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

1. Apr 29, 2010

### gards

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. May 9, 2010

### pongo38

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.

3. May 9, 2010

### pongo38

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.