Sum of reaction moments do not add up to zero

[Fabrice Demiere]

Hello everyone,

I am simulating a beam intersection, where the two bottom end surfaces are totally fixed (encastré). The end surface of the top arm has some applied displacement, with the rotations fixed to zero.

This is done by creating 3 reference points, and doing a kinematic coupling between each reference point and its face. (RP_arm, RP_left,RP_right).

The model is 2D.

It looks like this:

Deformed it looks like this:

Now I want to obtain the reaction moments along z (RM3), and plot them.

I did it by requesting an xy data ODB field output on my three reference points, and I get this:

As you can see, the reaction moments don't add up to zero, and I don't understand how this is possible, do you have any explaination?


For verification, I plotted the reaction forces along the y-axis, and they do add up to zero, see picture:

Thanks for your help!

 

[StefCon]

I think you have to use forces and distances to calculate the moment around a point that you chose, just like you do by hand. Sum of moments and force*distance should equal 0.
The sum of forces can be done as you did.

Test: Lets say you would allow two of the boundary conditions to rotate (no reaction moment) you would see that a reaction moment still develops in the top boundary condition. That value will not be 0.