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Fall Factor Explained


 

by Editor

December 29, 2001

From Petzl Catalog:
http://www.petzl.com

"A lot of climbers don't really understand the fall factor concept; however, it's pretty simple, even if you hated math (this is math you can use in later life. In fact , you can use it to have a later life) Fall Factor is simply the length of the fall divided by the length of the rope from faller to belayer. The equation looks like this;

Fall Factor = Length of Fall / Length of Rope

Fall Factor 2 is the maximum you should encounter in a typical climbing fall, since the height of a fall can't exceed two times the length of the rope. Normally, a Fall Factor 2 can only occur when a leader who has placed no protection falls past the belayer, or the anchor if it's a solo climb. As soon as protection is placed, the distance of the fall as a function of the rope length is lessened, and the Fall Factor drops below 2.

Your life depends on the stretch of the rope...

Shock load is the result of three factors; The nature of the rope, the fall factor, and the weight of the falling object. That is you.

Obviously, the only part of this equation that can reduce the force of a fall is the bungee-like stretch of the dynamic rope (unless, of course, you can lose weight really fast). Thus, climbing safety systems are designed around the shock-absorbing quality of dynamic rope. It cushions the fall, reducing the impact force and the chance of system failure. In fact, the dynamic rope is the one "given" in the whole system. It is designed to limit the force of one climber's weight (80 KG) in a worst-case fall (Fall Factor 2) to not more that 12 kN. Thus, the rest of the gear can be designed to work with this known maximum force.

More rope means more stretch to absorb a fall. Which explains why a Fall Factor 2 drop of 4 meters develops the same shock force - 9 kN - as one of 20 meters, assuming a dynamic rope is used that conforms to UIAA standards. What's happening is that the increasing length of the fall ( and the greater shock force that goes with it) is compensated by the greater length of the rope available to cushion its arrest.

Static rope doesn't stretch enough....

Static ropes - traditionally used mostly in caving and rescue but now also used for sport rappelling and even in climbing gyms - are designed to minimize stretch ( cavers hate feeling like yo-yo's). So their ability to absorb shock is marginal, particularly along short lengths of rope. What's more, static ropes aren't as well defined by industry codes as dynamic ropes, so they vary in elasticity according to the manufacturer and the country of origin. They're often about as non-dynamic as a cable, and transmit virtually all the shock load to the safety system and the body. And in a climbing situation, a very short fall can develop enough force to be critical.

Slings and runners are just like static rope...

Used for security, without a dynamic rope, runners are just as dangerous as static rope. As the diagram shows, a Fall Factor 2 develops enough shock load to risk failure of the runner, the harness, carabiners, not to mention a lot of failure in the climber's skeletal system.

This is worth saying again:

A fall of less than four feet on a static rope or sling can create enough shock force to cause serious injury or death. Bearing in mind that the human body can only handle, for a brief instant , a shock force of 12 kN without risking serious injury, you don't want to go around absorbing 18 kN. And you should know that 18 kN is getting real close to, or over, the minimum limits set by the UIAA on all the gear in your safety system.

For purposes of comparison, here are the UIAA limits;

Anchors: 25 kN
Carabiners: 20 kN
Slings: 22 kN
Harnesses: 15 kN

Meanwhile up at the 'biner....

Physics isn't our friend in a fall. The same mechanical advantage we use in pulleys works against us when we're on the end of a rope. Because at the point where the rope returns, normally a carabiner, the force of the fall is increased by approximately 66% (it would be doubled except for the friction of the rope against the metal).

So, starting with our 9 kN maximum shock force with a dynamic rope, the force on the carabiner becomes 15 kN in a Fall Factor 1.9 fall. That's a lot. You better hope it's a good anchor or placement .

Now apply that same math to a static rope, The Factor 1.9 fall, with is normal shock force of 18 kN, becomes a shock force of 30 kN (multiply 18 kN by 1.66) In this case, you couldn't even count on a stout tree. And it wouldn't matter if the anchor held, because something else would undoubtedly fail."



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