Design of steel truss footbridge

[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
??

 

[Studiot]

You said yourself you are using the trial and error method or if you like
pick a section and match its properties against the loads.


I note you have used partial factors for the loads, but not the material properties?

It is normal to have only one bearing at each end for a footbridge. You will obviously have to brace the ends further against torsion and differential thermal expansion.

Talking of thermal expansion, take note that there will be considerable differential expansion on a steel bridge of this length when the sun shines on directly one side, but not the other.

 

[gards]

Hi Studiot,

One end is pinned jointed, other on a roller.
This project is only a couple weeks in duration, thus thermal expansion is not a necessity.

My main concern is determining the design moment, so i am able to check if these members agree with the N* loading...

 

[Studiot]

Not sure why the moment is a problem.

You either design as a truss, in which case all the loads are considered axial and there are no bending moments.
In this case the main extra issue is to ensure that compression members are safe from buckling.

Or

You design as a latticed beam. The moments in a latticed beam are no different from any other type of beam, they are determined by the geometry and the loads.
In this case the extra design is mainly to ensure the member connections are stiff enough, as well as checking the compression members against buckling.

Or you could go for plastic analysis, based on the beam approach.

 

[gards]

Could you possibly explain the plastic analysis, based on the beam approach?

:)

 

[Studiot]

Only in principle since I don't know what your members look like.

In principle you locate the position of the plastic hinges at the positions of maximum moment and then use the fact that for a beam the hinge cannot develop until all the material at that location is stressed to the elastic limit.

If you look in Universal Beam tables you will find plastic moments of resistance, based on this principle.

I had a thought is your difficulty finding the moment of resistance?

 

[gards]

Yeah, I think that's another way of expressing it.
I am trying to find the maximum design moment