Forces on a Truss Homework: Find Tension/Compression

In summary, Brad is trying to solve a truss problem and is having trouble. He has completed one part of the problem, but needs help with another part.
  • #1
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Homework Statement



Ok, you'll have to wait for my picture. Basically we are doing trusses, and I'm not sure where I'm going wrong here.

We are to find the tension/compression in each member(line) of the truss.

Ok, from my picture. F1 is directly at B(in neg y direction), and is 2800N. F2 is at point D(in the neg x direction) and is 2100N.

The distances between A&B, and B&C are 4m. A is a pin system(basically has x and y forces)...and C is held up by a cable(I believe only supports in the y direction).

I also drew another picture, which consists of little circles on each joint, and arrows pointing in the directions of the tensions(and forces acting on the "circles").

Homework Equations





The Attempt at a Solution



Ok, I'm only going to post some of this, to see if I started correctly(where I think I messed up).

So, from my picture, I took overall equilibrium. So, I chose to sum moments about point A=0(with a counter-clockwise rotation I chose).

moments at pt A = 0= 8Cy -2800(4) - 2100(3.4641)
Cy = 1259.33N up

Sum Forces in +x direction=0

Ax - 2100 = Ax=2100N to the right

Sum forces in +y direction = 0

Ay + Cy - 2800 = Ay = 1540.67N up

If this is correct, then I'll post more. Next, I drew that other picture, and started at points and found the tension(as I draw them all in tension to begin with) at each point.

Thanks,
Brad

View attachment problem 6.bmp
 
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  • #2
Looks okay to me.
 
  • #3
For point A:

Sum Forces in +x=0:

Tab + Tadcos(60) =0 Tab = -3558.02N
Sum Forces in +y=0:

Ay(or 1540.67) - Tadsin(60) = 0 Tad = 1779.01N

I'll stop here again too.

The book says Tab = 2.38kN and Tad = 0.567kN

So, what did I do wrong?
 
  • #4
Is this portion correct?
 
  • #5
trying to do another one of these now...so any help would be nice:)
 
  • #6
Is my drawing or the last post not clear...I'd like to know where I've went wrong. I'll do the calculations, I can't find any other way to do them, as of now...need to know if I'm right up to this point.
 
  • #7
Bradracer18 said:
Is my drawing or the last post not clear...I'd like to know where I've went wrong. I'll do the calculations, I can't find any other way to do them, as of now...need to know if I'm right up to this point.
You have a math error up front when you calculated C_y. Right equation, careless math.

Then you left out the A_x reaction when you summed forces in x direction at A.

And one of those members at A will not be in tension.
 
  • #8
Ok...I figured it out...I guess I didn't go over my work as carefully as I thought I did. Thanks for the help...I'm sure by the end of the day, I'll have another one of these posted...ha ha...Thanks again!
 

What is a truss?

A truss is a structure made up of multiple triangles connected together to form a rigid framework. It is commonly used in bridges, buildings, and other structures to distribute weight and forces evenly.

What are the forces acting on a truss?

The forces acting on a truss include tension, compression, and shear. Tension is the pulling force that stretches the truss, compression is the pushing force that compresses the truss, and shear is the force that acts parallel to the cross-section of the truss.

How do I find tension and compression in a truss?

To find tension and compression in a truss, you can use the method of joints or the method of sections. In the method of joints, you analyze the forces at each joint in the truss, while in the method of sections, you cut through the truss and analyze the forces in the resulting section.

What factors affect the tension and compression in a truss?

The tension and compression in a truss are affected by the load placed on the truss, the angle and length of the truss members, and the support conditions of the truss. The material properties of the truss members, such as their strength and stiffness, also play a role.

Why is it important to find tension and compression in a truss?

Finding tension and compression in a truss is important because it allows us to ensure that the truss is strong enough to support the intended load. It also helps us identify any potential weak points or areas of failure in the truss, allowing us to make necessary modifications or repairs.

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