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Statics struss tension/compression

  1. May 31, 2012 #1
    1. The problem statement, all variables and given/known data


    see attachment
    2. Relevant equations



    3. The attempt at a solution
    Why is DG, DE in tension while HG is in compression according to the FBD? How did the book determine this? Do you just randomly guess tension/compression and then do the calculations and see if you get a positive/negative number? Is there a set procedure? Thanks.
     

    Attached Files:

  2. jcsd
  3. Jun 2, 2012 #2

    PhanthomJay

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    It's ok to make assumptions, consistent with newtons 3rd law, but limit the number of assumptions you make or else the plus and minus sign will devour you. I notice that the solution shows Ax pointing right, when obviously it must point left, so don't assume when you don't have to. Always calculate support reactions first, numerically, that's a big help. When you look at the right side cut section, since R points up, producing ccw moment about G, then Fde must point inward toward the joint, to produce a cw moment about G . Fde is compression by the way, since it points toward joint . Forces pulling away from joint are tensile.
     
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