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Moment in truss

  1. Jan 11, 2017 #1
    1. The problem statement, all variables and given/known data
    for the moment about A , why there 's moment generated by the forces of 0.8NB (circled part) ?


    2. Relevant equations


    3. The attempt at a solution
    I think it's not necessary to include the moment 0.8NB(4) because the d = 0.4m is not measured directly from point A ....it's measured from the red circled part ...
     

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  3. Jan 11, 2017 #2

    BvU

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    1. It's not 0.4 m but 4 feet and it's the vertical distance between B and A
    and that is multiplied with the horizontal component of NB
    2. the 10 feet is the horizontal distance etc.
     
  4. Jan 11, 2017 #3
    btw, this is not a truss, but a frame. Trusses are designed only under tensile and compression forces, no bending moments
     
  5. Jan 11, 2017 #4
    In the diagram attached below , we can see that the d (prepedicular distance) for NBy is measured directly from A , but for the NBx ( blue part) , it's measured directly from a point which is 10 m from A . So , for the moment 4NBx , it's not moment about A , am i right ? Why the author consider it in the calculation ?
     

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  6. Jan 12, 2017 #5

    BvU

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    No. The line of action of NBxis at a perpendicular distance of 4 feet from A
     
  7. Jan 12, 2017 #6
    why ?
     
  8. Jan 12, 2017 #7

    BvU

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    upload_2017-1-12_11-19-59.png

    A force acts along a line of action. You can always move it along that line without changing anything.
    Only when you move a force perpendicular to its line of action you have to add a torque.

    To reassure yourself: calculate the perpendicular distance of the line of action of NB itself to the point A and compare with the total torque from x and y components.
     
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