Understanding Moment Direction: Analysis and Solution for Homework

  • Thread starter Thread starter chetzread
  • Start date Start date
  • Tags Tags
    Direction Moment
Click For Summary
SUMMARY

The discussion centers on the analysis of a moment diagram for a beam subjected to a moment of +400 Nm at point B and a reactive moment of -400 Nm at point C. The moment at B is a couple that causes the beam to concave downwards, while the moment at C is necessary to maintain static equilibrium. The participants clarify that the moment diagram does not reflect a continuous change from +400 Nm to 0 Nm but rather indicates a constant moment of +400 Nm from B to C, with a reactive moment at C to ensure the net moment is zero.

PREREQUISITES
  • Understanding of static equilibrium principles in mechanics
  • Knowledge of bending moment diagrams and their construction
  • Familiarity with the concept of couples in mechanics
  • Ability to interpret free body diagrams for beams
NEXT STEPS
  • Study the construction of bending moment diagrams for various loading conditions
  • Learn about the principles of static equilibrium and how they apply to beams
  • Explore the concept of couples and their effects on beam deflection
  • Review examples of free body diagrams and their role in structural analysis
USEFUL FOR

Students and professionals in civil engineering, mechanical engineering, and physics, particularly those focusing on structural analysis and mechanics of materials.

  • #31
chetzread said:
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.
 
Physics news on Phys.org
  • #32
SteamKing said:
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
 
  • #33
  • Like
Likes   Reactions: chetzread
  • #34
SteamKing said:
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
 
  • #35
chetzread said:
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.
 
  • #36
SteamKing said:
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 ?
 
  • #37
SteamKing said:
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.
 
  • #38
  • #39
SteamKing said:
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?
 
Last edited:

Similar threads

Why is the 400Nm straight line drawn from the moment point in moment diagrams?
  • · Replies 3 ·
Replies
3
Views
2K
Moment in beam (caused by influence line)
  • · Replies 3 ·
Replies
3
Views
2K
Engineering Where does the bending moment exerted by a wall on a beam come from?
  • · Replies 2 ·
Replies
2
Views
3K
How to determine direction of a moment
  • · Replies 5 ·
Replies
5
Views
5K
Engineering Statics - Determine the Reactions on this bent bar levering between two surfaces
  • · Replies 27 ·
Replies
27
Views
3K
Understanding Moment Diagrams: Why is the Maximum Moment at Point B?
  • · Replies 5 ·
Replies
5
Views
3K
Solving Beam Deflection with Singularity Equations
  • · Replies 2 ·
Replies
2
Views
2K
Moment applied causing bending stress in beam
  • · Replies 1 ·
Replies
1
Views
2K
Beam Deflection Homework: Missing Moment Calculation for Beam Circled Section
  • · Replies 6 ·
Replies
6
Views
2K
Moment at Two Ends of Beam: Understanding Variations and Direction
  • · Replies 4 ·
Replies
4
Views
2K