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Which is better for structural support PVC pipe or plywood?

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  1. Jan 14, 2017 #1
    I'm building a structure that will weigh approximately 200lbs (100kg) but that is only for the shell of the structure. So to support it I want something strong with as little deflection as possible as well as being as light weight as possible and I've narrowed down my options to PVC piping or plywood however I've been having some trouble finding out which would work best. I'm sure there's some sort of equation out there for determining the weight and amount of bend in a length of plywood and PVC pipe so if anybody knows that
    I'd love to hear it but if not just what do you think would be best?

    Edit: What I'm making is a long wing like structure for display, it will be about 10ft (3m) long and I'd like it to last a long time. I'm unsure of what type of plywood I would be using and am open to any suggestions on what would be best
     
    Last edited: Jan 14, 2017
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  3. Jan 14, 2017 #2

    berkeman

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    Can you post a drawing of what you are building? How are you planning on joining the pieces of PVC or plywood? How long is this supposed to last? How are you going to treat/finish the plywood? What grade of plywood are you considering?
     
  4. Jan 14, 2017 #3
    You give no indication as to the type of loading that you need to support, the time duration of the load, the criticality of the loading, or any of the other necessary information required to give sound advice. Please try again.
     
  5. Jan 14, 2017 #4
    Well I'm just asking, I assume in general plywood is stronger, lighter, and more susceptible to bend than PVC piping but I'm not sure.
     
  6. Jan 14, 2017 #5

    berkeman

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    That still is no help. Please answer as many of our questions as you can. Thank you.
     
  7. Jan 14, 2017 #6
    Ok, thank you for being patient with me
     
  8. Jan 14, 2017 #7

    Stephen Tashi

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    That assumption doesn't make sense because the susceptibility of a structural member to deformation depends on the members shape as well as its material composition. If you used a flat strip of plywood as a beam it would bend and twist easily. If you cut the strip into 4 pieces and put them together to form a box beam, the beam would have a greater resistance to bending and twisting - hence the need for you to be more specific about your designs.
     
  9. Jan 14, 2017 #8
    Which is why I'm asking about the materials, thank you by the way for answering some of my question by telling me that the plywood would bend easily. The design is still open for being changed. I just want to know which material is stronger, which would bend less and which would be lighter for a given length. I'm just trying to understand which material would work better and if making a box beam out of the plywood would make is stronger and less bendable than pvc pipe then that's what I'll use. I'm just looking for a general idea
     
  10. Jan 14, 2017 #9
    Since it is confusing I'll make a hypothetical. If I have a 1 meter by 20 cm thick by 20 cm wide piece of plywood and a PVC pipe that is 1 meter by 20 cm diameter with walls 1 cm thick which would be stronger, which would bend less, and which would be the lightest
     
  11. Jan 14, 2017 #10

    berkeman

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    Comparing a sheet with a tube shape is not a good comparison, IMO. You've already been reminde of a box beam shape, what about an I-beam shape? A triangular beam versus a tubular beam?

    What is your design background. It sounds a bit like we should send you off with some reading assignments before we can be of much help. What ME reading have you been doing so far?
     
  12. Jan 14, 2017 #11
    some reading ideas might be helpful, I'm aware that a triangular or box like beam would make plywood stronger and less flexible but I don't know how that would then compare to the PVC
     
  13. Jan 14, 2017 #12

    berkeman

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    There are so many things involved in this. Like my joining question, and the questions about beam shapes. There's lots to learn, and it's fun learning that helps you build better structures.

    Okay folks -- let's make some suggestions of some good intro ME/structural education sites that the OP can read through. I'm off to make a list... :smile:
     
  14. Jan 14, 2017 #13
    Thank you very much, I'm sorry I'm ignorant about this I thought it would be simple. You're an amazing help ^-^
     
  15. Jan 14, 2017 #14

    berkeman

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    No need to be sorry! We all learned this stuff at one time or another. :smile:
     
  16. Jan 14, 2017 #15

    Stephen Tashi

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    We can answer the question of which beam would be heavier by computing the volumes of the two constructions and looking up the density of PVC and the density of plywood. As I read the internet, the density of pine plywood is about 600 kg/ m^3 and the density of PVC is about 1300 kg/m^3.

    The question of how much each beam will bend depends on what kind of load we assume, but we could assume a uniform load. We need to look up the modulus of elasticity for plywood and for PVC.

    As I read the internet, the modulus of elasticity for plywood is about 6.8 x 10^9 Pa and the modulus of elasticity for PVC is about 3.2 x 10^9 Pa.

    Other forum members should check whether I looked that data up correctly before we proceed.

    If those numbers are correct then, as a generality, plywood is stiffer and lighter than PVC, but , as another generality, beams with a round cross section have some advantage over solid cross section beams if both are constructed using the same amount of material. It would involve considerable labor to make a round beam out of plywood.
     
  17. Jan 14, 2017 #16

    berkeman

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  18. Jan 14, 2017 #17

    Stephen Tashi

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    Courses in "The Great Courses" series of DVDs aren't cheap, but I recommend "Understanding The World's Greatest Structures" as one of the best of that series. It contains an intuitive discussion of structural elements - columns, beams, arches, etc.
     
  19. Jan 14, 2017 #18
  20. Jan 15, 2017 #19
    So I read all the pages online, very interesting
     
  21. Jan 15, 2017 #20

    rbelli1

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    How did you come to this conclusion?

    There may be other options you didn't think of. A drawing would be helpful as berkeman suggested.

    BoB
     
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