How to calculate guy line strength vs. wind load

  • 1
  • Question
  • Updated 10 months ago
Well, after posting my reference architecture I was so proud of the day before Christmas, I'm somewhat amused to report that my precious Spiderbeam came crashing down during a wind storm. This morning when I went to rotate the antenna, it didn't move, so I drove to the remote station to find what you see attached.

Fortunately, I was able to take the antenna down and repair it. That HD Spiderbeam is a beast. The top of the mast was not so fortunate and must be replaced.

As we say in Texas, I obviously misunderestimated the forces on the guy lines and need to recalculate for version 2. I assumed the nylon paracord rated for up to 485 pounds breaking pressure should be sufficient. I found the paracord snapped like a twig...

So my question is, given the spiderbeam has an estimated wind load of 0.5 meters squared, plus the 47' aluminum mast that's around 2.5" in diameter on average, what strength guy lines are actually required... for 75 / 100 / 125 MPH winds?

I looked around for a calculator and what I found suggested my guy lines should've been sufficient, but clearly they were not.

Suggestions?

Thanks
Rick
W5FCX


Photo of Rick - W5FCX

Rick - W5FCX

  • 144 Posts
  • 24 Reply Likes

Posted 1 year ago

  • 1
Photo of Chris Tate  - N6WM

Chris Tate - N6WM, Elmer

  • 759 Posts
  • 196 Reply Likes
I am not sure I know of a calculator for this, but you could try consulting with US tower. I an not certain that Aluminum is your optimal material for this.  perhaps 2-3 sections of Rohn 25, guyed and ancored in a couple yards of cement with a thrust bearing and a steel mast would be better, guying based on Rohn specs on their website.   I am sure there is a tubular tower that would fit the bill.

The failure looks like its favored toward the water..  were your guy anchor points evenly distributed and spaced?

125Mph is a lot of wind for any structure and places you into the high reinforcement category.  I have verticals and towers on a 2000ft hill that gets up to 125 mph.  Something like this would not last up there at all.  My 10m tower shown here is several sections of R25 setup as I have suggested, a taller mast can be used then I would have to brace or guy.  Attached is a photo, the steppIR in the foreground has survived such winds as well. 

Just my 2 cents.

Photo of Rick - W5FCX

Rick - W5FCX

  • 144 Posts
  • 24 Reply Likes
The failed guy line was on the north side. The lines are pretty evenly distributed. The pole bent straight south away from the guy line that broke.

I thought I had replaced all the guy lines with military grade paracord that's rated for 650 pounds, but the one which failed was an original cord I picked up from Walmart while waiting on Amazon to deliver the military grade cords. It's clear now that I missed replacing that one, which turned out to be the weak link.

Given this failure, it seems prudent to determine the theoretical load limits to expect at different wind speeds. If we get a 125 MPH wind, a lot more than the antenna will be at risk...

Nice tower, beam and vertical setup. I'm leaning toward getting a tower and SteppIR as next step one way or another at this point.

Looks like a nice QTH up on that hill!
Photo of k3Tim

k3Tim

  • 803 Posts
  • 149 Reply Likes
I recall from long ago a PhD - ME telling a similar story about how he miscalculated wind load on a mobile antenna. Seems the static load calculation is not correct but a dynamic load calculationi si required. I thought it ended up being a 3x to 5x higher load force. 

I would look at the Chrome-Moly masting DX-Engineering sells and forget Aluminimum. 

Hope the beam didn't get damaged. More importantly no one was hurt.
Photo of Rick - W5FCX

Rick - W5FCX

  • 144 Posts
  • 24 Reply Likes
Thanks Tim. Will look into that, and thinking about towers again... it's apparent the force was greater than the guy line could handle just unclear what it needs to handle at various wind speeds.

The beam was not damaged permanently. One of the fiberglass poles was bent from the impact into the ground, but after relieving the pressure from the support Kevlar ropes it snapped right back. The HD Spiderbeam is a beast and apparently can take some punishment.

I'll post this on a tower forum to see if there's some MEs lurking who know how this should be calculated.
Photo of Michael Coslo

Michael Coslo

  • 720 Posts
  • 162 Reply Likes
Ohh, the rare folded spider beam antenna! Sorry. 
Photo of Rick - W5FCX

Rick - W5FCX

  • 144 Posts
  • 24 Reply Likes
Isn't ham radio fun?  It takes us on learning journeys we wouldn't normally go.  With some research, and for future reference, here's the solution. It took downloading a commercial pole load calculator, combined with calculating the force on the antenna itself, to get the combined estimated force on the mast and antenna system, which I have documented in the attached drawings.  

I'm certain this is only an approximation and doesn't take into  account the moment at each guy point and other more advanced aspects, but hey, I'm no mechanical engineer and just need to get into the ballpark here.



I used the highest PsF from this table from K7NV:



Based upon this, in a 100 MPH wind, there is direct force on the antenna and mast of around 335 pounds.

Clearly the nylon cord from WalMart P/N NPC5503250W, which upon further research shows a "safe load" of just 110 pounds was the root cause of the failure.  It's now clear that I failed to replace one of those guy lines with the MIL-C-5040, which resulted in the guy line breakage.

Sorry to pester everyone with this... looks like I resolved this myself - but it helps to talk through it :)

Best,
Rick
W5FCX
Photo of Michael Coslo

Michael Coslo

  • 718 Posts
  • 162 Reply Likes
I'll bet the Walmart "nylon' is a mix of polyester and nylon. Although I thinnk the more robust small tower is the way to go, there are military surplus fiberglass mast kits that might do the trick for you. 
Photo of KY6LA - Howard

KY6LA - Howard, Elmer

  • 3401 Posts
  • 1289 Reply Likes
Rick

Remember Engineering Calculations may seem scientific but they are only best guesses for standard conditions. When dealing with wind loads you also have to consider exceptions such as gusts that can easily exceed design wind speeds by 100%.

I. Normally upsize support structures by at least 100% or preferrably 200% as an Engineering Safety Factor. Never had a support structure failure in any of our 100's of antenna support structures. It is much less expensive to overbuild it the first time than to have to repair damages and rebuild again
Photo of Rick - W5FCX

Rick - W5FCX

  • 144 Posts
  • 24 Reply Likes
Indeed
Photo of Rick - W5FCX

Rick - W5FCX

  • 144 Posts
  • 24 Reply Likes
I'm going to use redundant 770 lb upper guy lines to different guy points as a result of this great suggestion, Howard. Thank you!!

The mast will have to crumple or compress downward next time before a guy line at the top gives way.

Rick
Photo of Chris Tate  - N6WM

Chris Tate - N6WM, Elmer

  • 759 Posts
  • 196 Reply Likes
I would still suggest a more robust mast, particularly since you need to replace this one.  I steel mast with a base tilt mount with your Mil grade guys should hold up..  GL OM.
Photo of Jay / NO5J

Jay / NO5J

  • 1366 Posts
  • 196 Reply Likes
Bigger, should not be done in moderation. Belt and Suspenders, prevent things falling down better than miscalculations.

  SDRgadgets

#FlexRadio IRC chat

  73, Jay - NO5J

Photo of Mark - WS7M

Mark - WS7M

  • 953 Posts
  • 334 Reply Likes
Jay... I have the perfect place in Fort Collins to test your Spider/Hex beam that is in your garage to see if it can withstand a fall like this.  Ship it on up and I'll let you know!  :-)
Photo of Rick Hadley - W0FG

Rick Hadley - W0FG

  • 529 Posts
  • 116 Reply Likes
I'm with Jay.  I'd be scared spitless to put any kind of a rotating antenna on a mast like that for anything except a 2-day FD installation.  At just 4 sq ft windload for the Spiderbeam, even 3 sections of 25G should be self-supporting and a better answer than a light mast.   My lightly-guyed 65' 25G tower with nothing but a 2m Ringo Ranger on top survived a 135mph derecheo that destroyed most of the big trees in town in 2011.  The Ringo was bent in half, but the tower survived just fine.  If you need to go above 30', guys will be mandatory, I'm sure.
Photo of Rick - W5FCX

Rick - W5FCX

  • 144 Posts
  • 24 Reply Likes
Good points all. I'm giving this mast one more go with 770 pound Kevlar guys before tossing in the towel, just to see if it will work out. I'm convinced that if the guy lines had held firm the antenna would still be up.

Meanwhile, I'm starting to ponder a tower with SteppIR - now that the XYL is opening up to the idea.
Photo of Jay / NO5J

Jay / NO5J

  • 1366 Posts
  • 196 Reply Likes
My next planned @ 30' mast will be going up using ...
https://www.dxengineering.com/parts/msg-mp04100
When that comes down, I'll be using. ...
https://www.dxengineering.com/parts/msg-mp05100
Until that comes down.
Rinse, Repeat, until it stays up.

  SDRgadgets

#FlexRadio IRC chat

  73, Jay - NO5J

Photo of Rick - W5FCX

Rick - W5FCX

  • 144 Posts
  • 24 Reply Likes
Jay I get your point about steel vs aluminum. In my case the mast bent because of the failed guy line. If the mast fails and the guy lines are good then I will probably just move to a steel tower and be done.

Thanks
Rick
Photo of Rick - W5FCX

Rick - W5FCX

  • 144 Posts
  • 24 Reply Likes
This is what I ended up using...

http://www.radioworks.com/crope.html

KEVLAR 1/81/8" diameter, 770+ pounds test, Dacron outer jacket for UV protection