Understanding Moment Direction: Analysis and Solution for Homework

[SteamKing]

Why the moment is restricted about A, but not at C?

I don't know what you mean by "restricted about A, but not at C".

The beam is fixed at C, so a reactive moment is supported there. The beam is free at A.

 

[chetzread]

I don't know what you mean by "restricted about A, but not at C".

The beam is fixed at C, so a reactive moment is supported there. The beam is free at A.

https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
I am sorry for the previous post...
I mean why in this thread, the moment of the beam is not restricted about B , but is restricted about A ?
I

 

[SteamKing]

https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
I am sorry for the previous post...
I mean why in this thread, the moment of the beam is not restricted about B , but is restricted about A ?
I

I still don't know what you mean by the moment being "restricted" about A. There is no moment about A.

 

[chetzread]

I still don't know what you mean by the moment being "restricted" about A. There is no moment about A.

As you can see, the moment 400Nm graph is drawn from A to the point where moment 400Nm act, so there's moment restriction about point A, right? Why there's no moment restriction about point B?
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349

 

[SteamKing]

As you can see, the moment 400Nm graph is drawn from A to the point where moment 400Nm act, so there's moment restriction about point A, right? Why there's no moment restriction about point B?
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349

There is no moment applied at point A, so there won't be one showing up on any moment diagram.

There is a moment applied at point B, which is why the moment diagram starts there.

 

[chetzread]

There is no moment applied at point A, so there won't be one showing up on any moment diagram.

There is a moment applied at point B, which is why the moment diagram starts there.

Sorry, I think u misunderstood me... I refer to the thread below...
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
why the moment of the beam is not restricted about B , but is restricted about A ?

 

[chetzread]

If you look at the moment plot at C, it clearly indicates that the reactive moment there is -400 N-m, which, when added to the moment at B of +400 N-m, equals zero net moment.
Do you mean that since the moment at B is positive 400Nm, so the moment -400Nm must be restricted at any points along beam BC to keep it in equilibrium?
Your interpretation of what the plot would look like would require a net change in moment of -800 N-m, which is not supported by the loading of the beam.

 

[chetzread]

There is no moment applied at point A, so there won't be one showing up on any moment diagram.

There is a moment applied at point B, which is why the moment diagram starts there.

sorry , can you help me in the following thread
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
?? Thanks in advance!

 

[chetzread]

It's not clear what you mean here.

The moment diagram doesn't plot discrete values of BM in isolation, like you apparently want to do.

At point B, the moment goes from 0 to +400 N-m, which is why there is a jump at this location.

The moment remains constant at +400 N-m from point B to point C, where the reactive moment of -400 N-m is added to the +400 N-m, to bring the net moment back to zero.

You should review how shear force and bending moment values are plotted for regular beam problems.

it it because of the reaction moment and moment causes the part of beam to become concave downwards, so bending moment is negative?