I made the mistake recently of posting on UKClimbing.com. There had been some discussion about multi-pitch belays, and this deviated to someone posting about sling strength, and quoting DMM’s testing of sling strength.
It’s worth watching the DMM video:
My interpretation of this was that not a good idea to clip straight into an anchor with a sling –using it like a lanyard- and take a fall factor 1 or 2 fall on the sling; nor should you have slack in the system if connecting directly to an anchor with a sling. The impact forces generated are greater than 10KN for both dyneema and nylon slings for both fall factor 1 and 2. Knotting the sling weakens both sling types, causing them to fail at a lower impact forces; but still greater than 10KN. It’s worth noting that if you took a fall on a sling and genertated 10kN of force in your body, it would really, really hurt. Have a look at this report from the Health and Safety Executive.
That sounds all pretty nasty. But what does it actually mean in a typical climbing situation?
Climbers use slings to join individual pieces of gear together to make a better anchor, like in the picture below.
They also use them to extend running belays, like in the photo below:
So, is it safe to knot the slings and use them in these scenarios?
In the tests knotting a sling does weaken it – from around 22kN –where it didn’t break – to 10.7kN to an fall-factor 1 drop test .
If you knotted a sling, could it break in normal climbing situations?
This web page from Beal shows lots of research into fall-factors and impact force.
In general, taking a leader fall using a rope and runners will generate peak impact forces between 5.1 kN for low-impact, dynamic rope using a belay plate and 14.5 kN for high-impact rope using a “gri-gri” like device.
For most climbing situations, the impact forces are below that of the breaking point of the knotted sling in the tests. BUT it is conceivable to generate very high impact forces using less-stretchy ropes and auto-locking type devices.
So, for most climbing situations using dynamic ropes, classic belay devices, the forces generated in a fall probably won’t cause a knotted sling to break.