UKC

Had this debate last night and we could not resolve it. We were wondering how much weight a sinlge rope (standard size) could hold statically. Just take 10 feet of rope tie a knot in it and hang the weight of it. No falling no sharp edges, no trickery. Anyone know the answer?

In reply to Aigen:

Needlesports sell 9mm dynamic off the reel and give it a breaking strain of 19.5 kN (i.e almost 2 tonnes).

I realise this doesn't actually answer your question, but is the only citable information at my fingertips. I seem to remember that a single rope is normally rated to something like 22-25kN.

If what I have learned is correct:

100g = 1N.
100kg = 1kN

So my rope at 9.40kN could take

9.40 x 100kg = 940kg static weight?

Is this right?

In reply to Milesy:
9.4kN sounds more like a peak impact force than a breaking strain for a rope! I'd expect you to get a couple of tons on a single rope depending on knots etc.

In reply to Milesy: The math is right but we dont know what weight the rope can take. If your rope is 9kn it means it takes a dynamic weight of 900kgs falling onto it.

In reply to Aigen:

Not quite, if you drop the standard mass (55kg or 80kg depending on the type of rope I think) with the standard fall factor then the peak force on the mass during the fall will be 9 kN.

In reply to Aigen:

So the math is wrong. Multiplying the weight by g is only good for a static load.

In reply to Monk:
This also doesn't quite answer the question, but the standard for a low-stretch rope ('type A' - the kind usually used for caving etc.) says it should hold 2200kg when its clamped, and has to withstand 1500kg for 3 mins when it's tied with a fig-8:
http://www.bealplanet.com/portail-2006/index.php?page=normes_static&lan...

The standard for climbing rope (en892) doesn't specify a minimum breaking strength in the same way, its all about impact forces, number of falls and wotnot: http://www.bealplanet.com/portail-2006/index.php?page=normes_dynamiques&...

Prolly fairly safe to assume that the ultimate breaking strength of a dynamic rope will be kinda sorta the same ish as a low-stretch rope of the same diameter though, so I'd say somewhere between 1500 - 2000kg tied with fig-8 knots.

In reply to Aigen:


What on earth does that mean?

OP: I've always assumed the breaking strain of a full rope is somewhere in the 20-30kN region but since they're not rated in this way it is little more than a guesstimate. It's also of no real significance when using it for climbing so I'm quite comfortable not knowing.

jk

In reply to Reach>Talent:

Yeah you are right. I am assuming the breaking strain would also change with the length of the same rope?

In reply to deepsoup: Doesn't tying a knot in it reduce the strength of the rope by a 1/3? Unless it is a super-strength tie-off?

In reply to Richiehill:

Depends on the knot...

"No knot was found to reduce rope strength to less than 55% of its absolute strength"

http://www.hse.gov.uk/research/crr_pdf/2001/crr01364.pdf

In reply to Aigen:
They vary quite a bit (as does the weight) but in the region of 1800 to 2100kg seems to be usual, there was recently some concern about one that got down in the 1600´s but I´d have to remember where I read that.
Don´t forget rope and tape tensile testing is done over a large diameter drum and you need to de-rate for smaller radii such as karabiners and knots.

The good old hawser laid ropes to BS where another beast, had to hold 2.5tons tensile, no drop testing in those days!

Jim

In reply to Milesy:

Shouldn't do, since the hypothetical case we're talking about doesn't involve any kind of shock load. The weakest point would be at the knot, so the length probably wouldn't have any effect at all.

In reply to Richiehill:

It certainly reduces it by something. Probably a bit more than 1/3 I should think, though it'd vary with the kind of knot and all sorts of other variables so I think you'd probably struggle to predict it accurately.

The standard I quoted above specifies 1500kg *with* fig-8 knots though, as a minimum.

In reply to Aigen: it could take much more than stated I wouldn't be surprised if there was a factor of safety of 2,0 on them.

Also you'd want to tie using a fig-9 as it will reduce the breaking strength by 15-30%, as opposed to a fig-8 which would reduce it by 25-35% (according to Dr Merchant).

In reply to Swig:


Thats a great read, though it kinda makes me want to bin my WC ropeman now!

In reply to Milesy:

Don't be daft, do you think a longer chain is stronger than a shorter one?

In reply to Toerag:

No need to call me daft. It is only a theoretical discussion. I asked a question. Not make a statement.

My ex-rope could pull 1800kg of car sideways. That is why it's now an ex-rope. It went very tight and I watched from a safe place...

In reply to Aigen:

15 goats, 2 cows and a shaved red panda

In reply to Milesy: OK no worries, I apologise. I'm genuinely interested to know why you thought a longer rope might be stronger though.
If it helps answer the original question, I've towed a VW camper up to 40mph on an old 9mm to try to bump start it. I've always worked on the value of 2.2tonnes - probably from the old 'modern rope techniques' booklet.

In reply to Toerag:

I was thinking the rope stretching would distribute the force across the rope, so the longer the rope then the smaller the force on each area of the rope? Like I said I was merely musing - I am not a physicist.

In reply to Milesy: Ah OK - that's reasonably logical.

In reply to Milesy:

And if you keep going with that thought: if your rope is long enough, it can hold any weight you want.

In reply to PM:

...give me a long enough lever and a firm place to stand....

Oh, no, that doesn't really translate to rope lengths does it?

*grins*

In reply to Milesy:



bit thin skinned there- which brings us nicely to the word 'area' which is what you need to focus on, cross sectional area (proportional to diameter (pi * r * r) eg 9mm 11mm etc) force
will be same all along it. stretching distributes the energy absorption over time (but also makes the cross sectional area smaller!)

HTH

In reply to Aigen:

Funnily enough i'm writing a dissertation on this kinda stuff, from my research, dynamic ropes [be it single, double or twin] have two main criteria to fill to be approved:

firstly that using the Dodero machine [the drop tester], the peak force in the first drop must not exceed 12kN [8kN for a half]

secondly the rope sample must withstand 5 consecutive drop tests without breaking [12 for twin ropes]

these are fall factors of two btw, more than most peopel will ever experience.

the rest of the standards include sheath slipage, elongation etc. but those are the two UIAA and European/British Standards that go as far as how much the rope can hold.

for the static load you just times the weight by gravity, but during a fall you would use potential and kinetic energy equations, so a bit more force...

off the top of my head i remember reading an article stating that current ropes have roughly twice the strength of nuts, so in the 40-50kN range i'd guess, ropes that pass the standard are highly unlikely to break, more often the thing it would be attached to would break way before.

the knot will reduce the strength of the rope, tests on 8mm static shown that a stopper knot on a loop [not a prussik] will break the rope at 50-60% min rated strength, but as the dynamic rope would tighten, would get thinner, reducing the ropes cutting force on itself.

In reply to Matthew Edwards:

what nuts have BS of 20..25Kn ? most are around 10..14.
Krabs have 20..30Kn ratings

In reply to Aigen:

Slight tangent on the subject but which is stronger in a fall?

Figure of 8 knot, or

Bowline

In reply to Aigen: Iv had ropes up to 30kn in the test machine - the gear tends to break at this point, so its probably higher.

In reply to Trangia:
fig 8 according to all the literature I've read.

In reply to Milesy:

This isn't the case. As the force is applied constantly not over a short time period. If you apply 10 kn to a 50 meter rope each 1 meter section would stretch the same as if you applied 10kn to a 1 meter section.

A used 50 meter rope would be more likely to break at a lower force than a shorter rope because you increase the chance that more of the fibres accross the length of the rope will be damaged / cut through prior to the pull test (much the same as with new acessory cord we use to sling hexes).