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The best place for a beam support?

  1. May 16, 2012 #1
    Question 1: On a typical wooden hanging sign you have three pieces of beam. One main beam extends upwards from the ground and at the top a beam slices through this (so that on one side of the slicing beam there is more beam than the other) with the sign hanging down from the longer section. From the short section another beam (quite short) extends diagonally back down to the main beam. Why is the diagnal beam (could be chain) placed here rather than diagonally on the longer side (same side as the sign) ?

    Question 2: If, instead of being used to hold up a sign, the beam structure was used to support an object which had been placed so it was sitting on the very end of the long section of the slicing beam why would the best place for a supporting arm be diagonally from the longer section to the main beam rather than from the shorter section to the main beam (the shorter section being the section on the other side of the main beam after the beam has sliced through the main beam)
     
    Last edited: May 16, 2012
  2. jcsd
  3. May 16, 2012 #2

    haruspex

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    When the diagonal is a chain it has to be on the opposite side from the sign. There, it will be under tension. A diagonal on the same side as the load would be under compression, not a suitable use of a chain, and not ideal for a metal strut either.
    For a wooden diagonal it's less critical, so that could be placed on either side. The main consideration in this case may be that on the load side it would obstruct the load, requiring a longer horizontal.
    That leads into your Q2. Where the load is necessarily displaced some distance from the upright member, providing the diagonal support on that side can greatly reduce the bending moment on the horizontal. If the diagonal meets the horizontal very close to the load there is almost no bending moment, and the only strain on the horizontal is tension.
    In short, if the upright can be close to the load, that is usually the way to go. If it can't, you want the diagonal close to the load.
     
  4. May 17, 2012 #3
    Thanks a good clear reply
     
  5. May 17, 2012 #4
    On question 1 why is there tension on the opposite side and compression on the other exactly? Is tension caused because the chain supports from the far end of the horizontal (utilising the entire horizontal and upright structure) instead of part way along and at a diagonal away from the sign in which case you are saying it is simply pressed to the upright? (compression)


    Question 2: Does the diagonal support have to to be on the same side of the horizontal as the load (ie from the top part which the load is resting on to the upright rather than from underneath the horizontal to the upright) because the weight is pressing down here? Why can't it extend on the same side of the upright as the weight but from underneath the horizontal? Would this cause the horizontal to sag?
     
    Last edited: May 17, 2012
  6. May 17, 2012 #5

    haruspex

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    From your new q2, I see there's an ambiguity needs clearing up. I have assumed throughout (as I thought you had) that the diagonal is below the line of the horizontal. It could instead be above (assuming the vertical extends above the horizontal). In that case, matters are reversed. It would be under tension on the side towards the load and under compression on the other side. Indeed, it is not unusual to see a chain connecting the very top of the upright to the end of the horizontal beyond the load.

    Think of the cross-beam as able to pivot on the vertical. When you add the load, the cross-beam will fall on that side and rise on the other. Hence a constraining diagonal will be under compression either under the horizontal & near the load or above the horizontal on the side away from the load; and under tension in the other two locations.
     
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