The energy generated by a fall can be substantial. The rope, or dynamic, part of the system has to be able to absorb this energy which it does by stretching. However this critical factor has to be considered. How much force can be absorbed in the integrated system before injury or catastrophic failure occurs?
The more stretchy the rope, i.e. the higher percentage of the energy it absorbs, the less shock loading on the static gear. But too much elongation might be equally dangerous
However in order to simplify what is a complex mechanical calculation we use Fall Factors which assume a dynamic rope certified for climbing will perform within certain parameters.
A fall factor calculates the force on the climber given two variables:
The length of the rope in use between belayer and the climber. ( e.g. 15 metres)
The length of the fall. (e.g. 3 metres)
Fall Factor = Length of fall = 3 = 0.2
Length of rope 15
What it means:
0.1 - 0.6 Fall is within safe limits
0.7 - 0.9 Survivable
1.0 - 1.4 Survivable but gear at risk. Rope should be discarded
1.5 - 1.9 High risk of gear failure and injury to climber, e.g. whiplash
2.0 Maximum fall factor. Gear failure and climber injury is likely
All this is irrelevant if you hit a ledge or the ground first
Slings and Daisychains
The safety of slings and daisychains is now something that climbers take for granted, given that most are rated at 22K newtons. However it is important to understand a little bit more about the dynamics of slings.
Slings are broadly made from two different types of material, dyneema and polyamide, or often a combination of the two.
Dyneema® slings are predominantly white as the material cannot be dyed. It is extremely strong, lightweight and has minimal stretch.
Polyamide/nylon which is the material used in rope and therefore has some dynamic properties. They are more bulky than a dyneema sling.
Why we should be concerned.
If you are sitting on a ledge level with the ring to which your dyneema daisy chain is attached and you slipped, the resulting fall would have a fall factor of 1. That is to say that you would
fall the full-length of the daisy chain.
While Dyneema® has a much greater strength-weight ratio (static load) than nylon, its elasticity is far less. Even a 60 cm fall-factor 1 fall on to an open Dyneema® sling can generate enough impact force (16.7 kN) at the anchor to pull a Wallnut 11 wire (12 kN) apart.
Tying a knot in a Dyneema® sling weakens it even further leading to sling failure in a fall-factor 1 loading on to a 120 cm sling. Knots in a sling mean that the webbing material is subject to smaller deflection radii. This reduces the strength of the sling. Knots in slings (tied into the double strand) reduce the breaking strength by an average of approx. 45%.
Knots in webbing slings should therefore be avoided as far as possible. If you cannot do without a knot in your sling when climbing/mountain climbing, you should use knots that have the least effect on the strength of the sling. Your choice should be polyamide slings, as these are less weakened by knots.
As stated above, most leader falls are between 4 – 7 kNs and forces above 10kN will start to cause internal injuries.
Guidelines for the use of slings
Only use undamaged webbing slings that have a known history (age, number of climbing tours, number of falls etc.).
If several strength-reducing factors occur at the same time, the strength reduction of the sling can become critical.
Tying knots into slings during belaying should be avoided as far as possible (if you need to extend a sling, you can also do this with a carabiner)
Slings must never lie on top of each other
It is important to ensure that no rope is in frictional contact with the sling as this can cause the sling to burn through
If slings have to be knotted, always tie the knot into both strands of the sling. Use a strop bend. If your belaying technique is based on a series construction, polyamide slings are the preferred choice since they are less weakened by the knots
Due to the considerable reduction in strength caused by knots, Dyneema® slings are less suitable for belaying purposes
A summary of this sling-on-sling connection testing is:
1. Joining two slings together using a hitch reduces the ultimate strength of the material up to and over 50%. Only a nylon-on-nylon connection with a hitch seems to result in a reasonable amount of strength loss compared to other sling materials.
2. In general, the narrower the material used, the greater the reduction in strength when joined together with any hitch.
3. In general, materials of different width joined together with a hitch results in a significant reduction of strength.
4. If you must link two runners together, a carabiner is stronger. If you must use a girth hitch then put the girth hitch on the web that has higher strength or use a strop-hitch.
5. Better yet - use a longer runner altogether or a thick nylon daisy-chain for harness connections.
For more information please visit the following sites from which I have drawn most of this material.
You would be very wise to know
The risk you take when you let go.
Three metres from a Camalot
Hastily placed in a narrow slot
Rope stretches, cams jam the rock.
But if there is too greater shock
There’ll be just seconds to reflect
Fall factors were not one to neglect
Simply divide how far you fell,
By the length of the rope run out
Less than one all should be well
Near two your future is in doubt
You’re feeling fit and well alive
On pitch 4 of that new E 5
Do not start your vertical dance
Above the safety of the stance
Without clipping some solid pro
To catch you as you start to go
So you need not wave goodbye
To your mate as you fly by
High on the Via Ferrata
There is some scary data.
If your grip should ever fail
When clipped on the safety rail
The force could be off the scale.
Make sure your clipper is a Zyper
In case you become a slipper
Next time you try to see the sights
High in the Italian Dolomites
So, now you know, don’t be dumb
Visit Dom’s emporium.
There’s a point
Two all this
To pearly gate