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Homework Help: Design of individual members in a steel truss bridge?

  1. Apr 29, 2010 #1
    Hi, I have to design a steel structured truss footbridge.
    Im using beam anology to determine the correct member sizes.
    Having trouble determining the maximum design moment (M*)

    Here's what I have calculated thus far:

    Q=6kN/m
    G=1kN/m
    50m long, 3m depth, 3.125m spacing

    Self Weight Preliminary Design
    Chords :150UC30.0
    Tension-Web :90x90x6EA
    Comp-Web :150PFC
    therefore SW=4.48kN/m

    W*=1.2(1+4.84)+1.5(6)
    W*=16/2
    W*=8kN/m per truss
    M*=2500/3m depth
    N*=833kN

    After trial and error, all satisfy N*<Nt,Nc
    Chord :200UC46.2
    Tension-Web :125x125x12EA
    Comp-Web :380PFC

    From this point onwards, how do I solve for M*<Mb?
    And do I solve M*<Mb individually for all members?:
    M*<Mb,200UC46.2
    M*<Mb,125x125x12EA
    M*<Mb,380PFC
    ??
     
  2. jcsd
  3. May 9, 2010 #2
    M* was for the beam analogy, and it produced N*. Provided the loads are at the nodes only, then the only M is from self-weight of a member, and you need not check any further.
     
  4. May 9, 2010 #3
    On second thoughts, if the compression flange at the top is unrestrained against lateral buckling, then you do need Mb for the whole truss. That's a hard one. It depends on your whole design concept and where restraint is.
     
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