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Concrete beam question

  1. Dec 10, 2011 #1
    I saw in some building constructions where they pour concrete in half of the horizonal beam (with rebar skeleton). Then the next day, they pour the other half of the rebar skeleton. This means the center of the concrete is not contniuous and only the rebars in the mid position holds the strength. Isn't this a bad idea? Or since constructions use it. What's the explanation why they allow that?
     
  2. jcsd
  3. Dec 11, 2011 #2
    As long as the beam is not in pure tension this will not be a problem as there would be zero forces on the very center of the beam. If the beam is flexed, one side will only be in tension and the other side will only be in compression. The side which is only in tension will be the side that needs the rebar, the compression side does not require it.
     
  4. Dec 11, 2011 #3
    In some 2 or 3 storey buildings in Turkey damaged by earthquakes where only one side of the foundations is affected. They remove the concrete cement in the column-beam joint and one meter of the beam and remove the column then put new column rebars and tie it to the existing beam rebars. Then pour concrete into it the joint and new column. Would this have similar strength as the original or would this be more weaken? How does this compare to the United States practice (like do they even allow it)?
     
  5. Dec 11, 2011 #4
    That is a common practice to repair damaged concrete structures, as long as the concrete mix is proper, and the rebar joints are put in the correct places it will be just as strong as the original. Think of it this way when buidling a structure out of concrete they don't pour it all at once! :smile:
     
  6. Dec 11, 2011 #5

    Thanks.

    An unrelated question. How come I can see construction techniques where steel horizontal beam is put on concrete column.. yet I don't see reinforce concrete beam being put on steel I-BEAM column?
     
  7. Dec 12, 2011 #6
    Because concrete is heavier than steel.
     
  8. Dec 12, 2011 #7
    What I just said is technically incorrect, steel is denser than concrete, but it usually takes more concrete to perform the same function in a building.
     
  9. Dec 13, 2011 #8
    Or maybe because of ductility. To attach horizontal reinforced concrete beam to a vertical steel i-beam. The point of attachment can break the cement due to unequal steel to cement stress and tension. Maybe this is why you don't see this technique being done. Right?
     
  10. Dec 13, 2011 #9
    It can be done, but the point you are making is generally why it is not done.
     
  11. Dec 13, 2011 #10
    Why would anyone do it that way?
     
  12. Dec 13, 2011 #11
    Perhaps the price of concrete is cheaper than steel, that would be a reason to do it that way.
     
  13. Dec 14, 2011 #12
    but how do you connect concrete with rebars to i-beam? maybe by soldering the rebars right to the i-beam? Does any construction do that?
     
  14. Dec 14, 2011 #13
    let's just use a concrete support column for example
    on top there's a bracket
    anchored in the column
    the steel i beams are BOLTED to the bracket
    i don't know if it's practice to
    also weld the bracket to rebar before pouring concrete
     
  15. Dec 14, 2011 #14
    But it's the other way around.. the support column is i-beam and the horizonal beam is concrete. Has anyone actually seen this technique being done in original or most likely retrofit situations anywhere? If yes, where? I just want to understand the stress in the column-beam joint in such scenerios as I'm studying about column-beam joint seismic shear resistance and failure.
     
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