Design of steel truss footbridge

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Discussion Overview

The discussion revolves around the design of a steel truss footbridge, focusing on determining the maximum design moment (M*) and the appropriate member sizes using beam analogy and trial and error methods. Participants explore various aspects of structural design, including load factors, member properties, and considerations for thermal expansion.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • The original poster (OP) presents calculations for loads and preliminary member sizes, seeking guidance on how to solve for M* in relation to member capacities (Mb).
  • One participant suggests that the OP's use of trial and error is valid but notes the absence of material property factors in the calculations.
  • Another participant emphasizes the importance of bracing against torsion and thermal expansion, particularly for a footbridge with a pinned and roller joint configuration.
  • A different viewpoint posits that if the design is treated as a truss, bending moments should not be a concern, focusing instead on axial loads and buckling safety for compression members.
  • Another participant introduces the concept of plastic analysis, explaining that it involves locating plastic hinges and understanding the material limits at those points.
  • The OP expresses that their difficulty lies in finding the maximum design moment, indicating a need for further clarification on the topic.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of bending moments in the design process, with some advocating for a truss approach while others consider a beam analysis. The discussion remains unresolved regarding the best method to determine the maximum design moment and the implications of thermal expansion.

Contextual Notes

Participants mention various assumptions, such as the duration of the project affecting the relevance of thermal expansion considerations. There is also a lack of consensus on whether to treat the structure as a truss or a latticed beam, which influences the approach to calculating moments and member safety.

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

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

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