Design of individual members in a steel truss bridge?

In summary, the conversation discusses the design of a steel structured truss footbridge using beam analogy to determine member sizes. The maximum design moment (M*) is a challenge and calculations have been made for self-weight preliminary design. Trial and error has led to satisfactory results for the chords, tension-web, and compression-web members. The question arises about how to solve for M*<Mb and whether it needs to be done individually for each member. It is determined that Mb may be necessary for the whole truss if the compression flange at the top is unrestrained against lateral buckling. However, this depends on the overall design concept and restraint placement.
  • #1
gards
5
0
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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  • #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.
 
  • #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.
 

1. What materials are typically used in the design of individual members in a steel truss bridge?

In the design of a steel truss bridge, the most commonly used material for individual members is high-strength steel, such as ASTM A992 or A572. Other materials that may be used include weathering steel, which can provide a rust-like appearance without compromising structural integrity, and stainless steel, which is typically used for special applications where corrosion resistance is a major factor.

2. How are the dimensions of individual members determined in the design of a steel truss bridge?

The dimensions of individual members in a steel truss bridge are determined through a combination of structural analysis and design codes. Engineers will first analyze the loads that the bridge will be subjected to, such as traffic, wind, and seismic forces. Then, they will use design codes, such as the American Institute of Steel Construction (AISC) Manual of Steel Construction, to determine the appropriate dimensions for each member based on the loads and the material properties.

3. What factors are considered in the design of individual members to ensure structural stability?

In the design of individual members in a steel truss bridge, engineers consider a variety of factors to ensure structural stability. These include the loads that the member will be subjected to, the material properties, such as strength and stiffness, and the geometry of the member, such as its length and cross-sectional shape. Additionally, the connections between members are carefully designed to ensure that they can transfer the loads effectively without compromising the stability of the structure.

4. How are individual members connected in a steel truss bridge?

The connections between individual members in a steel truss bridge are typically made using bolts or welds. The type of connection used will depend on the specific design and the loads that the connection will be subjected to. Bolted connections are typically used for members that will experience tension or shear forces, while welded connections are used for members that will experience bending or torsion.

5. What are some common design considerations for individual members in a steel truss bridge?

Some common design considerations for individual members in a steel truss bridge include the strength and stiffness of the member, the potential for corrosion, the effects of fatigue and stress concentrations, and the constructability of the member. Engineers must also consider the potential for different types of loading, such as live loads from traffic and dead loads from the weight of the structure itself, in their designs.

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