Odd Arc'teryx instruction

[Pete04]

Reading the fine print of my new harness (Arc'teryx AR 395a), I read a warning not to girth hitch anything to the belay loop because it can focus abrasion on a single location. Huh? Does anyone have any insight into why Arc'teryx would give this instruction as girth hitching seems pretty common if not fundamental whether girth hitching a PAS or dyneema. Is it just legal cya or is there something to that guidance?

 

 

[jfs1978]

From a "best practices" standpoint, attaching a PAS or sling to both the leg and waist tie in points is considered optimal. The concern is that people leaving their PAS or rappel extension girth hitched to the same, single point increases the chances of long term abrasion degrading the integrity of the belay loop. This has been a factor in some reported accidents. Both the leg loop and waist belt tie in points are typically reinforced with material intended to decrease or delay this abrasion as well as providing some redundancy by being through two points.

 

In addition, anecdotal evidence points to an increased reduction in ultimate breaking strength (of the extension material) when girth hitching tightly around a single point. Typically this can result in a (roughly) 50% reduction in ultimate breaking strength of the dyneema webbing material.

 

The upshot is that yes, while occasionally utilizing the belay loop as your attachment point will likely not result in failure (50% of 22kn is still an extremely strong attachment assuming no material degradation), running your attachment through both of the standard tie in points removes some of the risk factor and introduces little in the way of inconvenience. So go this route if using a PAS or sling in this way regularly.

[Pete04]

Interesting, thanks. I never even thought of leaving a PAS girth hitched to one point as that would obviously create wear, but I guess I could see folks doing that.

 

I would think that girth hitching through both tie in points would weaken the dyneema/PAS/etc; but, I've got 0 evidence for that so I'll defer to your 50% number. As you pointed out 50% of 22kN is still pretty darn good and as long as you don't take a fall on the static dyneema/PAS/etc (which could be catastrophic) it probably won't result in failure.

 

 

[Ben Beckerich]

Todd Skinner.

[jfs1978]

Yes there are a lot of factors and that 50% rough number is variable. Also, there are variations in failure modes and which material fails depending on whether the test is done in a static vs. dynamic load test. So a correction, my comment about the "extension material" being the specific failure point is probably not always accurate.

 

In the end it doesn't matter unless you just like geeking out on these things.

 

The larger points to consider are redundancy and material wear distribution. There are a million forum threads discussing the ins and outs of destructive testing and material strengths. Interesting reading on a slow evening ... like a REALLY slow evening ... maybe. =)

Edited October 16, 2015 by jfs1978