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Homework Help: Statically Indeterminate Bar with alternating area and length

  1. Sep 15, 2009 #1
    http://img197.imageshack.us/img197/5981/probie.png [Broken]

    So first I did a force balance (dFx=0)


    RD= RA - P
    RA = RD + P

    I know the total change in length is equal to 0, so

    dAB + dBC + dCD = 0


    (RA*L1)/(E*A1) + (P*L2)/(E*A2) + ((RA-P)*L1)/(E*A1) = 0

    but when I do the algebra I can never get it to work. my numbers never match the answer no matter which way I approach it. I can do problems with only 2 different sections but this 3 sectioned one has me baffled.

    the answers are
    RA= 10.5kN to the left
    RD= 2kN to the right

    anyone have any insight?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 15, 2009 #2


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    Science Advisor
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    Gold Member

    You are assuming that the force in the middle segment is P. It is not. Cut a section (Free Body Diagram) thru the middle bar and determine the force in that bar as a function of the known and unknown variables.
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