KNCalculating Member Forces in Truss Structures

In summary, the conversation is about calculating the member forces of a truss by taking moments and using the equation -(2KN * 2.4) + 1*ReactionC = 0. The person initially gets confused about the direction of the 2kN force and its moment, but eventually understands how to correctly calculate the moment. They end up with a final equation of 2.8/cos(38.66) = 3.59.
  • #1
MMCS
151
0
Attached is a picture of the truss. I am trying to calculate the member forces.

I know a moment is = force * perpedicular distance, but in this case, for example, if i am taking moments about A, Would the equation be

-(1*2KN)+(1*c) = 0

or using an alternative perpedicular distance

-(2.4*2KN)+(0.8*c) = 0

For my preliminary working, assuming i have used correct perp distance i have

ƩFx = 0
ƩFy = 0
ƩMoments about a point = 0

Ay + By - 2KN = 0

Momments about A = -(2KN * 2.4) + 1*ReactionC = 0
-4.8KN = reactionC



Sub into first eq

Ay +(-4.8) -2 = 0
Ay = 6.8

To find out the reaction in member AB

I have

6.8 - Tab Cos(tan-10.8/1) = 0
Tab = 6.8/Cos(tan-10.8/1) = 8.738

I have the answer to be 2.56
 

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  • #2
In taking moments, the 2kN force has the same direction as the vertical distance on 1m, so its moment will be 0. If you get confused, you can always calculate M = r x F, where x is the cross product operator.
 
  • #3
SteamKing said:
In taking moments, the 2kN force has the same direction as the vertical distance on 1m, so its moment will be 0. If you get confused, you can always calculate M = r x F, where x is the cross product operator.

Im don't quite understand sorry, i would understand if the force was acting through the point i am taking momoments about but how can the 2KN force x distance away not cause a moment
 
  • #4
  • #5
Using the value of of 4.8 but the same working i end up with a final equation of

2.8/cos(38.66) = 3.59
 
  • #6
Ok i realize where my confusion was on this problem thanks for your help
 

1. What is a truss structure?

A truss structure is a framework made of interconnected bars or beams that are designed to support loads and distribute forces. It is commonly used in the construction of bridges, roofs, and other large structures.

2. Why is it important to calculate member forces in truss structures?

Calculating member forces in truss structures is important because it helps engineers and architects ensure that the structure is strong and stable enough to support the intended loads. It also allows for the optimization of materials and cost-effectiveness in the design process.

3. What factors affect the member forces in a truss structure?

The member forces in a truss structure are affected by the type of loads it is designed to support, the arrangement and size of the truss members, and the overall geometry of the structure. Other factors such as wind, temperature, and seismic activity may also play a role.

4. How are member forces calculated in a truss structure?

Member forces in a truss structure are typically calculated using principles of statics and equilibrium. This involves analyzing the external loads acting on the structure and solving for the internal forces and reactions at each joint using equations such as the method of joints and method of sections.

5. Are there any limitations to calculating member forces in truss structures?

While calculating member forces is an important aspect of truss design, it is not the only factor to consider. Other factors such as material properties, connections, and construction methods can also affect the overall stability and strength of a truss structure. Additionally, idealized assumptions made in calculations may not always accurately reflect real-world conditions, so it is important for engineers to use their professional judgment and experience in the design process.

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