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4' fiberglass military surplus masts- wind load?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC8PJS, Dec 3, 2009.

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  1. KC8PJS

    KC8PJS Ham Member QRZ Page

    It has come to my attention that in order to erect antenna supports around my city lot I must submit a City building permit. The permit is only to prove that the structure I erect will be able to withstand the "typical" maximum wind for the region. This is a safety issue, so the supports don't fall and hurt someone. The building department is not requiring approval to install an antenna in general.
    So, I would like to use the 4' sections of military fiberglass mast. They are 1-7/8" outer diameter with a wall thickness of 0.110", and do have the stiffening rings. The catch is, I cannot find any sort of recommended stacking height or wind load specifications. Does anybody have this sort of information, or know where to find it?
    I would like to stack 5-6 sections tall to hold a single 18AWG wire loop (80 meters). The bottom section would be slid over a 6' t-post sunk into the ground. The mast will also be attached to the top of my 4x4" (4') fence post with angle iron, and steel muffler clamps. This will place 4-5 sections above the fence. While I don't plan to guy it, the loop wire itself will help the top of the mast from whipping about.
    Any help in calculating wind load and such would be greatly appreciated!! 73's
  2. KB9MZ

    KB9MZ Ham Member QRZ Page

    No problem at all. The aluminum poles fit inside the fibre glass poles also so you could go half and half so you have fibre glass where the loop is positioned
  3. AE5JU

    AE5JU Ham Member QRZ Page

    You can buy guy rings for the fiberglass poles.

    Rather that fit them together on the ground and try to raise the assembled mast, I would suggest you get some help, and put it together vertically. As you lift the mast, another person fits a section into the bottom.

    I don't go more than 5 up.

    There are aluminum mast pieces the same size, and will interchange with the fiberglass.

    Check this out, the new club Field Day antenna I just finished last month. 75, 40, 20 meters, and 17 and 15 with a tuner.

    The guys ARE the antenna.


    Stacking height: The poles are 48" long, but the plug end is 4", so you actually get 44" per section.

    I would suggest using the aluminum pieces for the lower half, at least, and just 1 or 2 fiberglass sections at the top.

    The plastic rings about the socket end break off. Put a radiator hose clamp on the end and that will help prevent the end from cracking.
    Last edited: Dec 4, 2009
  4. K4TBX

    K4TBX Ham Member QRZ Page

    nobody read the op ????
  5. K4PP

    K4PP Ham Member QRZ Page

  6. K4SAV

    K4SAV Ham Member QRZ Page

    Are you sure you need a permit to put up a pole? I wonder what they are going to inspect. Most permits require supporting documentation, either manufacturers data or engineering stamped data. I hope you don't need that. I thought I read somewhere that those fiberglass masts were originally intended for use as tent supports. If that is the case, it is not likely there will be data to support its use as an antenna mast, and you sure don't want to pay an engineer to generate it.

    Most structures also have set-back requirements. Where is you fence located? If you use a fence pole as a support they will probably want to inspect the concrete frame (if it has concrete) for the pole before you pour it (checked against the documentation), which means you have probably already violated the permit requirements. Normal fence poles would not quality as antenna mast supports.

    The reference listed by K4PRP shows about what you would need to survive 80 mph winds, and you are at least in a 70 mph wind area. Notice that the diameter of that mast is 2.36 inches and it requires guys at three levels.

    You didn't give any information on how the antenna wires would be attached, but I think it is unlikely that this would support a horizontal 80 meter loop, unguyed. What supports the other ends? Are you going to use 4 of these?

    You had better recheck the requirements for a permit.

    Jerry, K4SAV
  7. K6OK

    K6OK Ham Member QRZ Page

    KC8JPS: Back to your question...

    wind load per Uniform Bldg Code in lbs per sq ft: 0.00256 x V^2 where V is wind in mph

    at 100 mph, load = 25.6 lbs per sq ft

    24 ft high x 2-in dia pole = 4 square ft area presented to the wind

    distributed wind load = 102.4 pounds

    must use a factor of safety, typically 3. Assume design load of 310 pounds. This should be enough to include the point load at the top from the wire antenna.

    For a freestanding pole, unguyed, must consider two possibilities: pole will snap and buckle at the base; or, the soil will fail and allow the whole thing to become uprooted.

    For the pole buckling question: stress at the bottom of the pole will be s = WL/2Z, where W=load, L=length, and Z=section modulus. Section mod for a pipe is
    (pi x (OD^4 - ID^4)) / (32 x OD). Fiberglass has a crosswise flex strength of 10,000 psi ( sheets/fiberglass_technical_data.htm).

    For the soil question (depth of embedment): you might try

    this calculator has a 45-day free trial. Calculates foundation strength of a pole embedded in soil.

    Good luck and 73...
  8. AE5JU

    AE5JU Ham Member QRZ Page

    Yes, and I suggest guying.
  9. AC0GR

    AC0GR Ham Member QRZ Page

    Must have missed this part....

    He needs hard specs to submit for his permit, not anecdotal 'It worked for me' stories.
  10. AE5JU

    AE5JU Ham Member QRZ Page

    Well, all I have is anecdotal. Sorry, the eBay sellers of these fiberglass masts aren't big on engineering specs. :D

    But the fiberglass poles are good for handling force straight down, but not side forces.

    And without guying, I don't think it's going to work out well for him. The aluminum poles would be a lot better.

    So not specs, just that I've actually used these things.
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