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Victoria, BC
Canada
83 Posts |
Posted - 06/07/2007 : 11:51 PM
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It makes sense that a self-equalizing anchor system is always equalized. As the load shifts, the anchor adjusts so that the load is evenly distributed.
However, if you build a so-called "manually" equalized anchor system where the sling lengths of the anchor are static, is it ever really equalized?
In theory, if the fall/load happens at the exact angle you set your anchor, then everything is fine. But if the load occurs at a different angle (even a miniscule difference) than expected, then one side of the anchor will pick up the entire load and you've lost equalization.
The reply I always get to this is "your anchor may not be completely equalized, but if you get your angle right, it will be close and your system will be partially equalized".
But once that one side of the anchor becomes the main load bearing side, it will take 100% of the load, so there really is no such thing as "partial equalization". Either you're sharing the load exactly, or its all on one side.
So if thats the case, and its also reasonable to assume that it is impossible to have the exact angle of the load determined, isn't it safe to say that the only anchor system that can be truly equalized is a self-equalizing system?
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Vancouver, BC
Canada
1422 Posts |
Posted - 06/08/2007 : 12:26 AM
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Assuming nothing stretches, for a 2 point anchor it's a rather simple physics problem. If the rope (or whatever connects to the anchor) comes straight in to the triangle of two equalized pieces, then it's perfectly equalized. As the angle shifts, the leg of the anchor that is more ini line with the rope gets more load. When it's aligned straight with the rope, 100% of the load is on that strand.
Say the angle between anchor point A and the load is "a" and the angle between anchor point B and the load is "b"
Then the force balance equations are
Load = Load_A * cos(a) + Load(B) * cos(b)
Load_A*sin(a) = Load_B*sin(b)
or
Load = Load_A * cos(a) + Load_A sin(a)cos(b)/sin(b)
or
Load_A = Load / (cos(a) + sin(a)cos(b)/sin(b))
and
Load_B = Load / (cos(b) + sin(b)cos(a)/sin(a))
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Edited by - scottN on 06/08/2007 12:32 AM |
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Finally stopping that crazy suffering that is ice, climbing to concentrate on great ski tours!
3505 Posts |
Posted - 06/08/2007 : 06:04 AM
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Yes, I think only a self-equalizing system would ever be truly equalized. However, personally I would NEVER use the "sliding" equalizing system without the knot. Though it is equalized, it is NOT redundant. If the sling/cord breaks in any spot, that's it. Also, if one anchor blows, it's got a good amount of extension. That will shock load the second anchor, your only remaining anchor.
There are trade offs to every system, but for me in trying to get "SRENE" the self-equalizing system misses too many other knock-outs. |
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Edmonton, Alberta
Canada
797 Posts |
Posted - 06/08/2007 : 07:44 AM
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Reading the above it seems to me the best option would be a self-equalizing system with two knots tied to minimize extension if one anchor fails.
Why do you still favour the other method John? (I have noticed it seems the choice for most people).
(I only started climbing in Feb...so always trying to learn) |
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| Paulo
Intermediate Member
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Vancouver, BC
Canada
768 Posts |
Posted - 06/08/2007 : 08:01 AM
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| I knot a three point system (two if bolted) after getting it as equalized as possible (without spending a lot of time adjusting for a few millimetres here and there) based on the line of pull discovered while ascending to the belay station. Now if the next climber takes a significantly different line then it's not going to be as equal as I'd planned for but, under the circumstances, it'll have to do. In that case, if the overloaded anchor starts to breakdown under stress the next anchor(s) will take more of the stress and, hopefully, not suffer from a sudden shock load. Bottomline... climbing's a risky activity. Thanks for bring up the topic... good to think about such things. |
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Alberta-based choss climbin', flame throwin', rappel lovin', ass talkin' hater who doesn't like "Gumby" for a descriptor
6302 Posts |
Posted - 06/08/2007 : 09:00 AM
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quote:
However, if you build a so-called "manually" equalized anchor system where the sling lengths of the anchor are static, is it ever really equalized?
Nope.
quote:
trying to get "SRENE" the self-equalizing system misses too many other knock-outs.
I am curious to know what you mean by "too many other knock outs"? The "no extension" part can be minimized alot by the use of knots.
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Finally stopping that crazy suffering that is ice, climbing to concentrate on great ski tours!
3505 Posts |
Posted - 06/08/2007 : 09:24 AM
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I should be clearer. I figure that the OP is about a simple sling/cord that is run through two anchors and then the 'biner is clipped through it as it is looped, creating a "full-floating" no-knots type of self equalizing set-up. That system extends and has no redundancy. It's not until you begin tying knots can you begin to get SRENE, but I figure that once a knot (or several, depending on how you do it) is introduced, it will begin to hamper the self-equalizing compared to the "full-floating" set-up.
http://www.uoregon.edu/~opp/climbing/topics/anchors.html
At this page, check the "self equalizing anchor" (red slings) and you'll see what is in my head. That system is equalizing, but with it made from one continous sling and having only a loop and no knots, it will extend and is not redundant. Also, if the sling material were to be released, the whole thing would come down. I don't like this set-up.
 
Now, you can easily add some knots that still let it self-equalize quite a bit, but not to the extent that the "no-knots" set up does.
For something quick, I'd carry a double length sling, with a knot tied in the middle, and another knot about half way between the middle and the end. Then you can clip different spots depending on where the anchors are situated in relation to each other. |
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 | Dru
Mountain Grammar Police
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Sardonic sandbagging scoundrel, Cascade Climbers lobotomized spraymeister, space blanket flyer, new millennium vulgarian betaboy and friend to all squids
Climbing, a mountain
Canada
∞ Posts |
Posted - 06/08/2007 : 09:53 AM
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| Tests have shown that with a sliding X, if one end fails, the extension occurs over time and so the load is not "shock loaded". On the other hand, with a knotted anchor like a cordelette, the loading is never even between the multiple strands, with the result that one anchor piece is preferentially loaded. This was the mechanism of anchor failure in a double fatality at Tahquitz a couple of years ago. |
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Alberta-based choss climbin', flame throwin', rappel lovin', ass talkin' hater who doesn't like "Gumby" for a descriptor
6302 Posts |
Posted - 06/08/2007 : 09:57 AM
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quote:
That system is equalizing, but with it made from one continous sling and having only a loop and no knots, it will extend and is not redundant. Also, if the sling material were to be released, the whole thing would come down. I don't like this set-up
...which is why you should use knots to limit extension and add redundancy to the sling. Personally I don't, nor does anyone I know, use the sliding x without adding limiter knots. I always thought about limiting extension as par for the course. |
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Alberta-based choss climbin', flame throwin', rappel lovin', ass talkin' hater who doesn't like "Gumby" for a descriptor
6302 Posts |
Posted - 06/08/2007 : 10:01 AM
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| With that said if some one was to make a tr setup off of 2 bomber bolts / sliding x and not add knots to it, I would not make a fuss over it. The chance of a bolt failing on tr or a good condition sling spontaneously breaking is slim to none. |
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Alberta-based choss climbin', flame throwin', rappel lovin', ass talkin' hater who doesn't like "Gumby" for a descriptor
6302 Posts |
Posted - 06/08/2007 : 10:04 AM
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quote:
Tests have shown that with a sliding X, if one end fails, the extension occurs over time and so the load is not "shock loaded".
Do you have a link to the test results handy Dru? |
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| Dru
Mountain Grammar Police
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Sardonic sandbagging scoundrel, Cascade Climbers lobotomized spraymeister, space blanket flyer, new millennium vulgarian betaboy and friend to all squids
Climbing, a mountain
Canada
∞ Posts |
Posted - 06/08/2007 : 10:09 AM
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| It was on gravsports |
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Victoria, BC
Canada
230 Posts |
Posted - 06/08/2007 : 11:18 AM
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quote:
But once that one side of the anchor becomes the main load bearing side, it will take 100% of the load, so there really is no such thing as "partial equalization". Either you're sharing the load exactly, or its all on one side.
This is not really true, because there IS some elongation of the slings. If you have one sling that's, say, 2cm longer than the other. If the 'biner connecting these two slings is loaded at an angle that is CLOSE to equal relative to both slings, then one sling may take up more load, however it will also elongate more. Once it elongates to the same length as the other sling, then the other sling will share the remaining load. So you can in fact end up with 'partial' equilization. Obviously webbing and cord does not elongate that much for a typical load, but I would think that, provided the line of action of a top rope (for example) bisects the line between the two anchor points (ie. bolts), and the slings are as close to the same length as possible, then reasonably effective 'partial' equalization can be achieved.
When toproping off of bolts, I think the use of two independent, appropriately sized slings is fine. In other scenarios the loads (both magnitude AND direction) on the system are less predictable, and the anchors may be natural/off of gear, so self-equalization is more important.
As an aside: I don't like the sliding x without knots because of the lack of redundancy. |
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| Dru
Mountain Grammar Police
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Sardonic sandbagging scoundrel, Cascade Climbers lobotomized spraymeister, space blanket flyer, new millennium vulgarian betaboy and friend to all squids
Climbing, a mountain
Canada
∞ Posts |
Posted - 06/08/2007 : 11:41 AM
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| in theory it may lack redundancy, but in practice it is safer. i like to look at actual reported accidents. hard to find any using the sliding-X |
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Victoria, BC
Canada
230 Posts |
Posted - 06/08/2007 : 11:45 AM
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In theory it may lack redundancy, but in practice it is safer. i like to look at actual reported accidents. hard to find any using the sliding-X
I think it's relative to what your anchor points are. If they're not bolts, then yes, the sliding-x is probably safer. |
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Alberta-based choss climbin', flame throwin', rappel lovin', ass talkin' hater who doesn't like "Gumby" for a descriptor
6302 Posts |
Posted - 06/08/2007 : 12:00 PM
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quote:
Originally posted by retep
quote:
In theory it may lack redundancy, but in practice it is safer. i like to look at actual reported accidents. hard to find any using the sliding-X
I think it's relative to what your anchor points are. If they're not bolts, then yes, the sliding-x is probably safer.
There are no cut and dry "rules" of which anchor method is the the best or "safest". As always, the best method is completely dependant on the situation. |
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Kootenay Bud
2695 Posts |
Posted - 06/08/2007 : 1:09 PM
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Over how much time? If I simulate this on my climbing wall, by having one bolt suddenly fail, the load very rapidly comes on to the remaining bolt, at least to the degree it can when a limiting knot is in place - which seems pretty much the definition of shock loading. Do you mean that some of the force is taken out by friction as the cordelette extends?
quote:
Originally posted by Dru
Tests have shown that with a sliding X, if one end fails, the extension occurs over time and so the load is not "shock loaded". On the other hand, with a knotted anchor like a cordelette, the loading is never even between the multiple strands, with the result that one anchor piece is preferentially loaded. This was the mechanism of anchor failure in a double fatality at Tahquitz a couple of years ago.
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Victoria, BC
Canada
230 Posts |
Posted - 06/08/2007 : 3:51 PM
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The term 'shock loading' is also somewhat ill-defined. Any time you slow something down there is a force required (Newton's second law, F=ma). If you slow something down very fast (ie. a=some large value opposing the direction of motion) the corresponding force is typically considered to be this 'shock loading' that we talk about. But at what point is an acceleration great enough to produce a shock load?
With respect to the sliding-x and shock loading: There will certainly be some force on the remaining anchor(s) greater than the static weight of the climber in the event of an anchor failure. I guess Dru's implication is that this dynamic load (call it what you want) is typically not very large. I can't agree or disagree with this assertion. I would have to see some empirical evidence.
I agree with T2C that it's all very dependent on the particular situation. |
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Vancouver, BC
Canada
1321 Posts |
Posted - 06/08/2007 : 4:17 PM
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| John Long's latest anchor book also says that the "shock loading" that occurs on anchor extension is not as big of a deal as people once thought. I don't remember if he included data on this or not. There is a bunch of data in there comparing various equalization schemes though. |
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Finally stopping that crazy suffering that is ice, climbing to concentrate on great ski tours!
3505 Posts |
Posted - 06/08/2007 : 5:30 PM
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The point about not any one system being appropriate for every circumstance seems bang on. It's going to depend on your anchors (bolts, bomber pro or manky) and how much time you have. Of course, on belay stations, the objective is to be bomber every time as that is the anchor that really is the last thing between you and "the big fall."
If the station is shiny big bolts with nice shiny hangers, then shock loading and equalizing might not be much of a concern. If your anchor consist of micronuts, you might think about it differently. |
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Vancouver, BC
Canada
1422 Posts |
Posted - 06/08/2007 : 6:08 PM
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| So let's so you're just starting out climbing from a belay on a multi pitch climb and the leader takes a 15 foot fall onto the belay. That's already a lot of shock loading right there. So if one piece in your multi point anchor fails and it extends an additional 2 feet, the added energy from the anchor extension is pretty small compared to the energy from the falling climber. Personally I'll take the equalization over the extension. |
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is it really equalized?
