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Direct Loading of Dyneema Slings


ajfclark

12-Apr-2010
10:28:08 AM

http://rockclimbingcompany.blogspot.com/2010/04/dyneema-slings-breaking-under-direct.html

dieselcam

12-Apr-2010
4:30:07 PM

"Clipping directly into a belay with a sling, not keeping it under tension, allowing slack to develop and then slipping or falling onto the belay"

If this did happen, surely you aren't going to fall very far so I would hardly think that there could be enough force generated to make anything much fail.

"clipping into the belay with a sling and then moving around to get comfortable/keep the climber in sight. If you then slip or have to hold an unexpected fall the belay and sling will be shock loaded"

Half of that is the same as the first example (slipping and falling a small distance onto the belay). As for the second part about an unexpected fall (as opposed to an expected fall), I don't understand how clipping in with a sling creates a "shock loaded" situation on the belay.

"Working a move on a route whilst on the lead and using a sling to keep yourself in position, then trying the move with the sling still attached to the anchor. If the rope is slack and you fall you will be falling directly onto the sling."

Really? Would you have a sling attached to the anchor while going for the move rather than (or as well as) the rope to catch you in a fall? And if you did fall on the sling again it's not going to be a big fall. Unless of course your sling is one of those 4 metre cordelette chaps!

patto

12-Apr-2010
4:42:13 PM

Diesel your doubt shows a fundamental lack of understanding of the basic physics of fall arresting. The length of the fall matters little. Forces are much more dependent on the fall factor and the strechiness attachment material. Furthermore their results show that significant forces can be generated. (though given that bodies themselves are elastic the forces are likely to be a fair bit lower for short falls)

Anyway this information is far from new. In general it is far better to tie in with climbing rope or with a Nylon sling. Save the dynema for your runners.

dieselcam

12-Apr-2010
5:45:33 PM

On 12/04/2010 patto wrote:
>Diesel your doubt shows a fundamental lack of understanding of the basic
>physics of fall arresting.

I'm not claiming to be an expert and I am new to climbing but I understand about fall factors and dynamic vs static etc. My doubts are mainly that you would generate that much force from falling the small distance you would fall slipping off a belay ledge onto the belay and being caught by the sling you've clipped in with.

mikllaw

12-Apr-2010
5:51:49 PM

It's the difference between falling onto concrete (very little movement to cushion the fall) and onto a mattress. Distance doesn't matterthe relative length of fall and length and elastcicity of your 'rope' are all that count.

BTW the 50% mortality height in the building industry is about 1.6m. Concrete floors.


ajfclark

12-Apr-2010
6:49:09 PM

On 12/04/2010 dieselcam wrote:
>If this did happen, surely you aren't going to fall very far so I would hardly think that there could be enough force generated to make anything much fail.

That was the point of linking to this. While at first it may seem that a short fall couldn't cause much trouble, as soon as you are falling onto something static, it becomes a problem. Even with a stretchy material involved, short lengths have less give and therefore less shock absorbency.

>"Working a move on a route whilst on the lead and using a sling to keep yourself in position, then trying the move with the sling still attached to the anchor. If the rope is slack and you fall you will be falling directly onto the sling."

If you doubt that this even happens, in one of Sharma's videos there's a snippet of him hanging off a bolt with a quickdraw clipped directly to his harness. He forgets that he's clipped in and goes to pull a move and falls double the length of the draw. It looks really painful. I'll see if I can dig out the title of the video...

patto

12-Apr-2010
7:23:18 PM

On 12/04/2010 dieselcam wrote:
>On 12/04/2010 patto wrote:
>>Diesel your doubt shows a fundamental lack of understanding of the basic
>>physics of fall arresting.
>
>I'm not claiming to be an expert and I am new to climbing but I understand
>about fall factors and dynamic vs static etc. My doubts are mainly that
>you would generate that much force from falling the small distance you
>would fall slipping off a belay ledge onto the belay and being caught by
>the sling you've clipped in with.
>
>

Well its quite clear that you don't understand fall factors.
FALL FACTOR
"The significance of the fall factor is that, when a fall is arrested by a belayer, if the tension in the rope is assumed to develop according to Hooke's Law, then the maximum impact force of the fall depends only the climber's mass, the elasticity of the rope, and the fall factor. In particular, the maximum impact force does not depend on the length of the fall."

Thus if you understood fall factors you would understand that a factor 1 fall on indentical material produces the same peak force for 60m, 6m and 60cm. You really shouldn't be falling on static slings.

In practice for short falls your body becomes the primary energy absorber. The flexing of your body reduces the peak force felt by your body and the anchor. This is fortunate for the anchor but less fortunate for you. It will be painful and on a 120cm I would be worried about breaking my pelvis.

dieselcam

12-Apr-2010
7:24:55 PM

Point taken, I suppose I had never actually thought of being in a situation of falling directly onto a sling.

So even Sharma "can be a bit of a numpty" at times.

citationx

12-Apr-2010
9:40:11 PM

My thoughts are "who would really take a direct fall onto the piece?"
I would have thought that people rarely fall directly onto the sling, but instead would be sideways and after "falling" most of their catching motion would actually be in a swinging arc. Kinda like a bridge swing or so, where the initial fall is freefall but when the sling catches the fall you then swing the rest. I'd be more interested to see these sorts of forces. (Having said that, i'm aware of what others are referring to re: Sharma - but that's too numpty! ;-))


ambyeok

12-Apr-2010
10:19:39 PM

Most of us clip the anchor with a sling or daisy before weighting a rap. I then give a couple of good tugs to check everything is working, if my rap setup blows for some reason I can see a good amount of force hitting the sling/daisy.

patto

12-Apr-2010
10:45:15 PM

On 12/04/2010 citationx wrote:
>My thoughts are "who would really take a direct fall onto the piece?"
>I would have thought that people rarely fall directly onto the sling,
>but instead would be sideways and after "falling" most of their catching
>motion would actually be in a swinging arc. Kinda like a bridge swing or
>so, where the initial fall is freefall but when the sling catches the fall
>you then swing the rest. I'd be more interested to see these sorts of forces.
>(Having said that, i'm aware of what others are referring to re: Sharma
>- but that's too numpty! ;-))

A factor 1 pendulum results in forces 3x the climbers weight as opposed to forces the 20-30x reported in the above testing. A half fall, half pendulum will naturally result in peak forces somewhere between these two figures.

All this isn't to say that you should never use slings to anchor in. But you should be aware in the wrong set of circumstances falling on anchored slings can cause stuff to break, either you or the anchor. Given that a factor 1 fall can result in 25kN I wouldn't want to see the results of a factor 2 fall.

To keep youself safe anchor in whereever possible with your climbing rope. Use nylon slings instead of dynema/spectra. Nylon also has better abrasion and cut resistance to thing spectra.

On 12/04/2010 ambyeok wrote:
>Most of us clip the anchor with a sling or daisy before weighting a rap.
>I then give a couple of good tugs to check everything is working, if my
>rap setup blows for some reason I can see a good amount of force hitting
>the sling/daisy.

Its scary the number of abseiling accidents that I read about that could have been prevented by this minor precaution. Half a dozen worldwide at least. I do this and yes you are normally risking a factor 0.5 fall or something onto your sling. But it is not big deal considering the alternative!


aarond

13-Apr-2010
5:18:34 PM

On 12/04/2010 patto wrote:

>Thus if you understood fall factors you would understand that a factor
>1 fall on indentical material produces the same peak force for 60m, 6m
>and 60cm. You really shouldn't be falling on static slings.

Doesn't momentum come into play aswell? which is Mass x Velocity. so if you fall further you will be falling faster, means you have more momentum, means more energy and force. is that right??

Also from the basic Force = Mass x Accel

if you fall further, you will fall faster.
if you stop over the same distance, (which you can assume close to 0) from 5-0m/s will produce more force than from going from 0.5-0m/s
is that right or am i wrong???

isnt it the same as if you throw a steel ball at a glass window. throw it slowly (ie a small fall) it wont break the glass but if you throw it hard (a big fall, more speed) it will break the glass...

I mean i understand you don't want to fall directly onto a static sling because there is no shock absorption and there is still i large impact force but if your moving very slow doesn't it decrease the force?????


ajfclark

13-Apr-2010
5:49:03 PM

I think what patto is getting at is that all materials have some stretch, some more than others (climbing rope > nylon slings > spectra). Shortening the length reduces the impact absorption of the material and increases the forces. A 6m fall onto 12m of rope will have about the same forces as a 12m fall onto 24m of rope as they are both .5 factor falls (6/12 = 12/24 = .5).

I'm not sure how much this carries over to much less dynamic slings, but given they do have some stretch in them, I don't see why it wouldn't apply to some degree.

To use your window analogy, change the thickness of the glass in scale to the reduction in force of the throw and you'll still break the window.


aarond

13-Apr-2010
9:54:29 PM

yeh that makes sense i guess..

patto

13-Apr-2010
10:30:59 PM

On 13/04/2010 aarond wrote:
>yeh that makes sense i guess..

To further try to clarify things:

The energy of a fall increases linearly with fall distance. Double the distance then you double the fall energy.

The energy absorbtion capability of a spring be it a dynamic climbing rope, a static sling or a steel cable increases linearly with length.

Thus although long falls have more energy there is also more rope to absorb that energy. Thus it cancels out.

gfdonc

14-Apr-2010
9:26:21 AM

Just in case anyone misconstrues this based on the title, much the same principle applies to nylon slings.

Yes they do stretch slightly more than Dyneema but the shock-loading problem is the same.

A modern belay system has either a dynamic rope with a belay device - often something like an ATC which slips to reduce fall force - or alternatively a large pool of water below you.
Falling onto anything else is asking for trouble. And I'm not too sure about the water, either.

patto

14-Apr-2010
10:13:01 AM

On 14/04/2010 gfdonc wrote:
>Just in case anyone misconstrues this based on the title, much the same
>principle applies to nylon slings.
>
>Yes they do stretch slightly more than Dyneema but the shock-loading problem
>is the same.
>
>A modern belay system has either a dynamic rope with a belay device -
>often something like an ATC which slips to reduce fall force - or alternatively
>a large pool of water below you.
>Falling onto anything else is asking for trouble. And I'm not too sure
>about the water, either.
>

The stretching slightly more of nylon results i forces around half as much. This makes a big difference.

One further thing to note. When you take a lead fall you are still 'shock' loading your rope. It is just that your rope is MUCH stretchier so the sharp edge loading isn't a big issue.


wallwombat

14-Apr-2010
10:19:54 AM

I reckon if people don't already understand all this, they probably shouldn't be climbing outside.

It's hardly breaking news (excuse the pun).


aarond

14-Apr-2010
10:31:58 AM

from the listed article:

"A fall factor 1 using an unknotted 120cm x 11mm dyneema sling registered 25kN (surprisingly the sling survived, but whether ther climber or belay would have is another matter), a fall factor 1 with an unknotted 11mm x 60cm dyneema slings generated 14kN."

so falling a lesser distance does reduce the impact force even if they are both fall factors of 1??

davepalethorpe

14-Apr-2010
10:57:15 AM

>>so falling a lesser distance does reduce the impact force even if they are both fall factors of 1??>>

Surely this is not too suprising. From my understanding peak impact forces are effected by the duration of the deceleration. Eg stretchy rope equals longer time to dissapate energy over, resulting in a lower peak force. More rope, more stretch which is why a factor one fall onto rope generates a similar peak impact force whatever the distance. With a direct fall onto a dynema sling that has near zero elasticity no matter what length it is(?) the main factor on peak force is how quickly you are travelling (ie height you fell from).

Think the long and the short of this discussion is don't go wandering around on belay with yards of slack dynema and all will be good.

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