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Calculate the lateral strength / buckle point of a cylinder

  1. Sep 6, 2016 #1
    Here is my problem:
    I want to mount 2 solar panels on a horizontal 10 foot bar. (panels are 5' and 20-40/lbs each - so 80lbs)
    is there a formula i can use on each material option to figure out what material to use for the 10 foot bar (what wont break under the weight of the panels or bend or crack from the changes in temp and under load, and which is the cheapest option
    I'm thinking a piece of 2" PVC filled with cement would do the job but i wonder if it would crack. I don't think 1 1/2 like chain link fence post pipes would be strong enough. I'd like to be able to figure it out with nice cheap math before going out and buying lots of things to experiment with.

    google was little help with mostly similar questions with answer that say it depends on the material but that's what I'm trying to figure out

    Thank you in advance for any assistance.
  2. jcsd
  3. Sep 12, 2016 #2
    Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.
  4. Sep 13, 2016 #3
    Yes and No. Not as simple as you are hoping for I guess, particularly when using composite materials like concrete in a pipe.
    Concrete in plastic pipe might be a waste of time. It will probably break and just add weight on the pipe.
    A structural I-beam would be your best option, this is what they are designed for.
    Alternatives are steel / aluminium pipe, angle iron (steel / aluminium), c-section, or just the next size up PVC pipe.
    If you can find the lengths just lean on them to see if they have the strength you need.
    Otherwise, you will need to apply "s=My/I" to determine the maximum stress caused by the distributed load on the given cross section, and compare that with the yield strength of the material selected.
  5. Sep 14, 2016 #4
    Thank you for replying but a structural I beam might be my best bet, but way out of my budget.
    (cheapest I could find)
    S 3 x 5.7 lb (3.00" x .170" web x 2.33" Standard Steel I Beam 10' $85.50
    3" X 2.33" X .170" web 6061-T6 Aluminum I Beam - AS 12' $165.00
    I'm just going to have to go about this the old fashioned way and experiment and get a piece of PVC, cement, a metal pipe and 4 40 lbs bags of mulch and leave them for a few months and see which holds before I go spending any real money.
  6. Sep 14, 2016 #5


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    Is it possible to have some bracing ? If the bar was supported at the mid point by a bracket or strut it would make solving your problem much easier .
  7. Sep 17, 2016 #6


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    A PVC tube is useless with concrete filling. It will expand in the heat and slide free. To give concrete strength it needs a tensile core.

    If you are on a budget you should consider a hollow timber box beam. It can be screwed together from scrap pallet parts. You might also consider plywood side panels. A box beam could be tapered towards the ends. A coat of paint will preserve it.
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