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NJSlacker
Oct 28, 2008, 3:13 AM
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Hey guys, first thing's first. I've seen people use slings as anchors and I've seen people wrap trees directly for anchors, but which is stronger?
each case has the theoretically same amount of webbing, and in each case the biner is putting force on the same theoretical cross-section of webbing. Is there a theoretical difference?
That question probably ties into my second question, which is, how strong is a rope when the force is perpendicular to the direction it's designed to hold weight (down the length of the line, instead of across its diameter). In each of those anchors I drew, the biner is pulling across the diameter of the rope, though the tree raps are not a crimped as the slings. so based on that, which anchor would be stronger?
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-George
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Melon
Oct 28, 2008, 3:52 AM
Post #2 of 16
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Sounds like a riddle? Little too much thought into a slackline.I just girth hitch slings.The rig I use also has no knots for easy tear down.
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majid_sabet
Oct 28, 2008, 3:55 AM
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NJSlacker
Oct 28, 2008, 4:14 AM
Post #4 of 16
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Thanks for the quick replies
In this case, I'm only concerned with the forces on the rope, so lets assume the biner in infinitely strong and crossloading is negligible.
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majid_sabet
Oct 28, 2008, 4:54 AM
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NJSlacker wrote: Thanks for the quick replies In this case, I'm only concerned with the forces on the rope, so lets assume the biner in infinitely strong and crossloading is negligible.
when you say rope, are you talking about the anchor rope?
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NJSlacker
Oct 28, 2008, 4:58 AM
Post #6 of 16
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whoops, I was saying webbing in the OP
yea, I'm talking about the anchor rope, yellow in the picture.
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majid_sabet
Oct 28, 2008, 5:31 AM
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NJSlacker wrote: whoops, I was saying webbing in the OP yea, I'm talking about the anchor rope, yellow in the picture.
where do you think the weakest point is in your anchor?
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USnavy
Oct 28, 2008, 6:46 AM
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majid_sabet wrote: [IMG]http://img139.imageshack.us/img139/8417/2980391262271af824ccdx8.jpg[/IMG]
Trixaial loading, not crossloading. It’s not the same thing and that is not crossloading.
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NJSlacker
Oct 28, 2008, 12:31 PM
Post #9 of 16
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majid_sabet wrote: NJSlacker wrote: whoops, I was saying webbing in the OP yea, I'm talking about the anchor rope, yellow in the picture. where do you think the weakest point is in your anchor?
The point I'm concerned about is where the rope/webbing meets the biner in either case.
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patto
Oct 28, 2008, 12:52 PM
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I disagree that biner crossloading is a problem. Unless the angle is wide then it isn't a problem. And if the angles are wide then you are dealing with force magnification on the sling which isn't good.
Idealy the second is best. Use strong webbing, two wraps but only have one wrap in the biner. That way it tightens itself against the FULL cicumference of the anchor.
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milesenoell
Oct 31, 2008, 2:51 AM
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Sharp bends in rope or webbing reduce the strength, and the stiffer the fibers the greater the reduction in strength. I'll have to look for the article but there was a report that girth hitching slings made of Spectra or Dyneema could severely reduce their strength and was thus warned against. The thinner the sling the sharper the angle and the greater the reduction in strength. The anchor setup in the left create sharper bends than the other, and I don't know if it would be enough to matter, but I'd go with the anchor on the right. Then there's the triaxial loading issue just to further clinch it.
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basilisk
Nov 1, 2008, 5:04 PM
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None of these answer the OPs question.
Will loading the rope perpendicularly give it less strength?
Well, sort of.
Perhaps you've seen this diagram?
Normally this is for climbing anchors, but it applies for slacklines too. The closer you clip to the anchor (tree, etc) the wider the angle will be, and thus you'll lose strength in the rope. It's best if the anchor rope can extend out a but from the tree.
All that said, unless the rope is already damaged, the carabiner will probably fail before the rope.
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milesenoell
Nov 3, 2008, 12:57 AM
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basilisk wrote: None of these answer the OPs question. Will loading the rope perpendicularly give it less strength? Well, sort of. Perhaps you've seen this diagram? Normally this is for climbing anchors, but it applies for slacklines too. The closer you clip to the anchor (tree, etc) the wider the angle will be, and thus you'll lose strength in the rope. It's best if the anchor rope can extend out a but from the tree. All that said, unless the rope is already damaged, the carabiner will probably fail before the rope.
How is that a better answer to the OP? For one, both configurations would be improved by your suggestion, and neither stands out as better or worse. For two, the increased forces are for a pair of anchor points rather than around one big anchor. The tree is not really in danger of being squeezed together the way two independent anchor points would be. I know the rope/webbing is still under increased stress, but as you acknowledged, the biner will likely fail first anyway.
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basilisk
Nov 3, 2008, 3:55 AM
Post #16 of 16
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Looks like you understood my post perfectly.
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