Understanding bar element moments in Nastran F06 file

[marty8040]

Hi - first post here!

I have a beam that is a part of a more complex beam model. The F06 shows that the shears are balanced from end A to B, but the moments are not. This has also been verified using a Patran freebody plot.

Shouldn't the sum of the forces and moments be zero?

---Marty

 

[brianabreugomes]

Hi Marty,

My only suggestion to you is you need to dumb-down the model to understand what you are seeing. You should create a very simple cantilever model (rectangular cross-section with no offsets, x-dir. being longnitual and y & z follow the RH rule) using beam elements. Have it simple supported (constrain the LHS as pinned but allow the RHS to "roll" in the x-dir.) and apply a down load (opp. to the +ve z coord. system) at the center. Take care in defining the element orientation as beam elements are super tricky. Any moment generated follows that a compression on the top surface should produce a +ve moment (but only if the element orientation is correct). What you see in theory should match the FEM output results. You should have 0 moment at the ends and a peak moment +ve at the center.

Cheers,
-B

 

[marty8040]

Thank you very much for the reply!

I have run this test with a cantilever beam and the results were as expected, however, this didn't seem to translate well to my problem. When analyzing a single beam element of a built-up member, should I expect the forces and moments found in the F06 to equal zero? See below...

F O R C E S I N B A R E L E M E N T S ( C B A R )
ELEMENT BEND-MOMENT END-A BEND-MOMENT END-B - SHEAR - AXIAL
ID. PLANE 1 PLANE 2 PLANE 1 PLANE 2 PLANE 1 PLANE 2 FORCE TORQUE
787 -1.793988E+03 1.161979E+04 2.092177E+03 1.914107E+04 -1.077621E+02 -2.085625E+02 -6.519150E+04 7.374604E+02

 

[stresscalcs]

From your question the problem you have is unclear.

Create a complete (this includes any appropriate applied loads) free body diagram of a single element from your F06 data (ensuring the node axes are the same analysis coordinate system) and you should see a balanced element (unless a load is missing from the free body diagram).

Look at the attachment. The moments at A and B are not equal (I quote you "but the moments are not") but the element is in full balance.

Beware that the F06 file load values are applied to the nodes, not to the element.

Richard

 

[marty8040]

Okay, I'm showing a different moment on A and B because I am not asking for all of the loads in the system. Once I bring in the reaction moments, my system will be balanced. Thank you both for your time.

---Marty