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While browsing the DumbAnchors blog, I came across a post which points out the poor use of a single strand (not a loop) of webbing for building top rope anchors.

The blogger doesn't say why these are unacceptable to use, he only points out the mistakes people make.

Assuming the other components of the anchor are correct, is/why is a single strand of webbing unacceptable to use for a climbing anchor?

Edit: To clarify, assume the webbing is redundant above the master point, just not used in a loop above the master point. For example, it may have a figure eight clipped to a bolt with one single strand coming down to the master point. Also, here's another post with some commentary.

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He says "single strand webbing." When I first read the question, I thought this referred to a certain type of webbing, but from the web page it looks like he just means a single strand of webbing. I'll edit the question to clarify this. – Ben Crowell Mar 14 at 14:23
    
good point, I'm guessing he means no redundancy in the anchor – Liam Mar 14 at 14:24
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@Liam Looking through many other posts, the criticism (and my resulting question) is related to the fact that above the master point, there is one single strand used instead of a loop. I wanted to see if it has to do with a loop actually supporting double the strength of a single vertical strand. – Chris Mendez Mar 14 at 17:40
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Tensile strength of webbing appears to generally (fishproducts.com/tech/High_Strength_Cord.pdf) approximate (itrsonline.org/PapersFolder/2013/Spinelli2013_ITRSPaper.pdf) dynamic loads from drop tests. Used as a single arm of a multi-armed anchor I see no issue with single strand webbing. – requiem Mar 14 at 18:15
up vote 10 down vote accepted

As with most things in climbing, I myself would not go as far as saying this is generally unacceptable. In multipoint anchors there are often single strand connections between one point and the central point when building a cordelette or equalette. So the question reduces to whether it is (un-)acceptable when using a single point (like a tree) as an anchor.

Typical climbing webbing has a strength of 10kN or more and a general rule of thumb is that knots reduce the strength by 50%. This is an extremely conservative estimate derived from sewn slings, which have a strength of 22kN as a loop, neglecting the sewing, dividing by two and rounding down this results in this very conservative estimate. So using a single strand you obviously need a knot, resulting in a minimal strength of 5kN. While this is enough for top-roping with belaying correctly, it does not have much safety margin. The fact that there is only one anchor and thus no redundancy makes this very conservative (maybe overly) approach sensible.

When setting up an anchor on a single point you have no redundancy, so you want to be very sure that the point itself (tree, rock, ...) is actually stable and your extension is safe. Focusing on the webbing this means abrasion and cutting on anything along its way. It needs to be either not touching anything at all or be protected by laying something between the rock and the webbing. In both cases you need to consider that the rope with the climber may shift around. But all of this is true for a double strand of webbing as well. If you have a setup which leads to one strand getting cut there is a very high probability that the second one (being parallel) will be severed as well.

In the end the problem is the borderlineish strength: It is a good idea to be on the safe side and use two strands. The much bigger problem with single point anchors is the reliability of the point itself and abrasion/cutting of the webbing on rock. So you gain more using more than one point, where a single strand is certainly ok.

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I believe your strength numbers may have an incorrect assumption; the 22kN is for sewn slings (usually 5/8" nylon) tested as a loop. Similarly, 1" tubular webbing tests just under 18kN single-stranded and around 25-30kN as a loop. – requiem Mar 14 at 18:20
    
I was not clear on this. I am aware that tubular webbing can be stronger than the mentioned 10kN. This is just a very conservation estimate not knowing what webbing is actually used. I edited it to make it (hopefully) more clear. – imsodin Mar 14 at 18:35
    
+1, but I would say - the math looks great - until the loaded single stand passes over a sharp rock. – mattnz Mar 15 at 20:09

The blog post shows an anchor in which some kind of hitch is tied around a tree, and a single strand of webbing leads away from the tree horizontally to, presumably, the top of the climb, which is out of frame.

The blogger seems to be criticizing this setup because there is only a single strand. I suppose this is somewhat valid because if you load this anchor, all of the load is taken by a single strand of webbing. If they instead tied a big, long loop of webbing around the tree, then each strand would only take half the load. But this aspect of this anchor actually doesn't seem like a huge problem to me, since it's being used as top-rope anchor, and doesn't need to catch a dynamic fall. You can't break a piece of nylon webbing with a top-rope fall, unless there's something else going on (e.g., the webbing was damaged by rubbing against a sharp edge).

Assuming that the knot is a good knot, it's dressed nicely, and there are long tails, I would not have a problem trusting my life to this anchor. The tree looks gigantic, and it looks like its roots are solid. (I can't see in the picture whether the tree is alive.) It's true that the anchor is not redundant. However, redundancy is just a rule of thumb -- people often use a single non-redundant anchor when it's as huge and obviously bomber as this seems to be based on the photo. If I had a way to back it up, I certainly would, but using it as is is not stupid, IMO.

Note that tying a big long loop of webbing would not make this anchor redundant in terms of failure of the webbing, because if either strand broke, the loop would come apart.

Most likely the reason they used a single strand is simply that they didn't have enough webbing to reach with two strands.

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You can break a single small tree with a top rope fall though, seen it happen. Tree's on rock faces can have very, very shallow roots, especially if they are not well established. They also tend to fail catastrophically and unexpectedly! The person in the post seems to know that the tree's are not sound (mentions that several are dead and hollow) – Liam Mar 14 at 14:48

A single strand of 1" tube webbing has a breaking strength of almost 18kN. which is 50% more than the total amount of force any climber will ever be able to generate (12kN), but the safety standard is a breaking strength of at least 100% more than what a human body could generate (24kN).

That means a single strand of tube webbing may not be acceptable as an anchor for climbing because it does not meet the minimum breaking strength requirement for a top anchor. But more importantly, it's not redundant. It would be suitable as a rappel anchor, it could hold a fall, but it is not bombproof. A loop of webbing, or two strands sould be the minimum requirement.

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This is not correct. Firstly it is not about what a climber can generate, about what the standards for climbing rope allow. A climbing rope will stretch so much that the peak force will be under 12kN. The reason why it is still necessary to withstand double this force is, that at any point where the rope direction is inverted (toprope anchor) this force will pull on both sides. – imsodin Mar 14 at 17:06
    
After looking through more of the blogger's posts (and possibly answering my own question) I found some comments where he mentions your answer. – Chris Mendez Mar 14 at 17:55
    
In a toprope situation you're going to be hard-pressed to put more than about 6-7 kN on the anchor (yes, accounting for pulley effect). If using trad gear to build the anchor, the ratings of such gear will also be in the 10-14 kN range, (similar to what you'd get with single-strand webbing once you've added knots). What safety standard are you referencing? I'm familiar with the SWL numbers, but those are usually a 5:1 or 10:1 ratio which doesn't work well for climbing. (My concern with the anchor would thus be redundancy, not the strength of a single arm.) – requiem Mar 14 at 18:11

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