NASTRAN BEAM ANALYSIS NOT MATCHING HAND CALCS?

[s_jmenzel92]

Hi,

I’m playing around with Nastran 2025, trying to understand it a bit more, I’m sure I’m missing some sort of fundamental element here, I’m hoping someone can shed some light.

I’ve just created a beam (see sketch), for example purposes it represents a trailer beam with known reactions at the coupling and axle bogie centreline.

I’ve created ridged body elements to represent spring/hanger connections.

Rigid body elements are free to rotate in the Z axis, as is the coupling constraint.

The FEA result doesn’t seem to match the hand calculations, generally I think I’m heading in the right direction, but something is just a bit off.

I've calculated a maximum 600Mpa - NASTRAN is showing approx 505MPa at a similar location.

Any help would be very much appreciated.

Kind Regards,

Sam

[s_jmenzel92]

P.S Here is a screenshot of the NASTRAN output.

[John_Holtz]

Hi Sam,

I'm on vacation so did not spend a lot of time looking at this. These are the two things that I do not know:

1. What happens when 2 rigid connectors (not touching each other) are connected by a third rigid connector through the center points. That's an odd arrangement.
2. How the spring/hanger connections and axle bogie is supposed to behave.

In other words, it seems like the 3 rigid connectors do not behave like you think they will. I do know that this statement is not correct: "Rigid body elements are free to rotate in the Z axis, as is the coupling constraint." The checkboxes for the rigid connectors are related to the ends of the connector. The center point will transmit all 3 forces and 3 moments that are applied at the center point. See Difference between rigid and interpolated type of rigid body connectors in Inventor Nastran

for details.

You may want to use beams instead of rigid connectors, and use end releases on the beams to make the hinges where needed. (Right-click on Elements in the model tree.)

John