Mega Tree Guy Wires

[rlilly]

I was reading the Megatree construction details at Nevada Christmas and got to thinking that more thought needed to be put into the guy wire selection.

After doing a bit of research on wind loading and wire rope, I came up with the attached guy wire calculator.

In the Carson Valley, the wind can really blow and the 70 MPH gusts that Wayne describes in his write up are not uncommon. He opted to go 'one size up' from the 1/8" and described that as overkill. Using the calculator, turns out it's not overkill at all, and in fact may be undersized.

To use the calculator, you enter the data for your set up/location highlighted in red.
The blue values are the appropriate guy wire sizes.

Edit: Updated sheet to reflect Deplanche's comments #1 & 2 below.

 

[rstehle]

Great job, very helpful!
Thanks!

 

[vairmoose]

Not to criticize the table, it provides much food for thought, but i don't see where it allows for various configurations - i.e. 3 wire, 4 wire, 5 wire, etc. The 3 wire would generate the highest per wire loading, whereas the 5+ wire arrangement reduces the per wire loading and provides some measure of redundancy. 4 wire would reduce the per wire loading but not provide any real redundancy.

Larry

 

[Wayne J]

Don't forget the use of dual guy wire setups..... I use a set at top and midway up the mast. I use 1/16" wire cable, and I really think I could climb up the thing. LOL

 

[ben]

Don't forget the use of dual guy wire setups..... I use a set at top and midway up the mast. I use 1/16" wire cable, and I really think I could climb up the thing. LOL

That is what I am going to use with 3/32" steel braided cable. 3 in the middle and 3 at the top coming to a single ancor point on the ground.

Ben

 

[deplanche]

Your spreadsheet is a good start, but you are missing a few key elements:

1 - It appears you are assuming that the full wind load acts at the top of the structure (tree). If it were something like a billboard or even a sign on a post, that would be a decent assumption. But for a tree and lights that are supported at the base, you should be able to assume that the centroid (the point where the wind force occurs) is approximately 1/2 the height of the tree. If you have a wire attached to the top of the tree, it would only need to restrain have the load. So you are oversizing the wire with this assumption.

2 - The force, F, that you are assumining would act in the horizontal direction (perpendicular to the vertical pole). If your wires are also attached in a horizontal direction, your spreadsheet is fine. However, it is unlikely that you will have something nearby as all as the tree in each direction to anchor the wire to. Instead, you would be anchoring to the ground at an angle. If your guy wires are at a 45 degree angle (the distance from the center pole to the ground anchor is the same as the height from the ground to the anchor, (21 feet for both in your spreadsheet) then the force in the guy wire is actually 1.4 times bigger than what you are calculating. If the wires are anchored even closer to the base, say about 1/2 the height from the center pole, then the load in the wire is 2.25 times what you calculated. As you can see, this angle makes a huge impact on the load is the wire. It can even cancel out the oversizing that was made in the first assumption.

3 - Keep in mind that you are designing here only for strength, and have not considered deflection at all. A cable may be able to take the load and not break, but the top of the pole may deflect 6" in a wind storm. Depending on how you construct things, your display may fall apart with that much deflection, and a thicker wire may be needed.

4 - This is only accurace for the 4-wire configuration at 90 degrees apart from one another, as indicated by vairmoose. 3-wire confiurations will have more load on them.

As I said, it is a good start, just a few little flaws that I felt i needed to point out. Unlike most here, I am a Structural Engineer, not one of the electrical or computer gurus. If I didn't mention this, and some one got hurt when this fell over in the wind, I would feel terrible.

 

[deplanche]

That is what I wam going to use with 3/32" steel braided cable. 3 inthe missl and 3 at the top coming to a single ancor point on the ground.

Ben

By single anchor point, do you mean a total of 3 anchor points, with the top and middle wires or each 'side' coming to an anchor point? Or do you just mean 1 anchor?

 

[ben]

By single anchor point, do you mean a total of 3 anchor points, with the top and middle wires or each 'side' coming to an anchor point? Or do you just mean 1 anchor?

I man there will be 3 anchors in the middle, 3 anchors at the top. On top and one middle will meet at a same anchor point in the ground, 120 degrees from that another middle and top will meet at the same point then the final one.

Ben

 

[rlilly]

Great!

This is what I'd hoped to get started!

Your spreadsheet is a good start, but you are missing a few key elements:

1 - It appears you are assuming that the full wind load acts at the top of the structure (tree). If it were something like a billboard or even a sign on a post, that would be a decent assumption. But for a tree and lights that are supported at the base, you should be able to assume that the centroid (the point where the wind force occurs) is approximately 1/2 the height of the tree. If you have a wire attached to the top of the tree, it would only need to restrain have the load. So you are oversizing the wire with this assumption.

2 - The force, F, that you are assumining would act in the horizontal direction (perpendicular to the vertical pole). If your wires are also attached in a horizontal direction, your spreadsheet is fine. However, it is unlikely that you will have something nearby as all as the tree in each direction to anchor the wire to. Instead, you would be anchoring to the ground at an angle. If your guy wires are at a 45 degree angle (the distance from the center pole to the ground anchor is the same as the height from the ground to the anchor, (21 feet for both in your spreadsheet) then the force in the guy wire is actually 1.4 times bigger than what you are calculating. If the wires are anchored even closer to the base, say about 1/2 the height from the center pole, then the load in the wire is 2.25 times what you calculated. As you can see, this angle makes a huge impact on the load is the wire. It can even cancel out the oversizing that was made in the first assumption.

3 - Keep in mind that you are designing here only for strength, and have not considered deflection at all. A cable may be able to take the load and not break, but the top of the pole may deflect 6" in a wind storm. Depending on how you construct things, your display may fall apart with that much deflection, and a thicker wire may be needed.

4 - This is only accurace for the 4-wire configuration at 90 degrees apart from one another, as indicated by vairmoose. 3-wire confiurations will have more load on them.

As I said, it is a good start, just a few little flaws that I felt i needed to point out. Unlike most here, I am a Structural Engineer, not one of the electrical or computer gurus. If I didn't mention this, and some one got hurt when this fell over in the wind, I would feel terrible.

I hadn't seen anything before on this subject, and was of the opinion that most people 'winged' it when selecting guys.

My goal was to give people a benchmark to begin their thought process.

This spreadsheet could be made much more sofisticated. In my web searches, I saw software to compute this offered for up to $5k.

I'll make the changes you suggest in 1 & 2 and repost. (Great points) I also noticed a problem with the look up table I need to fix. Multiple guy arrangement was more than I was shooting for. Maybe you'd like to take a crack at it?

The other discussion that I'd hoped to start is what type of winds to prepare for. In my neck of the woods, there haven't been winds of 50 knots or greater in the Christmas display season for over 50 years.

 

[rlilly]

Sheet in first post is updated.

 

[ben]

As far as I can remember nothing over 50mph here for gusts. Steady 30-35 sometimes when its BAD.

Ben

 

[deplanche]

Bob,

I am having some issues with a nasty virus on my home PC right now, and can't seem to open the spreadsheet, but I will take a look as soon as I get that fixed. I'll see if I can also locate a few references I have on the design of radio towers with guy wires. What you have is a very good start, and there is no need to spend $5k on a program that a speadsheet can handle. Although I do have access to some of those expensive software packages thru work. I may run a sample thru one of those just to add some pretty graphics to this thread to show what it would look like in terms of layout and forces.

 

[rlilly]

Bob,

I am having some issues with a nasty virus on my home PC right now, and can't seem to open the spreadsheet, but I will take a look as soon as I get that fixed. I'll see if I can also locate a few references I have on the design of radio towers with guy wires. What you have is a very good start, and there is no need to spend $5k on a program that a speadsheet can handle. Although I do have access to some of those expensive software packages thru work. I may run a sample thru one of those just to add some pretty graphics to this thread to show what it would look like in terms of layout and forces.

That would be great! Looking forward to it!