Beam diagram - Beam moment END A-Y Plane 2 always shows "+" when "-" is correct and opposite

[jura.tomorad]

Assignment shown below was modeled in NASTRAN:

Please find solution attached. Beam diagram - Beam force END A-Z Plane 2 shows correct results. However, Beam diagram - Beam Moment End A-Y Plane 2 shows results with correct absolute value, but in opposite sign. I have tried to model multiple assignments, beam force diagram is always correct, and beam moment diagram is always wrong. Why is this happening?

1D element orientation can be seen in images in the attachment.

Thank you in advance for considering my inquiry.

Kind regards,

Jura Tomorad

[John_Holtz]

Hi Jura,

Think of it like this:

• The paper is 2D, and it is easy to define what convention is + and -. Note that the convention is entirely arbitrary.
• The model is 3D. It is not as easy to define an arbitrary convention on "up" and "down" or other arbitrary view. The beam diagram is based on each beam element orientation. + and - mean something based on the direction of the Y and Z axes, so if you switch those directions by rotating the orientation, the direction of the diagram will change. (The rotation is entered on the Idealization dialog.) 

John

[jura.tomorad]

Hello Mr. Holtz,

I am using Inventor NASTRAN 2022. 

I agree that convention is entirely arbitrary, but only in case in which both results (beam force and beam moment) are inverted. If one is correct, and one opposite, that means that the solution is wrong. Element direction and orientation is visible from attachment "SETUP". 

As displayed on the image below, local coordinate system is in accordance with global coordinate system (graph on the bottom right part of the image is a screenshot from Inventor Nastran support: "How to understand results in beam and bar elements in Inventor Nastran 

https://knowledge.autodesk.com/support/inventor-nastran/troubleshooting/caas/sfdcarticles/sfdcarticl...

:

According to Nastran support, yellow arrowhead represents "Direction", or local x-axis. Also, red arrowhead represents "Orientation", or local y-axis. It is my understanding that these local axes correspond to the axes in idealization dialogue box:

If this is so, solution would have to be the same as on the "ASSIGNMENT" image from previous post.

Please explain to me what am I missing.

Thank you for your time.

Kind regards,

Jura Tomorad

[John_Holtz]

Hi Jura,

You are missing two things:

1. What direction is defined as a positive shear or positive moment result. Nothing discussed so far provides this information. (Hint: they are not necessarily in the direction of the positive axis.)
2. What direction is the diagram drawn in. (Hint: this is undocumented.)

For 1., the definition of positive shear and positive moment, that is given in the documentation for CBAR. I have marked up the figures from that page and used 1-2-3 for the local beam axes to avoid confusion with global X-Y-Z directions.

Based on the figure, the following table of results can be used to understand what direction the shear (V) and moment (M) results are in at each end of the element.

Result	End A	End B
Shear (V) in 2	+V in opposite direction as 2 -V in same direction as 2	+V in same direction as 2 -V in opposite direction as 2
Shear (V) in 3	+V in opposite direction as 3 -V in same direction as 3	+V in same direction as 3 -V in opposite direction as 3
Moment (M) about 2 (right-hand rule at A)	+M in same direction about 2 -M in opposite direction about 2	+M in opposite direction about 2 -M in same direction about 2
Moment (M) about 3 (right-hand rule at B)	+M in opposite direction about 3 -M in same direction about 3	+M in same direction about 3 -M in opposite direction about 3

For 2, the diagrams, it is hard to know if they are right or wrong, logically or illogically designed. There are 4 options for the plane that the diagrams are drawn in (Auto, X, Y, Z) but what is unknown is which direction in the plane is used for a positive result. Based on your results and my results, there is a difference in how they are drawn. (I plotted my diagram using the "Z-Dir" plot orientation. I'm guessing that you diagrams were created using the "Auto" option. (What is consistent is a + result is shown in the same direction for both graphs. You will have a hard time convincing anyone that a + value should be "up" in one graph but "down" in another graph.)

In summary, the shear force and moment values are correct in all cases based on the definition in the table. The diagrams are consistent in that a positive value is drawn in the same direction in both the shear and moment diagrams. This does not agree with the engineering drawings because the definition of positive and negative are different: the engineering diagram has one positive (shear) and one negative (moment) where as Nastran has both values positive.

John

[jura.tomorad]

Hello Mr. Holtz,

I understood the point number 2 in your reply and have adjusted the diagram accordingly:

However, I don't understand how to change the definiton of positive moment result (point number 1). I wish to set the positive orientation of bending moment My in the direction of local y axis (which in my case corresponds to global y axis). 

Is it possible to do? Will it change the moment diagram from "+" to "-"?

[John_Holtz · Accepted solution]

Hi @jura.tomorad 

For your comment:

However, I don't understand how to change the definiton of positive moment result (point number 1). I wish to set the positive orientation of bending moment My in the direction of local y axis (which in my case corresponds to global y axis). Is it possible to do? Will it change the moment diagram from "+" to "-"?

You cannot change the definition of positive moment.

John

[jura.tomorad]

Thank you for your reply,

Kind regards.