load shared anchor-Dyneema vs nylon

When at Black Diamond, I was interested in the meme “spectra webbing cannot be tied”. So we took some ‘spectra webbing’ (which, like the Titan webbing, is 50% spectra (or dynex or dyneema, all equivalents)), tied them using normal knots for webbing and broke them on the machine. This was an informal test so I do not have a record of it, but…

From memory, the knot-loss-of-strength was roughly 50%, which is somewhat lower than the conventional all-nylon knot-loss-of-strength of 70%.

I don’t see a compelling reason to try to break the meme “spectra webbing cannot be tied”. More subtle memes like “can be tied carefully, dressed well, and your loss-of strength is higher” is unlikely to translate across the culture effectively.

But it does point to that knots in Spectra work, just not as well. So I suspect your know would work fine; and that given the strength of the components, it will be hard to get it to break unless you have a 20K capable test rig.

If you go into testing, I would suggest testing simple tied spectra webbing loops as a start.

Tom

— In Yahoo Canyons Group, “Sonny” wrote:

The question about a knot in a Dyneema sling came from Paul Stovall.

Two weeks ago I created a land navigation course for the search and rescue team I am on. For one of the stations, the participants had to read a flag placed 12 feet up in a tree. To do so, they had to rig a guided rappel at just the proper angle. We were at Pinnacles rock climbing area near Big Bear Lake, California. There is a large rock with two bolt anchors on top. I used those to build a load shared anchor using a Bluewater Titan 13 mm sling (rated at 27 kN). I used 30 kN carabiners, stamped Maillon Rapides and a solid belay ring. I figured this anchor would be more than strong enough to handle the loads from a guided rappel. I did not want members who are trained in technical rope rescue to question the strength of the anchor nor its components. It was set up so they could rig a pull down guided rappel (assuming they knew how). Participants would navigate to the anchor I built, call the command post with info about that station, then be told that the next station was due south and 4 meters horizontally away. One person would rap down, establish the bottom anchor, bring the second person past the elevated station so the required information could be obtained. > After we were finished, I broke down the anchor. Paul Stovall asked the question, if one leg of the anchor fails, will the knot in the Dyneema hold as when tied with nylon or will it slip apart as with Dyneema cordelette? I had not considered that when I built what I considered to be a rescue-load rated anchor. >

There are a few times when load-sharing is the highest priority, but 99% of the time, we are just trying to achieve redundancy, preferably without extension.

The “cordalette-style tie-off” works well for this, and that is the purpose of the knot.

My question is:

Sonny – why did you post this? Is this a guessing game for an upcoming test? Will the knot stay tied in the Titan 50/50 nylon spectra webbing, when only one leg of the anchor is anchored?

My guess would be YES.

Tom

— In Yahoo Canyons Group, rging@… wrote:

You want to equalize it use one long sling as in a sliding X. Any anchor that has one leg vertically higher than the other by more than a few inches is virtually impossible to equalize unless your material has zero stretch or you use a sliding something. Read John Long’s anchor book regarding this topic. He is one of the few people that performed measurable tests on such systems (for climbing). There are some lengthy threads on line regarding the topic as well. I would not have believed it myself if I had not seen the numbers. So the next question is, do you need equalization or just redundancy?

In the specific setup shown the best way to equalize it without unecessary extension would be a sliding X with stopper knots. >