Fall Factors and Sling Safety


 

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 Climber’s body. (Internal injuries 10Kn. av. Leader fall 4 -7 Kn)
  • The harness (Belay Loop 15Kn)
  • The static links (Open sling 22Kn)
  • The attachment point to the rock (Wallnut 11 wire 12 Kn)

 

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

 

Static Gear 

  • Karabiners quickdraws, nuts, cams, slings etc.
  • Function : To establish secure attachments points or links in the safety chain.
  • Characteristics; Very strong , able to with stand high levels of shock loading, 

 

Dynamic Gear

  • Ropes
  • Function : Sufficient energy absorption to provide acceptable loading to static gear
  • and the climber’s body, limited elongation to limit fall length. Climbing ropes are designed to balance these two requirements.
  • Characteristics : Elastic, flexible, abrasion resistant, can be  Elongation
  •  

Elongation: 

  • The amount a rope stretches when a static weight of 80kg is applied. Given as a percent figure e.g. 8% for single rope 10% for half rope

 

Impact Force: 

  • Force generated when 80kg mass is dropped through 4.8 metres. This is the critical number. 

See: http://www.theuiaa.org/safety_labels_products.php 

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:
Factor 
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.

An example:

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.


http://www.mammut.ch/en/slingswebbings_faq_belay.html

http://www.blackdiamondequipment.com/en-us/journal/climb/all/qc-lab-connecting-two-slings-together

 

http://climbinglife.com/tech-tips/sling-on-sling-connections.html

 

Risk Adverse

David Bruton

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.

 

 

 

Ten up

Down two

There’s a point 

Two all this

 

Twenty up

Down four

Same score

No floor

 

Ten up 

Down five

Point five

Still alive

 

 

Ten up 

Ten down

Lucky one

Scrap rope

 

Up four

Down eight

Go straight

To pearly gate

 

 

 

 

 

 

 

 

 

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copyright: david bruton