fall factor calculator

treebing

treebing

Been here much more than a while
Location
Detroit, Mi.
http://www.getbeta.com/fall_factor.asp

I am sure that this topic is somewhere in the archives of treebuzz somewhere, but to me it seems worth bringing up again, and then again.
The site above is a great place to get an understanding of fall factors. Fall factors are something I vaugely knew about from rock climbing but never really bothered to take the time to really understand what it meant until it was forced home to me at TCI in Baltimore by Chris and Mark in their presentation about their caribiner strength research. They discussed how some caribiners would break at around 800 pounds when used to choke off a limb with a small diameter. I asked "yeah that looks like fun research and all, breaking stuff and such, but when do we ever come close to putting that much force on a caribiner in a normal climbing situation?"

Using the fall factor calculator, it is easy to see how quickly amazing forces are generated. Mind blowing, really. Imagine, your up in a tree on a horizontal branch, you take your lanyard, clip one end to your bridge and you choke off the other end to the branch. Then you stand up on the branch thinking to yourself "i'm tied in". But you slip and fall straight down. Lets say, three feet of lanyard, thus a six foot fall total. Put that into the fall factor calculator and see what kind of force is generated on you and your equipment. It is something that you do not want to do! Regardless of your caribiner possibly busting, you will be in a world of hurt. Now, Im not sure but I think that tree climbing lines could be considered static compared to the ultra stretchy rock climbing lines? or would they still be dynamic compared to the KM III or a really true static line. Any way, the fall factor calculator doesnt have options for more dynamic or less dynamic, but either way, ouch.

Putting more rope into the system is critical in reducing crazy forces. This was also demonstrated at the expo by Tod K and crew, somewhat by accident I think. They were demonstrating forces of rigging by dropping a log on a dynometer. Because the GRCS was on the back side of the tree they had created a wide angle at the block. A wider angle decreases the force generated at the block and they wanted to demonstrate that the force would increase at the block if they closed that angle. Their solution was to insert another pulley directly under the block hereby closing that angle to zero. They dropped the log and lo and behold they had succeeded in decreasing the force at the dynometer even more! This was because by adding another pulley, they had added more line into the system which over-rided the effect of closing the angle.

The fall factor calculator at the site above I think can prove extremely usefull. In many ways it is more or less the same as Ken and Rips rigging software without a greenlog chart. It also does not acount for angle at the rigging point, any tricks to help figure that out easy? anyone? Interesting and life-elongating stuff. Thanks guys.
 
Just lost a long explanation post on this, so heres an abbreviated version:

1. Fall factors are simple concepts that don't calculate force.

2. The maximum fall factor can easily be many times more than two.

3. Rope stretch or no stretch won't make a difference to the fall factor.

4. Karabiner strength depends on many things. How tests were carried out makes a big difference to tests results. I do choke for life support, including your lanyard example. It depends how you do it and what type of karabiner and rope is used.

5 years ago AMT told shocking stories of how Kong karabiners were failing at half their rating in tests. It was nothing to do with a failing of the krab design, it was how the test was set up. Something to bear in mind with any research or statistics.
 
As I'm sure anyone who came to the presentation on the demo tree at the expo can confirm, we were not sharing "shocking stories", just highlighting relevant issues.

The choked carabiner research was really about showing the relevance of correct configuration and choice of equipment. The presentation is one of a series of presentations about this topic. In this case we tested three batches of eleven krabs on three different "branch" diameters, 15 kN for three minutes, in a single line straight pull.

The results are going to be written up for a future arborist news article, but don't hold your breath... but it was very interesting. One thing that became apparent to us was the consequences of the optimised design of carabiners, tailored to conform to the various tests, but that really struggles to handle the loads imposed in such a choked single line config that was never intended or forseen by the manufacturer.

The results in no way reflect a lack of quality of the karbiners used in the tests, but more the necessity to consider carefully the way we configure our gear. And certainly there are products out there of inferior manufacturing quality, so by using "budget" chepo equipment you can compile the problem.

The forces we can generate in our climbing systems is often underestimated. I think the point we were trying to put across - the risk of high peak loads and misconfiguration coinciding - is a vaild one worth discussing at such industry events.

We've discussed this issue at various shows theses last six months, two in the UK. One direct result is that LANTRA, one of the regulating bodies in the UK has responded by redefining their recomendations on how to choke around a limb.
 
your right that rope stretch or not wont make a difference to the fall factor, yet it does make a difference on the impact felt.
As for there being possibilities for a fall factor more than 2? I do not see how that would be possible. Fall factor simply means the length of fall divided by the length of rope in the system. It would be impossible to create a fall factor greater than two. Am I missing something? A six foot fall on a three foot rope is a factor 2 fall, which is worse than falling 15 feet with 15 feet of line in the sytem (think lead climbing)which would be a factor 1 fall.

Your right that fall factors do not equal force but they are important in calculating the force generated at the rigging point as is what kind of rope you are using, steel cable versus bungee cord.
 
I just punched in the numbers for the above example.
At 160 pounds (my weight) a six foot fall on three feet of dynamic rock climbing rope generates 8.16 kilonewtons at 225 pounds a kilonewton more or less, that is 1836 pounds at the rigging point. For a static rope KM III for example, the force generated is 16.329 KN. Or 3667.5 pounds. I would assume that with standard arborist line, the number would lie somewhere in the middle.

This is somewhat deceptive because the probability of you falling straight down is fairly slim. most likely in a fall scenario like that you would fall with some swing which would greatly reduce those numbers. But still...

Now an example of a factor one fall would be, you are up 150 feet in a tree with a 120 foot line, you tie one end of the rope to the tree and the other end to your ankle and jump. Assuming you miss all the branches on the way down, the shock calculator shows 6.123 KN (1350lbs) for dynamic, 12.246 Kn (2700 lbs) for static.
This would mean that falling 6 feet on your 3 foot lanyard is worse than falling 120 feet on your full climb line. Crazy and counter intuitive.
 
Treebing,

your're absolutely right about the angle of fall being relevant. One thing I've found when setting up demos of the dynamics involved in a fall is that to get the really high numbers you have to set the haul line that you have your dead weight/ lump of wood/ whatever on in line with the line you're dropping into. As soon as you create a pendular swing, peak loads decrease.

But this worse case scenario is not unthinkable:

A climber reaches the top of his or her access line, hooks his or her foot over the limb and stands on the branch above the anchor point of the access. If he or her were to loose their balance at this point in time, that's a vertical drop onto little line with a high fall factor... ouch.
 
"Now an example of a factor one fall would be, you are up 150 feet in a tree with a 120 foot line, you tie one end of the rope to the tree and the other end to your ankle and jump. Assuming you miss all the branches on the way down, the shock calculator shows 6.123 KN (1350lbs) for dynamic, 12.246 Kn (2700 lbs) for static.
This would mean that falling 6 feet on your 3 foot lanyard is worse than falling 120 feet on your full climb line. Crazy and counter intuitive. "


I was under the impression that the force generated by a falling object would increase exponentially as the distance of the fall increases.
Treebing just said that a six foot fall would generate 16.329 kN while a 120 foot fall would only generate 12.246 kN. This obviously has alot to do with the length of rope in the system, but it still sounds a little fishy.
Is that the actual force generated by the falling object or is that simply the impact felt at the rigging point or both rigging points, i.e. anchor and saddle?
 
The calculator is flawed.
It only uses the fall factor, not he distance one falls, to come up with a shock load.
Try a 200 pound person falling one inch. It darn near snaps your carabiner! :)
 
I was goofing around at work one day (don't tell the boss), but I was hanging on about 50 feet off of doubled rope, my normal climbing rig, and then droping off a limb (close to the ground in case of failure). I started at about a foot, and worked my way up.
I kept going higher until the pulley (rated at 5000 LBs MBS) I was using in my false crotch, failed to opperate properly.
I won't guess at how high I was dropping, because it might be irresponsible to post, but you book worm, chart reading, climbers might learn a thing or two by doing the same. If you want to know how far you can drop on your line, do it.
By the way, climb safe.
 
I think your right about this calculator being flawed. I have been thinking about that most of the day and your right, it doesnt add up. sorry. Maybe there is something else out there?

I dont think it would be irresponsible for you to post your guestimations. You survived didn't you? I would much rather hear a more detailed acount of your field research than to try and replicate it myself :)

Another question I had was,if you are in dDRT, that would effectivly double the amount of rope in the system correct?
 
I think now I'm re-confused. Please help me out here. Somebody who really knows what is up post a solid informative piece on what I am trying to wrap my head around but having a hard time with.

Does anyone remember Dan Osborne? He was a real bad [censored] crazy rock climber with enormous balls who would free solo all kinds of ridiculous climbs. When I was younger, someone gave me a "no fear" motivational poster with Dan doing a horizontal handstand 500 feet off the ground on el cap with no rope. Great role model eh?

Anyway, another thing he would do was full length rope jumps from the tops of Yosemite cliffs. I mean 200 foot jumps onto standard dynamic climbing line. If I'm not mistaken, he went as far as tying two lines together and hurling double that distance. He liked to push himself I guess. He succesfully walked away from dozens and dozens of them before, well, his last jump didn't turn out so good. But basically he used the principle of fall factor to know that most of the time he would be fine after hurling himself into space onto his rope.
So there is something to this, but how is it calculated correctly? Because your right Mike, a one foot factor 2 fall can't be as bad as a 10 foot factor 2 fall. Or is a factor two fall a factor two fall like the get-beta calculator states? Help, please.
 
Sorry, his name was Dan Osman. His final jump was meant to be 1,100 feet onto several climb lines tied together between the leaning towers in Yosemite. The fall ended up being 150 feet longer than intended with a not so soft landing. For some reason he had left his ropes unattended to the elements for a too long a period. The rope failed obviously at one of the knots. With 1,100 feet of line, there is going to be a weak point I guess. this happened a little over ten years ago.
 
Mike,

I don't think that poh-pohing this issue is very smart.

Fact is you can hurt yourself doing this. This is not coming from at it from book-worm perspective, during demos using a load cell we have consistently come up with peak loads of almost 500 kg after a 40cm drop (twice 20cm of line) into a low stretch access line... not very pleasant! Think of the slack we generate whilst footlocking, that loop of slack must be at least 40cm at times, which would mean a 80cm drop.

It also depends how you fall. If you are expecting a drop, obviously you will orientate your body accordingly in expectance of the impact. NCAP who do the vehicle impact testing with the crash test dummies talk about spinal damage from side impacts as low as 300kg. That's not a crazy amount, if you think back to the 500kg mentioned above.

Doubling the line reduces the amount of elongation further making the problem worse.

During demos I've found that using a 50kg lump of wood on a 1 meter lanyard a factor two drop can produce peak forces of one ton and more. Again, many variables involved here, but there is the potential to seriously hurt yourself.

We should not blow this out of proportion, there's no point in scaremongering. This situation is easy to avoid if you are aware of it and take appropriate precautions with standard techniques every arborist has in his or her mental tool box... but ignoring it won't make it go away. And belittling it isn't smart in a public forum.
 

Attachments

Here the discussion went to just about this very thing. (Like Mike has said, "so long as your hitch holds") The extra rope in the doubled-rope system acts to effectively increase the diameter of the rope, not the length. If it were to do the latter it would make the system springier. It instead makes it firmer. Each leg now has less weight on it so will stretch less than it would if carrying all the weight alone.

You might have some fun paying out several feet of your half-inch rope and jumping onto it, but I doubt you'd want to try it the same with a three-quarter-inch one of similar construction. A doubled half-inch rope has almost the same cross-sectional area as a three-quarter-inch rope. All else being equal, the larger the diameter the more maas :) it would take to get the rope to stretch and absorb energy.

Attached is page 15 culled from http://www.samsonrope.com/site_files/Arborist_Catalog.pdf

Note the charts.
 

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Quite a lot been said since I last posted!

Mark's examples of access and fall forces are one of the reasons to avoid footlock and take up SRT; If the rope is doubled back and tied off at the ground, you still have a lot of rope to absorb a fall as you approach the anchor. Plus built in energy absorbers and immediate rescue.

Couple that with the ergonomic issues, and my reasoning against footlocking is more understandable.

The tests I ran in the prusik research also showed up the same fall forces Mark talks about. Another reason why I advocate only using ropes that meet EN 1891 type A rather than B (type B ropes don't meet the drop test criteria for Type A).

Its also why I use a single line on a pole, where a true fall is more likely. This has been discussed with similar points in the 'Mark Chisholm tie in technique' thread.

As for choking lines with karabiners, it depends how the load comes on the line; not the same on a vertical pole compared to a horizontal branch, plus a few other variations.

As for higher fall factors than 2, a via-ferrata fall type situation is a good example.

As for LANTRA, I think you mean lantra awards Mark; they definitely aren't a regulating body, and they aren't democratically elected by industry to speak on their behalf. I look forward to hearing about the changes to their training programmes. And I wasn't implying you were 'scare mongering', but reminding anyone reading to look carefully at the details of research, to see how tests have been set up, as it can alter the results dramatically. That includes any research that I may have been involved with.
 
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belittling it isn't smart in a public forum.

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OK, Fair enough. I don't get accused of being smart too often.
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But you do agree the fall calculator is flawed in that it only uses fall factors, not distance, right?
Also to be considered, is we arborist rarely climb above our TIPs, there's just no need, and if we did, the hitch is not a static connection to the host line. It slips under higher loads and has an effect like a groundy letting the load run.
You did make a good point about side loading snaps and biners, cinching them to small diameter limbs is wrong. It only takes a second to tie a loop in the host line to snap to, eliminating the side load.
 
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