Prusiks weaker than expected?

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

itrsonline.org/wordpress/wp…

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.

Please show me how this research is fundamentally flawed so I can ignore it's conclusions.

Shane1234 wrote:Please show me how this research is fundamentally flawed so I can ignore it's conclusions.

It is well known that knots reduce the breaking strength of strands. While I haven't seen prussik data before, the conclusions are of no surprise. Expect 20-50% loss of strength when knotting ropes and slings. (Plenty of data around about specific knots etc...)

Either way. The strengths of prusssiks featured are hardly a concern unless you weigh 500kg or so....

Just don't shock load a prussik.

I'm trying to picture where this would be relevant/cause for concern. The highest load would (I guess) be using a Prussik as a foot loop for ascending...but that's static.

Playing tug of war with two pulley systems I've seen 6 mil burn through at less than 2kN. Different mechanism but eye opening nonetheless.

Agree with above...from a practical standpoint I was always told (by climbing people with engineering phD's ) that burning / melting of a prussic is the larger worry, like when using one for a back-up in rappelling. (The old way of using the prussic was to put the knot above any break-bar device, or no device at all with a body rap [..yes, I'm old!] ) No data or study, just how much heat can be generated and the melting point of nylon.
That being said, I can't remember ever reading (in "Accidents in North American Mountaineering, by the AAC) of a situation where a prussic failed by melting.

Ted Pinson wrote:I'm trying to picture where this would be relevant/cause for concern. The highest load would (I guess) be using a Prussik as a foot loop for ascending...but that's static.

Exactly,, I have seen a prussik that partially melted during a rap, but never a failure. It slid pretty easy after it cooled

A loop of cord should be considered at its single strand rating

In reality it will be a bit more .... But not double

;)

Cordage and webbing sold by the foot/metre are tested in a different way to slings etc, over a 10mm karabiner you can expect a 25-35% strength loss already and for a knot you can reckon on 40% of the original rated strength due to the even smaller diameter so as a loop you´ll be around 120% of a single strand. In new condition!
The DAV table isn´t exactly correct (well it is for cordage but not webbing). If you double a tape (webbing) sling over a karabiner and one strand lies over the other which is normal with modern karabiners you can expect a ca30% reduction over the single loop strength, the outer layer cuts through the inner one, not the 200% given.
Yer gonna die!

Yeah...I zipped down a rappel last weekend with an autoblock backup and definitely heard some crackling.

Tom actually does some solid hypothesis testing. The geology tie-in and rescue rigging deals with his application into transporting fossilized or geologically interesting materials down a hillside/technical terrain.

His testing really isn't alarming, but does show limitations of a rigging system in moving fossils, or rescue loads with static systems. About the only thing pertinent to climbers is having a hitch jam into a device and/or friction burning it. Most times it's just a stuck hitch which can be a pita.

One thing prussiks allow for is lightweight material. A loop of cord is much lighter than any rescue haul rig.

Now, realizing the limitations of a single loop, you can load-share between tandem prussiks and twining main-lines. More resilient than cable system, much lighter too. Essentially for what he tested, in reality of the loads he's considering, you essentially share a load between eight hitches and two mainlines (and sag) to move from point A to B. Works pretty well.