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Compression/Tension in Steel I Beams

  1. Mar 24, 2017 #1
    1. The problem statement, all variables and given/known data
    Why is a steel beam shaped in the form of the letter "I" still strong enough to bear a load?
    1) Its compression area is located on the top of the I beam.
    2) Its tension area is located on the top of the I beam.
    3) Its neutral area is located on the top of the I beam.
    4) Its compression area is in the middle of the I beam.
    5) Its neutral area is located in the middle of the I beam.

    Most of the material in these I-beams is concentrated in the top and bottoms parts, called the flanges. The piece joining the bars, called the web, is thinner. Stress is predominantly int he top and bottom flanges when the beam is used horizontally in construction. One flange tends to be stretched while the other tends to be compressed. The web between the top and bottom flanges is a region of low stress that acts principally to hold the top and bottom flanges apart.

    2. Relevant equations
    n/a

    3. The attempt at a solution
    I think its #5, only because the neutral layer is in the middle of the block and that's where neither compression nor tension occurs. I figured that's why it can support a load. I'm unsure though and would like a second opinion.
     
  2. jcsd
  3. Mar 24, 2017 #2

    berkeman

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    Staff: Mentor

    But the question is asking about why I-Beams are strong. #5 is a true statement, but doesn't really answer the problem's question, IMO. Try again?
     
  4. Mar 24, 2017 #3
    Then what's the answer? There are only 5 choices
     
  5. Mar 24, 2017 #4

    berkeman

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    You tell us! :smile:

    Are any of the other statements true, and dealing with the "strength" issue?
     
  6. Mar 24, 2017 #5
    I wouldn't be asking if I knew, haha
     
  7. Mar 24, 2017 #6
    I really don't know and that's why I posted the question.
     
  8. Mar 24, 2017 #7

    berkeman

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    Well, it's against the PF rules for us to give answers on homework / schoolwork questions. We can give hints, ask questions, etc.

    So, say in words what you think about each of the choices #1-#5. Which are true and which are false?
     
  9. Mar 24, 2017 #8
    oh my godddddddddddddddddd I'm just trying to finish this assignment and y'all tripping
    1) Its compression area is located on the top of the I beam.
    this one is true because that's where the weight is

    2) Its tension area is located on the top of the I beam.
    no, the bottom is where the tension area is

    3) Its neutral area is located on the top of the I beam.
    no

    4) Its compression area is in the middle of the I beam.
    that doesn't even make sense, there is little to no stress on the web of an i beam

    5) Its neutral area is located in the middle of the I beam.
    yeah but you said this wasn't right

    there are my reasonings
     
  10. Mar 24, 2017 #9

    berkeman

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    Is there a figure that goes along with this question? How is the beam loaded? I was assuming it was being used in a cantilever configuration, but it sounds from your answers like it is center-loaded with a weight down...
     
  11. Mar 24, 2017 #10
    if the beam is standing like an I, the weight is on top of it with shear (vertically downward force) force in the middle of the top flange
     
  12. Mar 24, 2017 #11

    berkeman

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    BTW, if it is center loaded with a weight down and supported on its ends, your reasonings are correct in your post #8.
     
  13. Mar 24, 2017 #12

    berkeman

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    Okay, then I agree with your answer of #1 being true and describing the strength of the I-Beam. Good job! :smile:
     
  14. Mar 24, 2017 #13
    What does the top of the beam being in compression have to do with the strength of the beam, especially since the bottom of the beam is in an equal amount of tension? I don't think any of the answers have anything to do with strength. Maybe they meant stiffness? This is a very puzzling question to me. The web part of the beam has lower tensile and compressive stresses, but the entire cross section has shear stress. Still, the highest stresses (combined shear and tensile) have to be near the top and bottom.
     
  15. Mar 24, 2017 #14
    That's why I'm so confused, all parts of the beam contribute to it's strength
     
  16. Mar 24, 2017 #15
    Well, if you are ever able to get them to make sense out of this question they have given you, I would be interested in hearing about it.
     
  17. Mar 24, 2017 #16
    My first answer was correct and as a result I got the question wrong since I put #1. Thanks, MENTOR.
     
  18. Mar 24, 2017 #17
    For someone a part of the staff, you don't know your physics very well. I will not be returning to THIS site.
     
  19. Mar 24, 2017 #18

    berkeman

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    Huh, strange. Sorry that it didn't work out this time. Were you able to ask the instructor why answer #5 was better than #1?
     
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