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How strong are prusiks?

Freddofrog

17-Jun-2016
6:49:24 PM

I've come across some research that shows prusiks repeatedly failed at the single strand rating. I thought it was double.

Http://itrsonline.org/wordpress/wp-content/uploads/2015/02/004.Evans_.2014.pdf

Given I've never come across anyone talking about this study, possibly because it's produced by a university geology department and not a climbing organisation, I thought I'd seek feedback.

What do you guys think? Dodgy research or not?


ChuckNorris

17-Jun-2016
7:07:03 PM

Prussiks pass a one Eduardo test which is equivalent to a two stugang test so they're good enough for me.

Oh and can people stop PM'ing me about ODH's goat. When I get the time I'll post the story I heard given ODH is so ashamed to tell his side.


E. Wells

17-Jun-2016
7:12:37 PM

Im pretty happy with 9.6kn for a 6mm prussic. Good data!

patto

17-Jun-2016
7:56:36 PM

On 17/06/2016 E. Wells wrote:
>Im pretty happy with 9.6kn for a 6mm prussic. Good data!

Pretty much. While I've never seen prussik specific data before there is nothing surprising coming out of this. Two strands is 2x the single strength in 'ideal' conditions. Knotted strands are generally 20-50% less strength depending on the conditions and knots. So the results are pretty much in line which what you would expect.


Eduardo Slabofvic

18-Jun-2016
8:25:40 AM

On 17/06/2016 Stugang wrote:
> one Eduardo is equivalent to two stugang
>
>

That's what she said

freddofrog

18-Jun-2016
11:44:01 AM

Actually it seems from this research, under ideal lab conditions with new rope the best you can expect from a prusik is 50%.

My own interpretation is the two strands don't equalise properly so one strand is holding all the weight and breaks at the knot's bridge.


ajfclark

18-Jun-2016
12:09:57 PM

The 6mm cord has an MBS of 7.2kN: http://shop.pmirope.com/webbing-and-cordage/cordage/accessory-cord-6mm/6mm-accessory-cord-solid-black-100-meter-spool-cc060sb100s

Doubled strength for the two strands, but derated ~50% for the knot is still 7.2kN.

The test data is 9.61kN on the 11mm and 9.73kN on the 11.4mm

Seems reasonable.

The other thicknesses:

MBS of 14.3kN,11.9kN, 5.4kN for the 8mm, 7mm, 5mm cords respectively.

Broke at 15.46kN, 12.36kN, 8.07kN on 11mm.

Broke at 15.59kN, 13.57kN, 9.73kN on 11.4mm

I'd be interested to see if thinner rope dropped the strength more as the radius of the bends in the prusik knot decreased further. The 5mm cord does surprisingly well.

Freddofrog

18-Jun-2016
4:39:15 PM

Yep. What the researches concluded was prusiks broke at the rated strength for a single strand plus the 20% safety margin rope manufacturers allow for in their ratings.

rightarmbad

18-Jun-2016
8:06:34 PM

What's more important is how hit and miss their ability to hold is.


ajfclark

19-Jun-2016
9:26:58 AM

On 18/06/2016 rightarmbad wrote:
>What's more important is how hit and miss their ability to hold is.

And yet the tandem prusik belay is one of the preferred belay methods in rescue.

kieranl

19-Jun-2016
1:50:32 PM

On 19/06/2016 ajfclark wrote:
>On 18/06/2016 rightarmbad wrote:
>>What's more important is how hit and miss their ability to hold is.
>
>And yet the tandem prusik belay is one of the preferred belay methods
>in rescue.

Because they slip at around 9Kn (8mm cord), well below their breaking strain. This little video shows a prussik slipping before catching a load during some testin in the Blueys a few years ago.

https://www.youtube.com/watch?v=VDTxFS33_TI

This was catching a 280kg load after a highline was failed. The black tape shows where the prussik was set before the catch. There's also a nice puff of smoke as the prussik sheath melts.

I'd prefer to use a mechanical belay system but the prussiks do work and are very light. A downside is that you probably have to cut the rope after a fall to get the prussik off.

Dave_S

19-Jun-2016
2:43:41 PM

On 19/06/2016 kieranl wrote:
>This was catching a 280kg load after a highline was failed

Worth noting that if there's a 280kg mass on a slackline, the load at each anchor will probably be 5-10 times higher.


ajfclark

19-Jun-2016
3:10:25 PM

I would've thought high line, slack line, whatever, the angle of deflection was what influenced the load on the anchors, or do you mean slack lines generally have less deflection?

Dave_S

19-Jun-2016
4:06:32 PM

I mean in general, the angles of a slackline are very much opposed, compared with, say, those in a climbing anchor. So if you've got a 10-20m long slackline which sags 0.5-1m under load, then each anchor point is actually seeing up to 10 times the load that was placed on the slackline.


shortman

19-Jun-2016
9:53:25 PM

http://www.balancecommunity.com/slack-science/standing-tension-versus-working-tension/

And

http://www.balancecommunity.com/slack-science/gear-test-leash-fall-simulations/

And

http://www.balancecommunity.com/slack-science/highline-shockload-simulation/

kieranl

19-Jun-2016
9:59:02 PM

The tests were on a highline for rope-rescue using semi-static rope. So there's already about 5% rope-stretch. Then the standard is to tension with just one person using a 3-1 mechanical advantage so there is nothing like the tension that can be got with a slack-line and there is significant sag in the middle of the span.
Here's another video of the same testing day, same test but using MPDs for belays instead of prussiks.
Actual mass was 272 kg. Peak force at anchor 7.24 kN.

https://www.youtube.com/watch?v=zOaKUJ0ZuRU

In the background you can see an observer on another highline which has very little sag. That highline is a double track on HTP (very low-stretch) rope. Would probably get very different results with that

Jayford4321

20-Jun-2016
8:24:24 AM

On 17/06/2016 Stugang wrote:
>Prussiks and
>
>Oh and can people stop PM'ing me about ODH's goat. When I get the time
>I'll post the story I heard given ODH is so ashamed to tell his side.

Was it a kind of goatfest bondage party?
Did widewetandslippery bring his goat chloe?


ChuckNorris

20-Jun-2016
1:58:38 PM

It's not that exciting but the story goes that ODH was at moonarie around Anzac Day with a young lady he wanted to get intimate with. So they shlepp up to top camp to deprive the universe of drinking water and get a bit of "alone time".

First night under the stars just as ODH is making his move a bloody goat starts bleating which sorta kills the mood so no action night one.

When they wake up it turns out there is a billy goat stuck on a ledge just above top camp. Damo wants to push it off the ledge to shut it up which goes down like a microwaved cane toad to his prospective conquest.

That day is spent trying to coax the goat along an escape path to no avail. That night no action either with billy bleating and concern for the poor walking curry.

So next day the blue balled hero rigs up a pulley system and rips down an enormous tree to build a bridge for poor billy. No luck cos billy is a mountain goat not a tree goat. Genius.

The goat stays put all week meaning no love for hero.... But on the last morning billy is not there. Turns out he didn't need any help and walked off the way he came.

I may have got some names wrong but other than that the above is more or less true.


Eduardo Slabofvic

20-Jun-2016
2:10:12 PM

So what happened to the microwaved cane toad?


ChuckNorris

20-Jun-2016
3:51:33 PM

On 20/06/2016 Eduardo Slabofvic wrote:
>So what happened to the microwaved cane toad?

What happened on straddie stays on straddie.

There are 23 messages in this topic.