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Grifted
from petzl. -- Shitbag
"I
want to know the thoughts of God. All the rest are details." Albert
Einstein
Understanding Shock
Load
Turning falls into
flight... Some basic truths, simple math and common sense...
Fall Factor Explained... Your life depends on
the stretch of the rope... Static rope doesn't stretch enough...
Slings and runners are just like static rope... Meanwhile
up at the 'biner'
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Turning
falls into flight...
It can happen. Concentrating,
studying the holds, absorbed in the route, you get run out above your
last piece. Suddenly your foot slips, a hold breaks off, you lose your
balance and the hard pull of gravity asserts itself. You're falling.
 There's the fear, the rush of adrenaline,
and then mechanical phenomena intervene. Your security system brakes
your fall and stabilizes you. For years we have played in the vertical
world, seeking, like everyone else, both excitement and safety.
Some basic truths,
simple math and common sense...
Okay, you're climbing,
your rope and harness
are secure, the anchor
is bombproof, and you feel pretty safe. The thought of falling
doesn't upset you. Everything's cool.
Maybe. But every fall
creates an enormous amount of energy. We are, after all, relatively large
creatures , and gravity is a formidable force - as any belayer who has
caught a screamer can attest.
What's more, the shock
load for the fall is transmitted all through your security system,
and is nearly doubled at the anchor or pro on top. And every element
in the chain has to sustain the shock without breaking if your fall is
going to cause you nothing worse than scrapes and bruises.
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Fall
Factor Explained...
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
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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.
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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
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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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