Support Cable Configuration

I'm sorry to say this, but I think you are in deep trouble. Unless some left-to-right compression members can be added, the tensioned cables are likely to collapse the pipe side frames inward. Bona fide calculations would need the various weights and tensions to be spelled out.

--Tornado

I didn't see any length on the pipe? More pipe length, more cable sag.

Put the cables one foot above ground level and you could tune the cables like a cheap guitar.

So, how high does it need to be? Or did I miss it?

The cables are 16 feet above the raceways ( fish tanks).

I suspect this is a cluster "you know what".....I was just brought in after the damage and wanted to run it past you all and see if there was even a partial solution.

Otherwise..... I just supply them with replacement netting and they watch the same thing happpen at some time in future......again.

Thank you.

You're screwed.

Just a note more or less to myself (although I'm not marking it off-topic)--I apparently, but don't know why I marked my post #5 off-topic--I guess I need to watch more carefully in the future.

Of course, someone else might mark this post as off-topic, as, indeed, it is. ;-)

GA.

I would point out to the OP that it is important that the cable fail before the structure.

If the net is so fine that the ice fills the voids, it could become a basin with the increased sag. If the net is that fine, it might pay to have some kind of vibration at various points, perhaps a small motor driving an eccentric wheel.

* file photo when the contractor installed everything brand new....circa 2009 *

K,

I am not totally understanding the calculator but thank you very much for doing the math for me. As you should know by now, I am not an engineer and possess limited advanced math skills.

You are saying that no matter what the cable or rope size used, we should look at allowing some sag. My thoughts as well as theirs' has been to make a tight, 'pretty looking' structure cover.

This makes some sense to me as in net design we always allow a 15% decrease in net spread for the same reason. (40 Meter headrope is fixed to spread 35 Meters to keep the lines and netting from over stressing and breaking .)

** The netting used is extruded PE which means it is flexible only to a point and then it snaps off. I am suggesting true , knotted netting in UHMWPE which is highly flexible and has 400% greater breaking strength while remaining in the same .75mm diameter.**

I will look into this more as it seems the only method of re-netting their enclosure.

Thank you.

You're welcome. Happy to help.

Another thing to consider, based on your photograph. It looks like cables might stretch not just from one support pipe to the next, but over the entire length, (maybe 4 or 5 times 160... or however many pipe structures there are... I'm not sure how many there are from the photo) so there is not a termination at each intermediate support. If this is the case, then the cable tension would add up. So if you used 200 lb tension to keep the sag to a foot or so between supports 160 feet apart, then to maintain that sag across (let's say) four spans would require 800 lb.

This is much better, though, than trying to maintain one foot sag across 640 feet, without the intermediate supports. Doing so would required about 3000 lbs tension per cable. (The tension increases with the square of the span, so if you multiple the span by four, you multiply the tension by 16.)

Another thing to consider: whether or not the cables are tensioned at each intermediate support, the tension will still add up as it is seen by the end supports. The tops of the intermediate supports will move under tension (the vertical posts will bend slightly) and so will pull on the next cable segment.

The underlying principal re tensioning horizontal cables is that to make the cable truly straight requires infinite tension. The simplified reason is that what you are trying to do is lift the center of the cable upward (which for these cable lengths and sizes, you could do with finger pressure) but you are applying the force in exactly the wrong direction to do so. The more you let the cable sag, the closer you get to applying the force in more or less the "right" direction.

(BTW the calculator uses a figure for "force perpendicular to cable length." The number is actually not a force but the acceleration due to gravity. There would be no need to change this figure [from about 32 ft/sec/sec] unless you are working on the moon, or on something that is accelerating.)

It would be worth giving a quite a bit of thought to the weight of ice. I'd think it could easily double the cable weight and maybe multiply the net weight by 10 times (perhaps more). (It is easy to imagine that each net thread could get to be 3mm diameter, which is 16 times the area of .75 mm.) If the cables were tensioned to keep the net area taut, they would certainly fail with so much weight.)

Also, as someone else mentioned, guy wires at the end supports would be wise. Let's say you have 10 guys along the length of the end pipe structure. How do you anchor those? Concrete? Ground augers? What happens if the ice load is higher than expected and one of these guy wires parts, taking off someone's head in the process? You can see where I am going here. You may want to look into getting a PE involved.

Also, as someone else mentioned, guy wires at the end supports would be wise. Let's say you have 10 guys along the length of the end pipe structure. How do you anchor those? Concrete? Ground augers? What happens if the ice load is higher than expected and one of these guy wires parts, taking off someone's head in the process? You can see where I am going here. You may want to look into getting a PE involved.

I have no idea as yet as to how they have the ends anchored. Its why they are asking me to fly up and look it over. I am just suspect that I'll need a little engineering help to mend ( NOT fully solve) this problem.

Properly sized auger type anchors would work fine (depending on the makeup of the substrate). After they are sized (and ok'd by a civil engineering firm) and quoted, it would be a matter of cost comparison between augers and concrete pylons.

Exactly!

Vibration to the wires would do the trick. The problem is how to accomplish that within the structure. see picture.

Thank you too. That would indeed keep the ice from forming as quickly.

At it's simplest, something like a bow, as in bow and arrow, with holes in the ends, threaded onto the cable with a small motor and eccentric wheel fixed in the middle of it.The cable and netting are so light that you shouldn't need many, depending on the size.

I had another thought, is there a coating that has no affinity for water? If the netting doesn't get wet....

I think this could be done . too along with the other suggestions...

Good Answer and thank you.

What happens in these Barrier nets ( Aviary nets) is that water spills over the thread or extrusion, wind effect and dropping temps freeze minute amounts. As the water sheds away it forms minute ice cicles. As more rain and sleet fall and the wind blows the netting, the ice now has a longer base to drip down and the icicles grow and grow....finally pulling down the net and cables.

Nothing is 100% unless you would use a severe over kill structure which seldoms happens within the budgets of fish hatcheries.

Nylon (polyamide) holds water ( 1.14 sg)

UHMWPE does not hold but sheds (.94 sg).

However , under really ice storm conditions both will produce cicles.....

The netting is 4" x 4" square/

The extruded netting used in their first attempt seemed to sheer away as it was forced down on the slightly rough edged galv. cable.

I'm not the OP, but that's a useful calculator--thanks very much for the link.

IIRC, somebody in an other thread the other day (I can be very specific when I try ;-) was looking for a cable sag calculator (or set of tables)--I think this would almost do the job for them.

The reason I say almost is because we used to have to provide the stringing sag at 0, 30, 60, and 90 degrees Fahrenheit so that the installers could interpolate the sag for the conditions in the field. I can't immediately think of how to use this calculator to get the sag for the different temperatures.

This is probably OT, but again, I won't mark it as such. ;-)

Tension can also be accomplished by installing pulleys and weights. Run the cables through a pulley on one end rather than attached directly to the pipe, then add a length of cable with an appropriate weight attached. It should keep the cables in tension regardless of temp, wind etc. I use this technique to keep antenna wires in tension. I find the correct weight by using a container that I can fill with water until the tension I want is achieved, then I weigh it, and use solid weights.

I have a suggestion. It's not fully thought through, so perhaps some input from the other members might be in order, but... how about this:

Extend the vertical pipe supports another 10' in height.

Add an additional horizontal pipe above each existing horizontal pipe.

Remove every other cable from its current position (this will leave spacings of 48")

Re-attach these cables directly above each existing cable, but instead of running the cable 160' to the next pipe, use them as guy lines, and run them down at an angle to the 1/3 position of the lower cables (53' in from each end). This will form triangles of x=53' and y=10'. Along the same principle as a suspension bridge.

Cut your net into 3 sections and lay it down. Two sections will lie within each triangle, and the middle section will be placed between the two triangles.

Add additional guy lines from the pipe structures, anchored down to the ground to keep the whole system from collapsing inward (due to the added height)

And as was mentioned, do not strive to remove all of the sag. Only what is needed to keep the original purpose of the cables as functional (which is to support a net above the fish tanks)

As an added benefit, your net sizes will be reduced, which will make them easier to deploy and to handle.

** Or, as a slightly modified version of what I just described, lay the full size netting on top of the angled cables instead of within the tringles. This will give you a series of valleys.

Did I describe this clearly? Comments?

As an added note, in order to keep costs down, you might possibly be able to use 5" or 6" pipe as your additional upper tier structure if the proper methods were employed in welding it to the lower tier. Perhaps even channel, for the horizontal sections. Of course, calculations by a qualified structural engineer would need to be done to confirm this as fact.

Com sa,? I mean, like this?

How high should the eleveated piece be?

Yep. That's pretty much what I had in mind. As far as the height, I used 10' mainly because that would allow you best use of 20' stock lengths, and at the same time give you maximum angular advantage while still using stock lengths. In other words, to a certain point... the higher those posts are (i.e. the more vertical your angled cable is), the closer to equilibrium your vertical vs. horizontal stresses will become.

I'll put that one in the book as a simple method using what is there already.

Thank you and Good Answer to boot!

You're welcome. Glad to help. I'm also a part-time wildlife biologist, so I take particular interest in helping to protect your little fingerlings. And if you are so inclined, feel free to rate it as a Good Answer, so I can perhaps get out of this embarrassing "1 Good Answer" slump

OK.

Good Answer for mentioning the elevated suspension system for bird netting.

Have you thought about using heavy mono-filament line rather than cables? It would reduce the weight considerably.

Mono filament ( Pa) rots very quickly in UV .

The Dyneema line is suited for heavy duty outside, wear and tear as well as very low stretch and extreme break strength loads for in small diameters.

Thank you anyway.

I'm guessing, but is this netting meant to protect the fish from birds?

Yes. Its a Depratory bird net.

the original netting used for this will wpork under most conditions. It just happened that THIS winter was extreme in their locale. they have asked me to upgrade using the existing support structure.

Just putting in my final $0.02. I think the least expensive solution for the client is simply allowing enough sag in the cables and net to reduce the tension to a level that can handle the added ice (and wind) loads under worst case conditions.

Possibly also adding some sort of guying system to the left and right--but, if the supports haven't bent so far, I don't see them bending with reduced tension in the cables.

Adding extra sag doesn't require additional structure, nor a change in materials of either the net or the cables.

I can imagine the client preferring to see something without sag, but then I would undertake to gently educate the client, starting with the old tie a pencil (or suspend a small weight) in the middle of a string and then pull on the ends hard enough to make it perfectly straight. (Choose a weight and a string so the demonstration works as intended--when he pulls hard enough to get the string almost straight, it will break. (Obviously, you can choose a rope and a feather and not have the demonstration work as intended.)

If he (or she, or they) gets real stubborn or even obnoxious, then I'd sell them an elaborate Rube Goldberg solution and retire ;-)

The one possible drawback of the solution could be the trapping of birds instead of simply preventing them from reaching the fingerlings. I don't think that's a realistic fear--if the nets were in some sort of configuration where nets folded over each other leaving trap like spaces, yes, but a little sag--even 8' of sag (from the 16' height) isn't going to make the net any more of a trap than it is as a flat trap.

And, thank goodness ;-), somebody else in this thread (I won't look for the name, but thank you!) came up with the sag calculator and it looks like the sag doesn't have to be anywhere near that--more on the order of a foot or two, which won't even be (imho) aesthetically displeasing.

Have a good day!

Well noted and thank you again for the input.

I have enough information on this to present some ideas at least.

Down here in Louisiana we haven't seen enough sneaux or ice to know much about it. However, as I get calls from all over the US, its something I will have to educate myself on quickly. Bird Netting is not a prime business of ours as we do not offer the cheaper variety of mesh. However, every now and then , one of our net customers refers us to a hatchery , land fill, air plane hanger, indoor parking garage, etc. that has a bird problem. Most of them have used the off-the shelf netting and they did not like it ( for whatever reason) . They then turn to us for something high end .

Thank you and everyone else who contributed. It is still the reason when I am in doubt, I run my situation through here. ......lots of smart people with smart answers!

Ok, so that wasn't my final $0.02. Somewhere there exist tables (from the weather bureau) that give things like 100-year worst storm conditions--wind speeds, and, iirc, ice build up levels. (Maybe the ice levels came from some other source.)

Anyway, we always looked at (and considered) those conditions when designing (overhead) power lines. I'm sure that information exists for Louisiana. In this day of (potential?) global warming, I'd tend to err on the more conservative side (that's always an interesting phrase--do I mean conservative in terms of trying to save money by designing to a lower standard, or conservative in the other direction--designing to a higher standard.

I guess I believe in global warming--I'd design to a higher standard.