Strength
of Slings in Three Different Anchor Configurations
A
few weeks ago a crew of super badass climbers
rolled through the QA lab and one question came
up more than just a few times: “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 weakens it.
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.
Testing
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, 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, etc). This is just an apples-to-apples strength
comparison of the three configurations.
Results
|
Configuration
|
Peak Load (lbf/kN)
|
Failure Point
|
|
Sliding X
|
8000/35.6
|
none (machine limit)
|
|
Sliding X with knots
|
4760/21.2
|
webbing @ knot
|
|
Figure 8
|
5272/23.5
|
webbing @ knot
|
 |
|
Sliding
X
|
 |
 |
|
Sliding
X with knots
|
Figure
8
|
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 —
KP
