The following article is courtesy of Black Diamond Equipment.
"What is the strongest way to rig an anchor at a belay?"
Now that's one of the most loaded questions I've ever
heard, because, of course, there is no real definitive answer. There are so many factors involved, including quality of the placements, quality of
the rock or ice, materials available, etc.
For the sake of the discussion, however, we narrowed it down to assuming two "perfect" bolt placements and using one equalized sling. My immediate answer
was that such a set-up would be plenty strong for most climbing applications no matter which way you slice it, but any time you knot a sling it undoubtedly
Remember I'm not a guide and don't pretend to be one, and I'm not suggesting which anchor equalizing method is better or worse. All I'm providing is some
data based on a very few (i.e., one) data point for each scenario.
My crack crew of QA engineers and I decided to check out the three most common equalizing methods using a single 48" runner: Sliding X [not redundant],
Sliding X with Knots, and Figure 8. Again, I'm not going to get into the merits or negatives of each situation (e.g., shock loading if one anchor placement
blows, how "equalized" they actually are, redundancy, etc). This is just an apples-to-apples strength comparison of the three configurations.
|| Peak Load (lbf/kN )
|| Failure Point
||none (machine limit)
|Sliding X with knots
||webbing @ knot
||webbing @ knot
So What Do These Numbers Mean?
A couple of things to remember:
- CE-certified slings are rated to 22 kN (4946 lbf)
- Typical CE-certified carabiners (e.g., lockers, wiregates, bent gates, etc) in closed gate are rated 20 kN minimum (4496 lbf)
- CE-certified cams are rated 5 kN, but most are over 10 kN
Using a Sliding X anchor, our tensile tester couldn't even break it. Now that is BURLY. And both configurations with knots were more than 20 kN in ultimate
strength. So just as we've seen in previous sling-on-sling girth hitch experiments, knotting slings, etc, knots reduce the ultimate strength by anywhere
from 40-60% and the failure mode is always at the knot. However, even though that seems like a big reduction in strength (which it is) the bottom line
is that the anchor is still plenty strong for most any typical climbing scenario thrown at it.
Climb safe —
Kolin Powick (KP) is a mechanical engineer hailing from Calgary, Canada. He has over 20 years of experience in the engineering field and served as Black Diamond’s Director of Quality for over 11 years. He is currently their Climbing Category Director. If you have a technical question for KP, please email him at firstname.lastname@example.org and he will TRY to respond.
To help make more climbers safer climbers, Rock and Ice has teamed up with Black Diamond Equipment to present the information here.