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Homework Help: Designing a truss (working stress method) Missing Info?

  1. Oct 11, 2009 #1
    1. The problem statement, all variables and given/known data

    "Design a bridge over a valley 300m wide. Road surface is supported by a truss underneath."

    My problem: With calculations
    Now calculation for buckling says i need to design the truss for compression such that

    I > (P. L^2) / (pi^2 x E )
    or Pcr > P
    I = reduction value
    P = unfactored load
    L = length
    E= modulus
    pi = Pi.
    Pcr= Critical Load

    Now the information I'm given doesnt give me a value for I, just the rest. Not sure what I'm meant to do, just estimate a value and insert it in and do calculations from this or another method or if I'm meant to know the value fro plastic piping (?)
    - diameter (can work out effective area)
    - modulus
    - yield strength
    - length

    2. Relevant equations

    Sum Fx= 0
    Sum Fy= 0
    Sum M(hinge) = 0

    3. The attempt at a solution

    Not sure where to start, as it doesnt seem to be enough information?
  2. jcsd
  3. Oct 11, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Why not calculate I using your above formula?
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