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Re: [rgold] Common KNs in real world falls:
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jt512
May 15, 2009, 4:46 PM
Views: 7180
Registered: Apr 12, 2001
Posts: 21904
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rgold wrote: 7. Halifax keeps saying that the fall factor calculation is a poor model. I guess this would need some kind of quantification, and then an agreement about what kinds of answers one wants from a model. If the spring model regularly overstates the peak load in a consistent way that is not "too far off," (definition required), then it has some value in providing a useful upper bound.
If we could determine that the error really was consistent, then we presumably could measure the error, and correct for it. Hence the the model would provide more than just an upper bound. Biased predictions are ok, if you know enough about the bias.
But I actually do think that there may be a fairly serious problem with the model that results in underestimation of peak force when the fall factor is less than 2. When the rope is clipped through a top anchor, the section of the rope between the anchor and the belayer stretches less than section of the rope between the anchor and the climber. Intuitively, this strikes me as a serious departure from an ideal spring with a single modulus of elasticity. However, if my reasoning is correct, then perhaps the error is mitigated, at least for fall factors near 1.78, by using a rope modulus that is calculated from the rope's UIAA impact force rating.
A related problem is that we seem to be treating the anchor as frictionless for purposes of calculating the peak impact force on the climber, but then incorporating a substantial frictional force when we calculate the impact force on the belayer. We then calculate the force on the anchor by adding together two numbers that were derived using inconsistent assumptions, a practice that strikes me as a little weird.
So, questions to rgold: Am I correct that friction through the anchor is not accounted for in the model, and that therefore the model substantially underestimates impact force for low-fall-factor-falls? If so, can we (where "we" = "you") improve the model? And if not, should we perhaps ignore the reduction of the impact force on the belayer's side of the rope by friction when calculating the total force on the anchor, to compensate?
Edit: I have derived an equation for maximum impact force that incorporates the effect of friction at the top anchor, and will post it soon.
Jay
(This post was edited by jt512 on May 19, 2009, 3:41 AM)
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Edit Log:
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Post edited by jt512
() on May 15, 2009, 5:04 PM
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Post edited by jt512
() on May 15, 2009, 5:52 PM
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Post edited by jt512
() on May 19, 2009, 3:41 AM
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