TIP load

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moss wrote:


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And us? We're non-professional climbers being theoretical on a pro climber message board.

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Just to clarify:


I'm not sure whom you meant by 'us'.

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Talking about me and Ron.
-moss
 
TreeCo,

What you are describing are the effects of dynamic forces. Dynamic forces will still create a 2x effect at the TIP. I think what's happening is we are confusing the claim that the force at the TIP will be twice the weight of the climber. That's true if we have only static loading. If we add dynamic loading then we will see twice the load of the dynamic force plus twice the weight of the climber. In a nutshell, any force, dynamic or static in a trunk-tie will be reflected to the TIP as twice the sum of the dynamic force and the climber's weight.

For example, suppose we have a 200 lb climber and something slips a bit and a dynamic force of 100 lbs is generated. How much force will be exerted at the TIP? We have the weight of the climber, 200 lbs, plus the 100 lbs of dynamic force creating a total of 300 lbs of tension in the rope. The climber is trunk tied so the force at the TIP will be 2x the tension in the rope or 600 lbs. That's three times the weight of the climber, but only 2 times the force in the rope. What happened is the dynamic loading on the rope changed from just the weight of the climber to the weight of the climber plus the dynamic load and it reflected to the TIP as 2x the tension in the rope.

So yes, loading at the TIP is not limited to 2x the weight of the climber when dynamic forces are added. But, the 2x is still in play. What ever force is applied to the climbing side of the rope will produce twice that at the TIP.

While we're at it let's take a quick look at the effect of rope angle. Yesterday I posted that if the tie side of the rope was at a 45° angle it would only make a difference at the TIP of 14.6%, but that was incorrect since it only took vertical loading into account. It really only makes a 7.6% difference!!!

Here's how that works:
Given a 200 lb climber hanging straight down on the climbing side of the rope, and the tie side tied off at a 45° angle, what would the force be at the TIP? For now let's assume the worst case - no friction. Well, obviously the climber exerts 200 lbs and hence there is 200 lbs of tension in the rope. So the TIP has to support the climber's weight plus the force generated by the tie side of the rope at a 45° angle.

Since the climber weighs 200 lbs, the tension throughout the rope is 200 lbs, again disregarding friction for simplicity sake, so there is 200 lbs of force on the rope at 45°. The way this kind of force is dealt with is to break the forces up into their x and y components. So the vertical force produced is 200 * cos(45°) which gives 141.4 lbs in the vertical direction. So the total VERTICAL force is 200 lbs (weight of the climber) plus the vertical component of the rope at 45° for a total of 341.4 lbs at the TIP. That's only 14.6% difference from two vertical lines.

But, that's where I made my mistake yesterday; I stopped with the vertical component and that was incorrect, there is also a horizontal component that adds to the force at the TIP. Since the rope is at 45° the horizontal and vertical forces are equal or 141.4 lbs. So we have a vertical force of 341.4 lbs (200 lb climber weight plus 141.4 lbs vertical component from the tie side) and a horizontal force of 141.4 lbs. Since these are vector forces, i.e. a force at a direction, the resulting force has to be calculated by the Pythagorin theorem. So the square root of 141.4 squared plus the square root of 341.4 squared is 369.5 lbs of force at the TIP. If both sides of the rope were vertical the force would be 400 lbs. The difference is 7.6% less, referenced to 400 lbs.

So, the load on TIPs with trunk ties with vertical ropes can easily approach twice the load exerted on one side of the rope. If dynamic loading increases the force on the climbing side of the rope, that force exerts twice that on the TIP. Even with the rope tied at a 45° angle, the force still comes within 7.6% of the vertical loading of 2x.

I'm tired. But, please, anyone/everyone check my calcuations and theory - that's why I went into such detail.

moss,
I've posted previously that I'm not even in the ball park compared to these guys when it comes to climbing/rigging, etc. But are we talking about climbing/rigging or forces generated in ropes and TIPs? We're talking about forces generated in ropes and TIPs. That deals with laws of physics. Aerospace engineers used physics to design the space shuttle and not one of them has ever been in space.
 
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I'm tired. But, please, anyone/everyone check my calcuations and theory - that's why I went into such detail..

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Just so you know Ron this isn't new stuff for a lot of us. Speaking for myself, I learned vector analysis in the mid seventies in my college electronics courses. I've still got my Texas Instruments SR50 calculator!(it's dead though)

It is new to quite a few and always good to review.....so carry on!
 
TreeCo,

Edit:
After thinking about what you said for a minute, I hope my explainations didn't come across as condescending. That wasn't my intent at all, but sometimes what we say doesn't come out right. My sincere apologies if I've presented this in a condesending way. I sure didn't mean or want to.

And, now back to the regularly scheduled program....

An SR50!!! I should have one of those; I haven't seen that thing in years! I'm gonna have to see if I still have it.

What I really regret is somehow the slide rules I used in college 'got gone'.

A young son came to his father and asked, "Dad, where'd I come from?" The dad, realizing it was time for the 'talk' carefully explained the reproduction process and concluded by asking if his son understood. The son said, "I think so, but I was talking to Johnny and he said he came from the hospital."

Sometimes I wonder if I've given the long answer when the short one was expected.
 
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...We're talking about forces generated in ropes and TIPs. That deals with laws of physics. Aerospace engineers used physics to design the space shuttle and not one of them has ever been in space.

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Most definitely.

So... what we got?

1. Re-verified that two vertical legs on trunk-anchored SRT system causes 2x static loading on the TIP.

2. Dynamic loading from climber motion causes significant load increase over 2x, don't have strictly scientific data here but it appears 3-4x loading can occur from Tod's dyno readings.

3. Ron's numbers show that varying the angle on the anchor leg reduces TIP load but not significantly*

*In item number 3, I think that what the anchor side of the rope goes over can make a big difference in reducing load on the TIP. Imagine multiple branches, some of them potentially higher than the last branch supporting the climber's side of the system. This is more likely in a hardwood tree with wide crown but can happen in a big conifer as well. I think of this as a distributed TIP. As Treespyder pointed out, there is more shock absorption in this scenario, increasing safety but reducing climbing efficiency

4. 2x loading is a static baseline, everything else is about characteristics of individual trees and the climber's ability to assess the quality of TIP.
-moss
 
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TreeCo,

Edit:
After thinking about what you said for a minute, I hope my explainations didn't come across as condescending. That wasn't my intent at all, but sometimes what we say doesn't come out right. My sincere apologies if I've presented this in a condesending way. I sure didn't mean or want to.



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Ron you are doing great! I love to see your enthusiasm for the subject.
 
The trunk-anchored SRT system causes 2x loading at the TIP period. If we're talking static then it's 2x the static load; if we're talking dynamic then it's 2x the dynamic load. It just happens that for a static load, the load at the TIP is 2x the climber's weight because that's the only load involved.

I guess the distinction I'm trying to make is that if we have a DDRT system, the loading at the TIP is 1x the total load on the rope(s). If we add dynamic loading to the DDRT, we could easily see 2 - 3 times the climber's weight at the TIP. But that exact same dynamic loading on a trunk anchor would produce 4 - 6 times the climber's weight at the TIP because of the inherent 2x loading effect of the trunk anchor.

moss,
Your comment about 3. is right on. But it is very, very situational dependent and could be extremely hard to make an accurate estimate without measuring forces with a dyno and hence could be very deceptive. For example, I was quite surprised to see how little difference a 45° angle makes. I was so surprise that I've checked that calculation and theory 5 times and ran it by a colleague as well.

As for point 4, the 2x is the maximum loading that can generated at the TIP. The only question is 2x what? The accurate answer is 2x the load applied to the climbing rope, be it static or dynamic. If we reference the total loading to the weight of the climber, then we could see loading far in excess of 2x the climber's weight. But I still like using the climber's weight as the reference as long as it is understood that whatever force is produced by the climber's weight plus the dynamic loads are multiplied by two at the TIP.

Edit:
Friction AT THE TIP or below the TIP on the climbing side, can reduce loading at the TIP significantly, but again, it depends on the specific situation. Friction on the rope on the tie side of the rope below the TIP will not reduce loading on the TIP. E.g. if the rope is over the TIP and then wrapped around the trunk, the loading at the TIP will not be reduced by that friction.
 
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Ron you are doing great! I love to see your enthusiasm for the subject.

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Thanks TreeCo, but don't hesitate to 'adjust my attitude' where needed -
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Stumper...you have such a clever mind! Thanks for the chuckle!

Pete's article was in Arborist News a number of years ago. It was very enlightening to see how an ineffecient ie: with high friction, changed the load on the anchor point.This is relevant here because we will find that an ineffecient TIP like a natural crotch for SRT should reduce the actual load on the limb at that point.

I'll be that the TB Archivist will be able to link us to Pete's article. If not, I think I have it on my desktop. The catch is that I only have my laptop with me here in Stockholm and the desktop is 5,100 miles away in Dallas...later...

I'm with Moss in this discussion. Loads can be reduced like he said. The actual exact mathematical calculations are less important to me.

During one of these TIP discussions it was shown that the load on the TIP varies in a DdRT setup. The mechanical advantage is variable from 1:1 to maybe 2:1. then add in some dynamics and you can see the loads that TK found using the dyno.

Since we all climb on gear that is designed to hold loads well above anything that our body can survive we should be paying more attention to the TIP loads. bounce testing, gently at first, then going up to two people...at least...bouncing is not a bad idea. This has the potential of giving us some sort of 2:1 up to maybe a 4:1 safety factor on the TIP.

It's rare to see climbers do bounce tests though. This is something that could be added into the Rope Installation step in The Tree Climbers Companion's PREP system.
 
Tom,

I didn't mean to imply we should use mathematical calculations in the field. I was using math to point out how tricky it can be to estimate loads, especially with unknowns. For example, how many have been kinda thinkin' they're gaining a significant margin of safety because the tie line is at an angle, i.e. tied to another tree? Only 7.6% difference with a 45° angle!

As for bouncing, I used to do pretty intense bounce testing and then thanks to moss posting something about high loads being generated from bounce tests, I decided to apply math.

I went to NE rope's website, and followed their procedure for calculating the shock load on 50' of rope by dropping my weight one foot - like dropping suddenly on my harness from a standing position. The shock load was well in excess of 1000 lbs, I just don't remember the exact number, but I remember it was more than 1000 lbs because I knew that most ascenders start to slip at 900 lbs (4kG).

The reason that's very significant is two fold. One, it overstresses the TIP and may set it up for a fatigue failure later, and two most ascenders start to slip at 4kG that's 900 lbs! So we could damage our rope from over bouncing.

I still bounce, but I do it a bit gentler now and sometimes I just rig a 2:1 up to double my weight on the rope. That's a very soft test.
 
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...bouncing is not a bad idea. This has the potential of giving us some sort of 2:1 up to maybe a 4:1 safety factor on the TIP.

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It also has the potential to give us a sore [censored]... if the TIP fails the bounce test catastrophically.
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Now I do the bounce test on a foot lock now... after landing a bit hard when one broke testing it in my harness.
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I test every TIP i set form the ground with as much force as i can manage. Like I said before... breaking out a TIP at height would be just... rude.
 
I also do the bounce test. Keep in mind though the cycles to failure. Maybe you are weekening it to a failure point several bounces later...ie... 1/2 way up the rope?
 
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I also do the bounce test. Keep in mind though the cycles to failure. Maybe you are weekening it to a failure point several bounces later...ie... 1/2 way up the rope?

[/ QUOTE ]Exactly what I'm wondering. How do we know the severe bounce testing didn't produce a crack that's just going to grow as we climb?
 
My opinion of the bounce test is that it's telling you that the branch doesn't have a hidden flaw. It's no guarantee, has to be factored in with the rest of your assessment. Excessively loading the rope (2 or more people) and bouncing hard could be counterproductive (to put it mildly).

I'm with sore butt Blinky, do bounce tests using footlock without being tied into the rope. The worst thing during testing would be land on your butt and then get whacked on the head. Escape potential is a good thing.
-moss
 
Being reasonable with bounce testing is the idea. Unless we do a two person bounce how can we tell what sort of [theoretical] safety factor we have at the TIP?

Is there any evidence that bounce testing has cracked a limb?

Anyone cracked a limb without knowing it? I did once and fell about six feet...with no injury. After I replayed the 'tape' in my head I realized that the branch had cracked but it didn't register in my head that what I heard was a crack. Now I have a more attuned attention for these sort of things.
 
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