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Shear flow in thin wall members

  1. Nov 9, 2016 #1
    1. The problem statement, all variables and given/known data

    In the notes , I don't understand that flange is thin , and the top and bottom surface are free of stress , can someone help to explain please?

    Secondly , why the q' is assumed to be q ' throughout the flange is assumed to be 0 ? why are they stress free ? We could see that , the shear force V is applied to the top of the beam right ?


    2. Relevant equations


    3. The attempt at a solution
    why are they stress free ? We could see that , the shear force V is applied to the top of the beam right ... So , there must be some magnitude of stress acting on top or bottom or the beam
     

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  3. Nov 9, 2016 #2

    PhanthomJay

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    that is right. The transverse Horizontal shear stresses ( and accompanying longitudinal shear stress) are non zero on the flange except at the ends. The reference to zero shear stress in the flange is talking about negligible vertical shear stress in the flange due to assumed thin wall.
     
  4. Nov 9, 2016 #3
    why the wall is thin , and the shear stress is negligible ? ?
     
  5. Nov 9, 2016 #4
    do you mean likt his ? in the notes , it's stated that the shear flow components that acts parallel ro the sides of flange will be considered ?

    Does it mean that the red shear flow will be considered ( since it's side of flange) ? while the blue shear flow will be ignored since it's top and bottom are free of stress ?
     

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  6. Nov 10, 2016 #5

    PhanthomJay

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    The plot thickens. The blue (horizontal shear flow) and the red (longitudinal shear flow) exist simultaneously. You can't have one without the other. What is ignored is the small vertical shear stress in the flange, assumed to be zero since the web carries almost all the vertical shear. Also, you have an extra red arrow in there pointing in the wrong direction.
     
  7. Nov 10, 2016 #6
    why the web carries almost all the vertical shear ? how to know it ? do you mean shear flow ? the web carries almost all the vertical shear ? (refer to the photos uploaded , do you mean it is the vertical arrow alll pointing downwards in the web ? while no vertical arrow at all in the flange , so the web is assumed to be taking all the vertical shear ?
     
  8. Nov 10, 2016 #7

    PhanthomJay

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    No upload. But web carries most of vert shear, flanges only a small amount
     
  9. Nov 10, 2016 #8
    sorry , image here
     

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  10. Nov 10, 2016 #9

    PhanthomJay

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    This is getting way too confusing. Image shows vert shear stress distribution in web, and horiz shear stress distribution in flange. Vert shear stress distribution in flange (the 3 MPa vert shear stress we talked about earlier) not shown.
     
  11. Nov 10, 2016 #10
    can you explain on why But web carries most of vert shear, flanges only a small amount ?
     
  12. Nov 11, 2016 #10

    PhanthomJay

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    Recall that vert shear stress is VQ/it. For flange, t is quite large , it is the full width of the flange, so when you divide by a large number you get a small number relative to the web shear stress where t is only the web thickness.
     
  13. Nov 13, 2016 #11
    Can i use your theory to explain the horizontal shear stress ?
    to find horizontal shear stress , we have to cut vertically , since the 'thicnkness' at the web is large , so the horizontal shear stress is so small compared to the horizontal shear stress at web , so , horizontal shear stress at web is negligible ?
     
  14. Nov 14, 2016 #12

    PhanthomJay

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    I am not following you. Horizontal shear stress is in the flange at a vert cut through the flange. Flange thickness is small. Regarding web, that is just vertical shear stress, which controls design..
     
  15. Nov 17, 2016 #13
    is my theory of

    to find horizontal shear stress , we have to cut vertically , since the 'thicnkness' at the web is large , so the horizontal shear stress is so small compared to the horizontal shear stress at web , so , horizontal shear stress at web is negligible ? only the horizontal shear stress at the flange is considered ?

    wrong ?
     
    Last edited: Nov 17, 2016
  16. Nov 18, 2016 #14

    PhanthomJay

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    For an I beam, in order to find the longitudinal shear stress in the flange at a vertical cut thru the flange, you use the area to the right of the cut in calculating Q, and then the longitudinal shear stress acts into the plane ( along the beam ) parallel to the face of the cut, and along the side of the cut. In order to find the longitudinal shear stress in the web at a horizontal cut thru the web, you use the area above the cut in calculating Q, and then the longitudinal shear stress acts into the plane ( along the beam ) parallel to the face of the cut, and along the top of the cut.

    In looking at a vertical flange cut near the web, the shear stress is about 1/2 the shear stress of a horizontal web cut near the flange, because the Q area for the former is about half the Q area of the latter. Neither stress is very significant unless you have a built-up beam requiring nails or welds in which case the concept of shear flow determines weld size or nail spacing.
     
  17. Nov 20, 2016 #15
    do you mean apply the cut at the center of the web ?

    just like the case below ?

    if so , then i found that the Q fir the vertical cut and horizontal cut are the same ... taking the diagram below as example , i have skteched the Q for vertical cut and horizontal cut .....
    so , the Q for the vertical cut is 30*50*15 =22500 ,, Q for the horizontal cut = 30*50*25 =37500 , so . it;s clear that the Q for the horizontal cut is higher , so the vertical shear stress is higher , $$\tau = VQ/It $$ , so the vertical shear stress at the web is higher , so it's considered , while the horizontal shear stress us lower at the web , so , it's ignored ?
     

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  18. Nov 21, 2016 #16

    PhanthomJay

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    I don't know what these numbers are that you are using.

    There are generally 3 directions of shear stress in a member......vertically up and down along the cross section, horizontally left to right or right to left along the cross section, and longitudinally into and out of the plane of the cross section.

    In the web, shear stresses are vertical and longitudinal. The stresses are determined by cutting horizontally at the point of interest, and using the Q value of the area above the cut, and then the longitudinal stresses act at the top and bottom of the cut face, into the plane. There are no horizontal shear stresses in the web , since if you were to make a vertical cut thru the entire web, the Q to the right of the cut would be zero, since the centroid of the area to the right of the cut is at the neutral axis, such that the vert distance from the centroid to the NA is 0.

    In the flange, shear stresses are vertical, horizontal, and longitudinal. The horizontal stresses are determined by cutting vertically at the point of interest, and using the Q value of the area to the right of the cut, and then the longitudinal stresses act at the side of the cut face, into the plane. There are small average vertical shear stresses in the web , which are typically ignored except when designing welds for a cover plate over the flange. The assumed zero vertical shear stress in the web, and its associated longitudinal stress , designated as (q'), is shown in figure e of the 1st post. The horiz shear stress , q, is non-zero.
     
  19. Nov 21, 2016 #17
    why ? can you explain it with diagram ?
    do you mean cut the web vertically thru the center of the web ?
     
  20. Nov 21, 2016 #18
    the diagram represent the web , if i apply a vetical cut thru the center of the web , the centroid of the area is the red dot , so , the y in Q = Ay is measured from the center to the centroid of the yellow area , why you said that y = 0 ? i'm confused
     

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  21. Nov 21, 2016 #19

    PhanthomJay

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    If you apply a vertical cut thru the web, the area to the right of the cut is the blue area. Its centroid is at the NA. The vertical distance from the centroid to the NA is 0.
     
  22. Nov 21, 2016 #20
    ok , for flange , when we apply a vertical cut , the area is the area to the right of the cut , then , the vertical distance from the centroid to the NA is 0. so , horizontal shear stress in flange also = 0 ?
     
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