Truss Bridge Tension and Compressions

In summary, the problem involves finding the tension and compression forces of the members of a truss bridge under the weight of a CL-625 design truck. Using the equations Mb = PL/4 and T=M/H, the total tension was found to be 625kN, but this may not be the most accurate method. It is suggested to use the Truss Method of Joints or Sections to find the forces in all members.
  • #1
noahsc
2
0

Homework Statement


Given the weight of a CL-625 design truck (625 kN) on a truss bridge of span 50m, divided into 8 sections (each 6.25 m) with a height of 6.25m making 45deg angles and a width of 6m, what are the tension and compression forces of the members?
pratt-1.jpg



Homework Equations



Mb = PL/4
T=M/H

The Attempt at a Solution


Using these equations, I got a total tension of 625kN.. I don't know if I'm using the equations wrong or I'm missing something completely, but that seems like a very small number.
 
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  • #2
You are not using correct equations...Trusses are subjected primarily to tensile and compressive stresses in each member from member forces under the applied loads, whereas Beams are subject to bending stresses from bending moments under the applied loading. Two different processes. You need to find the force in each member...where is the load placed?
 
  • #3
yeah I am going to restate the title.
this is only part of the problem. what i need to find is the area of the lower and upper chords with the equation T = phi*As*Fy with the Fy=350Mpa

So, what I have been instructed on doing is find the bending moment of the live load M=PL/4 and then find the tension using the moment arm of the height (or vertical truss)
 
  • #4
noahsc said:
yeah I am going to restate the title.
this is only part of the problem. what i need to find is the area of the lower and upper chords with the equation T = phi*As*Fy with the Fy=350Mpa

So, what I have been instructed on doing is find the bending moment of the live load M=PL/4 and then find the tension using the moment arm of the height (or vertical truss)
Your answer for the top chord max force of 625 kN compression, and bottom chord max force of 625 kN tension appears correct, but generally, this is not the way to proceed. There are forces in the diagonal and vertical members also,,,,,forces in all members should be found using the trusss Method of Joints or Sections.
 
  • #5
Could someone please help me out?

As a scientist, it is important to approach problems and calculations with a critical and analytical mindset. In this case, it is important to consider all factors and assumptions made in the problem before coming to a conclusion. Firstly, it is important to clarify if the given weight of 625 kN is the total weight of the truck or the weight per axle. This information can significantly affect the calculation of forces on the truss bridge.

Additionally, the given equations for determining tension and compression forces may not be applicable to this specific problem. It is important to consider the type of truss bridge and the forces acting on it, such as the weight distribution of the truck and the direction of forces. It may be necessary to use more advanced equations or even perform a structural analysis to accurately determine the tension and compression forces on the truss bridge.

Furthermore, it is important to note that the given dimensions and angles of the truss bridge may also affect the distribution of forces on the members. A more detailed and precise diagram or model of the truss bridge may be necessary to accurately determine the forces on each member.

In conclusion, while the given solution of 625 kN may seem small, it is important to carefully consider all the factors and assumptions made in the problem before coming to a conclusion. It may be necessary to use more advanced equations or perform a structural analysis to accurately determine the tension and compression forces on the truss bridge.
 

What is a truss bridge?

A truss bridge is a type of bridge that is made up of a series of interconnected triangles, which work together to distribute weight and support the structure.

How do tension and compression forces affect a truss bridge?

Tension forces occur when a force pulls on a material, while compression forces occur when a force pushes on a material. In a truss bridge, tension forces are present in the bottom chords, while compression forces are present in the top chords. These forces are essential for keeping the bridge stable and balanced.

How do engineers determine the tension and compression forces in a truss bridge?

Engineers use a variety of mathematical calculations, computer simulations, and physical testing to determine the tension and compression forces in a truss bridge. They also take into account factors such as the materials used, the weight of the bridge, and the expected loads it will need to support.

What happens if there is too much tension or compression in a truss bridge?

If there is too much tension or compression in a truss bridge, it can lead to structural failure and collapse. This is why it is important for engineers to carefully calculate and monitor these forces during the design and construction process.

Are there different types of truss bridges?

Yes, there are several types of truss bridges, including Pratt, Warren, and Howe truss bridges. Each type has a different design and arrangement of the truss members, which affects how tension and compression forces are distributed throughout the bridge.

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