Need help on some questions - Moments, Bearings & Trusses

[princejan7]

Need help on some questions -- Moments, Bearings & Trusses

Hi guys, just have some general queries on a couple of problems :

1) http://i1097.photobucket.com/albums/g357/princejan7/reactiondirection_zps85d28093.png

They got the moment of the force BC about A by splitting into components at B and then summing up the individual moments.

Would you get the samet magnitude for the moment if you split the force at some other point on BC's line of action, eg. at C?


2) http://s1097.photobucket.com/user/princejan7/media/bearings_zps342d5a2a.png.html

Don't the thrust and journal bearings prevent rotation about the x and z axis?
The solution doesn't seem to have included these reaction moments. Why is this?


3) http://i1097.photobucket.com/albums/g357/princejan7/truss_zps5c6d8f88.png

In general, when considering the forces on the joints of a truss, how should I know which direction the forces act in?
At the point C, why do BC and CG act in opposite directions?

thanks for any help!

 

[Maiq]

1) You wouldn't be able to use point C for this problem since you don't know where it is exactly and with problems that are in static equilibrium the moment about any point is 0. For this problem you would need to first take the moment about A because that would give you an equilibrium equation with one unknown.

2) Those Bx, Bz, Ax, and Az reactions are preventing the rotations about the x and z axises.

3) All those joints on the truss are in static equilibrium. So at joint B, draw a FBD but have your y-axis go along AC and x-axis perpendicular to AC. Sum up the forces and what do you get?

 

[princejan7]

1) You wouldn't be able to use point C for this problem since you don't know where it is exactly and with problems that are in static equilibrium the moment about any point is 0. For this problem you would need to first take the moment about A because that would give you an equilibrium equation with one unknown.

2) Those Bx, Bz, Ax, and Az reactions are preventing the rotations about the x and z axises.

3) All those joints on the truss are in static equilibrium. So at joint B, draw a FBD but have your y-axis go along AC and x-axis perpendicular to AC. Sum up the forces and what do you get?

http://s1097.photobucket.com/user/princejan7/media/reactiondirection_zps85d28093.png.html
Is there a way to find out if the force acts along the direction BC or CB using equilibrium equations?

I tried by taking the moment about A:

M(A) = [moment of BC about A] -125[4] -600[9]

+5175 = [moment of BC about A]

So I think the moment of BC should be positive, but how do I know which direction the force acts in, BC or CB?
Thanks

 

[Maiq]

Yes it is positive. If you look at the FBD you can see that all the forces besides BC are in the negative direction. So what would that mean for BC?

 

[Nickie]

Hi there,

I would be happy to help you with your questions on moments, bearings, and trusses.

1) The magnitude of the moment of force BC about A will be the same regardless of where you split the force, as long as you are considering the same direction for the force. This is because the moment of a force is dependent on both the magnitude of the force and the distance from the point of rotation. As long as you are considering the same force and the same distance from the point of rotation, the moment will be the same.

2) You are correct that the thrust and journal bearings prevent rotation about the x and z axis. However, it is important to note that in this problem, the only external forces acting on the system are in the y direction. Therefore, the reaction moments for the bearings would only be in the y direction and would not affect the rotation about the x and z axis. This is why they are not included in the solution.

3) When considering the forces on the joints of a truss, you can use the method of joints or the method of sections to determine the direction of the forces. In the method of joints, you would look at the equilibrium of forces at each joint and use that to determine the direction of the forces. In the method of sections, you would cut the truss at a specific point and analyze the forces on each side of the cut to determine the direction of the forces.

At the point C, the forces BC and CG act in opposite directions because they are acting along the same line of action but in opposite directions. This is similar to how a seesaw works, where the forces at each end are equal and opposite in direction. In this case, the forces are acting in opposite directions because they are balancing each other out in order to keep the truss in equilibrium.

I hope this helps clarify your questions. Let me know if you need any further assistance. Good luck with your studies!