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Sling angle


muki

20-Oct-2008
3:51:57 PM

Hi M9, I would avoid using any knot at all unless forced into that situation by the shape of the bollard.
I feel that even when threading a sling its best to just pass it through and clip both ends to a draw/biner.
I would say that 30% is a rough estimate of strength reduction barring any other factors like sharp edge.
As I said, two wraps in line are preferable to a girth or even a clove hitch in IMO, but horses for courses.
Sling ratings generally are about 22kn, that is so much more than even the biggest fall I've ever taken!


muki

20-Oct-2008
4:05:04 PM

Ronny, in reality, a tied or sewn sling will have two loops forming the two legs of the vector, each leg is
made up of two pieces of sling running down that fold and return under the biner and back up, if you
concede that each leg of the loop is holding half the load...... then you must realise that the sling is in
fact one giant loop that feeds around the bollard and comes together at the biner, one sling holding the
entire load!
Other wise if I follow your logic then each leg of the two individual strands of webbing is in fact only
holding a quarter of the load ? but the way I see it is that even then if I add the four quarters or two
halves ..... I still get one sling holding the entire load. the rest is just semantics!


wombly

20-Oct-2008
4:20:51 PM

Depends on the friction on the back of the bollard. If there's enough then each end of the sling will essentially be independant - i.e. you could tie it off on one end simply by wrapping it around the bollard enough times.

Ronny

20-Oct-2008
5:04:05 PM

Bomber - The issue is what tension is required along the length of the sling to hold the load applied. For the same load, the tension in the sling will vary depending on how tight the sling is around the bollard (and therefore the angle between the "legs" of the sling at the biner). This is the same whether the sling is doubled over and joined at a biner or knot, or simply looped over the bollard.

The fact that it is one sling is irrelevant save for the fact that it means the tension must be the same all the way along (not allowing for friction on the bollard obviously), and in any event, the same on each side of the biner clipped to it (which is holding the load).


Macciza

20-Oct-2008
11:50:41 PM

Hi All,
Most interesting discussions, with some bizarre bits along the way . . . .
I've given it a bit of thought and it is actually a complex problem.
We need to be be talking about the same setup but we may not be.
Sinplistically it does seem that a sling wrapped tight around a bollard would present a sling angle of
almost 180 degrees, so a load applied to it would be massively multiplied - 170 = 6x, 180 = inf?,
So what happens when it is a little bit loose? It does not take much to get a 120 degree angle.
If the sling angle is 90 then a departs a perfect round bollard with 90 degrees separation and load point
extends to almost half the radius away from the boulder. .
If the sling angle is 120 then its depart seperation is 60 degrees and extension is more like 1/8 radius.
So for a bollard 1m wide you want it at least 15 cm out from the boulder,
Does that makes sense I should have done the maths for it a bit better but its late . . . .
Cheers and goodnight


sliamese

21-Oct-2008
6:03:22 PM

>* At 120 degrees, each anchor bears a force equivalent to 100% of the
>original load. An angle this large should never be used.

why not?!?! even 200% of 100kg is still roughly 10% of the total breaking strain, well inside any Safe Working Load. yeah yeah its not ideal, but id rather use less sling and make more abseils than faf about getting angles right.


The good Dr

21-Oct-2008
8:43:52 PM

On 21/10/2008 sliamese wrote:
>
>>* At 120 degrees, each anchor bears a force equivalent to 100% of the
>>original load. An angle this large should never be used.
>
>why not?!?! even 200% of 100kg is still roughly 10% of the total breaking
>strain, well inside any Safe Working Load. yeah yeah its not ideal, but
>id rather use less sling and make more abseils than faf about getting angles
>right.

Other factors come into play when the forces increase. These include:

- roughness or edges on the rock that may damage the sling, slings cut easily under tension.
- any jerkiness, or more sudden stopping during the abseil - which produces a peak load significantly greater than the dead weight load,
- the strength of the karabiner, as once angle are greater than 60 deg significant cross loading of the karabiner occurs (6-8 kN across the gate, minimum 200kg load approx 2kN, with peak loads which can be 2-3 (or more) time higher, ie approaching 6kN and above).
- potential damaging surfaces on the karabiners.
- old or slightly damaged slings (as people will tend to leave tat)

Slings are cheap and sling breakages have caused significant numbers of accidents.


Chuck Norris

21-Oct-2008
9:05:49 PM

Going back to the original accident - isn't the key reason for the accident the fact that the primary
anchor (ie the nut) failed and was positioned "1.2m above the bollard"?

Then dropping 1.2m onto the 'back-up" bollard would have shocked the sling on the bollard (made
worse if there was no crab - as it would have added rope on sling friction during the drop). Given all the
stuff in the above posts regarding the forces in a tightly slung bollard it isn't hard to see how the sling
failed after being shock loaded.

Although I've taken the point re the length of sling for bollards, the thing to learn from this is primarily
to make a set up so as to reduce the shock on the backup (ideally equalised).

rolsen1

21-Oct-2008
9:16:34 PM

On 21/10/2008 sliamese wrote:
>
>>* At 120 degrees, each anchor bears a force equivalent to 100% of the
>>original load. An angle this large should never be used.
>
>why not?!?! even 200% of 100kg is still roughly 10% of the total breaking
>strain, well inside any Safe Working Load. yeah yeah its not ideal, but
>id rather use less sling and make more abseils than faf about getting angles
>right.

Tend to agree, at the moment.

This thread has got me thinking as well. I set up a top rope a few weeks before with a very tight sling, among other better pieces, yet the forces would have been really low.

On lead, I prefer to use a knot to make sure the sling won't pull off a bollard except when the bollard is absolutely bomber. At the moment I feel the risk of the sling popping off is greater than the risk of the angles being too high. Given that we're talking about a runner and not a belay then a high fall factor (except if its the 1st piece) is not gonna happen. I guess another reason for placing lots of gear.

And what do we do with a thread? Looping through the sling itself will cause high angles, while clipping both ends through a biner risks cross loading. Lose - lose situation.

I'm still gonna think about this more and talk about it with the guys in the car on the weekend.

PDRM

22-Oct-2008
8:08:14 AM

Wendy

22-Oct-2008
9:26:13 AM

On 20/10/2008 davids wrote:
>yes, the discussion has answered the question quite satisfactorily!
>So, rules of bollards and slings
>1. Long sling - ideally 3 times as long as the diameter of the bollard
>- so narrow V-angle

Some basic maths tells me that this sling won't even fit around the bollard .... the circumference of a bollard will be 3.14 times the diametre.


ajfclark

22-Oct-2008
9:53:04 AM

On 22/10/2008 Wendy wrote:
>On 20/10/2008 davids wrote:
>>1. Long sling - ideally 3 times as long as the diameter of the bollard
>Some basic maths tells me that this sling won't even fit around the bollard .... the circumference of a bollard will be 3.14 times the diameter.

If he means the length as stated when you buy them (eg a 60cm sling is actually 120cm around) then it works:

20cm diameter bollard = ~63cm circumference.
60cm long sling = 120cm circumference.

Wendy

22-Oct-2008
10:17:46 AM

On 22/10/2008 ajfclark wrote:

>
>If he means the length as stated when you buy them (eg a 60cm sling is
>actually 120cm around) then it works:
>
>20cm diameter bollard = ~63cm circumference.
>60cm long sling = 120cm circumference.

It does work when you are putting the sling over a stickyup type bollard, but it won't go around the original chockstone discussed.


ajfclark

22-Oct-2008
11:00:38 AM

On 22/10/2008 Wendy wrote:
>It does work when you are putting the sling over a stickyup type bollard

Isn't a bollard "stickyup" by definition?

There are 34 messages in this topic.