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Webbing strength confusion

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  • Webbing strength confusion

    Good morning everybody,
    During a my shift on a few days ago, we were training on anchors, 3:1 haul systems, and rigging a stokes. I had setup a W3P2 to anchor the safety line. In the middle of all this, one of our chiefs walked up and started asking “why does increasing the number of strands in the webbing make the webbing stronger?” He was convinced that a piece of webbing rated for 6,000lb is rated for 6,000lbs no matter how many times you wrap it or loop it or fold it. He held a straight piece in his hand and said “This is rated for 6,000lb.” Then he tied a water knot, held it up and said “And this is rated for twice as much now? Why?”

    So, I’m pretty new to this. Just got my FFI, I have 1 week of low angle rope rescue training. I have a LOT to learn, and while I have a comfortable understanding of this, I was unable to get him to that point.

    If I have that single loop of webbing, around a bollard, I understand that the load is distributed evenly (more or less) on each strand running back to the anchor. However, on the backside of the anchor, those two strands meet in one strand. Why does that one strand not get overloaded and fail?

    sorry for the book and the (likely) dumb question, but Chief asked me “Why?” And I’d like to learn the answer.




  • #2
    He's right.... sort of... I think.

    wrapping webbing does not increase it's strengths; the material itself still has the same breaking point at 6000 lbs.

    That being said, different configurations have different strengths, because of how the force is applied to the webbing, especially when discussing different methods of anchors. and if you have multiple strands going to multiple anchor points, you can distribute the load.

    check out https://www.cmcpro.com/one-inch-webbing-anchors-minimum-breaking-strength/ for some examples

    you can also check out http://www.mountaineeringlife.com/wp...strengths1.pdf

    Remember, knots affect the strength of the webbing, as do turns involving sharp angles.

    If my basic HazMat training has taught me nothing else, it's that if you see a glowing green monkey running away from something, follow that monkey!

    FF/EMT/DBP

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    • #3
      While a W3,P2 has 4 strands sharing the load it theoretically has 4x6,000 lbs of load carrying capacity (thus 24k), assuming the webbing is rated at 6k on a single strand. I think it's typically closer to 4k though for 1". Even though the theoretical load is 24k, in practice and testing it will actually be closer to 13-14k before it breaks. This is due to friction, primarily between the strands where they cross over each other on the load caribeaner as the load increases.

      Like stated above, knots do affect strength, regardless of whether it's rope, webbing, chain, cable, whatever. Again, that's mostly due to friction. The reason you wrap 3, but pull 2 is to leave the knot on the back side of the anchor where the friction is isolated and the load does not affect the knot for the most part. It's the same reason you can do a "friction-less" hitch as an anchor.

      You would probably be well served to get the book "On Rope" by Padgett and Smith and do some reading if this is something that interests you.

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