Webbing for anchors

[jbliss]

In the canyons group I recently initiated a discussion about webbing used for anchors. It seems that the general consensus among the more experienced canyoneers there was that 9/16 climb-spec was sufficient for many purposes, and that 11/16 and 1 inch were obviously acceptable as well. It was my contention that given its extra strength, that 1 inch was preferable in most instances. I have already communicated with "Mr. wild" on this topic.
Any thoughts?

[jbliss]

As a follow up related question, where did the 10 to 1 and 15 to 1 rules come from? That is that the rope should only be loaded 1/10 to 1/15 the load strength of the rope. Is there some scientific evidence to support these numbers?

[rcwild]

I came upon a fellow in a canyon once who was using orange and black braided hardware store rope. I asked him if he felt the rope was strong enough to be rappelling on. He informed me that the rope was rated to 275 pounds and he was the heaviest one in his group -- weighing in at ONLY 245 pounds -- so certainly the rope was strong enough. :confused:

Safety factors exist to ensure that users have adequate margins of safety. Factors of 10 to 15 are commonly used by manufacturers and the NFPA (National Fire Protection Agency). Are these factors overkill? Of course. There are liability issues to consider.

But how much overkill? Not as much as some people might believe. Tying a knot in webbing can significantly reduce its strength -- by as much as 40-50%. Typical 1 inch webbing can hold 4200 lbf. Typical 9/16 inch webbing can hold just 2200 lbf. Lose 40% with a knot and you are left with strengths of only 2520 and 1320 lbf.

Even in canyoneering, we subject ourselves to potential falls. A fall can generate dynamic force several times greater than the person's body weight. There is a cool little calculator at:

http://toad.stack.nl/~stilgar/calc.php

I just used it as follows:

Solve for rope modulus by entering:
Static elongation = 1
Weight = 80kg (approx 176 pounds)
Rope modulus = 78.48.

Solve for force generated in a fall by entering:
Fall factor = 1 (i.e. 5-foot fall on 5-foot rope)
Rope modulus = 78.48
Weight = 80kg
Force generated = 11.91 kN (approx 2625 lbf)

I'm not an engineer or physicist, but I do know that with more and more people using extremely low-stretch ropes, the force generated in a fall has to go somewhere -- your body, your anchor and your webbing.

The weight difference between 1 inch and 9/16 inch webbing is so insignificant, I'd rather have the extra margin of safety that 1 inch provides.

Mr. Wild