Science > Physics > The dynamics of a fall in rock climbing - shock loading from the sliding-x
| Topic: |
Science > Physics |
| User: |
"jd" |
| Date: |
04 Jan 2005 03:06:45 PM |
| Object: |
The dynamics of a fall in rock climbing - shock loading from the sliding-x |
Anybody here with a strong knowledge of clasical physics and/or
engineering (particularly dynamics and mechanics of materials) do any
regular rock climbing?
If so, there are a few topics I would like to discuss regarding the
forces generated in rock climbing falls. They have been discussed to
death on climbing forums and never get resolved, as it seems about 98%
of the people in these discussions are essentially physics hacks and
only can back up their arguments with heresay and a very poor
understanding of physics. I feel that when I try to discuss these
issues with most climbers, they are not able to follow what I am saying
and the conversation ends as these such conversations always do: with
zero progress. I come from a mechanical engineering background and so
feel I have a pretty good understanding of the issues at hand.
So, if anybody here has set anchors for a top rope, then you have
undoubtedly heard of a sliding-x (sometimes called a magic-x). (If you
haven't heard of it but are interesting in the discussion, I can
explain it for you ... just let me know ... otherwise, I'll assume you
know what it is.) Here is the issue:
Most climbers will tell you that the sliding-x is bad because, although
it is a self-equalizing anchor, if one bolt blows, the remaining bolt
is supposedly shock loaded. However, I disagree (with perhaps one
caveat I'll mention below). If one bolt blows, and so the anchor
extends by, say, 3 ft, then the argument goes that since the sliding-x
is made from static rope (cordelette or webbing), a 3 ft fall on said
static rope can generate a tremendous force both on the climber, and
more importantly, the remaining bolt (which would then also blow from
such a shock load leading to the undoubtedly disastrous case of total
anchor failure). Hence the reason a sliding-x is considered dangerous.
But I disagree.
The reason people are confused is they think that, because the "action"
is happening up at the anchor, it's the static rope of the anchor that
takes the load. What they don't understand is it's no different than if
the climber suddenly slipped and fell 3 ft, which clearly does not
generate dangerous loads (especially in a top rope system) because the
dynamic climbing rope has a high elasticity which stretches
significantly and hence "absords" the fall and decreases the
acceleration and hence force on both the climber and anchor. The result
of a bolt blowing is that all the sudden the climber drops 3 ft. The
system doesn't care if this happened because a bolt blew or because the
climber slipped on a hold and there was 3 ft of slack in the climbing
rope because of a sloppy belay: it experiences the same load in either
case. There is no shock loading, and no reason to be any more concerned
about a sliding-x extending than there is to be worried about a 3 ft
fall on top rope (which nobody I know is concerned about). Even worse,
lead climbers don't think twice to take tremendous falls (fall factors
of 1 or greater) on a single bolt which generates much greater forces
than a sliding-x extending due to a blown bolt. I think, given the
above understanding, people's concerns are misplaced and there is no
reason to worry about a sliding-x. Even further, it's good equalizing
properties are much more important.
With one exception which might explain why experienced climbers, even
with a poor understanding of physics, swear from experience that a
sliding-x extending from bolt failure is bad. I got to thinking about
it, and I certainly think my above explaination is correct ... except
it neglects one thing. I assumed that the climber was hanging on the
rope with his static body weight and then all the sudden one bolt
failed. If this is the case, then my above analysis is certainly
correct and there is no need to be concerned.
However, one hanging on the climbing rope with only their body weight
is most certainly not going to cause a bolt to blow. In reality, the
bolt will blow when the climber will is taking a fall, and as the load
approaches its maximum and the climbing rope is already significantly
loaded much beyond the climber's weight (at least twice the climbers
weight is the minimum, but most likely considerably more), this is when
the bolt is probably going to blow. In this case, the rope is already
stretched and, especially if it was already a big fall in the first
place, the rope will be effectively stiffer due to strain hardening and
hence will be more like a static rope. If the bolt blows in this
situation, then the rope is not as elastic and I can see how a
sliding-x extending would increase the loads, possibly enough to case
failure of the remaining bolt. It is for this reason that the sliding-x
might be bad. However, most climbers are not understanding that it is
due to the fact that the bolt will most likely blow when, due to strain
hardening, the climbing rope is already stiffer than when only loaded
under static body weight. It is not at all because a bolt blowing and
the sliding-x extending somehow only transmits the fall to the static
ropes of the anchor (which, if it was the case --and it's not, as I
outlined above--then it certainly would be bad, as any caver who
regularly uses static rope correctly understands).
So my hopes is that somebody on this forum who understands not only the
climbing aspects but, more importantly, the physics aspects of what I'm
talking about can chime in and see if my understanding makes sense. If
there is anything that is unclear, let me know, and I'll try to explain
more clearly.
Any takers? Much thanks in advance.
Jonathan
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| User: "BllFs6" |
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| Title: Re: The dynamics of a fall in rock climbing - shock loading from the sliding-x |
05 Jan 2005 07:54:44 AM |
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If
there is anything that is unclear, let me know, and I'll try to explain
more clearly.
Any takers? Much thanks in advance.
Jonathan
Hi there
take your question over to www.caves.org
that is the website for the USA National Speleological Society (NSS)
on the right side of the NSS home page you''ll see a link for a discussion
area...
go to that forum, then go to the vertical caving section
you'll find some vertical cavers there that are really into ropework/gear and
seem to know "climbing physics" pretty well
take care
Blll
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| User: "jd" |
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| Title: Re: The dynamics of a fall in rock climbing - shock loading from the sliding-x |
06 Jan 2005 06:30:05 AM |
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Thanks Bill. I posted over there and got many responses. However, I was
still hoping I would get some people to respond on this forum.
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| User: "BllFs6" |
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| Title: Re: The dynamics of a fall in rock climbing - shock loading from the sliding-x |
07 Jan 2005 07:41:47 AM |
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Thanks Bill. I posted over there and got many responses. However, I was
still hoping I would get some people to respond on this forum.
Glad to hear that you did!
I'll have to run over there sometime and read em
take care
Blll
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