Trees and Physics

[oldugly]

Calculating the physics of rigging is essential in some ways, very dangerous in others.
Noting the shock load and figuring out the physics to reduce it is, slow or stop the decent of falling wood, brush etc. controlling the landing zone, to eliminate damage, while safely securing the load away from the climber..etc.

These are essential in rigging, but coming up with a formula to push limits is the essential recipe for disaster. Your variables in rigging have very little to do with the rope strength...(any embicle can figure out when a load is too heavy for a rope)..your variables are the tree itself, and the working load and strength of your rigging point/s.

Ways of reducing stress on your rigging points, ways of reducing shock load on your spar, and different methods of strengthening your rigging points.ways of directing your load etc. are very helpful.

Formulas that allow you a false sense of security, and a major potential for injury...very damming, and you are playing with fire. I would much rather trust what I know is overkill, than to calculate what I do not know, and try to base the limits of my work on variables of which I am unclear.

I have very seldom seen men that consider themselves unlearned, or underinformed prior to a big decision, make dumb choices. Its almost always the one that thinks he has it all figured out. (Myself included).

 

[TheTreeSpyder]

Wow, quite a th-read!

While O.Ugly's post might carry some wait hear on the tests and theories; let's not miss the point.

Maximizing safety can sometimes only be done by examinations of pushing the limits to find where they lie, curve and are minimized too. So, even when we study where something breaks; the real study is how not to reach that point after defining it. Also, what variables have different effects, in what directions; and at what times altering said variables has maximum or conversely marginal effects for the expense of your time and efforts; also; when they are actually needed or not.

In the field; we can't stop and calculate everything; but in playing with these things just for observations, we can find and define the patterns of loading and changes; that then we can take the patterns and not the precise math into actual field work.

i see in Moray's test he used fairly inelastic StableBraid; at a very high tensile for the loading; over a very short piece of line before frictions. All 3 factors construction, loading to tensile and short length of line offer very little deformation response. The inversion doubles the fall impact. In this case, a less strong rope; even of the same construction would have given less loading because of more elastic absorption/ dampening of force. Most folks try to place the first catch/ Half Hitch just over the cut; though it might seem counter intuitive; placing this higher/farther from cut is a way to sneak more elastic line length into the system(but all catches/ Half Hitches and Running Bowls should be before the CG). i also like a separate tag line tied to the top of the load, to flex the load farther over on the hinge; to start it's drop later; or perhaps sometimes get total inversion before tearoff; from being 'muscled over'; but this is hard to due with anything of any size. And unless the rigging line was pre-stretched or is slack is being taken up along the way; can still give some impact force. If the line is pretightend well; and the first hitch higher; this can also give a stronger hinge. The more vertical a CG is from the compressed part/pivot of the hinge; the less it forces it's own hinge strong! A stronger hinge with no dutching, can give a much softer catch impact by the line.

Just as fall length gives distance to build force; deformations (elastic and non) give the inverse; distance to dissipate said dynamic forces. Sometimes on short lengths, we have placed a pulley block at base instead of a Porty, then stretched the redirected line across the yard to the Porty. Thus, more elastic line length in the formulae to absorb forces. This can also help to facillitate more tightening, by providing a very good sweating/ swigging in point; and even creatively a http://www.mytreelessons.com/farmer%20snob.jpg type strategy; once again using the less thought of science of perpendicular leverage force on a line. The tighter the line is; the more it resists bending, the more multiplier it presents to the application of perpendicular force to a straight line.

Another factor of deformations; is that of the wooden supports itself; once again whether permanent or elastic deformation; it all absorbs forces/ releases peak steam from the built up pressures. So, what works with one setup; may not work in the next tree, even if the setup is exactly the same!

Another counter-intuitive point; is that though a pulley redirect will give 2:1 on the support; and making it a redirect + 2:1 support on the load will give only 1.5:1 on support (and same as a 3:1 on load + redirect will give 1.3333:1 on support etc.); that is just statically. But, dynamically; especially under short lengths (which at the initial catch/top is shorter length and of peak force); the forces of the more legs to load are increased on the support, not decreased. This is due to less loading per line strength/per leg, once again receiving less elastic response/ dampening. So; in dynamic loading; less tensile can be better; in that it can be deformed more; so absorbs more impacting from all connected parts (knots, supports etc.); but at a cost of cycles to failure. So, the 'macho' view; of the more strength the better can fall short; in neglecting the 'softer' side of rope dynamics; it's elastic response, the wide band between pure static hold and pure failure.

Thanks for a good read all!

 

[cityboy]

What formula or computations are necessary to go from knowing how much an object weighs and how far it falls, to knowing how many ft/lbs of force are generated in the fall?

Hi there, found this thread doing a search on an unrelated topic, thought I'd try to help out a bit. Don't know anything about tree trimming, but can help a bit with the physics part.

Weight is simply a force. A force in physics is equal to mass*acceleration. Gravity causes acceleration, at a rate of about 32 ft/sec^2. This means anything falling for one second (from rest) with have a velocity of 32 ft/sec, after two seconds 64 ft/sec, and so on. This of course ignores air resistance and any other forces that would reduce the acceleration, so in real world applications its not always completely accurate unless you understand the exact nature of the opposing forces.

The part that isn't all that intuitive is (in a vacuum) no additional force is required to maintain a given velocity once something has been accelerated to that velocity. No force is required unless something is accelerated (or decelerated).

So the force you're trying to calculate has to do with how quickly the falling object is decelerated to a stop. Let's say the falling log gets to 64 ft/sec velocity before you try to stop it with the rope. If you stop it in two seconds, you would have applied the same force as gravity (but in the opposite direction) to stop it, or 1G. During that time the force on the rope was twice the normal weight of the object, because you will always have to overcome the 1G force of gravity. If the log is being held stationary in the air by a rope, with no acceleration, the log will always have 1G of gravitational force on it, which is counteracted by 1G of force on the rope. So whatever the deceleration force is, add 1G to that to get the force on the rope. So, if you stop it in 1 second, that would require 2Gs of deceleration force, so the rope sees 3Gs, or 3 times the normal weight.

In practice its going to be difficult to plan exactly how far the object falls before you halt it with the rope, and exactly how much force you apply on the rope, and for how long, and how the force on the rope might vary over time, rope stretch, etc. I think the best advice that was given is to not try to calculate the force precisely, because you will likely be wrong given the lack of control over the situation to reproduce the exact conditions used in the calculation.

If you're interested in a bit more math than I've given here, PM me and I can go into more detail if you like.

 

[TheTreeSpyder]

For those wondering about the Rigging Software by Sherrill and ArborMaster; i have dug up some pics.

Screen shots of Rigging Program

Some Outputs of the Rigging Software with manipulations of key variables

Also, i found these in my stash; that someone posted earlier:

 

[moray]

For those wondering about the Rigging Software by Sherrill and ArborMaster; i have dug up some pics.

Spydie, I really like your old tire shock absorber--very clever.

I assume you own a copy of the rigging software. You could run an interesting experiment. By choosing your input variables carefully, you could run several simulations for a single type of rope, say 1/2 in Stable Braid, and determine whether the software assumes the stress-strain relationship is linear or not. I would lay odds that it does assume linearity.

 

[oldugly]

While not as scientific as the previous posts I have learned and presently use some methods to lessen the load on the rigging point.
By taking one natural friction wrap of the lowering rope around the trunk before the rigging point, I can reduce the force on the block, crotch, etc. by spreading the dynamic of the opposing force and lessening the 1:2 ratio to approx. 1:1.5.
By my groundsperson using a soft-stop I can reduce the impact energy to near nil. Would I trust my life in a situation that would require my predictions to be perfect no.
Using a redirect before my rigging point can also reduce the load on the point of contact...but again, maximizing the load you can lower on any point requires the control of all variables. The variables of the internal dynamics of the tree itself are sometimes completely unknown to you, and therefore beyond any calculation or control.
Regardless of how you calculate, redirect, or reduce load, rigging limbs, and false crotching spars are some of the most dangerous aspects of our game...and should never be pushed to its limits, or approached in a manner of production over safety, rather it should be approached in a reverse order of safety over production.