Shock load

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The other half is to know how to put the brakes on easy

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Yep. The tough part of dismantling a tree in a residential area is not always having the right distance to let every load run well to absorb the best amount of shock.

And, I appreciate a good groundie knowing how to gauge these loads and ease into them.
 
there might be a few out there that never make a mistake. me however I've got the scars to remind me of mine, and some from other peoples mistakes.
i find that pain is sometimes an outstanding teacher.

allot of accident reports seem to begin something like: trimmer with 10 years experience
bucket opp. 8 years
39 year old tree worker
etc. etc.

my point being complacency is one of our worst enemies. the more we get away with certain actions the more we will repeat the same behavior. Even those close calls that we miraculously escape unscathed, where maybe a foot to the left we would have been crushed, are easy to shrug off as luck and we learn less than if we had gotten a paper cut.

there is no such thing as luck and there are no accidents everything is connected in a chain of events.

"if you don't change where you are heading you will wind up there" -old Chinese proverb

I try to always visualize what I am trying to accomplish and imagine several worst case scenarios. and try my best to chose the best escape rout.
 
Holy crap, I spent a couple hours last night trying to figure out this formula. I converted velocity and mass into joules and newtons, it was getting too complicated to understand but I figured I finally had it. In proving the formula I found a discrepancy due to the formulas being based on an object falling onto the ground, not hanging from a rope. I tried to find another formula I could use and found this one:

impact force=weight * fall distance / stopping distance.

This simple formula came within a few decimal places of the other one I spent so much time on.

As it works out the same, It still has the same flaw the first one had. You can change the numbers to see that if the stopping distance is more than the fall distance, you get an impact force that is less than the weight. I can only assume that we need to add the weight to the force to include it in the final position. I feel that without this the formula assumes that the weight stops just as it touches the ground and does not remain suspended.

Either way you can change the numbers and visualize how the force gets larger when the stopping distance shortens. try inputting ".16" (two inches) and see how the force skyrockets.

I welcome input on the math to see if we can get this right.

Dave
 

Attachments

since this string has led to all parts of the rigging system I feel this is a good place to post a question.

I am in the middle of a red oak removal about 100 ft tall 36" BHD the tree is in serious decline if not dead. root cavities
loose bark, looks like a lightning strike from the codom situation about halfway up all the way down. I spiked the tree to asses it further on the assent up. ive bored into it in multiple locations it feels solid.

ive done nastier trees but it still gives me the willies I used a ratchet strap to beef up the crotch and air mailed what I could get away with. the ratchet strap was mostly to give my mind peace i don't think it was necessary for not rigging real weight out of the tree yet.

my question is this:
considering worst case and a serious shock load is applied to one side could the ratchet strap increase the chance for failure in either side of the codom above the ratchet strap by not allowing the reaction wood to flex?

If it don't bend it breaks. that's always a calming mantra to me when the wind starts really picking up.

maybe I should use chains below the crotch instead?
 
Doc, you are certainly changing the load dynamic of the stem. EVERYTHING is a trade off. However , I have used ratchet straps to hold together lightening struck trees that were fractured before with great success. If you are going to rig you must take precautions at every level you can.Spreading out the load, like using the fishing pole technique, more rope in the system,your best rope runner on the line , and then go a bit smaller and test the behavior of the whole process.
Remember also do not be afraid to step back and re-evaluate at any time ! .... pride kills!! trust your gut if it doesnt look or fell right do not risk it.
 
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...impact force=weight * fall distance * stopping distance.

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I think the formula isn't quite right, but the idea is. The essential notion is that the ratio of fall distance to stopping distance is related to the ratio of impact force to weight.

Here's an idea that might be as close to a rule of thumb as we're going to get. Try to let the load run (at least) as far as it falls. Call the fall distance F. Then the stopping distance is also F, and the total distance is 2 x F. Call the weight W. The total energy developed by the falling load is W * 2 * F, as noted above by Dave. If we apply a uniform force to stop it in distance F, the force must be 2 * W. This would be the absolute best case. Using a more conservative approach, assume the ground person is only half that accurate, and starts with zero force and keeps increasing it smoothly until the load stops, again after distance F. The average force under this scenario is still 2 * W, but the maximum is twice that or 4 * W.

So there's a rule of thumb: let it run as far as it falls, apply as uniform a stopping force as you can, and the maximum force will be roughly 2 to 4 times as great as the load. If the stopping force is seriously non-uniform, of course, the maximum force could go much higher.

Note: I am talking about the rope itself. The pulley and sling will feel about twice as much force.
 
Just a comment on about the first post from Royce (I know that others may have already basically stated this but feel that adding my voice to the chorus might pay off in having fewer accidents)...I would like to make an appeal to instincts. I feel that you might have been ignoring the only voice in the situation who could have been silenced..and that was your own. Your voice in this situation is very valuable whether or not anyone gives you any credit for it. If you are really seriously concerned the rigging point is going to fail, you owe it to yourself to find a way to rig out the wood in a manner with which you feel comfortable. You might be the only one who actually senses the impending tragedy and I suggest that for that reason alone you need to speak up and not be afraid of losing your job over it. Your life is worth more than a load of wood to me and I don't even know you. I understand and have been in the situation my self and it sucks. I would like to thank everyone who does the testing and makes the results available to us. It's important to know the limits, of the gear but I've got three words for those who love to go big CYCLES...TO...FAILURE. I too love to go big when appropriate, but I have also been told by a couple of owners that, "It's too early to retire that rope, I just bought it blah, blah blah ago." I don't think it's appropriate for a professional to ever be putting him/herself and others, not to mention property, in danger by blowing out rigging points or equipment on the job. My point is it's a lot more economical to work well within the WLL of the piece and have the piece for a long life. Let the test monkeys do the testing and work within your specific parameters. Just want to reiterate that I LOVE going big...when appropriate
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...impact force=weight * fall distance * stopping distance.

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I think the formula isn't quite right, but the idea is. The essential notion is that the ratio of fall distance to stopping distance is related to the ratio of impact force to weight.

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I have edited my earlier post. The spreadsheet was right but the formula in the post should have read impact force=weight * fall distance / stopping distance

sorry for any confusion
 
of cycles to failure. any one got a good system on tracking that? more paperwork. I know it is important, but push for production is always a compromise of safety and training and family, new gear is a compromise of operating cost. sometimes makes me wanna puke.
 
Clearly there is no way to "safely" climb trees with chainsaws and climbing lines...safest thing to do would be to walk away...but that said...I don't have a good system for tracking...each job is different and the variables are often unknown...so...DON'T SHOCK LOAD YOUR SYSTEMS AND IF YOU HAVE TO, RETIRE THE ROPE AND BILL THE COST OF THE MATERIAL INTO THE JOB! If they can't pay for a rope they don't understand the actual cost of doing business and, again, the safest thing to do is walk away and not worry that someone is gonna risk their stupid neck for a low-ball job...take responsibility for your own safety for crying out loud! It's not that hard to say that you're uncomfortable, and in fact I do it all the time. Then there is an opportunity for learning as we discuss how we will do the operation in a way with which I (or even a greenhorn)feel comfortable. Thinking about safety and not worrying about the cost of safety is a good way to stay alive...You must be alive as a prerequisite for doing business, so to be in business it makes good sense to not skimp on safety.
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Probably too late now TreeDoc but given that it's a dead red oak, you can't bend it with causing damage because it's dead. The wood is strong but no longer flexible. In other words, it would be bad to rely on flexing wood to absorb shock. I think the binder is a good idea.
 
I guess I should have mentioned the tracking cycles to failure question was rhetorical it would take a physics genius to keep up with the exact load and forces of every piece of wood that goes on our ropes.

I appreciate the advice and I will admit I have taken jobs I should have turned down. As a survivor of a 32 foot fall even solid trees give me the willies now I question every tip and have to draw into my courage reserve often. However I love what I do and strive to become a better climber and rigger.

as far as the red oak which I finished today (delayed by weather) it was alive. I meant that the decline was evident enough that even the customer needed no convincing that the trees death was eminent and removal was best done sooner than later. Douse the fact that the tree was alive change the answer about the straps? Can they cause more of a hazard or are they always a good Idea?

after cutting more into the tree (hindsight is 20/20) I dont think I needed the straps but I am glad i did and the lower portions of the tree were quite solid I felt silly for being such a sissy.

Next time though I am not going to do a tree like that unless we can get our crain or a lift to it.
 
i think Dave's 2nd formulae, gives good guide, but must include other distances, like deformations (of breakage and elasticity) in chain (line, load, support etc.).

So, we have raw force potential (weight) X distance (acceleration factors)/distance (deaccelration factors). The whole chain matters, becasue for an exerted force to be passed as force, it has to be resisted, otherwise it is just distance; so it is a 2part consideration. All we can do is L-earn the patterns of forces, then orchestrate and dance appropriately with them!
 
I gotta disagree with you here spydey. 4 ft of stopping distance is 4 ft of stopping distance, no matter what causes that final distance, be it stretch, sway or braking. This is not something that can be used to predict in advance but if the formula is correct and the distances can be measured, you should be able to get a pretty close idea of the forces involved.
 
Ummm; i'd think that it wouldn't be so much the distance, but the work/frictions that it took to modify the distance force(so differnt categories?)? And, if we have a braided line, braid running at 30deg, then pull the braid almost straight; do we measure the inline(load) or sideways(braid)distance modified?
 
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do we measure the inline(load) or sideways(braid)distance modified?

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Since Ed Hockuley is not the head ref in the Super Bowl, I think we can rest assured that the Steelers first downs will be called correctly.



SZ
 
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...4 ft of stopping distance is 4 ft of stopping distance, no matter what ...

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This is good as far as it goes, but you are leaving out the force component. The 4 ft. determines the absolute minimum force you will need to stop the load, and that will be when a uniform stopping force is applied from beginning to end. As you say, the details of rope stretch, friction at the block and all the rest of it won't matter.

But what if for the first 3.99 feet I apply 1 lb. of stopping force, and then clench down with 87,000 lbs. force to bring the load to a stop in the last .01 foot? 4 feet is 4 feet, but how you apply the stopping force during those 4 feet has everything to do with the maximum force the rope will experience.
 
Moray, you have a great point. There is definitely a lot missing from the math here at this point. I know that a final equation for this is beyond the power of my brain. If someone comes up with it I would sure like to take a gander though. It is easy to calculate the energy of a falling object, but to figure out the energy dissipation on a rope has given me a headache.
 
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