Design of individual members in a steel truss bridge?

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SUMMARY

The discussion focuses on the design of a steel truss footbridge using beam analogy to determine member sizes and maximum design moment (M*). The user calculated various parameters including loads (Q, G), self-weight (SW), and axial forces (N*), ultimately determining preliminary member sizes such as 150UC30.0 for chords and 90x90x6EA for tension-web. The user seeks guidance on ensuring that the maximum design moment (M*) is less than the moment capacity (Mb) for each member, particularly addressing concerns about lateral buckling in unrestrained compression flanges.

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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
??
 
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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.
 
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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