Hoping that an engineer can help me with a question

[LashL]

This is a bit of an unusual request but I am hoping that some of the engineering wizards here can help me out.

The hypothetical scenario is this:

Old, redundant hydro wires are being removed along the length of a street.They are attached, of course, from pole to pole down the street. They are removed in sections which are equivalent to the distance between two poles. The sections are removed from east to west. As each section is removed, it is cut on the east side of the pole and lowered to ground. Before the cut is made on the east side of the pole, the wire to the west of the proposed cut is tied off and secured to the pole and J hook to ensure that only the cut portion to the east comes down. After each section on the east is lowered to the ground, the process is repeated while moving on to the next section to the west.

The problem is this:

Let's say that we are now at pole number 5, as we move from east to west, and that the previous sections between poles 1 & 2, between poles 2 & 3, and between poles 3 & 4, have all been lowered to the ground in accordance with the plan. So, now the section between poles 4 and 5 is on the ground at the easterly end but is still attached to pole 5 at height. To remove the section between poles 4 & 5, we secure the wire to pole 5 on the west side of where we are going to cut, and then cut on the east side of pole 5, so that the remaining part of the section between 4 & 5 will fall to the ground to join the easterly part of that section which is already on the ground.

However, something goes wrong and the wire not only falls to the east as planned but also falls to the west. On the west side, it hits a person who is situated between pole 5 and pole 6. The wire still remains attached at height, though, to pole 6.

The question is this:

How would one go about calculating the speed and acceleration that the wire fell and how would one go about calculating the force with which the wire hit the person?

(I can provide details of height, weight, circumference of the wire, distance, etc. for actual calculations as well, but I would really like to learn what criteria an engineer would require to figure out the answers and what formulae would be used to figure out the answers.)

Thank you in advance for your assistance.

Edit to add: As an aside, the reason I'm asking this is because I received a report late Friday afternoon that purports to calculate the items mentioned above, but the report strikes me as wholly, grossly, and flagrantly inadequate. I will be hiring an expert to respond to that report, but it strikes me as so hopelessly inept that it is bugging me, even on the weekend. Thus, I hope to get some input from engineers here on what information you would need and what formulas you would utilize to tackle the questions.

[cooper1958nc]

Sounds like you represent the injured party. Not a problem.

I don't quite understand the details of the wire fall. Can we simplify the situation? Did the wire fall from one pole?

It is possible to model the fall of a rigid beam supported on one side and falling in a vacuum. The beam rotates at one support as it accelerates downward. The gravitational force is considered acting at the center of mass. The angular acceleration is inversely proportional to the angular inertia. The speed of the beam at impact depends on the height. The part of the beam, i.e. how far from the fulcrum, impacts the victim, does matter.

If one knows the type of beam, it might be possible to estimate the effect of air resistance. Very heavy beams falling relatively small distances are not affected much by air resistance. Light beams falling far are.

A nonrigid wire introduces another level of uncertainty. Wires do not fall as a unit, and estimating their air resistance would be a challenge. Insulated wires are pretty light for the surface area. Perhaps some empirical measurements have been made on wires falling in air.

I would think it would be easier, frankly, to do a test. Mount the wire on an exemplar pole. Photograph the fall and compute the speed. The impact force depends on a lot of hard to measure things, like how quickly the wire is stopped by the victim. Probably it would be better to test that also, on something that acts like the victim's body when struck.

Don't the injuries speak for themselves?

[LashL]

Sounds like you represent the injured party. Not a problem.

Actually, no. I'm on the other side. And just so that it is clear, I am just trying to figure out what a legitimate expert would require in order to assess this situation in order to write a report (since the one I received appears to be totally looney tunes, even to my non-engineering mind). I am trying to figure out, for my own edification, what factors, distances, numbers, and information a legitimate engineer would require in order to make a realistic assessment of the events because, frankly, reading this report makes me doubt the writer's qualifications in their entirety and makes me think that he's a full blown woo of some kind.

I don't quite understand the details of the wire fall. Can we simplify the situation? Did the wire fall from one pole?

Yes. Only one end of the wire fell. The other end remained attached to the next pole to the west.

It is possible to model the fall of a rigid beam supported on one side and falling in a vacuum. The beam rotates at one support as it accelerates downward. The gravitational force is considered acting at the center of mass. The angular acceleration is inversely proportional to the angular inertia. The speed of the beam at impact depends on the height. The part of the beam, i.e. how far from the fulcrum, impacts the victim, does matter.

This is helpful, even though it was not a rigid beam. Some of the things that surprised me so much about this report are that there was no mention of the fact that only one end of the cable fell, no consideration of the location of the fellow who was struck by the wire, no consideration of the fulcrum, no consideration of the fact that the wire may very well have (and probably did) hit the ground first before striking the fellow, no measurements taken by the so-called "expert", etc.

In the report, the falling wire was treated as though it was a solid beam, unsupported at either end, being dropped much like one might imagine dropping a pencil held like this <=========o from a height of an inch or two from a desktop. But it was a semi-rigid cable that could not and did not fall in the manner of a solid beam, and that was still attached to a pole on one end.

It seems to me that all of the calculations this guy made were based on outrageously wrong premises, and ell completely vertically as if it was a completely distance of the victim from the fulcrum. I couldn't figure out, in my admittedly non-engineer mind, how one could assess the alleged force of impact without even accounting for the location of the person viz the falling wire.

If one knows the type of beam, it might be possible to estimate the effect of air resistance. Very heavy beams falling relatively small distances are not affected much by air resistance. Light beams falling far are.

I don't have the specific piece of cable from the incident, but can work out the weight from the sample that I do have. A 210 inch (17.5 ft.) sample of it weighs ~7.5 pounds. The actual piece that was being removed was ~100 ft. in length, and it was approximately 20 feet above the ground prior to being removed.

Given the height of only 20 feet, I suspect that air resistance would not have had much of a role, and I am content to disregard it for purposes of this discussion if it is negligible.

A nonrigid wire introduces another level of uncertainty. Wires do not fall as a unit, and estimating their air resistance would be a challenge. Insulated wires are pretty light for the surface area. Perhaps some empirical measurements have been made on wires falling in air.

This is also helpful, thank you.

I would think it would be easier, frankly, to do a test. Mount the wire on an exemplar pole. Photograph the fall and compute the speed. The impact force depends on a lot of hard to measure things, like how quickly the wire is stopped by the victim. Probably it would be better to test that also, on something that acts like the victim's body when struck.

Yes, this makes good sense, of course. I can and will have a recreation done, as you suggest, but my purpose in asking the questions that I have asked above is to try to comprehend this report which strikes me as completely bogus, and to try to understand what information, numbers, measurements, etc. would be required by a legitimate expert to come up with calculations of speed and acceleration of descent, and more importantly, force of impact, without recreating the scene, since this purported expert did not do any such thing. In other words, I want to understand what he should have done, what information he should have sought, what measurements he should have taken, etc., in order to give an informed expert opinion.

Don't the injuries speak for themselves?

I certainly think so. He received not a cut, not a scratch, not even a bruise. He didn't fall down, he didn't even fall to his knees, he didn't lose consciousness, he suffered no physical injury at all. But he now claims that he suffered a severe brain injury.

Which is where the force calculations come in. This writer of the report says that the force of the wire hitting the fellow was something in excess of 3,787 pounds of force per square inch, and that the contact location was ~ 0.62 square inches.

I have no idea what that means in real terms, but it sounds highly dubious to me that someone could be hit with that kind of force over a very small area and not even have a bruise, cut, scrape, fall, etc.

Anyway, as I said, this is just bugging me because I don't think I have ever seen such a terrible "expert" report - on any subject matter, and boy, I've seen a lot of them on a myriad of topics - to date. (There is a lot more crap in it that leads me to that conclusion, which is not relevant to this thread.)

Thank you for your assistance, cooper.

[WDB]

To get a figure for the actual force of impact like that you would need to calculate the rate of deceleration of the wire upon landing on the man. I'd say this would be extremely difficult as it would involve all sorts of assumptions about the softness of the man's head and the stiffness of his neck and even his knees cushioning the impact.

[Modified]

Which is where the force calculations come in. This writer of the report says that the force of the wire hitting the fellow was something in excess of 3,787 pounds of force per square inch, and that the contact location was ~ 0.62 square inches.

From what I can gather from a quick web search, that (the total force) is a bit more than the force necessary to break the average person's skull at the sides or back.

He received not a cut, not a scratch, not even a bruise.Certainly it is possible to be struck over a large area of the skull and experience brain damage without any visible cut or bruise, but with such a small contact area that would seem unlikely.

[mhaze]

Edit to add: As an aside, the reason I'm asking this is because I received a report late Friday afternoon that purports to calculate the items mentioned above, but the report strikes me as wholly, grossly, and flagrantly inadequate. I will be hiring an expert to respond to that report, but it strikes me as so hopelessly inept that it is bugging me, even on the weekend. Thus, I hope to get some input from engineers here on what information you would need and what formulas you would utilize to tackle the questions.


You need a drawing.

Lawyers tend to think in grammer, but this is a classic case of a problem in which - first of all - an accurate sketch is made - and then the force vectors are calculated.

That is how to start with the engineering method. Also, oftentimes drawing the little simple-but-accurate sketch is not so simple.:)

[rwguinn]

LashL:
Examining the worst-case "distance toward pole 6" from pole 5 , the distance is the height of the attachment to the pole.
That will be reduced somewhat because the wire will likely not lie on the ground at the bottom of pole 5 and make a 90 degree angle up to the pole 5 attachment, but the line will actually form a curve.
Air resistance will be neglegible, unless the pole is very tall, and can generally be ignored.
Velocity will be about 8.025*(sqrt(h)), in feet/sec (assuming h is in feet). That is at the very end of the wire.
What you will need is line material, diameter, length (total), pole height, and the type of surface it is laying on, as well as the line coating type and condition (for friction)--this, along with weight (from material and diameter), will help get you the distance from pole 5 the line actually leaves contact with the ground so you can calculate the true distance past pole 5 it will actually fall

[rwguinn]

You need a drawing.

Lawyers tend to think in grammer, but this is a classic case of a problem in which - first of all - an accurate sketch is made - and then the force vectors are calculated.

That is how to start with the engineering method. Also, oftentimes drawing the little simple-but-accurate sketch is not so simple.:)
What mhaze said...

[Gord_in_Toronto]

I am not an engineer and would not dare to play one on the Internet.:D But as a humble physicist, I note the the original calculations represent a worst possible case interpretation. Taking account of all the factors noted to date will produce a smaller figure for the force of impact. You don't necessarily need to include them all in your calculations; just as many as it takes to reduce the force to something "negligible".

[DanishDynamite]

No expert on this area at all, but aren't these wires under tension? I suspect the effect of even a fairly small amount of released tension would outweigh the effect of gravity.

[LashL]

Thank you, all of you, for your help.

I will scan in a drawing, as suggested, and will post it with the measurements, distances, etc.

The wire is normally under tension, yes, when it is in use. However, prior to removing the wire, the workers had already released the tension of the entire span that was to be removed, before they cut the first section. They do that precisely so that the sections come down without going flying due to tension.

I appreciate all of your assistance, and will try to get the drawing scanned in this afternoon.

[rwguinn]

No expert on this area at all, but aren't these wires under tension? I suspect the effect of even a fairly small amount of released tension would outweigh the effect of gravity.

Any tension on the wire in question is a result of the weight of the suspended portion of the wire and friction between the wire and ground.
One end has already been cut and is laying on the ground.

ETA:
Whoops==looks like that is wrong. If I understand LashL's post correctly, both ends were still attached to poles, and upon cutting, the cut end went away from the load direction?
I definitely need a picture!

[LashL]

Okay, here is a crude sketch of the scene.

The cable is approximately 2 cm (0.79 inch) in diameter.

According to the report, the sample piece is 532 cm. (210 inches) long and weighs ~3500 grams (~7.5 pounds) *note, by my calculation, 3500 grams = ~7.7 pounds.

The wire was attached to the poles at a height of ~20 feet.

The man was standing ~30 feet east of pole #6, ~70 feet west of pole #5.

The man is ~5'8" tall.

The distance between poles is ~100 feet.

http://i229.photobucket.com/albums/ee129/LashL2007/sketch.jpg

When they cut the wire at pole #5, the part to the east went down as intended and simply dropped to the ground to the east of that pole.

But the section of wire between pole 5 and 6 also went down, striking the man. The wire did, however, remain attached to pole 6 at all times.

[mhaze]

Actually, no. I'm on the other side. And just so that it is clear, I am just trying to figure out what a legitimate expert would require in order to assess this situation in order to write a report (since the one I received appears to be totally looney tunes, even to my non-engineering mind). I am trying to figure out, for my own edification, what factors, distances, numbers, and information a legitimate engineer would require in order to make a realistic assessment of the events because, frankly, reading this report makes me doubt the writer's qualifications in their entirety and makes me think that he's a full blown woo of some kind.



Yes. Only one end of the wire fell. The other end remained attached to the next pole to the west.



This is helpful, even though it was not a rigid beam. Some of the things that surprised me so much about this report are that there was no mention of the fact that only one end of the cable fell, no consideration of the location of the fellow who was struck by the wire, no consideration of the fulcrum, no consideration of the fact that the wire may very well have (and probably did) hit the ground first before striking the fellow, no measurements taken by the so-called "expert", etc.

In the report, the falling wire was treated as though it was a solid beam, unsupported at either end, being dropped much like one might imagine dropping a pencil held like this <=========o from a height of an inch or two from a desktop. But it was a semi-rigid cable that could not and did not fall in the manner of a solid beam, and that was still attached to a pole on one end.

It seems to me that all of the calculations this guy made were based on outrageously wrong premises, and ell completely vertically as if it was a completely distance of the victim from the fulcrum. I couldn't figure out, in my admittedly non-engineer mind, how one could assess the alleged force of impact without even accounting for the location of the person viz the falling wire.



I don't have the specific piece of cable from the incident, but can work out the weight from the sample that I do have. A 210 inch (17.5 ft.) sample of it weighs ~7.5 pounds. The actual piece that was being removed was ~100 ft. in length, and it was approximately 20 feet above the ground prior to being removed.

Given the height of only 20 feet, I suspect that air resistance would not have had much of a role, and I am content to disregard it for purposes of this discussion if it is negligible.



This is also helpful, thank you.



Yes, this makes good sense, of course. I can and will have a recreation done, as you suggest, but my purpose in asking the questions that I have asked above is to try to comprehend this report which strikes me as completely bogus, and to try to understand what information, numbers, measurements, etc. would be required by a legitimate expert to come up with calculations of speed and acceleration of descent, and more importantly, force of impact, without recreating the scene, since this purported expert did not do any such thing. In other words, I want to understand what he should have done, what information he should have sought, what measurements he should have taken, etc., in order to give an informed expert opinion.



I certainly think so. He received not a cut, not a scratch, not even a bruise. He didn't fall down, he didn't even fall to his knees, he didn't lose consciousness, he suffered no physical injury at all. But he now claims that he suffered a severe brain injury.

Which is where the force calculations come in. This writer of the report says that the force of the wire hitting the fellow was something in excess of 3,787 pounds of force per square inch, and that the contact location was ~ 0.62 square inches.

I have no idea what that means in real terms, but it sounds highly dubious to me that someone could be hit with that kind of force over a very small area and not even have a bruise, cut, scrape, fall, etc.

Anyway, as I said, this is just bugging me because I don't think I have ever seen such a terrible "expert" report - on any subject matter, and boy, I've seen a lot of them on a myriad of topics - to date. (There is a lot more crap in it that leads me to that conclusion, which is not relevant to this thread.)

Thank you for your assistance, cooper.

1. Did the guy have a construction helmet on? If so, it would seem possible to have no obvious trauma but to have a concussion.

2. Medical reports should show such a thing if it had occurred.

3. I hate to say this but the wire could have acted like a whip.

[LashL]

1. Did the guy have a construction helmet on? If so, it would seem possible to have no obvious trauma but to have a concussion.

2. Medical reports should show such a thing if it had occurred.

3. I hate to say this but the wire could have acted like a whip.

1) No, he was not a worker. He was just a guy standing on the boulevard beneath the wire, who refused to move when asked to do so.

2) Medical reports show only "minor head injury" and "muscle strain to neck", "no bruises, no swelling, no lacerations, no loss of consciousness", and no treatment was ordered except to apply heat to his neck.

3) No need to hate to say it. :) I am just after the truth and the facts.

[cooper1958nc]

If 210 inches weighs near 8 lb, then 1200 inches (100') weighs like 40 lbs. A 40 lb object falling 20 feet can injure. Not all 40 lbs fell, true. An analytic solution is difficult because wire can act chaoticly. Local areas can move at faster speeds than others. Nor will a single test tell much. It is a good problem to consider more.

[Tez]

Well the expert is a moron, so you're on safe ground.

Imagine it *was* a rigid rod stretched between the poles. Then when it falls it pivots (since its pinned at the top of pole 6), like a pendulum. The free end of the pendulum (the piece which was attached at the top of pole 5) hits the ground long before anything hits the guy (unless he's about 14 feet tall). So you can discount his analysis by that simple observation....

[Jekyll]

1) No, he was not a worker. He was just a guy standing on the boulevard beneath the wire, who refused to move when asked to do so.

2) Medical reports show only "minor head injury" and "muscle strain to neck", "no bruises, no swelling, no lacerations, no loss of consciousness", and no treatment was ordered except to apply heat to his neck.

3) No need to hate to say it. :) I am just after the truth and the facts.
I think any whipping effect is unlikely as the wire as not under tension and the tip of it did not touch him.
One thing that's always worth doing in these kind of situations is to take the assumptions the other guy makes and to apply them to what else you know about the situation.

So you're expected to believe that if a solid beam of weight ____pounds fell 94 feet and landed on the guy, he wouldn't move.

It can provide a quick and easy test of how plausible the assumptions are.

[LashL]

If 210 inches weighs near 8 lb, then 1200 inches (100') weighs like 40 lbs. A 40 lb object falling 20 feet can injure. Not all 40 lbs fell, true. An analytic solution is difficult because wire can act chaoticly. Local areas can move at faster speeds than others. Nor will a single test tell much. It is a good problem to consider more.

I find it fascinating myself, even though I do not have the technical background to figure out the appropriate tests to apply. I anticipate doing a "re-construction" of the event to see what actually happens when we recreate the circumstances, but since I received this report on Friday afternoon, I have been curious about the calculation methods, etc.

Well the expert is a moron, so you're on safe ground.

Imagine it *was* a rigid rod stretched between the poles. Then when it falls it pivots (since its pinned at the top of pole 6), like a pendulum. The free end of the pendulum (the piece which was attached at the top of pole 5) hits the ground long before anything hits the guy (unless he's about 14 feet tall). So you can discount his analysis by that simple observation....

Yes, he is a moron. He is not considering at all the fact that the wire was only detached from pole #5 and NOT from pole #6. He has treated it as though the cable was a rigid rod that fell in "free fall" parallel to the ground with no support on either side, like this:

http://i229.photobucket.com/albums/ee129/LashL2007/sketch2.jpg

I know how wrong he is, and I know that I can easily refute his report, but it has made me curious about it would accurately be calculated. :)

I think any whipping effect is unlikely as the wire as not under tension and the tip of it did not touch him.
One thing that's always worth doing in these kind of situations is to take the assumptions the other guy makes and to apply them to what else you know about the situation.

So you're expected to believe that if a solid beam of weight ____pounds fell 94 feet and landed on the guy, he wouldn't move.

It can provide a quick and easy test of how plausible the assumptions are.

Indeed. :)

[Tez]

it would be tricky to calculate accurately - once the one end of the wire hits the ground, quite how it impacts the guy will depend on how flexible the cable is. For instance if its not flexible at all (the "beam" limit) then it never hits him. If there is a lot of friction between the cable and the ground it may also not hit him at all!

Something can probably be computed in a worst case scenario by using a model of the cable as like a chain with independent links, but I'd have to think about it.

[LashL]

it would be tricky to calculate accurately - once the one end of the wire hits the ground, quite how it impacts the guy will depend on how flexible the cable is. For instance if its not flexible at all (the "beam" limit) then it never hits him. If there is a lot of friction between the cable and the ground it may also not hit him at all!

I understand what you are saying with regard to the difficulty in calculating it accurately. I do not understand the significance of the friction between the cable and the ground, but I do know that the cable did actually strike him, although somewhat glancingly as best as can be determined. The cable is somewhat rigid, as it is consists of a bundle of metal wires encased in a hard rubber sheathing, and if you held an 18 inch piece of it in your hand, it would make a pretty effective baton-like weapon. However, longer lengths are easily coiled, so it is also flexible. (I realize this doesn't help without being able to quantify its rigidity or its flexibility, but I add this only for context.)

Something can probably be computed in a worst case scenario by using a model of the cable as like a chain with independent links, but I'd have to think about it.

Ahh, that sounds interesting. Thank you!

[SezMe]

The wire can be modeled as a chain. Assuming a rigid body leads, I think, away from a reasonable approximation.

Such a model is not complex at all. I would think numerous numerical models are available for free.

ETA: Tez beat me to the chain analogy. This is an upperclass applied mechanics problem. Or Masters level at most. It is NOT a hard problem.

[rwguinn]

it would be tricky to calculate accurately - once the one end of the wire hits the ground, quite how it impacts the guy will depend on how flexible the cable is. For instance if its not flexible at all (the "beam" limit) then it never hits him. If there is a lot of friction between the cable and the ground it may also not hit him at all!

Something can probably be computed in a worst case scenario by using a model of the cable as like a chain with independent links, but I'd have to think about it.
Yeah--the flexibility issue is kinda important here. Now that I see the scenario, it gets a bit wierd.
Ifthe whole thing free-fell, max velocity is about 30ft/sec. This is going to be slowed a bit by flexibility, because the entire wire is going to want to rotate about the fixed end until the free end hits the gound. This is modified by the flexibility to some extent, but a rigid body rotation is not a bad approximation.
Then, after the end hits the ground, it will be dragged across the ground toward the still-attached pole at an acceleration much lower than what falling would cause, due to friction with the ground, and the stiffness of the wire itself. It will have an initial velocity laterally--36 ft/sec*sin (20/100), or about 10.6 ft/sec.
If we assume frictionless and absolute wimpiness (dishrag stiffness), the max velocity it hits him with is sqrt(10.6^2 + 30^2) feet sec, which is about 1/4 the speed of the Lubbock, Tx Little League pitcher's fastball . (he throws about 80 mph)

The opinions and calculations above are based on assumptions, but should be an absolute worst-case scenario. I welcome corrections and critiques.

ETA==> the FREE end hits at 36 fps. the 30fps number takes into account the guy's 5'8" height...

[LashL]

The wire can be modeled as a chain. Assuming a rigid body leads, I think, away from a reasonable approximation.

Such a model is not complex at all. I would think numerous numerical models are available for free.

ETA: Tez beat me to the chain analogy. This is an upperclass applied mechanics problem. Or Masters level at most. It is NOT a hard problem.

Well, I'm not an upperclass or Masters applied mechanics student, of course. :)

I will be retaining a properly qualified expert to do a complete analysis and report, but as mentioned previously, I am curious about the mechanics of the scenario and my inquisitive nature led me to make inquiries via this thread. I love the flow of these discussions and seeing "science in action" so to speak.

[LashL]

Yeah--the flexibility issue is kinda important here. Now that I see the scenario, it gets a bit wierd.

If the whole thing free-fell, max velocity is about 30ft/sec. This is going to be slowed a bit by flexibility, because the entire wire is going to want to rotate about the fixed end until the free end hits the gound. This is modified by the flexibility to some extent, but a rigid body rotation is not a bad approximation.

Then, after the end hits the ground, it will be dragged across the ground toward the still-attached pole at an acceleration much lower than what falling would cause, due to friction with the ground, and the stiffness of the wire itself. It will have an initial velocity laterally--36 ft/sec*sin (20/100), or about 10.6 ft/sec.

If we assume frictionless and absolute wimpiness (dishrag stiffness), the max velocity it hits him with is sqrt(10.6^2 + 30^2) feet sec, which is about 1/4 the speed of the Lubbock, Tx Little League pitcher's fastball . (he throws about 80 mph)

The opinions and calculations above are based on assumptions, but should be an absolute worst-case scenario. I welcome corrections and critiques.

ETA==> the FREE end hits at 36 fps. the 30fps number takes into account the guy's 5'8" height...

Thank you, rwguinn!

A couple of questions, if you don't mind?

How would I translate ft/sec into mph?

Is it accurate to say that "sqrt(10.6^2 + 30^2) feet sec" is ~20 mph?

Am I understanding correctly in thinking that as the rigidity of the cable increases (from dishrag stiffness --> starched dishrag stiffness), the lower the lateral velocity to the point of impact with the guy?

ETA: Following from the calculations above, how does one calculate the force (in newtons or pounds) exerted upon the guy's head by the cable?

[rwguinn]

Thank you, rwguinn!

A couple of questions, if you don't mind?

How would I translate ft/sec into mph?

88 feet/sec=60 mph


Is it accurate to say that "sqrt(10.6^2 + 30^2) feet sec" is ~20 mph?that value is 32.2 ft/sec, which is 21.9 mph...

Am I understanding correctly in thinking that as the rigidity of the cable increases (from dishrag stiffness --> starched dishrag stiffness), the lower the lateral velocity to the point of impact with the guy?actually, both velocities will be reduced, but they are also both dependent on friction.

ETA: Following from the calculations above, how does one calculate the force (in newtons or pounds) exerted upon the guy's head by the cable?There is where the calculations get complicated--see all the stuff in the CT forum on "Energy" and "force"
Force is a function of the mass involved--how much of the cable do you assume is involved--a value certainly far less than the whole cable length of 100 feet--, and how long it took to decelerate. It is complicated by the fact that the guy moved--he had to, since he is neither rigid nor welded to the ground, in addition to reflexive actions..
The force required to break bones--skulls, collar bones, arms,etc, and to cause bruising is certainly defined in the literature somewhere. It was obvously less than that. Surely you, of all people, have access to some forensics folks?:D..

[LashL]

88 feet/sec=60 mph
that value is 32.2 ft/sec, which is 21.9 mph...

Thank you.

actually, both velocities will be reduced, but they are also both dependent on friction.

Ah, yes, of course.

There is where the calculations get complicated--see all the stuff in the CT forum on "Energy" and "force"

Force is a function of the mass involved--how much of the cable do you assume is involved--a value certainly far less than the whole cable length of 100 feet--, and how long it took to decelerate. It is complicated by the fact that the guy moved--he had to, since he is neither rigid nor welded to the ground, in addition to reflexive actions..

The force required to break bones--skulls, collar bones, arms,etc, and to cause bruising is certainly defined in the literature somewhere. It was obvously less than that. Surely you, of all people, have access to some forensics folks?:D..


Thank you. This is all very helpful.

And yes, I have carte blanche to hire all the experts that I need. I'm just impatient and wanted to figure some things out over the weekend. That said, I need not go any deeper into the actual force calculations now. I will leave it to the hired guns, since it's Sunday evening now. :D


Thank you very much to all of you for the excellent, useful, enlightening, and educational posts, folks. I really appreciate it!

[Tez]

I'm sure if you were really, really nice to rwguinn he could produce a plot with tradeoff curves of friction, stiffness etc. It would be a useful double check on your experts who may, in contrast to their guy who oversimplified, go to the other extreme and produce way too complicated calculations (to justify their expense). I've only ever been informally consulted about one legal thing along these lines, and that was exactly what happened. IIRC the jury went for the simpler calculation they could understand.

(If I was teaching undergrad mechanics I'd set it as a problem, and then we could average all their answers, it would probably be quite accurate despite a huge standard deviation!)

[rwguinn]

I'm sure if you were really, really nice to rwguinn he could produce a plot with tradeoff curves of friction, stiffness etc. It would be a useful double check on your experts who may, in contrast to their guy who oversimplified, go to the other extreme and produce way too complicated calculations (to justify their expense). I've only ever been informally consulted about one legal thing along these lines, and that was exactly what happened. IIRC the jury went for the simpler calculation they could understand.

(If I was teaching undergrad mechanics I'd set it as a problem, and then we could average all their answers, it would probably be quite accurate despite a huge standard deviation!)


Actually, I have sorta discussed this sort of thing with LashL before, and the reasons I do not make a good expert witness. I have given her enough to go to somebody who is both a good engineer and capable of verbally defending the situation.
I don't do well with non-engineers...

ETA: I would, however, be interested in what such an expert has to say and how things work out.

[LashL]

I'm sure if you were really, really nice to rwguinn he could produce a plot with tradeoff curves of friction, stiffness etc. It would be a useful double check on your experts who may, in contrast to their guy who oversimplified, go to the other extreme and produce way too complicated calculations (to justify their expense). I've only ever been informally consulted about one legal thing along these lines, and that was exactly what happened. IIRC the jury went for the simpler calculation they could understand.

(If I was teaching undergrad mechanics I'd set it as a problem, and then we could average all their answers, it would probably be quite accurate despite a huge standard deviation!)

You make a very good point about over-complicating vs. over-simplifying. In this case, the trial will be heard by a judge without a jury, so I am not too worried about it. Still, I will be cognizant of ensuring that a responding expert not over-complicate the matter.

Actually, I have sorta discussed this sort of thing with LashL before, and the reasons I do not make a good expert witness. I have given her enough to go to somebody who is both a good engineer and capable of verbally defending the situation.
I don't do well with non-engineers...

ETA: I would, however, be interested in what such an expert has to say and how things work out.

I really appreciate your help, rwguinn; you have, indeed, given me what I need to go to an appropriate expert.

Also, I have a pre-trial conference in this matter on Friday and you have given me enough ammunition to seriously criticize the report to the judge who presides at the pre-trial. (Since the other side just served the report on Friday afternoon, in non-compliance with the rules, the judge will let me comment on it without requiring me to have a responding expert report at the ready.)

The trial is scheduled to begin in September, but if the other side wants to use this report or call its author as a witness, they will have to ask for an adjournment because the rules require that a new expert report be served a minimum of 90 days before trial, and when they served it on Friday, it was only 37 days before trial. So, I guess I will find out this Friday what their intentions are. If they do seek and obtain an adjournment, as it appears they will, then I will have an expert on board next week.

I will certainly let you know how it all plays out.

[Dan O.]

You are wrong to say the cable is not under tension. There is a minimum of 20 feet of cable pulling on the free end with about 8.6lb force. But this would mean the remaining cable would sag 103 feet (according to this calculator (http://www.spaceagecontrol.com/calccabl.htm?F=8.6&a=100&q=.43&g=32.18503937&Submit+Button=Calculate))

Even if 100 ft of cable were providing tension, the sag would be almost 13 feet or more than half way to the ground. You should have photos from after the event from which you can estimate the sag on the remaining wire.

Also consider, as the tension is released, some of the cable is going to be pulled over poll 6, 7, 8 etc. depending on the friction and stiffness.

If this were a lighter cable that could have whipped between the guys legs, he may have been a candidate for a darwin award.

[Jimbo07]

A 210 inch (17.5 ft.) sample of it weighs ~7.5 pounds.


Assuming some standard conversions, that works out to 3.3 kg/5.3 m, or a linear density of ~0.62 kg/m. This is an interesting problem. For a first order approximation, I'd start with the beam idea. Next, I'd move on to the chain idea, assuming it started in a rigid position. Someone could probably get down to finite element analysis, but maybe that's over-complicating the problem!

but... I am not yet a professional engineer...

Have you covered all the legal details? Is this opposing 'expert' a registered professional engineer in that jurisdiction? You should be able to check, if they have stamped the report. If you're in Ontario, you can start with PEO (http://www.peo.on.ca/) for contact info.

[rwguinn]

You are wrong to say the cable is not under tension. There is a minimum of 20 feet of cable pulling on the free end with about 8.6lb force. But this would mean the remaining cable would sag 103 feet (according to this calculator (http://www.spaceagecontrol.com/calccabl.htm?F=8.6&a=100&q=.43&g=32.18503937&Submit+Button=Calculate))

Even if 100 ft of cable were providing tension, the sag would be almost 13 feet or more than half way to the ground. You should have photos from after the event from which you can estimate the sag on the remaining wire.

Also consider, as the tension is released, some of the cable is going to be pulled over poll 6, 7, 8 etc. depending on the friction and stiffness.

If this were a lighter cable that could have whipped between the guys legs, he may have been a candidate for a darwin award.

All tension is due to weight. It goes away immediately the cable is cut.
I ignored sag, as it can only reduce the velocity with which the cable hit the guy. In fact, the worst case scenario is a 30' long cable swinging down and hitting the guy. However, that is not something that could actually happen, because the stiffness of the cable does exist.

[rwguinn]

Assuming some standard conversions, that works out to 3.3 kg/5.3 m, or a linear density of ~0.62 kg/m. This is an interesting problem. For a first order approximation, I'd start with the beam idea. Next, I'd move on to the chain idea, assuming it started in a rigid position. Someone could probably get down to finite element analysis, but maybe that's over-complicating the problem!

but... I am not yet a professional engineer...

Have you covered all the legal details? Is this opposing 'expert' a registered professional engineer in that jurisdiction? You should be able to check, if they have stamped the report. If you're in Ontario, you can start with PEO (http://www.peo.on.ca/) for contact info.

I had not looked at that. That is wierd. Something is wrong there--that's a density of .07 lb/in^3,

(.078in/2)^2*pi*210=102.25 in^3, divided into 7.5 lb) Aluminum is 0.1 lb/in^3...

[Blue Mountain]

I had not looked at that. That is wierd. Something is wrong there--that's a density of .07 lb/in^3,

(.078in/2)^2*pi*210=102.25 in^3, divided into 7.5 lb) Aluminum is 0.1 lb/in^3...

The wire may be hollow. (Just a guess; I'm not a linesman!)

[Dan O.]

All tension is due to weight. It goes away immediately the cable is cut.
I ignored sag, as it can only reduce the velocity with which the cable hit the guy. In fact, the worst case scenario is a 30' long cable swinging down and hitting the guy. However, that is not something that could actually happen, because the stiffness of the cable does exist.

I took from the OP that the cable is not tied at each pole (or had been untied) because they were able to remove the original tension. At some point, there is a free end on the ground and the rest of the cable is suspended from the remaining polls. The tension on the cable at this time is limited to the weight of the free end which is a maximum of 100 feet+/-. Where the free end is dragging on the ground, it can only contribute it's weight times the coefficient of friction which is probably less than 1. Using the weight of 100 feet of cable will give a reasonable upper limit on the cable tension which then defines the maximum hight of the low point in the cable sag between successive polls.

Unless something has been left out of the story, I estimate that the cable only had 18 inches or less to fall before striking the man.

[fishbob]

1) No, he was not a worker. He was just a guy standing on the boulevard beneath the wire, who refused to move when asked to do so.


Unfortunately, if this part of the situation is accurate, I think your engineering calculations are pretty much irrelevant.

This will boil down as follows:

Your crew recognized a hazard.
Your crew proceeded with cutting while the guy was still in the location of the recognized hazard.
Personal injury attorneys love this kind of situation.

Sorry, because if everything else in your description is accurate, this guy is a dipwad and deserved to be knocked senseless.

[rwguinn]

Unfortunately, if this part of the situation is accurate, I think your engineering calculations are pretty much irrelevant.

This will boil down as follows:

Your crew recognized a hazard.
Your crew proceeded with cutting while the guy was still in the location of the recognized hazard.
Personal injury attorneys love this kind of situation.

Sorry, because if everything else in your description is accurate, this guy is a dipwad and deserved to be knocked senseless.:D:D:D
yeah, but, but....

Who cares about the truth?
I recollect a bus accident. 45 passenger bus. Driver was on lunch, so he parked it and went to the back for a nap. brake wasn't set, it rolled down the hill and hit a building. No injuries, just property damage.
nearly 100 people filed claims as having been passengers...

[LashL]

I had not looked at that. That is wierd. Something is wrong there--that's a density of .07 lb/in^3,

(.078in/2)^2*pi*210=102.25 in^3, divided into 7.5 lb) Aluminum is 0.1 lb/in^3...

The wire may be hollow. (Just a guess; I'm not a linesman!)

I suspect that the discrepancy is due to the fact that I left out a detail that did not seem relevant to the discussion insofar as it was designed to figure out the speed, velocity, distance, etc. relating to impact rather than to the minutiae of the configuration of the cable.

The cable consists of a bundle of wires (~50) encased in an outer rigid shell, but it also has a ridge of the same shell material at the top side of the cable (which has metal inside as well), so that is probably where the extra weight comes from.

Here is a crude sketch showing, roughly, what a section of the cable would look like if you hacked off a piece, to try to give you a visual:

http://i229.photobucket.com/albums/ee129/LashL2007/sketch3.jpg

I will take some photographs of the actual cable tomorrow at work to give you a more accurate representation, but I think that this omission on my part may explain the weight issue (assuming that the "expert" weighed it properly in the first place).

[Dan O.]

If your crew was supposed to remove the hydro cable you have more problems because I'm pretty sure that is a telephone line. The individual wires would be insulated copper. I would expect the upper ridge to contain a steel cable to support the weight of the span since copper itself can't take the strain. Something like the second picture here (http://www.cliffordvt.com/outsidetelecable_text.html).

[LashL]

Unfortunately, if this part of the situation is accurate, I think your engineering calculations are pretty much irrelevant.

This will boil down as follows:

Your crew recognized a hazard.
Your crew proceeded with cutting while the guy was still in the location of the recognized hazard.
Personal injury attorneys love this kind of situation.

Sorry, because if everything else in your description is accurate, this guy is a dipwad and deserved to be knocked senseless.

Thank you for your input but, in my view, the engineering calculations are very relevant.

There is no question about liability. I have admitted liability on my client's behalf since that is a no-brainer. However, contributory negliglence is very much an issue, given that the guy was (a) warned about the potential for danger (even though it had never happened before that the wire fell in both directions instead of only the correct direction), and (b) could see full well that there was work going on overhead, and that reduces the damages payable by whatever percentage the court attributes to his own contributory negligence.

The only real dispute in this case is damages and, frankly, the guy's injuries were pretty minor but he's now trying to claim that he was hit with such great force and at such speed and velocity that it caused a brain injury, even though there is no objective medical evidence to support that claim.

The engineering report has been submitted in order to try to support the brain injury claim, and probably for purposes of muddying the waters.

Thus, the engineering results are, in fact, very relevant. It is important to refute every piece of faulty evidence that is proffered.

[rwguinn]

I suspect that the discrepancy is due to the fact that I left out a detail that did not seem relevant to the discussion insofar as it was designed to figure out the speed, velocity, distance, etc. relating to impact rather than to the minutiae of the configuration of the cable.

The cable consists of a bundle of wires (~50) encased in an outer rigid shell, but it also has a ridge of the same shell material at the top side of the cable (which has metal inside as well), so that is probably where the extra weight comes from.

Here is a crude sketch showing, roughly, what a section of the cable would look like if you hacked off a piece, to try to give you a visual:

http://i229.photobucket.com/albums/ee129/LashL2007/sketch3.jpg

I will take some photographs of the actual cable tomorrow at work to give you a more accurate representation, but I think that this omission on my part may explain the weight issue (assuming that the "expert" weighed it properly in the first place).
Actually, LashL--
It ain't heavy enough! For 2cm diameter, it should weigh at leat 9lb for 17 ft. However, your description of it would make that a lot better...

[LashL]

If your crew was supposed to remove the hydro cable you have more problems because I'm pretty sure that is a telephone line. The individual wires would be insulated copper. I would expect the upper ridge to contain a steel cable to support the weight of the span since copper itself can't take the strain. Something like the second picture here (http://www.cliffordvt.com/outsidetelecable_text.html).

The crew was removing the correct cables. They were old, redundant interconnection cables but for the sake of simplicity in the "hypothetical" that I spelled out in the OP, I referred to them as old, redundant hydro wires. In retrospect, perhaps I should have gone into more detail from the outset but I thought that would only bog things down in the minutiae of details that were not necessary to the discussion.

The actual cable looks very much like the second photo in the link you posted, and yes, it has metal in the upper ridge.

[rwguinn]

I took from the OP that the cable is not tied at each pole (or had been untied) because they were able to remove the original tension. At some point, there is a free end on the ground and the rest of the cable is suspended from the remaining polls. The tension on the cable at this time is limited to the weight of the free end which is a maximum of 100 feet+/-. Where the free end is dragging on the ground, it can only contribute it's weight times the coefficient of friction which is probably less than 1. Using the weight of 100 feet of cable will give a reasonable upper limit on the cable tension which then defines the maximum hight of the low point in the cable sag between successive polls.

Unless something has been left out of the story, I estimate that the cable only had 18 inches or less to fall before striking the man.
I think yopu are misusing your calculator in that link
tension in the cable is a function of sag, not the other way around in this case. There is a fixed length of cable between towers, and it is not going to be a whole lot more than 100 feet.
At 100 feet of cable between 100 foot supports in a 1 g field, the tension is theoretically infinite.(100/sin(0)). if the cable length is 140 feet (100' between poles+20' down one pole+20'up the other), the tension is ~10 lb at the attachment decreasing to 0 in the first 20 feet, 0 in the next 100 feet, and 0 increasing to ~10 at the next attachment. sag is 20 feet. In other words, it would be slack. So we are somewhere in between. I am too lazy tonight to calculate the actual length, but we can assume that sag was less than 3 feet for safety reasons (vehicles and people must not be able to reach it easily)

[LashL]

Actually, LashL--
It ain't heavy enough! For 2cm diameter, it should weigh at leat 9lb for 17 ft. However, your description of it would make that a lot better...

Oh! How odd, indeed.

I haven't actually weighed it, of course. The figures are taken from the "expert" report. But it sounds as though you're saying that even without taking into account the additional ridge with the metal in it (which I left out of the initial description), the 2 cm. diameter of the main part of the cable should still weigh at least 9 lb for 17 ft?

[TjW]

Yes, but he probably calculated it as being more-or-less solid metal inside the jacket. If it's a 250 twisted pair cable, or something like that, a good deal of the volume will be air and insulation, and perhaps some packing fiber, all of which are much lighter than aluminum.

[rwguinn]

Oh! How odd, indeed.

I haven't actually weighed it, of course. The figures are taken from the "expert" report. But it sounds as though you're saying that even without taking into account the additional ridge with the metal in it (which I left out of the initial description), the 2 cm. diameter of the main part of the cable should still weigh at least 9 lb for 17 ft?

Without actual measured dimensions,I am conjecturing. But aluminum is the lightest metal used for wiring, and it is generally 0.098 to 0.1 lb/cubic inch density. If it were indeed a signal cable of some type, it could be lighter due to individual insulatied wires and the fact that it is not solid. It is something your expert should verify. That will also change the stiffness some.
I was assuming a 2cm power-type cable. As I said, your picture makes it better. The additional ridge likely contains a twisted wire rope, and would be the tension carrying part of the system, as Dan O. pointed out earlier. While the wire rope could be steel, it is likely aluminum due to the weight.

RW goes to bed muttering to himself about lawyers and what they consider relevant...:D

Dadgumit--TjW beat me by "Thasaat much!"

[LashL]

Yes, but he probably calculated it as being more-or-less solid metal inside the jacket. If it's a 250 twisted pair cable, or something like that, a good deal of the volume will be air and insulation, and perhaps some packing fiber, all of which are much lighter than aluminum.

It looks pretty tightly bundled to me, but I am not qualified to opine on the specific qualities or quantities of the bundle of wires so, as I said, I'll take some photographs tomorrow and post them for reference.

[LashL]

Without actual measured dimensions,I am conjecturing. But aluminum is the lightest metal used for wiring, and it is generally 0.098 to 0.1 lb/cubic inch density. If it were indeed a signal cable of some type, it could be lighter due to individual insulatied wires and the fact that it is not solid. It is something your expert should verify. That will also change the stiffness some.
I was assuming a 2cm power-type cable. As I said, your picture makes it better. The additional ridge likely contains a twisted wire rope, and would be the tension carrying part of the system, as Dan O. pointed out earlier. While the wire rope could be steel, it is likely aluminum due to the weight.

Ahh, I understand now.

It is my fault entirely for over-simplifying in the OP. I was trying to avoid going into lengthy explanations about things that did not seem relevant to the initial question, but of course, those very things have become relevant now.

I should know, however, from my time here at JREF that every detail is likely to be considered by somebody with relevant knowledge and expertise at some point, and that the subject matter will be assessed and expounded upon by lots of intelligent people with knowlege and expertise on different details of the overall scenario, and that the input of every one of the contributors with knowledge on a particular detail or subject will be valuable and relevant.

So, I should have set out the additional details from the outset. Again, my apologies, and my thanks to everyone who has assisted on this thread. I really appreciate it.

RW goes to bed muttering to himself about lawyers and what they consider relevant...:D


Yeah, sorry about that. :blush: See my post above.

My apologies. And my gratitude to everyone who has posted in this thread for making my apologies necessary. It is a tribute to this forum that such depth and breadth of knowledge and expertise exists here, and I am truly grateful for that.

In other words, you all rock! :D

[Dan O.]

I think yopu are misusing your calculator in that link
tension in the cable is a function of sag, not the other way around in this case. There is a fixed length of cable between towers, and it is not going to be a whole lot more than 100 feet.
At 100 feet of cable between 100 foot supports in a 1 g field, the tension is theoretically infinite.(100/sin(0)). if the cable length is 140 feet (100' between poles+20' down one pole+20'up the other), the tension is ~10 lb at the attachment decreasing to 0 in the first 20 feet, 0 in the next 100 feet, and 0 increasing to ~10 at the next attachment. sag is 20 feet. In other words, it would be slack. So we are somewhere in between. I am too lazy tonight to calculate the actual length, but we can assume that sag was less than 3 feet for safety reasons (vehicles and people must not be able to reach it easily)

Using the cable sag calculator, I figured that to maintain 3 feet of sag you would need a tension of 180 lb at the poll. That's not unreasonable but it won't be achieved in the operation detailed in the OP. If the tangent clamps had been removed to detention the cable, the cable would be hanging over a pulley or J-hook on the poll which cannot provide any tension. The free end of the cable would itself need to be tied off to the previous poll to provide the necessary tension until a clamp is secured to the cable on the next poll.

This whole operation doesn't make sense to me and I wonder if LashL has gotten the picture right. For instance, things you want to avoid on utility polls are lateral loads and sudden changes in loads. You get both of these when the cable is cut on the poll not to mention the bulk of the work taking place above ground. The operation that would make the most sense would be a reversal of the instillation procedure. The free end of the cable would be suitably tied off at the base of the previous poll. The cable could then be detached from the tangent clamp on the current poll and lowered to the ground. The west end of that cable could then be tied off at that poll and the previous 100 ft to the east cut off and removed. This avoids the strain on the polls and would probably take about 1/2 the time.

[Crossbow]

This is a bit of an unusual request but I am hoping that some of the engineering wizards here can help me out.

The hypothetical scenario is this:

Old, redundant hydro wires are being removed along the length of a street.They are attached, of course, from pole to pole down the street. They are removed in sections which are equivalent to the distance between two poles. The sections are removed from east to west. As each section is removed, it is cut on the east side of the pole and lowered to ground. Before the cut is made on the east side of the pole, the wire to the west of the proposed cut is tied off and secured to the pole and J hook to ensure that only the cut portion to the east comes down. After each section on the east is lowered to the ground, the process is repeated while moving on to the next section to the west.

The problem is this:

Let's say that we are now at pole number 5, as we move from east to west, and that the previous sections between poles 1 & 2, between poles 2 & 3, and between poles 3 & 4, have all been lowered to the ground in accordance with the plan. So, now the section between poles 4 and 5 is on the ground at the easterly end but is still attached to pole 5 at height. To remove the section between poles 4 & 5, we secure the wire to pole 5 on the west side of where we are going to cut, and then cut on the east side of pole 5, so that the remaining part of the section between 4 & 5 will fall to the ground to join the easterly part of that section which is already on the ground.

However, something goes wrong and the wire not only falls to the east as planned but also falls to the west. On the west side, it hits a person who is situated between pole 5 and pole 6. The wire still remains attached at height, though, to pole 6.

The question is this:

How would one go about calculating the speed and acceleration that the wire fell and how would one go about calculating the force with which the wire hit the person?

(I can provide details of height, weight, circumference of the wire, distance, etc. for actual calculations as well, but I would really like to learn what criteria an engineer would require to figure out the answers and what formulae would be used to figure out the answers.)

Thank you in advance for your assistance.

Edit to add: As an aside, the reason I'm asking this is because I received a report late Friday afternoon that purports to calculate the items mentioned above, but the report strikes me as wholly, grossly, and flagrantly inadequate. I will be hiring an expert to respond to that report, but it strikes me as so hopelessly inept that it is bugging me, even on the weekend. Thus, I hope to get some input from engineers here on what information you would need and what formulas you would utilize to tackle the questions.

Hello 'LashL'!

I am an engineer and I would say that it would be rather difficult to calculate the effects of the falling cable due to the large number of variables involved (height of the cables above ground, amount of separation distance between the breaking points, the rigidity of the cables, the linear density of the cables, and so on).

If all of these values were well known, then I expect that one could calculate things to say about a 10% accuracy.

However, I also expect that the effects on a human would vary considerably based upon many other factors as well (height of person, what direction he was facing, exactly where he was struck, and so on) which would be quite difficult to determine.

Therefore, I expect that even if the calculations were 100% accurate, then the determination of just how much physical harm was actually inflicted would be quite difficult to determine, thus the calculations would not be of too much help.

Therefore at the risk of speaking out of my depth, I suggest that you re-create the accident instead of trying to calculate its effects.

With a good record of several such re-creations, I expect that one could have a much easier time of demonstrating just what happened and just much physical damage was incurred.

I hope this helps!

[Dan O.]

My question is: Where are you going to get the volunteers to stand in for the victim in the recreations? :)

A computer simulation is much cheaper than a recreation. If you do just 1 recreation to prove the physics of the simulation model, the simulations will provide the same accuracy as you are able to measure the results in the recreation. The simulation has the advantage that you can then see how important each parameters is. For instance, it might be argued that the cable that has been in the air for 30 years is stiffer than new cable or even the same cable after it's been dropped to the ground, rolled up, unrolled and then rehung. You could rerun simulations to find out exactly how much difference a change in stiffness makes in terms of the final impact force.

[Crossbow]

My question is: Where are you going to get the volunteers to stand in for the victim in the recreations? :)

A computer simulation is much cheaper than a recreation. If you do just 1 recreation to prove the physics of the simulation model, the simulations will provide the same accuracy as you are able to measure the results in the recreation. The simulation has the advantage that you can then see how important each parameters is. For instance, it might be argued that the cable that has been in the air for 30 years is stiffer than new cable or even the same cable after it's been dropped to the ground, rolled up, unrolled and then rehung. You could rerun simulations to find out exactly how much difference a change in stiffness makes in terms of the final impact force.

Well, actually I was thinking of using something like "Buster" from the Myth Busters show instead of a real person.

While a computer simulation could be quite useful, however one should be advised that to do it accurately would require a good knowledge of the aforementioned factors (cable stiffness, linear density, height above ground, and so on). Therefore, I expect that producing an accurate computer simulation would be about as difficult as working out the problematic calculation that I was trying to avoid, so I really do not think that that computer simulation approach would be of much help.

But thanks much for giving the matter some thought all the same!

[rwguinn]

Well, actually I was thinking of using something like "Buster" from the Myth Busters show instead of a real person.

While a computer simulation could be quite useful, however one should be advised that to do it accurately would require a good knowledge of the aforementioned factors (cable stiffness, linear density, height above ground, and so on). Therefore, I expect that producing an accurate computer simulation would be about as difficult as working out the problematic calculation that I was trying to avoid, so I really do not think that that computer simulation approach would be of much help.

But thanks much for giving the matter some thought all the same!
Actually, recreating it is not that difficult, and probably worth doing.
Just put a witness specimen out there at the proper height, and measure it 3 or 4 times.
You can calibrate any number of soft materials for "depth-of-dent" at varying forces-and there are lots of poles around...

ETA: We might not be able to get 100% accuracy on the value of the hit with simulations, but you can truly envelope it by analysis. That's pretty much what I did--minimum is zero, maximum can be calculated using the most liberal values (K=0, u=0) for variables...

[LashL]

Yes, but he probably calculated it as being more-or-less solid metal inside the jacket. If it's a 250 twisted pair cable, or something like that, a good deal of the volume will be air and insulation, and perhaps some packing fiber, all of which are much lighter than aluminum.

It isn't entirely clear to me whether your post #46 (quoted here) was in response to my post #45 directly above it, but if so, the answer is no. The engineer who inspected the sample piece of cable and wrote the report referenced in this thread actually weighed it, so the weight is not a calculated value but a hard number.

[TjW]

It isn't entirely clear to me whether your post #46 (quoted here) was in response to my post #45 directly above it, but if so, the answer is no. The engineer who inspected the sample piece of cable and wrote the report referenced in this thread actually weighed it, so the weight is not a calculated value but a hard number.

No, I was referring to the estimate made by rwguinn, who pointed out the cable was lighter than it would be if it were made from aluminum. I was just saying that his estimate was probably made on the assumption that it was a single, possibly stranded, conductor of large diameter. Taking the phrase "hydro cable" at face value, that wouldn't be an unreasonable assumption. Sorry.

[LashL]

<snip>This whole operation doesn't make sense to me and I wonder if LashL has gotten the picture right.

Well, it may not make sense to you, Dan O., but I am pretty sure that I have the picture right, and it makes good sense to those who have been carrying out these operations for years. If it doesn't make sense to you, perhaps it is just that I have not explained it in enough detail for you. But, for legal reasons, I am not going to elaborate any further on the details of operations on this thread.

Suffice it to say that when setting up for the day's operations, the crew would scope out the entire length of the cable to be removed that day, and do their prep work by loosening the cable (and relieving the tension) but the cable was still supported by the J hooks as mentioned previously, and each section over the entire course of the span to be removed was further supported by tying off every section on both sides of each cut that would have to be made, prior to any cuts actually being made.

And yes, they were most definitely working from east to west. Perhaps you didn't notice the North indicator in my first sketch, but they were in fact dropping the line to the east of each tied off segment.

[LashL]

Hello 'LashL'!

I am an engineer and I would say that it would be rather difficult to calculate the effects of the falling cable due to the large number of variables involved (height of the cables above ground, amount of separation distance between the breaking points, the rigidity of the cables, the linear density of the cables, and so on).

<snipped for brevity>

Thank you for your input, Crossbow. As I mentioned previously, I anticipate conducting recreations if it becomes necessary. For current purposes, however, I am interested in the engineering opinions of members here as they relate to the questions posed in the OP as a result of the report I received from the other side about speed and velocity of impact, etc. I think that those questions have been answered quite succinctly and quite nicely by rwguinn and others, which I appreciate very much.

The only outstanding engineering question, really, is that of the force of impact, but it seems that it is pretty much impossible to calculate that without far more detail of the logistics, and those details are not available. There is no evidence from any source as to the site of impact, the angle of impact, or anything of that nature. I am okay with this being an outstanding question, since it is not me who has the burden of proof. In other words, the answer is, "it is impossible to determine the force of impact in the absence of evidence of the details of impact, none of which have been forthcoming." That's fine with me.

As for the effects on a human, that is something entirely outside of the engingeering realm, of course, and thus it is not something that I am asking about on this thread. I do not anticipate any problem in establishing that had the force of impact actually been what this "expert" claims it was, the man would have suffered far, far more serious and obvious injuries than he did. I will rely upon the medical evidence and, if necessary, upon expert evidence about studies regarding forces necessary to cause certain injuries. Frankly, if this "expert" were correct in his calculations as to the force of impact, the man would have suffered massive skull fractures, if not death. In reality, he didn't even suffer a bruise, scratch, or cut. But, again, that is outside the scope of the engineering questions.

In any event, I think it will be fun to engage in recreations if we get that far (I'll see after the pretrial on Friday whether it will be necessary). In fact, I'm quite looking forward to it. Recreations will be very easy in this case, as the utility poles are still there, and the J hooks that held the wires prior to their removal are still in place, and all of the other hardware is still in place. We can, literally, recreate the scene at the very site of the events with almost 100% accuracy, which is almost unheard of in recreations. That alone makes it an interesting endeavour. :)

[LashL]

No, I was referring to the estimate made by rwguinn, who pointed out the cable was lighter than it would be if it were made from aluminum. I was just saying that his estimate was probably made on the assumption that it was a single, possibly stranded, conductor of large diameter. Taking the phrase "hydro cable" at face value, that wouldn't be an unreasonable assumption. Sorry.

Ahh, thank you, TjW, for clarifying. Much appreciated, and no apology necessary at all. I just wasn't sure which you were referring to. It was my failure to comprehend, not your failure to explain. :)

[LashL]

My question is: Where are you going to get the volunteers to stand in for the victim in the recreations? :)

Hee hee. A dummy will suffice. Not the same dummy as the first time around... er, um, never mind... ;)

A computer simulation is much cheaper than a recreation. If you do just 1 recreation to prove the physics of the simulation model, the simulations will provide the same accuracy as you are able to measure the results in the recreation. The simulation has the advantage that you can then see how important each parameters is. For instance, it might be argued that the cable that has been in the air for 30 years is stiffer than new cable or even the same cable after it's been dropped to the ground, rolled up, unrolled and then rehung. You could rerun simulations to find out exactly how much difference a change in stiffness makes in terms of the final impact force.

If I choose to obtain recreations in this case, I think I will opt for the real thing rather than a computer simulation. Why? Because in this case, it can be done at the actual site, the actual poles are still there, the actual cable still exists, and everything about the event is reproducible. I think that in a case like this, it would be far more compelling to do a recreation on site than to use a computer simulation. A field trip to the site during the trial is also on the menu. There is no substitute for actually going to the location and taking in the surroundings to understand much that is otherwise merely abstract.

[LashL]

While a computer simulation could be quite useful, however one should be advised that to do it accurately would require a good knowledge of the aforementioned factors (cable stiffness, linear density, height above ground, and so on). Therefore, I expect that producing an accurate computer simulation would be about as difficult as working out the problematic calculation that I was trying to avoid, so I really do not think that that computer simulation approach would be of much help.

I agree that in this case it would be better to do a recreation than a computer model, because all of the materials, the site, the poles, the wires, the people involved, etc. are all readily available, so there is no need to speculate with modelling, and no need to imput ranges of possibilities, etc., when you can simply recreate the scene easily and measure everything on the recreation. But, again, it's there, it's available, everying is so easily reproduced that it cries out for live rather than Memorex computer simulations.

Obviously, the recreation will have to be repeated several times in order to analyze the data and the differences between attempts, but I think it is far preferable in the unique circumstances of this case to go with the real thing rather than a computer sim.

[LashL]

Actually, recreating it is not that difficult, and probably worth doing.
Just put a witness specimen out there at the proper height, and measure it 3 or 4 times.
You can calibrate any number of soft materials for "depth-of-dent" at varying forces-and there are lots of poles around...

ETA: We might not be able to get 100% accuracy on the value of the hit with simulations, but you can truly envelope it by analysis. That's pretty much what I did--minimum is zero, maximum can be calculated using the most liberal values (K=0, u=0) for variables...

Yes, I agree on the recreation point. I think that is the best way to go in the circumstances of this case, and admittedly, I think it will be fun, too.

[LashL]

Oh, I forgot to mention that I did take photos of the sample piece of cable earlier today, as I said I would, but it seems almost redundant now to post them, so I haven't.

If anyone wants to see them out of interest, though, just say so and I'll post them.

[Dan O.]

I realized something I forgot to add in was the friction of the cable over the J-hook. Assuming from the OP that the cable has not been tied to poll 4 (or any other poll) as it is lowered to the ground from poll 3 we have the following configuration:

~100ft of free cable east of poll 4 dropping 20 ft and remainder dragging on ground.
Assuming a maximum CoF=1.0 this can produce up to 43 lb of tension on the east side of poll 4.
The cable goes over the J hook on poll 4 and each successive poll where the weight of the suspended cable times the CoF adds another 43 lb force to the tension.

Tension west of poll #4 = 86 lb max
Sag of loop between polls 4 and 5 = 6.25 ft

Tension west of poll #5 = 129 lb max
Sag of loop between polls 5 and 6 = 4.18 ft

Tension west of poll #6 = 172 lb max
Sag of loop between polls 6 and 7 = 3.13 ft

The coefficient of friction is undoubtedly less than 1.0 which would reduce the tensions on the cable and allow larger loops.

If the cable is tied off at the next poll before the west end is lowered, A higher tension will be maintained on the cable and the sag will be less.

Here is an approximate picture of what the cable looks like with the above numbers.

http://forums.randi.org/imagehosting/thum_1514446cbda1d356fe.jpg (http://forums.randi.org/vbimghost.php?do=displayimg&imgid=7867)

[rwguinn]

A "Pole" is a long slender rod of undetermined cross section, used to support things or touch the untouchable.
a "Poll" is a series of questions used in an attempt to determine what people think of a particular issue
[/quibble]

[slyjoe]

And, a "Pole" may be someone of the Polish persuasion. :)