This is what we are talking about:
It’s an easy way to adjust your spansets to get reasonable rigging angles. You should click on the aforementioned link, if you don’t know what it is.
Cihan and I had a long discussion on the strength of said rigging method. The reasoning being that you basically girth hitch the tree, spin the girth hitch around and thereby creating a mechanical advantage on the part of the sling that opposes the girth hitch (you can read more about this on this discussion from freaking 2005!!). Also the sharp 180° bend is at least somewhat problematic.
Since noone has any published tests on the actual strength of this anchoring method, I decided to do a little experiment:
(Un)fortunately it is really difficult to break spansets..
That’s why I used 6mm hollow braid polypropylene instead. The rope was labeled as 0.57kN strong and was of core-sheath construction. I removed the core and spliced the sheath into 4 identical loopie slings. Since I removed the core, I wanted to re-evaluate the strength of the rope in single strand so I also spliced an eye-to-eye piece for reference. Those were then slung around a tree covered in double layered carbon fibre infused tree protection (more on that in a different article) and pulled to destruction!! A webbing sling was used as a backup to catch the shackle (red webbing).
The forces were monitored with a dynamometer and the maximum reading was used to determine the breaking strength of the slinging method. There was only one test for each method, since the test isn’t very representative of spansets anyways and I justed wanted to get a general idea on how strong the bernese anchor is.
The results were very surprising.
Eye-to-eye (single strand strength): 3,5kN
So, apparently the rope is a lot stronger than advertised. Even without the core it broke at 7 times the indicated strength. The rope broke at the end of the splice, which is normal for this kind of sling and is usually within 5% of the actual line strength.. At this point I was beginning to wonder if I would be able to break the other configurations with the rig I had on me..
End-to-end sling (double strand strength): 6,8kN
The sling broke at the shackle at almost twice the single strength. This seems rather reasonable and wasn’t surprising.
Girth hitch: 6kN
The sling broke at the girth-hitchy part. As the sling was stretching the girth hitch audibly readjusted 3 times before finally breaking slightly below the end-to-end strength. This was expected to happen.
Basket configuration: 10,2kN
The sling was installed in a 90° angle, which was closer to 60° at breaking due to the stretch of the polypropylene (similar to nylon). The sling broke at the shackle, which would explain the slightly lower than expected value for this configuration.
Bernese anchor: 11,2kN
The sling broke at a higher than expected value into more pieces than expected. It appears, as if the sling failed in 2 places simultaneously. Once at the girth hitch part, where only one of the strands failed (this might suggest uneven loading of the strands, since the other one was pretty much undamaged) and secondly at the sliding-x part, where both strands were totally destroyed. The higher break strength in comparison to the basket is probably due to the better load distribution on the shackle, which is the weak point of the basket.
This test was done with polypropylene rope instead of spansets, because it is a lot cheaper and easier to break. It is however, not the same, so these results aren’t to be relied upon. If someone has the time and machinery necessary to perform this test on actual spansets, please do so.