here is that you can try to pose numbers as responses to these types of questions, , but in the end they just don't hold much water. It's just not that simple. An acceptable anchor in one situation may be totally unacceptable in another situation. As we love to say when asked if something is acceptable in a climbing situation, "It depends..."
To be able to build a safe anchor, you need to consider all of the factors involved. You also must know the limits of your gear. Not only that, but also know your own limits. And last, but not least, you need a basic knowledge of the physics involved in an anchor system. More than anything, people seem to be lacking in the first and last categories. The first, I feel, comes with experience, and the last moreso from research. "Freedom of the Hills" is a great source for info on the basic physics of anchors. This website's forums are not! :roll:
~Adam~
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jt512
Aug 6, 2004, 4:17 PM
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In reply to: He seems to be assessing 1) the risk of a fall, 2) chances of placing additional protection of a fall (presumabley protecting the integrity of the the anchor?) and then climbing past it 'cause he feels he will not fall on it or downclimbing 'cause tedc's anchor is (going to be tested in real life and jt512 is concerned that it's) too sketchy. That is the questions point, right?
Keep in mind that all of the above only comes into play way for the 1% of the time you can't build a bombproof anchor.
In reply to: Acceptable - Climb up or Unacceptable - Go down?
Unless going down means rappelling off a questionable anchor, in which case going up might be better. This is a decision I've only had to make once -- that one dubious bolt in sandstone anchor I mentioned earlier.
Jay
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jt512
Aug 6, 2004, 4:21 PM
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In reply to: In reply to: I think you need a little work on your deductive reasoning, ... Do tell.
All I said was that I wouldn't reject an anchor out of hand because it had one weak arm, implying that there are other criteria, and then you challenge me, and give an anchor that proves my point.
-Jay
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jt512
Aug 6, 2004, 4:31 PM
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In reply to: "Freedom of the Hills" is a great source for info on the basic physics of anchors. This website's forums are not! :roll:
What I've been saying since I joined this web site is that the Internet is best suited for climbing information that is too specific (eg, beta) or too advanced for broad publication. People shouldn't be coming here for basic information that is published in a zillion climbing instruction books. The effect of the angle between arms of an anchor on the force applied to the anchor is basic knowledge that every lead climber ought to know, and every decent basic climbing instruction book explains it. On the other hand, subtleties, like whether a tied pseudo-web-o-lette is as good as a sewn one, or some of the advanced theory on anchor forces that rgold discusses aren't published, and it's for stuff like that these forums can really be valuable. If only we could improve the signal-to-noise ratio.
-Jay
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vivalargo
Aug 6, 2004, 4:54 PM
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[quote="trenchdigger"]What is the minimum single arm strength you're willing to accept in an anchor?
One that would actually hold.
You cook up some abstract number, then what happens out in the field when you can't meet the criteria?
JL
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reno
Aug 6, 2004, 5:04 PM
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In reply to: One that would actually hold
Best answer so far...
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coylec
Aug 7, 2004, 1:30 AM
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In reply to: In reply to: "Freedom of the Hills" is a great source for info on the basic physics of anchors. This website's forums are not! :roll: What I've been saying since I joined this web site is that the Internet is best suited for climbing information that is too specific (eg, beta) or too advanced for broad publication. People shouldn't be coming here for basic information that is published in a zillion climbing instruction books. The effect of the angle between arms of an anchor on the force applied to the anchor is basic knowledge that every lead climber ought to know, and every decent basic climbing instruction book explains it. On the other hand, subtleties, like whether a tied pseudo-web-o-lette is as good as a sewn one, or some of the advanced theory on anchor forces that rgold discusses aren't published, and it's for stuff like that these forums can really be valuable. If only we could improve the signal-to-noise ratio. -Jay
Like shoe threads? :lol:
seriously, though ... nice job jay.
coylec
ps - yeah, i know this post is just noise ...
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