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L (Angle) & C (Channel) beam sections versus an I section

  1. Jun 3, 2014 #1

    rollingstein

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    In terms of unit material spent my impression is that an I beam uses it most efficiently at least for non-torsional loads? i.e. putting most material far from the axis into the flanges giving higher moments.

    If so, what are situations when an angle or channel cross sections are preferred in structural work? Per unit mass is the rolling mill cost of an I beam higher?

    For beams & columns would an I section always be preferred? Or not?
     
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  3. Jun 3, 2014 #2

    Baluncore

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    RSJ or “I” beam is optimised for horizontal weight bearing members. Failure of RSJ is usually buckling of the top flange in compression. RSJ is especially convenient where the top flange can be controlled by the supported structure or slab.

    A closed section tube is best for columns. But in a large structure where column stability or torsion is not critical, an “I” section Universal Column is often used. UC in walls can be oriented so that their directional strength keeps the wall flat.

    Angle is better for tension members, angle is also good in some compression situations. Channel can be used for horizontal tension members where angle would sag.

    Channel generates twist or side forces when loaded as a horizontal beam. Where attachment to the web of an RSJ is needed, a channel section may be a better choice.
     
  4. Jun 5, 2014 #3

    rollingstein

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    Thanks @Baluncore.

    Can you explain why an angle is better as a tension member? Say it was a 2D truss with a member in pure tension, no shear nor Bending Moment; is that what you mean?

    Even if so, the tension resistance should depend only on C/S area right? So an angle, channel, I beam etc. all seem equivalent so long as same area is maintained?

    Or not? Am I making a mistake?
     
  5. Jun 5, 2014 #4

    Baluncore

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    You are correct for simple 2D trusses. But we live in the real world.

    Consider an electrical transmission line tower. The legs are angle in compression, girts are also angle but closer to neutral so must handle compression, all the diagonal bracing is angle in tension.

    Where tension members cross they are bolted together to stop them rattling against each other in the wind. By using angle rather than straps of flat bar, the tension members will be more rigid and so will not sympathetically oscillate, or flutter in the wind, which would lead to rapid fatigue failure.

    Angle has the advantage of being easier to join since the 90° angle gives more degrees of freedom.
    Angle can have a flat surface oriented uppermost, so it is easier to climb than flat bar on edge.
    Angle is easier to roll and packs better than channel or RSJ.
     
    Last edited: Jun 5, 2014
  6. Jun 5, 2014 #5

    rollingstein

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    All excellent reasons. Thanks.

    So, it seems the reasons you'd use an angle instead of, say, an I beam is join-ability, fabric-ability, cost, transportation logistics, & other such concerns.
     
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