Is a single strand of webbing unacceptable for climbing anchors?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.