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Compression and Tension

  1. Feb 6, 2007 #1
    Hey, first post! I happened upon this forum and after about ten seconds of perusing topic titles, decided this forum was definately for me! Here is my question. What type of cross section would a beam need to be the most efficient at handling 1) tension and 2) compression.

    I am working on a science olympiad boomilever, kind of like someone else who made a thread recently. I attached a picture (hopefully correctly) that shows what the boomilever will look like basically. It has to support a 15 kg weight 40 cm from the wall, and be as light at possible, like 10 or 15 grams (!). Heres what I have in mind There will be two 40 cm compression chords, and one 45 cm tension chord. The lever will be mounted by one bolt just above where the tension chord meets the wall. The chords have to be smaller than 1/4" by 1/4", and I will make them by laminating several 1/16" sheets of balsa or bass. I have heard that bass is better for compression and balsa is better for tension, but bass is heavier, and I dont want 2/3 of my lever to be bass, so I think I will try first with all balsa, and switch to bass if the compression chords are too weak.

    OK so for my second question. Would having only one tension chord be alot weaker? Am I correct in thinking that the tension chord should be rectangular, with the thickest dimension supporting the downward pull? What about the cross section of the compression chords? Terribly sorry if all of this is unclear, be sure to look at the picture and ask if anything needs clarification.


    Attached Files:

  2. jcsd
  3. Feb 7, 2007 #2
    How long does it have to be, and how will it attach to the wall? There will be bending stresses that increase as the length of the compression member gets longer.

    I would make the tension member out of something lightweight and strong, like fishing wire. Also, what do you mean by "compression chord"? Wires only work in tension.
    Last edited: Feb 7, 2007
  4. Feb 7, 2007 #3
    Everything has to be made out of wood and glue. The weight must be held 40 cm from the wall, so that is why the compression chords are perpendicular to the wall, and not the tension, because tension doesnt increase with length.

    I think that I decided that I will have two tension chords, I will just angle them so that they are next to each other at the wall, and then farther apart at the end. The reason I need them to be close together is so that the attachment base (the heaviest part) is as small as possible.
  5. Feb 7, 2007 #4
    What do you mean 'two tension chords'?

    ....and what is a chord? Do you plan on playing music?
  6. Feb 8, 2007 #5


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    "chord n.
    1. Math. & Aeron. etc. a straight line joining the ends of an arc, the wings of an aeroplane, etc.
    4. Engin. one of the two principal members, usu. horizontal, of a truss."

    (Concise Oxford Dictionary, 9th edition).
  7. Feb 8, 2007 #6
    This is not an arc, or an airplane wing. Calling it a truss....maybe. It's a stretch.

    The correct term is called a memeber.
    Last edited: Feb 8, 2007
  8. Feb 8, 2007 #7


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    Well, I suggest you get in touch with the Science Olympiad and tell them they don't know what they are talking about. See
    http://www.soinc.org/events/boomilever/Boomilever_Design.pdf (page 2)

    BTW what language is "memeber"? British English, not... :rolleyes:
  9. Feb 8, 2007 #8
    The science olympiad refers to it as a chord, but member is probably more accurate. Putting semantics aside, is one cross section inherently better at handling tension and or compression? Like an I-beam or what? Maybe the member just needs to be the full 1/4" in the direction it needs to provide support and thin(ner) in the direction it does not?
  10. Feb 8, 2007 #9
    How about I just open my statics or mechanics of materials book, which no where does it call its members a chord?

    Obviously, they DONT, or they would not use terms for an airfoil to talk about a structual member.
    Last edited: Feb 8, 2007
  11. Feb 8, 2007 #10


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    I've come across a few engineers who used "chord" in this sense in the past 30 years, and they all came from America. So I assumed it was an American English thing, like sidewalks and elevators. The Oxford (British English) dictionary is generally "pro-American" in spelling etc, so I wasn't surprised (or even surprized) to find the meaning there.

    Personally I would call them "struts" and "ties" (depending whether they were in compression or tension) - not "structural members". But part of the "north/south" divide in the UK is that northerners prefer short germanic or norse origin words, and southerners prefer long latin or french-origin words instead...
  12. Feb 8, 2007 #11
    Either way, calling it a chord just sounds awkward to my ears. I dont want to bicker about something so trivial. :smile:

    Classic example, the "two force member."

    Last edited: Feb 8, 2007
  13. Feb 12, 2007 #12
    The term chord is typically used for trusses. You have a top chord, and a bottom chord. The are different from the diagonals, or struts.
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