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Determine the load

  1. Oct 26, 2015 #1
    1. The problem statement, all variables and given/known data
    I am asked to find the load in AD, BD, and DC....
    I have AD(2/surd38) +BD(2/surd29)=800
    AD(3/surd38)=DC
    I can only formed two equation.... But , so many unknown... How to solve this???

    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Oct 26, 2015 #2
    Here's the diagram
     

    Attached Files:

  4. Oct 26, 2015 #3

    SteamKing

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    Instead of just writing down a random equation, you should:

    1. Write down the known information in a logical order
    2. Write the general equations of static equilibrium (ΣF = 0; ΣM = 0)
    3. Analyze the problem to determine if there are simplifications or special conditions which may apply.
    4. Write the equations of equilibrium for this problem.
    5. Solve these equations to determine the unknown reactions / internal loads.

    For this problem, you could first calculate and write down the lengths of the members involved: the bar BD and the lines AD and CD.
    Step 2 is pretty much taken care of.
    Step 3: The bar BD can take either tension or compression. The lines AD and CD look like they can take only tension. There is only one applied load of 800 kN.

    Here's the critical step:

    Write the equations of equilibrium for this structure after completing steps 1-3.
     
  5. Oct 26, 2015 #4
    how do u knw The bar BD can take either tension or compression. The lines AD and CD look like they can take only tension??


    since this is an exercise that involves 3D , then we can only consider moment about x , y and z axis , right ? we cant consider moment about a specific point ?
     
    Last edited: Oct 26, 2015
  6. Oct 26, 2015 #5

    SteamKing

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    Because that's the nature of stiff members like bars. Bars can be stretched in tension, compressed, even bent by applying moments to the ends.

    Ropes and wires can only sustain tension loads. Repeat after me: You can't push on a rope.

    I'm curious: haven't you ever tried to build anything from scratch, played with Tinker Toys, hung a picture on a wall with a nail and some wire, heck even picked up a stick?
    This is how we learn about the mechanical nature of different things, like bars, strings, rope, and the like. There's more to experience in life than banging keys on a keyboard.

    I don't know what "moment about a point" means. You can evaluate moments only about an axis.
     
  7. Oct 26, 2015 #6
    moment about a point means we simply pick a point , and claculate the moment at it . For example , we take the point A , and we calculate the moment ( force x distance ) about it .
     
  8. Oct 26, 2015 #7

    SteamKing

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    You can calculate moments about an axis which passes thru a point, but you can't calculate moments about a single point. :wink:

    In any event, have you progressed any further with your analysis of this problem?
     
  9. Oct 26, 2015 #8
    I have ΣFx = -AD(5/surd38) -BD(5/surd29) =0
    ΣFy = AD(3/surd38)-DC=0
    ΣFz= BD(2/surd29) +AD(2/surd38) -800= 0
     
  10. Oct 26, 2015 #9

    SteamKing

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    You might want to check the x, y and z-components of AD and the length of AD.
     
  11. Oct 26, 2015 #10
    i couldnt see whats wrong with it
     
  12. Oct 26, 2015 #11

    SteamKing

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    Look again at the diagram carefully. The length you have calculated for line AD is incorrect.

    What's the location of point A?
     
  13. Oct 27, 2015 #12
    The z coordinate for B is 2, for A is 4?
     
  14. Oct 27, 2015 #13

    SteamKing

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    And the length of line AD is?
     
  15. Oct 27, 2015 #14
    Surd50... The diagram is unclear
     
  16. Oct 27, 2015 #15
    Surd50... The diagram is unclear
     
  17. Oct 27, 2015 #16

    SteamKing

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    Seemed pretty clear to me. :wink:
     
  18. Oct 27, 2015 #17
    The 2 m looks like B to Z
     
  19. Oct 27, 2015 #18

    SteamKing

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    The dashed lines have some significance in the diagram here.

    There is no purpose in locating the position of the letter Z; there is a purpose in locating the position of point A.
     
  20. Oct 27, 2015 #19
    here's a case . it give the moment about the x , y , x' and y' axis .... but not moment about a specific point
     

    Attached Files:

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    • 01.png
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  21. Oct 30, 2015 #20
    so when we are asked to find the moemnt for a question involving 3D model , we can only use the moemnt about a specific axis to find the moment ? but not a specific point ? (for example. in the above case , we cannot find the moment about the O ( centroid) ) ?
     
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