Using 9/16" webbing instead of 1" webbing

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Using 9/16" webbing instead of 1" webbing

Postby Jeff Bartlett » Nov 19, 2011 10:37 am

I've started carrying 9/16" instead of 1" webbing, since it's so much easier to carry a 20ft length in a pack. Primarily, I use webbing for two things on caving trips: for constructing backup anchors for scary TAG bolts, and for when a handline is needed.

I do have one remaining question, however, and I can't seem to find the answer in any of my references. I know a W3P2 in 1" webbing -- Bluewater says 4,000# / 17.7 kN breaking strength on 1" -- is one of the strongest points in a system and perfectly suitable for use in single-anchor rigging. I can't remember how to calculate the strength of that anchor, however, so I can't determine what the strength of the same anchor is in 9/16" webbing.

Bluewater says 2,300# / 10.2 kN is the breaking strength of its 9/16" (15mm) tubular webbing. What, then, is the strength of a perfect-world Wrap 3 Pull 2 in 9/16" webbing, tied around a bedrock pillar or similar solutional feature?



Edit: I realize I could just email one of my rope nerd friends -- Tim White and Bill Putnam, I'm looking at you -- but it's more fun to ask the question publicly, and more useful to the caving community for the answer to be available on cavechat later.
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Re: Using 9/16" webbing instead of 1" webbing

Postby Caver John » Nov 19, 2011 11:42 am

So if your willing to go off of bw specs, then didn't you just answer your own question?
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Re: Using 9/16" webbing instead of 1" webbing

Postby Chads93GT » Nov 19, 2011 1:21 pm

Is 4000# the strength of a 1" single strand or is that the breaking strength of the actual w3p2? If its the prior doesn't that make the anchor 8000# so theoretically 9/16 would be 4600# right? I have honestly never read how the strength of that anchor is calculated. Interested.
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Re: Using 9/16" webbing instead of 1" webbing

Postby Jeff Bartlett » Nov 19, 2011 4:19 pm

Caver John wrote:So if your willing to go off of bw specs, then didn't you just answer your own question?


Did I just answer what the breaking strength of a W3P2 is in 9/16" webbing based on webbing rated to 10kN? No, no I didn't. That's what I am asking. I can't seem to recall if a Wrap 3 Pull 2 is simply 2x the breaking strength of the webbing -- two separate loops of webbing with the knot removed form the equation -- or not.

Certainly, if that IS the answer, 20 kN is more than enough for a single anchor. I believe Life On A Line specifies 12 kN as the requirement for an anchor in a rescue situation. I'd also be curious to know the value for 9/16" webbing rigged in a "basket" orientation, in which case the water knot is part of the equation.
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Re: Using 9/16" webbing instead of 1" webbing

Postby NZcaver » Nov 19, 2011 5:30 pm

If the short answer "strong enough" is not satisfactory for you, this is a brief summary from the 2011 ITRS. Scroll down and read the last part talking about a series of testing conducted by a couple of dodgy cavers I happen to know. :wink:

PS - I realize there is no mention of 9/16 webbing, but it does illustrate how conventional W3P2 wisdom is not an exact science.
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Re: Using 9/16" webbing instead of 1" webbing

Postby LukeM » Nov 19, 2011 6:10 pm

More importantly, how strong is that pillar you're rigging off of? :P

Going off of the numbers given in the link from Jansen, it looks like the theoretical average breaking strength of the 9/16" wrap-3-pull-2 would be around 5270# or close to 23.5 KN, or like Jansen was getting at: "strong enough". Of course, until you do pull tests with that particular webbing you can only guess. Keep in mind the reduced abrasion resistance.
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Re: Using 9/16" webbing instead of 1" webbing

Postby Jeff Bartlett » Nov 19, 2011 6:58 pm

Thanks guys! This is exactly what I was asking about -- theoretical W3P2 strength = breaking strength of webbing x 4 -- AND some test data that allows me to scrap the "theoretical" and focus on whether 9/16 provides enough real-world strength to be feasible.

So, yes, if a W3P2 in 1" webbing is a theoretical 17.7 kN x 4 = 70.8 kN, even assuming HALF of this strength (less than the testing indicates) means approximately 35 kN for a 1" webbing basket or 1" webbing W3P2, both significantly greater than the strength of a piece of 11mm rope.* Using the ~10kN rating of 9/16 webbing as our starting point, we can still expect a breaking strength of greater than 20kN for a basket (at least one with a properly-tied and dressed water knot that isn't girth-hitched to the anchor) and similar figures for a W3P2. While 20kN isn't stronger than the rope, it's plenty strong for a caving anchor in my book, and just about equivalent to the strength of that same rope once you tie an eight-on-a-bight in it and attach it to the webbing with a carabiner.

Luke, agreed on reduced abrasion resistance; the trade-off for lighter weight and smaller size in this instance, in addition to lower strength, is the need to retire webbing more frequently. While I'd definitely considered this, it's good to put that caveat on the board for anyone reading this thread later on.



* 11mm PMI Pit Rope is rated as 30kN; 7/16" Bluewater II+ is rated as 32kN
Last edited by Jeff Bartlett on Nov 19, 2011 8:12 pm, edited 1 time in total.
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Re: Using 9/16" webbing instead of 1" webbing

Postby Cody JW » Nov 19, 2011 7:35 pm

I have been using 9/16 webbing for hand line over 20 years. So far abrasion has never been an issue. I can carry 50 feet in a small lost creek pack and not know it was there, unlike caving rope.
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Re: Using 9/16" webbing instead of 1" webbing

Postby Jon » Nov 20, 2011 1:17 am

Take Yoda and forget all that stuff, as long as he's paying attention dental floss will do!
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Re: Using 9/16" webbing instead of 1" webbing

Postby ron_miller » Nov 20, 2011 4:20 pm

Jeff - another important factor to consider is the "critical angle" - the interior angle formed by the webbing at the connector. The tests presented at ITRS 2011 were done with a very low critical angle, about 12 - 13 degrees, IIRC, thus allowing each leg of the webbing to see about half the load. At a critical angle of 120 degrees, each leg sees the full load; for field calcs, I would suggest (until testing at higher critical angles suggests otherwise!) that you would want to reduce the anchor's estimated strength based on Tom and Aaron's ITRS paper by half. At a critical angle of 90 degrees, each side sees 70% of the load. At critical angles above 120 degrees, the force multiplier effect ramps up pretty quickly, further reducing the overall anchor strength.
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Re: Using 9/16" webbing instead of 1" webbing

Postby snoboy » Nov 20, 2011 6:12 pm

Angle is important in any anchor building, but in practice I have found it extremely difficult to tie a W3P2 with an interior angle of greater than 90°. If you were to tie it and slide it down a formation that tapered then you could get into extreme angles.

Since the W3P2 fails by squishing the inner loop of webing, I wonder how much the angle actually substantially affects the strength? Interesting...
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Re: Using 9/16" webbing instead of 1" webbing

Postby Stridergdm » Nov 22, 2011 12:53 am

ron_miller wrote:Jeff - another important factor to consider is the "critical angle" - the interior angle formed by the webbing at the connector. The tests presented at ITRS 2011 were done with a very low critical angle, about 12 - 13 degrees, IIRC, thus allowing each leg of the webbing to see about half the load. At a critical angle of 120 degrees, each leg sees the full load; for field calcs, I would suggest (until testing at higher critical angles suggests otherwise!) that you would want to reduce the anchor's estimated strength based on Tom and Aaron's ITRS paper by half. At a critical angle of 90 degrees, each side sees 70% of the load. At critical angles above 120 degrees, the force multiplier effect ramps up pretty quickly, further reducing the overall anchor strength.


I'm going to quibble with the wording here, but I think it's an important distinction.

The overall anchor STRENGTH doesn't really change. The safe working LOAD changes.

A double loop of webbing (which is essentially what a W3P2 is) will have theoretically 4x the strength of a single strand of webbing.

So, if your angle is 12 degrees or 90 or 120 degrees, the strength remains the same, but as Ron's overall point is, the LOAD you can put on it changes dramactically.
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