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Chockstone Forum - General Discussion

General Climbing Discussion

 Page 1 of 2. Messages 1 to 20 | 21 to 34
Author
Sling angle
davids
16/10/2008
2:56:03 PM
The fatality report from the "bailing" thread today wrote

54-year old male rock climber with over 30 years experience died
while abseiling off "Battlements" a 118m, multi-pitch route in the
Columns area of the Organ Pipes on Mt Wellington.
The climber and his partner were abseiling off the climb due to the
late hour and impending darkness. The climber placed a large nut anchor
in a vertical crack. He clipped the abseil rope to this anchor and then
backed it up with a sling placed around a large rock on the ledge. The
sling was approx 1.2m below the nut. The sling was 25mm tubular-type,
knotted and rated at 18kN (1800kg) and was a snug fit around the rock
with an approximate sling angle of 150 degrees. It is not known whether
the rope was threaded through the sling or attached by a carabiner


What does the 'sling angle' mean in this report? Is it the angle that the sling makes as it comes round the rock? If so, I imgine that the sling angle is always quite large if there is a snug fit of a sling around any rock or bollard. A large rock, with a snug fit would always make nearly a 180 degree angle. Even an extra long sling with a knot cinched up onto the rock would have a 'sling angle' at the knot of 180 degrees still.
Should one always loop the sling loosely around the rock/bollard so there is a long V hanging down giving a smaller sling angle. Or have I got this wrong? I think there may be a safety issue here that I don't understand.
Advice anyone?

ajfclark
16/10/2008
3:18:49 PM
I gather they mean the V angle of in sling. The below is about equalising anchors, but I don't see why this doesn't apply to the V in a sling too. From http://en.wikipedia.org/wiki/Anchor_(climbing)#Equalization:

If the load force is FLoad and the V-angle is θV, then the force on each anchor is given by:



Resulting from this expression, we can deduce:
* At a V-angle of 30 degrees, each of the two anchors bear a force of about 52% of the original load.
* At 45 degrees, each anchor bears about 54% of the load.
* At 60 degrees, each anchor bears about 58% of the load.
* At 90 degrees, each anchor bears about 71% of the load.
* At 120 degrees, each anchor bears a force equivalent to 100% of the original load. An angle this large should never be used.
rb
16/10/2008
3:19:12 PM
The angle between the 'arms' of the protection effects the force within the system. To
put it simply, wide angles can result in an increase in the force applied. This is
particularly relevant if you are building an anchor with a cordelette or similar. There's
heaps of info on the net about this, try the link below or google 'force multiplication
cordelette' or something similar.

Check it out in more detail here:
http://books.google.com.au/books?
id=0OQZ8LUJYCAC&pg=PA97&lpg=PA97&dq=force+multiplication+cordelette&source
=bl&ots=bOhWaQ78hu&sig=m4L4hxCnlg1UVLVbNODWFXt6HVI&hl=en&sa=X&oi=boo
k_result&resnum=1&ct=result#PPA98,M1

The good Dr
16/10/2008
3:44:19 PM
Also of interest here is that even at an included angle of 90 degrees the forces on the karabiner become cross loading forces as well (ie not aligned down the karabiner spine). Steel locking karabiners have been tested to failure loaded at 90 degrees and fail at approx 30% of their rated capacity (along the spine rating).
davids
16/10/2008
4:16:47 PM
yes, I know about V angles and anchors/cordelettes - I'm wondering whether it applies to the a single sling that is passed round a rock, and then a biner for abseil/setup or quickdraw for leading placed on the sling, with the weight on the biner creating the V. is that clear?! It would seem that the v angle will always be nearly 180 degrees, and thus force magnified, when the sling is tightly round a large rock. Obviously a loose sling round a bollard or such runs the risk of being displaced, but is that better?
maybe there is so much redunancy in sling ratings that the main point of the report is not to shockload.
David

ajfclark
16/10/2008
5:01:42 PM
On 16/10/2008 davids wrote:
> It would seem that the v angle will always be nearly 180 degrees, and thus force magnified, when the sling is tightly round a large rock.

I've always been told to avoid that and anything else that makes large angles. It makes sense that if the two independent arms of an anchor would each receive 70% of the force from a given loading at a certain angle that each side of a sling would receive 70% at the same angle.

>Obviously a loose sling round a bollard or such runs the risk of being displaced, but is that better?

From a load perspective I think so.

I'm now wondering about what forces are in play when you loop a sling around a bollard twice and pull one of the loops in a tight cinch around the bollard and put the carabiner through the other. That's a lot harder to dislodge and (if a long enough sling is used) forms a low-angled V... Does the loop around the bollard significantly weaken the setup?

shamus
16/10/2008
5:13:46 PM


I'm guessing this is your question? That once you tie the knot, it essentially becomes like a pulley on the sling, will tighten and the green downwards force from the carabiner gets translated into opposing forces on the knot, "crossloading" the knot and creating a large force across the black length of sling as it goes through the knot? If so, I cant help you with an answer, I thought about your question and came to the same problem myself. It clearly helps create a straight single-line force on the carabiner, avoiding crossloading it, but the angle of forces on the sling still seem bad. Anyone able to help?

shiltz
16/10/2008
5:26:07 PM
I don't see any cross loading of the carabiner in either case.
It is more interesting to consider which is better for the sling. In my opinion the knot is better for the other reasons suggested. As far as breaking strength goes the unknotted version may actually have a slightly advantage.

shamus
16/10/2008
5:51:53 PM
Yeah, the carabiner can only get crossloaded if you clip both ends of the sling to it independently, such as was done in the picture mentioned in the Crux #7 thread.
Wendy
16/10/2008
6:00:14 PM
I think there's a difference between the anchor angles and the slung bollard angles. The anchor angles are between multiple bits of gear and increasing angles increases forces, as per vast amounts of info previously referred to. With the slung bollard, the sling isn't distributing forces between multiple pieces but just around one bollard. Thus I don't see how there can be any increase in force put on the bollard by the sling angle. Ok, it might be possible that a tight wrap increases force on the sling, but there's also the friction around the bollard itself. And with an extra wrap to choke the bollard, there's bucket loads more friction. Surely a round turn and half hitches would be just as problematic (that is, if you were to consider it problematic, which I don't). It's a pain in the bum when the sling's really too small and you are just trying to make it do the job, and not a good idea to bung a biner in to get the last couple of inches and then load said biner downwards as well, but the actual slinging bit, I happily sling away with minimal looseness and wrap away with abundance.

muki
16/10/2008
9:52:56 PM
On 16/10/2008 shamus wrote:
>

>help?
the angle of the sling is not an issue! the sling runs flat around the bollard, and at the last point of
contact runs directly to the carabiner, as Wendy says, this is not a two point set up where the
increased angles add extra load to the two individual points, but rather relies on the strength of the
sling in a straight line around the bollard, and then a straight line to the carabiner, with no three way
loading going on at all.
If the bollard was a concern in the regard that the sling could lift off it when climbing past, or with some
rope movement, then the best option is to do multiple wraps on the bollard with a long enough sling,
the next solution on the list would be to girth hitch the bollard, and finally to resort to the knot in the
sling, bearing in mind that that weakens the slings load bearing considerably!
As previously mentioned, joining a sling at its terminating ends with a carabiner, and then running a
rope through the biner as well will create three way loading, and possibly re-orient the biner so the gate
is involved in holding a fall if it happens, this is the worst possible situation, join two slings instead if
the sling you have is not long enough.
one day hero
16/10/2008
11:14:02 PM
I'm a bit suss about that sling breaking due to excess force (in the organ pipes case). Check out slacklining sometime for very obtuse angle loading, I've often been surprised that the forces don't exceed the breaking strength of the single biner/sling which usually holds these things up.

Also, don't forget that the guy was abseiling, so any force transmitted to the anchors would have had to be held by his hand/rap device as well......I doubt I could hold a sling snapping force under such circumstances.....of course, there may be stuff in this case which I'm not aware of, so it's all a bit speculative.
mikl law
17/10/2008
9:14:21 AM
Sling angle matters around bollards too- Treat the points where the sling loses contact with the bollard as the anchor points. The forces on these points is still v high AS IS THE SLING TENSION. (in reality, the small amout of strech in the sling chages the situation only slightly)

I believe in the organ pipes fatality that the back of the sling may have pulled up the bollard, and cut easily under this extreme tension

IdratherbeclimbingM9
17/10/2008
10:47:52 AM
On 17/10/2008 mikl law wrote:
>Sling angle matters around bollards too- Treat the points where the sling
>loses contact with the bollard as the anchor points. The forces on these
>points is still v high AS IS THE SLING TENSION. (in reality, the small
>amout of strech in the sling chages the situation only slightly)
>
>I believe in the organ pipes fatality that the back of the sling may have
>pulled up the bollard, and cut easily under this extreme tension

Further from the AAR link.

At around 1920 the climber started the abseil. He was carrying the
climbing rack. Both climbers were wearing helmets. On weighting the nut
it pulled out of the crack. The climber (with abseil rope attached)
then dropped about 1.2 metres onto the sling anchor which immediately
failed by ripping completely through at a point other than the knot.
The climber then dropped
(further) ...

I think davids has hit the nail on the head with
>maybe there is so much redunancy in sling ratings that the main point of the report is not to shockload.

It also highlights the importance of 'no extension' in belay / abseil backups scenarios, should the 'main' anchor fail.

As an aside,~ given the tension forces mikl refers to, it makes using old tatt* (as some have/do) even more dodgy.

(*Before being taken to task; I also note no mention of using old tatt in the AAR acount).

Link to Australian Accident Register report here.


Ronny
20/10/2008
9:21:26 AM
On 16/10/2008 bomber pro wrote:

>the angle of the sling is not an issue! the sling runs flat around the
>bollard, and at the last point of
>contact runs directly to the carabiner, as Wendy says, this is not a two
>point set up where the
>increased angles add extra load to the two individual points, but rather
>relies on the strength of the
>sling in a straight line around the bollard, and then a straight line
>to the carabiner, with no three way
>loading going on at all.

In the top left drawing in the diagram it is a "three way loading" situation in the sense that the tension in the sling will not be equal to the load placed on the setup (by the abseiler or whatever). If the angle between the slings is less than a certain size (about 90deg?) the tension will be less than the load. If the angle is greater than that amount the tension will be greater than the load. So if you can only just squeeze the sling over the bollard it will likely have quite a high tension (whether this is enough to be dangerous will depend on all sorts of other factors). Tying a knot won't change this - the angle still makes the differnce (as shown on the diagram).

As I read it, this is what davids was asking about.

BP is right though that it is not a "three way loading" in the sense that the force on the bollard will be equal to the load on the biner.

muki
20/10/2008
11:04:36 AM
On 20/10/2008 Ronny wrote:

>In the top left drawing in the diagram it is a "three way loading" situation
>in the sense that the tension in the sling will not be equal to the load
>placed on the setup (by the abseiler or whatever).

No, not three way loading, just has to do with vector loading, and that is all about angles.

>If the angle between
>the slings is less than a certain size (about 90deg?) the tension will
>be less than the load.

No again, the the tension can never be less than the load, if there is no angle ie slings running side by
side, then the tension is equal to the load.

>If the angle is greater than that amount the tension
>will be greater than the load.

The tension gets greater as the sling angles depart from zero (side by side) and increase in vector,
more than 90 degrees is considered to place too much load on two individual points,eg anchor points,
because they are loaded with more force/weight than the original load placed on the focal point, but
this loading will not fail the individual legs of a vector set up unless the slings pass sharp edges, rather
it will fail the anchor point at the end of the sling.

>So if you can only just squeeze the sling
>over the bollard it will likely have quite a high tension (whether this
>is enough to be dangerous will depend on all sorts of other factors).

the tension will be high, as described in my comments above, but the slings overall strength rating will
be the one to consider, not an individual anchor point involved in an equalised system, but again sharp
edges will compromise this rating much sooner than it is normally reached.

>Tying a knot won't change this - the angle still makes the differnce (as
>shown on the diagram).

Tying a knot will make a big difference, as the knot will now become the weak point in the sling, and
as the load is placed on it, will be the place the sling will fail, even if the load is less than the rating on
the sling!

>As I read it, this is what davids was asking about.

then davids needs to bone up on vector loading, this will teach him what he needs to know.

>BP is right though that it is not a "three way loading" in the sense that
>the force on the bollard will be equal to the load on the biner.

but the force on the sling might exceed its rating if the other factors such as sharp edges, knots, or
age of the nylon, wear and tear ect are combined with a bad vector.
To sum up I feel a 1 foot bollard connected with a 3 foot sling is optimal, if the bollard is a bit difficult
to sling without the sling coming off, then give it an extra wrap, this still leaves the sling with an
excellent vector and the added security of a firm sling placement, using short slings, connecting them
with biners to extend, or failing to check for sharp edges is asking for trouble.
davids
20/10/2008
1:09:00 PM
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
2. Don't join up short slings with biners to make long slings
3. Don't join ends of a "just long enough" sling with a biner
4. Check back/top of bollard for sharp edges
5. If using for a back-up for set-up/abseil, then equalise - don't allow shock-loading.
6. An extra wrap/loop around the bollard can conveniently shorten the sling and stabilise the placement.

Thanks
Ronny
20/10/2008
1:29:37 PM
On 20/10/2008 bomber pro wrote:
...

>No again, the the tension can never be less than the load, if there is
>no angle ie slings running side by
>side, then the tension is equal to the load.

If the two sides of the sling are running side by side then the tension will be half the load. The tension will increase from there as the angle between the halves of the sling increases.

>then davids needs to bone up on vector loading, this will teach him what
>he needs to know.

...

Otherwise BP is right...

>if the bollard is a bit difficult
>to sling without the sling coming off, then give it an extra wrap, this
>still leaves the sling with an

...and this is a very good idea.

IdratherbeclimbingM9
20/10/2008
1:41:26 PM
Bp wrote;
>To sum up I feel a 1 foot bollard connected with a 3 foot sling is optimal, if the bollard is a bit difficult to sling without the sling coming off, then give it an extra wrap, this still leaves the sling with an excellent vector and the added security of a firm sling placement, using short slings, connecting them with biners to extend, or failing to check for sharp edges is asking for trouble.

No argument from me about that.

What are your thoughts on the strength of a long sling when used in that application when it is tied with an overhand knot that is cinched up on itself, to prevent the sling from coming off the bollard?

Also what are your thoughts on the same situation where the sling is girth hitched to prevent it coming off the bollard?

My uneducated guess is that they are probably all academic, ie the sling strength is not compromised too much as a runner, and both situations are probably up to the required strength.

For many years now I have been a fan of overhand knots cinched up in this application, instead of girth-hitching, though I am hard pressed to think of a time when I have put that particular piece of pro to a fall test where it was the first runner below the fall point, and thus took the main load.

ajfclark
20/10/2008
2:24:04 PM
On 20/10/2008 IdratherbeclimbingM9 wrote:
>My uneducated guess is that they are probably all academic, ie the sling strength is not compromised too much as a runner, and both situations are probably up to the required strength.

Appendix C of "Climbing Self Rescue" has a decent explanation of the loss of strength due to knots and a list of knot efficiencies. I'm really surprised by how much some knots impact the strength.

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