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Homework Help: Design a tension member w/ cross section

  1. Sep 27, 2010 #1
    design a tension member w/ cross section....

    1. The problem statement, all variables and given/known data

    You are to design a tension member (square rod) with a cross section to carry a mass of 575 000 kg with a safety factor of 2 against yield and a safety factor of 3 against static fracture (overload).

    a) What is the design stree that you will use for the application? Why?

    b) What are the appropriate bar dimensions for use?

    2. Relevant equations

    FS = UTS / R

    [sigma] = F / Ao

    Ao = L * w

    3. The attempt at a solution

    FS = 1279MPa / 3 = 426.33333... MPa ~ 426 MPa
    that's the design stress considering static fracture

    FS = 862 / 2 = 431 MPa
    that's the design stress consider yield stress/strength

    So, I'm considering using a design stress that is the average of these two values, which would be: 429 MPa

    I'm just confused, because we haven't done anything like this in class.

    Now for part b)

    [sigma] = F / l*w

    F = 575 * 103(kg) * 9.81(m/s2)
    F = 5.641 MN (mega Newtons)

    429 = 5.641 MN / Ao[/SUP]

    Ao = 13148.6014 MM (mega meters? I'm not sure about the units on this one)

    l = w = 6574 MM

    Please help and thank you.
  2. jcsd
  3. Sep 27, 2010 #2


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    Homework Helper

    General_Sax: First, where did 1279 MPa, and 862 MPa, come from? I do not see that in the problem statement. Secondly, MN/MPa = MN/(MN/m^2) = m^2. Or N/MPa = mm^2. Third, use the lowest allowable stress, not the average.
  4. Sep 27, 2010 #3
    Re: design a tension member w/ cross section....

    Thanks for the help!
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